> YI Me Ye ~ 6 os ek SS te 4) tte AOE LG) LEAP EL AAALER PATOL SS tte ms ey Wi; ij if (ies UY, VEE ‘ J i es aS ‘WOOOVA JO ULY ‘NVALS oy YYA oyBs9do 0} poystuany ‘saoAlIq Jo sated 910M 10 OMY TOF ‘IMVUG AUNSsA_ GAZIIVAOY AdIsSLAG GAUVANVIS ‘VS ‘a “OM ‘SIA0T “ES ‘SHOIAHO TVHANAD ‘Sr sivVv wo.” HATGEOMNO0oO'!L "IHEOH O©LAIOVd GNVUYD HO SUPUALOVEONVN yuesy ‘fury gy ugog SAumpvoig O9T :aolaao OpvoIHO. .-~=s-: £89SS97] SANWdIWOO Z2NVHE HIV SSNOHONILSSM JHL :HOlddO WUOA MAN ‘ANVAWOO SESIWUua NVOIdNV ydng .U9D ‘WVHOIH *0 ‘0 \ % “TaseuvW ‘NOAUCV “I “A PesQeteG=<8=-B=-Be: ox s shaqunog sv) fo quawdinby WMO WHVIAT avyua ay} fo *yWad wad cy UNY} 2, SAA 242 URN pany sivQ WYHIAIT LOT FEL ZO\‘pSl [ . rr emeereecmerceessmm Z(i(" 1G 068I go9‘col ‘ jeanne : ¢90°9¢ 6881 ovs'ZZ O 96912 8881 vrs'‘6p : 183°6 L88I €9S‘Op ? = 976'8 9881 Zi9‘is | y ‘Soe en PUL gg8l Z02‘1% : seed Ci uery es 10S] F981 9c 1‘9 —~,am | 996'F E91 O6I'} . —= ¢80'T G88 gol oe Sa | = ¢0L I88T *[Z10._ puwsry ; Bin | | "1VOK 1G ‘ON “BOR 2w0izdaour ray, fo worywasnyy ponydv19 9 8, No; ‘poddmbo os o18 S1Bod 949 JO uoMm.a0d q ATOATSNTOUOD UeAOCId SBy eoudlIed xo SsuBod 1 JYUSloIJ UL pourezqo o1B S}[NSed 4S0q OTL ‘qstopag ‘ssed Avul Aoy} Gory 1oA0 SABATIBA 1} 09BJSIOJUT Ul posvsus oie oso} JO \LNHO [ 9L NoQwB St TOIT “srBo IYSLOAF OOO TIL (SOMVLG Wd OLLVWOLOV ASQOHONILSAM OL \ 0u} Jo SADUNJOVNUeW Ts ope — & NONILSAM ANL *quopIsoid 8]l VOWS SUDO WYOLa4T 072 AyD OYDWOINP - RB 4Nq WOM SUIBIY JYSIOIy TO pasn ATQeIY [BAdAVS INQ ‘soyBaiq JaMod WIM poy ULBIZ */997 “OO GOUIS SONBIG UOTZOV-yornb poaow UO osn AIOYA AQ SUTBIY JO poods oYY SUTTTOI Waid 08 JnogeB puv ‘A1ZUNOd SIqy JO yuouIdIY UIR[d YIM) SOPN[OU! SIT, “S1Bd QOO'OSS PUl ANVUG OLLVINOLN “ANWdWAIOS AM ‘IoSvUB] [BIOUOY) *£1B1919¢ ‘TSNOHDNILSA A “H “H ‘duvoO "MM RBIBRARY OF iversity O Un —_THp— BALIONAL scoottrve BUILDER Is READ MORE BY GENERAL RAILROAD OFFICERS, MASTER MECHANICS AND MASTER CAR BUILDERS, THAN ANY OTHER PAPER PUBLISHED. DURING THE TWENTY-ONE YEARS OF ITS EXISTENCE THE PAPER HAS STEADILY INCREASED IN VALUE, AND CONSEQUENTLY IN POPU- LARIT Y. Se Se ak ee The paper is edited by Mr. ANGUS SINCLAIR, a practical railroad engineer and mechanic, who was for many years engaged in the building, repairing, and operating of railroad machinery on British and American railways, and who is considered reliable authority on matters pertaining thereto. His book on Locomotive Engine Running and Management is in its seventeenth edition ; his book on Combustion in Locomotive Fire Boxes has met with an extraordinary demand, over twenty thousand copies having been sold in one year. EHVHRY ISSUH CONTAINS THOROUCHLY PRACTICAL ARTICLES ON TOPICS RELATING TO THE OPERATION OF RAILROADS AND TO THE DESIGN, CONSTRUCTION, REPAIR AND ECONOMICAL OPERATION OF ALL KINDS OF RAILROAD MACHINERY, WRITTEN BY MEN WHO CAN DO THE WORK THEY WRITE ABOUT. THE ARTICLES ARE NOTED FOR CLEARNESS AND SIMPLICITY OF DICTION, AND THEY ARE ALWAYS PRACTICAL. NO WRITING OVER THE HEADS OF ORDINARY BUSINESS MEN. Subscription, - - $2.00 a year. Paice, per copy. - - = 20 cents: National Car and Locomotive Builder, 140 NASSAU STREET, NEW YORK. WEEN oe AR FHUR T? WOODS, ‘M. ME. PRoFrssoR OF MECHANICAL ENGINEERING, UNIVERSITY OF ILLINOIs, , PRICE, TWO DOLLARS. , NEW YORK: ‘PUBLISHED BY R. M. VAN ARSDALE, ® Proprietor “‘ National Car and Locomotive Builder,”’ MorsE BUILDING. ) 1891. CoPpvRIGHT, 1889, By AntHuR T. Woops. PREFACE, In the preparation of the series of articles for the columns of the National Car and Locomotive Builder which are here collected in book form, the aim of the author was to combine the description of the various forms of com- pound locomotives which have been actually used, with so much of the theory of the design of compound engines as would seem to be directly applicable to locomotive prac- tice. An effort has been made to present an unprejudiced analysis of each type, and to point out such advantages and disadvantages as are apparently clearly demonstrable, while carefully avoiding matters of individual preference. Free use has been made of all available material, and the authority for data is in general given in the text. The author wishes to specially acknowledge his indebtedness to Engineering and to Mr. Anatole Mallet, civil engineer, Paris; Mr. A. von Borries, locomotive superintendent of the Han- over Railroad; Messrs. Henry and Baudry,of the Paris, Lyons & Mediterranean Railway, and Mr. G. Du Bousquet, of the Northern Railway of France, for courteously supply- ing him with information concerning their designs. CHAMPAIGN, Illinois, January, 1891. IE %\ CONTENTS. CHAPTER I. THEORY OF DESIGN OF TWO-CYLINDER ENGINES. CHAPTHEGIL THEORY OF DESIGN OF TWO-CYLINDER ENGINES. CHAPTER III. DESCRIPTION OF THE WORSDELL-VON BORRIES SYSTEM. CHAPTER IV. DESCRIPTION OF MALLET TWO-CYLINDER TYPE; SIZE OF CYL- INDERS, PORTs, ETC. CHAPTER V. STARTING POWER OF TWO-CYLINDER ENGINES—THE LINDNER TYPE. CHAPTER VI. EcoNoMy OF TWo-CYLINDER ENGINES. CHAPTER VIL. STEAM DISTRIBUTION IN THREE CYLINDER ENGINES—DESCRIPTION or THREE-CYLINDER ENGINES. CHAPTER VIII. DISTRIBUTION OF WORK BETWEEN CYLINDERS—STARTING POWER AND PERFORMANCE OF THREE-CYLINDER ENGINES. CHAPTER IX. FOUR-CYLINDER RECEIVER ENGINES. CHAPTER X. FOuUR-CYLINDER TANDEM ENGINES. CHAPTER XI. COMPARISON OF TYPES—PISTON SPEED—W EIGHT OF RECIPROCAT- ING PARTS—RUSSIAN TESTS. CHAPTER XII. AMERICAN COMPOUND LOCOMOTIVES. 7 a3 | ~ Compounn Locomotives. * OL ALL Bey if The elementary theory of compound locomotives does not of course differ from that of other compound engines, whether they are condensing or non-condensing, or for stationary or marine purposes; but in applying the prin- ciples of compound engines to locomotives, while careful attention must be given to each of the many items which enter into the problem of designing any well-proportioned compound engine, it will be found that some factors, which are of comparatively small consequence in marine or sta- tionary work, become of importance in the locomotive. These differences arise largely from the wide range of power required from locomotives, and the practical neces- sity of keeping the valve gear and operating mechanism as: free from complication as possible. On the other hand, the recent introduction of higher pressures and greater piston speeds in marine practice has rendered some of the work. ing conditions of marine engines and locomotives much more nearly alike than they have been heretofore. ELEMENTARY INDICATOR CARDSs.—The theory of the com- pound engine has been so thoroughly discussed by able writ- ers, particularly in text books on the marine engine, that any further discussion seems almost superfluous; but as the most satisfactory method of designing any steam engine is to start with the elementary theoretical indicator card, and alter and adjust it here and there as experience and the special requirements of the case dictate, we will follow that 10 COMPOUND LOCOMOTIVES. methcd as far as practicable in considering compound loco- motives. The type of compound locomotive which most nearly resembles the ordinary form of simple engine is the two-cylinder receiver engine having the cranks at right angles, the high-pressure crank leading. In this form, the steam, after expanding in the high-pressure cylinder, is ex. hausted into an intermediate receiver from which the low- pressure cylinder is supplied. We will first consider the form of the elementary theoretical indicator cards from such an engine, and for the present will assume that there are no clearance spaces, that steam is admitted and exhausted ex- actly at the beginning and end of the stroke respectively, that the point of cut-off is sharply defined, and that irreg- ularity, caused by the angularity of the connecting rods, may be neglected. If with these assumptions steam is cut off at one-half stroke in each cylinder, the indicator cards would have the general form shown in Fig. 1. The upper card, a, b, c,d, e, f, a, is from the high-press- ure cylinder and the lower card, e, f, g, h, k, e, is from the low-pressure cylinder. The cards are shown as they would be traced by the pencil of an indicator, which is applied. first to one cylinder and then to the other, the same spring being used and both cards having the same length. The two cards are then fitted together as shown in Fig. 1, so as to represent the successive actions of the steam in the two cylinders, the cranks being at right angles. Referring to the high-pressure card, a b is the admission line and b ¢ the expansion line, asin a card from a simpleengine. Atc the high-pressure exhaust opens, connecting the high-press- ure cylinder with the receiver, and the pressure falls to d. The conditions which control this drop in pressure ¢ d will be discussed later. As we have assumed a cut-off at one- half stroke ineach cylinder, the steam valve of the low- pressure cylinder closes just as the high-pressure piston reaches the end of the stroke at c. Hence, as the high-press- ure piston makes the back stroke, the steam is compressed COMPOUND LOCOMOTIVES. Tt in the high-pressure cylinder and receiver until one-half the back stroke is accomplished at e. Here the low-pressure valveopens, admitting steam from the receiver to that cylinder, and from eto f there is free communication be- tween the exhaust side of the high-pressure cylinder, the receiver, and the steam side of the low-pressure cylinder. At f the high-pressure exhaust valve and the low-pressure steam valve close, and the steam expands in the low-press- Kk ATMOS? LINE! ZERO LINE OF PRESSURE Fig. 1 ure cylinder to the end of the stroke g, when it is exhausted into the atmosphere as froma simple engine. The remain- ing lines of the cards do not differ from the corresponding lines in cards from simple engines. FORMULAS FOR CALCULATING PRESSURES.—For calculat- ing the pressures at the various points in the cards we can without serious error make use of the ordinary formulas for ‘*‘moist steam,” or in other words, assume that pressures vary inversely as the volumes, the curves of expansion and com- pression then being rectangular hyperbolas. On this basis 12 COMPOUND LOCOMOTIVES. mean pressures for such lines as a b care determined by the 1 + hyp. log. r "s nized as the ordinary formula for mean pressures, and in which p, is the absolute initial pressure, r is the ratio of expansion, and pm is the gross mean forward pressure, The absolute pressure is the gauge pressure plus the atmos- pheric pressure, which is ordinarily taken as 14.7 pounds per square inch. Tables of hyperbolic logarithms or of 1+ hyp. log. r : formula pm = Pp, (1). This will be recog- values of the fraction for various values of r are given in almost all books on the steam engine, and will therefore be omitted here. Asan example of the ap- plication of the above formula, let the gauge pressure at the point b in Fig. 1 be 145.3 pounds per square inch, so that the absolute pressure will be 160 pounds. As cut-off takes place at one-half stroke the ratio of expansion r = 2, and therefore the final pressure in the high-pressure cylinder will be one-half of 160 = 80 pounds. From the tables pre- viously referred to we find that for r = 2, pm = .847 p,, and therefore pm = .847 X 160 = 185.5 pounds absolute pressure, which is the mean pressure between a and c measured from the zero line of pressures. This formula is applicable to such lines of the card asa be whena bD is parallel to the atmospheric line, as it is practically in engines supplied from a boiler and working at slow speeds. For calculating the mean pressure between b and ¢, d and e, e and f, or for other expansions or compressions in which the part of the card considered is wholly curved, or where no line of con- stant pressure as a b is included, the formula pm =P a (2) is to be used. This formula is de- duced from the same theoretical considerations as the one given above, and the letters represent similar quantities. The former covers the whole stroke from a to c, and the latter only the curved part of the card as from btoc. For COMPOUND LOCOMOTIVES. 18 example, the pressure at b is 160 pounds and the volume at c is twice that at b. The ratio of expansion is therefore 2, and by reference to a table of hyperbolic logarithms we find pm = 160 — = 160 x .693 = 110.9 pounds between b and c. This is for one-half of the stroke, and for the first half, from a to b, the mean pressure is 160 pounds, therefore the 160 = 185.5, which is the same as calculated by the first formula. To avoid needless repetition, the following symbols, which will hereafter be used, are here inserted: v = volume of high pressure (h. p.) cylinder in cubic inches, V = volume of low pressure (Il. p.) cylinder in cubic inches, C = volume of receiver in cubic inches. R = ratio of the two cylinders. PRESSURE IN THE RECEIVER.—As before stated, there is in the case illustrated by Fig. 1, compression in the h. p. cylinder and receiver from d to e, and expansion in the h. p. cylinder, the receiver and thel. p. cylinder from e to f. How much the pressure varies between d, e and f depends upon the capacity of the receiver as compared with the h. p. and l. p. cylinders. In the present case, assume the capacity of the receiver to be equal to that of the h. p. cylinder, or C = v, and let the pressure at e be taken at 96 ponnds. At d the steam fills the h. p. cylinder + receiver, and at e it fills one- half the h. p. cylinder + receiver ; therefore the compres- sion isfromv+ OC = 2vto.5v+ C=1.5 v, and the ratio of compression is 2 v + 1.5 v = 1.83. The pressure at dis then 96 x .75 = 72 pounds, and by formula (2) the mean pressure between e and d is 96 x .86 = 82.6 pounds. At /f the vol- ume occupied is that of one-half the 1. p. cylinder + receiv- er. Assuming for the present case that the I. p. cylinder is 2.5 times the h. p. cylinder, or R = 2.5, the expansion will average for the whole stroke would be 14 COMPOUND LOCOMOTIVES. 2.5 V 2 sion is 2.25v+1.5v=1.5. The pressure at fis then 96 x .67 = 64 pounds, and by formula (2) the mean pressure be- tween e and fis 96 X .81 = 77.8 pounds. The following table shows the pressures at the points d, e and f of the h. p. indicator card with receivers having capacities of v, 1.5 v and 2 v, and with cylinder ratios of 2 and 2.5, which give a sufficiently wide range to cover present practice in com- pound locomotives: be from 1.5 v to +- C = 2.25 v, or the ratio of expan- l as) S é S “a 4 3S 3 ae 3 o> © os o Os © oo ey * ff O 5 as | ai = a= a a3 mn As a r=] 3) ao a Ry nN e 2 OQ 2 oe 2 oe py = Ay = Ay = C= v,R=2..... 80 91.8 | 1067] 91.8 80 91.8 C=15v, R=2..... 80. 88.9 | 100 88.9 80 88.9 C=2 v,R=2..... 80. 87.4 87.4 80 87.4 | C= v,R=2.5 72. 82.6 96 77.8 64 80.2 — C=2 v,R=2.5 69.3 | 75.7 83.2 | 72.4 64 : An examiration of this table shows that, with the as- sumed cut-offs and relative volumes, the actual pressure in) the receiver during one stroke may vary as much as 26.7— pounds, and that in all of the cases given the pressure at f, or the point of 1. p. cut-off, is considerably below that at e, while the mean pressure between e and/f does not differ much from the mean pressure in the receiver. In design- ing compound engines the pressure in the receiver is frequently assumed as constant. It will be seen from the above that this assumption does not introduce any serious error as far asthe h. p. back pressure and the 1. p. mean pressure up to the point of cut- off are concerned. But as the pressure at f is considerably below the mean receiver pressure, the mean pressure be. ‘1 all ye COMPOUND LOCOMOTIVES. 15 tween f and g, calculated on the basis of a constant receiver pressure, will be too high. In many cases, however, it is well-nigh impossible to predetermine the receiver pressure by calculations, and the best that can be done is to estimate it from the known pressure found in similar engines in practice. The nature of these difficulties will appear later, and for the present we will continue to calculate pressures, assuming as before that there is no condensation in the re- ceiver. The table also shows clearly one result of chang- ing the relative capacity of the receiver, viz., that the larger the receiver thesmaller are the variations in pressure in it during a stroke. FINAL PRESSURE; TOTAL EXPANSION.—In a compound en- gine, a certain fraction of the h.p. cylinder is filled with steam from the boiler at each stroke, and after expanding in both cylinders this mass of steam finally fills the 1. p. cylinder be- fore it is exhausted into the atmosphere or condenser. For example, if the cylinder ratio is 2.5 and the h. p. cut- off is at one-half stroke, .5 v is the volume admitted from the boiler at each stroke, and this finally fills the volume 2.5 v before it is exhausted. The steam is therefore ex- panded to five times its initial volume, or the ratio of total expansion is 5, and the final pressure at which it is ex- hausted will be one-fifth of the initial pressure, or 32 pounds in the case we have used for purposes of iJlus- tration. Similarly, if the h. p. cut-off was at three- eighths stroke the ratio of total expansion would be 2.5 X § = 22 = 62, and the final pressure in the I. p. cylin- der would be three-twentieths of 160 = 24 pounds. It will be noted that the only data required in determining the total expansion and final pressure are the ratio of the cylinders and the h. p. cut-off, or, in other words, these results are independent of the capacity of the receiver and of the l. p. cut-off. The effect of the size of the receiver is seen in the shape of the indicator cards due to the compres- sions and expansions ; but how many or how large these 16 - COMPOUND LOCOMOTIVES. variations are does not affect the final pressure. The office of the 1. p. cut-off is to control the division of the work be- tween the two cylinders. In acompound engine, which exhausts into the atmosphere, the steam can be expanded economically until the boiler pressure is reduced to the at- mospheric pressure. Steam at 160 pounds absolute could, therefore, be expanded 77; = 11 times, nearly, if it were 160 14.7 not for losses of pressure by wire-drawing, condensation, etc. In two-cylinder compound locomotives, large ratios of expansion are practically unattainable on account of the large 1. p. cylinders necessary. DROP IN PRESSURE IN RECEIVER.—We are now prepared to show how the receiver pressures which have been as- sumed can be calculated. Taking as before, R = 2.5, C = v, h. p. cut-off at one-half stroke, and I. p. cut-off at one- half stroke, we have the final pressure at the end of the ex- pansion in the 1, p. cylinder equal to one-fifth of 160, or 32 pounds. The ratio of expansion in the 1. p. cylinder is 2, therefore the pressure at the point f is 32 x 2 = 64 pounds. Then, knowing the ratio of expansion from e to f, as al- ready calculated to be 1.5, we have the pressure at e = 64 x 1.5 = 96 pounds, which was assumed for the time in calculating the variations of pressure in the re- ceiver. Working back from this still further, we find the pressure at d as before, 72 pounds, and as the pressure at c is 80 pounds, there has been a drop in pressure of 8 pounds when the h. p. exhaust opened. When the I. p. steam valve closed at f, the pressure of the steam left in the receiver was 64 pounds. Then when the h. p. exhaust opened, the steam which filled the h. p. cylinder at a pressure of 80 pounds mixed with this, and gave a resulting pressure of 72 pounds. This drop represents an actual loss in the effi- ciency of the steam in the engine, since when it occurs the steam expands witlout doing useful work. Itcan be readily avoided, but in remedying this defect we may introduce others which are of moré importance. COMPOUND LOCOMOTIVES. x To prevent drop, it is only necessary to adjust the cut-off of the |. p. cylinder so that the volume of steam drawn by it from the receiver equals that of the h. p. cylinder. For instance, with the dimensions already given in this para- graph, it will be evident that when the l. p. cut-off is at ss or two-fifths of the stroke, there will be no drop, be- cause two-fifths of the 1. p. cylinder is equal to the whole h. p. cylinder in volume, and if we withdraw from the receiver at each stroke a volume which is equal to that re- ceived from the h. p. cylinder, the pressure in the receiver will not be reduced. This can also be readily shown by calculating back from the final pressure in the 1. p. cylinder as before. Suppose ef to represent two-fifths of the l. p. stroke instead of one-half, as shown in the figure, then the pressure at f would be 32 x § = 80 pounds, which would be the pressure in the receiver when the h. p. exhaust opened ; and as this is also the final pressure in the h. p. cylinder, there would be no drop. There is, of course, always more or less drop due to wire-drawing and friction in passages which cannot be prevented, and it must also be borne in mind that all of these calculations are based on the assumption that pressures vary inversely as the volumes. MEAN EFFECTIVE PRESSURE ; EQUIVALENT PRESSURE IN ‘ONE CYLINDER.—With the data already used the mean for- ward pressure in the h. p. cylinder was found to be 185.5 pounds, The mean receiver pressure, or h. p. back press- ure, is 80.2 pounds, and thus the mean effective pressure in the h. p. cylinder is 135.5 — 80.2 = 55.3 pounds. For the l. p. card, the mean pressure between e and f was found to be 77.8 and the pressure at f was 64 pounds. By formula (2) the mean pressure between fandg is 64 x .693 =z 44,4 pounds, The mean forward pressure for the stroke 77.8 + 44.4 2 pressure of 18 pounds, or 3.3 above the atmospheric press- is then = 61.1 pounds, and assuming a back 18 COMPOUND LOCOMOTIVES. ure, the 1. p. mean effective pressure will be 61.1 —18 = 43.1 pounds. As the ratio between the cylinder areas is 2.5, assuming the stroke to be the same in both cylinders, as it generally would be in practice, one pound per square inch on thel. p. piston is equivalent to 2.5 pounds per square inch on the h. p. piston. We can thus readily find the effective pressurein a single cylinder, which is equivalent to the effective press- ure in the two cylinders of the compound engine. Ordi- narily the mean pressure is thus referred to the 1. p. piston, although a reference to the h. p. piston is more convenient for some purposes. In the present case, the effective h. p. pressure referred to the 1. p. piston is — 22.1. The total effective pressure referred to the l. p. piston is then 22.1 + 43,1 = 65.2 pounds. From this we find that the proportion of the total work which is done by each cylinder is, in h. p., ae - = = .84, and in I. p. = .66. If the press, _ es are re- ferred to the h. p. piston, we have 48,1 x 0.5 + 55.3 = 107.8 + 55.38 = 163.1 as the equivalent pressure in one cylinder of the same size as the h. p. cylinder. A common practice has been to make the h. p. cylinder of a compound locomotive of the same size as one cylinder of the simple engine which it is intended to replace. On this basis the theoretical compound engine under discussion would be developing the same work as the simple engine when the latter was developing a mean effective pressure of one-half of 163.1 = 81.6 pounds in each cylinder, EFFECT 07 CHANGING THE POINT OF CuT-OFF.—If in Fig. 1 the h. p. cut-off is made earlier, while the 1. p. cut-off re- mains as before at one-half stroke, a series of changes will be introduced, which are shown in full lines in Fig, 2, the lines of Fig. 1 being repeated in dotted lines. Assuming a cut-off at three-eighths stroke, the final pressure in the h. p. cylinder is 160 X # = 60 pounds, or at c’ instead of ce. Also, COMPOUND LOCOMOTIVES, * 19 as the total expansion is now 2.5 x § = 2° = 63 instead of 5, the final pressure at g is reduced to g’, which repre- sents 160 < 38, = 24 pounds. Then, as the ‘h. p. cut-off is unchanged, the pressure at fis reduced to f’, or 24 x 2 = 48 pounds. The steam which fills the h. p. cylinder ata pressure of 60 pounds is mixed with an equal volume in the receiver at a pressure of 48 pounds, giving a resulting pressure at d of 54pounds. Theresults of this change are, then, that the forward mean pressure in the h. p. cylinder, Fig. 2 the pressure in the receiver, the initial pressure in the 1. p. cylinder, and the mean pressure in that cylinder are all less than before. The work done by the]. p. cylinder is therefore less, while for the h. p. cylinder we have taken from one part of the card and added to another part. The total work done by both cylinders is, of course, less than before, but the proportion done by the h. p. cylinder is . greater, and, in fact, the mean effective pressure in that cylinder has been increased. When both cut-offs were at the same point considerably more work was being done in the 1. p. than in the h. p. cylinder, but by making the h. p. 20 COMPOUND LOCOMOTIVES. cut-off the earlier of the two there is less difference in work than before, or, in other words, the work may be equal- ized by this means. A similar effect will, of course, be pro- duced by making the 1. p. cut-off later than that of the h. p., and conversely by making the l. p. cut-off earlier than that of the h. p. the proportion of the total work which is done by the 1. p. cylinder will be increased. The following table calculated for R = 2and C = 1.5 v will illustrate this: Cut-off.| Mean | Mean | Mean h. p. |Total mean in|Propor'’n press. | press. |press. referred; one cyl. of work. h.p.|l. p 5) De LoD: to 1. p. h.2pold, op. 4 | 4 | 46.6 54.0 23.3 77.8 a Be ary f 4 ae, Os 39.6 25.7 65 4 431 38 es psa ie 48.9 19,6 68.4 soo | etd 4 | $1] 81.5 60.3 15.7 76.0 .21| .79 In practice the pressure in the receiver may be less than that calculated, on account of losses in the h. p. cylinder and passages. The effect of a lower receiver pressure is to increase the proportion of work done in the h. p. cylinder, so that by adjusting the valve gear to give an.earlier cut-off in the h. p. cylinder than in the 1. p., the total work may be very nearly equally divided between the two cylinders. In Figs. 1 and 2 thel. p. cut-off has been taken at one- half stroke, and it was assumed that release occurred in the h. p. cylinder exactly at the end of the stroke. If now we make the |. p. cut-off later than one-half stroke, leaving everything else unchanged, there will he an exhaust from the h. p. cylinder, while the 1. p. steam valve is still open, which will increase the pressure in the receiver and cause what may be called a re-admission in th~ 1. p. cylinder. This is illustrated by Fig. 8,in which the h. p. exhaust oc- curs at b, causing a rise in pressure to c, from which there is expansion as before in the h. p. cylinder, the receiver COMPOUND LOCOMOTIVES. 21 and the 1. p. cylinder until the 1. p. steam valve closes at d. A similar effect will be produced by pre-release in the h. p. cylinder. An examination of a diagram such as Fig. 4 a 0 Fig. 3 may make this subject more clear. In this Fig. bc repre- sents the stroke of the pistons and the circle the path of the crank pins. Taking the direction of revolution as indicated by the arrow, when the h. p. piston is at the end of a stroke or its crank is at ac, thel. p. crank will beat ac’, and the exhaust from the h. p. cylinder which takes place at this position of the cranks will cause the rise in the 1. p. card shown atc, Fig. 3. Ifthe h. p. exhaust occurs before eo the end of the stroke, for example when the piston is at d, the 1, p. crank will be at a e’, and the 1. p. piston at g, caus- ing a rise in the 1. p. card as shown at k, Fig. 3. In cards taken from an engine this increase in pressure will of course be more gradual, and at high speeds may simply cause the 1. p. admission line to be more nearly parallel with the atmospheric line, ORAL. RATIO OF CYLINDERS.—In treatises on compound engines formulas are generally deduced for determining the ratio between the volumes of the two cylinders, so that the total work done by the engine will be equally divided between them. As usually given for receiver engines, these for- mulas are based either on the assumption that there is no drop in pressure in the receiver, or else it is assumed that the receiver pressure is constant. The rule most commonly given is that R equals the square root of the total number of expansions. In designing compound locomotives any such rule could be used only for a rough approximation at best, and in general would be of no value, since the require- ments of construction place a maximum limit upon the size of the 1. p. cylinder, which is less than that theoretic- ally advisable. The ratios which have been used for two-cylinder com- pound locomotives range from 2.74 for small engines to 1.77 for large engines. Mr. Mallet has stated that the ratio should not be less than two. Mr. von Borries, in his pamphlet on compound locomotives, recommends ratios of from 2 to 2.05 for large locomotives with tenders, and from 2.15 to 2.2 for tank locomotives. These ratios have apparently been adopted by other de- signers, as we find that for the greater number of locomo- tives, of which records are published, the ratios lie be- tween 2 and 2.1. With such ratios the division of the work between the cylinders is regulated by adjustments of the valve gear. Larger. ratios than these are used for small two-cylinder locomotives, but it does not appear that ratios smaller than 1.9 have been used except in converted inside cylinder engines. COMPOUND LOCOMOTIVES. 23 CLEARANCE.—In discussing the distribution and action of steam in a cylinder, the term clearance space, or simply clearance, is understood to mean the volume included be- tween the piston when at the end of a stroke and the valve face at that end, and thus includes the steam port, the space between the piston and the cylinder-head, and any other spaces which are in communication with the forego- ing, such as indicator pipes and cylinder drains. One of the principal effects of clearance is to make the effective cut-off later than it would be without clearance, and this produces results which can be best illustrated by reference to a figure. Referring to Fig. 5, let ed represent the stroke of a piston, and assume a cut-off at one-half stroke and 10 per cent. clearance. Then a b is one-half of e d, and the apparent ratio of expansion is two. Lay off e f equal to one-tenth of ed,then f e or ag represents the clearance. The volume which is filled with steam when cut-off takes place is g b, and this expands until it fills the volume of f d. The actual ratio of expansion is therefore fd divided by g }, or in the present pe ose aa ea OS ee eee aaa 1.83 instead of 2. Expressing this +k as a formula, the actual ratio of expansion = —, in which kis the clearance expressed as a decimal of the volume displaced by the piston in one stroke, and n is the apparent cut-off, or one divided by the apparent ratio of expansion. The point c on the expansion curve is, of course, higher with a ratio of expansion of 1.83 than with a ratio of 2, and hence the mean pressure between 6 and c is higher. In making calculations the actual ratio of ex- _ pansion should of course be used, but formula (1) will not then give correct results,as by it the mean pressure be- tween g and c is found, and not that between a and c, and acorrection must therefore be made which necessitates ad- ditional calculation. It is somewhat better to use formula (2), taking the corrected value for 7, and still better in 24 COMPOUND LOCOMOTIVES. most cases to make use of a graphical construction. As an example of the application of formula (2), let the apparent cut-off be at one-third stroke with eight per cent. clearance, Pe ES | The actual ratio of expansion is then 33.08 the mean pressure between b and c will be p, sadl =.594 p,. This is for two-thirds of the stroke, and for the first third the mean pressure equals p,. The mean for the stroke is 2 therefore mes TES bs tl = .738p,. The mean pressure calculated by formula (1) without correction would be .70 ‘Dis oe AS ee eee ba. | ae { ote | oe iS om — oo” N ‘ \ \ > ' ' ' ' 4 4 : ! ' t © Fig. 5 CONSTRUCTION OF THE EXPANSION CURVE.—A simple method of plotting points on the expansion curve is the fol- lowing, which requires only a triangle and a straight edge. In Fig. 5 let O V be the zero line of pressures, O P the zero line of volumes, and p a known point on the hyperbola, Through p draw p s parallel to O V, making it of any con- \ COMPOUND LOCOMOTIVES. 25 venient length. Draw p k and s ¢t perpendicular to O V and draw Os. Through the point u where O s crosses p k, draw u q parallel to O V, and where this line cuts s ¢t at q isasecond point on the curve. Any number of other points can be found from p or q in a similar manner, as in- dicated in Fig. 5. An advantage of this method is that the distance of a point from O P can be selected at pleasure, as it will be always directly under the point to which the diagonal is drawn, as q and s, or « and w. COMPRESSION.—In considering compression or cushion in compound locomotives, we find that it is a factor of the steam distribution which it is more difficult to dispose of satisfactorily than in simple engines. For economy of steam, the pressure in the clearance space when the steam valve opens should be equal to the initial pressure, while the necessary pressure for cushioning the reciprocating parts is a problem in itself. There is, of course, no advantage in compressing to a pressure higher than the initial pressure. In a simple engine having an initial pressure of 175 pounds absolute, and a back pressure of 18 pounds absolute, it is possible to compress to 9.7 times the back pressure before the initial pressure will be exceeded. If the same pressures are taken in acompound engine, for the h. p. initial pressure and the 1. p. back pressure, we have the same possible range of compression, but it is divided between two cylinders. If the receiver pressure is 70 pounds absolute, the possible range of compression is for the h. p. cylinder from 70 to 175 pounds, and for the 1. p. cylinder from 18 to 70 pounds, or 2.5 times in the former and about 3.9 times in the latter. It will be at once apparent that the valve adjustment for compression in the compound is a much more difficult problem than in the simple engine. For example, with 5 per cent, clearance, and the pressures as just stated, the pressure in the clearance space at the end of the stroke would equal the initial pressure in the h. p. 26 COMPOUND LOCOMOTIVES. cylinder when the exhaust closed at 2.5 x .05 — .05 = .075 of the stroke from the end, or at 92.5 per cent. of the stroke, as it is frequently stated. In thel. p. cylinder, an exhaust closure at 85.5 per cent. would fill the clearance space with steam at receiver pressure. With 10 per cent. clearance, and the same pressures as before, the earliest allowable points of exhaust closure would be 85 per cent, in the h. p. and 71 per cent. in thel. p. cylinder. It will be seen from this that a large percentage of clearance in a compound engine may be a positive advantage. The rela- tions between the back pressure, the pressure from com- pression, the point of exhaust closure and the clearance, can be expressed in a general formula as follows: Refer- ring to Fig. 5, let p’ represent the back pressure and p” the pressure in the clearance space at the end of the compres- sion, both measured from the zero line of pressures; let J be the point of exhaust closure, 1 m the compress n curve which is considered asa rectangular hyperbola, d e the stroke of the piston, and fe equal k, the clearance as be- fore. Then the fraction of the stroke at which the ex- haust should close to produce a pressure p” is d 1 +? tai et —1 Jb. (3) The problem of determining the amount of compression necessary to cushion the reciprocating parts does not differ essentially in compound and simple engines, and for that reason will not be discussed at length at this time. The work done in compressing the steam from / to e, Fig. 5, ex- ceeds that done by the steam on the other side of the pis- ton during the same time, and this excessof work tends to retard the piston. It will be evident that in determining the amount of this excess we can regard the back pressure line as the zero line of pressures, if it is parallel to the at- mospheric line, and then determine how much the area 1m e exceeds the area of an equal length from the other end of the card. The actual pressure of the back pressure line, COMPOUND LOCOMOTIVES. 27 or whatever line is used for a zero line from which to measure pressures, is therefore of no importance in this connection, and hence the problem will be very similar in all steam cylinders. INDICATOR CARDS IN PRACTICE.—The indicator cards taken from compound locomotives in practice will, of course, differ greatly from those which have been called elementary theoretical cards, and which were illustrated by Figs. 1, 2 and 38. The cards taken from the engine should agree very closely with theoretical cards which are drawn according to a complete and correct theory. What has been called the elementary theory is not strictly correct, andis incom- plete ; but it is preferable for practical purposes to a more accurate construction on account of its simplicity, and be- cause we cannot predict the exact form of an indicator card from an engine, which differs from existing types to an appreciable extent. The causes which produce modifications of the elementary theoretical card, are chiefly the initial condensation and re-evaporation during expansion; the size, shape and loca- tion of the steam passages and the receiver; pre-release and compression ; wire-drawing due to gradual opening and closing of ports, and the effects of high piston speed. Indicator cards from a compound locomotive, in which steam was cut off at about four-tenths of the stroke in both cylinders, are shown in full lines in Fig. 6. The clearance space is 10 per cent. of the piston displacement in the h. p. cylinder, and 7.5 per cent. in thel. p. cylinder. The vol- ume of the receiver is one and one-half times the h. p. cylinder. With this data the theoretical lines shown dotted in the figure have been constructed, making allowance for the excessive drop shown between the two cards. The differences between the actual admission and expansion lines of the h. p. card are the same as in cards from simple engines, and are due to the wire-drawing during admission and at cut-off, and to the re-evaporation during expansion. ’ 28 COMPOUND LOCOMOTIVES. The extent of these departures from the assumed theoretical curve varies greatly in simple engines, and probably depends upon the piston speed, together with apparently small differences in valve gear and steam passages. The only satisfactory way of determining the probable loss in a pro- posed engine, whether simple or compound, is to examine Sot aah o- Be oceocme es = woe mmm em eer eee Ss SH RH PR SH Se Se Sew owe ora Fig. 6 indicator cards from an existing engine of the same general proportions, and having a valve gear of the same type and dimensions. This is also true of the loss of pressure be- tween the boiler and the cylinder. Indicator cards taken from engines of various makes when on similar service show variations in these particulars of as much as 20 per cent., and it is obvious that no general rule can be laid down. COMPOUND LOCOMOTIVES. 29 In Fig. 6, when the h. p. exhaust occurs at }, the 1. p. piston is at m, and re-admission to the 1. p. cylinder takes place, causing a rise in pressure to m. The l. p. piston moves from this position to that of cut-off f, four-tenths of the stroke, before the h. p. piston has moved over the re- —170 H. P. Cut-off. 70% LP” ” 84.5% Rev. p.min,. 42 H, P, Cut-off. 50% Depa Be Rey. p. min. 147 Fig. 7 mainder of its stroke from b toc, The pressure at c was calculated approximately on the basis of the receiver pressure when the h. p. exhaust opened, being that at f; but if the valve opened with sufficient rapidity this point would be found directly below b. From c to d there is compression as shown, and from d there is expansion until 30 COMPOUND LOCOMOTIVES. compression in the h. p. cylinder begins; but with the proportions above given, this expansion line is nearly parallel to the atmospheric line, or in other words, there is practically no expansion. Turning now to the l. p. card, and taking the pressure at e as that of the steam in the re- —140 H. P, Cut-off. 30% LP.” ” 40% Rev. p. min. 150 — 145 Rev. p. min. 180 Fig. 8 ceiver, we find that the line from e ton is practically at constant pressure, and that the rise in pressure from n to m is comparatively slight. Also, that during the expansion from m to f the fall in pressure is not great. The drop be- tween the h, p. and the 1. p. cards in this figure has been referred to as excessive, and data which would furnish an COMPOUND LOCOMOTIVES. 31 explanation are not at hand, but this does not lessen the value of the cards for our present purpose. In Figs. 7 and 8 are shown indicator cards from two- cylinder compound locomotives at different speeds and points of cut-off. The shape of the h. p. back-pressure line is to be noted. Cards Nos. 2 and 3 are from the same engine, and it will be noticed that the compression up to about the middle of the back stroke is quite marked, and that the remainder of the back-pressure line is nearly horizontal, as it was found in Fig. 6. In Nos. 4 and 5 the compression appears to continue during the whole of the back stroke. This is found to be the case in a considerable number of cards which have been examined, and is particu- larly noticeable at high speeds. It will also be seen that the 1. p. cards do not differ much in appearance from cards taken from simple engines. This would seem to indicate that we can without great error consider the receiver pressure as constant, and therefore treat the 1. p. cards as if they were actually taken from simple engines. This arises largely from the effects of re-admission. It is not likely that, with the ordinary valve gear, the h. p. release will occur later than at 90 per cent. of the stroke, and an examination of a diagram of the crank positions such as Fig. 4, will show that the 1. p. piston has moved over 20 per cent. of its stroke at this position. The |. p. cut-off will not generally be earlier than three- tenths of the stroke, and hence it is safe to say that re- admission will always occur in practice. We have already seen that the practical effect of this is to make the 1. p. ad- mission line more nearly parallel with the atmospheric line, or, in other words, causes the l. p. admission line to more nearly resemble the admission line of a card from a simple engine. In Fig. 9 are shown the admission and expansion lines of four indicator cards from the 1. p. cylinder of a compound 32 COMPOUND LOCOMOTIVES, locomotive. The points of cut-off given are those which were recorded on the cards. The dotted lines indicate the form of the theoretical card for these points of cut-off and for the initial pressures as shown. Cut-off. 60 % R.p.m. 120 OSD re am ome ee oe ee ee ee Cut-off. 50% Cut-off, 40 % R.p.m. 180 Cut-off, 30 % R. p.m. 170 lL SOO 2 PCOS S 222822 22 OSES EH € BEG SSlPF*SVPASCA SVIZFEQSFeCSS er Sat So a oe On card No. 6 a curve which agrees with the actual curve very closely is indicated by dots, and shows an earlier cut-off than that recorded. On card No. 9 the irregular dotted line shows the form of the card from the other end COMPOUND LOCOMOTIVES. 33 of the cylinder with the same nominal point of cut-off. It will be apparent from Fig. 9 that the 1. p. cards do not differ more from the elementary theoretical cards for simple engines than do the actual cards taken from these engines, To obtain a fair idea of the probable form of the cards from a compound locomotive but one factor now remains to be determined, and that is the probable pressure in the receiver. On this hinges the division of work between the cylinders, and there is, in fact, no general rule by which it can be calculated with any certainty. The size of steam passages, and the size and location of the receiver, all steam space between the valves of the two cylinders being in- cluded in this term, will control this pressure to a great ex- tent, as upon these depend the losses of pressure due to friction in passages and to condensation. Mr. von Borries states, in his pamphlet on Compound Locomotives, that for cut-offs of from 30 to 40 per cent. the pressure in the receiver should be from 30 to 33 per cent. of that in the boiler. If the pressure maintained in the receiver of an engine in practice is known, the probable receiver pressure in asimilar proposed engine can of course be predicted with some degree of certainty; but when any different ar- rangement of valves and passages is used, the distribution in previous engines can serve as a guide only. The follow- ing is suggested as a method of approximating to the prob- able receiver pressures. With the same initial pressure in the h. p. cylinder, as the h. p. cut-off is made later the receiver pressure will become higher, and a similar effect is produced by making the 1. p. cut-off earlier. Hence a formula for calculating the receiver pressure should have the form p =c X p; he wae In this formula p is the absolute receiver pressure, p, the absolute h. p. initial pres- sure, and c is a numerical coefficient. An examination of a considerable number of indicator 34 COMPOUND LOCOMOTIVES. cards from compound locomotives gave an average value for c of 0.46, but this value is not recommended except for approximations, and of course no such formula can take the place of direct experiment. COMBINED INDICATOR CARDS.—It is a common practice to combine the indicator cards taken from the several cylin- ders of compound engines in one diagram, and to compare i ps) ee aes ee re Fig. 10 this with an assumed curve of reference. The expansion of the steam in two or more cylinders can thus be compared approximately with an equal expansion in one cylinder, but itis not clear that much can be learned from such a diagram. The reference curve is ordinarily the rectangular hyperbola. Fig. 10 illustrates a combined diagram from a compound locomotive, of which the separate cards as taken closely resembled card No. 4 (Fig. 8). In Fig. 10 vertical distances represent pressures, and horizontal distances rep- COMPOUND LOCOMOTIVES. 35 resent volumes as usual, Both cards must first be reduced to the same scale of pressures. Then take any convenient distance such as bc to represent the volume of the l. p. cylinder, and let a b represent the volume of its clearance space. Then Oa P isthe zero line from which to measure volumes, and O V drawn as usual is the zero line of press- ures. Lay off ad equal to the h. p. clearance space, and d e equal to the volume of the h. p. cylinder, both on the same scale as that of the 1. p. cylinder; or de should equal bec divided by theratio of the cylinders. The outlines of the cards are then found by plotting points as usual. OPA tue eee GENERAL ARRANGEMENT OF PaRTSs.—While the disposi- tion of cylinders and steam chests with regard to the boiler and running gear in two-cylinder compound locomotives does not differ from European practice in simple locomo- tives, the same diversity of design, which has heretofore been remarkable in that as compared with American prac- tice, is found in their compound locomotives. The designer will thus find precedent in existing engines for almost any arrangement of the principal parts, and for any type of valve gear which he is likely to adopt. The credit of having inaugurated the present era of com- pound locomotives in Europe is due to Mr. Anatole Mallet, who designed successful two-cylinder compound locomo- tives for the Bayonne & Biarritz Railroad in 1876, and has since brought out many different designs. While it would not be incorrect to class the greater number of compound locomotives as belonging to the Mallet system, this term as applied to two-cylinder engines is usually re- stricted to those which can be operated either as simple or compound engines at the will of the engineer, as distin- guished from others which are necessarily worked as com- pound engines, except for a brief interval in starting. Engines of the latter class have been built in consider- able numbers since 1880, especially after the designs of Mr. A. von Borries, Locomotive Superintendent of the Hanover Railroad, and Mr. T. W. Worsdell, Locomotive Superin- tendent of the North-Eastern Railway, England. This class of engines comprise what is known as the Worsdell and von Borries system, the essential difference between their designs being in the method of accomplishing the automatic change from simple to compound working. COMPOUND LOCOMOTIVES. 37 THE VON BoRRIES SySTEM.—Figs. 11 and 12 illustrate the arrangement of cylinders and steam connections in two designs of compound locomotives according to the von Borries system. In both figures h is the h. p. cylinder, 7 is the 1. p. cylinder, A is the steam pipe from the boiler to the h. p. cylinder, C is the receiver connecting the two cylinders, V is the starting and intercepting valve, B is the auxiliary steam pipe from the boiler to the starting valve, and D is the exhaust pipe from the 1. p. cylinder. It will be noted that in Fig. 12 the steam pipes, receiver and exhaust pipe are almost entirely inclosed in the smoke-box, and it is very desirable that they should be so placed. 38 COMPOUND LOCOMOTIVES. The arrangement in Fig. 11 is that of asix~ seas at este nee bE 38 3 _ Weight in'-working order, total... 0. .<.:ses sn cease enue .97,000 lbs. ‘ cy = es on driving wheels............. 68,000 ‘* ValVe SCaty .as set tile 0 62 2714 70 Bo See ay tka. cs 3654 1394 eecceee eveces . ola 4 4 i) 2.905} 0:16 0.69 3134 "7 Babe ses Bosh ta shh fe O00 0:84 47 81 ip 0.13 0.78 4384 8214 hol ie ee late oh ig 0-08 0.87 BBG FE ; 0:12 0.87 53h4 8614 See site| te | sh | we A 4 A. 4 Similar variations in the points of cut-off can be obtained with the other types of valve gears which have been men- tioned, for example, by placing the arms on the lifting shaft of the Allan motion at an angle with each other, 48 COMPOUND LOCOMOTIVES, and by inclining the sliding links of the Jey gear. Other minor changes which will produce similar results will naturally occur to the designer, For inside valve gear and outside steam chests Mr. von Borries advises the use of the American form of the Stephenson motion; for inside gear and inside steam chest, the Stephenson and Allan types; and for outside gear, the Walschaert motion. All of the recent Worsdell engines, concerning which information is at hand, are fitted with the Joy. valve gear. SAE ee Ve THE MALLET SysTtEM.—The Mallet system of two-cylin- der compound locomotives has been already referred to as that in which by means of suitable valves the engine may be operated as a simple engine not only in starting, but at any time when in service. Such an engine, while having all the advantages of compound working, possesses an emergency power eyual, or possibly superior, to a simple engine having the same general dimensions, Figs. 17 to 20, inclusive, illustrate the arrangement of this system as applied to a converted six-coupled engine of the Western Switzerland Railroad. For the drawings from which the illustrations were made, and also for other data, I am indebted to Mr. Mallet. The cylinders are 17.7 and 25.6 inches in diameter by 25.6 inches stroke, one of the old cylinders having been retained as the h. p. cylin- der. The driving wheels are 58.4 inches in diameter, and the working weight, which is all on the driving wheels, is 79,350 pounds. In Fig. 17, h and 7 are the h. p. and L. p. cylinders, re- spectively, A is the main steam pipe from the boiler to the h. p. cylinder, B is the receiver, Cis the l. p. exhaust pipe, Dis the starting valve which is connected to the boiler by the pipe EL, F'is the intercepting valve, and G is the exhaust pipe from the h. p. cylinder when working as a simple engine. The construction of the starting valve is shown in Figs. 18 and 19. It consists primarily of a short slide valvea, which, as shown, covers two ports leading to the receiver. The pipe p connects the starting valve chamber with the main steam pipe. On the back of the valve ais an inverted slide valve b, which slides on a seat formed in ‘the valve- 4 50 COMPOUND LOCOMOTIVES. chest cover. A small pipe c connects the starting valve chamber with the intercepting valve, on the other side of the smoke box, as shown at c, Fig. 20. Referring now to Fig. 20, it will be seen that the intercepting valve consists of two circular valves and a piston, all being mounted on Fig. 17 one stem, and so forming a sort of balanced double poppet valve. The connections to the intercepting valve are as in- dicated in the figure, the central opening connecting with the h. p. exhaust, the left with the common exhaust nozzle and the right with the receiver pipe. The operation of these valves is as follows: They are shown in the illustrations in the positions which they COMPOUND LOCOMOTIVES. 51 ordinarily occupy, or when the engine is working as a com- pound. Under these circumstances steam from the boiler is admitted to the space d back of the piston e by way of the small pipe c, the starting valve chamber, and the pipe p. The pressure thus acting upon the piston e keeps the valve g closed against the ordinary receiver pressure. The intercepting valve can, of course, be connected so as to be worked by hand in connection with the starting valve. If now the starting valve is opened, or moved to the right in Fig. 18, steam from the boiler is thereby admitted to the receiver, and at the same time the pipe c is placed in com- munication with the atmosphere by means of the cavity in the top of the starting valve. The pressure back of the NSSSININ TS i= ey Wy GZS Vp Y SSS Fig. 18 piston e being thus reduced, the valve g is opened by the receiver pressure, and the valve his closed, in which posi- tion it isretained by the excess of the pressure in the re- ceiver k, Fig. 20, or that on the l. p. side of the valve, over that on the h. p. side, which is now in communication with the exhaust nozzle. It will be seen that the loco- motive will now work asa simple engine, and will con- tinue todo soas long as the starting valve is kept open. As soon asitis closed, the intercepting valve will be re- turned to the position shown in Fig, 20. On the engine illustrated by Fig. 17, a pressure-reducing 52 COMPOUND LOCOMOTIVES. valve is inserted between the starting valve and the re- ceiver. This reducing valve is of the common differential piston type, adjusted by springs. In addition to this the receiver is fitted with a spring safety valve loaded to 70 pounds pressure. It would seem when a starting valve of this form is used in conjunction with a safety valve, that the introduction of a reducing valve is unnecessary, as the receiver pressure can be regulated by the starting valve. In earlier designs Mr. Mallet has combined the starting and intercepting valve in one distributing valve. This is illustrated by Figs. 21 and 22, which are reproduced from Engineering. The distributing valve and a reducing valve are enclosed in a casing which is fastened to the smoke box. The main steam pipe is connected at a, and thence by a passage b, back of the valves, to the h. p. steam chest. An opening at c admits steam from this pipe to the reduc- COMPOUND LOCOMOTIVES. ; 53 ing valve chamber and thence to the distributing valve chamber. The distributing valve is a slide valve, and covers three ports, as shown. Of these d is theh. p. exhaust, e connects with the receiver, and hence with the 1, p. steam chest, and g leads to the exhaust nozzle. The valve is shown in the position for compound working. If it is moved forward, or to the left in the illustrations, the passage d is connected with g, and the h. p. cylinder exhausts directly to the exhaust nozzle, and at the same Z | Sb ow fli SS Ws I Fig. 20. time by means of the passages c and e boiler steam at reduced pressure is admitted to the receiver and the l. p. steam chest. In the earlier Mallet engines the lifting shaft is divided so that the valve motion of each cylinder is to a certain extent independent of the other. The h. p. valve gear is controlled by a screw and nut, which takes the place of the ordinary quadrant. The nut which is on the h. p. re- verse lever carries a short sector or quadrant and 54 COMPOUND LOCOMOTIVES. a latch on the 1. p. reverse lever works in this sec- tor. The effect is that both cylinders can be re- versed by moving the h. p. lever; while by adjusting SEZ = I a - PYSSSSSSS SONS SSS NSS wy mb 00001 Y lof 4+ aN NP, Z De mea Y y Ya) ames) W bie a A (eS q 57,320 73.2 176 22.8 17.3 24.8 27.3 17.5 57,300 68.1 176 22.8 16.5 23.6 26.4 16.9 88,250 52.4 174 24.8 18.1 25.6 27.6 Lice 80.25 170 24.0 18.0 26.0 30.9 19.8 40,320 85.25 175 24 0 18.0 26.0 24.2 15.5 ; 61.25 160 24.0 18.0 26.0 32.0 20.3 101,600 61.25 160 24.0 18.0 26.0 34.0 21.8 46,48 42.5 160 21.6 10.6 17.0 20.2 13 0 AS 60.00 120 24.0 18.0 24.0 *16.0 79,350 58.4 140 25.6 47.7 25.6 30.4 19.5 * Original diameter, simple engine. 60 COMPOUND LOCOMOTIVES. In the last column of the table are given the diameters of cylinders for simple locomotives having the same boiler pressure, adhesion weight, and diameter of , drivers, calcu- lated by the Master Mechanics’ formula. In the col- umn preceding this are given the diameters of the low-pressure cylinders calculated by the von Borries formula, as already quoted. It will be seen that this for- mula gives, in general, larger results than are found in practice. The same formula, as quoted in the 1886 edition of ‘‘Recent Locomotives,” contains values for Z and p, which will give diameters about 7 per cent. smaller than with those quoted above, which indicates that Mr. von Borries has concluded that larger cylinders are ad- visable. It appears to the writer that no general rule can be de- vised which would be applicable to all cases, and that the character of the work which is to be required of the com- pound engine, as shown by a study of the work done by the simple engine which it is intended to replace, is the only safe guide. The maximum and minimum limits of mean pressure between which the engine will probably work, the usual demand upon it as determined by the character of service, and the type of starting device which is to be employed, must all be given consideration. In brief, if full advantage is to be taken of the economical possibilities of the compound locomotive, the first requisite in attempting its design is a thorough understanding of what it is expected to do. The most satisfactory method of comparing the work done by simple and compound locomotives is a comparison of indicator cards taken from the two forms of engines under the usual working conditions. Complete sets of such cards from compound locomotives have not been obtain- able, but a sufficient number of cards are included in the accompanying table to make it of some value. The figures given for the compound engines were selected COMPOUND LOCOMOTIVES. 61 principally from reports of the performance of various two-cylinder engines which have appeared from time to time in foreign technical journals, and may be assumed to be at least a not unfavorable representation of their capa- bilities. For the simple engines cards have been selected, which were taken at nearly the same number of revolutions COMPOUND. SIMPLE. ' 2 ° ° s : ee oO e e ' o 18 |e |e) ei ge ig lag jae | € 2 ale IB |4alale (2 “ale 8 2 a |e Sey ona ae eo 2 |S No. |e, o|mt & : NH -|\oAn| An | Pe\Ss/Sa) 9) 9) gd lagless|>33 $9 | 9 |8s ofSshad/ |) 8] 84 |Sq'SASIdES| SA 8 Ss S18 mi DlOlha la la (a) | (b) | O |Q i Dol rete 27; 170} 160) .73) .86) 109.0/53.0} 110.0) 105.0) 121 eee CES 2 _| 42] 170] 170| .70) .85| 106.6/43.0| 89.4] 98.0) 122.0, .71| 148 8...0222-| *a7| 162] 160! .70) .73| 92.2/44.9) 89.8] 91.0) 119.0) .75| 146 Eee *74| 173) 165) .60| .65) 75.6|37.1| 74.2) 74.9) 111.0)....| 139 5...e.2.-| 114/ 170) 154) .50} .60} 89.4/45.1) 91.5} 90.5! 85.0)....| 130 Grae... .| *80| 162) 150] .60| .65] 68.3/37.3| 74.61 71.4} 96.0, .46| 146 7....----| 135} 170) 138] .5u| .60| 60.9/35.3! 71.7) 66.-| 101.0] .58) 140 yeh Status: *128| 176] 132} .80) .80) 65.3/31.7| 63.4) 64.3) 80.0) .46| 140 9........| 147| 170] 160! .50| .73| 67.5/29.8| 62.0} 64.7] 62.0] .34/ 130 Hy Bees 120} 162) 157] .50| .50) 57.2/33.2} 83.0) 70.0) 60.0] .42) 130 ieee 150} 168} 143) .30! .40) 46.7/29.4| 59.7) 53.2) 59.5) .42| 135 5 hae ee 161| 176) 162} .30| .40 52.0 26.7 54.5} 53.2) 53.0} .34| 130 Li ear ee 161} 176) 166) .40) .50) 62.9/33.6) 68 5) 65.7] 58.5] .33] 138 Nae See 162) 177) 145] .40) .50| 61.4/36.3) 73.7) 67.5) 75.6] .42) 128 UE Sede 171| 176) 152}; .20) .30} 47.0/23.9|} 48.8] 47.9) 53.8] .29) 136 RG ees 180} 172) 131) .30| .40) 48.2/28.9) 58.7] 53.4) 59.0] .38] 144 Viete: 195) 170) 141} .40) .50| 53.9/27.6| 56.0) 54.9} 53.5] .36] 136 TSosee ece| 209). 160) 147). .50| .73| . 56.5/26.0) 54.1). 55.3) 52.0) .29) 148 By ee c kote 230) 175) 148] .75|....) 55.7/24.8) 516) 53.6} 49.2] .36} 137 ue 41.1/19.2} 39.9) 40.5! 43.0) .36) 135 * Rev. of simple engine; speed of compound not known. as those of the compounds, in order to eliminate the ques- tion of speed as far as possible. For convenience in com- paring the mean pressures in the two types of engines, the average of the h. p. mean effective pressure and that in the |. p. cylinder referred to the h. p. cylinder is given in column (a). It is, of course, not to be assumed that the 62 COMPOUND LOCOMOTIVES. figures given in the same line necessarily represent the best that either engine can do, and isolated examples form no basis for argument, but a comparison of columns (a) and (b), together with the points of cut-off, clearly shows some of the advantages and disadvantages of the compound as compared with the simple system. It is to be borne in mind that the results given in the table for compound loco- motives represent compound working in all cases, and not their capacity when working as simple engines by means of starting or distributing valves. The question of starting power will be discussed subsequently. The table shows that the maximum average mean pressure of the compound is less than that readily attain- able.in the simple engine at slow speeds and late cut-offs. For example, in line 2 of the table, the points of cut-off - being nearly identical, the compound gave an average mean pressure of 98 pounds, with 170 pounds boiler press- ure, as against a mean pressure of 122 pounds for the simple engine, with 148 pounds boiler pressure. The compound is evidently deficient here, and it is necessarily so. The total possible range of pressure in the two cylinders is from 170 pounds to the atmospheric pressure. If there were no losses and the full initial pressure could be maimtained throughout the h. p. stroke, the maximum mean pressure, assuming two as the cylinder ratio and an equal division of work, would be two-thirds of 170 in the h. p., and one- third in the 1. p. cylinder, or about 113.38 and 56.7 pounds respectively. These are the maximum mean pressures at- tainable with 170 pounds boiler pressure. It follows di- rectly from this that if it is required that the compound shall have tractive power equal to that of the simple en- gine in this case, the area of the high-pressure piston would have to be greater than that of the simple engine, in the proportion of about 122 to 100, even though the boiler pressure be increased 20 pounds. On the other hand, a further inspection of columns (a) COMPOUND LOCOMOTIVES. 63 and (b) shows that at the earlier points of cut-off and higher speeds, the compound engine is about equal to the simple engine, from which it follows that a high-pressure cylinder of the same size as one cylinder of the simple engine would be sufficient. The correct size for the h. p. cylinder is undoubtedly somewhere between these two limits ; for if designed for the power necessary at high speed it will probably be deficient at slow speed and late cut-off ; and on the other hand, if made large enough to meetall emergencies, the engine will be over-cylindered for ordinary running. If over-cylindered, the effect will be that, when the only work required of the engine is to maintain the speed on level parts of the road, the necessary mean pressure will be obtained with an earlier cut-off than is advisable with the common forms of valve gear, and asa resultof the large ratio of total expansion, the final press- ure in the 1. p. cylinder wiil be very low, possibly below the atmospheric pressure. The conclusion which seems evident is, that in designing a compound to take the place of a simple locomotive, the basis for calculation should be, as has been already stated, a detailed record of the work done by the simple engine, taking into account the number and rise of grades, the variation in weights of trains, and the frequency of stops. The table also shows clearly one of the causes of the economy of the compound locomotive. Comparing lines 11 to 17, inclusive, we find that the average mean effective pressure of the compound engines is 56.5 against 58.7 in the simple engines, the average cut-off being .33 in the h. p. cylinder of the compound and .36 in the simple engine. It appears from this that the compound engine will do the same work with about one-half the volume of steam. This is not, of course, a measure of the economy, but it is an indication of one of the causes of the economical per- formance of compound locomotives. In the National Car and Locomotive Builder of Sep- 64 COMPOUND LOCOMOTIVES, tember, 1889, were given a number of indicator cards which serve admirably, in connection with data given in the accompanying description, as an illustration of the ap- plication of the principles of design just discussed. Partic- ulars are given of twenty-seven cards, for which the speec ranges from 120 to 312 revolutions, and the mean effective pressures from 71.3 to 41.8 pounds, the average mean effec- tive pressure being 49.8 and the mean cut off about .384. The engine was hauling the New York and Chicago Limited, and only one stop was made between New York and Albany. Anexamination of the above table indicates thata two-cylinder compound locomotive having a high-pressure cylinder of the same size as a cylinder of the simple engine, and with 170 or 175 pounds boiler pressure, could haul this train under ordinary circumstances without dif- ficulty, the only additional requirement which is apparent being the adoption of some form of starting valve which will insure a starting pressure of about 122 pounds in the h. p. cylinder and its equivalent in the l. p. cylinder. This appears to be a very simple case, the principal remaining COMPOUND LOCOMOTIVES. 65 question being, how often would the engine be required to develop more power than is shown by these indicator cards ? The records of the road should furnish the answer which will indicate whether it is necessary or not to increase the size of the cylinders, and if so, the amount of such increase, CHAPTER V. STARTING POWER OF TWo0-CYLINDER COMPOUND LOCO- MOTIVES.—So much has been written on this subject, and so little has been really logically demonstrated, that it may be well to first briefly investigate the starting power of locomotives of the ordinary form in order to compare the two systems intelligently. It is the almost universal practice to measure the trac- tive power of locomotives by applying the formula, Y= ons + *in which d = the diameter of the cylinders in inches, p = the mean effective pressure in pounds per square inch, s = the stroke in inches, D = the diameter of the driving wheels in inches, and T = the tractive power or pull at the railin pounds. This formula is based upon the fact, that, neglecting friction, the work done in both cylinders during any period, such as one revolution, is equal to that done at the circumference of the driving wheels during the same time. The work done in the cyl- inders in inch-pounds is 2 X area in square inches X mean effective pressure xX twice the stroke in inches = 2 X 4 xd? X p X 2s.; that at therim of the driving wheels is the pull in pounds x the circumference of the wheel in inches = 7 x 2 D; therefore, a Xi ede ie Oya pe nx D REP avis bya It follows from the method of deduction that this formula gives an average value for the pulling power, and therefore that, while it furnishes a ready method of comparing the pulling power of locomotives under ordinary conditions, itisof very little use in estimating the starting power, hk, ee COMPOUND LOCOMOTIVES. 67 since the minimum pull, and not the averuge, is then the measure of the power of the locomotive, In the ordinary locomotives, assuming that steam can be admitted during the full stroke, and neglecting the effect of angularity of connecting rods, the minimum pull oc- curs when one crank is on the haif center, the other being at a dead point, and the maximum pull is developed when both cranks make an angle of 45 degrees with the center line through the dead points. This can be readily demon- strated by calculation, or by a graphical construction. There are several methods of representing rotative efforts graphically, one of which is shown by Fig. 24, in which 1 i ' | ! NI | i : 7 | on j ne, 4 ! : mee | le PH Dg yg, eo I ie Xa ae, Le ee aio 4 WA es tN Suk \I7 N A % %JS% % % % % % B Fig. 24 the dotted line a. . a represents the rotative effort, or the tangential pull or push, on one crank pin, and b. . b is that of the other at right angles to it, the steam pressure being assumed as constant throughout the stroke. The method of construction is as follows: Let A B be the length of the circumference of a circle, of which C D, Fig. 25, is the radius. It can be readily shown that the component D F, of the pressure on the piston D H, which tends to produce rotation, is proportional to the sine of the angle a, through which the crank has turned from a dead point. Divide the line A B and the circumference in Fig. 25 into the same number of equal parts. Then through the points of division on A B lay off perpendicular distances, such as kd, equal to the lines which represent the sines of the angles in Fig. 25, such as K D. 68 COMPOUND LOCOMOTIVES. The dotted curve aa represents the variations in rotative efforts on the crank starting from C Z during one revolu- tion, and the curve b b, shown by a broken line, represents the variations in efforts on the crank starting at C M, or at right angles with the first. The total rotative effort is shown by the ordinates of the full line curve in Fig. 24, which is obtained by adding the ordinates of the curves for the single crank, for example,fm=fg+fh. It is evident that the value of the total effort varies between ANandke,. In the first case, one crank is on a dead point, and the other is on the half center, or midway be- tween the two dead points. The pull at the rail is then ¢ zd? Xp X s+ D, which is .7854 of the tractive power as found by the ordinary formula. In the second case the pull is twice that of one crank when making an angle of 45 degrees with the center line, or it is } 7d? Xp X2X .707%s + D, which is 1.11 of the tractive power as usually estimated. Itis also clear that there are four maximum and four minimum points during a revolution. These values are determined, as has been said, on the basis that a constant steam pressure can be maintained throughout the stroke, which would be the case in starting if steam could be admitted to the cylinder during the whole stroke. But when the latest cut-off takes place, when the piston is some distance from the end of the stroke, as, for example, COMPOUND LOCOMOTIVES. 69 at 21 inches with 24 inches stroke, the engine may be ina worse position for starting than that given above as a minimum, When one piston is 21 inches from the be- ginning of its stroke the other will be about four inches from the beginning of its stroke, and its crank will have turned through about 50 degrees from a dead point. If cut-off takes place at 21 inches, no steam can be admitted to that cylinder during the remainder of the stroke, and the work of starting devolves upon the other cylinder. When the piston has moved four inches from the be- ginning of the stroke the rotative effort is about three- fourths of the maximum for one cylinder, and is, there- fore, about .589 of the tractive power as usually estimated. This corresponds to an ordinate of the curve a a, a little to the right of k d, and is evidently the most difficult posi- tion from which to start the ordinary locomotive. The re- duction in the rotative effort on account of the fall in pressure due to the expansion after cut-off and release will be slight. It can be shown on the diagram by laying off radial distances such as C Pand C R on the proper radii to represent the pressures for these crank positions, and using the lines PQ and RF S for ordinates in Fig. 24 instead of those used before. The final effect is shown by the dotted curve at n, Fig. 24. As the locomotive starts the mean effective pressure in the cylinders will be somewhat reduced, but the reduction will not be of large amount within what may be called the starting limits, or until the link would ordinarily be hooked up. As the speed increases the inertia of the reciprocating parts, etc., will be sufficient to modify the form of the diagram of crank efforts, but it is not necessary to consider that in estimating the starting power. Turning now to the compound locomotive, it is apparent that in the Mallet system the starting conditions are almost identical with those in the simple locomotive. If the h. p. cylinder is of the same size as one cylinder of the simple 70 COMPOUND LOCOMOTIVES. locomotive, and the cylinder ratio is two, it is only neces- sary to admit steam of one-half the boiler pressure to the 1, p. cylinder in order to have starting power equivalent to that of the simple engine, the same boiler pressure being used. If thel. p. initial pressure is greater than one-half the boiler pressure, the starting power of the compound will be greater than that of the simple engine in all positions in which the I. p. cylinder is available for use in starting, that is, except when the l. p. crank is on a dead point, or when the l. p. valve is in such a position that steam cannot be admitted. If the boiler pressure of the compound is higher than that of the simple engine, and the h. p. cylinder is the same size as one of those of the simple engine, the starting power of the compound engine will be the greater in about the proportion of the two boiler pressures. In the Worsdell and von Borries type of compound loco- motive the conditions in starting are quite different from those just described. When steam is admitted to the re- ceiver by means of the starting valve, the intercepting valve is closed, and the h. p. piston therefore starts against the pressure of the steam or air which filled the receiver just before the starting valve was opened. The amount of this receiver pressure will depend upon the length of time during which the engine has been standing, - the condition of valves, etc. Ifat starting the h. p. crank is at a dead point, the pencil of an indicator, which is ap- plied to the steam end of the h. p. cylinder during the first stroke, will trace a line similar to abe, Fig. 26. The back pressure acting against the other side of the piston during this stroke is shown by a line such as d e, the press- ure at e being somewhat greater than that at d on account of the compression in the h. p. cylinder and receiver. The initial back pressure is assumed in the present case as equal to the atmospheric pressure. The diagram, a bced, thus represents what may be called the effective indicator card for the first stroke of the h. p. piston. COMPOUND LOCOMOTIVES. 71 When the h. p. exhaust opens the pressure in that cyl- inder and the receiver will fall to some point g, which can be only approximately determined by calculation. It is located on Fig. 26, by calculation on the basis of no conden- Fig. 26 sation or evaporation during the exhaust. The forward pressure on the h. p. piston during the second stroke will be similar to that during the first stroke, and is shown in Fig. 26 by hkl. The back-pressure line during this stroke will consist of, first, a curve g m, which represents the com- pression by the h. p. piston of the steam which fills the space between the h. p. piston and the intercepting valve, until that valve opens; and second, of a line m n, of nearly constant pressure, which represents the back pressure dur- ing the remainder of the stroke, after the intercepting valve opens and the starting valve is closed. It is generally stated that the pressure of the steam, which is admitted di- rectly to the receiver in startihg is reduced by wire draw- ing to about one-half the boiler pressure. Assuming this to be correct, the h. p. back pressure will become sufficient to open the intercepting valve when about five-eighths of the second stroke has been accomplished, as indicated at m, Fig. 26. The net diagram from which the effective pressure on the h. p. piston for the second stroke can be obtained is then hklnmg. A diagram of rotative efforts constructed from these indicator cards is shown in 72 COMPOUND LOCOMOTIVES. Fig. 27 by the curve A EF C FB, from which the reduced effort resulting from the increasing back pressure during ~ the second stroke is apparent. The distribution of work in the 1. p. cylinder in starting does not differ from that in the simple engine. The rota- tive effort will, therefore, be represented by a curve such as H K L D M, Fig. 27, which has the same form as the single crank curves in Fig. 24. The curve in Fig, 27 is constructed on the basis of the initial 1. p. pressure, being one-half of the boiler pressure. If the initial pressure is greater than this, the ordinates of the curve between H and K, K and D, etc., should be proportionately increased. The combined effort of the two cylinders is shown in Fig. 27 by the full line curve. The intercepting valve opens at about the point /, and from that point the engine will work asa compound. It has been already shown that when so working with the customary pressures the power developed at late cut-offs is less than that of the simple engine. The location of the point at which the intercepting valve opens depends upon the pressure in the receiver before starting, the pressure of the steam admitted to the receiver by means of the starting valve, and the size and location of the receiver. For any given combination of conditions it will be found at a definite distance from the point C, or from the end of the first stroke of the h. p. piston. In the present case this point was found to be about five-eighths of the stroke from C. It is obvious that COMPOUND LOCOMOTIVES. uo this action is not at all dependent upon the first stroke of the h. p. piston, but only upon the exhaust from that cyl- inder. It follows from these considerations that, if the h. p. crank is at a dead point at starting, the engine will move through something over three-fourths of a revolution be- fore compound working begins; but, on the other hand, if the h. p. piston is at the position corresponding to P, or near the point at which cut-off takes place, the compound working will begin after about seven-sixteenths of a revolu- tion. If theh. p. crank is in some position such as Q, at which the steam valve is closed, the starting must be ac- complished by the 1. p. cylinder_alone; but after a slight movement, sufficient to carry the h. p. crank over the dead point, the cycle will continue as if started at A, the effect being to prolong the time of direct working of the I. p. cyl- inder to about seven-eighths of a revolution. After compound working commences, and while ad- mitting steam for as much of the stroke as possible, the combined diagram of rotative efforts would be similar to Fig. 24, but with a smaller mean effective pressure, the proportion being with boiler pressures of 170 and 150 pounds in the two types, not greater than 110 to 122, as has been already mentioned. The two diagrams Figs. 24 and 27 are not drawn to the same scale of pressures, but the shape of the full line curves represents with reasonable accuracy the variations in rotative efforts in the simple and compound locomotives. In conclusion, it appears that, with the pressures customary in the two forms, the pulling power of the Worsdell and von Borries’ type of compound locomotive in starting may be greater than that of the simple engine having cylinders of the same size as theh. p. cylinder, during the first half revolution ap- proximately, but that after this the power of the compound engine diminishes until it is from eighty to eighty-five per cent. of that of the simple engine. There is another type of two-cylinder compound locomo- ° 44 COMPOUND LOCOMOTIVES. tive which has not as yet been described in these pages and for which somewhat different conditions exist in starting. In this form the marine practice is followed of placing a valve or cock on the receiver, by which steam from the boiler can be admitted to the receiver and the 1. p. steam chest, without the addition of intercepting valves or other complications. This arrangement was applied to a two- cylinder compound locomotive in India by Mr. E. W. McK. Hughes, in 1882, and isin use in Germany in a modified form known as the Lindner starting valve. The latter is -_———~—. -_ —— - ~~. ee — ee ae | ill | | Yt || UL AVY N , WUdddddddddidirrrerrés Fig. 28 illustrated by Fig. 28, which is reproduced from the Rail- road Gazette. In Fig. 28, C is the receiver, # is a small pipe connecting the receiver and the main steam pipe, and J is the starting valve, which has two ports, H and J, formed in it at right angles. The lever K by which the valve is operated is connected to the reach rod, and the proportions are such that K turns through ninety degrees, COMPOUND LOCOMOTIVES. %5 as indicated in the figure when the reverse lever is moved from one extreme position to the other. The effect is that steam from the boiler is admitted to the receiver when the valve motion is in either the extreme forward gear or the extreme backward gear, and that the cock is closed for intermediate positions. Another feature of the Lindner system is the introduction of two small ports, each having an area of about 0.17 square inch, in the high pressure slide valve, which are so located that when the valve covers the steam port, as after cut-off takes place, that end of the cylinder is connected by means of one of these small ports with the exhaust side of the valve and thus with the re- ceiver. The effect is to admit steam at low pressure to the end of the h. p. cylinder, which is covered by the slide valve, and as the other end is then open to the exhaust and hence to the receiver pressure, the pressure on the two sides of the h. p. piston is partially equalized. In other words, the effective back pressure on the h. p. piston is more or less reduced, so that it offers little resistance in starting. This device is, of course, useful in starting only for the piston positions between full gear cut-off and the end of the stroke. The possible effect of this arrangement of starting gear will depend upon whether or not a safety valve is provided to limit the maximum pressure in the receiver. If this re- ceiver pressure is equal to one-third of the boiler pressure, with a cylinder ratio of two, the effect of the starting valve is to enable the engine to start with very nearly the same distribution of pressures on the pistons as would be found when it is working as a compound in full gear. The re- sulting rotative efforts will then be represented by a curve such as the full line curve in Fig. 24, the ordinates or actual pressures, however, being less than those for the simple engine in about the proportion of 113 to 150, with boiler pressures of 170 and 150 pounds. If the receiver pressure is allowed to become higher 76 COMPOUND LOCOMOTIVES. than one-third the boiler pressure, the back pressure on the h. p. piston is increased proportionately, and the result is, that the power of the h. p. cylinder is reduced, while that of the l. p. cylinder is increased. The advisability of using the higher pressure depends upon the positions of the cranks at starting. Ifthel. p. crank is at a dead point, the maxi- mum effort will be obtained by not admitting any steam to the receiver at the instant of starting, but before the en- gine has made one-eighth of a revolution pressure in the receiver will be necessary to enable the 1. p. piston to act. The other extreme is when the h. p. crank is at a dead point in starting. When this is the case, the l. p. crank being then on the half center, full boiler pressure could be ad- vantageously used in the 1. p. cylinder, with the result of obtaining a rotative effort about four times as great as ina simple engine s.arting with the same crank positions. But similarly to the first case, the receiver pressure should be reduced almost as soon as the engine begins to move, or else the h. p. piston will be practically thrown out of ac- tion, and the engine might be stalled after making one- fourth of a revolution. It appears, then, that with this starting valve and a properly loaded safety valve the starting is very simple; but the power is less than that of the simple engine having cylinders of the same size as the h. p. cylinder of the compound, the boiler pressures being 170 and 150 pounds, respectively. With no safety valve, the utility of the device depends upon the position of the crank and the judgment of the engineman, and to this it should be added that the 1. p. cylinder, piston, rods, etc., must be made strong enough to bear the full boiler pressure. It is evident that each of the several types of starting arrangements has decided advantages and disadvantages, With the Mallet type considerable complication seems un- avoidable, but the cylinders can be proportioned for the usual work on the road, and the engine will nevertheless COMPOUND LOCOMOTIVES, 77 be very powerful in starting. With the Worsdell and von Borries type there is also complication, and the engine if - proportioned as above will be powerful at the start, but the power rapidly decreases. The Lindner type has the advantage of great simplicity, but the cylinders must be considerably larger than for a Mallet engine of equal starting power. CONDENSATION IN CYLINDERS.—It is not intended to dis- cuss under this heading the effects of initial condensation and re-evaporation during expansion and during exhaust, but rather to call attention to some of the special require- ments of the compound locomotive in this connection. It has been repeatedly shown by writers on compound en- gines that the loss from condensation should be less in the compound than in the simple engine, under similar condi- tions, but the gain in this direction may be more than overcome by faulty mechanical arrangement. That is to say, if the cylinders, steam passages and receiver are poorly protected from losses by radiation, the doss by condensa- tion may become sufficiently great to seriously diminish the theoretical thermal advantages of the compound en- gine. When steam which is initially dry is expanded in a non- conducting cylinder, or without gain or loss of heat, a par- tial condensation takes place, and the greater the range of pressure is during expansion, the greater will be the con- densation. As the transfer of heat between the steam and the cylinder walls is less in the compound than in the simple engine, the conditions in the compound are more nearly those of non-conducting cylinders, while the ratio of ex- pansion is greater, and hence a greater amount of conden- sation during expansion might be expected. Therefore, if the initial steam is dry in both cases, we would expect to find more moisture in the I. p. cylinder of the compound at the end of the expansion than in the simple engine. This is by no means a complete statement of the case, and is 78 COMPOUND LOCOMOTIVES. only intended to indicate why the presence of considerable moisture in the exhaust from a compound locomotive is not necessarily evidence of uneconomical working. In discussing this subject, Mr. von Borries has written substantially as follows: ‘‘Since in compound engines re- evaporation is very much lessened—one of the advantages of the system—the steam in the cylinders is always more moist than usual, and in the large cylinder always con- denses to some extent. In order that the water may not have an opportunity to re-evaporate, it should be removed. This is most readily accomplished by cutting notches about 2 millimeters (0.08 inch) in width in the cylinder cocks with a sharp-edged file ; by this means the cocks are kept suffi- ciently open to allow the water to escape.” ‘‘ The steam escaping from the stack is always wet; this, however, attends the system, and is no sign of priming, but of the fact that much heat has been withdrawn and converted into work.” In some designs of compound locomotives ‘‘safety valves,” or, more properly, automatic water valves, are fitted to the ]. p. cylinder to prevent damage by possible accumulation of water. This is, however, good practice for all large steam cylinders. The above remarks simply emphasize the statement already made as to the advisability of carefully lagging the cylinders, and of inclosing the receiver in the smoke-box in order that the steam delivered to the 1. p. cylinder may be as dry as possible. ExHAUST NozzLes.—There is a very noticeable difference between the exhaust from compound and simple locomotives. Asthe averaze expansion in the compound is considerably greater than in the simple engine, the final pressure in the low-pressure cylinder is comparatively low and the intensity of the blast is therefore much reduced. Also, instead of four exhausts in rapid succession during a revo- lution there are but two, and the total volume of steam COMPOUND LOCOMOTIVES. 79 exhausted in a revolution issomewhat greater. The re- sult isa more even and less intense urging of the fire, from which more perfect combustion and transfer of heat to the boiler are to be expected, while the quantity of small coal drawn through the tubes will be proportionally less. The single circular nozzle appears to be the favorite with the designers of compound locomotives abroad, the diameter of the nozzle being in the two-cylinder compound engines concerning which the writer has information, about one-fifth of the diameter of the low-pressure cylin- der. The best proportion will naturally have to be deter- mined by experience for different kinds of coal and for the various kinds of service. SEQUENCE OF CRANKS.—The natural sequence of the cranks in two-cylinder compound locomotives would seem to be to place the low-pressure crank 90 degrees behind the high-pressure crank for forward running. Mr. Urquhart is reported as having tried the effect of placing the low- pressure crank leading in forward motion. The following is quoted from a paper recently read by him in England, as given by the Master Mechanic: ‘‘From indicator diagrams taken in forward gear with the latter arrangement, it ap- pears that inconvenient and very excessive compression takes place at the first notch in the high-pressure cylinder; so much, indeed, that this notch is not used, but in all other notches a good distribution takes place. In the low- _ pressure cylinder a much better distribution takes place at all notches compared with engines having the high- pressure crank leading; and this engine with the low- pressure crank leading, which has only recently been put to work, seems to develop more power and to burn less fuel than the others having the high-pressure crank lead- ing. . . . The receiver capacity is equal to that of the high-pressure cylinder, and all the dimensions are the same in both the above cases. The idea of making one engine, by 80 COMPOUND LOCOMOTIVES. way of trial, with the low-pressure crank leading in forward running originated in its being’ accidentally noticed that one of the other engines, compounded with the high- pressure crank leading in forward running, seemed to work better and to be more powerful, developing its full tractive force with an earlier cut-off and making steam with greater freedom, when running backward with its train, in which case, of course, the low-pressure crank became the leading one. . . . The only objection the author now sees to the low-pressure crank leading is that it gives too much power in the large cylinder and too little in thesmall, thus putting the engine out of equilibrium; but this irregularity will in a great measure be obviated by having a receiver about half as large again as those now used on all the present compounds.” The writer has not as yet been able to discover any reason why, with a valve gear which gives an approxi- mately equal distribution for both ends of the cylinders, and with the usual long connecting rods, there should be any practical difference in the steam distribution with the two arrangements of cranks mentioned. Neglecting for the moment the irregularity caused by the connecting rods. for any position of the high-pressure crank, the low-press- ure is at the same relative point in its revolution whether it is leading or following. For example, if the high-press- ure crank is at a dead point, the low-pressure crank will be at either the upper or the lower half center, and the piston and valve will be in the same relative positions, the only difference being that the piston is making the forward stroke in one case and the backward stroke in the other. The same is evidently true for all positions of the high- pressure crank, The relative direction of the stroke will be changed, but the high-pressure piston will be at the same distance from the beginning of a stroke, whether its crank is leading or following. The irregularity in steam distribution caused by the connecting rods must be slight. COMPOUND LOCOMOTIVES. 81 with the long rods which are commonly used in locomotive practice. It would seem that there must be some other explana- tion of the difference found by Mr. Urquhart than the sequence of the cranks, and that the real cause has escaped his notice during the short time which this engine, with the low-pressure crank leading, has been in operation. ORLA TEE Vite ECONOMY OF TWo-CYLINDER COMPOUND LOCOMOTIVES, — The various reasons why compound locomotives should be more economical than those of the common type have been so often stated at length before the railway clubs in this country, before engineering societies abroad, and by the advocates of the different systems of compound loco- motive construction, and the whole theory of compound or multiple-cylinder engines has been so thoroughly devel- oped, especially in the numerous books on the steam en- gine, that an extended discussion of the question in this chapter seems to be uncalled for. Nor does the compound locomotive need the support of arguments to show why it should be more economical than locomotives of the or- dinary type, inasmuch as the economy of the compound has been repeatedly demonstrated by that best of all argu- ments, actual tests in service. Admitting, then, that a noticeable saving in fuel follows the use of compound locomotives, there remains to be shown the actual amount of this saving, under what cir- cumstances it will pay to build compound locomotives, and the conditions which will make it advisable to alter existing simple locomotives. The accompanying table contains a list of comparative tests of compound and simple locomotives which would seem to be sufficiently comprehensive in number, duration, boiler pressures, location and kind of service to convince the most skeptical that there is considerable saving in the use of the compound system. The data for this table have been collected from various sources which are presumably reliable. A considerable number of tests are quoted from Mr, Lapage, and for the more recent results in Saxony, I COMPARATIVE 4 | Where Tests No. were made. Date. Dur: vee es 1./Prussia.....- 1883 | 2 mo ae Sad blac washes 1883 | 2 mo a lS Aiea: 1883-4) 9 mo 4, hh ae 1883-4' § mo 6. Be we eS 1884 2 mo 6.) os ‘ 1884 2 mo TeNIHOLA Us. Siok TSE IT cies 8.|Prussia ..... 1884-5| 2 mo 9, se 1884-5! 2 mo 10. be, paper reas 1885 | 4 mo 11. England.... |1886 | 3 mo 12. ‘ Los ey | { 132 Prussians... sc 1887 | 6 mo 14. Saxony.......1887 | 6 mo iby VERE 1887 | 4 mo Argentine | al Republic \ (1887 1 mo 17. England.,.... 1888 | 2 mo 18 |Saxony...... 1887-8 18 mo 19. pa Gere i , 1887-8 18 mo 7) a | 4 . : 9 Cylinders. S Whore Tet ead ener pa ere Tests No. were made. Date.| Duration. Hs oa ‘p ‘ 2o| «8 oo E%| “it | Diameter,| <= i: Geet Inches. i ie 4 2 TL eS ee! nae 3 ie 1 esl SS Ga a, ‘tae az - 1.'Prussia....... 1883 | 2 months {| ; eaee are eg pd foxes se, an 1883 | 2 months {|} |-------: SB tts fa 8 an ieee-dt Ssrionthas be, fentecan| Ae ae O28 3 4. et he Ni 1883-4, § months { : eet Sa Sopa ane 4 Qo. OS, 1884 | 2 months { : a aed eter G oe a gst 2 months {) fa | OFS | iB: = & 2 Me} 7.\India........ 1884 eee: Rees ag pines ie a : | 23 69 8.'Prussgia .....| siete 2 months { : eee At oR ‘ia oa a5 bt y) 1884-5 2 months {| 7777777) "tg 51 "| So'08 t 10. See 1885 | 4 months : i < af vies ais 3 oe: & 11. England.... 1886 | 3 months { te tari a or , uae ee Be = 10 | 185,091 |18 26 | 24 | ae, 1886-7) ....-- seers { AR RN) 1320 aS own, mein Reaeg ake 4 3.372 |18.11 25.6 | 24.8 fe 13.’ Prussia...... 1887 | 6 months \ 4 ey 17.79 04.8 3 | fon 6.77 18.12, 25.6 at } il 4,426 16.75 Pegacany.. «11887 | 6 months 4 | 4 9°91 18 4 , M2 | 1808 17.25 24.75] 21.6 Mig |... oP CACTI Ee | a a oe, OSS ames 19 Fs Erg eae 3,638 [16 23.25) 24 16. ATE lie { 1887 | 1 month t | 3937 |..:... Sr Wes ete ee eee: | | i9? 58 1098.9 COMPARATIVE FUEL CONSUMPTION OF TWo-CYLINDER COMPOUND AND ORDINARY LOCOMOTIVES. nae eel Square Inches. face, Diameter, feet. 52.36) 16.47|1308. ‘ 54.02/18 .30|1259 .4 52.36/16.47)1308 .9 51.10/21.53|1345.5 52 .36|16.47/1308.9 52.36|16.47|1340 .2 44.49} 5.81) 247.6 44.49} 5.81) 247.6 52..36|10.47|1308.9: 52 .26|16.47/1340.2 52.36|16 47/1308 .9 52.36) 16.47/1340.2 18.84|1054.9 1938/1038 .7 18 .84|1054.9 16 .68)1334.8 16.47|1308 9 16.47)1340.2 73.23 73.23 73.23! 77.95 52.36 52.36 84 52.3 |16.68'1291.7 52.3 |16.68 1345.5 55.6 (15.17 1238.1 55.6 '15.17 1288.1 55 6 15.17 1237.8: 55.6 15.17 1235.7: 73.75 | os. me @ | Feese 2 6 eo e &% «¢ Bigs o'2 & Ss eee ere e's. 6.0 @ 1 D6) 6 peas 61, @°0 © | Sen.e ace ia 75 75 19.58 1098.9 55 6 15.17 1238.1 “65.6 15-17 72384 55.6. 15103233 55.6 15.17 1235.7 +t Engine and tender. af © 2S Viele Sie wee) ©. 2 or Oe © CP OL igig ac PED 12 2 2 2 999 ts lee Wie SS 6. 14% Mat ft DN a Sg eT SS on BE may te Ao | bo aa) oh an Remarks. DRO] & o od s | &f SES] SS 88 eG = nl E O In 45.4 149 ae Freight. 40.8 | 59.96*|18 (Two snecia: trains on 43.1 | 73.80*| . \f mountain sections 7.4 | 92. 25% 20 |* only. Max. grade, 1 in \ 64. (S Saving calculated from SRR rene ae average actual con- 42.4 eri ptt { sumption and ordinary ' consumption. 1h Ga eis Poy Local passenger. Saving UBS ths ae { as in No 43.1 | &7.98* Freight 42.4 | 67.06*|14.3) *§ 43.1 | 46.48%), - |Special. 42.7 | 55.35” 16 33 10 Both types of engines do- 82 98 1435 ing ee same class of wo 41.9 127.73") ....|Special, passenger. 38.5 |152.56*|16 sb 41.9 |115.47*|. Passenger. 47.9 |170.30*|14.5 43.1 | 64.58%}. -|Freight. 42.4 | 76.64*|16 49.8 | 29.4 |....|/Passenger. aks da.8 113 <¢ 45.2 | 34.9 |..../Freight. Seipaiee ELL! RMeG 8 0: Sy # fetes 9 Coal trains—load drawn 42.9 | 40.08 by compound 8 per cent. a 9 | 47.55 |16_ aie 45.1 | 43.1 Freight 44.4 | 56.7 as A250 22.0 oe 43.5 | 55.7 (22.6) < | Ce Oh renee eral oat eee et ae ot era, 18) Sac eae See ee 40 22.36 |....|Passenger. es 28.19 |20.7 < | | 91 1+} 28.3 |....|/Passenger. S025 30 O21 e 47.4 | 25.2 |....|Express, 46.1 | 31.9 (21 3 45.8 | 43.4 |..../Freight. 44.4 | 52.9 |18 a 42.9 | 522 \i7 tr 43.5 bo.2 Aa as ene et COMPOUND LOCOMOTIVES. 83 am indebted to Mr. D. L. Barnes. The table shows that the saving in fuel is from 13 to 24 per cent., and it will be noticed that the highest percentage of saving is for a six months’ trial (No. 14) of two engines working with the same boiler pressure and of almost identical dimensions. Other cases in which the same boiler pressure was used are Nos. 2, 4, 6, 7,16and19. In No.7 the boiler pressure was but 120 pounds, which is generally admitted to be too low for the best results ot compound working. The saving in this case was 13.5 per cent., and the next lowest, with equal boiler pressure, is(No, 6) 16 per cent, It has been claimed by several engineers, whose reputa- tions give weight to their opinions, that a large part of the economy of compound locomotives is due to the higher boiler pressures employed, and that equally good results can be obtained with simple engines. On the other hand, engineers of equal prominence have failed to find any ma- terial advantage in the use of what may to-day be called high pressures in ordinary locomotives. There is undoubt- edly from theoretical considerations a marked economy in the use of high pressures, provided that full advantage is taken of the greater range of expansion which the higher pressure makes possible. It is not practicable to obtain economical results if a large ratio of expansion is used in one cylinder, on account of the greater condensation, to say nothing of the great variation in the pressure on the crank pin during a stroke. We thus find two conditions which are conflicting as the pressure is increased, and in the loco- motive we have the further obstacle that with the ordinary forms of valve gear it is not practicable to cut off steam very early in the stroke. There is no advantage in using ‘higher pressures and correspondingly smaller cylinders, so that the points of cut-off will remain about the same as with the lower pressure, because while the volume used per stroke to obtain the same total pressure on the piston is less, the weight of each cubic feot of steam has increased 84. COMPOUND LOCOMOTIVES. with the pressure, and the amount of heat drawn from the boiler per stroke will be very nearly the same. In the compound or multiple-cylinder engine advantage may be taken of the possibilities of high-pressure steam without encountering the losses which follow from extremely early cut-offs in a single cylinder. This is another point of superiority of the compound engine, and while some saving may be accomplished in the simple locomotive with higher pressure by means of somewhat smaller cylinders and a slightly earlier cut-off where practicable, it does not appear that a saving of fuel equal to that of compound engines can be reasonably expected. The general average of the results in the table, computed by giving each test a value proportionate to the time covered by it, is 18.5 per cent. Making allowance for the probability of -special care having been exercised in some of these tests in favor of the compound, it would seem to be safe to estimate upon the basis of 15 per cent, saving. - The annual saving per engine on any road on the basis of an assumed percentage, such as 15 per cent., is of course easily calculable from the record of engine mileage and fuel consumption. If the ordinary engine for which it is desired to substitute a compound makes M miles per annum and 7’ miles per ton of coal, which costs D dollars per ton, the annual saving by the compound would be in LLB ONT OKT, dollars a For example, with coal at $2 per ton, 30 miles per ton, and 20,000 miles per annum, the : : 20.000 x 2 xX .15 annual saving per engine would be =o ee = $200. It is obvious that the advisability of using the compound system varies directly with the total cost of fuel per engine per annum, and not necessarily upon the cost of fuel per ton. For instance, with 25 miles per ton, 25,000 miles per annum, and coal at $1.33 per ton, the annual saving per engine will be the same as that given above. COMPOUND LOCOMOTIVES. 85 Mr. von Borries gives the cost of his compound locomo- tives in one case as from 2 to 3 per cent., and in an- other case 4 per cent., more than ordinary locomotives of equal weight. He also states that ‘‘ The power of com- pound freight locomotives is known by experiment to be Sto 10 per cent. and of compound passenger and ex- press locomotives 10 to 15 per cent. higher than that of ordinary locomotives of equal weight, so far as not hindered by the adhesion of drivers to rails. For an equal power the compound passenger locomotive is lighter and cheaper than the ordinary locomotive.” Accordingly, for equal powers, the first cost is ‘‘ in freight engines two to five per cent., andin passenger engines eight to twelve per cent. lower than ordinary locomotives, similar construction of course being assumed in both cases.” Mr. Lapage makes a similar argument in comparing a compound with a simple engine weighing about 364 tons full. ‘‘ As there is a saving of from 144 to 20 per cent. of fuel, the grate area and heating surface may be reduced in proportion, and also the boiler slightly; and the boiler not being so large will bear a somewhat higher pressure, while the weight of water in it will also be less. These items make a consider- able difference, for the boiler alone weighs about 8.75 tons, and the water it contains about 2.4 tons, making a total weight of 11.15 tons, of which about 1.5 tons may be saved with the compound. Again, the motion work, springs etc., may be made somewhat lighter, as the strain on the compound is not so great as on the ordinary engine. With- out taking this into consideration, however, there is a sav- ing in metal of about .75 ton in the boiler alone.” The ‘saving in the weight of the tender, as less fuel and water ‘are required for the same service, is estimated at about two tons, thus making the saving of metalin the engine and tender about three tons. ‘‘ The cost of the compound for the same power is thus brought to about the same as that 86 COMPOUND LOCOMOTIVES. of the ordinary engine, or rather less, The engine and tender being lighter and requiring less coal and-water, there is a consequent saving in the hauling.” These conclusions are logical, but it does not follow that it will be considered advisable in many cases to reduce the capacity and consequently the weight of the tender, as the increased endurance of the engine with the present tank and coal space may be of more importance. However, the subject of the most economical capacity of tenders for a particular line of service has not in many cases received the attention to which its importance entitles it. On the ° assumption that the proportions of the boiler of the simple engine which is to be replaced by a compound are satisfac- tory, including grate and heating surface and steam space, there is a possible reduction in these which is worthy of consideration. To offset this we will have the increased weight of cylinders, pipes, etc., in the smoke-box, and or- dinarily some increase in the thickness of boiler plates fol- lowing the increased pressures, In view of the lack of ex- perimental data of compound locomotives in this country at the present time, it would seem to be advisable to disre- gard these possible reductions in first cost. The figures given by Mr. von Borries, from two to four per cent. more than the cost of ordinary locomotives of equal weight, or, on the above basis, for the same work, are pre- sumably based upon the cost of a number of engines, and would not therefore be sufficient for a single engtne if the cost of drawings and other expenses incident to getting out a new design are to be charged to it. But even if all ex- penses which are properly chargeable to the engine are in- cluded, itis apparent that the probable saving of 15 per cent. will secure a good return from the investment. It is claimed by the advocates of some of the various systems of compound locomotives that the cost of their mainte- nance is no greater than that of the ordinary engines. This claim appears to be reasonable, as the slight increase COMPOUND LOCOMOTIVES. 87 in the cost of repairs which would naturally follow the in- troduction of additional parts may be balanced by the in- creased endurance of the boiler, taken all together, on ac- count of the diminished intensity and frequency of the ex- haust. The cost of converting simple locomotives to compound will depend largely upon the type which the working con- ditions make it advisable to adopt. The simplest case will be that in which one cylinder of the ordinary engine can be retained for the high-pressure cylinder of the compound, andasimpie form of starting gear such as Lindner’s is adopted. The minimum changes required will then be a new cylinder and piston for the low-pressure, new pipes for the receiver, etc., a starting valve and connections, a safety valve on the receiver, and probably two new slide valves and slight changes in the valve motions. The other extreme is apparently that in which two new cylinders are necessary, and starting and intercepting valves of the Mallet type are adopted. In any case the probable cost can be closely estimated from a detailed cost sheet of ordi- nary engines of similar dimensions in the shops in which the alteration is to be made. Mr. Hughes gave the cost of converting a 16 X 24 engine in India, which has been already referred to, as about two hundred dollars, which is probably considerably lower than the same work could beedone for in this country. Judging from such cost sheets as are at hand, it seems probable that the minimum cost would not be less than three hundred dollars, and may be as high as seven hundred dollars for the more complicated arrangements. But, even with the latter amount as a basis, it is evident that there are many cases in which the expenditure would be advisable. CHAPTER VII. THREE-CYLINDER COMPOUND LOCOMOTIVES. The first question which naturally arises in beginning the consideration of three-cylinder compound locomotives is, what are the reasons why that form should be selected in any case in preference to the two-cylinder type,. which is evidently so much more simple? In answer to this, it may be said that the expansion of steam can be carried further in some designs of three-cylinder engines"than in the two- cylinder type without using cylinders which are excessively _ large, and, therefore, that the advantages of high-pressure steam can bemore fully realized. The ratio of the volume of the low-pressure cylinders to the high-pressure can be made greater than is practicable with two cylinders, and a more nearly equal distribution of work can, therefore, be obtained. By a. proper arrangement of cranks a more uniform rotative power can bessecured, and further, a bet- ter distribution of weights can be obtained than is possible with the two-cylinder outside connected type. To avoid possible misunderstanding, attention is called to the distinction between three-cylinder compound en- gines and triple expansion engines. Inthe latter the steam performs work in three cylinders in succession, the high- pressure, the intermediate, and the low-pressure. In the former, when released from one high-pressure cylinder the steam is divided between two low-pressure cylinders, or else the exhausts from two high-pressure cylinders unite in the receiver and further expansion takes place in one low-pressure cylinder. In general the name compound is given to those engines in which the steam acts in but two cylinders in succession, whether two, three or four cylin- ders are used, while triple and quadruple expansion mean COMPOUND LOCOMOTIVES. 89 that the steam is used in three or four cylinders in succes- sion. The two possible arrangements of three-cylinder com- pound engines have been applied to locomotives ; that with one high-pressure and two low-pressure cylinders by the Northern Railway of France, while that with two high- pressure cylinders and one low-pressure is the arrangement of the well-known Webb compound. Steam Distribution in Three-Cylinder Compound Loco- motives.—The fundamental theory of three-cylinder com- pound engines does not, of course, differ from that of two- cylinder compound engines. The only differences which exist are the result of the relative angies of the cranks, and are to be found in the variations in the turning moments and in the variations in pressures in the receiver. Each case must be individually analyzed, and the only difference between such analyses and those already given for two- cylinder engines is the greater complication which arises from having three cranks to consider instead of two. As an example of the method to be preferably followed in at- tempting such an investigation, an arrangement of cranks which has been used for a locomotive is selected. In this form the low-pressure cranks are at right angles and the high-pressure crank makes angles of 185 degrees with them. In the first place we assume the following data: In the high- pressure cylinder, cut-off, .75; release, .90; compression, -90; in the low-pressure cylinders the same distribution. In Fig. 29 are shown successive positions of the three cranks, h representing the high-pressure crank, L one low- pressure crank, and 7 the other. Assuming the direction of the revolution to be as indicated by the arrow, an ex- haust takes place from the high-pressure cylinder when its crank is ath,. One low-pressure crank, 7,, is then just commencing a stroke, and the other, Z,, has accomplished about .57 of a stroke, the effect of the angularity of the connecting rods being neglected. From these positions 90 COMPOUND LOCOMOTIVES, there is free communication between the three cylinders and the receiver until Z moves to L,, where the cut-off takes place in that cylinder, the other low-pressure crank Fig, 30 being then at 7, and the high-pressure crank ath,. From these positions’ expansion continues in tke cylinder J, while there is still freecommunication between the other COMPOUND LOCOMOTIVES. 91 low-pressure cylinder, the receiver and the high-pressure cylinder until the low-pressure crank L arrives at L,_, when steam is again admitted to that cylinder for the re- Fig. 32 turn stroke. The other low-pressure crank is then at 7,, and the _ high-pressure crank is at h,. All three cylinders are now again in communication, and remain 92 COMPOUND LOCOMOTIVES. so until the cut-off position 7, is reached, the other cranks then being at L,andh,. ‘The two cylinders which are represented by A and Z remain in communication until the positions numbered ,; are reached, when steam is again admitted to the cylinder/. Soon after this the high-press- ure exhaust takes place at h,, and a fresh supply of steam is admitted to the receiver, from which it enters both low- pressure cylinders whose cranks are at L,and7l,. These positions correspond to those numbered ,, the direction of the piston movement only being changed. It is clear that, when the exhaust takes place from the high-pressure cyl- inder, the low-pressure piston corresponding to / is always near the beginning of a stroke, while the other is near the middle of its stroke. The effects of this distribution in the low-pressure cylinders are shown in Fig. 30 by indicator cards, which are constructed on the assumption of rapid valve movements and neglecting the irregularities which are caused by the connecting rods. The cards are not drawn to a scale and the variations in pressures are pur- posely exaggerated. Witha relatively large receiver the drop in pressure at /, and L, will be very small. In prac- tice the readmission at , would produce a hump in the card L, while the card? would have a form which would apparently indicate that the valve was late in opening. At earlier points of cut-off somewhat different results will be found. These are illustrated by Figs. 31 and 82, in which it is assumed that cut-off takes place at .4 and re- lease at .75 of the stroke in all three cylinders, Taking the direction of revolution as before, when release occurs in the high-pressure cylinder at h,, one low-pressure crank is at L, and the other is at 7,. A very slight movement brings the crank Z to its cut-off position Z,, soon after which steam is admitted to the other low-pressure cylinder at 1,, and that cylinder isin communication with the receiver and the high-pressure cylinder until its cut-off point is reached atl,. There will then be slight compression in the SOMPOUND LOCOMOTIVES. 93 high-pressure cylinder and the receiver until steam is ad- mitted to the L cylinder at the beginning of its next stroke. The remaining events of the revolution are similar to those already noticed and will be made clear by a study of Fig. 31. It will be seen that there is still readmission to the low-pressure cylinder LZ, but that this does not affect the form of the card from the other low-pressure cylinder. With this arrangement of cranks and with the same valve adjustment the indicator cards from the two low-pressure cylinders will be unlike for all points of cut-off. There is in fact but one arrangement of cranks for which the dis- tribution in the low-pressure cylinders will bethe same, and that is when the low-pressure cranks are both at right angles with the high-pressure, and therefore either directly opposite each other or parallel. Assuming an equal di- vision of work between the three cylinders, the most uni- form turning moment will be obtained by placing the cranks at angles of 120 degrees with each other, but the difference in the distribution in the two low-pressure cylinders will still exist, although it is not probable that this will be of great importance at ordinary running speeds. An examination of the crank positions for the form of three-cylinder engine having two high-pressure cylinders and one low-pressure cylinder shows similar peculiarities in the distribution. This will be evident from Figs. 33 and 34, which are lettered similarly to Figs. 29 and 31, Hand h representing the two high-pressure cranks, which are at right angles, and 7 the low-pressure crank, which makes angles of 135 degrees with the others, The distribution in Fig. 33 is the same as that in Fig. 29, and that in Fig. 34 is the same as that in Fig. 31. It will be seen that there is readmission to the low-pressure cylinder in both figures; but at the earlier cut-off of four-tenths it is not probable that the effect on a low-pressure indicator card would be noticeable. Placing the cranks at angles of 120 degrees 94 COMPOUND LOCOMOTIVES. would, as in the first arrangement of cylinders, produce very little change in the indicator cards. It is evident, from the preceding partial analysis of the steam distribution, that the construction of theoretical indi- cator cards for three-cylinder compound engines will be considerably more difficult than for the two-cylinder type, but that the same formulas and methods of construction Fig. 34 can be used. The remarks which were made in discussing two-cylinder compound locomotives in regard to the effect of varying the capacity of the receiver and the results of changing the points of cut-off are equally applicable to three-cylinder engines. In fact, the only differences are those in the steam distribution, which have been already discussed, and which depend upon the angles made by the three cranks. A mathematical discussion of the three-cylinder type of compound engine, having one high-pressure cylinder and two low-pressure cylinders, and with the cranks placed at angles of 120 degrees with each other, will be found in the appendix to ‘‘ The Marine Steam Engine,” by R. Sennett. The form having two high-pressure cylinders and one low- pressure cylinder does not appear to have been used in COMPOUND LOCOMOTIVES. 95 marine practice, and its use is not to be expected, inas- much as one of the chief reasons for using three cylinders instead of two is to avoid excessively large low-pressure cylinders. In attempting to determine the size of cylinders for three-cylinder compound locomotives, the best guide will undoubtedly be the results obtained with locomotives of that form in practice. When such information is not obtainable, the most satisfactory method will be that advocated under similar circumstances for two-cylinder compound engines, 7. e., the construction of, what were called for convenience, theoretical indicator cards, and the alteration of these as experience dictates, to allow for wire-drawing during the opening and closing of valves, drop in pressure, etc. The proportions which appear to have been generally adopted by Mr. Webb are, high- pressure cylinders, 14 inches in diameter; low-pressure cylinder 80 inches in diameter; stroke of all pistons, 24 inches. The ratio of the volume of the low- pressure cylinder to that of both high-pressure cylinders is thus about 2.3. Assuming a mean forward pressure of 175 pounds gauge, in the high-pressure cylinders, and a back pressure in the low-pressure cylinder of 3 pounds above the atmospheric pressure and an equal division of work, we can make an approximate estimate of the maximum power of the engine as follows: The area of the low-pressure piston is 4.6 times that of one high-pressure piston, and, if the work is to be the same in both, the mean pressure in a high- pressure cylinder must be 4.6 times that in the low-pressure cylinder. As the total range of pressure is 172 pounds, and as the mean receiver pressure is approximately the same as the mean high-pressure back pressure and the mean low-pressure forward pressure, we have: 4.6 x l. p. mean effective pressure = 172—1. p. mean effective, whence 1, p. mean effective = 172 + 5.6 = 30.7 pounds. The mean 96 * COMPOUND LOCOMOTIVES. receiver pressure is then 30.7 + 3 = 33.7 by gauge, and the mean effective in the high-pressure cylinders is 175 — 338.7 . =141.38 pounds. A similar calculation can, of course, be made with any assumed mean forward pressure, and this method can also be used for making an approximate comparison of the maximum work done in the cylinders of the three-cylinder compound with that in ordinary loco- motives. For example, if the mean forward pressure in the latter is 150 pounds and the back pressure is 3 pounds as before, the total effective pressure during a stroke will be 2 x 147 x area of one piston. To be the equivalent of the compound locomotive this must equal 3 x 141.3 x area of one high-pressure piston. This gives in the present case 221.9 square inches as the piston area of the simple engine, or in other words a simple engine having two cylinders about 16.8 inches in diameter would be equal in power, with the assumed pressures, to the compound engine having cylinders 14, 14 and 380 inches in diameter, the stroke being the same in all cylinders. The same method can be used to find dimensions for an equivalent three-cylinder engine having one high-pressure and two low-pressure cylinders. If the ratio of the volumes of the two low-pressure cylinders to that of the high- pressure cylinder is 2.3, each low-pressure cylinder will be 1.15 times as large as the high-pressure. Therefore 1.15 X 1, p. mean effective pressure = 172 — 1. p. mean effective, whence 1. p. mean effective = 80 pounds. The mean re- ceiver pressure will be 83 pounds gauge, and the h. p. mean effective pressure will be 175 — 83 = 92 pounds. To find the piston areas we have 92 x area of the high-press- ure piston for this engine = 141.3 X area of a 14-inch cylinder, which gives an area of 236.3 square inches, 17.35 diameter, for the high-pressure piston, and 1.15 times this or 271.8 square inches, 18.6 diameter, for each low-pre sure piston. An engine having one high-pressure cylinder 17.35 inches in diameter and two low-pressure cylinders 18.6 COMPOUND LOCOMOTIVES. 97 inches in diameter, is thus equivalent with the assumed pressures toone having two high-pressure cylinders 14 inches in diameter and one low-pressure’ cylinder 30 inches in diameter. The distribution of work among the three cylinders will be considered in a subsequent chapter. ILLUSTRATIONS OF THREE-CYLINDER COMPOUND LOcOMo- TIVES. —Before discussing other special features of three- cylinder compound locomotives, illustrations of the two forms will be given. The compound locomotive having Fig. 35 ene high-pressure cylinder and two low-pressure cylinders, which was built by the Northern Railway of France, and to which reference has already been made, was illustrated in Engineering of Dec. 6, 1889. The general arrangement of the cylinders and steam connections of this locomotive 7 98 COMPOUND LOCOMOTIVES. is shown in Fig. 35. Referring to this figure, his the high- pressure cylinder; 7,7 are the low-pressure cylinders; A is main steam pipe to the high-pressure cylinder; C, C is the receiver; and D, Dare the low-pressure exhaust pipes. The low-pressure cylinders are placed as usual and have the valve chests above. The high-pressure cylinder is placed below the smoke-box with its valve chest B below it, and is inclined at an angle of one inten. The locomotive is of the Mogul type, having six coupled driving wheels, the middle axle being the main driving axle for all three cylin- ders. The low-pressure cranks are at right angles, and the high-pressure crank is midway between them, thus making an angle of 135 degrees with each low-pressure crank. It will be noticed that the receiver is formed in the cylinder castings, and not by pipes, as in the- locomotives previously illustrated. The high-pressure valves are a special feature of this engine. These consist of a main valve and a cut-off valve, which slides on the back of or below the main valve, the whole forming a combination which in principle is the same as the Meyer and Ryder cut- offs. The edges of the cut-off valve form an oblique angle with the axis of the cylinder, as in the Ryder valve gear, and the ports in the main valve are correspondingly in- clined at the back of that valve, but are twisted so that on the face next to the cylinder they are placed as is custom- ary. The edges of the exhaust port in the cylinder casting are, however, inclined, and the exhaust cavity in the main valve is formed to correspond. The yoke which drives the main valve does not fit it at the sides, and so permits a transverse movement while controlling it longitudinally. A second yoke incloses the valve, and permits a longitudinal movement, but holds it transversely. This yoke is con- nected to a stem, which passes through a stuffing-box in the side of the valve chest, and is operated from the cab by lever connections. It is clear that the high-pressure cut-off can be adjusted at any time by means of this connection, COMPOUND LOCOMOTIVES. 99 while the valve is so proportioned that in its extreme posi- tion the steam and exhaust ports remain open for all posi- tions of the high-pressure piston, and steam is thus allowed ‘to blow through the high-pressure cylinder without doing work. The engine can, therefore, be started by the low- pressure cylinders with steam from the boiler, the high- pressure piston being then practically inoperative; and as ‘the low-pressure cranks are at right angles, the starting ‘conditions will be the same as for a simple locomotive. The principal dimensions of this locomotive are as follows : ‘Diameter of high-pressure cylinder (original). ..... 18.11 inches. 66 (present)....... 17 ne ‘7 ** low-pressure cylinders (2)........ .... 19.69 <* Re MRMOR GL DISLONE. be) sas cuivesae case senses .508s rea ct ee Ma ‘Diameter of driving wheels............ eH eip Bee Gl Sirk y £8 MOM CTS PLOASUTO; LAM LE ees cc cineaebetesessenvceccccce 199 pounds. ‘Diameter of boiler, smallest inside............. ... 52.3 inches. Pintbos 208, 1.17 inches, lOng@th. Jace cccocs dense ceesses 13 ft. 1.48 inches. ONES oc hapwaipartadwesveaas's PRL a titarn ee dees 22.5 sq. ft. PORE OISUT TAGE 5 Hat ota. sein ehpre neat a wramm ca Ceres eb esas 1224,9. <° ‘Weight in working order, ROUAL AEE Se alte a eens 106,176 pounds. on driving wheels...... 90, 944 LOw-pressure Valve Gear........sseee o---- cess Walschaert. Exhaust nozzle, 4.21 inches in diameter, originally 5.51 inches. The general arrangement of the cylinders and steam connections of compound locomotives of the Webb system is illustrated by Fig. 36. In this figure h, h are the high- pressure cylinders, which are placed so that the centers are in a transverse line about four feet back of the front tube sheet, and which are connected to the second pair of Griving wheels. The low-pressure cylinder / is placed be- neath the smoke box and is connected to the forward pair of driving wheels. The course of' the steam from the boiler is through the pipes A, A to B, B, and thence back to the high-pressure cylinders. The exhaust from these cylinders is led through the pipes D, D, and thence around the smoke box through two pipes C to the low-pressure steam chest. The course of the exhaust from the low-pressure cylinder is clearly in- dicated in the figure. The disposition of the cylinders and 100 COMPOUND LOCOMOTIVES. steam pipes is essentially the same in the Webb compounds for passenger and freight service. The most noticeable peculiarity of the system is the absence of driving connec- tion between the high and low pressure axles, there being Fig. 36 no coupling rods on engines having two pairs of driving wheels. In one design for freight service there are three driving axles, the first Leing driven by the low-pressure cylinder, and the second and third, which are coupled, be- ing driven by the high-pressure cylinders. It will be seen COMPOUND LOCOMOTIVES. 101 that even in this case there is no connection by coupling rods between the high and low pressure cylinders. The principal dimensions of a recent Webb compound locomo- tive are as follows: Diameter of high-pressure cylinders (2).............. 14 inches. - low-pressure Cylinder.......-.sscecs-eee SOLR SSG Stroke Of all pistons.............secccccecsccsccccccese 24 «ff Diameter of driving wheels (4).......cccscsescccccces ESD eee PIPOCURUITO, PAUSE. 6.5 ccs teccacecn shesesupeseese 175 pounds. Diameter of boiler, gin llest MBG sss as cst ek sce 49 inches. Tubes, 235, 1% in., length ........... Mest eisiatee #1 naie' bs ae 11 ft. 3in. MTN eho Sk wala oa cee vep av ua teas eae ee Waste awgawesede eae 55 sq. ft. MEMO PANE MUTI CE... sass cesta de spincindeenecvaensnc scsi 1456.7 ‘* Weight i in working GIaers LOtAle tect cee oe tween ee 99,350 pounds. on driving wheels........ 66,700 MIEPs)! oa 5252 = od chk ok Sete stint Sisve Ga Peeewes'e res Joy ETE EG OZZIO. co cdcde'cden cvs ckeencdanvees esetnscedsee-454 10. in diam, OT ASE aay Gti DISTRIBUTION OF WORK IN THREE-CYLINDER COMPOUND LOCOMOTIVES.—It was shown in the theoretical discussion of the distribution of work between the cylinders-of two- cylinder receiver compound locomotives, that with the same points of cut-off in both cylinders and with the ratios of cylinder volumes which are practicable in locomotives, considerably more than one-half of the total work will be: done by the low-pressure cylinder. It was also demon- strated that the work can be to a great extent equalized by making the cut-off in the high-pressure. earlier than eae in the low-pressure cylinder. The same process of reasoning can be applied to the threes cylinder type of compound engines, inasmuch as we may regard this form as a development of the two-cylinder type, produced by substituting either two smaller high- pressure cylinders for the original high-pressure cylinder, or else two smaller low-pressure cylinders for the original single low-pressure cylinder. It is, therefore, to be ex- pected that, with the same points of cut-off in all three cylinders, considerably more than one-half of the total work will be done in the single low-pressure cylinder of the Webb type of compound locomotive, and in the two low- pressure cylinders of the other form of three-cylinder com- pound locomotive which, for the sake of brevity, may be called the French type. We may even go a step further and say that, with the ratios of cylinder volumes which are practicable, the total work cannot be so divided that much less than one-half of it will be done in the low-press- ure cylinders. This statement is borne out by the pub- lished indicator cards of the Webb locomotive and leads to some interesting conclusions. COMPOUND LOCOMOTIVES. 103. These indicator cards show that the proportion of the total work which is done in the low-pressure cylinder is from 50 to 65 per cent. at various speeds, with the low- pressure valve in full gear. As making the low-pressure cut-off earlier would increase the proportion of work done in that cylinder, it follows directly that the low-pressure cylinder’s share of the total work is at least 50 per cent. As the Webb locomotive has no coupling rods between the high and low-pressure axles, and as the weight on each pair of -drivers is very nearly the same, it is evident that this division of work is the best under the circumstances. This point and others can be well illustrated by a diagram A + a ST S { res i ! | "I maeN | Cal as | 2 Lo Ler NE _ . ! aa was eA oy a ea ~ } Pie 3 ‘hi 19 eas | eet Lay: Se peek he ie ane AON TE bt | Wate. fl { , | | Ny Al i x | H | ee. L/ H Ve | | | 3% SY Y fond | \y ia | : ol ea ee | bert ah Be a Va a ee ae | Lea AS en se ff EN AOR 4 eee ae ves Be tet EO PAA TN oc= an ! { \/ H ou \ rs | | 7 Ns 4 ae | pee A ined Eee Ee CGN SS on Bs a he fit A hoch RT EN Ps,0 Lege ey SR Sh a Rd ga Py te a oe bet Ce ead Ue UAL Bee ee Fig. 38 it is doubtful if there would then be any practical gain by the introduction of coupling-rods. Turning now to the French type of three-cylinder com- pound locomotives, it will be found that an application of the same method of reasoning leads to very different re- sults. As has been pointed out, the steam distribution is different in the two low-pressure cylinders, but it is never- theless to be expected that more than one-half of the total work will be done in the low-pressure cylinders with the same points of cut-off in all three cylinders. Also by ad- justing the points of cut-off, the proportion of the total work done in the high-pressure cylinder can be decreased. It is therefore possible with this type of engine to divide the total work equally among the three cylinders. A diagram of crank efforts COMPOUND LOCOMOTIVES. In this locomotive the two low-pressure cranks are placed with this crank arrangement and on the basis of an equal at rizht angles, and the high-pressure crank is placed at 135 degrees with the otbers. 106 a fas UJ ee Ry oe one Bee Roy aes pert / , / Fa Ne ee 9 qt a0 | ee ey ee A. Xx o, ~~ a a 3 {- nN “ Ss od & i a eras ee om \ —-f-—~-- Sea ose Saxe Oe arae we & eae q x ; sme mete os K---- = Q Se sees rar oak em rel \ *. ee iS o { Noo teks SS ) Yok oe ee a oN Pens oo aa caer be Bis 8 t meee \ ere 2B mee Lacie les De ae sree as Pras ae Sa 4 Vet eS f= TEN xe ato eee (a) ° fy n ta Ea asrarcae dete Vee ve , Ps is os = z bs VW / ‘ Ps MD Pr 7) ah S Ree Meili op pep fete fet wpe SE are eg pre ao i Page a6 g Bp wes a oj 7g . ia] ‘ ’. fe, ahi nt eel a ty” oe . a Lx =| E Sloe. >} SSS —s== a" vegit cas eominmeae eae Maer air yh 8 some / . 4 \y ae ° (e) So ‘, ¢ sien RRS A AN NAGY ey Mena SW [aes Mga s 1S i eee OTN Damien ee a ke ae Bert erase a" a ee Aa eS Vv. wa Wo 5, A A i 1 ed Bae ie Sehr at oe qe Be jmeoes t s t Ss re iN Ne hog ite maT aby af ee #—-*%-->3 == san 9 On 1 at Sa / NS Gis dn ee eg. x a aide eee SoHo s GEL yes. eee We \ Sarg, S ea Ipc gem A 5 apo & PTR ee 3oODs0 Fig. 40 is shown by the full-line curve. gram for all three cranks If the cranks were placed at angles of 120 degrees, the combined diagram would have the form shown by the full- line curve in Fig. 40, from which it is clear that this dis- position of cranks would give a very constant turning moment. COMPOUND LOCOMOTIVES. 107 It follows from these diagrams of crank efforts, in con- nection with the previous statements regarding the division of the total work, that coupling rods are as much a necessity for the French arrangement of cylinders as they are a useless complication in the Webb system. The work cannot be so divided that as much as one-half of the total will be done in the single high-pressure cylinder of the former, while nearly one-half is necessarily done in the single low-pressure cylinder of the latter. STARTING POWER OF THREE-CYLINDER COMPOUND LOcOMOTIVES.—It has been already stated that the starting conditions for the French type of three-cylinder compound locomotive are the same as for an ordinary locomotive, inasmuch as the low-pressure cranks are at right angles, and the form of valve gear adopted for the high-pressure cylinder makes it possible to let steam blow through that cylinder without doing work. It is, therefore, possible to admit steam at nearly boiler pressure to the low-pressure cylinders, and the starting power of the locomotive would then be about the same as that of an ordinary locomotive having cylinders of the same size as the low-pressure cylinders, 19.69 x 27.56 inches, and 199 pounds boiler pressure. But as it is not probable that a valve gear of that form would be adopted in American practice, it may not be out of place to consider the effect of some other possible arrangeinent. There would not apparently be any practical difficulty in the application of starting gear of the Lindner or the von Borries-Worsdell type to three-cylinder locomotives of thisform. With the Lindner starting gear, steam at re- duced pressure is admitted to the receiver, and therefore acts as forward pressure on the low-pressure pistons and as back pressure on the high-pressure piston. With an ar- rangement of cranks such as that of the French locomo- tive, this amounts to having an ordinary locomotive with 19.69 x 27.56 inch cylinders and witha pressure in the 108 COMPOUND LOCOMOTIVES. cylinders equal to about one-half the boiler pressure, aided by a single cylinder, 17 inches in diameter, with about the same effective pressure. The position of the cranks, for which the combined turning moment will apparently have its minimum value, is that at which one low-pressure pis- ton has passed just beyond the point of latest cut-off. The high-pressure crank has then moved through but a small angle from a dead point and the other low-pressure crank is not far from the half-stroke position. An approximate calculation, which is sufficient for our present purpose, shows that the combined effort on the crank pins is then equal to that of an ordinary locomotive having 19 x 24 cylinders, at its most disadvantageous position for starting, with 174 pounds effective pressure in the cylinders. As the weights and diameter of driving wheels of the French locomotive do not differ much from those of well-designed six-coupled American locomotives having 19 X 24 cylin- ders, it is evident that the compound locomotive with the simple Lindner starting gear would be fully equal in start- ing power tothe ordinary locomotive. It would, there- fore, seem to be unnecessary to introduce intercepting valves, or other, and more complicated, starting arrange- ments. That the power of the compound locomotive at other crank positions is superior to that of the other will be made clear by an inspection of diagrams of crank posi- tions, such as Fig. 29. It has been shown that if the cranks of a French type of compound locomotive were placed at angles of 120 degrees the combined turning moment would be considerably im- proved, and it appears from an examination of a diagram, such as Fig. 29, that, with a starting gear of the Lindner or von Borries form, the starting power will be increased. As before, the most disadvantageous position for starting will be that at which one of the pistons has passed just be- yond the latest cut-off position. Thecranks of the other two pistons will then be at approximately 25 degrees and 85 de- COMPOUND LOCOMOTIVES. 109 grees with the center line, and the combined crank effort will be equal to about 1.4 times the maximum of one cylinder as against about 1.13 times that of one cylinder for the most disadvantageous position with the arrangement of cranks actually used on this locomotive. The latter is, of course, the best disposition of cranks with the system of steam distribution which was adopted. It may be useful to note that on the assumption of a uniform steam press- ure throughout the stroke on all thiee pistons inversely proportional to their areas, the combined effort on the crank pins varies from 1.4 to 1.73 times the maximum for asingle cylinder for the 90 and 135 degree arrange- ment, and from 1.73 to 2 times the maximum for one cylinder for the 120 degree arrangement. In the Webb compound locomotive, as now built, no provision is made for admitting steam from the boiler directly to the low-pressure cylinder. The work of start- ing therefore devolves at first upon the high-pressure cyl- inders, which is equivalent to saying that the locomotive is primarily equivalent to a simple locomotive having 14 x 24 inch cylinders and 175 pounds boiler pressure. It has been shown that the turning power on the cranks of the ordinary locomotive in starting may vary from about .75 to 1.4 times the maximum for one cylinder, the lower limit corresponding to the position at which steam cannot be admitted to one cylinder. The tractive power of the Webb locomotive, of which the principal dimensions have been given, would then vary at starting from about .19 to .34 of the weight on the high-pressure driving wheels. It is per- fectly possible that the lower value may be encountered in practice, but it isnot probable, the case being about the same as that of an ordinary locomotive. The probability is that the high-pressure cranks will be in such positions that the tractive power at starting, with full boiler pressure on the high-pressure pistons, will be from .25 to .384 of the weight on the high-pressure driving wheels. If this is sufficient to 110 COMPOUND LOCOMOTIVES. slip the driving wheels, steam will then be admitted to the receiver and so to the low-pressure cylinder until the press- ure in the receiver rises to such a point that the effective pressure on the high-pressure pistons is not enough to slip the wheels. The locomotive will then be in condition to start as a compound, all three cylinders being available. Another factor has now to be taken into consideration, which is the position of the low-pressure crank. Owing to the absence of coupling rods, this may be anywhere be- tween a dead point, in which position itis of course useless, and a half center where it can exert a pressure equal or superior to the combined high-pressure pistons. It is evi- dent that there is a wide range of possibilities in the start- ing power of the Webb locomotive, using the term starting power as we have heretofore in discussing two-cylinder’ locomotives. The exact starting conditions will be partly accidental, and partly dependent upon the skill of the en- gineer in stopping the locomotive in a good position for starting. The starting power of locomotives having the Webb ar- rangement of cylinders, but with the cranks at angles of 120 degrees and coupled driving wheels, will be similar to that of the French type having a like disposition of cranks. Some form of starting valve would then be necessary or else the locomotive would be reduced, for all positions at starting, to a simple locomotive having two small cylinders and cranks 120 degrees apart. Without entering into a de- tailed investigation, which the practical value of this form of construction does not seem to warrant, it appears that two cylinders would always be available in starting, and that if one of these was the low-pressure cylinder, the loco- motive would have ample power, while if the low- pressure was unavailable, there would be an evident de- ficiency. THREE-CYLINDER LOCOMOTIVES IN PRACTICE.—The pub- lished records of the comparative performance of ordinary COMPOUND LOCOMOTIVES. 111 and three-cylinder compound locomotives are not as com- plete as those for locomotives of the two-cylinder type. It is to be remembered, however, that any saving which has been effected by their use is not due to their having three cylinders, but is the result of the application of the com- pound principle, which necessitates a more economical use of steam in ordinary running. The following is a sum- marized statement of the work of compound locomotives on the London & Northwestern Railway, as given out by Mr. Webb in June, 1889. Seventy-five compounds had run 11,644,222 miles since the introduction of the system in © 1882, with an average fuel consumption of 32.9 pounds per mile. One éxpresslocomotive had run 154,342 miles in about 414 months on an average coal consumption of 36.5 pounds per mile, including that burned while standing in steam or issued for all purposes. The average of forty of the same class was 86.8 pounds per mile. On the Metro- politan District Railway a converted locomotive had run 178,337 miles on an average coal consumption of 23.2 pounds, against 32.4 pounds average for six months for an ordinary locomotive doing the same work. A report of the results obtained with three Webb com- pound and three ordinary locomotives in India, doing the same class of work, for the month of Janury, 1886, shows an average coal consumption of 37.57 pounds per mile for the compounds, against 47.14 pounds per mile for the ordinary locomotives. The average mileage for the month was 2,448 and 1,700 miles respectively, No reports of fuel tests of the French three-cylinder locomotive have as yet been made public. It is reported as having hauled very heavy trains (Hngineering, Dec. 6, 1889) up grades of 1 in 200at speeds of from 124 to 80 miles per hour, but the records do not appear to be valuable for comparison with American practice. It was found that the exhaust nozzle (4.21 inches in diameter) was too small for satisfactory work at the higher speed. 112 COMPOUND LOCOMOTIVES. COMPARATIVE SUMMARY.—The reasons which would lead to the selection of the three-cylinder type of com- pound locomotive in preference to the two-cylinder form have been already mentioned. The better distribution of weights is obvious; it has been demonstrated that the combined turning moment is better, and the greater pos- sibilities in the direction of utilizing high boiler pressures economically are apparent, as we find a cylinder ratio of 2.3 in the Webb locomotive and 2.68 in the French, which would be impracticable ratios for heavy two-cylinder loco- motives. To this may be added that with the French arrangement of cylinders it is possible to construct a very powerful and an economical locomotive, for which the low-pressure cylinders will be no larger than those now in common use. To offset these advantages we will have a cranked axle, a considerable increase in the number of parts and a con- sequent increase in first cost and expense of maintenance. It is not the intention of the writer to discuss the merits and demerits of cranked axles nor to advocate their use. But in view of the fact that there are hundreds of them in use on locomotives in Great Britain and in Europe, the necessity of using them should not stand in the way of a trial of three cylinder locomotives if other considerations make such a trial advisable. In this connection it should be noted that the Webb com- pound locomotive, probably from the fact that it has been the most widely illustrated of any type of compound, has apparently come to be regarded by many as the standard by which ail compound locomotives are to be measured. It has been unreasonably attacked and unjustifiably vaunted. The weak features in its design, from an Ameri- can point of view, have been charged to the compound system in general, and its successes credited to the per- sonal superintendence or ‘‘nursing” of the inventor. That the latter 1s a factor which is at least worth considering in COMPOUND LOCOMOTIVES. 113: estimating the value of the reported results, should be evi- dent to all who are familiar with the management of steam machinery. It will nct probably be questioned that a sup- erintendent of motive power can, by paying special per- sonal attention to some one item of expenditure, secure a notable saving therein, although it is not unlikely that this concentration of supervision may make more important losses possible. And it has been shown that a change in engineers may result in a greater saving of fuel than can ordinarily be expected to. follow the introduction of compound locomotives. But there is reason to believe that with the same handling and on the same work a properly proportioned compound locomotive, regardless of the ‘‘system” to which it be- longs, will be much more economical in the consumption of fuel than the ordinary locomotive. Whether it has two, three or four cylinders is a secondary matter. Its value as a fuel-saver compared with other means of econo- mizing lies largely in the fact that its success depends less upon the care and attention of the engineer than does. that of any device which can be added to the ordinary locomotive. Failures due to errors in design or to unsuit- ability for the service demanded do not form a sound basis for arguments against the practicability of applying the principles of compound working to locomotives. ORLA Re HEE coe FOUR-CYLINDER COMPOUND LOCOMOTIVES. There are two general classes of four-cylinder compound locomotives, one of which embraces all designs in which a receiver of large capacity is used, and in which the angles between the cranks are comparatively unimportant, while the other includes only those engines in which the high and low pressure pistons are on one piston rod, or are other- wise rigidly connected, and the ‘‘ dead space ” between the cylinder is reduced toa minimum. The first class includes those locomotives which have two inside connected high- pressure cylinders and two outside connected low-pressure cylinders, and also Mr. Mallet’s double bogie compound locomotive. The second class comprises tandem engines and those of the Vauclain type, in which the high and low pressure cylinders are placed side by side and both pistons are connected to one crosshead. The elementary theory of four-cylinder compound loco- motives of the first class is essentially the same as that of two-cylinder receiver engines, and the four-cylinder type may be regarded, as far as the cylinders are concerned, as formed from the two-cylinder type by substituting for each cylinder of the latter two cylinders having a joint volume equal to the corresponding single cylinder. It was shown in discussing two-cylinder receiver engines that, in making approximate calculations to determine proportions, the receiver pressure may be regarded as constant without serious error, assuming that the capacity of the receiver is large compared with that of a high-pressure cylinder. It follows from this that the distribution of work in the cyl- inders is practically independent of the angle between the COMPOUND LOCOMOTIVES. 115 high and low pressure cranks when a large receiver is used. If in a four-cylinder engine both high-pressure cylinders exhaust into one receiver, which is the reservoir from which both low-pressure cylinders are supplied with ‘steam, the variations in pressure in this receiver during a revolution will presumably be less than in a two-cylinder engine. Wecan, therefore, in the design of four-cylinder engines, make use of formulas which are based upon a constant receiver pressure, proceeding at first asif the en- gine were to have but twocylinders. The formulas are those which are usually given for two-cylinder receiver engines, and are notof special value in the design of two- ‘cylinder compound locomotives on account of the necessity of a very careful analysis of the steam distribution in that type of locomotive if the possible advantages of compound working are to be realized. In subsequent formulas the letters have the following meaning: v = volume of high-pressure cylinder. V= ~*~ * low-pressure cylinder. R = ratio of the cylinders, V= Rv. y = ratio of expansion in h. p. cylinder. 7’ — 66 6é 66 66 1. p- 6é p, = pressure in h. p. cylinder during admission. 2 = pressure in h. p. cylinder when exhaust opens, 3 = mean pressure in the receiver. ‘¢ = pressure in l. p. cylinder during admission. 4 = mean l. p. back pressure. All pressures are absolute pressures. Neglecting the effects of clearance, the mean forward pressure in the high-pressure cylinder is 1+hyp. log. r ed alld I. cae ANS Pm S The mean effective pressure is (~Pm—>p,) and the work done in the high-pressure cylinder during a stroke is 116 COMPOUND LOCOMOTIVES. U(Pm—p;). Similarly, the mean forward pressure in the low-pressure cylinder is, " 1 +- hyp. log. r’ f'n = Map log. the mean effective pressure is (p’m—p,) and the work done in the low-pressure cylinder during a stroke is V(p’m— p,). If the work is to be equally divided between the two cylinders, v (p m— pP3)=V (p’m —p,). On the basis that volumes vary inversely as the pressures, we have, 8 Piva tPsVEePs R eae ic Ta. By substituting the value for p, obtained from this. equation in the preceding one, and reducing, the following — is obtained: Pe ra log. No. = +p4= 0.4 By means of this equation the ratios of expansion in. each cylinder (r and r’) for which the work done in each will be equal can be determined for any assumed values of p,p,and Rk. If it were required that there should be no: drop in pressure at the end of the expansion in the high- pressure cylinder, p, must equaz p,, from which it follows. that 7’ must equal R. It will be found that equation (4) will give impossible values for r’ for many values of 7, As r becomes less, or steam is admitted to the high-pressure cylinder during a large part of the stroke, r’ will be found to be less than one which is manifestly impossible, and shows that with a late cut-off in the high-pressure cylinder the work cannot be equally divided between the cylinders. On the other hand, as 7 is made large, 7” also increases. until it is greater than R, which is an impracticable result, as the receiver pressure would then be higher than the pressure in the high-pressure cylinder at the end of the expansion. For example, if we take R= 2.3, Pp, = 190 pounds absolute, p, = 20 pounds absolute, and. COMPOUND LOCOMOTIVES. 117 v7 = 1.38, or cut-off at0.75 of the stroke in the high-pressure cylinder, the equation reduces to hyp. log. r’ + .4848 7’ = 0.3904 from which 7” = 0.97. As steam is admitted during the whole stroke when 7’ = 1.0, it is clear that with the above proportions more than one-half of the total work is necessarily done in the low-pressure cylinder. If ris taken as equal to 4, with the other data the same as before, the value of r’ will be found to be 3.75, or the low-pressure cut-off would have to be placed at 1 + 3.75 or (0.267 of the stroke. But as there will be no Baa te fA 0 0.10 0.20 0.30 0.40 fp 0.50 0.60 0.70 0.80 Fig. 41 drop in pressure between the cylinders when 1’ = R, or when steam is cut off in the low-pressure cylinder at 1 + 2.3 = 0.485 of the stroke, it follows that to equalize the work in the two cylinders at the earlier cut off the re- — ceiver pressure would have to be higher than the pressure at the end of the expansion in the high-pressure cylinder. The engineers of the Paris, Lyons & Mediterranean Rail- way have applied a formula similar to the above in the determination of the proportions for a class of four-cylin- der compound locomotives, and have shown the proper re- 118 COMPOUND LOCOMOTIVES, lations existing between the points of cut-off in the high and low pressure cylinders graphically by a diagram sim- ilar to Fig. 41. This diagram is reduced from that given in a pamphlet by Mr. C. Baudry, assistant engineer-in-chief of motive power and equipment. Formulas similar to the above will be found discussed at greater length in ‘‘Com- pound Engines,” by Mr. Mallet; but in some editions of his book there are many confusing typographical errors. In Fig. 41 the horizontal distances represent the points of cut-off in the high-pressure cylinder, and the vertical distances represent the points of cut-off in the low-pressure cylinder. The inclined lines are curves ‘which represent the solution of equation (4) for dif- ferent values of R, the pressures used in the construction. of the diagram probably being 213 and 21 pounds. For ex- ample, if A = 2.5 when the high-pressure cut-off is at. 0.4, the low-pressure cut-off should be at about 0.5 in order to equalize the work. If the ratio R = 2, a cut-off at 0.4 in the high-pressure requires a cut- off at about 0.58 in the low-pressure cylinder. For the cases in which the equation gives values of r’ which are too small, the cut-off for the low-pressure cylinder is fixed at 0.8 or the maximum for full gear. For instance, taking R = 2, the low-pressure cut-off would re- main at 0.8, or full gear for all values of the high-pressure: cut-off greater than 0.58, although more than one-half of the work would then be done in the low-pressure cylinder, The other limit to the application of the formula is fixed by making the earliest low-pressure cut-off that at which there will be no drop in pressure between the cylinders. So that finally, the relation between the points of cut-off in the two cylinders is shown by broken lines such as a bc d, for which R = 1.82. For example, if R = 2, the diagram shows that the points of cut-off should vary as follows: High-pressure... 4. whenss seer eee 10 .20 .30 .40 .50 .60 .70 .80: AiO W-PLESBULC ss csa Sosa cs ceiecs ces .50 .50 .50 .58 .70 .80 .80 .80 120 COMPOUND LOCOMOTIVES. It does not of course follow that it will be advisable in practice to make use of the early cut-offs given above for the high-pressure cylinder, and it will be remembered that Mr. von Borries has stated, as the result of his experience with two-cylinder compound locomotives, that the most advantageous points of cut-off in the high-pressure cylin- der are from 0.3 to 0.4 of the stroke. Before calling attention to some of the special problems which wi!l present themselves in the design of four-cylinder compound locomotives, illustrations will be given of three experimental types which have recently been constructed by the Paris, Lyons & Mediterranean Railway, and which are described in the pamphlet by Mr. Baudry, to which reference has been made. Fig. 42 shows the general ar- rangement of the compound locomotives intended for fast passenger service. The principal dimensions of these Jocomotives, and of the type of simple locomotive which — formed the basis for the design, are given in the ac- companying table under the heading Fig. 42. The table shows that two compound locomotives of this and of each of the succeeding types have been built, which differ only in the number and diameter of the tubes. It will be seen that in the type of locomotive illustrated by Fig. 42 all four cylinders are placed beneath the smoke-box, witb their axes horizontal. The two high-pressure cylinders are between the frames and are connected to the forward driving axle. The low-pressure cylinders are con- nected to the rear driving axle. The axles are so coupled that the high-pressure crank on each side leads the low-pressure crank on _ the same side 198°. The object of this arrangement is to ob- tain as large a value for the minimum starting power as possible. In Fig. 43 is shown the general arrangement of the four-cylinder compound locomotives for freight service. In this locomotive the second driving axle is connected to the low-pressure cylinders, and the third axle to the high- COMPOUND LOCOMOTIVES. 121 122 COMPOUND LOCOMOTIVES. pressure cylinders. The high-pressure crank on each side leads the low-pressure crank 232° 48’. In the correspond- ing simple locomotive, of which the dimensions are given in the sixth column of the table, the rear axle is nota driving axle. Fig. 44 illustrates the arrangement adopted for locomotives for steep grades. The high-pressure cylin- ders are connected to the second axle, and the low-pressure cylinders to the third axle. The high-pressure cranks lead the adjacent low-pressure cranks, as in the other designs, but in this case the angle is 235° 54’, The Walschaert valve gear is used for all of these loco- motives, and the points of cut-off in the high and low- pressure cylinders are adjusted by means of a complicated cam arrangement, designed to fulfill the requirements in- dicated by Fig. 41. The starting gear adopted for these locomotives consists of simply an auxiliary steam pipe and cock for admitting steam from the boiler to the receiver, which is fitted with a safety valve as usual. In determin- ing the sizes of the cylinders the basis of calculation ap- pears to have been the weight of steam which the boiler may be expected to produce without being forced. This quantity had been found by previous tests to be about 14,500 pounds per hour for the boiler of the express loco- motive, and the cylinders should be capable of dispos- ing of this weight of steam. Mr. Baudry states that the steam may, with due _ regard to the tractive power necessary, be expanded to a pressure of about 44 pounds absolute at a speed of 31 miles per hour, and to about 19 pounds absolute at a speed of 68 miles per hour. The high-pressure cylinders should be capable of utilizing the above weight of steam at slcw speeds when the full tractive power of the locomotive is required and also at high speeds. The dimensions selected for the cylinders are thought to meet these requirements, the cut-off in the high-pressure cylinders being of course made earlier as the speed is increased. The method by 124 COMPOUND LOCOMOTIVES. which the angles between the adjacent high and low-press- ure cranks were determined is not explained. Consider- ing the question of balancing the locomotive, the best re- sults would apparently be obtained by placing each high- pressure crank at 180° from the adjacent low-pressure crank. To obtain the greatest value for the minimum turning moment in starting, the angle between the high and low pressure cranks should be 225°, assuming that the pressure of the steam admitted directly to the receiver is such that the total pressure on each piston is the same, and that the cranks on each axle are at right angles. The angles which were adopted appear to be to acertain extent a compromise. For the express locomotive, Fig. 42, in which the question of the balancing of the reciprocating parts at high speeds would be of most importance, the angle selected, 198°, is approximately half way between 180° and 225°. For the freight locomotive the starting power was apparently given greater weight in the prob- lem, the angles in each design being 225° plus the angle of inclination of the high-pressure cylinders. It will be interesting in this connection to note the rea- sons as given by Mr. Baudry for the adoption of such high steam pressures and the four-cylinder type. ‘‘The sole object of the study of new types has been to secure as great economy in fuel as possible. It has therefore seemed nec- essary to increase the boiler pressure considerably and to adopt the compound system. It is true that some saving in fuelcan be realized by the use of the compound system ° without increase of pressure; but the ultimate advantage seems to be in this case at least questionable. With the pressures of from 128 to 156 pounds per square inch used for locomotives, it is not necessary to resort to this system to obtain a large expansion. Its adoption could then only diminish to some extent the losses resulting from conden- sation in the cylinders; but as the difference between the extreme temperatures is not very great, and considering COMPOUND LOCOMOTIVES. 125 the high piston speed, the economy which would result’ does not seem to be sufficient to compen- sate for the mechanical complication and_ the additional expense for repairs which it entails. With high pressures, on the contrary, a little more coal is re- quired to heat the steam, but the possible work of this steam when suitably expanded is increased in a much lar- ger proportion; but the expansion can no longer be accom- plished in a single cylinder with slide-valve distribution, the only method which seems to be practicable for locomo- tives; moreover, the variations in temperature are much greater, and the effect of condensation in the cylinder may become considerable. For these two reasons the compound system becomes a direct consequence of high-pressures in locomotives. It was thus that Mr. Henry was led, without any other reason, to adopt for the new P. L. M. engines the pressure of 213 pounds and the compound system. The employment of four cylinders is not a necessary conse- quence of the adoption of the compound system. Indeed, the greater part of existing compound locomotives in France and in other countries belong to the Mallet system, and have consequently only two cylinders. But the adoption of this system for locomotives as powerful as those under consideration would have led to very large dimensions, both for the cylinders and for their connec- tions. With four cylinders, acting two by two on differ- ent axles, these dimensions are more moderate; there is the further advantage of perfect symmetry, and what is still more important, a notable diminution in the strains on the driving axles and on the rails. The axles are less strained because each transmits but one-half of the work of the locomotive; and the strains on the rails are less since the rapid shocks due to the obliquity of the connect- ing rods are proportional to the efforts transmitted by the latter, and since these are one-half less when the same work is transmitted by four connecting rods instead of by two.” 126 COMPOUND LOCOMOTIVES. We are indebted to the engineers of the Paris, Lyons & Mediterranean Railway for their very thorough investiga- tion of four-cylinder receiver compound locomotives, and the report on the performance of these locomotives will be awaited with interest. But the writer is not prepared to admit that all of the points in Mr. Baudry’s argument are well taken, or that the advisability of using the four- cylinder receiver type of engine has been demonstrated. A reference to the accompanying table shows that the weights of the compound locomotives are in all cases con- siderably greater than those of the corresponding simple engines, and they should therefore be more powerful, as they are in fact reported to be. But it does not appear that it was necessary to resort to this type of compound locomotive in order to secure the advantages of com- pound working. The adhesion weight and the diameter of the driving wheels of the compound express locomotive (Fig. 42) are nearly the same as in the Webb engine and in the Worsdell engine, which have been illustrated in these pages. The total weight of the four-cylinder locomo- tive is from nine to ten tons greater than that of the two and three cylinder locomotives, and there is, therefore, that much more dead weight. In the designs for the freight locomotives there is more reason for the use of four cylin- ders, although the three-cylinder.compvund locomotive of the Northern Railway of France, which we have illus- trated, is comparable with the simple locomotive of which the dimensions are given in the sixth column of the table. It certainly cannot be justly claimed that the necessary complication of two-cylinder compound locomotives of the von Borries, Worsdell, or Lindner types, is sufficient to have much weight as an argument against iheir use. Nor does it follow that pressures as high as 213 pounds per square inch are a necessity in order to secure good results from the use of the compound system. We have no ex- perimental data by which the advisable maximum and COMPOUND LOCOMOTIVES. 127 minimum steam pressures can be determined, but we have reports of good economical results from two-cylinder com- pounds working with a steam pressure of kut 120 pounds. The economical advantages of compound working aoe in general more marked as the pressure is increased, but there is obviously a limit beyond which there will be no gain in economy by increasing the pressure. The reasons for this are similar to those which make the adoption of the compound system advisable in any case; briefly, that to secure the advantages of high-pressure steam it must be sufficiently expanded. Therefore, as pressures are in- creased, a point will be reached beyond which no greater proportion of the energy in the fuel can be utilized with two cylinders of practicable dimensions than can be utilized in a single cylinder with lower pressures. To secure full advantage of the high pressures, triple or quadruple expan- sion engines must be used as in modern marine practice. It is not, of course, to be inferred that it is advisable to con- struct triple or quadruple expansion locomotives, although such construction would seem to be more sensible than that of some of the ingenious monstrosities which appear perennially, and which frequently receive the favorable indorsement of men who should know better. We are not prepared to assert that such pressures as 213 pounds per square inch are too high for good work in compound loco- motives, but wish simply to call attention to the fact that there are both maximum and minimum limits to the press- ures which are properly applicable to compound engines. The principal advantages of the four cylinder receiver type of compound locomotives appear to be the possibility of obtaining a very uniform turning moment, together with a better running balance than is possible with any other form of locomotive. Whether or not these advan- tages are sufficient to compensate for the greatly increased number of parts must remain a matter of opinion. Mr. Mallet’s ‘‘ articulated” four-cylinder compound lo- 128 COMPOUND LOCOMOTIVES. comotives do not present any special features as far as the steam distribution isconcerned. In this form the high- pressure cylinders are fastened to the rear part of the main frames, and drive one set of driving wheels, and the low- pressure cylinders are mounted on a front bogie with a second set of driving wheels. There is, therefore, no dead weight, and the locomotive is specially adapted for use on very sharp curves. At the same time, as the low-pressure cylinders are on the bogie, the joints in the steam connec- tions are not subjected to high pressure and therefore will not be difficult to keep tight. This system has been used with success on narrow gauge roads, while on the other hand it has been adopted for a locomotive for the St. Got- hard Railway, which weighs about 85 tons (2,240 pounds) in working order. There are six axles, coupled in two in- dependent groups, the low-pressure cylinders being placed with three axles on the front bogie, while the high-press- ure cylinders are fastened tothe main frames and are con- nected with the other three axles, COMPOUND LOCOMOTIVES. fF 9C1 €°eIZ % Ori €°E1Z FP 9ST "IZ **qoulr *bs od spunod ‘93nes ‘oinssoid 19 [log doll 6h ¢ ees °F nee ge"e JapurpAd “d *Y 9UO 04 OUINJOA IOATAIOI JO OTFVY sees CZ'Z sees ZC°% see O22 ae She See eee ee ** xopurpso ‘d "TY 04 -d *T Jo oney 008021 068°E31 090‘F1T OCF ‘ZI ore TTT 0c6‘L11 "+" spunod ‘T2304 ops0 SULYION ur qU 310 MA 008 ‘0ZT 068°C2T OZ1'E6 Oh *ZT OF6°29 096°¢9 Sak ae ae ag ee : spunod ‘g[90Y M SULALIp UO Jop1oO SULYIOM UL YS1O MA 9 6P 9°6P 90° 6¢ 90° 6S PL SL P18) pte it * “SsoYour ‘spooyM SUTALIp JO LoJOUUVICY ¥'2S C°EZ 6'%Z G'CZ L'#Z Z° CZ a eR owes Sean eS e's 6 sp ae 4ooy 9renbs ‘a1 eid jo RVolV T6¢ T'69 T'g¢ T9¢ L’6P 9°69 SOYOUL ‘SULI 4So9S1V] OpIsUl Io[loq JO LoJoULVIy 8°8Z1Z | 4°8O9T | 6°9G9T | S'T99T | 8°SCLT | L°TOST | L°SESL | L°LLET | S°982L | 777 900s orenbs *[eqo} ‘o08JINs SulyeoH €°F20G | “16FL | ‘“GLSL | ‘“GESE | “SPOT | ‘“G8FL | L°S2FL | 9°CSZE | FISTIL | °° °°" 90s erenbs B9q D7 SOBsIDS sulyeoH G'pOL | L°LIL | GLI | S'S | 8° SIL | L sir "SIL | LSE | T'Ser ira GOO] erenbs xoq-o1y ‘soejns SulyVoH 1G¢°LE =|29°SE |29°Sk |6F'9T [So FL [8a tt [€2°9r |Fosk |Past | "**5" "9993 ‘Sjooys oqn4 we0Mjoq Y4SUI'T L6°T LIS 16°T 16°T 8o°T LL’T L6°T LL*T L6°T Ss i BOUSGL Opsqno (soqny 70 ToJoULVId Ch O14 L¥G 002 LOE L¥G S8T WG C8T ae sseeeeeeeses saqny JO toquUnN (2) (1) (2) (1) (2) (1) 60°) 60°24 ¥0°6 6o°P 60°L €l°OT |cE°s STL cE °6 vsseeeees pare UOISTd JO *70900 iod ‘sq.10d umre94S TESS «=6|IL'SSZ) «= |9S' FL) =80'9T)§=TESS)=S|S6"ST = [80° OT PI ISR OL tke cet iea eh ne erenbs wore ‘sy10d UIB949 6, £08 §°6L 9° LL PPL 6° LL 9°91 6°69 ¢'T8 CTL “+ *gyOajs Jo ‘quod rod Uo serurpsS UONUITXe AL c0°0 400 ¥0°0 70°90 50°0 40°0 ¥0°0 0 1 el ce ie Sea tA ated ako he Jo a prvun CE iia ltek STOLL Sele Tar Ek gle Lata (re ae onesies: ihe! Pah bal p ge bia meet ters 1¢°¢ €9°¢ g¢"¢ 68°F €9°¢ OFS 98°F 6¢°¢ Lot seeeee see" SOUOUT ‘BATVA JO [OABIY WINUIIXB]L of of PS o0T 00 0) Fol of of 00 gel a OO anid eee ‘sroput[s£o jo uoljeUulpouy 86°9C] 6 69°SS)Sssj6S"sm]SCec'ceSC6S"ceSCs6S"SS]SCSST FSCO FGCU BOS neem ad Fe Nee 6 oo SOGOU suojstd Jo 990199 94 1e— (92°12 -|ZT°FL (92°TS: (96°1S A |SE°SE [89°6T -|80°GE |12:2k fro c rs" s** eeqouy s1opul[ AD JO IoJOUIVL(T (6 S VE Z Z Z Z Zz z soccccvccsecscovsccessesssnzonut £0 JO requinN cs ad Red Bare Oi © fi Bale ek aL). dH giduig| *puroduog j|oefdug| *punoduog jo[dulg| *punoduop ‘PP SLA "Sh “SLA "Ch SLA *SHAILLOWODOT HIGWIS GNV GNONOdWO) HHANITAD-HOOY HONHUY LO SNOISNAWNIC Ce Aer ee aes FOUR-CYLINDER COMPOUND LOCOMOTIVES, Tandem Engines.—The problems to be solved in connec- tion with the steam distribution in compound engines in which the pistons move simultaneously are in some re- spects quite different from those which are encountered in the design of compound receiver engines. The tandem ar- rangement is one form of what are frequently called Woolf engines, or continuous expansion engines, the dis- tinctive feature of the class being that the pistons move simultaneously, and that there is no receiver. In the sim- plest forms of this type, as applicable to locomotives, the high- and low-pressure pistons are attached to the same piston rod, and the slide valves of both cylinders are oper- ated by the same link motion. The peculiarities of the steam distribution in this arrangement of cylinders can be best examined by means of theoretical indicator cards, such as Fig. 45. Referring to this figure, a, D, d, e, f, g, h,k,ais the high-pressure card, and g, h,l, m, n, q, g is the low-pressure card. In the high-pressure cylinder cut- off takes place at 6, and there is expansion in that cylinder until the exhaust opens at d. There is then a drop in pressure to e as the steam in the high-pressure cylinder mingles with that in the passages which connect the cylin- ders. From eto f there is further expansion in the high- pressure cylinder and the connecting passages. At f the low-pressure steam valve opens and there is another drop in pressure to g. From g to hf the cylinders are in com- munication, and there is expansion until the low-pressure steam valve closesath. From h to k there is compression in the connecting passages and the high-pressure cylinder, 131 COMPOUND LOCOMOTIVES. eed _ | ! ft Fee Ree | Ny a 132 COMPOUND LOCOMOTIVES. and when the high-pressure exhaust closes at & there is further compression in that cylinder. In the low-pressure cylinder the steam expands from h to 1 where release occurs and the pressure drops to the ordinary back press- ure line. The features of this diagram which require special at- tention are the losses in pressure at d and / and the com- pression in the high-pressure cylinder. In order to prevent the drop at d, either the pressure in the connecting pas- sages when the high-pressure exhaust opens must be the same as that at d, or else the volume of the connecting passages must be practically nothing. The former result can possibly be obtained by adjustments of the low-pres- sure cut-off, but it is not practicable on account of the unavoidable complications. The only feasible method of reducing this loss to an inappreciable amount appears to be to make the volume of the connecting passages very small compared with that of the high-pressure cylinder, The drop in pressure at f can be prevented or reduced by compressing to the pressure f in the low-pressure cylinder, or by making the low-pressure clearance very small. The question of compression in the high-pressure cylinder in this type of engine iseven more troublesome than in receiver engines, In order to avoid compressing to a higher pressure than the initial pressure with the usual forms of valve gear, it is necessary that the volume of the high-pressure clearance space should be made large, since the pressure at k, where the compression caused by the exhaust closure begins, is unavoidably high. This pressure can of course be somewhat reduced by making the volume of the passages connecting the cylinders large, but, as has been shown, this involves a considerable drop in pressure atd. The expedient of giving the high-pressure valve in- side clearance may also be employed in connection with a large clearance space to assist in keeping down the com- pression. In any case in which the shifting link motion is COMPOUND LOCOMOTIVES. 133 used early cut-offs are to be avoided, both on account of this compression and o avoid the wiredrawing which re- sults from a small port opening. lt is, however, not necessary to resort to very early cut- offs in order to obtaina sufficiently great expansion, as this may be secured by using a comparatively large cylinder ratio. If, for example, we assume a cylinder ratio 3 anda boiler pressure of 160 pounds, a final pressure of about 2.5 pounds above the atmospheric pressure can be secured with a cut-off in the high-pressure cylinder at three-tenths of the stroke at slow speeds, and at high speed a later cut-off would be necessary to maintain the same terminal pressure. In determining the proportions for the valve gear and the size of the cylinders advisable for a tandem compound which is intended to take the place of an ordinary locomo- tive, the most satisfactory mode of procedure will be to construct theoretical indicator cards, using the term as we have heretofore, for various points of cut-off, measure the area of these cards or calculate them by the formulas al- ready given in discussing the theory of two-cylinder re- ceiver engines, and finally to adjust or ‘‘ doctor” the cards for losses, as experience with ordinary locomotives has shown to be necessary. Anexample of indicator cards constructed in this way is given in Fig. 45,0n a much smaller scale, however, than is advisable in practice. The assumed data in this case are as follows: Initial pressure, 175 pounds absolute; cylinder ratio, 3; low-pressure back- pressure, 17 pounds absolute; cut-off in both cylinders, 0.5; release and compression in both cylinders, 0.78; volume of high-pressure clearance, 15 per cent.; volume of low-pres- sure clearance, 6 per cent.; volume of connecting passages, 0.3 of high-pressure cylinder. The scale of pressures used in the diagram is 80 pounds tothe inch. For the benefit of those who may wish to construct such diagrams we will follow through this case in some detail. 134 COMPOUND LOCOMOTIVES. ‘The following symbols will be used: v = volume swept by high-pressure piston. ge = 4) 4 ‘* low-pressure a C= ‘* of high-pressure clearance. C= ‘* of low-pressure “ tox ‘* of intermediate or connecting passages. The volumes occupied by thesteam at the several lettered points on the diagram are, then, Atb,= 5v+c=.6dv, Atd, = .78v+c= .93 v. Atf, =vtc+t7=1.45v. Atg, =1.45v+ C= 1.63 v. At h, before cut-off, = 5u+e+i+C+.5V= 2.638». Ath, in 1. p. after cut-off, = .56 V+ C= .56 V. Ath, inh. p. and passages after cut-off inl. p., = .6v + c+t= .95v, At k, before valve closure, = .22v+e +7 = .67 uv. Atk, inh. p. after valve closure, = .22v+¢ = .37v. Atl,=.78V +0 = OY. Atn, = 22 V+ C= .28 V. The pressure at d and the curve between b and d may be found by constructing the curve through b with Bas the origin, A B being ,15 of A D;.or by calculation as the pres- sures may be taken inversely as the volumes, whence pressure at d = 175 X .65 + .93 = 122.3 pounds. The drop in pressure from d to e depends upon the pressure at k, that in turn depends upon h, and so upon g. The pressure at g depends upon that at g and at f/, and so upon e. Inany case, there is but one pressure at h which will fulfil the conditions, and that pressure must be determined by calculation. Assuming for the moment that we know the pressure at e to be 112.5 pounds, the pressure at f will be 112.5 K 1.23 + 1.45 = 95.4 pounds. The pressure at g is determined by the mixture of the volume at f at 95.4 pounds with the volumeof the low-pressure clearance at COMPOUND LOCOMOTIVES. 135 pressure g. To find the latter we have pressure at g = 17 X .28 + .06 = 79.3 pounds. Then pressure at g = 79.8 x .18 + 95.4 x 1.45 18 + 1.45 The pressure at h = 93.7 X 1.63 + 2.63 = 58.1 pounds. The pressure at k = 58.1 X .95 + .67 = 82.3 pounds. We can now find the pressure at e which is 122.3 x .93 + 82.3 x .3 93 + .3 By combining these various expressions for pressures wecan readily form a single equation from which the pressure at h can be calculated, which is, in fact, the method by which it was determined in this case. Having found the pressures at e, g and h by calculation, the various curves of the diagrams can be readily con- structed. For the curve between e and f a point C is used for the origin, which is found by laying off B O equal to .80f AD. Thecurvehkis constructed from the same origin. The compression curve k wu is laid off from B. To find the origin for the curve gh, we proceed as follows: At g the steam occupies the volumev + c+ i+ (0, and = 93.7 pounds. =e L120, at h the volume occupied is .J5v+cec+i+C+4+ (5V= \ 1.5v), The increase in volume is therefore equal to v, and therefore the scale of this part of the diagram must be such that the horizontal distance from g to h represents v, the volume of the high-pressure cylinder. With this scale of volumes lay off DK = .06V = .18v, K L = .3v, LN=vand NE = .15v; then EF 1s the origin from which to construct the curvegh. For the curves h/ and nq the origin is taken at H, whichis found by laying off D H= .06 of A D, which for these curves represents the volume of the low-pressure cylinder. This diagram illustrates the difficulty of keeping the high- pressure compression within reasonable limits. The ex- haust closure at 0.78 when cutting-off at .5 is approximate- ly that obtained with the valve gear, of which the distri- 136 COMPOUND LOCOMOTIVES. bution was shown in tabular form in the Third Chapter. The valve has 0.25-inch inside clearance, and there is 15 per cent. cylinder clearance. But the pressure at k is 82.3 pounds, and the pressure at the end of the compression u will therefore be 82.3 x .87 + .15 = 203 pounds absolute, or 28 pounds above the initial pressure. To overcome this difficulty it will probably be necessary in most cases to make use of a combination of inside valve clearance, large cylinder clearance and considerable lead. As a first approximation in determining the size of cylin- ders the following method may be used. Assume, for example, that a tandem compound is to be substituted for an 18 X 24 simple locomotive, the same boiler pressure of 160 pounds to be retained. With a cut-off at seven-eighths of the stroke, and 160 pounds initial pressure, the mean forward pressure in the high-pressure cylinder will be 173.3 pounds absolute, and the final pressure in that cylin- der will be 153.1 pounds absolute. We may assume a drop in pressure of about 5 pounds between the cylinders on account of resistance of ports, etc., so that the initial pressure in the low-pressure cylinder may be taken at 148 pounds. With acylinder ratio of three, the steam which now fills the high-pressure cylinder and the low-pressure clearance will be expanded to about three times its pres- ent volume, and the mean forward pressure in the low- pressure cylinder will be 81.3 pounds absolute. These mean pressures are calculated by the formulas already given in discussing the theoretical distribution in two-cylinder re- ceiver engines. The low-pressure back pressure may be taken at 17 pounds absolute, which gives 64.38 pounds for the mean effective pressure in the low-pressure cylinder. For our present purposes we will take the mean high- pressure back pressure as five pounds above the mean low- pressure forward pressure, which will give a mean effective pressure in the high-pressure cylinder of 173.3 — 86.3 = 87.0 pounds. If the maximum mean effective pressure COMPOUND LOCOMOTIVES. 137 which can be developed in the cylinders of the simple locomotive be taken as 155 pounds, we will have, in order that the maximum work of both engines may be the same, assuming that the stroke is the same in both, 87. a + 64.3 x 3 a = 155 X 254.5, in which a is the area of the high-pressure piston and 254.5 is the area of the 18-inch - piston. From this equation we find a = 140.9 square inches, from which the area of the low-pressure piston is 422.7 square inches. The corresponding diameters are approximately 13.4 and 23.2 inches. In the above approximate calculation, the effects of early exhaust opening, clearance, compression, and low-pressure cut-off have been neglected, and it will be apparent that a calculation in which these factors were duly considered would be very laborious. But knowing the approximate dimensions of the cylinders the design can be sufficiently completed to determine the volume of ports, passages, etc., and accurate indicator diagrams can then be constructed, by the method already given, by which the final dimensions for cylinders and valves can be determined. The starting power of a tandem compound locomotive, having cylinders of the dimensions given above, will be about the same as that of the simple 18 x 24 locomotive, when steam at about 82 pounds absolute pressure is ad- mitted directly to the low-pressure cylinders; The steam so admitted acts as forward pressure on the low-pressure pistons, and as back pressure on the high-pressure pistons, so that the locomotive may be said to start as a compound engine. There would, therefore, be no advantage in keeping the starting valve open after the first exhaust from the high-pressure cylinder had taken place. But there is another view of the question of starting which should be noted. If full boiler pressure were admitted to the high- pressure cylinder by the main steam pipe, and the same pressure to the low-pressure cylinder by means of the starting valve, the high-pressure piston would be 188 COMPOUND LOCOMOTIVES. —\y \) Xe LL ZZ) une 0 pl 22 RTD, GZ GZ umn Camu Fig. 46 145. Cut-off .7 Revolutions, 61, O, Fig. 47 Cut-off .55 Revolutions, 86, 0. Fig. £8 COMPOUND LOCOMOTIVES, 139 Cut-off .45 Revolutions, 86, 0. Fig. 49 Cut-off 55 Revolutions, 192, Fig. 50 Cut-off .36 Revolutions, 82, 140 COMPOUND LOCOMOTIVES. practically thrown out of action, and the starting power would depend simply upon the area of the low-pressure piston. Therefore, if this piston were but 18 inches in diameter, the possible maximum starting power would be equal to that of the simple engine. Such an engine as this would be deficient in power as a compound,. while with an engine having cylinders proportioned for the maximum pressure, as already given, it might be neces- sary to employ earlier cut-offs for ordinary work on a level than are advisable. It is probable that the best dimensions for cylinders will be found to be those which will permit the engine to do the greater part of its work at cut-offs of from .4to.5 in the high-pressure cylinder. It is evident that the only starting gear necessary is an arrangement by which steam can be admitted directly to the low-pressure cylinder for a short time in starting. A successful application of the tandem form of com- pound engine to a locomotive has been made by Mr. G. Du | Bousquet, of the Northern Railway of France.