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IC 
 
 9000 
 
 Bureau of Mines Information Circular/1984 
 
 Overspeed Protection for Mine Diesels 
 
 A Literature Review 
 
 By Lito C. Mejia and Robert W. Waytulonis 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
Information Circular 9000 
 
 w 
 
 ^ 
 
 u 
 
 v\ \ Vei S Vii-W ^ « 'feu^reA.U o V ^^ v w^e fe J 
 
 Overspeed Protection for Mine Diesels 
 
 A Literature Review 
 
 By Lite C. Mejia and Robert W. Waytulonis 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 William P. Clark, Secretary 
 
 BUREAU OF MINES 
 Robert C. Horton, Director 
 
Library of Congress Cataloging in Publication Data: 
 
 
 Mejia, Lito C 
 
 Overspeed protection for mine diesels. 
 
 (Information circular / United States Department of the Interior, Bu- 
 reau of Mines ; 9000) 
 
 Bibliography: p. 12-13. 
 
 Supt. of Docs, no.: I 28.27:9000. 
 
 1. Mining machinery— Safety measures. 2. Diesel motor— Safety 
 measures. 3. Governors (Machinery). 4. Methane. I. Waytulonis, 
 Robert W. II. Title. III. Series: Information circular (United States. 
 Bureau of Mines) ; 9000. 
 
 TNaeSrtM [TN345] 622s [622'. 2] 84-600201 
 
^ 
 
 CONTENTS 
 
 <^ Page 
 o^ 
 
 \^ Abstract 1 
 
 Introduction 2 
 
 Mine accident data 2 
 
 J. Compression ignition of methane in a diesel engine 3 
 
 ^ Effects of various methane concentrations and engine conditions 3 
 
 J Duel-fuel engine studies 5 
 
 N^) Fuel governors 6 
 
 Types of governors and principles of operation 6 
 
 , Special conditions 8 
 
 p^ Positive-shutdown devices 9 
 
 vi^ Summary 11 
 
 ^ References 12 
 
 ILLUSTRATIONS 
 
 1. Autolgnltion of methane-air corrected for Induction heating by normalizing 
 
 on 8 .8 vol pet methane 3 
 
 2. Engine speed versus methane concentration. 5 
 
 3. Typical diesel fuel-governor operational characteristics curves 7 
 
 -5^ 
 

 UNIT OF MEASURE ABBREVIATIONS USED IN 
 
 THIS REPORT 
 
 cm^/min 
 
 cubic centimeter per minute pet 
 
 percent 
 
 »F 
 
 degree Fahrenheit rpm 
 
 revolution per minute 
 
 hp 
 
 horsepower s 
 
 second 
 
 mln 
 
 minute vol pet 
 
 volume percent 
 
OVERSPEED PROTECTION FOR MINE DIESELS 
 A Literature Review 
 By Lito C. Mejia and Robert W. Waytulonis 
 
 ABSTRACT 
 
 Diesel-powered equipment operating in a gassy underground mine could 
 conceivably ingest a methane-air mixture from the mine atmosphere that 
 could cause the diesel engine to overspeed, or exceed its rated speed. 
 Engine overspeed, particularly if extreme (engine runaway), could re- 
 sult in personal injury, a possible mine explosion, and/or catastrophic 
 engine failure. In this report, the Bureau of Mines reviews the liter- 
 ature on the potential hazards of me thane -induced diesel engine over- 
 speed. Also included are the Bureau's findings from consultations with 
 representatives of the diesel engine industry to determine the specific 
 engine behavior that could be expected under the overspeed condition. 
 The report summarizes data on mine accidents involving methane, dis- 
 cusses results of tests on the compression ignition of methane in die- 
 sel engines , and examines fuel governors and intake-air-cutoff devices , 
 two kinds of devices used to prevent overspeed. 
 
 Analysis of the information gathered suggests that the conditions 
 necessary to cause a diesel engine to overspeed uncontrollably are not 
 likely to occur, particularly if the engine is equipped with flame- 
 proofing devices. The available literature and lack of actual over- 
 speed case histories suggest that the safety devices currently in use 
 are sufficient to prevent diesel engine overspeed. 
 
 ^Mechanical engineer. 
 ^Supervisory physical scientist. 
 Twin Cities Research Center, Bureau of Mines, Minneapolis, MM. 
 
INTRODUCTION 
 
 In 1976, 135 mobile diesel units were 
 in use in 18 underground coal mines and 
 366 such units were in use in 11 gassy 
 metal and nonmetal mines O).^ Currently, 
 mobile diesel units are used in approxi- 
 mately 82 underground coal mines (a more 
 than fourfold increase since 1976) and 15 
 MNM mines (2^). Since 1976 there has been 
 a sevenfold increase in the number of 
 diesels in use in coal mines , and the 
 number has almost doubled in gassy MNM 
 mines. About 1,000 diesel units operate 
 in coal mines, and another 621 operate in 
 underground gassy MNM mines (2^). A gassy 
 classification by the Mine Safety and 
 Health Administration (MSHA) means either 
 a flammable gas has been ignited or a 
 concentration of 0,25 vol pet or more of 
 flammable gas such as methane has been 
 detected in the atmosphere of any open 
 working area (_3) . All coal mines are 
 gassy. 
 
 Engine surface and exhaust temperatures 
 are limited for diesels approved under 
 30 CFR 36 O ) . Dieselized equipment ap- 
 proved under this regulation is referred 
 to as permissible equipment. Not all 
 diesel units operating in gassy mines are 
 permissible since only dieselized units 
 that are to be operated at or near an ac- 
 tive working area (within the last open 
 crosscut) require approval. Thus, the 
 number of diesel units operating in gassy 
 MNM and coal mines is a combination of 
 permissible and nonpermissible equipment. 
 It is not known at this time how many of 
 
 the total number of units are permissible 
 units and how many are nonpermissible, 
 Dieselized equipment can be operated if 
 the concentration of methane in the mine 
 air is less than 1.0 vol pet in any ac- 
 tive working area (30 CFR 57) (^) . 
 
 A diesel engine operating in a gassy 
 underground mine could possibly encounter 
 a methane-air mixture of sufficient com- 
 bustion energy to accelerate the engine 
 uncontrollably. The ingestion and com- 
 bustion of the fuel-air mixture could 
 produce engine power that is not under 
 the equipment operator's control, a con- 
 dition that could result in engine run- 
 away (speeds greater than 150 pet of the 
 rated engine speed) and catastrophic en- 
 gine failure due to excessive inertial 
 loads (_5) , 
 
 The increasing use of diesel equipment 
 in mine working areas where combustible 
 methane-air mixtures may be present has 
 prompted this study of the potential haz- 
 ards of methane-induced overspeed and the 
 precautionary measures that have been 
 taken to prevent its occurrence. It is 
 stipulated in 30 CFR 36.23 (3) that the 
 intake system of an approved diesel en- 
 gine operating in a gassy atmosphere must 
 include a valve, operable from the opera- 
 tor's compartment, to shut off the air 
 supply. The question remains, however, 
 whether this precaution is sufficient to 
 prevent potential overs'peed and/or run- 
 away conditions upon methane ingestion. 
 
 MINE ACCIDENT DATA 
 
 Methane concentrations greater than 
 1,0 pet occasionally occur in mines, as 
 shown in the records of mine explosions 
 (^-_7) , The approximate flammability 
 range wherein an ignition or explosion 
 is possible is 5 to 15 pet methane in 
 air (8;"9^) . The autoignition temperature 
 of firedamp, or mine methane, is about 
 
 ■^Underlined numbers in parentheses re- 
 fer to items in the list of references at 
 the end of this report. 
 
 1,049° F, as compared to 1,000° F for 
 commercial-grade methane ( 10 ) . 
 
 From 1941 to 1969, 740 methane igni- 
 tions or explosions occurred in coal 
 mines, and since 1969 another 690 have 
 been recorded (_7) , indicating dilution to 
 less than 1.0 vol pet methane in the mine 
 air is not always maintained. These data 
 are approximate, since some minor explo- 
 sions go unreported. About 85 pet of the 
 incidents since 1969 occurred at the face 
 
and were caused by friction from the 
 cutting bits of continuous miners. The 
 other 15 pet were caused by miscellaneous 
 sources such as weld sparks, overheated 
 
 roof-drill bit, blasting, etc. No re- 
 cords of mine explosions attributable to 
 diesel engines ingesting methane were 
 discovered. 
 
 COMPRESSION IGNITION OF METHANE IN A DIESEL ENGINE 
 
 EFFECTS OF VARIOUS METHANE 
 CONCENTRATIONS AND ENGINE CONDITIONS 
 
 Compressed ignition of methane-air mix- 
 tures was investigated by the Canada Cen- 
 ter for Mineral and Energy Technology 
 (CANMET) to determine the possibility of 
 diesel engines continuing to operate on a 
 methane-air mixture after diesel fuel 
 delivery has been shut off by the fuel- 
 injection pump (11), The engine used for 
 this study was a four-stroke single- 
 cylinder liquid-cooled test engine with 
 variable-compression ignition and indi- 
 rect injection; the engine was rotated 
 at 900 rpm by a constant-speed motor. 
 Audible knock determined whether the in- 
 jested methane-air mixture was igniting 
 in the cylinder (11-12) , Knock is a 
 characteristic violent detonation pro- 
 cess associated with methane-air mixtures 
 (low-cetane, high-octane rating) at high 
 engine compression ratios (_5, 13-16), 
 The test procedure was to set the engine 
 compression ratio and methane ingestion 
 rate, run the engine on diesel fuel for 2 
 min (to reach thermal equilibrium), and 
 follow immediately with 15 s of methane- 
 air ingestion without diesel fuel. If 
 the methane ignited on almost every com- 
 pression stroke for more than 15 s, con- 
 tinuous combustion was said to have 
 occurred. Several different methane in- 
 gestion rates were used to determine the 
 minimum continuous-combustion compression 
 ratios of four combinations of engine in- 
 take and coolant temperatures. 
 
 The test results indicate that under 
 certain conditions, high-compression 
 indirect-injection diesel engines can 
 operate on ingested methane-air mixtures. 
 Figure 1 shows the compression ratio re- 
 quired for autoignition at various meth- 
 ane concentrations. It is apparent that 
 an 8,8-vol-pct methane concentration 
 (nearly stoichiometric) is most sensitive 
 
 to compression ignition and that the hot- 
 ter the engine, the lower are the com- 
 pression ratios required to autoignite 
 the methane-air mixtures. 
 
 The CANMET study also suggested that 
 glowing combustion chamber deposits could 
 act as an ignition source and allow 
 the engine to ignite methane at lower 
 compression ratios. In addition, fuel 
 leakage — even a small volume of fuel nor- 
 mally insufficient to maintain idling in 
 plain air — could result in a dual-fuel 
 operation, with the engine continuing to 
 
 28 
 
 26 
 
 24 
 
 O 22 
 
 20 
 
 16 
 
 KEY 
 
 Symbol 
 
 Intake air 
 
 Engine coolant 
 
 • 
 
 83° F 
 
 90° F 
 
 o 
 
 88° F 
 
 2I5°F 
 
 D 
 
 92° F 
 
 2I5°F 
 
 A 
 
 I46°F 
 
 2I5°F 
 
 A 
 
 IOO°F 
 
 352°F 
 
 5 7 9 II 13 15 
 
 APPROXIMATE INGESTED METHANE CONCENTRATION, vol pet 
 
 FIGURE 1. - Autoignition of methane-air cor- 
 rected for induction heating by normalizing on 
 8.8 vol pet methane. 
 
operate on methane combustion Initiated 
 by the compression ignition of the diesel 
 fuel. The CANMET study also indicated 
 that in larger engines, compression igni- 
 tion of methane can occur at lower com- 
 pression ratios because the greater com- 
 bustion volume to wall area ratio results 
 in lower compression-heat losses, CANMET 
 recommended that quick-closing intake 
 valves continue to be required on flame- 
 proof diesel-powered equipment certified 
 for use in coal and other gassy under- 
 ground mines. 
 
 The occurrence of continuous combustion 
 in the CANMET study indicated that under 
 certain conditions a high-compression 
 diesel engine can operate on an ingested 
 methane-air mixture. This knocking comr- 
 bustion was also observed by the South- 
 west Research Institute (SwRI), which 
 found it to be insufficient to success- 
 fully power a test engine O ) . SwRI per- 
 formed tests sponsored by MSHA to evalu- 
 ate the potential hazard of a diesel 
 ingesting combustible methane-air mix- 
 tures. The types of diesels used in un- 
 derground mines are usually rated near 
 2,100 rpm, and previous SwRI experience 
 had shown that engine speeds greater than 
 3,500 rpm can cause engine disintegration 
 due to excessive inertial loads, (That 
 is, the engine becomes mechanically over- 
 stressed due to the inertia of its rotat- 
 ing parts.) 
 
 SwRI's experimental design used a sin- 
 gle-cylinder test engine with a compres- 
 sion ratio of 18:1 coupled to an eddy- 
 current dynamometer and direct-current 
 motor. The test procedure involved run- 
 ning the engine at constant speed, at a 
 sufficient diesel fuel rate to produce 
 a measurable torque. Commercial-grade 
 natural gas was introduced into the in- 
 take duct in increasing concentrations 
 until the methane-air mixture was too 
 rich to sustain combustion. Testing was 
 repeated at engine speeds of 1,000, 2,000 
 and 3,000 rpm. Collected data were used 
 to calculate the volumetric and thermal 
 efficiency of the engine and the methane 
 fuel-air ratio. 
 
 The test procedure required the use of 
 diesel fuel as an ignition source for the 
 methane-air mixtures. Attempts to run 
 the engine on methane alone were unsuc- 
 cessful. With the engine hot, ignition 
 of methane without diesel fuel could be 
 accomplished, but severe knocking re- 
 sulted. In this experiment, the combus- 
 tion process produced insufficient power 
 for runaway. 
 
 Owing to its specific properties, espe- 
 cially its low propensity to ignite under 
 compression, methane is a poor substitute 
 for diesel fuel. Theoretically, the min- 
 imum ignition temperature of industrial 
 methane (1,000° F) requires an adiabatic 
 compression ratio of approximately 19:1 
 (at an intake temperature of 60° F) (17) , 
 and the previously discussed CANMET test 
 results support this. This ratio, 19:1, 
 is typical of present mine diesels, but 
 the quality and quantity of fuel, and ac- 
 tual nonadiabatic conditions, affect the 
 true compression ratio necessary for 
 ignition. Firedamp has an ignition tem- 
 perature of 1,100° F (10), due to its 
 relative impurity. Therefore, the com- 
 pression ratio necessary for the ignition 
 of firedamp will be higher. 
 
 The SwRI investigators also estimated 
 the thermal efficiency of methane and 
 used this value to calculate the addi- 
 tional power produced by the combustion 
 of methane. The power contributed by the 
 diesel fuel was subtracted from the ac- 
 tual power measured. The engine was not 
 able to operate on methane alone, so the 
 thermal efficiency of methane was not 
 readily calculated. When methane is in- 
 gested, the engine will accelerate until 
 the power produced by the methane is 
 exactly balanced by the power required 
 to overcome engine friction. For the 
 calculation of a runaway condition, the 
 methane-produced power was equated to the 
 engine friction. The engine-power equa- 
 tion used was based on the calculated 
 methane thermal efficiency and the engine 
 volumetric efficiency. Engine friction 
 data from Taylor ( 15 ) were used in 
 the calculations. For each methane-air 
 
ratio, the equation was solved for the 
 engine speed at which methane-produced 
 power equaled friction power. 
 
 Plotting engine speed versus the meth- 
 ane-air ratio gave the methane concen- 
 tration range at which runaway speeds 
 (greater than 3,500 rpm) could be at- 
 tained. As shown In figure 2, this range 
 Is about 3,5 to 15 vol pet. The steep 
 slopes In this figure clearly show engine 
 speed to be highly sensitive to the meth- 
 ane concentration. Given an Ignition 
 source, the theoretical results of meth- 
 ane Ingestion for an unloaded engine 
 would be Instantaneous acceleration to 
 very high and destructive speeds. Where 
 an engine In a vehicle moves across a 
 nonhomogeneous methane-air mixture, er- 
 ratic engine behavior and severe vehicle 
 surging could result since the transmis- 
 sion would be engaged. 
 
 Actual testing never proceeded beyond 
 running the engine at 3,000 rpm. Running 
 the engine at this speed collapsed the 
 
 7,000 
 
 5,000 
 
 uj 4,000 
 
 ^ 3,000 - 
 
 1,000 
 
 4 6 8 10 
 
 METHANE, vol pet 
 
 FIGURE 2. • Engine speed versus methane 
 concentration. 
 
 piston crown and cracked the cylinder 
 head, terminating the experiment. This 
 failure was attributed to the knocking 
 characteristics (loss of combustion con- 
 trol) that are observable in methane com- 
 bustion when compression ratios are high 
 (15). 
 
 The SwRI investigators concluded from 
 their experiments that runaway can occur 
 only under abnormal engine conditions and 
 only if an ignition source is present. 
 If an ignition source is present, runaway 
 can only occur if the methane concentra- 
 tion range is between 3,5 to 15 vol pet. 
 Also, the most likely result of methane 
 ingestion will be knocking combustion 
 producing insufficient power to cause 
 runaway , 
 
 SwRI's unsuccessful attempts to operate 
 its diesel test engine on methane alone 
 was in agreement with a similar experi- 
 ment conducted by Deutz Corp, , a manufac- 
 turer of diesel engines in the Federal 
 Republic of Germany (FRG) , As a result 
 of Bureau Inquiries to Deutz, tests were 
 performed on the manufacturer's dual-fuel 
 engines in a small-scale experiment. A 
 dual-fuel engine is a diesel engine that 
 operates on natural gas combustion initi- 
 ated by the compression ignition of a 
 small amount of diesel fuel, 
 
 DUEL-FUEL ENGINE STUDIES 
 
 Testing at the Deutz R&D Center in 
 Porz, FRG, defined the minimum value of 
 diesel fuel needed to support combustion 
 (18). The testing procedure Involved 
 bringing the engine to operating equilib- 
 rium and decreasing the amount of diesel 
 fuel that was mixed with natural gas un- 
 til the engine stopped. The dual-fuel 
 engine tested had a compression ratio of 
 17:1. It was shown that the Deutz dual- 
 fuel diesel engines required a minimum of 
 15 pet diesel fuel mixed with natural gas 
 to operate. Natural gas by itself did 
 not support combustion under the diesel 
 cycle (18) . It was also discovered that 
 the Injector tip (a possible ignition 
 source Implied by CANMET and SwRI) can 
 overheat if too little diesel fuel is in- 
 jected. A hot Injector can serve as an 
 
ignition source for the natural gas, but 
 testing showed that even with an over- 
 heated injector tip, diesel fuel was 
 still required to sustain combustion. To 
 avoid injector-tip overheating, which 
 would necessitate the costly replacement 
 of burned-off injector tips, Deutz recom- 
 mends using a minimum of 25 pet diesel 
 fuel for its dual-fuel engines. 
 
 A number of dual-fuel combustion stud- 
 ies are reported in the literature. The 
 difficulty of operating a diesel engine 
 solely on a methane-air mixture has been 
 well documented O, j3 , 18-19) . Although 
 methane was found to be a very suitable 
 primary fuel, it requires, as do other 
 gaseous fuels, an independent ignition 
 source owing to its poor self-ignition 
 quality (high-octane, low-cetane rating) 
 
 (13, 19). Only with pilot (diesel) fuel 
 injection was it possible to run a diesel 
 engine with a high-octane fuel as the 
 main energy source ( 19 ) . Most high- 
 octane gaseous fuels require about 30 pet 
 (on an energy basis) pilot-fuel injection 
 (19) . For methane, as much as 43 pet 
 pilot fuel was needed to achieve optimum 
 engine efficiency (19) . As rediscovered 
 by SwRI and Deutz, both the performance 
 and use of dual-fuel engines are limited 
 by knock, a characteristic loss of com- 
 bustion control especially with the igni- 
 tion of methane at high compression 
 ratios ( 13 , 15 , 19 ) . Knock is the com- 
 bustion process experienced by CANMET in 
 the motoring action of its test engine 
 (11-12) . Knocking combustion has limited 
 the normal operation of dual-fuel engines 
 (5, 13, 18-19). 
 
 FUEL GOVERNORS^ 
 
 The fuel governor, a part found on 
 every diesel engine, controls speed and 
 torque by regulating the fuel flow rate. 
 Because the engine has no control over 
 the contents of the intake air, and the 
 combustion of methane in the engine in- 
 creases power and speed, the fuel gover- 
 nor acts to compensate for the speed and 
 power fluctuations caused by methane in 
 the intake air. How several types of 
 fuel governors regulate the fuel flow and 
 engine speed is summarized below. 
 
 In order to maintain desired speeds un- 
 der variable loads, as in mobile diesel- 
 powered mining equipment, the fuel rate 
 must be metered to correspond to the re- 
 quired torque. In diesel engines, fuel 
 metering is achieved by changing the 
 
 '*This section includes considerable 
 information from technical and product 
 literature from American Bosch Div. of 
 United Technologies, Springfield, MA; 
 Barber-Coleman Co., Rockford, IL; Cater- 
 pillar Co., Peoria, IL; Detroit Diesel 
 Div, of General Motors Corp,, Detroit, 
 MI; Hoof Products, Chicago, IL; LUCAS- 
 CAV, London, England; Robert Bosch GmbH, 
 Stuttgart, FRG; Terex Div, of General Mo- 
 tors Corp., Detroit, MI; and Woodward 
 Governor Co,, Fort Collins, CO, 
 
 quantity of fuel injected by movement of 
 the fuel-injection pump's control rod. 
 Control-rod movement is accomplished by 
 the acceleration pedal or by a signal 
 from an engine revolution-per-minute 
 sensor, 
 
 TYPES OF GOVERNORS AND PRINCIPLES 
 OF OPERATIONS 
 
 Four basic types of governors are com- 
 mercially available: mechanical, pneu- 
 matic, electric, and hydraulic (20-22) . 
 The mechanical governor is a speed-sensi- 
 tive control that uses tbe movement of 
 flyweights under the influence of centri- 
 fugal force. The pneumatic governor uses 
 a diaphragm activated by the vacuum pres- 
 ent in the engine's intake manifold. 
 Where constant-speed (isochronous) opera- 
 tion is required, electric governors are 
 used. They are activated by engine-speed 
 signals obtained from a magnetic pickup 
 that monitors the teeth of the flyweel's 
 ring gear. Off-speed is corrected elec- 
 trically through the magnetic pickup sen- 
 sor and linkage to the fuel system. Like 
 mechanical governors, hydraulic governors 
 depend on the variation of centrifugal 
 force created by flyweights in the gover- 
 nor. However, this force does not di- 
 rectly operate the fuel-control mechanism 
 
as in the mechanical governor. Instead 
 the fuel rod is connected to a piston- 
 type pilot valve that controls the flow 
 of high-pressure oil to a cylinder-type 
 servomotor. 
 
 Two kinds of governors are commonly 
 used in mobile mining diesels: minimum- 
 maximum and variable-speed governors (_5) . 
 Both are mechanical governors. Typical 
 operational characteristics of a vari- 
 able-speed governor are shown in figure 
 3. Speed control is exerted over the en- 
 tire engine speed-load range. The curves 
 represent various throttle and fuel-cut- 
 off positions for given horsepowers and 
 speeds; they also represent the amount of 
 fuel injected at fixed accelerator pedal 
 
 positions. With the pedal position con- 
 stant, methane ingestion increases the 
 engine horsepower and thus engine speed; 
 and the governor compensates by reducing 
 the diesel fuel flow rate, as indicated 
 by the downward curves of figure 3. At 
 any pedal position, a further increase in 
 engine speed will result in the fuel be- 
 ing shut off. Downhill travel or some 
 other reduction in engine load will re- 
 duce the amount of diesel fuel. If the 
 reduction in load is significant enough, 
 continued increases in engine speed will 
 cause the fuel to be shut off completely 
 (23). 
 
 A minimum-maximum governor exerts speed 
 control only near the idle speed and at a 
 
 1,000 
 
 1,500 
 
 2,500 
 
 3,000 
 
 2,000 
 ENGINE SPEED, rpm 
 
 FIGURE 3. - Typical diesel fueUgovernor operational characteristics curves (23). Downward curves 
 represent governor actiono 
 
maximum setting (usually the rated 
 speed) . In an engine operating at a con- 
 stant load and speed (constant pedal po- 
 sition) , a sudden load decrease will 
 cause the engine to accelerate up to the 
 maximum governed speed. Only then will 
 the governor react to reduce the rate of 
 fuel flow. This surging will normally be 
 countered by the operator easing off the 
 accelerator pedal. An increase in engine 
 speed from idle or from the maximum rated 
 speed (no load) will cut off the diesel 
 fuel (2, 20-22). 
 
 In principle, governor control allows 
 enough fuel to be injected so the engine 
 can attain its rated speed. If a rated 
 speed of 2,100 rpm is exceeded, for exam- 
 ple, the flyweights override a prese- 
 lected spring force and move the fuel 
 rack linkage to reduce the rate of in- 
 jected fuel. For an engine running at 
 its rated speed under zero load, only 
 enough fuel is injected to overcome 
 internal engine friction, A further in- 
 crease in engine speed (either by motor- 
 ing or ingestion of a combustible meth- 
 ane-air mixture) to above the rated speed 
 moves the linkage to a position that al- 
 lows no fuel to be injected. Even fur- 
 ther engine speed increases will continue 
 to move the linkage but will have no 
 practical effect because no fuel is being 
 injected (24) . 
 
 For a loaded engine, methane in the in- 
 take air will produce additional power 
 (19, 25-26) , The operator or fuel gover- 
 nor will reduce the fuel to maintain the 
 injection-rate power level, or the speed 
 will increase until the maximum speed 
 regulation described above becomes ac- 
 tive. In effect, the engine is operating 
 at a lower-than-noinnal throttle position, 
 A sudden load reduction, such as a clutch 
 disengagement, will cause the engine to 
 accelerate. Fuel reduction and speed 
 stabilization will occur in the zero-load 
 condition previously described, 
 
 Diesel engines accelerate rapidly, and 
 some "overshoot" can be expected. With 
 methane-air ingestion, engine speeds to 
 2,500 rpm can occur before stabilization 
 is achieved. Acceleration from idle to 
 
 zero-load maximum speed can take less 
 than 1 s, and stabilization at the maxi- 
 mum zero-load speed can occur within an 
 additional 2 s (24). 
 
 Mechanical governors react quickly un- 
 der changing load, and the response to a 
 change in engine speed is immediate, A 
 typical response time for a change in en- 
 gine load is less than Is, Some vari- 
 able-speed governors, if subjected to in- 
 stantaneous speed change, have a typical 
 response time of about 0,05 s to cut back 
 fuel delivery (27) , This response is in- 
 dependent of load. Mechanical fuel gov- 
 ernors are rated to regulate within 0,5 
 to 1 pet of the rated engine speed ( 28 ) . 
 It is estimated that one model of mechan- 
 ical governor would reestablish engine 
 speed within 0.2 to 0.3 s, even with 
 methane ingested ( 15 ) . In applications 
 where electric governors are used, speed 
 control within 0.2 pet of the selected 
 speed is common ( 28 ) . For a mining en- 
 gine encountering a gassy air mixture, 
 the mechanical governor will provide al- 
 most instantaneous control, 
 
 SPECIAL CONDITIONS 
 
 Because diesel fuel or some other igni- 
 tion source is required to initiate com- 
 bustion of methane-air mixtures , and be- 
 cause of the governor's quick reaction 
 time (to cut off diesel fuel if the rated 
 speed is exceeded) , it is improbable that 
 destructive speeds could be attained ( 23 , 
 25 , 28-29), There are, however, specific 
 engine and governor conditions that could 
 alter this assessment. 
 
 The possibility of fuel remaining in 
 the fuel-injection system after shutoff 
 was brought up by governor and engine 
 manufacturers (^3, 25.* 2:2)* ^^^^ ^^® 
 governor in the full cutoff position, 
 small amounts of fuel can still be in- 
 jected into the combustion chamber by 
 residual pressure. Normally, however, 
 these injections are not adequate to sus- 
 tain combustion (23), For one flyweight- 
 type governor, leakage can occur (at 
 about 35 cm^/min) at the complete cutoff 
 position (29) . It is not known how much 
 fuel actually gets into the combustion 
 
chamber. When an electric governor is 
 used, the failsafe mode returns the fuel 
 throttle activator to a "minimum fuel" 
 position, but this minimum fuel position 
 is not necessarily a complete cutoff 
 condition. 
 
 Another consideration is the role of 
 the governor as a speed-controlling de- 
 vice and not as an engine-shutdown de- 
 vice. When diesel fuel is cutoff and a 
 condition exists where combustion of 
 methane delivers torque greater than that 
 required for the existing load, a violent 
 instability with the governor cycling 
 from full on to full off at the system 
 natural frequency is possible. This cy- 
 cle would continue unless there is an in- 
 dependent overspeed shutdown ( 28 ) . The 
 severity of this cyclic response can be 
 undesirable, depending on methane concen- 
 tration, engine conditions, and the iner- 
 tial forces attained. Stable speed could 
 not be achieved, since the governor has 
 no control over the energy source. If 
 the methane did not provide sufficient 
 additional power to deliver the torque 
 required for the existing load, speed 
 stability might be achieved, because some 
 control would be exerted by the governor 
 (30). 
 
 In effect, a diesel operating in a gas- 
 sy mine will behave as a dual-fuel en- 
 gine. The most likely ignition source 
 for the methane-air mixture is the avail- 
 able fuel supplied by the engine's fuel- 
 injection system. However, other engine 
 conditions that potentially could provide 
 an ignition source for methane (5, 11, 
 
 18) include: (1) injectors getting stuck 
 open as a result of a gummed-up fuel sys- 
 tem; (2) bad enhaust valves — improperly 
 seating valves cannot perform the neces- 
 sary heat transfer to the head (which 
 will erode the valve or seat and provide 
 a hot spot for ignition); (3) carbon de- 
 posits providing a "glow plug" effect; 
 (4) an overheated engine caused by poor 
 cooling; (5) a badly worn engine which 
 consumes lubrication oil; (6) an over- 
 turned engine or an engine operating at 
 a large tilt causing lube oil consump- 
 tion; (7) overfilling of the oil-bath air 
 cleaner; (8) failed turbocharger bearing 
 seals, with leaking lube oil ingested 
 through the intake; and (9) abnormally 
 high compression ratios — for example, a 
 high-altitude engine with high-compres- 
 sion pistons operated at or near sea 
 level . 
 
 The above conditions cannot be con- 
 trolled by a properly working governor. 
 A malfunctioning governor can fail to 
 properly regulate the quantity of diesel 
 fuel being injected into the engine. The 
 governor, though simple in principle, is 
 complex owing to its many parts. Mis- 
 adjustment or failure of any part may 
 take the following forms: (1) flj^eights 
 "frozen" because of insufficient lubri- 
 cation or excessive bearing-pin or gear 
 wear, (2) improperly adjusted set screws, 
 
 (3) broken throttle rods and stops, 
 
 (4) fatigued inner springs, (5) excessive 
 friction in linkages, (6) engine parts 
 rubbing on throttle rod, (7) slipping 
 drive belts, or (8) pilot-valve plungers 
 sticking because of dirt in the channels. 
 
 POSITIVE-SHUTDOWN DEVICES 5 
 
 Several overspeed and emergency intake- 
 air-shutdown protective devices are 
 
 -*This section includes considerable in- 
 formation from technical and product lit- 
 erature from A^DT Controls, Richmond, CA; 
 Detroit Diesel Div. of General Motors 
 Corp., Detroit, MI; Progress Equipment 
 Co., Inc., Houston, TX; Pyroban Ltd., 
 Sussex, England; and Special Products 
 Div, of Otis Engineering Corp., Dallas, 
 TX. 
 
 available for diesels used in underground 
 gassy mines. All of these are based on 
 the premise that either the governor can 
 fail or combustible fuel from an uncon- 
 trolled source can self-ignite during the 
 diesel cycle. If methane were to ignite 
 under compression, or if the fuel system 
 is not failsafe, protection would be pro- 
 vided with an air shutoff system. 
 
 The most common air~shutoff device is 
 an air flapper valve. Upon release of a 
 
10 
 
 latching mechanism, a thin steel plate 
 covers the air inlet. An electronic 
 speed switch (a speed-sensing device) can 
 be used to trigger the latching mecha- 
 nism. Another device uses a spring- 
 loaded poppet valve to block the air 
 inlet. This valve is entirely self-con- 
 tained and does not need an external 
 power source. It is fitted upstream of 
 the air-intake manifold and actuated by 
 an air-pressure differential. Closure 
 occurs when the pressure difference 
 across the valve becomes great enough to 
 overcome the spring tension, which is set 
 at the desired overspeed limit. Little 
 or no modification is needed to adapt the 
 design to a diesel's air-intake manifold. 
 
 Other types of air-control devices are 
 maintained in the open position by an ac- 
 tuator supplied with engine oil at normal 
 pressure through an orifice. In this 
 design, oil pressure keeps three valves 
 closed. Should the exhaust-gas tempera- 
 ture, coolant temperature, or engine 
 speed exceed preset limits, one of the 
 valves would open and return oil to the 
 engine. With the loss of oil pressure, 
 the actuator would close the air valve 
 and stop the engine. Another triggering 
 device is an electronic impulse that 
 operates an actuator in a cylinder. A 
 spring-loaded release mechanism, trig- 
 gered by a gas-operated cylinder, closes 
 the air valve. Another approach to keep- 
 ing air out of the engine is to use (CO2) 
 to displace intake air. The inert CO2 
 prevents combustion in the chamber. 
 
 The air-shutoff devices described above 
 are activated by certain engine condi- 
 tions such as overspeed, high tempera- 
 ture, etc. Use of a methane monitor 
 could be another triggering method for 
 air shutoff and may be appropriate where 
 diesel operate in gassy atmospheres. The 
 monitor would be set at the 1.0-pct 
 methane-in-air concentration specified in 
 30 CFR 57. In noncoal mines, this regu- 
 lation prohibits the operation of diesel 
 equipment if the methane concentration 
 exceeds 1.0 pet in any active working 
 area. The methane monitor could be used 
 to simultaneously shut off air and fuel 
 
 as an additional safeguard to the normal 
 governor shutoff mechanism. 
 
 Use of positive-shutdown devices on 
 diesel engines operating in hazardous 
 areas where gassy air mixtures may occur 
 is a common practice in industry and is 
 required by MSHA in underground mines. 
 Current U.S. regulations require the use 
 of a manual air shutoff that is operable 
 from the operator's compartment (_3 ) . In 
 Canadian underground coal mines, mobile 
 diesels are required to have a manually 
 operated air-shutoff valve that automati- 
 cally stops the engine after the actua- 
 tion of fuel shutoff in the safety shut- 
 down system (31). In the United Kingdom 
 (U.K.), the approval of diesel-powered 
 equipment used in underground mines re- 
 quires the air-inlet systems to control 
 the effct of methane in the intake air 
 on the engine's ability to continue to 
 run after the fuel supply has been cut 
 off (32). Where an air-shutoff valve is 
 fitted, it must automatically cut off the 
 air supply to stop the engine if the en- 
 gine speed exceeds the governed speed by 
 more than 20 pet ( 32 ) . 
 
 Recognition of the potential danger of 
 diesels operating in hazardous areas is 
 also found in other industries ( 10 , 33). 
 Incidents in the petroleum and petrochem- 
 ical industries have shown that diesel 
 engines can provide a source of ignition 
 for flammable vapors and can also create 
 a hazard by overspeeding as a consequence 
 of ingesting of these vapors (33). In 
 order to provide a consistent approach in 
 formulating safety requirements for the 
 abatement of such hazards , the Oil Compa- 
 nies Materials Association (OCMA) has 
 developed recommendations for the pro- 
 tection of diesel engines used in poten- 
 tially hazardous areas. Those who pre- 
 pared these recommendations considered 
 the regulations of the U.K. Safety in 
 Mines Research Establishment (SMRE) and 
 those of the French and West German 
 Governments ( 33 ) . Current practices and 
 intentions of U.K. petroleum and petro- 
 chemical companies were also considered. 
 OCMA recommends the use of a manual air- 
 shutoff device as sufficient for abating 
 
11 
 
 the hazards of flammable vapor inges- 
 tion by an attended engine but con- 
 siders an automatic device to be neces- 
 sary for unattended machines (33). This 
 
 recommendation represents a standard of 
 good practice and exists as mandatory in 
 the regulations and/or codes of practice 
 in other countries. 
 
 SUMMARY 
 
 Conceivably, a diesel engine operating 
 in a gassy underground mine could ingest 
 a methane-air mixture from the mine at- 
 mosphere that could cause the engine 
 speed to increase, resulting in engine 
 overspeed or runaway. 
 
 High-speed diesel engines such as those 
 used in mobile mining equipment are gov- 
 erned not to overspeed (exceed the rated 
 engine speed) under any load condition. 
 The maximum operating speed for these 
 diesels is approximately 150 pet of the 
 rated engine speed; at higher (runaway) 
 speeds, excessive inertial loads will 
 destroy the engine. The speed-limiting 
 device used in these engines, a mech- 
 anical-type fuel governor, is designed to 
 maintain a desired operating speed as 
 well as completely shut off injected die- 
 sel fuel in the event of overspeed. Com- 
 plete fuel shutoff occurs when the rated 
 engine speed is exceeded by more than 20 
 pet. The normal time between a signifi- 
 cant change in engine load and the gover- 
 nor's response to maintain the balance 
 speed or stop diesel fuel injection is 
 nearly instantaneous. 
 
 The fuel governor plays a critical role 
 in controlling a possible overspeed con- 
 dition. At the onset of methane inges- 
 tion, combustion of the gas-air mixture 
 is initiated by the compression ignition 
 of the diesel fuel supplied in the normal 
 manner. Even if the governor is success- 
 ful at cutting off the diesel fuel flow, 
 the question remains whether the methane- 
 air mixture would autoignite under the 
 diesel cycle — without the aid of pilot 
 (diesel) fuel injection — and continue to 
 run the engine to destructive speeds. 
 
 Methane makes a poor fuel for diesel 
 engines owing to its low compression- 
 ignition quality (as indicated by cetane 
 nimber) and will not normally ignite un- 
 less the compression ratio is high or an 
 
 ignition source is present. The diffi- 
 culty of methane combustion through com- 
 pression ignition has been well demon- 
 strated in single-cylinder engine tests 
 and duel-fuel engine experiments, Diesel 
 engines will not operate on methane alone 
 under normal conditions. Even with a hot 
 injector tip as an ignition source, one 
 dual-fuel engine still required a small 
 amount of diesel fuel to initiate combus- 
 tion. Motoring tests on a variable- 
 compression-ratio engine produced contin- 
 uous knocking combustion at compression 
 ratios in the range of mining diesels, 
 but this was achieved at the most sensi- 
 tive methane concentration (stoichiomet- 
 ric) and at abnormally hot engine condi- 
 tions. Other tests have determined that 
 this knocking combustion causes some en- 
 gine damage, but insufficient power to 
 produce runaway. 
 
 Considering only the possibility of 
 diesel fuel ignition of the combustible 
 mixture, an uncontrolled engine is im- 
 probable. However, undesirable engine 
 behavior can result from the cyclic be- 
 havior in which the governor tries to 
 maintain the balance speed, leakage from 
 remaining fuel in the injection system 
 after fuel shutoff, and the extent of 
 overspeed. There are also potentially 
 dangerous engine conditions that could 
 sustain methane combustion. Although 
 such conditions are abnormal, the engine 
 could achieve destructive speeds because 
 the fuel governor, under certain condi- 
 tions, has no control over the ignition 
 source, A malfunctioning governor would 
 worsen the problem. 
 
 Mobile machinery receive much abuse in 
 the mining industry and poorly maintained 
 engines are not uncommon. The engine's 
 fuel governor does not have complete con- 
 trol over the effects of combustible 
 methane-air mixtures that could be en- 
 countered in underground gassy mines. 
 
12 
 
 Requirement of air shutoff systems for 
 diesels operating in hazardous areas 
 where gassy-air mixtures may occur is 
 currently a readily accepted practice. 
 Potential engine damage, personal in- 
 jury and possible mine explosions all 
 prescribe an air shutoff system as a 
 reasonable requirement. The available 
 
 literature and the lack of documented ac- 
 cidents suggest that a manual device is 
 sufficient control for the potential haz- 
 ards of methane ingestion by an engine in 
 an attended vehicle with a trained opera- 
 tor. For an unattended engine an auto- 
 matic air shutoff system should be used. 
 
 REFERENCES 
 
 1. LeFranc, C. I. (MSHA) . Private 
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 2. Brash, J. K. (MSHA). Private com- 
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 f rom L. C. Mejia, BuMines, Minneapolis, 
 MN. 
 
 3. U.S. Code of Federal Regulations. 
 Title 30 — Mineral Resources; Chapter 1 — 
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 Part 36 — Mobile Diesel-Powered Transpor- 
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 7. U.S. Mine Safety and Health Admini- 
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 9. Zabetakis, M. G. Flammability 
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 12. Stewart, D. B. , J. P. Morgan, and 
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13 
 
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 20. Straubel, M. The Robert Bosch In- 
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 1979, SAE preprint 790901, 11 pp. 
 
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 Robert Bosch GmBH, Dep. for Tech. Publ. 
 (Stuttgart, Federal Republic of Germany), 
 1975, 48 pp. 
 
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 23-27, 1981, SAE preprint 810516, 10 pp. 
 
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 1981; available upon request from L. C. 
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 available upon request from L. C. Mejia, 
 BuMines, Minneapolis, MN. 
 
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 BuMines, Minneapolis, MN. 
 
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 29. Finkbiner, K. B. (Cummins Engine 
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 available upon request from L. C. Mejia, 
 BuMines, Minneapolis, MN. 
 
 30. Wimp, J. W. (Woodward Governor 
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 available upon request from L. C. Mejia, 
 BuMines, Minneapolis, MN. 
 
 31. Dainty, E. D. , and J. P. Morgan. 
 The Certification of Flameproof Diesel- 
 Powered, Rubber-Tired Trackless, Self 
 Propelled Vehicles for Use in Underground 
 Coal Mines in Canada. Canada Center for 
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 Rep. ERP/MRL 79-68 (TR) , July 1979, 
 83 pp. 
 
 32. Health and Safety Executive (Lon- 
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 Diesel and Storage Battery Powered Loco- 
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 of Diesel Engines in Hazardous Areas. 
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 MEC-1, 1977, 14 pp. 
 
 ftU.S. CPO: 1984-505-019/5070 
 
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