3 40 1.96 ,,^ TMISC. PliSa i ] H.B. 809. \ MINISTRY OF MUNITIONS. Technical Department — Aircraft Proouction. Central Housh, KiNGSWAV, W.C.2. THE 12 CYLINDER rerty Aero Engine, CONFIDENTIAL Attention is called to tbe penaltiej* attaching t") any infringement of the Official Secrets Act, SEPTEMBER, 1918. FOR USE OF MEMBERS OF THE ROYAL AIR FORCE. J. G. WEIR, Brigadier-General, Controller, Technical Dep'irtment. T. 5— 372/4131— 2000/9/18. P. 34. / y ^ ^ ■*-er rev. (27,040 cub. cms.). Area of one piston. — 19.63 sq. ins- (126.6 sq. cms.). Total piston area of engine. — 235.6 sq. ins. (1,520 sq. cms.). Clearance vo'Aime of one cylinder. — 36.4 cub. ins. (naval type), 31.9 cub. ins. (army type). Compression ratio. — 4:78 (naval type), 5:3 (army type). Normal B.H.P. and speed. — 405 h.p. at 1,650 r.p.ni. Maximum r.p.m. near ground. — 1,500 r.p.m. Maximum rp.m. at altitude. — 1,750 r.p.m. (a1)o\e 6.000 ft. only). Piston speed. — 1.925 ft./min. at 1.650 r.p.m. Brake mean effective pressure. — 118 Ibs./sq. in for 405 h.p. at 1,650 r.p.m. Cub. in. of stroke volume per b.h.p. — 4.07 cub. ins. (66.69 cub. cms.). Sq. in. of piston area per b.h.p. — .582 sq. in. (3.75 sq. cms.) H.P. per cub. ft. of stroke volume. — 424 (15.0 h.p. per cub. metre). H.P. per sq. ft. of piston area. — 247 (2.666 h.p. per sq. metre). Rotation of crank. — Right-hand tractor (left-hand pusherj. Rotation of propeller. — As above. Speed of propeller. — Ungeared. Lubrication system. — Force feed to main bearings, big ends, and camshafts. Splash to pistons and gud- geon pins. Dry sump. 0\\ recommended. — ^slobiloil BB. ( )il pressure recommended. — Minimum, 25 lbs. per sq. in.; ^Maximum, 35 lbs. per sq. in- — at 1,650 r.p.m. Oil temperature recommended. — 40° Centigrade. Oil consumption per hour. — 8 to 12 pints per hour. Oil consumption per b.h.p. hour. — Average .03 pint H.P. hour. Specific gravity of oil — .9. Carburettors. — Two American Duplex Zenith, type 52 D.F., with modified altitude control (vacuum); or two Claudel-Hobson, type H.C. 7. Fuel consumption per hour. — 198.5 lbs. Fuel consumption per b.h.p. hour. — .49 lbs. Specific gravity of fuel. — .72. Ignition. — Delco system. Two distributors: four con- tact breakers in parallel ; dynamo generator, supplemented by accumulator for starting and slow running- hairing sequence of engine. — 'Left, I 9, 5. II, i, 7 ' Pi-opeller. Right, 8. 4, 12, 6. 10, 2 I ^ Numbering of cylinders. — Left, i, 2, 3, 4, 5, ^Ip^.^ jj^j. Xumbering- of cylinders. — Right, i, 2, 3, 4, 5, 6 I ^ Speed of low tension generator. — i^ times engine speed. ]\Iaximum ignition advance. — 30°. Inlet \ alve opens. — 10° after I'.D.C. Inlet valve closes. — 45° after B.D.C. Inlet tappet clearance. — .015 in. (.0381 cm.). Number of inlet valves. — One per cylinder. Maximum lift of inlet valve- — fg in. (i.ii cms.). Smallest diameter of inlet valve. — 2h ins. (6.35 cms.). Area of inlet valve opening. — 3.44 sq. ins. (22.2 sq. cms.)- Mean gas velocity through inlet valve. — 177.7 ft- P^'' sec. Exhaust valve opens. — 50° before B.D.C. (now 48°). Exhaust valve closes. — 10° after T.D.C. (now 8°). Exhaust tappet clearance. — .020 in. (.0508 cm.) (now .019 in.). Maximum lift of exhaust valve. — | in. Rev. counter drive rotates-— Clockwise, facing shaft on engine. Weight of engine, nnnus water, fuel and oil. — 820 lbs. Weight per b.h.p., minus water, fuel and oil. — 2.02 ibs. Weight of exhaust manifold. — Weight of oil carried in engine. — Nil. Weight of starting gear. — Weight of water carried in engine. — 45.8 lbs. (4.58 gal- lons. Weight of fuel Der hour. — Weight of oil per hour. — Fig. 2. Cylinders. Total fuel and oil weight per hour. — Gross weight of engme in running order, minus fuel and oil. — 1,083 lbs. Weight per b.h.p. minus fuel and oil. — 2.67 lbs. Gross weight of engine in running order, plus fuel and oil for six hours. — Weight per b.h.p. plus fuel and oil for six hours. — 1 1 in the followin,:^- descriptions, the propeller end of the en.L^ine is termed the "front"; the words "rig-ht" and "left" are ttsed on the supposition that the engine is viewed from the rear or distribution q-ear end. CHAPTER I. General Description. Fig. 3- Section of a C3-linder (showing welded joints). 12 CYLINDERS. Each cylinder is formed from a steel tube, the head, barrel, flange and spigot being- all in one piece. One end of the tube is closed do//n to form the cylinder head, and seating-s for the two valves are machiu'cd in it. The holding'-down flange is "jumped up" after local heating, leaving a spigot 2\% in. deep. The water jacket is made of two sheet steel pressings, welded together : the line of the main vertical joint is at right angles to the crankshaft axis. There are further welded joints at the two valve pockets, sparking plug 'bosses, npper and lower water connections, camcase studs, and round the base of the jacket. Fig. 4. Cylinder Head (plan view). The cylinders are interchangeable, though the long and short studs supporting the camcase must be inter- changed before a right-hand cylinder can be mounted in the left-hand block and z'tce versa. (See Fig. 2.) The holding-down flanges are drilled for ten studs apiece. Flats are cut through the two stud holes on the fore and aft sides of each flange, so that the two studs between a pair of cylinders secure both flanges. The holding-down nuts are made with circular bases of greater diameter than their hexagons. 13 Fig". 4 affords a plan viewi of a cylinder head, illus- trating the large water space round each valve pocket and the positions of the sparking plug" ,bosses, camcase studs, and water exit pipe: the latter is jointed by a rubber connection to a short pipe projecting from the water jacket of one of the four induction) manifolds. The circumference of each cylinder jacket increases gradually towards the top of the piston stroke. The water entry connections are short pipes, curving" down- wards from the base of each jacket on the outside of the block. Fig- 5- Side View of a Cvlinder. PISTONS. The high compressicn pistons (Fig-. 7) are of cast aluminium alloy with externally domed tops, flattened at the centre over a circle 2| in. in diameter. (The low- compression pistons have flat tops). There are no internal ribs, but the crown is | in- thick, and the upper part of the wall tapers down towards the gudgeon pin bosses. There are three compression rings above the 14 Fig. 6. Cylinder : Section through Valve Gear. gudgeon pin : each ring has a diagonal gap, and a shallow circumferen|tial oil groove is machined round the centre of its working face. No scraper rings are employed. Beneath the rings seven shallow oil grooves are machined circumferentially on the outside off the skirt, which is not drilled at any point, The hollow steel i^udi^eon pin is parallel-sided, and is a push fit in the bosses, which are cast on the skirt- It is located endways by the two caps shown hi Fig'. 8. Their outer faces are radiused to the contour of the cylinder walls, and they are positioned in the bosses by the rectangular projections shown in the photograph. They were originally fixed by spring' wnre clips, which have since been dispensed with. The holes in their centres are threaded to take an extractor. The gudgeon pin is allowed to float in the piston bosses and in the small end bush. Fig. 7. Broken V^iew of " Army " Piston. CONNECTING RODS. The connecting rods are illustrated in Fig. 9. The forked rods may be fitted on either side of the engine, provided that they are all assembled on the same side. The forked 'big ends grip a split phosphor bronze shell, which is fixed in them by dowel pins, whilst annular grooves machined on its exterior locate the forked rods and prevent them from "spreading"; the caps are not bridged. The shell is lined with white metal, and works on the crank-pin in the usual way. The single rod works i6 on the outside of the phosphor bronze shell between the forked ends of the double rod. The little ends are bushed with phosphor bronze. Details of the lubrication are given on page 36. Fig. 8. Pistons and Gudgeon Pins. Fig. 9. Connecting Rods. •7 Fig. lo. Pair of Connecting Rods (dismantled). CRANKSHAFT. The crankshaft runs in seven bearings, consisting of horizontahy spHt phosphor bronze shells, lined with white metal and housed in the crosswebs of the base chamber. The bearing next the propeller is 115 mm. in length, and the six reari bearings are 49 mm. long. The ends of the journals and crankpins are sealed on early engines by brass discs, soldered into position, but the standard system will consist of steel caps, drawn up by bolts- Details of the lubrication are given in Chapter 11. The main driving bevel is machined in one piece with a starting claw, and is bolted to a flange machined on the rear end of the shaft. Shims are threaded over the bolts between the flange and the bevel for the pur- pose of adjusting the mesh of the distribution gears. The centre of the bevel is internally splined to provide a drive for the gun interrupter gear. Fig. 85, p. 114, illustrates in detail the double thrust bearing, which is located between the propeller hub taper and the front main bearing, and is provided with cast housings in the base chamber. Its terminal stops con- sist of a flange machined on the shaft, and a large flanged and castellated nut, screwing on to a thread on the shaft. Fig. II. Lower Half of Main Bearing'. The maximum end play should not e cceed .008 in. The washers locating the thrust in the crankcase are thick and thin respectively. They should be selected so that when assembled they leave .001 in. end play in the grooves in the crankcase. The propeller hub is mounted by a taper and key in the usual way. The key is fixed by a grubscrew at its centre, fore and aft of which there are two threaded holes in the key. by means of which the grubscrew may be utilised to lift tlile key. The locking device is illustrated in Fig. "j"] , p. 105. Fig. 12. Crankshaft Taper and Key. BASE CHAMBER. The base chamber is made of two aluminium alloy castings bolted together: the horizontal joint comes at the level of the main bearing" centres, as usual- There are no "feet" or bearer brackets on the upper half: its flange, overhangs the lower half for the full length of both sides, and is attached direct to the engine bearers in the fuselage by seven bolts on each side. This pro- jecting flange is stiffened by seven pairs of vertical ribs on both sides. 19 H 20 Fig. Id. Top Half of Crankcase. The main bearings are housed in the cross webs, so tliat the crankshaft can be freed by unbolting- the sump. Similarly, the cylinders can be removed without dis- turbing the crankshaft. The two parts of the base chamber are secured together 'by fifty small bolts distributed along the flange. In addition, eight pairs of heavy studs are fixed in the lower half along the centre line, two pairs at the front bearing, and one pair at each of the six shorter bearings; and two pairs of external bolts are employed at the ex- treme front and rear ends of the base chamber respec- tivelv. 1 - m t ^Hf" ' ' : ■m - jamwmr^'-' ^^- K^ '£t, '-■^ ' -^^rjs ■ ^7-7 f i^ L W L,UL. ^ ML. . ■ w^ ■>*»•-- v'«f~ ~ '■ '- • - <»*«»» /■ ' • "^umm^-.-Mmm' Fig. 15, Top Half of Crankcase, viewed from beneath. 21 The top half is cross-stiffened by eight webs, which incorporate tubular housing-s for thie studs above men- tioned. The rear web and the two nose [)iece webs are solid, but the other five webs have panels cut out of them. The formation of the rear end is unusual, being designed to provide a moisture-proof chamber for the ball bearings of the distribution gears. This chamber is enclosed by means of the rear wall and top end of the casting, together with solid vertical and horizontal webs. In the top of the compartment so formed there are four apertures, respectively closed (when the engine is assembled) by a breather cap, the vertical shaft, and the two inclined shafts. In the base of the compartment Fig. 1 6. Bottom Half of Crankcase. there are two apertures, closed respectively by the vertical shaft and the oil trap shown in Fig. Sy, p. Ii6. This trap is kept permanently brimming with oil, which drains into it from the casings of the inclined shafts. When con- densed moisture is surging about in tlie main base chamber under the influence of crankcase compression, it can find no entry into the distribution gear compart- ment, seeing that the oiltrap is sealed by oil. Thus the ball races within the compartment are protected from rust. Two large breathers are bolted to the right-hand side of the upper half of the base chamber. An en- graved plate on the rear end gives the firing sequence of the cvlinders, and the name of the manufacturer. 23 The lower half of the base chamber is flat-bottomed, with fore and aft oil wells. It is cast with cross webs at each of the six shorter main bearings, and its nose piece is heavily ribbed, both laterally and longitudinally, t'j support the front main bearing.. The lower half of the housing for tlie thrust race is cast in the nose piece. Towards the rear the casting is given an approximately rectangular section, and houses the drive for the oil and water pumps. The base of the sump is cast with a narrow trough to accommodate the main pressure oil pipe and the oil return pipe from the front oil well. The lower halves of the main bearings are housed in tlie cross webs, as stated above, and are not slung in caps from bolts. .'\ cylindrical housing for the plain bearing of the lower vertical shaft is cast as part of the lower half at the rear end, immediately below the main crankshaft bevel. The water pump unit is bolted to the rear end of the sump, and driven by bevel gearing off the lower vertical shaft. A splined extension of this shaft drives the oil f'Umps, the oil pump unit being bolted beneath the rear end of the sump. Two straight steel oil pipes are inserted from the rear end, and pushed through housings in the bases of the cross webs. The right-hand pipe is the main supply pipe, and communicates with the bottom centre of each main bearing by means of a steel pipe screwed verti- cally into each web ; the supply for the camshaft is taken from the front main bearing- through a lead in the upper half of the base chamber. Screwed plugs at either end facilitate the cleaning of the main pressure pipe : the forward plug is beneath the sump; the rear plug is visible when the pump unit is dismounted. The left- hand pipe connects the forward oil well to the scavenging pump in the rear oil well. The front oil well contains a filter: its cover is secured by a spring clip. A felt oil-retaining washer is fitted at the front of the nose piece. The circular orifice opposite the rear end of the crankshaft may be closed by a coverplate if a gun interrupter gear or mechanical starter are not fitted. VAL,VES. There are two valves per cylinder, working in cast iron g'uides pressed into the valve pockets. The valve stems are inclined towards the cylinder axis at 13^*^, the exhausts being on the outside of each cylinder block, and the inlets on the inside. The camcase is accommodated within the included angle. All the valves are inter- changeable, their smallest diameter measuring 63.5 nun. ^4 Fig. 1 8. Valve and Fittings. Double concentric coiled springs are iitted. The inner springs are interchangeable. The outer springs of the inlet valves are longer than the corresponding exhaust spring's, which are of stouter gauge. A female taper in the upper spring cap presses a pair of internally ^^€:>'^^ Fig. 19. Camshaft and Bearings. shouldered split cones ag"ainst three annular grooves machined on the valve stem. Fig. 20. Camcase. CAMSHAFTS AND VALVE GEAR. The camshafts are not interchangeable, and are marked "R" and "L" respectively. The camcases are interchangeable. They are machined in one piece, and drilled for pressure lubrication, of which details are printed on p. 37. The driven bevel pinions are bolted to tlanges on the rear ends of the shafts. Notched flanges or " hubs," secured to these studs behind the bevel pinions, drive the rotating' portions of the distributor heads. Each camshaft runs in seven bushes (see Fig. 81, p. 1 10), which are a driving fit in the camcase. They are "stepped" in outside diameter, that at the rear end being the largest, the next ^in. smaller, etc. The five intermediate bearings are cast in aluminium ; they are horizontally split, assembled by screws, and located in the camcase by large grubscrews entered^ from the side. The front bearing of each shaft is a one-piece cylindrical bush of aluminium : the oil supply enters the camshaft bv this bush, as described on page 38. The rear bearing is of special pattern, as illustrated in Fig. 81. It con- sists of two phosphor bronze shells, lined with white metal, and is mounted between two flanges machined on the shaft, so as to take the thrust. A camcase is illustrated in Fig. 20. It is an alu- minium alloy casting, of tubular section between each pair of cylinders, and widening into approximately rectangular compartments with detachable lids over each cylinder head. It is secured to each cylinder head by a couple of studs- 26 The inlet and exhaust rockers are machined from stieel forgings, different in shape for the right- and left-hand sides of the cylinders, though either shape may be used to operate the inlets or the exhausts. The spindles are integral with the cam and valve arms. The lower half of each rocker spindle bearing is machined in the camcase, auid the upper half in one of the re- movable lids. The spindles are located endways in their bearings by the valve-actuating arm at one end, and lat the other end' by a flange turned on the spindle. Two arms project from opposite sides of each spindle. A short forked arm, projecting inwards near the centre of the spindle, carries a hard steel roller which engages the cam. A longer arm, projecting from the end of the spindle, carries an adjustable bolt which actuates the valve. The bolt has a square on it, which is a loose fit in a sc[uare hole at the outer ei^d of the arm ^ the bolt is packed to the height required for correct valve clear- ance by means of shim washers, and is then locked by a castellated nut and split pin. The engines are delivered with three shims of different thicknesses threaded on each bolt. ^^ D Fig. 21. Valve Rockers. DISTRIBUTION GEAR. Fig. 22 is a diagram of the general layout of the dis- tribution gears. The main bevel on the crankshaft drives upper and lower vertical shafts. The upper vertical shaft drives the two inclined shafts which form the camshaft drives; and its splined extension drives the low tension generator. The lower vertical shaft drives the horizontal water pump shaft through bevel gearing. 27 and its splined extension drives the oil pumps. These transmissions are next described in closer detail. The mesh of the main crankshaft bevel can be adjusted by means of shims threaded over the bolts by which it is secured to the flang-e on the crankshaft. Camshaft bevel J^ Inclined shaft Generator drive Upper vertical shaft Main crankshaf bevel Lower vertical shaft Water piim|5 drive Oil pump spindle Fig. 22. Diagram of Distribution Gear. The lower arc of the main crankshaft bevel (t,^ teeth) meshes with the upper bevel (212 teethj of the lower vertical shaft. This lower shaft is made in one piece with its top and bottom bevels, and runs in a split 28 phosphor bronze bush, held in ahmiiniunii housings, which are mounted in a cyHndrical lug cast in the base- chamber. Provision is made for oil to drain down through this bearing. The bottomi Ibevel (21 teeth) of the lower vertical shaft drives another bevel (21 teeth) on the front end of the horizontal water pump shaft : behind this bevel is a ball bearing, housed in the water pump casing. The lower vertical shaft is hoIlow\ and is internally splined to drive the oil pump spindle, which is provided with a phosphor Ibronze bush at the top of the pump casing. The puimps run ,at li times engine speed. The bevel pinion at the base of the upper vertical shaft has 22 teeth, and the three bevels composing the transmission at the base of the inclined shafts are all of 22 teeth, so that all three shafts run at li times engine speed. The top of the vertical shaft drives the generator shaft by splines. For ease of assembly and adjustment of mesh, each inclined shaft is arranged in two pieces, coupled together by a splined' joint at the point of exit from the Ibase chamber. The driving bevels at the top of the inclined shafts have 16 teeth, an^l the large driven camshaft bevels are of 48 teeth. As the illustrations show, the upper A^ertical shaft runs in two l>all bearings. The short shafts at the base of the inclined drives run in two ball bearings apiece. The upper end of each long inclined shaft is provided with a phosphor bronze 'bush, housed in the shaft casing. A tachometer drive is taken off the generator shaft by skew gearing, and runs at half engine speed. COOLING SYSTEM. The water pump is of the centrifugal type, its impeller revolving in the vertical plane. The unit is bolted to a circular aperture in the rear end of the sump, and its spindle is driven through bevel gearing off the lower \ertical shaft at li times engine speed. The spindle is supported by a ball bearing housed in the pump casing. A douible stuffing gland is provided on the engine side of the impeller (Fig. 79, p. 108). Graphited asbestos is wound round the shaft inside two sleeves ; a star washer is threaded on the shaft over each packing, and a spring between the washers keeps the packings compressed ; the projections of the washers engage slots in the sleeves. Supply pipes run from the two delivery ports of the water pump towards the right- and left-hand sides of the base-chamber respectively, and are coupled by rubber connections to long' water manifolds, lying along each side of the engine 'below the cylinder base level. The -9 n:ain pipes of these manifolds are tapered towards the front of the engine; short branch pipes project upwards. and are jointed to rubber connections by stub pipes ctn"ving' downwards from the base of each cylinder jacket. The type of water cHp employed throitghout the Liberty engine is illustrated on p. I 19: its band is made of mild steel, and is extremely flexible. The water return is taken through the induction nuinifold jackets. There are four of these manifolds, each serving a group of three cylinders. A short retm'u \)\\)e in each cylinder head is jointed by rubber hose to a pipe projecting from the adjacent inlet manifold. An elbow is bolted down over the adjacent water exits of the rear pair of manifolds, and a pipe runs forward to a similar elbow uniting the water exits of the front pair of manifolds. From the front elbow there is a single return pipe to the radiator. A plugged connection for a thermometer is pro\ided behind the front water elbow- The cooling svstem can be drained through a plug beneath the pump. Fig. 23. Water Puiuj) (sjiown (lisin;intle(i). 30 STARTER. At the moment of goings to press an electric start- ing motor has been adopted for the Liberty engine. A special handbook pf instructions for its use and main- tenance will be issued in due course. Relief valve. Fig- 24. Broken \'iew of Oil Pump Unit. 31 Lubrication. CHAPTER II. The lubrication system is simple in outline, but demands somewhat lengthy description, as some of the individual imits are complex. A large pump unit, con- sisting of a three-pinion duplex scavenge pump mounted above a pressure pinnp, is assembled in a casing bolted beneath the rear end of the sump. Both pumps are driven by a spindle which is jointed by splines to tlK lowe.'" vertical shaft. The supply pump forces oil under pressure along" a manifold in the sump, which serves all the main bearings. A lead from the front main bearing conducts oil to a three-way branch at the front of the base chamber, whence three pipes are connected to the pressure gauge and the front ends of the two camshafts respectively. The oil drains back to the sump down the casings of the two inclined drives, lubricating the bearings and pinions of the distribution gears on its way. The pistons and gudgeon pin> are lubricated by splash. The double scavenging pump is located in the puni]) unit beneath the oil well at the rear end of the flat- bottomed sump, and is connected by a pipe in the sump to the oil well at the front end. Both oil wells are pro- vided with filters, and their respective dehvenes meet in a common return pipe to the tank. A detailed description of the various components follows : — OIL PUMPS. It may be found difficult to grasp all the details of the pump until it has actually been d'smantled; it i-^ plentifully illustrated by sectioned and persjiective drawings' in Figs. 24-29. The diagram in l"ig. 25 and the sectioned perspective in Fig. 24 are the simi)lest to stud v. when the actual pump is not available. "As the section in Fig. 24 shows, the double scavenge ])ump and single pressure pump, together with the filters and relief valve, are accommodated in a casing, which is bolted beneath the rear end of the sump; the main pres- sure pipe and the return pipe from the front catchpii are connected to the pumps via ducts drilled in the i)ump casing. At the top of the unit is a phosphor bronze bush.' in which the driving spindle runs. The spindle drives the centre ])inion of the scavenging ])unip, and passes through a steel i)artition into the driven ])inion ol the lower oV pressure pump. The loo'^c gears arc 32 mounted on steel spindles fixed in the casing'. The oil is filtered before entering either pump : the upper filter is locked in position by a nut and tab washer: the lower filter is secured by the bottom cover. The relief valve is situated immediately outside the delivery port of the pressure pump. From rear oil well to scavenge pump From front oil well to scavenge pump To pressure manifold Relief vahe Filter of scavenge pinnp Return from scavenge pump to tank Filter of pressure pump F"rom tank to pressure pump , ... i^il ■ Fig. 25. Diagram of Oil Pump unit, showing leads. (N.B. — This sketch is purely diagrammatic.) Fig. 25 is purely diagrammatic, and does not show the actual disposition of the various parts and leads. At A the supply pipe from the tank enters the filter chamber surrounding" the lower or pressure pum.p, into which the oil passes at B- Oil under pressure emerges from the pump at C, and enters the sump pressure manifold at Ci, passing on its way the relief valve I). Turning" to the scavenge pump, it must be remem- bered that the sump is flat-bottomed, and that when the aeroplane is diving or climbing", the drainag'e oil will run exclusively into the front or rear oil well. The scavenge pump consists of three pinions, driven by the centre gear. It is th-erefore the equivalent of two separate pumps, each composed of two gears : as its gears have deeper teeth than the gears of the pressure pump, each of its units is singly morie powerful than the supply inimp. If the aeroplaiie is diving, the drainage oil collecting in the front catchpit will be sucked along the pijie Ki (Fig. 25), and enter one unit of the upper or scavenging pump at E. The pump will deliver it into the main return pipe to the tank at F. On the other hand, if 33 P--T 34 Fig. 27. Plan View of Scavenge Pumps. Fig. 28. Plan View of Pressure Pump. 35 Fig. 29. Cross Section of Pump Unit (showing rear half). Fig. 30. Cross Section of Oil Pump Unit (showing front half). 36 the aeroplane is climbing, the oil will drain into the rear catchpit, ie., the pump casing. It will enter the second unit of the scavenge pump at G, leave the pump at H, and at Fi it will enter the main return pipe F, leading back to the tank. When the aeroplane is flying hori- zontally, drainage oil will collect in both oil wells simultaneously, and both units of the scavenge pump will return oil into F. As there is a popular impression that gear pumps will not exert strong suction, it may be of interest to state that they suck o;l quite efficiently over a much greater distance than is required in this engine : but they will only suck when they are primed. The two scavenge pumps being formed cf only three gear wheels, which- ever pair is working at any moment keeps the remaining wheel primed. Figs, 27-30 show various sectioned views of the pump, indicating' the details of the various components and ducts with accuracy. The only point requiring further mention is the lead between the delivery port of the pump unit scavenging the front oil well, and the main return pipe : this lead consists of a duct machined in the casing of the scavenge pump, crossing over the top of the pump from one side to the other. The relief valve is of the mushroom type; when an excessive pressure is reached, the valve is forced from its seating, and oil drains through the valve box back into the casing, until the pressure sinks to the normal. LUBRICATIOX OF THE MAIN AND BIG EXD BR.\RINGS. The bottom of the sump is cast with a narrow trough along the centre line. This trough accommodates two straight steel pipes, secured in housings at the cross webs. The right-hand pipe is the main supply pipe: it is pushed in from the rear end of the sump, and provided with screwed plugs at both ends, so that it can easily be cleansed. Vertical branch pipes, screwed into each of the cross webs, connect it with the bottom centres of the main bearings, in which the oil is distributed -by suit- able gTooves, a supply being trapped by milled notches in the white metal along the joints of the bearing" on both sides. At each bearing some of the oil enters the crankshaft through a suitable hole, travels up the rear web of the throw into the crank-pin. and is forced out of another hole into the big end bearing. Suitable grooves distribute oil over the crank-pin bearing, and 37 four holes, spaced out at 90°, allow oil to penetrate to the centre of the otiter circivmference of the bush, on which the single connecting rod oscillates. The ends of the journals and crank-pins are sealed by brass discs, tinned and sweated into position. Future engines will have steel plugs drawn up by bolts. LUBRICATION OF THE GUDGEOX PIXS. The little end bushes are lubricated sclely by splash, the oil entering through a large hole drilled in the top centre of the little end. Dowel hole- Oil entry to bearing From pressure manifold — Oil entry to bearinf — Groove in housing (to camshaft lead) Dowel registering ?hel! in housing Fig. 31 Front Alain Bearing, showing oil leads. Top: Bearing shell. Bottom : Bearmsr housing. LUBRICATION OF THE CAMSHAFTS. Fig". 31 illustrates the manner in which the camshaft oil supply is taken from the front crankshaft bearing". A branch from the pressure manifold in the sump delivers oil into a longitudinal groove niachined in the lower housing" of the bearing^. This groove is connected by circumferential g'roo\es on both sides of the bearing" to a longitudinal groove in the upper housing, which con- ducts the oil back to a vertical lead in the web of the base chamber. Through this lead the oil reaches a three-way connection on the top of the base chamber, from wliich pipes are led to a pressure gauge and to the front ends of the two camshafts respectively. 38 Fig'. 32 indicates how the oil suppHes enter the camshafts. The front end of each camcase is sealed by a screwed cap. beneath which there is a CA. washer: the oil pipe is connected tO' the centre of the cap. The front cam- shaft ibearing" is a cylindrical bush of aluminium : its front end is sealed by a metal plate, attached to the bush by four screws, and rendered oiltight by a paper washer. Thus an oil recess is formed between the metal plate and the screwed cap already mentioned. An oil duct is machined in the thickness of the cylindrical bush. ]:)arallel with its axis. The front end of this duct is open to the oil chamber; the rear end of the duct turns at right- angles, and opens into the centre of the bush, where it ■registers once in each revolution with a hole drilled through into the centre of the hollow camshaft- Oil under pressure thus enters the camshaft, and is sprayed Oil Connection to Camshaft. out of holes positioned at the centres of the six remaining bearings, along which it is distributed by suitable grooves. There are no oil exits from the camshaft in the cam compartments, but an ample supply of oil leaks along the bearings into each compartment, and is splashed up by the cams. The rocking lev^r spindles are hollow, and drilled with one oil entry and three oil exits apiece, their ends being plugged with brass. The oil entry is above the point at which the roller arm joins the spindle • a small baffle cast on the imder side of the camcase lid directs the oil splash into this hole, and it escapes into the spindle bearing through the oil exits aibove men- tioned. Deep grooves (Fig. 81, p. iio) are machined in the outer circumference of the lower half of each camshaft 39 Fig- 33- Oil Pump Unit (dismantled). Fig. 34- Oil Pump Unit from below; cover removed 40 l)ush, so that the drainage oil can flow along the casing. It finally reaches the bevel gears at the rear ends of the camshafts, and drains down the casings of the inclined shafts, lubricating the bearings and bevels of the distri- bution gear on its way back to the sump. As described in a previous chapter, the compartment in which the upper vertical shaft and lower inclined shafts are housed, is separated by webs from the main base chamber; the oiltrap shown in Fig. 87, p. 1 16, is the sole outlet for the drainage oil from this compartment. The construction of this trap produces an oil seal, which prevents condensed moisture in the main base chamber from reaching the ball bearings of the distribution shafts. Provision is made for tlie oil to drain through the bearings of the various shafts, e.g., at the heads cf the long and short inclined shafts and along the bush of the lower vertical shaft, which is illustrated in Fig. 88, p. 1 1 34a- Oil exits Details of Valve Rocker Lubrication. LUBRICATION OF THE THRUST BEARING. The main thrust bearing is lubricated by surplus oil from the front crankshaft bearing. A spring-lid lubricator is provided on the top of the nose piece, and the thrust housings in the nose piece are recessed to act as a reservoir. This reservoir should be replenished by hand before starting up an engine which has not been running for some time. GENERATOR DRIVE BEARING. It is recommended that the generator be removed after every ten hours running, and the ball bearing at the top of its drive, together with the shaft and splined ccupling, be freely lubricated with Vacuum B.B. oil. 41 Fig. 35 Top view of Eni^ine from Front, showing Zenith Carburettor and Induction Manifolds. 42 CHAPTER III. Carburation. Two duplex carburettors are bolted beneath the junctions of the forward iand rear pairs of induction manifolds respectively, American Zeniths, type 52 D.F. being fitted to some engines, and Claudel-Hobsons type H.C.7. to others. Fig. 36. Photograph of Zenith Carburettor, Type 52 D.F. (minus main air intake scoop). 43 Zenith Carburettors. Examination of these carburettors will show that they have been modified in this country as the altitude control criginally fitted was not satisfactory. The g'eneral functioning of the carburettor has also been much improved. Each carburettor is built up of three castings, viz., a throttle chamber, a main casting embodying duplex mixing chambers, together with a single float chamber. and an air intake. The latter is quickly detachable, being secured by spring clips to a couple of studs on the sides of the main casting'. The throttle chamber contains a pair of butterfly throttle valves. The maintenance of an approximately constant petrol-air ratio at all engine speeds is attained by the familiar Zenith device of a compensating jet. If petrol is supplied by a single jet located in the centre of a choke tube, high engine speeds will result in an excessive flow of petrol, due partly to the inertia of the colunm of petrol, and partly to the increased depression or suction. Conversely, there will be a deficiency of petrol at low speeds, owing to the weakness of the depression round the jet. and the low momentum of the stream of petrol between the float chamber and the jet. The com- pensating jet corrects these faults, because it possesses exactly opposite tendencies, supplying an excess of petrol at low speeds, and a reduced cjuantity of petrol at high speeds. Consequently, if the main and compensating jets are properly balanced, they cooperate to furnish a correct flow at all speeds. Diagram illustrating the working principle of Zenith carburettors. I 44 Broken View of Zenith A — Petrol supply from tank. B — Petrol recess below needle valve. C — Needle valve seating. D — Petrol duct to jets. E — Compensator jet. F — Duct to main jet. G — Duct to shroud tube. H — Main jet. I — Shroud tube. J — Air supply to well Y. K — Air holes in dipping tube L. L — Dipping tube (slow-running device. MM — Restrictions at head of dipping tubes. Fig. 38. Carburettor, Type 52 D.F. N — Slow-running by-pass. Ni — Mouth of slow-running by-pass. O — Leakhole. P — Air supply to float- chatrtbcr. Q — Orifice in altitude control cock housing. Qi-.— Central slot of altitude control cock. R — Suction hole (from choke tube into cock housing). Ri — Holes in altitude cock registering with R. S — Throttle stop. X — Orifice in base of dipping rube. V— Well. 45 i;iS- Z7 is ^ diagrammatic sketch of the Zenith principle in a simple form. G is thie main jet, drawing petrol from the float chamber in the usual way. I is the compensator, or compensating jet, which controls the flow of petrol from the float chamber into the well J. The well J supplies petrol to a secondary spraying tube H parallel to the main jet G. It is clear that the flow of petrol from H will depend upon the height of the column of petrol in J, and this depends soldly on the amount of petrol which can pass the compensating orifice I in a given time. The hole at I, namely the orifice of the compensating' jet, is much smaller than the bore of the passage which supplies petrol tO' the main jet G. When the engine is not working, petrol rises in J, and a maxi- mum flow is ensured from H for starting purposes. When the engine is running slowly, a maximum supply is obtainable from H, which balances the minimum supply drawn from G by the depression. When the engine is running at full speed, a minimum supply will reach H, whereas the increase in the depression is drawing" a maximum supply from G. The action of the two jets is therefore mutually opposed : and if the orifices at I and G are accurately calibrated, a predetermined petrol- air ratio can be ensured at all engine speeds. Fig. 381 gives a sectioned view of the application of this principle employed in the American Zenith carburettor, type 52 D.F., as fitted to some of the Liberty engines. H is the main jet. Its petrol supply is received direct from the float chamber via the ducts D and F, and is mainly conditioned by the depression in the mixing cham^ ber. The jet H delivers its minimum flow of petrol at low engine speeds, and its maximum flow at full throttle openings. The main jet H is balanced by the outer or "shroud" tube I, which surrounds H concentrically. The tube I receives its supply of petrol from the well Y. which in turn is supplied via the compensating orifice or jet in the plug E. Thus the supply of petrol to the tube I is not conditioned by the depression in the mixing chamber, but by the calibrated orifice in the plug E (i.e., the com- pensating jet). When the engine is running slowly, the petrol flowing through the compensator E will build up to a comparatively high level in the well Y : as the engine is accelerated, the petrol level in Y will sink. Consequently, a maximum flow of petrol is drawn from the tube I at low engine speeds, and a minimum flow at high speeds. Thus its action is directly opposed to that of the main jet H; the main jet and compensator balance each other, and co-operate to supplv an approxi- mately constant petrol : air ratio at all engine speeds. 46 The slow-running device is in duplicate. Each con- sists of a tube M dipping- at its lower end into the petrol in the well Y and connected at its upper end by the small duct N to the throttle chamber, into which it opens en a level w^ith the lower edge of the butterfly throttle, when the latter is almost shut. With the throttle in its mini- mum position, the depression is insufficient to bring the jets H and I into action, but is concentrated upon the mouth of the slow-running by-pass at N'. Petrol is drawn up the dipping tube through the pilot jet at X and mixed with air inhaled at K. Fig. 39. Section of Zenith Carburettor (between the two mixing chambers). No adjustments to the main spraying devices are possible or necessary, the choke tubes, the main jets H and the compensators I, being standard calibrated sizes for all Liberty engines. A throttle stop, controlling the quantity adjustment for slow running, is located beneath the throttle control si:)indle. (See pp. 142 — 3 for important note on adjusting the carburettor controls.) Both throttles are operated by a single lever through a pair of toothed quadrants. 47 D B GF EX Fir. 40. Section of Zenith Carburettor (through the jets), (For reference letters, see p. 44.) Fig. 41. Plan of Zenith Carburettor. I 48 THe official Zenith jets for the Liberty engine are: — Choke Tube ... ... 36 mm. Main Jet 280 cc. Compensator ... ... 340 cc. Shroud Tube ... ... 3.9 mm. diameter. THE ALTITUDE CONTROL. The density of the air decreases rapidly within the altitudes now reached by aeroplanes. The deficiency of oxygen then results in an over rich mixture, which is corrected by reducing the flow of petrol in proportipn to altitude. This correction is effected by the pilot at lils discretion by means of a separate control lever. On the carburettor under notice the "vacuum" system is em- ployed. The flow of petrol from the float chamber to the main jet depend's upon a difference between the pressure ruling in the float and mixing chambers respec- tively. Whereas the float chamber is subject to atmos- pheric pressure, the mixing chamber is governed by a "depression," which may alternatively be described as a partial vacuum, due to the descent of the pistons in the cylinders Oif the engine. Thus, if the petrol in the car- burettor be regarded as a L'-shaped column, there is atmospheric pressure at the float chamber tip of the U, and a depression at the mixing chamber tip of the U. The flow of petrol depends upon the difference between these two pressures. It is thus a simple matter to cut down the supply of petrol at high altitudes by reducing the pressure in the float chamber. , Under the vacuum system of altitude control, the float chamber is fitted with an airtight lid, and is supplied with air at ground level and at moderate altitudes through a small hole communicating with the atmosphere and termed a "leak hole." So long as the leak hole alone is in action, atmospheric pressure governs the float chamber, and the maximum supply of petrol is assured. As the aeroplane gains altitude, and the petrol supply becomes excessive, the control cock is operated to connect up a '^suction hole" with the float chamber. This suction hole draws air to the depression area at the narrowest diameter of the choke tube : the pressure in the float chamber is proportionately reduced, and the petrol supply is cut down. The details of the control on this type of Zenith carburettor have been modified for the British Services, and are shown in Figs. 38 and 42. The bridge piece between the choke tubes contains a hollow chamber, connected as follows : — Ill 49 By the hole P to the float chamber. By the pipe O to the main air intake. This pipe O is the "leakhole." . By the duct O to the housing of the ahitude control cock, from which the "suction holes" R open into the mixing chamber. X ^Control Cock. B.tV P [ f \ r^ 1 1 / f 1 — I Mg. . \2. Diagram illustrating Zenith Altitude Control O — Leakhole. ; P — Lead to float chamber. R — Suctio n hoi ?. The pipe or leakhole O is permanently open. So long as the suction holes R are closed by the altitude control ccck, O forms the sole air supply to the float chamber, and the flow of petrol to the main jet is at its maximum. As the control cock is progressively operated to open up the suction holes R, the pressure in the float chamber is weakened, and the flew of petrol to the jet is reduced. Claudel-Hobson Carburettor, Type H.C. 7. This carburettor aims at providing a predetermined petrol-air ratio over the full range of engine speeds by a principle quite different to that employed in tlie Zenith. in outline the system is as follows : — (aj Throttle shut to slow-running position. The petrol supply is restricted by a calibrated pilot jet : the air supi)ly is automatically Fig- 43- H.C.' 7 Duplex Carburettor. restricted by the slow speed of the engine : indeed, the velocity of the air would be too low to pick up petrol if the air were not drawn through narrow passages, e.g.. a strangling slot in the throttle barrel, and a small bore by-pass : these narrow passages increase the velocity of the air stream. (b) Full throttle opening. The petrol supply is determined by a calibrated main jet of the submerged type, mounted at the base of a "diffuser" in the mixing chamber. The volume and velocity of the air stream are conditioned by the diameter of the choke tube, etc. (c) Intermediate throttle openings. As ex- plained on page 43, the tendency of a crude carburettor is to supply an excess of petrol when the depression or suction reaches a maximum at high engine speeds. This ten- dency is counteracted in the H.C. 7 by the calibrated main jet; its maximum delivery is correct for high engine speeds. Such a system introduces a fresh difficulty. If the petrol supply is correct at full speed, it is likely to be deficient at quarter, half, and three-quarter throttle openings, when the depression is less- This difficulty is over- come by employing a varying petrel level in the diffuser tube. At intermediate speeds the main jet passes slightly more petrol than can be atomised. This small surplus of spirit forms a reserve in the diffuser, and the amount of the reserve will vary in proportion to the engine speed. As the diffuser is formed from tubes of small diameter, the level of the petrol in the diffuser wi^^l also vary. As the level falls, the main jet is progressively relieved of suction by air entering through the holes in the diffuser tube. Consequently, as the engine is acceler- ated from its minimum towards its maxi- mum speed, the petrol level is gradually and automatically lowered by the depletion of the reserve in the diffuser tubes; and, contrariwise, as the engine is slowed down, the reserve is built up again, and the level rises. Thus the tendency of an increasing depression to enrich the mixture is counter- acted, and vice versa. The diffuser tube also serves a further purpose. In carburettors fitted with a simple jet, a stream of pure petrol issues from the top of the jet, and is not mixed with air until it enters the choke tube. With a diffuser type of jet, 'a small quantity of air is drawn at high velocity through the jet, and is mixed with the petrol before the latter issues into the choke tube. (The quan- tity of air depends upon the number of air holes in the diffuser tube uncovered by the falling petrol level.) Thus, instead of delivering pure petrol, the diffuser sprays out an "emulsion" of mingled petrol vapour and air into the stream of air drawn up from the main intake. This emulsion impinges. into the main air stream at high speed, I 52 Fig. 44- Section of H.C. 7 Carburettor. and at a sharp angle ; atomisation is consequently thorough. The sectional drawing, Fig. 44, and the photograph, Fig. 43, illustrate the construction of the carburettor, and its appearance. The throttle is shown in Fig. 45; the slot in the barrel closes round the top of the slow- running position when the throttle is closed, and acts as an air strangler, concentrating the depression round the head of the tube- The slow-running mixture is drawn into the engine partly via the by-pass shown in Fig. 44, and partly through the V in the back of the throttle. Fig. 45- The H.C. 7 Throttle Barrel. 53 The ditfuser is illustrated in Figs. 46, 47. Petrol passes through the main jet A into the base of the diffu- ser : the main jet A determines the rate of the petrol sv;pply, and its effective diameter must on no account be tampered with, e.g., by cleaning" with a broach. The petrol then fills the three concentric tubes B. X and ]\I to a maximum height, determined by the adjustment of the float needle- This maximum height will be set at 8 mm. below the lip of the "guard" tube M; other considerations, as is explained below, affect the level while the engine is running. The action of these tubes will now be described in detail. Fig. 46. Section of the H.C. 7 Diffuser. The Slow-Running Tube B. — The slow-running- tube B is mounted in series with the mainj jet A, i.e., it receives its petrol supply via A and D, instead of through a separate duct : the method of supply makes for fuel economy, as B tends to go out of action when the throttle is opened. A constriction in the inner diameter of B at the point X forms the pilot jet. There are two separate air supplies for slow-running- purposes. Air from the main intake enters the "air" tube P through the holes H, passes up P. over the lip of the "guard" tube 54 M, and through the depression holes F F into the ''diffuser" tube N. From N the air passes into the slow- running tube B via the holes E E, immediately above the pilot jet X. This air is mixed with petrol before it emerges from the head of tube B. A second air supply is drawn up through the main air intake, its velocity being greatly increased by the strangler slot in the throttle barrel, which slides down round the top of B in the closed position of the throttle- The slow-running mixture passes towards the engine through the by-pass shown in Fig. 44, which is adjustable by means of a screw. ^ O Dj Fig- 47- Parts of the Diffuser (shown dismantled). The main jet A passes more petrol than can be atomised through the tube B. Consequently, while the engine is running- slowly, the tubes N and M fill up with petrol to the fixed maximum level, in readiness for the liberal supply of petrol required for acceleration. The Depression Tube N. — On the throttle being opened, the depression immediately begins to act upon the depression tube N through the emulsion holes G G. Petrol is drawn out through G G, and is already mixed with air drawn thruigh the diffuser holes F F, over the lip of the guard tube M, out of the air tube P. This air and petrol are already mixed together when they emerge fromi G G, and impinge upon the stream of air drawn up outside the diffuser from the main intake. 55 As the engine speed rises there would lj«e a tendency for an excessive amount of petrel to be atomised, if the level in the diffuser remained constant. But the main jet A is not large enough to maintain the maximum level of petrol in the tubes 5l, X. and B permanently: so the mixture remains constant. If the engine is throttled right down again, the slow- running tube B, comes into action afresh, and a reserve of petrol again accumulates in ]\I and X. The guard tube AI is obviously provided to retain petrol in the depression tube X\ whilst allowing air to enter N at the holes FF: it is essential that some of the holes F F shall be near the surface of the petrol at all its varying levels, so that they are drilled at intervals from the bottom to the top. The air tube P increases the velocity of the air which is drawn through the diffuser. (Readers familiar with the earlier issues of the H.C. 7 type should note that no air cones are now fitted round the diffuser.) Adjustments. — X^o adjustments to the diffuser assem- l)ly are possible or desirable, as its working depends upon a variety of orifices, all of which are calibrated. X^or must any attempt be made to dismantle the imit consisting of tubes X^, M and P. The tube X^ is expanded into position, and the parts are soldered together: any tampering will merely result in wrecking the assembly. Cleaning should be done by blowing through, and not with broaches. The sole permissible adjustment which affects the diffuser. is the setting of the petrol level, which should be kept at 8 mm. below the lip of the tube M : the collar is a tapping fit on the float needle, to which it is lig'htly soldered after setting. The slow-running mixture can be adjusted for quan- tity by setting the adjusting screw of the by-pass fFig". 44) : and for quality, by setting the adjustable throttle stop screw. It might appear that the latter would only affect the quantity of the mixture: but it governs the strangling effect of the throttle slot round the head of tube B, and so varies the quality. These adjustments will usually repay trouble after an engine has been overhauled, as the sum total of the air leaks at the various joints in the induction system must inevitably vary. The adjustment should be made v.ith the engine warm, as its purpose is not so much to secure the best starting adjustment, as to obtain an economical setting on which an engine can be trusted not to choke when throttled down during flight. The procedure is therefore as follows : — i. Warm up the engine. ii. Set the throttle stop, till the engine just keeps running, and no more. iii. Unscrew the by-pass adjustment a little, so that the engine runs a shade faster. iv. Slow the engine down a little by resetting the throttle stop. By the above method, the desired minimum engine speed is attained, plus a little extra strangling of the air, which will facilitate starting. The standard Claudel jet sizes for the Liberty engine are : — Main ... ... ... ... 490 cc. Pilot 200 cc. FITTING NOTES. If a light carburettor is bolted up to an untrue induction pipe flange, the carburettor body may he dis- torted, and the throttle will bind. It is therefore essential to ensure a true jointing face on the induction manifold. This is peculiarly difficult witTi the Liberty engine, on which one carburettor face beds against ports in two separate induction manifolds : and the need for extreme care in erecting the cylinders and their mani- folds is indicated. Thin fibre washers are exclusively used for jointing, except at the joint between the two halves of the car- burettor, where a thin (.01 in. maximum) paper M-asher is employed. The main jet must make a petrol tig"ht joint with its .seating, or the calibration of the jet becomes futile- The p-iovement O'f the altitude control lever is short, owing to the limited travel of the small cock; the leverage should therefore be multiplied in linking this lever to the cock it controls. Care must be taken to synchronise the controls, so that movements of the cockpit levers affect both car- burettors identically: this, of course, applies to the alti- tude controls as well as to the throttles. Spare vacuum cocks are interchangeable, in the event of wear or damage. Under no circumstances must the holes or slots be tampered with in squadrons. If thie petrol is pressure-fed, the relief valve must act at 3 lbs. per square inch (absolute maximum). If fuel is fed by gravity, the head nmst not l>e less than two feet. 57 POSSIBLE CAUSES OF TROXTBLE. I. Absence or Shortage of Petrol. 1. Choked jets. Clear by blowing; the use of broaches is forbidden- 2. Dirt in tanks, supply piping, or filter. 3. Air lock in supply piping. 4. Needle valve stuck. 5. Toggle levers jambed. 6. Empty tanks. 7. Excessive heat, producing" vapour lock in supply pipe. II- Excess of Petrol: '"Flooding." 1. Damaged, ;bent, worn, or eccentric needle. 2. Faulty needle seating (removable without dis- mantHng carburettor). 3. Damaged washers on jet or needle seatings. 4. Other forms of leak at main jet seating. 5. Petrol level too high. 6. Toggle levers jambed. 7. Excessive pressure in tank. 8. Punctured float. III. Defective Altitude Control. — When this is in order, opening the cocks fully should reduce the engine speed fromi 1,400 r.p.m. to about 1,200 r.p.m. at ground level Needless to say, this test should only be made with the machine chocked and stationary, and the test should be barely long enough for the tachometer needle to record the drop. Such a test will not be made in flight; and if the altitude control should be held open for more than a few seconds in a ground level test, the engine might catch fire. No trouble can arise with the altitude control ex- cept as the result of wear, dirt, careless adjustment of the control linkage, or unwarranted interferences with the parts. THE ALTITUDE CONTRDL. As explained on p. 48, a carburettor setting Nvhich is correct at ground level will be far too rich at altitude unless special precautions are taken. The vacuum sys- tem is adopted in the H.C. 7 carburettor. The petrol supply from the float to the jet depends on differences of pressure, as explained on page 48. The rate of supply can thus be slowed down by reducing the pressure in the float chamber. This is easily arranged. The cover of the float chamber is provided with an air- tight joint, and air is supplied to the float chamber through a permanent "leak hole" of small diameter, 58' opening into the atmosphere- In addition, the air space in the float chamber is coupled tO' the depression area in the mixing; chamber by means of a duct known as a "suction hole"; this duct is controlled by a cock and lever. At ground level the suction hole is shut : the float chamber receives air at atmospheric pressure through the leak hole : the maximum contrast exists between the air pressures in the float and mixing cham- bers, respectively: and the full petrol supply is obtained. As the aeroplane climbs, and a reduction in the petrel supply becomes essential, the suction hole is progressively opened by means of the cock. The pressure in the float chamber is proportionately reduced; and the petrol supply is correspondingly cut down. Figs. 48-50 illus- trate the manner in which this system is applied to the H,G. 7. Fig. 48. Diagram of the H.C. 7 Altitude Control. F — Float chamber. V — Control cock. D — -Air lead to float chamber. Pi- — Leak hole. F2- — Suction hole. THE USE OF THE VACUUM CONTROL. The cook should always be kept closed at ground level : otherwise the weak mixture will overheat the engine, and, in addition, there is real danger of fire if the cock is opened wide for more than a few seconds. 59 hulicious use of the control will ijegin to make an appreciable difference in power and i evolutions when a height of 10,000 feet is exceeded. At lower altitudes the control will effect real fuel economies, even if its use is not reflected on the tachometer dial- The control, if already open, is automatically re- turned to the shut (position whenever the throttle is closed. There is therefore no lonL;er any risk of the engine being starved through the control being open when the pilot flattens out after a long dive. ( )n the other hand, pilots should remember to reopen the control after a dive which terminates at a high altitude, e.g.. after descending from 20,000 feet to 15,000 feet: otherwise, the nu'xture will be excessively rich when the engine is accelerated. Suction ht)li I.ealc hole To flont chamber V 7 1 Z \ Fig. 49- Diagram illustrating three positions of H.C. 7 Altitude Control Cock. Top: Suction holes shut. Centre: Suction holes just opening. Bottom : Suction holes at maximum opening. 6o Fig. so. Photograph of H.C. 7 Altitude Control Cock. It is obviously impossible to recommend definite positions of the lever for specific altitudes. As a general rule, pilots will make a practice of opening" the control as far as is possible without sacrificing revolutions. 61 CHAPTPZR IV. IGNITION. The ignition s\stein is a highly developed form of battery-coil ig'nition, designed as an integral part cf the engine. Owing to the angle between the two banks of cylinders being 45°. it is not possible to use a conven- tional type of i2-cylinder magneto, and although various schemes for the use cf magnetos were considered, the present arrangement was tina'ly adopted as providing the best compromise and being capable of rapid production. The general lay-out comprises a direct current dynamo-generator, gear driven from the engine, and charging an 8-volt accumulator battery : current is sup- plied by the generator or battery to two complete ignition units wdiich are mounted on the rear ends of the two camshafts. Fhe necessary switches and instruments are made up into a neat switch-board, and carried in the pilot's cockpit. Each ignition unit consists of a contact breaker for the low tension primary current, an induc- tion coil in which high tension current is induced at the moment of "Ijreak," and a distributor which passes the H.T. current to the plugs in their proper sequence. Each ignition unit is complete in itself as regards the contact breaker mechanism, the coil, and the distributor, and each unit fires a plug in each cylinder. So far as this portion of the system goes, there is absolute dupli- cation, with a further duplication of the main contact breaker levers — as described in detail later. Current for ignition is supplied at starting, or when running with only one unit switched on, bv the battery. As soon as the engine has run up to about 650 r.p.m. the generator takes up the ignition load, and on a further increase of speed begins to charge the battery, which is thus kept in good condition and well charged. In the event of a breakdown of the battery, the generator will run the ignition service, but the engine cannot then be restarted after a stop. THE GENERATOR. The generator, Fig. 51, is of the 4-pole, totally en- closed type, shunt wound for a maximum output of about 15 ampeVes at 10 volts, the normal output being consider- ably lower, or about 5 amperes. In constructional details the generator is quite simple; the tubular steel voke A is held between cast aluminium end shields or bousings B-C, which are bolted up with four long screws of which the heads are locked by a simple ring of steel wire D. The armature runs in a ball bearmg in the t 62 H Fig- 51- The Generator. i 63 upper or brush-gear end shield, but the driving end is merely steadied by a plain bush containing a felt oil retaining" washer. The driving' end shield B is provided with three lugs by wdiich the generator is studded down to the crankcase between the cylinders, and is extended to accommodate a pair of spiral gears w^hich serve to drive the revolution indicator. The armature is built up of the usual laminations upon a steel spindle, the driving end of the latter being splined to fit a corresponding socket on the driving gear shaft, which runs in ball bearings mounted in the crankcase. Xo unusual pre- cautions are required when mounting or dismounting the generator, but care should be taken tc see that the splined spindle end is not burred or damaged before assembly. The revolution indicator drive gear E must be unscrewed complete before the armature can be withdrawn. The armature is wound with 21 coils in series, the conductors being double-cotton-covered copper, with 20 wires per slot total. The commutator has 21 sections, and is i^ in. in diameter. The four field coils each con- sist of 145 turns of single-cotton-covered copper, the resistance being approximately f chm per coil at 60° F. Two of the carbon brushes are insulated, but the other two are earthed : the whole system is connected up with one side insulated and the other earthed, a method which results in a considerable simplification of wiring and detail work. The details of the brush gear are extremely simple, the brushes being held up to their work by helical springs, which should maintain a practically constant pressure throughout the life of the brushes, the correct load being from I to 1 1 lb. per brush. The brush gear is enclosed bv a light pressed aluminium cover F. through which the twc cable terminal studs project : one of these. G, is connected to the msulated brushes, and is marked "GEX. ARM": the other. H. takes the field current, and is marked "GEX. FIEED." The end of the cover is lined with insulating sheet, but some early models have a couple of loose fibre cans to protect the brush gear stud ends, instead of a complete lining. Practically the cnly attention required by the genera- tor should be cleaning of the commutator, and an occa- sional drop or two of thick oil to the ball bearing at the upper end of the spindle. At intervals corresponding to the stripping of the engine, the brush gear should be carefully looked over, and any necessarv adjustments made : the windings should be well dusted bv the use of bellows or an air jet ; and the re\-olution indicator drive repacked with grease. The conunutatcr must be watched for signs of scoring or unevenness of the bars : it should 64 be trued up in the lathe before being- allowed to become really bad. Should the insulating" material between the copper bars tend to stand proud cf the surface, it should be carefully scraped down by the use of a keen hooked scraper tool, w'hich may conveniently be made from a hacksaw blade. The commutator and brushes should become nicely glazed, and this condition is not to be interefered with. New brushes must be fitted by rubbing them down on a strip of fine glass paper wrapped round the commutator, so as to bring" them to the proper section. To do this, one of the four long assembly screws should be removed, when a brush arm may be lifted well clear cf the commutator. In the field no attempt should be made to effect any actual repairs to the generator in the shape of re-winding and so on; such work is for experts at base depots only. tig. 32. Voltage Regulator. THE VOLTAGE REGULATOR. Xo provision is made in the generator itself to effect any regulation of the voltage or current output under varying conditions of load or speed, but an automatic device is employed to regulate the voltage, and conse- quently the current, by intermittently breaking the direct earth connection of the field winding, thereby weakening the field and keeping the voltage down to the desired value. 6s The regulator, Fig. 52, is mounted within a cast aluminium cup A, which normally is fixed behind the switchboard. It consists of a soft iron core wound with three windings of differing natures, magnetisation of the core due to the flow of current in these windings serving to pull down an "armature" blade B. and sc break the direct path to earth for the field current. When the generator is not running, or when its voltage is below that to which the regulator is set — say 10 volts, the pair of contacts C are clcsed, and the field current can pass to earth across them. As the vohage rises, however, the pull of the core opposes, and eventually overcomes that of the spring D, resulting in the opening of the contacts at C. The alternative path for the field current is through two of the coils wound upon the core, one of these being of T15 chms resistance, and the other — wound non-induc- tively — of 44 ohms. There is, therefore, a resistance of 159 ohms inserted into the field circuit by the opening of the contacts, the result naturally being an inmiediate fall in the current flowing in the circuit, and a corre- sponding weakening of the field. Adjustment cf the regulator to maintain any line voltage within limits is made by changing the tension on the spring D; tightening up the nut E raises, and slackening' it off reduces, the voltage. In making the adjustment, only the nut is to be turned upon the screw, the latter being held by the knurled head F while the nut is lurned. The correct gap of the contact points C is from .C05 to .007 in. when the armature blade B is pulled down upon the stop pin; the pin projects from .043 to .045 in. beyond the end of the core. It is important to maintain the correct setting of the gaps, as successful operation will depend largely upon the air gap in the magnetic circuit when the contacts are closed. In action, the armature blade is in a state of ccntinual movement, the contacts opening and closing rapidly — just like those of a trembler coil or a bell. The adjustable contact screw- is locked by a spot of solder, and requires touchmg only at quite long intervals. The sole attention the regulator should need is periodic cleaning and adjustment of the contacts, and light lubrication of the bearing on which the armature oscillates. As the regulator is very inaccessible when fitted behind the switchboard, a large opening must be cut in the wood board on which the two are mounted, as indi- cated by the dotted line A of Fig. 55, so at to permit of access to the contacts after the switchboard has been 66 bX) > -M (U lU < - So ^ o S pq.S TJ' rci >i O 1-1 1 ) JD OJ •- -M 4-1 ^ 3 C 3 . :? ^ , O C bjcco rt ^ o [in <+-! C 5 -l-> O f^ tD C ,^ >-^ bJO c e read and adjusted in the following manner : — (i) Battery should be flushed, as described above. (2) If battery is in a discharged condition, it should be put on charge at i ampere, and charged 7^ .Standard Aviation Myorovieteir SorT Rubber Tube Fig. 58. Removing Surplus Electrolyte. 7Z until the terminal volta,^e with this cnrrent flowing' has risen to a maxinunn, i.e., shown no rise for a period of one hour. ( \ ent ping should he left out during- this charge.) The -battery should then be tipped on its side and gravity taken with the hydro- meter provided with a bent, hard rubber tube, as shown in Fig. 59. Be sure to return electrolyte to the cell from which it was taken. Fig. SQ- Taking the Gravity of Electrolyte. In batteries fully charged, gravity should be between 1.290 and 1.310. If. gravity is not correct on taking gravity readings, battery should be held upside down for live or six minutes and electrolyte allowed to run into a rubber or glass jar. The elec- trolyte removed should be adjusted to 1.300 specific gravity by the addition of distilled water, or 1.400 acid, as the case may be; then each cell should be filled with this electrolyte until the level is i in. above plate " E," as shown in Fig. 57. Battery should then be allowed to stand a minimum of five minutes (not more than ten minutes), and surplus electrolyte re- moved to top of plate " E " with hydrometer syringe, battery in upright position. Replace vent plugs- FITTIXCx THE BATTERY. The battery should be fitted in a wooden outer box. and secured against movement by suitable packing of 74 % • ► •' »'** )'i Fig. 60. An Ignition Unit. (Distributor head and cables removed to expose Hie contact breakers, condenser, and H.T. rotor.) 75 rubber strip or hosepipe in short lengths; it should be held down by buffers pressing- upon the lead connecting lugs between the cells. The cable terminals are of a rather unsatisfactory pattern, and need careful treatment to avoid permanent damage to the lugs on the cells. The cables must be well sweated into the thimbles, and the latter should be disturbed as seldom as possible, to reduce wear and tear. Fig. 6i. The Contact Breaker (showing H.T. Rotor). The battery connections are, positive to the terminal at back of switchboard marked "POS. BATT.". and nega- tive to earth upon the engine, preferably on one of the studs fixing the generator. They are to be made with flexi- ble cable of not less than 110/36 or 64/33 conductor; a special cab-tyre sheathed cable of 6 m,m, diameter will eventually be supplied for the purpose, the same cable being used also for the lead which connects the three terminals marked " GEN. ARM." on the generator, voltage regulator, and switchboard respectively. 76 THE IGNITION UNITS. In Fig. 60 is shown an ignition unit in its place upon the engine, the distrihutor head having been removed to expose the contact breakers. A detailed description follows, the like parts having like reference letters in the several figures. At the rear end of each camshaft housing there is a flange with six bolts to take the corresponding flanged base A of the contact breaker shown in detail in Fig. 61. Fig. 62. Contact Breaker Base Plate (back view). The six bolt holes are slotted to permit of the base plate A being adjusted 10° relatively to the camshaft casing, so providing for fine setting of the ignition timing. Mounted upon the base plate A is the base B of the con- tact breaker proper: the back of B is shown in Fig. 62, with four studs' C which pass through slotted holes in lugs cast on A. The slots are long enough to permit a timing control movement of 20°', equal to 40° on the crankshaft, and the movement is effected by the usual 77 control lever coupled up to the arm D, which may be reversed for use on either right- or left-hand units. The base B' has a brass-sleeved boss E, which registers A Fig. 63. Attachment of Contact Breaker Base (B) to Main Flange (A). and B concentrically; there is a thin brass friction washer F to prevent the rub being between two aluminium sur- faces. The studs C (Fig. 63) are provided with special nuts with collars which pull up against springs, and with fibre washers bearing on the back of the lugs cast on A; the studs limit the travel, and the springs absorb chatter. The nuts should be tightened up as far as they will go Fig. 64. The Cam and Cam Spindle. without causing binding of the control gear. Mounted in the base B are two annular ball bearings provided with grease-retaining felt washers, and in these bearings the cam spindle runs. In Fig. 64 the 12-lobed L 78 cam G is seen apart from the spindle H, to which it is keyed and secured by the screw I (Fig. 6i); the screw also retains the high tension dis'tributor rotor J (Fig. 6i) in place, the rotor being driven by a shallow stud pro- jecting from' the cam disc. The thin dished flange K is merely an oil t'hrower, to prevent an excessive amount of oil working along from the camshaft housing into the Fig. 65. Cam Spindle Drive (early pattern). contact breaker. In early models, the cam spindle was driven by a laminated spring coupling engaging with slots in the camshaft end (Fig. 65), with a dowel peg to take the drive in the event of the springs breaking. In later models, the spring drive is dispensed with, and a plain solid dowel peg engages a notch in a steel flange nipped under the seven nuts which bolt up the driven bevel wheel to the camshaft (Fie. 66). The body of the cam is hollowed out from the back and filled in with a felt oil pad, held in place by a light pressed steel plate. In the face of the cam there is a hole through which the felt may be oiled, and six tiny holes, drilled radially through the broad noses of the cam, feed oil to the fibre striker pads of the contact breaker levers. As the merest trace of oiliness is sufficient for these, care 7Q should be taken not to overdo the oiHng here : it may be found that no special provision was necessary after all. The cam has six broad and six narrow noses or lobes, corresponding to the angular intervals between firing points — namely, 75° and 45° successively. THE CONTACT BREAKERS. On the external face of the base plate B (Fig. 61) there is mounted a pressed steel ring L carrying the earthed contacts, the three insulated contact breaker levers, and the condenser M. The ring L is pivotted on a stud to the right of the condenser, and has three other Fig. 66. Cam Spindle Drive (later pattern). holes which are slotted for two plain studs and a screwed stud; the screwed stud is to the left of the condenser, and has a nut and split pin- The ring can, therefore, be moved slightly in relation to the cam in order to correct the setting of the contact breakers. There is also an insulated half-ring N, carrying the ends of the three spring blades controlling the levers, and the small resis- tance coil O. One end of the half-ring N is connected 86 to the insulated stud P, and primary current from the generator or battery (after passing" the coil) goes to earth via this connection, the sprint^s. and the levers and con- tacts. At Q there is another insulated stud to which the primary lead from the switch is connected, and which, with its fellow P, serves to secure the distributor, and at the same time makes the primary circuit complete throug^h the coil. The distributor cap is held in place by four spring-clips R in addition to these two studs, but the knurled-headed socket nuts of the studs P-Q must be pulled down very tight to make a good electrical circuit, apart from the mere holding of the distributor. -W Fig. 67. A Main Contact Breaker Lever. The arrangement of the three contact breaker levers presents several novel features, and must be described at some length. To secure an increased measure of reliabihty, two main contact breaker levers S-S (Fig. 61), are provided, either one of which will operate the ignition in the event of a failure of the other. Probably the most likely lever of the three to break down is the small auxiliary one T, and if this fail the other two may become useless, depending upon the nature of the fault. Should it remain permanently closed, the main levers cannot break the circuit, but if the auxiliary lever should open and remain open, the operation of the main levers will not be interfered with. In any case should one of' the main levers, or the auxiliary lever, break down, it may be removed, and the engine run on one main lever- only. A main contact breaker lever S is shown apart in Fig. 67; the fibre striker pad U, the rivetted-in tungsten contact \\ and the vulcanised-in rubber buffer pad W upon which the end of the spring X rests, are all clearly seen. The auxiliary lever T (Fig. 61) is generally similar, but cranked instead of straight, and the spring is connected to a small resistance coil O of about 8 ohms, instead of direct to the half-r.ng X already described. This auxiliary lever is timed about 5° in advance of the main levers, both for opening and closing. When running normally in the forward direction (which is clockwise as viewed from the face of the unit), the sequence of operation is thus : (a), all contacts closed and current flowing; (b), auxiliary contacts open, and current flow's across main contacts only; (o. main con- tacts open^, circuit is broken, and spark occurs; (d). auxiliary contacts close, and current flows through the resistance coil; (e), main contacts close, and current flows across these almost exclusively, as they offer a path of negligible resistance; (f), the cycle now recommences. Should the engine try to run backwards, the ignition is not actually cut out, but the flow of current (or, rather, the change of rate of flow) at each break of the contacts is so reduced that no spark can occur. The sequence on reversal is: — (a), all contacts closed, and current flowing; (b), main contacts open, but no spark occurs because the circuit is closed through the alternative path offered by the auxiliary contacts and the resistance, and consequently the current in the coil winding, does not fall sufiiciently rapidly to induce a high voltage in the secondary winding; (c), atixiliary contacts open, and again there is no spark, because the current flow is at a very low value; (d), main contacts close, and current flows; (e), auxiliary contacts close; (f), the cycle now recommences. The auxiliary contact breaker fills its function as a safety back-firing device perfectly, with the qualification that it cannot pre- vent a single back-fire if the circuit is broken when the crank is in advance of top dead centre. THE HIGH TENSION DISTRIBUTOR. The distributor head (Figs. 68 and 69) is an ex- tremely fine piece of mouldmg m the insulating material known as Bakelite, a ring of different composition being moulded in to form the- rubbing track for the carbon brush Z of the rotor J (seen in Fig. 61). The distributor head has a spigot edge fitting into a bored register on the contact breaker base (B, Fig. 61); it is firmly held in place by four spring clips ( R, Fig. 61 ), and two insulated 82 knurled headed socket nuts which screw down on to the insulated studs P and Q (Fig. 6i). Fitting snugly into the domed head is the induction coil, one end of which can be seen in Fig. 68. The coil is built up, as usual, with two windings of insulated copper wire upon a core of soft u'on; the two ends of Ficr. 68. The Distributor flead (exterior). the primary whiding are electrical'.y connected to the metal portions of the knurled nuts fi.xing the head. As the battery lead is connected to the terminal stud Q, and the terminal stud P is in connection with the con- tact breakers, it follows that the primary winding of the coil forms part of the complete circuit, and that current will flow in the coil when either or all of the contact breakers are closed and the switch is on. One «3 end of the secondary winding of many turns of fine wire is connected to the primary winding, but the high potential end is connected to a fixed carbon brush projecting shghtly from the centre of a disc of insulating sheet, which closes the inside of the distributor head and hides the coil. This brush, when the unit is Fig. 69. The Distributor Head (interior). assembled, presses upon a light bowed steel spring blade Y (Fig. 61), carried on the rotor J; as the free end of the spring blade touches upon a brass liner into which the carbon brush Z fits, the brush is brought into connection with the secondary winding of the coil. Moulded into the brush track in the distributor head 84. there are twelve circular studs or " segments," each in connection with a screwed terminal knob, by which the corresponding high tension sparkmg plug cable is fixed. The bosses uito which the insulated terminal knobs are sunk are notched on one side to clear the cables, and on the other side are very clearly marked with the numbers of the plugs; each terminal is pro- vided with a spring washer. It will be noticed that the distributor segments are not placed equally round the brush track; they are at angles correspondmg to those of the cams, so that the rotor brush must always be upon a segment when the cams open the circuit bv breaking the contacts apart. The edge of the brush track is bevelled, so that the head may be the more 4L 3(2 i L o o o 612 5L 2C o o o 5R 2 L 1 e O O O 6L 4 (3 3 L o c o Fig. 70. High Tension Cable Carrying Plate (showing the numbering). readily slipped on over the carbon rotor brush; great care is necessary when putting the head into place, or the brush may be broken. There is no special safety gap in the high tension circuit, but there is an air gap of about 14 mm. between the spring blade Y and the head of the screw I, which ftxes the rotor and cam to the cam spindle, and in the event of excess potential in the coil winding (due, for example, to a lead having come off a plug), a spark may pass here and save the coil from a breakdown. HIGH TENSION CABLES. ' The twelve high tension cables for each distributor go respectively to one plug in each cylinder, the left- hand unit firing the forward set of plugs and the right- hand unit firing the after set. The leads are of the usual pattern of 7 to 7 A mm. rubber insulated flexible cable, 85 LEFT DISTRIBUTOR RIGHT DISTRIBUTOR Fig. /I. Diagram indicating Sequence of Firing and Distributor Connections. 86 8? fitted with spring clip terminals to take ball knobs on the plug electrodes. Carried bv six steel clips or brackets lixed under the heads of screws securing the water pipe flanges to the induction manifolds, there is a " U " shaped aluminium conduit tube with a large opening at the base of the " U " by which the 24 leads enter, and a series of small holes by which single leads issue near their respective plugs. Between each distri- butor and the conduit tube the leads are passed through a pair of ebonite plates (Fig. 70), moulded with numbers corresponding to the numbers of the distributor terminals and plugs to which the leads are connected. The firing order of the cylinders, as indi- cated by Fig. 71, and by the metal name-plate on each engine, starting at the ignition gear end of the engine, is on left-hand side, i, 9, 5, 11, 3, 7, and on right-hand side, 8, 4, 12, 6, 10, 2. The sequence through the whole engine is iL, 6R, 5L, 2R, 3L, 4R, 6L, iR, 2L, 5R, 4L, 3R. TIMING AND CONTROL GEAR. The two ignition units are timed alike, and are moved in unison by operation of the hand control. Each contact breaker base B has a cranked lever arm D (Figs. 61 and 62) fixed by a pair of studs and nuts, the two arms being cross connected by a coupling rod having screwed end jaws and locknuts for adjustment to length, as shown in Fig. 72. Upon one side of the web of the cranked levers will be found the letter "R," upon the other side the letter " L." When used on the right-hand unit, the letter " R " is to be out- ward and visible: when used on the left-hand unit, the letter " L " must be outward. In every respect, save for the particular way of fixing these little lever arms, the two units are ahke and interchangeable, and both run right-handedly. The timing control must provide for a movement of if in. The method of timing the ignition will he fairly obvious from Fig. 72, in which the contact breakers are shown just breaking at the firing point on full retard, which is 10° of crankshaft movement beyond top dead centre for crank No. i on No. i left-hand cylinder. In this position the spring blades of the driving couplings, or the driving dowels when fitted, are to be in the plane of the cylinder centres as indicated, and the distributor rotor brush pointing midway between the auxiliary contact breaker points and the little resistance coil, thus coming on No. iL distributor segment. The exact firing point may be regulated bv 88 slackening off the six bolts securing each ignition unit base flange to its respective camshaft housing, and turning the unit in the required direction. The total range so obtainable is io° on the unit, or 20° on the crankshaft, which is not only ample to cover any accumulated errors in machining, but will enable the timing of the ignition to be varied irom the standard setting if required. The regular setting is to fire at 30° of crankshaft angle before top dead centre at full advance, and 10° after at full retard, and this setting should not be departed from without proper authority. When correctly set a notch is cut in the two main flanges, so that there can be no doubt of the position when reassembling. To check the timing electrically two means are available, and either will give the required result. One way is to assemble the distributor head, turn the engine forward slowly by hand, and watch the ammeter to show when the circuit is broken and the firing point reached; this must be done on only one unit at a time, unless a lead is first removed from the generatcsr ter- minal marked " GEN. ARM," as otherwise the battery would discharge through the generator (vide page 68). The alternative method is to wire up a couple of lamps or bells as indicated in Fig. 72. While either contact breaker is closed the current will flow in the corresponding circuit; but when all three contact breakers of the unit have opened, the circuit will be broken, and the lamp or bell will be dead. If, then, one unit be timed correctly in reference to the crankshaft, the other may be synchronised exactly by moving the base plate until the circuit is broken, when the six bolts may be locked up. The coupling rod should be dis- connected when timing up for the first time, and both control levers set to full retard; the coupling rod can be adjusted to length and coupled up after the units have been set independently. Finally, the timing should be checked electrically for synchronism at full advance, and if necessary any slight adjustment can be made on the coupling rod to bring the two units dead together. It must be remembered that, as the ignition gear runs at camshaft speed, all angles as measured on the contact breaker and distributor will be doubled on the crankshaft. Hence the 15'^ of advance on the contact breaker, shown in Fig. 72, corresponds to 30° on the crankshaft, and the 5° of retard to iqo on the crank. The correct setting of the contact breaker gaps is 0.012 in. at full lift, but the gaps of the two main breakers of either unit may be set between the limits of 0.0 10 to 0.013 in. if it should be necessary to bring 89 the timing of the pair closer together. In any case, the spark occurs when the later of the two main breakers opens, and there should be no more than 1 1° between them. The fibre striker pad of the auxiliary contact breaker should be central on a narrow nose of the cam when the mam breakers are just opening on two other narrow cams. To correct the relative settings of a pair of mam breakers, take out the spht pm and slacken the nut of the stud which secures the ring L (Fig. 6i). The ring may then be slewed about the stud at the other end of the condenser, and by resetting the con- tacts a close adjustment may be obtained before re- locking the fixing nut. The correct pressure between the contacts of the contact breaker levers is from 26 to 30 oz. The piessure may be adjusted by altering the effective lengths of the respective springs, which, as may be noted from Fig. 67, have slotted fixing holes. POSSIBLE TROUBLES. In the whole ignition system by far the most likely items to give trouble are the sparking plugs, and these should be examined and cleaned as a matter of routine. Misfiring to an ordinary degree, or total cutting out on cf«rtain cylinders, will most probably be due to defective plugs; misfiring equally all round, especially if following difficulty at starting, will be due to a defect m the system apart from the plugs. The running on each unit may be tested independently by switching on and off one at a time, as usual with magneto ignition, but a certain amount of study is necessary to attain that most desirable semi-instinctive knowledge of what is happening, which can come only with practice. In connection with testing on one unit at a time, it should be noted that the voltage on the primary circuit to the coils will not be constant, as in a simple battery system, but may vary considerably according to the conditions of the moment. Running slowly, the supply is from the battery and the voltage will depend upon the condition thereof; should the battery have been standing some time, or need attention to the electrolyte, the voltage will be low, and running may be irregular, and starting difficult or impossible. As the engine speeds up the generator voltage will rise until it equals that of the battery — say at 600 r.p.m.; while a further rise of speed will cause the generator voltage to go still higher, when the battery will be on charge at a rate depending upon its condition and the engine speed. If the battery be in poor condition, its voltage when supplying ignition current at starting or low engine GEINEIRATOR LARTHED TO LNGINL VOLTAGL RLCULATOR LEFT. DIST SWITCH BOARD \v /f \ // 8-VOLT BATTERY ^, FOR ALL LE.ADS MARKED ;— HLAVY"uSL64./33 OR 110/36 ) oh LIGHT'usL 30/35OR4o/35\^o^au Keeip all Le.adscle.ar Fig. 73. OF COMPASS) Main Wiring Diagram. speeds may be no higher than about 7, or even less with both ignitions on. As soon as the engine is run up to some 800 r.p.m., the generator voltage will be from 9 to 10, and the ignition may therefore function properly. But on switching off one ignition for testing purposes, the generator circuit will be broken, and the ienition current will be supplied from the battery again. Now, when a battery which is in poor condition is put on charge, it will stand at first a high charge rate with- out harm, and will almost immediately show a consider- able rise in voltage. If, however, the charge be stopped in a few minutes — as it would be in our case- — the voltage will drop again on current being drawn from the battery for a very short while. It is necessary. 91 then, to allow for this, and when it is known that the battery is in need of charging, the engine should be run up as quickly as possible to about 700 r.p.m. on one igni- tion onh-; if all is well with the lubrication and cooling system, and the engine is running passably well, the second ignition should be switched on and the speed increased until the ammeter shows at least 6 amperes charge rate, or even 8 or 10 amperes in extreme cases. This should be continued for about half an hour, when the two ignition systems may be tested independently as usual, as by that time the amount of current put into the battery should have brought its voltage up to a workable value. This process should be followed only if no well-charged spare battery be available; if possible the battery should be charged before starting, as such treatment is good for neithec engine nor battery. If the ammeter continues to show discharge (at about 3 amperes) when both ignitions are switched on with the engine running at 800 r.p.m., or faster, it is evident that the generator is not delivering current. This may be due to a broken or disconnected lead, a defective switch, or a broken spring on a generator brush holder. If the discharge rate, on switching on the second ignition set just after starting, is abnor- mally hip^h, it may be due to a burnt-out or shorted voltage regulator coil, a short circuit on either generator field or arm.ature winding, or on a brush holder, or even to a broken generator spindle. If the ammeter stands at zero, it is evident that the generator is carrying the ignition, but not charging the battery. This will probably be due to the battery having been connected up wrongly; the leads should be reversed, and another trial made. Unusual readings on the ammeter, but with the whole system functioning, and especially if the needle is far off zero with both switches off, denotes a fault in the instrument itself, which must be changed for a new one. No attempt may be made to repair these ammeters; they must be returned to the makers for attention. Failure of an ignition unit may be due to any one of several causes, apart from a broken wire or con- nection, which are always possible; but if the battery is being charged properlv, there is not likely to be any- thing wrong with the switch. The fault may lay in the ballast resistance behind the switch, in the windings of the coil, in the condenser, or, most probably, in a faulty contact breaker. In any case, the engine can be run for several hours on one ignition supplied from the batte*ry alone; so even if the fault be one which necessi- 9- tates the particular switch being off, the engine need not be stopped. Stuck regulator contacts are indicated by an abnormally high charge rate at engine speeds of over i,ooor.p.m. or so; this should be put right immediately, or the ignition coils may be damaged. To obtain access to the regulator, it is merely necessary to remove the two nuts from the long studs fixing the switchboard, when the latter can be removed and the regulator inspected through the holes cut in the instrument board. (See Fig. 55.) Should the ammeter not indicate charging until an unusually high engine speed is reached, the cause may be the regulator contacts having remained open; but this is an unlikely trouble to develop. The possibility of trouble can be reduced to a mini- mum by the exercise of care m the daily routine inspec- tion, and by cleanliness throughout. The detailed in- structions for the maintenance of the battery should be carefully followed, as the amount of electrolyte is so small that there is no margin for neglect. The distri- butor heads should be removed daily, and the brush tracks wiped with a soft rag and a little petrol At the same time the contact breakers should be dusted with a soft brush, and once a week the lever pivots shoulf' have a drop of oil. Otherwise, practically no attentior is needed, and the contacts should run for many hours without requiring adjustment. DAILY ROUTINE. 1 . Take the voltap^e of the battery before each day's flying. On open circuit — that is to say, with both ignitions switched off — the voltage should not be less than 8 volts; with one ignition switched on, and the ammeter showing a discharge of from 4 to 5 amperes, the reading should be not less than 7^ volts. Should the Voltages be lower, a freshly charged battery should be fitted. 2. Remove and clean the plugs, and set their gaps to 0.015 in., renewing any plugs that appear to be of doubtful soundness. 3. Remove the distributor heads and examine the contact breakers. The gap should be between the limits of 0.0 10 and 0.013 in., and the points should meet squarely. 4. Clean the carbon brush tracks of the distributors with petrol on a piece of cloth. Do not use any abrasive material. 93 5- Run over all leads, " especially those on the engine, examine for deteri&ration from oil or burning, and see that all terminals are tight. 6. Before starting the engine, switch on each ignition system separately, and note the ammeter read- ings, which should be approximately equal. It may be necessary to turn the engine a few degrees backzvard, in order to bring the main contact breakers to the closed position before the full current can flow. Should one system show no discharge reading, there is a break in the circuit, probably due to a loose termmal or very dirty contacts. Should one show an excessively high discharge, there is a partial short circuit some- where. To detect a short in a coil as distinct from the wnring, change over the primary leads to the distri- butor heads. /. Immediately the engine stops, put both ignition switches to the OFF position. WEEKLY ROUTINE. 1 . Flush the battery as already detailed, and correct the acid density- if necessary. Wipe the battery dry and clean from acid, and smear a little vaseline on the terminals before replacing it. Wipe out the battery box. 2. Remove the dust cover from the generator brush gear, dust the commutator, etc., and, if necessary, clean with petrol. See that the brushes are in order, and put a few drops of thick oil on the ball bearing at the top of the armature spindle. 3. Clean out the contact breakers, and put a drop or two of oil on the pivot bearing of each lever. Check and readjust the points if required. The amount of clack in the driving coupling should not allow of more than about -^ in. of play at the end of the distributor rotor. MONTHLY ROUTINE. 1. Let the weekly clean-up and inspection be, if possible, more thorough than usual. 2. Open out the switch, clean all contacts, and put a little vaseline on the track. Inspect the resist- ance coils. 3. Inspect and clean the voltage regulator. The gap between the contacts should be from 0.005 fo 0.00/ in. when the blade is pressed down upon the scop- pm. Work a drop of oil into the pixot bearing. 94 LIBLRTY. 12. P. R.M. 32E, Fig. 74. Rear View of Engine, showing Distributors and Generator. CHAPTER V. Running Instructions. MAXIMUM R. P.M. The army type of Liberty engine has a high com- pression ratio, for the purpose of obtaining a high power 95 output at altitude : it must never be run all out near the ground. Pilots and mechanics should carefully note the following r. p.m. limits: — Maxnnum speed " running up " on the ground, 1,400 r.p.m. Maxmium permissible speed flying at ground level, 1,500 r.p.m. Normaf full speed at or above 6,000 ft. — 1,650 r.p.m. Maximum permissible speed at or above 6,000 feet. — 1,750 r.p.m- If a suitable propeller is fitted, it is practically im- possible to exceed i ,400 r.p.m. in ground tests. The naval model has a lower compression ratio, so that its maximum speed, irrespective of height, is 1,750 r.p.m. PRECAUTIONS WITH A NEW ENGINE. It is highly inadvisable to run a new engme at all hot, as there is a considerable risk of "baking" the rings into their grooves with burnt oil, a mishap which can only be remedied by dismantling the engine com- pletely. For this reason it is inadvisable to run on a weak mixture — e.g., by testing the altitude control for more than a few seconds at ground level. FILLING WITH OIL. The recommended oil is Vacuum Mobiloil B B. the average consumption being eight to twelve pints per hour. A safety allowance of three gallons over and above the calculated consumption tor an intended flight must be provided. When the engine is stopped after a flight, about a gallon of oil will slowly drain down into the sump. The scavenge pumps have a. greater capacity than the pressure pump, and this gallon will quickly be returned to the tank when the engine is next started. It is therefore necessary to allow ample clearance in the tank when re- plenishing. As allowance must further be made for frothing and expansion, clearance should be left in the tank for about i^ gallons of oil, whenever the sump has not been drained. In winter the tank should be replenished with heated cil, unless facilities are available for keeping the engine warm whilst in the hangar. The main thrust bearing must be filled with oil by means of an oil can before each flight. M-echanics should realise that the thrust housing will hold a good deal of oil, and that the race cannot run dry during a loner flieht if the housing is filled. 96 COOLING SYSTEM. A plug beneath the lower delivery connection on the pump permits all the water to be drained in frosty- weather. In refining the cooUng system, it is advisable to raise the tail of the aeroplane, so that the system will fill from the bottom upwards. Otherwise air may be trapped, when the circulation will be imperfect, and the machine will start on its flight with less than the full complement of water. OIL PRESSURE. After first starting up, therefore, if the pressure begins to fall, and finally becomes constant at about 25-30 lbs. at 1,650 r.p.m., when the oil has warmed up, the circulation is almost certainty correct. In very cold w^eather the safest method of warming up the oil is to run the engine for several spells of two or three minutes each, switching" off between spells, so that heat can reach the base chamber by conduction. The camshaft is remote from the pump, and may be the last component tO' receive proper lubrication, when the circulation is very sluggish. Urtjder such conditions one of the lids may be removed from the camcase, and hot oil poured in- When the engine is accelerated towards its maximum r.p.m., the pressure gauge should register not less than 25-30 lbs. A WARNING ABOUT THE SWITCHES. The Liberty engine is liable to be started accidentally, when it is warm or has just been doped, ready for starting. The type of ignition employed affords a very- good spark : any rapid switching on and off produces a crude form of make and break, and may lead to spark- ing at the plugs. Special care should therefore be taken to see that nobody is within reach of the propeller when the switches are operated. One switch only must be on contact at starting: if both switches are " on " during starting or s^'ow-running. the heavy rate of discharge will run down, and will injure the battei'v. Needless to say. both switches must be kept in the off position when the eng-ne is out of use, or the battery will run down. (See pp. 68 and 03.) STARTING THE ENGINE. Owing to the excellence of the spark at low speeds, the engine is easv to start, despite its size. It is doped via the compression cocks above the inlet manifolds or by means of the doping pump in the cockpit. ONE distrib^itor switch is then put on contact, the ignition being fully retarded, the throttle lever in the be drawn off by either of two methods, viz.; — 97 minimum, position, and the altitude control shut. On the propeller being swung or the sl^arter (if any) being operated, the engine should start. (The Americans usually pull the propeller over compression by means of three mechanics who link hands in line, the left-hand man pulling the propeller with his left hand, the centre man pulling on the right-hand of the man at the propeller, etc.) As scon as the engine fires, the ignition should be fully advanced, and the second distributor head switch put to "contact." The engine cannot fire backwards, as auxiliary contact-breakers cut out the ignition when a backfire occurs. AFTER STARTING UP. Warning is given above against accelerating before the oil is sufficiently fluid to circulate properly. The larger the quantity of oil in the tank, the longer will be the delay. A satisfactory oil temperature is 40° Centi- grade, and this may not be attained for ten minutes in cold weather, if a supply of five gallons is carried. When the oil is circulating properly, the engine may be acceler- rated for a revolution test up to 1,400 r.p.m. Make a revolution test on each ignition set separately. Should a test of the altitude control be made, the tachometer should register a drop of approximately 200 r.p.m. if the altitude control is fully opened for a few seconds with the engine running at 1,400 r.p.m. The altitiide control must not be kept open for more than a few seconds at ground level, or the slow-burning mi.xture may cause the engine to catch fire. .SMOKY EXHAUST ON WIDE THROTTLE OPENINCJS. A smoky exhaust is noticeable at or near full throttle opening in the case of engines equipped with the H.C.7 carburettor. This effect passes off as the machine climbs into a less dense atmosphere, and is apparently due to "cracking" of the oil and petrol vapour, due to the high compression ratio df the engine, the volumetric efficiency of the H.C./, anid local causes peculiar to individual cylinders .(e.g., overheated valves or pistons, carbon de- posit, or uneven mixture). The matter is not serious, as the exhaust will become clear, owing to decreased cylinder pressure, before reaching 10,000 feet. The smoke may be de- creased by shghtly retarding the ignition. In any case, the ignition, in flight, should be set to the point at which the engine runs best from the point of view of revolutions; and it should be noted that with its special form of ignition the Liberty engine is sensitive to its ignition control to a greater extent than occurs where magnetos are used. 98 This paragraph is inckided to prevent mechanics from ascribing- the smoky exhaust tc over-large jets. The standard Claudel jets (main 490 cc, pilot 200 cc.) are the smallest compatible with full r.p.m. at ground level. AFTER THE PRELIMINARY RUN. If circumstances permit, switch off the engine after its preliminary run and acceleration tests, and carry out ihe following^ inspections: — i. The sump and camshafts should be warm- ii. The cyHnders, water pipes, and radiators should be hot. iii. See that the water pump is not excessively hot, indicating that the stuffing glands are too tightly packed, iv. There must be no oil, petrol or water leaks. AFTER A FLIGHT. Take whatever precautions are locally authorised to facilitate the next start, e.g., inject a little paraffin into the compression cocks, and on to the valve stems, giving the engine one or two turns. Refill the oil tank, unless this job is postponed in order to use warm oil immediately prior to restarting- Test, and, if necessary, reset the tappet clearances. Overhaul all nuts. Fill the main thrust with oil. Clean the petrol filter, and, if ordered, all three oil filters. Examine, and, if necessary, clean or renew the sparking plugs. Inspect all ignition connections. In winter, take precautions aganst frost : in summer, replenish with water. Verify the condition of the ignition, especially the accumulator. Test the firing on each distributor separately. As this ig"nition is of a type unfamiliar in the British Services, Chapter IV. should be carefully studied. Refill the petrol tanks. PERIODIC ATTENTIONS Cleanse the oil and petrol filters. Overhaul and test the ignition. (See Chapter IV.) Examine the high tension cables for worn or perished insulation. Remove and clean all the sparking plugs. Cleanse the carburettors. Examine the stuffing glands of the w^ater pump. Oil the joints of all the engine controls. 99 (\i Fig. 75- Three-ciuarter Side \'iew of Engine. lOO CHAPTER VI. Dismantling &: Reassembling. I.— PRELIMINARY XOTES. The following" points are standard practice in dis- niantlitig" an engine, and must be scrupulously observed. 1. An engine must not be dismantled in any place to which dust, sand and grit can find access. 2. There must be no mixing of parts. The various parts of the Liberty engine are perhaps less freely stamped with identification and assembling n%arks than is customary w^ith European aero engines. As each part is removed, it should be disposed of in a way which will insure its being refitted in its original location, e.g-, valves may be inverted, and slipped into their guides : nuts loosely screwed back on to their studs or bolts, etc. 3. As each part is removed, it should be cleaned, w^ashed in paraffin, and set on a clean, dry surface (wood, not concrete) to drain. The wiping down of parts is not recommended, owing to the risk of fluff and lint getting into the eng'ine. 4. All petrol and oil pipes or leads must be blown out with a powerful syringe. 5. When a joint is made with an adhesive (e.g.. gold size) or with a paper washer, all traces of the old jointing" material must be carefully removed before the joint is remade. SPECIAL HINTS. 1. A number of "soft" washers are fitted to various joints in the distribution gear. New washers will probably be required in reassembling, and it is usually important that the new washer should be of the original thickness, as one or two of the washers affect the depth to which the bevel pinions ane meshed. If washers are improvised, owing to lack of manufacturers' spares, this point requires watching". 2. Some of the studs in the Liberty engine are not too firmly fixed, especially the small studs in the car- burettor, camcase, etc. The nuts on such studs should be removed and replaced with care. Always refit the nuts tc their original studs. 3. Before assembling" thle camshafts with their respective inclined shafts, the directions on page 121 should be studied : otherwise the valves and ignition will have to be set without the assistance of the timing marks. lOI 4. If the main driving' bevel of the distribution gears is removed from the crankshaft during dismantling operations, it must not be refitted without reference to the special instructions on page 115; otherwise the marks will be useless, and the engine will have to be timed without regard to them. The shims between this bevel and its flange are provided for setting the mesh of this bevel with the horizontal bevels on the upper and lower shafts- As aluminium expands under heat more than steel, the depth of mesh alters as the engine heats up: and the mesh should therefore be set as directed on page 115. In an overhaul after a period of work sufficient to produce perceptible wear, this point will need careful watching. ][.— ACCESSIBILITY OF INDIVIDUAL COMPONENTS. Tlie next few paragraplis deal with the handling of individual components, which can be dismantled without stripping the entire engine. Directions for a complete overhaul follow on page 103. THE CRANKSHAFT. The main bearings are housed between the cross webs of the two halves of the base chamber, and are not slung in caps secured by bolts to the top half, as is the case with some engines. The crankshaft is therefore freed whenever the sump is unbolted : and the simplest way of exposing the main; and big end bearings for examination consists of inverting the engine on a suit- able stand and unbolting the sump, leaving the cylinders in position. If the bearings are in need of any serious attention, the engine will of course be stripped. CARBURETTORS. The carburettors can easily be cleared from the Vee without dismantling the induction manifolds. The air intakes of the Zenith carburettors can be freed by slipping their spring hooks off the securing studs, and the carburettors can be dropped by unscrewing the bolts which support them. INDUCTION MANIFOLDS, The induction manifolds are removable without dis- mantling the cylinders. The wiring V is removed,, the 102 water connections broken, the carburettors dropped, and the necessary nuts removed. The holes in the manifold flanges which register with the lower stud of each cylinder induction port are cut in the form of a deep tapering oval, . so That the manifold can swing on the upper studs, as shown in Fig. 78. If one manifold is iield down, and the corresponding manifold of the oppo- site cylinder block is pulled or tapped up gently with a soft hammer, removal is usually easy. If the vertical faces of the manifolds at the centre bear too hard on each other, the clearance may be eased by judicious filing at the point indicated in Fig. 78. This clearance does not affect any joint. CAMSHAFTS AXD CAAICASES. Each camshaft, with the upper member of its in- clined drive, will usually be lifted ofT and replaced as a complete unit. The long" inclined shafts are jointed by splining to short inclined shafts, the joints coming level with the point of entry into the base chamber (see diagram on page 27). Disconnect the oil connection at the front end of the camcase. Take the nuts off the twelve studs securing' each camcase to the cylinder heads, and free the packing nut at the base of the casing of the inclined drive. Raise the camcase evenly from 1)oth ends. If a camshaft unit is to be stripped, the seven large grubscrews which fix the camshaft bushes in the cam- case must be removed, or serious damage will result. The camshaft may then be driven out of the case with a soft drift from the front end, and will bring away the l:)ushes with it. VALVES. Minor attentions can be p^id to the valves without removing the cylinders, or even the camcase : and this makeshift method may Ibe adopted in emergencies, e.g., if one or two exhaust valves are inclined to stick. Remove the camcase lid concerned. Take out the rocking levers. Put the piston on T.D.C. Press down the spring cap and release the split cones- Obviously, if the crankshaft is turned so that the piston descends whilst the valve is free, the valve can only be recovered and the spring refitted by dismounting the cylinder. CYLINDERS. Each cylinder is held down on the base chamber by ten studs which perform no other function. One or 103 more cylinders of each block may be separately dis- mantled by removing the wiring tube, induction mani- fold and camshaft. MAIN DRIVING BEVEL OF DISTRIBUTION GEAR. This bevel can be viewed through a detachable cover plate at the rear of the base chamber, but the bevel cannot be reset through this aperture, as it is secured to a flange on the crankcase by bolts, not by studs, and cannot be remounted through the orifice if the nuts are unscrewed. Should the timing prove faulty after the first assembly (an inexcusable blunder, seeing that the various gears, etc., are plainly marked), cor- rection cannot be made by freeing the camshaft bevel. The simplest method of correction is described on page 127. DYNAMO GENERATOR. The dynamo generator can be lifted off after its flange is unbolted, as its spindle is coupled by a splined joint to the vertical shaft. It is not advisable to turn the crankshaft when the generator is out of posi- tion : the bevel drive will then exert a lifting' action on the vertical shaft, which is only held down (in the absence of the generator) by a small grubscrew in the top flange. Should this screw have been removed, the gears may conceivably jump a tooth when the crankshaft is turned, and derange the timing. OIL AND WATER PUMPS. All the pumps can be dismantled at any time without disturbing other components : it is consequently a simple matter to keep the oil filters (two in the rear catchpit. and one in the front catchpit) absolutely clean. III.— GENERAL OVERHAUL. The numbered sequence gives the order in which the various units should be removed from the engine. These reference numbers are retained in the first two sets of notes which follow the sequence; the notes deal with : — (a) The removal and dismantling of each unit (pp. 105 — 108). (b The inspection and adjustment of the r04 . Fig. ](). Front End View of Engfine. parts after dismantling, and the re-erec- tion of individual units (pp. log — Il6). (c) The reassembling of the engine as a whole (pp. ii6 — 129). III.A.— DISMANTLING SEQUENCE. Dismount the units in the following order : — • I. Propeller hub. This should only be removed from the crankshaft if the shaft thrust bearing or hub are defective. If no defect is suspected, this operation will 105 probably be left till the last. Otherwise the propeller will be removed, the hub beings left on the shaft. (In some cases the hub is shrunk on with hot water, and wall be particularly tight. ^ 2. Wiring U and cables. Distributor covers and bases, (iienerator. Sparking plugs. 3. Carburettors. 4. Water elbows and induction manifolds. 5. Camshafts, complete with upper inclmed shafts. 6. Lower inclmed shafts. 7. Vertical (generator) shaft. 8. Oil pump unit. 9. Water pump and lower vertical shaft. 10. Cylinders. I I . Pistons. 12. Lower half of base chamber. i^. Connecting rods. 14. Crankshaft. Loose flange Locking wire Lock nut Retaining nut Hub barrel Fig. 77- Details of Propeller Hub. NOTES ON DISMANTLING OPERATIONS. Crankshaft I . Propeller Hub. — As stated above, this should be left on the shaft unless the shaft, thrust bearing, or hub are defective. Remove the lockhig wire or plate. Un- screw the locking (outer) nut (left-hand thread) and the retaining (inner) nut (right-hand thread), taking them right out of the hub. Oil the threads of both nuts liberally. Screw in the inner nut as far as it will go, and then unscrew it five turns. Screw in outer nut til', it touches the shoulder on the inner nut- The hulj may now io6 (i) Hold the inner nut and shaft stationar}-, and screw in the outer nut till the hub is drawn. (li) Alternatively screw out the inner nut whilst holding the outer nut and the shaft stationary. It may be impossible to draw the hub without heat. In this case the blow-lamp should be applied evenly round the barrel close up to the flange. Fig. 78. Mode of removing Induction Manifolds. (In the sketch the R.H. manifold is being swung up.) A — Tapered holes for lower studs. B — Point where slight clearance is allowed. 107 2. Ignition Details. — Disconnect cables from spark- ing plugs and from distributors. Free clips of wiring U. Then remove cables and wiring U as one unit. Remove six bolts from each distributor flange, and lift off the bases, having marked or noted the angular position of each unit on the cam.shaft housing. Unbolt base flange of generator, and lift it off^. 3. Carburettors. — Mode of removal obvious. The air intake scoops of Zenith carburettors can be released from their spring clips before the carburettors are un- bolted. 4. Water Elbows and Induction Manifolds. — Fig. 78 indicates the manner m which the induction manifolds are removed. 5. Camshafts. — Each of these is removed as a unit, complete with the upper portion of its driving shaft. Disconnect the oil pipe at the front end of camcase. Unscrew packing nuts at base of long inclined shafts; slide nut and felt washer up housing, tying them in position if necessary. Unscrew the twelve nuts securing each camcase to the studs in cylinder heads. Lift off unit, raising it evenly from both ends. In dismantling the units notice that — (i) The seven grubscrews fixing the bushes m the camcase must be removed before the shaft can be withdrawn. (ii) The lids of the cam compartments are not interchangeable, having been machined in position. (ill) The camshafts are not interchangeable, (iv) The bearings are " stepped " in external diameter from the driven end towards the front. Each bearing is 3V in. larger than the bearing next in front of it. (v) Do not remove driven bevel gear wheel from camshaft unless it is defective. 6. Lower Inclined Shafts. — These lift straight out when the flanges at the top are freed. The units should not be dismantled, unless they are obviously defective. If either unit must be taken apart, draw off the gear with the special tool provided; on no account attempt to drive the shaft out through the gear, as the keys will not pass through the lower ball bearing. 7. Vertical Shaft (Generator Drive). — Remove grubscrew from flange of top bearing housing, when unit will lift out. Linit must not be taken apart, unless ob\iously defective, in which case draw gear with special io8 tool, and on no account try to drive shaft out, as the keys will not clear the lower ball bearing. The top bearing is clipped in its housing by a spring locking wire, which can be compressed with round-nosed pliers. The shims between the top ball race housmg and the crankcase set the mesh of the lower bevel pinion with the main bevel pinion on the crankshaft. 8. Oil Pump Unit. — The unit is freed from the base chamber by removing nine nuts Remove the base plate and lower filter. Take out the driving spindle. Free the tab washer and unscrew large nut on top of unit. Unscrew four bolts attaching upper cover, etc. Packing =:r > " Washer at this joint. Fig. 79. Water Pump Lnit. 9. Water Pump Unit. — The dismantling of this unit is perfectly straightforward, except that a special tool, provided in the kit, is needed to withdraw the impeller from the shaft This should be done in order that the shaft may be thoroughly inspected. 10. Cylinders. — Method obvious.. Take care that pistons are not damaged against studs of base chamber. 11. Pistons. — The caps at the ends of the gudgeon pins may be quite^ free If firm they can be pulled out with a screw^ed extractor, or tapped out from behind with a soft drift The gudgeon pins should be pressed out with the drift provided in the kit. The remaining operations are too simple to require annotation. III.B.— REASSEMBLY OF UNITS. These details have been dehberately separated from the notes on dismantling. Experienced mechanics re- quire no mstructions on the taking down of an engine, but require details as to clearances, etc. Such informa- tion is supplied below under the numbers previously applied to the various units. I. Propeller Hub. — Should be lapped to a fit on the shaft, about .001 in. tighter at the large end of the taper than the small. There must be a minimum clearance of .010 in. between the top of the key and the bottom of .'the kevwa\' in the hub. Fig. 80. Low Compression Piston (Navy type). 2. — Ignition Details. ^See p. 6i ct seq. 3. Carburettors". — Cleanse the filters. See that tlie floats are petroltight. Blow out the various jets,, etc.;, broaches must on no account be used. See that the diffusers of Claudel carburettors are perfectly firm. Remove traces of old adhesive from all jointing faces,, and fit new washers, where employed. 110 4- Water Elbows and Induction Manifolds. — Pay special attention to interior condition of all rubber hose connections. Scrap old water clips. Examine all joint- ing washers, seeing that they do not restrict effective area of gas and water ports. 5. Camshafts. — Rocking lever spindles should have from .005-. 010 in. end play, and from .001-. 0015 in. diametrical clearance. Renew any badly worn tappet studs. See that rollers are free from flats and cracks; rollers should have .010 in. side play in their forks, and the maximum shake permissible on the roller spindle is .001 in. Oi( qnoovi; REAR BEARING Doujel holes INTERMEDIATE BEARINGS (or VARYING DlAMLTtRS) FRONT BEARING P-gper uMshcr Clobing plate A and A' — oil holes in plate and washer, to be registered with oil duct A2 in bush. Fig. 81. Camshaft Bearingfs. The camshaft bushes should have a clearance of .00 1 -.003 in. on the shaft— -i.e., the shake should just be perceptible with oil in the bearing. The rear bearing should have .001 -.003 in. end plav between the camshaft flanges. Should either of the upper inclined shaft bushes need replacement, the new bushes should be oiled on the outside and pressed into place. They should be reamed to the following sizes: upper bush, .875 in. plus or minus .0005 in.; lower bush, 1.125 in. plus or minus .0005 in. The camshaft bushes should have no shake in their housings, but must be free enough to be turned for lining up with the grubscrews. Total end play on the camshaft must not exceed .008 in. Ill Fig. 82. Camshaft Bearing (showing assembhng marks). The backlash in the camshaft gears must not be less than .005 in. or more than .010 m. Reassembhng. — (a) Fit the bearings on the cam- shafts in correct order, the smallest at the front end, etc., remembering they are stepped by variations of ^2 in. in outside diameter from front to rear. Line them up to register with their grubscrews. See that the oil holes in the cover plate, washer, and front bearing are lined up accurately. If a new Camshaft or new gear wheel has to be fitted, defer fixing it until the valves are timed ; but be sure to insert bolts in flange before rear bush is mounted on camshaft. Fig. 83. Camcase Cover (showing location mark). (b) Fit rocking levers and covers of cam compart- ments, retaining original locations in both cases. (c) Fit screwed caps and C.A. washers at front of camshafts. (d) Fit upper inclined shafts, with washers between flanges of housing. If the original gears are refitted, mesh them so that the marks are aligned on the centre line of the cylinder, with the marked tooth of the driving bevel at the rear of the engine. 112 6. Lower Inclined Shafts. — The end play of the shafts in their bearings should not exceed .004 m. 7. Vertical (Generator) Shaft. — Its end play in the bearings must not exceed .004 in. Shims (.002 in. thick) are provided between the top ball race housing and the crankcase for setting the vertical mesh with the crankshaft bevel. Shims are also provided between the crankshaft bevel and its flange for setting the m^esh m a horizontal direction (Fig. 86). The correct backlash between the crankshaft bevel and the bevels of the upper and lower vertical shafts is minimum .005 in., maximum .010 in. (cold). 8. Oil Pump Unit. — Scrap the old tab-washer. Examine the splines for wear. The minimum diametri- cal clearance for all five gears in their housing is .001 in., maximum, .005 in., select for .004 in. Their end play should not e.xceed .003 in. Reassembling. — (a) Gears of pressure pump and spindle of driven gear. (b) Separating plate. (c) The three gears of scavenge pump, and spindles of driven gears. (d) Upper half of pump body (register its two dowel pins carefully). (e) Fit washers and nuts to the four bolts of pump body, and insert split pins in three of the bolts. (f) Fit relief valve; pass locking wire through its cage, and through the fourth bolt of pump body, left without a split pin for this purpose. {rr^ Fit lower filter, washer, and cover. (h) Fit upper filter and nut, using new tab- washer, bending one ear into slit in filter and another ear against nut. 9. Water Pump. — Examine rubber connections, especially for loose inside layers. Renew pump shaft if seriously roughened by rust. Examine key and keywav. End play at the ball bearinp- should not exceed .00 ^ in. Scrap the old packing, and fit two new pieces, -f^ in. in diameter and about 8J in. long, rubbing them with p-raph'te grease. The end play of the pump spindle should not exceed .010 in. The lower vertical shaft ooerating the pump should have a diametrical clearance of between .001^; in. and .0025 in. in its bush, and the end plav should be between .005 in. and .008 in. Reasserr^Muig. — (a) Fit pump shaft and bearing in bearing housing. 113 (b) Wrap a new piece of packing round shaft, and press it down with the front gland. (c) Compress the coiled spring and tie it up. Thread it on the shaft. (d) Fit the second gland and packing to the shaft. (e) Fit the pump casing; insert key in shaft key- way; fit impeller, and draw it into position by means of the nut. Bolt up casino to ball bearing housing. (f) Cut and remove the tie from the spring. Impeller and shaft should turn freely; permissible end play on shaft, .010 in. (9') Fit washer and pump cover. Fig. 84. Valve. 10. Cylinders. — The valves and cylinders will, of course, receive the usual attentions. The inside springs are interchangeable; outer inlet springs have twelve coils, and a strength of 23i lbs. when compressed to 2\- in.; outer exhaust springs ten coils, and a strength of 45 lbs. when compressed to 2^ in. Test the valve faces with petrol. The exhaust valve stems should 114 show a clearance of .004-. 0065 in. in their guides; inlets, .002-. 0045 in. In reassembling, note that each valve is stamped *' Ex " or " In," together with the cvlinder number — e.g., " R4." Cvlmders are marked with " R " or " L " and a number on the edge of the base flange near the water inlet connection. Fig. 85. Main Thrust Bearing. II. Pistons. — Aluminium pistons alwa}'s dexelop score marks in use; but a piston need not be scrapped unless the scores appear fresh and extend above the rings. If a piston shows marks of excessive wear on one side at the bottom, and not at the top, its connecting rod is untrue. The limits of clearance in the cylinders are .018-.022 in.; the gudgeon pin fit in the piston, minimum .00025 in. tight, maximum .00075 '^^- tight; the limits of the gudgeon pin fit in the small end of the connecting rod are minimum .00025 ^^-i maximum .00125 in. Examine the piston rings for even bearing m the cvlinder. The gap should be between .021 in. and .041 in. Select the top ring for .003 in. up and down play in its groove, the two lower rings for .002 in.; but old rings should not be scrapped unless they show excessive wear. As alloy pistons are soft, care should be taken not to scratch them in removing rings. 115 When a piston requires replacement, the new piston must be of exactly the same weight as the scrapped part; the number of ounces by which each piston exceeds ^ lbs. is stamped on one of the flats outside the crudgeon pin bosses. Reassembling. — The pistons are stamped with location marks — e.g., " 4R." The pistons should be mounted so that these marks face the rear of the engine. M. Connectmg Rods. — The bronze bush should have from .010-. 020 in. side play on the crankpin, and from .003-. 004 in. diametrical clearance. The single rod should have from .004-. 008 in. side pla^- and from .005-. 0065 in. diametrical clearance. Worn narts should only be refitted by a skilled man. Shim Fig. 86. Mounting of Main Driving Bevel on Crankshaft. 14. Crankshaft. — The crankshaft must not show more than .008 in. end play. In reassembling the thrust, select thick or thin washers, so that the assembled bearing will not have more than .001 in. end play in the crankcase grooves. Examme plugs in ends of crankpins and journals. Swill and blow out all oil passages. The crankshaft diametrical clearances should be between .0025 in. and .00325 in. ; the limits for end play in the journal bearings are .0575 in. mini- mum and .0775 in. maximum. Examine key, kevway, and hub taper, if the hub is removed. Insnect main distribution gear wheel for wear. Reassembling. — Bolt the gear wheel to its flange so that tooth marked " O " is m line with " O " on crank- shaft flange. The shims behind this gear wheel deter 116 mine the longitudinal mesh; if nothmg has been done to the engine which may affect an originally correct setting, replace the shims; otherwise test for backlash carefully during erection, and, if necessary, vary the shims. Propeller hubs should only be refitted by a skilled man. A clearance of .010 in. must exist between the top of the key and the bottom of the hub keyway. When the fit is known to be perfect, the hub may be re- mounted by immersing it for several minutes in boiling water, tapping it lightly into place, and drawing it tightly home by means of the retaining nut. The lock- nut need onty be screwed up so that it bears snugh" against the flange on the crankshaft nut; see that the tongue of the locking wire engages both nuts properly. See that the base chamber studs are tio-ht. Be t^areful not to injure the machined faces of the joint between the two halves. Blow out all oil ducts. Breather trap Fig. 87. Lpper Half of Crankcase (rear end), showing Oil Trap. in. c— NOTES ON RE-ERECTION. N.B. — Reassembling notes are combined with the sequence of operations, because the engine presents a few small traps for the unwary mechanic. I. Place the upper half of the base chamber upside down on the stand, and make sure that it is spotlessly clean. Oil the bearings. Ascertain that the oil trap is already fitted in the rear compartment (Fig. 8y). 117 2. Before placing the crankshaft in position, ascer- tain that (a) The main distribution gear wheel is assembled with the tooth marked " O " in line with the " O " on the crankshaft pange. (b) The washers (or " sleeves ") of the thrust bearing are selected from "thick" and " thin," so that the end play m the crankcase grooves does not exceed .001 in. on assembly. On placing the crankshaft in position, see that the thrust seats in the base chamber without end play. 3. Fit the connecting rods. It does not intrinsi- cally matter on which side the forked rods are assembled, provided, of course, they are all on the same side. In- structions are that they shall be fitted on the right-hand block. Each rod is stamped with a cylinder number (I — 6) on its web near the little end, the cylinders being numbered from the rear of the engine, and not from the propeller, as in British practice The numbered side of each rod should be towards the rear of the engine. Oil all bushes, and fill the crankpins and journals with oil. N.B. — The split pins in the bolts of the forked rods should be entered so that their heads are towards the single rod; otherwise their bent ends will foul the single rod. 4. Place lower half of base chamber in position. Do not insert anv flange bolts, and tighten the nuts on the front and rear bearing bolts only. 5. Assemble the 'ower vertical shaft (pump drive) with the deeo-faced pinion at the cupped end of the housing. Fit the shaft, and test the mesh of its uDoer ninion with the crankshaft gear wheel. This should be .007 in. cold, with h'mits of .005 m. and .010 in. If the mesh is outside these limits pack shims (.002 in. thick) between the crankshaft bevel and its flancre until the mesh is correct. Then t^'o-hten up the nuts on the intermediate crankshaft bearing bolts. Invert the assembl-" and fit all the bolts to the flanged joint of the base chamber. 6. Fit the pistons. Each piston is marked for location (e.^., " R^ ") and for we^'g-ht in one of the flats on its sk'^t. Tb^se ma'-ks should fare the rear of the eng-'ne. The we^Vht ma^ks ind-'CPte tb'^ number of ounces by which a n^'ston e"^'reeds t, lbs.; if a piston is scranned. the replace^^ent shonM be of the original weight to an ounce. 0^1 the p-ndgeon pins before inser- ii8 tion. Replace the capping pieces, which should be a light tapping fit. 7. Each cylinder is numbered for location (.e.g., " R3 ")on the edge of its base flange near the water inlet. Before fitting any cylinder, be sure that the washer is in place on the base chamber studs; oil the bore of the cylinder, and put the piston on the top of its stroke. Push the cylinder down into place, and run the holding-down nuts up with the fingers, but do not tighten them. Fit the inlet port washers, and put the induction manifolds in position, but delay replacing the washers and nuts by which the manifolds are secured to the cylinders. It is advisable to stop up the snark- ing plug orifices at this point to prevent small parts from being dronoed into the combustion chambers. Fig. 88. Housing and Bush of Lower Vertical Shaft. 8. Next put a washer and nut on each of the studs by which the manifolds are fixed to the cyhnders. and run the nuts up with the fingers. For tightening up ti'c cylinder, holding-down nuts and induction manifold nuts, work by groups of three cylinders, and tighten each nut a quarter of a turn in order, otherwise serious air leaks at the inlet ports cannot be avoided. 9. Soak the washers for the water joints between the cylinders and the manifold jackets in water, and refit them. Leave six of the screws at these joints loose for the present (three down each side of the engine), as they will be needed to fix the clips of the wiring U. 10. Fit the front and rear water elbows with washers beneath them. Before they are bolted down, slio the couphng tube and its rubber connections into place. 119 1 1 . Insert the bolts by which the carburettors are slung from the induction manifolds; fit the carburettors. Particular care is required in refitting either type of carburettor, owing to the general design of the induc- tion assembly. Serious Habihty to air leaks exists at four points, viz. : — (a) Float chamber lids, where any appreciable leak will reduce the range of the altitude control. (b) Joints between mixing and throttle chambers. (c) Joints between carburettor flanges and ports in underside of induction manifolds. ankshaft. Diametrical Clearance ... .0025 .00325 End Play ... .0575 0775 Connecting Rods. FoRKKD End — Diametrical Clearance ... .003 .004 End Play ... .008 .020 PtAiN End — Diametrical Clearance ... .005 .0065 End Play ... .004 .008 Gudgeon Pin. Fit in Rod Fit in Piston Piston Rings. iMt in Grooves .00025 00125 Select for .001 Clearance .00025 tight .00075 tight Select for light drive fit Gap .00125 .021 .003, .041 Top .003 Mid. & Bot. .002 .030 Piston. Fit in Cylinder .018 .022 Select for .020 Clearance Camshaft. Diametrical Clearance End Play 001 .000 .003 .004 Min. .002 Camshaft Upper Drive Shaft. Diametrical Clearance — Large Bushing Small Bushing End Play .0005 .0005 .002 .0025 .0025 ,008 Min. .0015 Min. .0015 Min. ,004 141 Standard Fits and Clearances. — con'imied. MlNIMTM, ]\I.\XIMrM. Dksirkd. Rocker Levers. Diametrical Clearance ... .00025 .00175 Mi in. .001 Eud Play ... 005 .010 .0075 Valves. Fit ok Stp;ms in Guidks. Diametrical Clearances— P^xhausi Valve* ••• 004 .0065 .005 Inlet Valves ... .002 .0045 .003 Water Pump Shaft. DiametricHl Clearance ... .0015 .0035 .M in. .0025 End Play ... OOfS .010 .010 Water Pump Bevel Driver. Diametrical Clearance ... .001 .0025 End Play ... .005 .008 Oil Pump. Fit ok Gears in Housing. Diametrical Clearance End Play Tappet Gap. Exhaust Valves Inlet Valves Contact Breaker Gap Sparking Plug Gap Regulator. Contact Gap Height of Pin .001 .005 Select for .004 Clearance 002 .007 Select for 003 Clearance .019 .021 014 .016 .010 .013 ,015 .018 .015 005 .007 .043 .045 142 ADJUSTING CONTROLS OF ZENITH CARBURETTOR. Each duplex carljiirettor is fitted with two butterfly thrott'.e valves, and it may occasionally be found that the pair are not accurately synchronised (see dotted throttle positions shown in Fig". 98). No adjustment is provaded, and in such cases the butterfly must be very carefully "set" so that the closing" points agree. It is further necessary to synchronise the throttles of the fore and aft carburettors in coupling" up the con- necting" rod, and this should be done as closely as is compatible with taking" up the backlash and torque in the connecting" rod. This is done by means of the screw adjustment "X" in Fig. 99. If it is impossible to syn- chronise both carburettors, and also to set the connecting" rod perfectly by one and the same adjustment, a slight variation in the throttle closing points is preferable to a sloppy settmg of the rod. In this case the throttles of the front carburettor should close very slightly m ad- vance of the rear throttles. Fmally. the throttle stop screws ("Z" in Fig. 99) must be set so that they relieve the butterfly valve of excessive pressure when the control lever is slammed back. 14:^ Fig. 98. Screw stop '1' Fig. 99. N^ ^ Pipe Ta p fOfi PfflMIMG~CupS . Fig. lOO. Installation Drawing'. (Front Elevation j. Ai)ril. lyiS Fig. loi. Installation Drawing (Rear Elevation. 1 DfK N'/i B. 4^tS c April. 1918. Fig. 103. Installation Drawing. (Plan View). April. 1918. 148 Fig-. 104. Rear Elevation. 149 Fig. 105. Transverse Section through Cyhnders. 1=^0 Fig. 1 06. Front Elevation. Fig. 107. General Arrangement (Longitudinal Section). c ™S,'^.f'"^nE2iow™ A 000 117 860 .^^ .\^' Q>- •s?-, d- ti »^ ^.^ .