INDUSTRIAL AND ENGINEERING CHEMISTRY
The writer gratefully acknowledges the assistance of many members of the Research Department for much help in collecting data presented in this paper, LITERATURE CITED
Vol. 43, No. 12
(2) Elliott, M. A, S.A.E. Quort. Truns., 3, 490 (1949). (3) Grunder, L. J., ‘ T e s t G a s t Diesel Smoke and Odor Problems,” paper presented at the Y.A.E. Diesel-Engine Meeting, Si,. Louis, Mo., N o v . 2 , 1949. (4) Holtz, John C., Berger, L. B., Elliott, M. A., and Schrenk, 11. H., U. 8. Bur. Mines, R e p f . Inoest. 3508 (1940). ( 5 ) Landen, E. W., S.A.E. Quart. Trans., 3,200-6 (1949).
(1) Boerlage, G. D., and Rroeae, J. J., IND.ENG.CHEM.,28, 1229
RECEIYED April 19, 1951.
DUAL-FUEL COMBUSTION IN DIESEL ENGINES MARTIN A . ELLIOTT AND ROGERS F. DAVIS U. S. Bureau o f Mines, Bruceton, Pa.
Synthetic Liquid fuels Branch,
Current interest in the commercial application of dualfuei combustion in Diesel engines has prompted an interpretation of data on this type of combustion obtained incidental to studies of the hazards of operating Diesel engines in flammable gas-air mixtures. The lower limit of flammability of the combustible gas in the compressed gas-air mixture is an important consideration in dual-fuel combustion. If the concentration of gas is greater than the low7er limit, flame propagates throughout the gas-air mixture and the attendant rapid rate of heat release results in engine operating difficulties and “knock.” If the concentration of gas is below the lower limit of flammability, the gas does not react completely with oxygen unless it is in or immediately adjacent to an inflamed or high temperature region. The fraction of gas reacting increases with an increase in either liquid fuel-air ratio or concentration of gas and is affected by type of engine and engine speed. The results of tests made with natural gas, propane, butane, and hydrogen can be applied to the design and operation of dual-fuel combustion systems.
VAL-fuel combuetion as applied to the Diesel engine signifies the simultaneous combustion of gaseous and liquid fuel. When a Diesel engine operates as a dual-fuel engine, the gaseous fuel predominates and generally is mixed uniformly with the intake air. The liquid fuel is injected into the compressed, homogeneous, gas-air mixture and furnishes a source of ignition for this mixture. From this brief description, it is apparent that an understanding of the fundamentals of dual-fuel combustion depends on knowledge of the properties and behavior of the gasair mixture under the heterogeneous conditions that attend combustion in the Diesel engine. Information of this type has not been generally available, although it is extremely helpful in establishing proper operating conditions for dual-fuel Diesel engines. The purpose of this paper is to furnish basic data that will be useful in solving combustion problems in such systems. Dual-fuel engines ma? utilize any type of gaseous fuel, but natural gas is most frequently used, although the use of manufactured gas and sewage-disposal gas has been mentioned (1). The extensive application of dual-fuel engines is a comparatively recent development in the United States. In the large Dieselengine field (8 to 32 inch bore, 84 to 1200 r.p.m.) approximately 300,000 horsepower in dual-fuel engines had been installed up to July 1950 (8). Comparable statistics are not available for the Diesel engines of higher speed and smaller bore, but many dualfuel applications have been reported in this field. The economic
importance of the dual-fuel development is evident vhen it is realized that in areas where the cost of natural gas is low it is possible to show savings as high as s%20,000 per year for every 1000 horsepower installed ( 2 ) ?Then dual-fuel combustion is compared with combustion of Diesel fuel alone. Information on dual-fuel combustion has been obtained by the Bureau of Mines in connection with studies of the hazards of operating Diesel engines underground. In certain studies natural gas was added to the engine intake to simulate operation iii gassy atmospheres. These tests had as their principal objective the evaluation of the toxic exhaust-gas hazard and the ignition hazard presented by a Diesel engine operated under these conditions. The results are also of interest in relation to dual-fuel combustion in Diesel engines. Many of the data presented here were obtained incidentally in other studies, and the present, paper is not an exhaustive study of t’he subject. The results illustrate ~ l e a r l ysome important factors to be considered when a Diesel engine is operated on a mixture of gaseous and liquid fuel. The present discusion of dual-fuel combustion is limited to Diesel engines in which gaseous fuel is mixed with the intake air and the source of ignit,ion is flame or Oxidation reactions from combustion of liquid fuel. The combustion process of the liquid fuel may have a significant effect upon combustion of the gaseous fuel. Therefore, it is desirable to review briefly the combustioii process in a Diesel engine operating on liquid fuel alone before discussing the more complex dual-fuel combustion. COMBUSTION IN DIESEL ENGINES OPERATING ON LlOUlD FUEL
There are a t least three basic requirements in any combustion process: formation of a mixture of fuel and air, ignition of the fuel-air mixture, and completion of combustion of the fuel-air mixture. I n the Diesel engine the fuel-air mixture is formed by disintegration or atomization of a continuous jet of fuel moving a t a high velocity, mixing of droplets of fuel with air, vaporization of fuel, and mixing of fuel vapor with air. Ignition occurs as the result of direct oxidation of the fuel, the rate of which is comparatively slow a t first and rapidly accelerates until the initial inflammation. In the inflammation phase, combustion proceeds either by the rapid oxidation of mixtures of fuel and air or by the oxidation of products of thermal decomposition of the fuel. These stages of combustion have chronological significance for a particular fuel particle, but there is no delineation of stages in the process as a whole. In other words, many of the individual steps occur simultaneously because of the inevitable heterogeneous conditions attending the injection of liquid fuel into either air or an inflamed fuel-air mixture.
I I SECTION
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
Schematic Representation of Fuel Distribution and Mechanism of Inflammation In a m a l l local region in Combustion chamber of Diesel engine
The heterogeneity of the fuel-air mixture has a significant effect upon the normal combustion process in the Diesel engine and also upon the dual-fuel combustion process. If a small enough volume of the combustion chamber could be examined just before ignition, it probably would contain droplets of fuel, fuel vapor unmixed and mixed with air, and air containing no fuel. This i3 shown diagrammatically in Figure 1, A , which emphasizes the extreme fuel-concentration gradients existing in the Diesel combustion chamber. After ignition occurs, flame propagates through continuous regions in which fuel vapor and air are mixed. Adjacent regions containing a mixture of fuel vapor and air will ignite, either by contact uith the advancing flame front from an ignited region or by autoignition accelerated by increases in temperature resulting from radiant heat transferred from a flaming region. Regions in which fuel is not yet vaporized or in which fuel vapor is not mixed with air will undergo extensive thermal decomposition, producing products that must be mixed further with air. Similarly, regions in which there is a deficiency of air also produce some products that must be mixed further with air before oxidation is complete. These points are illustrated diagrammatically in Figure 1, B , which also shows the extreme temperature gradients that may exist in Io’cal regions, These temperature gradients are significant in relation to dualfuel combustion. Their existence has been demonstrated by Landen’s (8) pictures of combustion in a Diesel engine, which showed streaks or striations of light caused by incandescence in flaming regions. These streaks were separated by dark lower temperature regions, which presumably consisted of air containing substantially no fuel. In the normal operating range of the Diesel engine (fuel-air ratios from approximately 0.01 to 0.055 pound of fuel per pound of air) combustion is substantially complete (6), although the exhaust gases contain low but significant concentrations (generally less than 0.1%) of such products of incomplete combustion as carbon moaoxide and aldehydes. However, in the normal operating range of the Diesel engines tested by the Bureau of Mines, and with the intake consisting of normal air, methane and hydrogen generally have not been detected when the exhaust gases were analyzed in a Haldane apparatus. In the few in-
stances in which methane has been detected, its concentration was less than 0.05%. This is an important observation in relation to dual-fuel combustion, because if methane or other low-boiling hydrocarbons are detected in the exhaust gas from an engine operating with a hydrocarbon gas added to the intake, it can be concluded that some of the gaseous fuel is passing through t h e engine unreacted. The foregoing outline omits many important details of the normal Combustion process in the Diesel engine, which are discussed elsewhere (4, 6). The information p r e s e n t e d here was selected to emphasize those aspects of the process that are significant in relation to the dual-fuel combustion process.
C O M B U S T I O N IN DIESEL E N G I N E S O P E R A T I N G ON G A S E O U S AND L I Q U I D FUEL
The operation of a Diesel engine on a mixture of gaseous and liquid fuel differs from operation with liquid fuel alone, in that part of the fuel enters the engine as gas and is mixed uniformly with the intake air. Therefore, the concentration and type of gas are important factors to be considered in dual-fuel combustion along with the liquid fuel-air ratio, which is one of the most important variables in the normal combustion process in the Diesel engine. The effect of any variable on a combustion process that is inherently efficient can be evaluated most accurately and conveniently by determining precisely the products of incomplete combustion in the exhaust gases from the process. In the present investigation this was done by analyzing the exhaust gases in a Haldane-type apparatus. EFFECT OF C O N C E N T R A T I O N AND TYPE O F G A S
The results of a series of tests in which natural gas was added to the intake of a CFR Diesel engine are presented in Table I. Figure 2 shows the proportion of natural gas that reacts as a function of the concentration of natural gas in the intake and the ratio of liquid fuel to air. In these tests, selected liquid fuel rates were held constant to determine the effect of the concentration of natural gas in the intake. The engine was operated a t 900 r.p.m., the liquid fuel being injected 13 crank angle degrees before top center. Commercial Diesel fuel was used. A typical natural-gas analysis (by mass spectrometer) is as follows: Constituent CH4 CzHa CaHa C4HlO CZH4 CyHa C4H8
% by Volume 88 9 7 3 2 3 0 3 0 1 0 5 0 1 0.3 0.2
Figure 2 shows that the proportion of natural gas reacting is a function of both the liquid fuel-air ratio and the concentration of natural gas in the intake. Considering, first, the effect of the concentration of natural gas, it will be observed that a t a given liquid fuel-air ratio the proportion of natural gas reacting in-
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atmospheric pressure) on the loa-er limit of flammability of methane in air. In connection with the so-called lower limit of flammability of natural gas under conditions esisting during combustion in the engine, it is of interest to note that a value of 5% natural gas in air has been given (3) as the 1on;er limit of flammability in B commercial 2-stroke-cycle engine, and a value of 4.2% has been given (3)as the "lean limit" below which combustion of the gas was considered unsatisfactory. The excellent agreement between the value for the lower limit obtained in a large-bore, 2stroke-cycle engine with the value obtained in a small-bore, 4stroke-cycle CFR Diesel engine shows that this basic property of the gas-air mixture is not affected to a great extent by the type of engine. Effects similar t? those shown in Figure 2 were observed in tests made with commercial propane and commercial butane in the CFRengine operated a t a 21 to 1compression ratio, 900 r.p.m., and with a liquid fuel-air ratio of 0.0066 pound per pound (see Table I1 and Figure 3). The concentration of gas in the intake a t which substantially all of the gas reacts is approximately 1.9% for propane and 0.9% for butane. The lower limit of flaniW Y
2 100 5
NATURAL GAS IN INTAKE. PERCENT 8 Y VOLUME
Figure 2. Relation between Proportion of Natural Gas Reacting, Concentration of Gas in Intake, and Liquid Fuel-Air Ratio In trrb of CFR Diesel engine
creases as the concentration of natural gas increases, until substantially all of the natural gas is burned. This occurs at a concentration of about 4% with the higher liquid fuel-air ratios and 5% a i t h the lowest liquid fuel-air ratio studied. It is I apparent from these results that flanie does not propagate coms o I 2 0 CONCENTRATION O F G A S N I N T A K E , PERCEhT BY VOLUME pletely through the natural gas-air mixture, under conditions Figure 3. Relation between Proportion of Propane or existing during combustion in the Diesel engine, until some lowerButane Reacting and Concentration of Propane or Butane limit concentration is reached. This lower-limit concentration in Intake can be determined most reliably under these conditions when CFR Diesel engine operated at 21 to 1 compression ratio and liquid the li~uid-fuel air ratio is a minimum because of the marked luel-aii ratio of 0.0066 pound per pound effect of liquid fuel-air ratio on the proportion of natural gas reacting (see Figure 2 ) . The Table I. Results of Tests with Natural Gas A d d e d to Intake of CFR Diesel Engine at 900 R.P.M." minimum liquid fuel-air ratio that gives consistent ignition Aldehydes Concn. in Liquid5 Reacting6 Gas Peak of Gas in in the CFR engine is about Intake, Coinposition of Exhaust Gas % Exhaust, Fuel-Air Fuel-Air Reacting, Pressure, Test yo by ~701~me P.P.M. by Ratio, Ratio % of Gas Lb./Sq. 0.0066 pound of fuel per pound KO. Volume COa 02 CO CnIInn 2 iY2 Volume Lb./Lb. Lb./Ld. in Intake InchAbs of air at a cornpression ratio 0.00 68 15J 0.00 ... 674 1.18 19.27 0.14 of 21 to 1. With this liquid 111 0 . 3 3 33.5 0.50 708 l5B 1.16 1 8 . 9 8 0 . 2 2 0 . 5 5 178 51.5 1 . 1 6 718 1.42 15C 15.44 0 . 2 6 fuel-air ratio, the lower limit 0.72 167 56.9 1.71 734 l5D 1 . 9 7 17.39 0 33 870 118 3.38 0.68 of natural gas is approximately 15E 78.7 3.66 14.47 0 . 3 0 4.17 1008 0.48 87.6 97 4 . 9 1 12 23 0.34 15F 5%. The calculated lower 1126 0.12 97.3 9.11 0 . 1 1 'ff4 6.88 4.88 l5G 1142 5.85 0.12 97.7 l5H 8 . 2 8 6.40 0 . 1 4 limit of flammability in air at 536 13A 0.0 1 . 6 8 18.47 0 . 2 3 0.00 19.62 55 0.011 0.0110 .. . normal pressure and tenipera596 l3E 0.61 2 . 1 8 17.60 0 33 0.25 (9.74 46 0.011 0,0120 61.0 595 ture of a natural gas having 13B 1.02 2.34 17.03 0 . 3 5 0.34 79.94 94 0.011 0.0133 65.9 616 0.011 0,0202 58 82.2 80.86 0.37 3.80 14.67 q . 3 0 13F 2.17 the composition given in the 0,011 0.0269 91 91.8 731 5.03 1 2 . 4 8 0 . 3 1 0.25 81.93 13C 3.25 0 , 0 1 1 0,0334 0 . 1 2 83.31 158 9 6 . 8 834 0 . 1 8 9 . 5 1 13G 4 . 1 7 6 . 8 8 foregoing is 4.8%. The small 0.011 0,0406 170 97.8 982 84.54 0.10 13D 5.36 8.27 6.97 0 . 1 2 0.0403 97.9 1062 80 0.011 0.11 85.74 difference between these two 5.96 10.00 4 . 0 8 0.06 13H 652 values, in spite of the large 12A 0.0 3.69 l 5 , 6 5 0.21 0.00 80.48 17 0,020 0.0200 ., . 689 12F 1.046 4.34 14.40 0 . 2 4 0.15 80.87 19 0,020 0.0222 85.1 d i f f e r e n c e s between the re752 12G 2.14e 6 , 5 3 12.19 0 . 3 0 0.14 81.84 91 0.020 0.0285 93.1 863 12B 3.00e 6.62 10.32 0.18 0.11 82.77 25 0,020 0.0340 96.0 spective pressures and tem932 12C 4.14e 8 . 7 7 6.83 0.10 0.08 84.22 25 0.020 0.0432 97.8 996 peratures, agrees with observa12D 5.048 10.18 4 . 3 6 0 . 1 2 0.08 85,26 32 0,020 0.0498 98.2 1074 12E 6.04e 11.50 2 . 0 3 0 . 1 0 0.10 86.27 31 0.020 0.0565 98.1 tions of others (9, 10) which a Fifteen series of tests at, 21 t o 1 compression ratjo; al1,other tests a t 16 t o 1 compression ratio. have s h o r n a comparatively b Calculated from liquid fuel rate and volumetric efficiency. C Calculated from liquid fuel rate and exhaust gas analysis. small effect of pressure (up to d Not determined. 10 atmospheres) and teme Calculated from metered gas rate and volumetric efficiency. perature (up to 700" C. a t
December 1951 Table
I!. Results of Tests with Propane and Butane A d d e d to Intake of a CFR Diesel Engine Operating at Compression Ratio of 91 to 1
Conon. of Gas in Intake, Test % by No. Volume
P1 P2 P3 B1 B2 B3
1 2C 1 9C
Aldehydes in Liquid5 Composition of Exhaust Gas, % b y Exhaust, Fuel-Air Volume P.P.M. by Ratio COZ 0% CO CnHm+z Nz Volume Lb./Lb. 80 60 63 0 0066 0 2OC 3 49 15 30 0 41 82 3 1 79 0 0066 0 06C 6 54 10 90 0 19 83 92 301 0 0066 0 066 8 98 6 89 0 15 0 0066 79 123 79 0 08d 1 82 17 80 0 51 81 31 92 0 0066 0 034 4 74 13 62 0 30 159 0 0066 82 14 0 03d 6 42 11 73 0 08
0 4d 0 9d 1 2d 4 Calculated from fuel rate and volumetric efficiency. b Calculated from fuel rate and exhaust gas analysia. C Propane. d Butane.
mability of these gases in air a t normal temperatures and pressure is 2.4% for propane and 1.9% for butane, The so-called lower limits of propane and butane as determined in the CFR engine are less than the values determined a t normal pressures and temperatures, probably because of the higher reactivity of these gases when compared qith methane. Additional information on the so-called lower limit of flammability of gases under the conditions existing during combustion in the Diesel engine was obtained in tests in which hydrogen was added to the intake of a 4-stroke-cycle commercial Diesel engine having a turbulence combustion chamber (engine B). These data are piesented in Table 111. Figure 4 shows the proportion of hydrogen reacting ais a function of the concentration of hydrogen in the intake and the liquid fuel-air ratio. From the results a t the minimum liquid fuel-air ratio it appears that the lower limit of hydrogen under these conditions is between 10 and 11% by volume. Repeated backfires were observed in the intake manifold when attempts were made to operate the engine with concentrations of hydrogen in the intake ranging from 12 to 14%, These backfires resulted from the ignition of the intake mixture when the intake valve x a s opened. The observations of backfiring and the exhaust gas analysis indicate that the lower limit of flammability of h y d r o g e n 4 mixtures in the engine tested is approximately the same for conditions a t the end of compression as it is for conditions existing in the intake system when the intake valve opens. The lower limit of hydrogen in air a t normal temperatures and pressures is 4.1% which is considerably less than the value determined under engine conditions.
Peak Reacting6 Gas Pressure, Fuel-Air Reacting, Lb./Sq. Ratio, of Gas Inch Lb./Lb. in Intake Abs. 0 0185 82 8 821 0 0324 96 6 1089 0 0436 97 4 1421) 0 0132 79 7 792 0 0300 96 5 1139 0 0382 97 4 1374
fso z W 0
80 Y i
z W 0 1
rotio, pound per pound
J 2 6 IO HYDROGEN IN INTAKE, PERCENT BY VOLUME
Figure 4. Relation between Proportion of H drogen Reacting, Concentration of Hydrogen in Intaie, and Liquid Fuel-Air Ratio Tests of engine
OF LlOUlD FUEL-AIR RATIO
nificant effect a t gas concentrations less than the so-called lourerlimit value. The effect of liquid fuel-air ratio can be presented more clearly by replotting the results of Figure 2 to show the proportion of natural gas reacting as a function of the liquid fuel-air ratio (see Figure 5 ) . Figuie 5 shows that the proportion of natural gas reacting increases very rapidly with increasing liquid fuel-air ratio a t the lowqr liquid fuel-air ratios, the rate of increase being greater a t the higher concentrations of gas. Nearly all of the gas reacts, regardless of gas concentration, a t a liquid fuel-air ratio of about 0.033 pound per pound (50% of the stoichiometric value). In the absence of liquid fuel, stoichiometric natural gas-air mixtures do not react to A significant extent in the CFR engine a t a compression ratio of 21 to 1. Under these conditions, carbon dioxide, carbon monoxide, or aldehydes were not detected in the exhaust
The foregoing results and discussion show that the concentration of gas is an important factor in, dual-fuel combustion in the Diesel enginefirst, because a t concentrations of gas below the lower limit of flammability a s i g n i f i c a n t proportion of the gas does not react; and, secondly, because a t gas concentrations a t or above the lower limit the y p i d rate of reaction of the gasair mixture can result in cucessive rates of pressure rise and so-called “knocking.”
As shown in Figure 2, the liquid fuel-air ratio also has a sig-
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Figure 7. Relation between Proportion of Natural Gas Reacting, Liquid Fuel-Air Ratio, and Concentration of Gas in Intake Ensine B at 600 r.p.rn.
during its history was present in ii region of intense combustion or, in other words, present in an inflanied region. Therefore, if the results of Figure 2 are extrapolated to zero concentration of natural gas, we obtain a value for the proportion of the fuel-air mixture t'hat is traversed by flame a t some time during t,he coinbustion of liquid fuel alone. The result of such an extrapolation is shown in Figure 5 , Thus, the results of operation on liquid and gaseous fuel furnish a criterion of the combustion perfornia i m of an engine operating on liquid fuel alone. Regions of intense combustion occupy only a portion of the combustion space a t the lower liquid fuel-air ratios, because fuel can find the oxygen for completing its combustion without' traversing the entire combustion space. A s the fuel-air ratio approaches thrx stoichiometric value, an increasing proportion of the fuel-air mixture must be inTable 111. Results of Tests with Hydrogen A d d e d to Intake OF Engine B at Approximately 700 R.P.M. flamed, because an increasing Concn. p r o p o r t i o n of t h e oxygen of HydroBldehydes Gas presrnt is required for com&en in in Liquid" Reactinga Reacting, Intake, Exhaust Fuel -Air Fuel-Air 7' of plete combustion of the liquid ,rest % by C o m p o s i t l o n g E x h a u s t Gas, % by Volume p,p.&f.b; Gas in Ratio, Ratio, fuel. Lb./Lb. Iiltake Lb./Lb. COz 0 2 CO CnHan+ a Hz Nz Volume KO. Volume 0 . 0 0 79.91 6 0.0098 0.0098 0.00 I n dual-fuel combustion the 1 0.00 1 . 9 0 18.02 0 . 1 7 12 0.0106 0.0109 5k:9 2.07 17.67 0.13 0.03 0.39 79.71 2 0.90 proportion of the fuel-air mix10 0.0082 0,0092 51.5 0.02 1 . 2 9 79.17 3 2.74 1 . 5 9 17.82 0.11 0.02 1 . 9 6 79.57 , , 0.0108 0.0128 57.9 4 4.92 2 . 1 8 16.22 0 . 0 5 ture that is inflamed a t a 2.23 79.67 10 0,0099 0,0124 63.8 0.00 9 6.64 1 . 9 4 16.10 0 . 0 6 0.00 0.0144 78.3 1 . 4 9 81.46 1 0,0100 10 7.64 2.10 14.94 0 . 0 2 given liquid fuel-air ratio in91.9 1.94 13.87 0.04 0.00 0.67 83.48 1 0.0093 0.0159 11 9.58 creases as the concentration 5 0.0 3 . 8 6 15.58 0 . 0 8 0.00 0.00 8 0 . 5 8 13 0,0184 0.0184 , 6 1.01 4 . 0 2 14.85 0.07 0.05 0.26 80.65 18 0.0195 0.0200 73:5 of natural gas increases (at ' 7 2.81 4 . 3 9 13.83 0 . 0 8 0.04 0.74 80.92 10 0.0217 0.0233 72.2 gas concentrations beloF the 8 5.14 4.29 13.29 0 . 1 0 0.03 1 . O Q 81.20 10 0.0205 0.0231 77.1 s o - c a l l e d lower limit. See 2 0 0291 0.0304 81.1 12 2.13 6 . l b 11.36 0.16 0.00 0 . 3 8 81.96 0.42 8 2 . 0 3 20 0.0281 0.0300 84.7 13 2.94 5.94 1 1 . 4 8 0.10 0.03 Figures 2 and 5 ) . This oc14 1.24 8 20 8 . 3 7 0 . 2 3 0.04 14 0.0390 0.0405 85.4 0.17 82.99 curs because the natural gas 16 2.93 8 . 2 0 8.17 0.20 0.04 0.16 83.23 10 0 0386 0.0407 94.1 burning in inflamed regions Calculated from exhaust gas analysis. extends these regions just as gas. Therefore, natural gas must be in a comparatively high temperature region j f it is to react when its concentration is less than the so-called lower limit concentration. Conversely, the presence of unreacted natural gas in the exhaust gases from the engine indicates that relatively cool regions existed Tithin the combustion space during combustion. Thus, the results s h o w in Figure 3 present additional evidence of the existence of extreme temperature gradients during combustion in the Diesel engine. This has also been indicated hy photographs ( 8 ) of combustion in the engine, which have shown that certain regions in the combustion space are not traversed by flame. It appears that the proportion of natural gas reacting is a measure of the fraction of the fuel-air mixture that at, some time
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they are extended by increasing the liquid fuel-air ratio. This extension increases as the concentration of natural gas increases and is manifested by an increase in the proportion of natural gas reacting. The foregoing results and discussion show the importance of liquid fuel-air ratio in dual-fuel combustion in the Diesel engine when the concentration of gas is less than the so-called lower limit of flammability. Under such conditions, flame does not propagate throughout the gas-air mixture, and therefore the gas reacts only when it is in or adjacent to an inflamed region. Obviously, the liquid fuel-air ratio does not affect the completeness of combustion of the gas-air mixture when the gas concentration is above the lower limit of flammability and the over-all fuel-air ratio is less than the stoichiometric ratio. EFFECT O F TYPE OF E N G I N E AND E N G I N E SPEED
Figure 8. Relation between Proportion of Natural Gas Reacting, Liquid Fuel-Air Ratio, and Concentration of Gas in Intake Engine B at 2600 r.p.m.
The extensiveness of the inflamed regions existing during combustion is likely to be affected by type of engine and engine operating conditions. In Figures 6, 7, and 8, the proportion of natural gas reacting is shown as a function of liquid fuel-air ratio for two different 4-stroke-cycle engines. These figures are based on interpolation of previously published results ( 7 ) . The circled points shown in Figure 7 are extrapolations made from the results of tests on the addition of hydrogen t o the intake of engine B a t 700 r.p.m. The general agreement between the extrapolation based on natural gas and that based on hydrogen is good. Comparing these figures with Figure 5, it will be observed that all the curves have essentially the same shape; however, the actual proportion of natural gas reacting depends upon the type of engine (compare Figures 6 and 7) and on the engine speed (compare Figures 7 and 8). The effect of type of engine and engine operating conditions can be compared most clearly by showing the proportion of the fuel and aip mixture that is traversed by flame as a function of liquid fuel-air ratio when liquid fuel alone is burned. Such a comparison is made in Figure 9, which is based on extrapolation, of dual-fuel combustion results t o a zero gas concentration. Figure 9 shows that a t the lower liquid fuel-air ratios the proportion of the fuel-air mixture traversed by flame is substantially the
LIQUID FUEL: AIR RATIO, POUND PER POUND
Extent of Inflammation of Fuel-Air Mixture
In different engines and at different fuel-air ratio]
Relation between Proportion of Natural and Concentration of Gas in Intake 2-drake-cycle Dlerel engine
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Diesel engine. For example, it has been shown that gases exhibit so-called lower limits of flaninisbility under the conditions existing in the Diesel engine. Tests in 4-stroke-cycle cngines indicated that this lower limit for natural gas was between 4 and 5%. Above these values, substantially all of the gaseous fuel reacted regardless of other operating variahles. Figure 10 show that above a natural-gas concentration of about 5% there was no change in the fraction of natural gas reacting. If the socalled lou-er limit of natural gas were 5%, then a t greater gas concentrat'ions all the nat'ural gas that was present during combustion would react. Therefore, any unreacted gas must have passed t'hrough with the scavenging air. Because of the necessity of scavenging in the 2-stroke-cycle engine, it is not possible t o achieve complete reaction of gas if it is added to the intalic of 3uc.h engines. [In commercial 2-et1,oke-cycle dual-fuel engines gtrs is added directly t'o the combustion space ( S ) . ] Similar considerations apply t'o the results of the tests of the 3-cylinder engine, but t'here are not. enough data in the range of gas con(*erituitions of 5 to 6% to establiph the so-called loiver limit. 0
2 3 4 5 GAS IN INTAKE, PERCENT BY VOLUME
Figure 11. Relation between Concentration of Carbon Monoxide in Exhaust and Concentration of Gas in Intake CFR Diesol engine operated at 21 to 1 compression ratio
0 Nolurol g a s plus 0 0 0 6 6 pound liquid fuel per pound air
same for engine B and the CFIt engine but significantly grratcr for engine A. Engine -1has a precombustion chamber, whereathe other two engines have turbulence or air-swirl chamhers. In the precombustion chamber the fuel is injected into ahout 50% of the total air present, whe in the turbulence chainbcr the fuel is injected into about 90yoof the total air present. It would seem, therefore, that, on the average, local fuel-air ratios \~.oultl be much higher initially in the precombustion chamber thiin in the turbulence chamber. Higher local fuel-air ratios mean niow intense local inflammation regions, and this, along Rith thc secondary mixing that occurs as the inflamed fuel-air inist,urc issues from the precombustion chimiber, may account for thc higher proportion of the fuel and air mixture traversed by flanicl in the engine having a precombustion chamber. The cffect of increased speed in promoting turbulence and thereby increasing the proportion of the fuel-air mixture that is traversed by flame is indicated in Figure 9 by comparison of the results of tests of engine I3 ut GOO and 2GOO r.p.m. The rcaults shown in Figure 9 are derived froin dual-fuel combustion data; howver, they are of interest chiefly from the standpoint of the performance of a combustion system operating on liquid fuel alone. This suggests t h s t the study of certain heterogeneous combustion problems may be facilitated by the addition to the combustion air of a combustible gas in concentrations below the lower limit of flammability. It mould serin that hydrogen might be particularly suitable for this purpose. All the test results presented previously were obtained 011 4stroke-cycle Diesel engines. It is of intercst, therefore, t o coinpare these results with data shown in Figure 10, n-hich were obtained on 2-stroke-cj~cle engines with natural gas added t o the intake. Figure 10 s h o w that at a given liquid fuel-air ratio the fraction of natural gas reacting reaches a maximum of about 47% in the &cylinder engine and about 57% in the 3-cylinder engine. This maximum value is reached a t a natural gtw concentration in the int,ake of about 4 t o 5% in the 4-cylinder engine and remains substantially the same as the concentration of natural gas increases above 5%. The point a t which this masimum is reached is not clearly indicated in the results of the tests of the 3-cylinder engine. The results shown in Figure 10 can be explained on the basis of the characteristics of the 2-st,roke-cycle engine and the previously discussed behavior of a gas-air mixture during combustion in the
Propane plus 0 0 0 6 6 p o ~ n dliquid f u e l per pound air x Butone plus 00066 pound liquid fuel per pound air
Figure 12. Relation between Concentration of Aldehydes in Exhaust and Concentration of Gas in Intake CFR Diesel engine operated at 21 to 1 compression ratio
It \?':is shown above t'hat unreacted natural gas passes through the engine with the scavenging air. -4t natural-gas concentrations greater than the lower limit, the fraction of unreacted natural gas is therefore a measure of the fraction of the total air that is used for scavenging, because under these conditions substantially all t,he natural gas present during combustion will react. Therefore, by the addition of gaseous combustibles in concentrations above the lower limit to the intake of 2-strokecycle Diesel engine we may have a means for readily determining the proportion of the total air that is used for scavenging. This determination is not made so easily by other methods, and it appears desirable t o give more detailed consideration to the application of dual-fuel combustion techniques t o this problem. The results of Figure 10 are only suggestive, and further work on this application of t,he results is planned.
INDUSTRIAL AND ENGINEERING CHEMISTRY
In connection with the discussion of unreacted natural gas that passes through the engine with scavenging air, it is of interest to compare the results obtained with the 4-stroke-cycle CFR engine (see Figure 2). The natural-gas reacting never increased above 98% even a t concentrations above the lower limit. It is possible that chilling of the reacting mixture was responsible for the 2% unreacted gas, but it seems more likely that this gas passed through the engine during the period of valve overlap. This possibility should be investigated further t o determine whether dual-fuel combustion techniques may have an application in certain fluid-flow problems of the type just mentioned. EFFECT ON C O M P O S I T I O N OF EXHAUST GAS
Dkcusbion of the effect on exhaust-gas composition of the additio of combustible gas to the intake of a Diesel engine is limited t o considerations that relate to the behavior of the gas-air mixture during combustion. A detailed discussion of the exhaust gases produced when natural gas was added to the intake of engines Band B has been published ( 6 , 7 ) . Figure 11 shows the carbon monoxide concentration in the exhaust gas from a CFR engine operated a t a 21 to 1compremion ratio with a liquid fuel-air ratio of 0.0066 pound per pound and with different concentrations of natural gas, propane, and butane added to the intake. In every instance, the carbon monoxide concentration first increases as the gas concentration increases, then passes through a maximum, and finally decreases as the lower limit of flammability is approached. As the liquid fuel-air ratio is constant, it is unlikely that the addition of natural gas causes an increase in the carbon monoxide produced by the liquid fuel. Therefore, any increase in carbon monoxide must come. from partly reacting natural gas. The most probable source of the carbon monoxide from natural gas under the conditions of these experiments where excess air is present is incomplete direct oxidation reactions occurring under overlean conditions in regions of comparatively low temperature. The results in Figure 11 indicate that a maximum of about 10% of the methane or propane and about 15% of the butane reacted to form carbon monoxide. The concentration of aldehydes in the exhaust gas from the CFR engine under conditions comparable to those of Figure 11 is shown in Figure 12. The relation between concentration of aldehydes and concentration of gas in the intake is complex. It is possible that a similar relation would have been observed for propane if more tests had been made at the lower concentration of gas in the intake. All the relations shown exhibit a sharp increase in the aldehyde concentration a t a gas concentration in the vicinity of the so-called lower limit. It is of interest to compare the foregoing results on the variation of carbon monoxide concentration with those obtained in the tests with hydrogen added to the intake of engine B when it was operated a t the minimum liquid fuel-air ratio (see Figure 13). The concentration of carbon monoxide decreases as the concentration of hydrogen in the intake increases. This occurs because the combustion of hydrogen extends the regions of high temperature and minimizes the production of carbon monoxide by the direct oxidation of the liquid fuel in locally overlean regions. A similar effect probably exists in the tests with hydrocarbon gases. In connection Rith such tests, the results with hydrogen support the conclusion that a small but significant fraction of the hydrocarbon gas is partly oxidized, yielding carbon monoxide as a product. The foregoing discussion has shown the effect of concentration of gas in the intake on the concentration of certain exhaust gas constituents produced a t a given liquid fuel-air ratio. Under such conditions, the ratio of reacting fuel (oil plus reacting gas) to air increases as the concentration of gas increases. The reacting fuel-air ratio is an important factor affecting exhaust-gas composition and should be used as the argument i s shoving the
composition of the exhaust gas produced under a wide range of dual-fuel operating conditions. Figure 14 shows the concentrations of carbon monoxide, aldehydes, and oxides of nitrogen produred by engine B a t different reacting fuel-air ratios and different concentrations of gas in the intake. These results have been discussed in detail (6, 7 ) , and only certain points are mentioned briefly in this paper. The relations shown are typical of the results obtained on other engines tested by the Bureau of Mines and operating with natural gas added to the intake in concentrations below the lower limit of flammability The characteristics of the relation between carbon monoxide or aldehyde concentration and reacting fuel-air ratio when natural gas is present in the intake in concentrations below the loaer fhmmable limit are similar to those observed when a Diesel engine is operated on liquid fuel alone (see Figure 15). The shape of thebe plations may be accounted for qualitatively, as indicated in Figure 16.
VOLUME Relation between Concentration of Carbon
.HYDROGEN IN INTAKE, PERCENT BY
Figure 13. Monoxide in Exhaust and Cancentration of Hydrogen in Intake Engine B operated at liauid fuel-sir ratio of 0.01 Dound per pound
The results on oxides of nitrogen are of interest because of the marked increase in the concentration of oxides of nitrogen a t fuel-air ratios greater than about 0.03 pound per pound when natural gas was added to the intake. The general shape of the relations shown would be predicted from thermodynamic considerations for the reaction '/2N2
+ '/~OL i NO
The effect of temperature controls a t the lower fuel-air ratios and an increase in fuel-air ratio (increase in average temperature) results in an increase in the concentration of oxides of nitrogen. At the higher fuel-air rat;os, the effect of oxygen concentration controls, and an increase in fuel-air ratio (decrease in average oxygen concentration) results in a decrease in the concentration of oxides of nitrogen. The results shown in Figure 14 a t the higher fuel-air ratios, when the effect of oxygen concentration controls, indicate that in regions where nitric oxide was formed the concentration of oxygen was, on the average, greater as the concentration of natural gas increased. If it is assumed that nitric oxide is formed principally in regions of intense combustion, then it would appear from the foregoing that when natural gas was added t o the intake at a given reacting fuel-air ratio, intense combustion occurred in regions in which the concentration of oxygen was, on the average, greater than when the intake consisted of normal air. This occurs as a result of the mixture of natural gas with the intake air, which in effect provides a uniform distribution of that portion of the combustible throughout the combustion space, and thus minimizes the tendency to form locally overrich regions.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 43, No. 12,
Figure 14. Relation of Carbon Monoxide, Aldehydes, and Oxides of Nitrogen in Exhaust Gas to Ratio of Reacting Fuel to A i r Engine B at 600r.p.m. wilh different concenlrations of natural gas in intake 4r
I 01 0
0.02 0.03 0.04 0.05 0.06 RATIO OF RACTING FUEL TO AIR, POUND PER POUND
EFFECT ON ENGINE OPERATION
Combustion under the conditions discussed above has a significant effect on the peak pressures attained during conibustion, and consideration muqt be given to this in any dual-fuel application. Figures 17 and 18 show the relation between reactingfuelair ratio and the maximum pressure in the combustion chambei of a CFR engine operated with different hydrocarbons added to the intake and a t different liquid fuel-air ratios. The effect of the presence of hydrocarbon gas in increasing the maximum pressure above that observed when the intake consists of normal air 18 clearly shown in Figures 17 and 18. The results shown were obtained with fixed liquid-fuel injection timing a t 13 degrees crank angle before top center. Obviously, the effect of gas concentration on peak pressure will depend on the type of engine and the timing of the fuel injection, However, Figures 17 and 18 emphasize the importance of considering peak pressure in dual-fuel combustion. With fixed injection timing, the increase in peak pressures in dual-fuel combustion, as compared a i t h combustion of liquid fuel alone, occurs as a result of the premixturr of the gas with air. -4s soon as the liquid fuel ignites, a gas-air mixture is available for combustion, and it is the rapid combustion of this mixture that leads to higher peak pressures, When the concentration of gas IP aboTe the lower limit, the rate of reaction of the gas-air mixture is high and leads t o so-called knocking. Conditions are therefore someirhat analogous to those that obtain in some instances with a Diesel fuel having a long ignition delay. Figure 17 shows that the peak pressures at a given reacting fuel-air ratio are substantially the same for natural gas and
propane but somewhat higher for butane. Butane exhibits radically different behavior from natural gas in certain coniinercia1 Diesel engines ( 3 ) . SUMMARY
An attempt has been made t,o point out some of the important factors affecting the dual-fuel combustion process in the D i e d engine. I n this process, gaseous fuel generally is mixed with the intake air or with the combuftion air in 2-stroke-cycle engines. Liquid fuel is injected into the compressed homogeneous gas-air mixture and, upon inflammation, furnishes a source of ignition for this mixture. A study of combustion under such conditions involves considcration of the factors affecting the combustion of gas-air mixtures, the factors affecting the combustion of liquid fuel alone under the heterogeneous conditions existing in the Diesel engine, and the interrelationship between three factors. Tests in which combustible gases were added to the int,ake of a Diesel engine have shown that the lower limit of flammability of the combustible gas in the compressed gas-air mixture is an important factor t o be considered in dua,l-fuel combustion. If the concentration of combustible gas is greater than this lower limit, flame will propagate throughout the gas-air mixture when the liquid fuel ignites. The rapid rate of reaction of the gas-air mixture under such conditions results in a rapid rabe of heat release. If the system is not capable of converting this energy t o useful purposes, it is dissipated in thermal, frictional, and vibrational effects, TTith attendant so-called lmoclr. Continuous op-
INDUSTRIAL AND ENGINEERING CHEMISTRY
' 6 '
;& z lk
FUEL: AIR RATIO, POUND OF FUEL PER POUND OF AIR
Figure 15. Effect of Fuel-Air Ratio on Concentration of Carbon Dioxide, Carbon Monoxide, Aldehydes, and Smoke
A st,udy of the composition of the exhaust gases produced when hydrocarbon gases are added t o the intake of a Diesel engine shows that a significant proportion (10 to 15%) of the hydrocarbon gas reacts only partly with oxygen a t gas concentrations below the lower limit' of flammability. This is indicated under the foregoing dual-fuel combustion conditions by higher concentration of carbon monoxide and aldehydes, which are products of incomplete direct oxidation reactions. Tests made at, constant liquid-fuel injection timing in a CFR Diesel engine have shown that peak pressures are greater under dual-fuel combustion conditions than those observed under comparable conditions when the engine is operated on normal air. Incidental t o the studies of dual-fuel combustion, several observations are of possible interest in studying the combustion performance of the Diesel engine operat,ing on liquid fuel alone. If it is assumed that gas reacts wit8hoxygen only in inflamed regions or high-temperature regions, then the fraction of gas reacting is a measure of the fraction of the fuel-air mixture that waR traversed by flame at some time during combustion. Thus, by determining at a constant liquid fuel-air ratio the fraction of gas reacting with oxygen a t different concentrations of gas and extrapolating t o zero gas concentration, the value obtained should be a measure of the fraction of the fuel-air mixture that, is traversed by flame when liquid fuel alone is burned. This furnishes a criterion of the combustion performance that may be useful in certain st'udies.
In exhaust gases of CFR Diesel engine
eration of Diesel engines under such conditions is not feasible. Tests in several Diesel engines have shown that the lower limit of flammability of natural gas in air under conditions existing at the end of compression is approximately 5% by volume. Tests in single engines have indicated lower limits for other gases, as follows: propane, 1.9%; butane, 0.9yo; and hydrogen, about 10%. If the concentration of gas in the gas-air mixture is below the lower limit of flammability,
a portion of the gas will react with oxygen if a t some time during combustion it is in a region of sufficiently high temperature. However, tests have shown that, a significant portion of the gas does not react with oxygen when the concentration of gas is below the lower limit of flammability. Extreme temperature gradients exist under the heterogeneous conditions existing during the combustion of liquid fuel in the Diesel engine. Therefore, it is apparent that the minimum temperature is not sufficient, within the time available, for complete reaction with oxygen of the gases studied. The proportion of gas react'ing with oxygen depends on the proportion of the fuel-air mixture that is traversed by flame or high temperatures at some time during the combustion process. This, in turn, depends on the liquid fuel-air ratio, the concentration of gas, the type of engine, and engine speed. The proportion of gas reacting increases as the liquid fuel-air ratio increases. As the combustion of gas in regions of high temperature tends t o extend these regions, an increaee in gas concentration also increases the proportion of gas reacting with oxygen.
8 6 4
.Ol ,008 ,006 ,004 ,002
0.001 0.01 0.1 RATiO OF REACTING FUEL TO AIR; POUND PER POUND (Logarithmic scale)
Diagrammatic Representation of Probable Origin of Carbon Monoxide I n exhaust gases horn Diesel engines
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 43, No. 12
6 0 ~
Concentration of gos i n i n t a k e
:: 800 W
0: 2 v)
N o t u r o l gas plus 0 0066 pound l i q u i d f u e l per pound oir Propane plus O 0 0 6 6 p o u n d liquid f u e l per pound air
A Butone plus 0 0066 pound l i q u i d fuel
0 L i q u i d f u e l only
001 002 003 004 005 RATIO OF REACTING FUEL [LlOUlD PLUS GAS) TO AIR, POUND PER POUND
Noturol gos plus 0.01 pound l i q u i d fuel per poundoir
t ,Natural gas p l u s 0.02 pound liquid fuel per pound oir
0.01 0.02 0.03 0.04 0.05 RATIO OF REACTING FUEL ( L I O U I D PLUS G A S ) TO AIR, POUND PER POUND
0 Figure 18.
Relation between Maximum Pressure end Reacting Fuel-Air Ratio
In CFR Diesel engine operated at 16 to 1 cornpression ratio J 006
17. Relation between Maximum Pressure and Reacting Fuel-Air Ratio In CFR engine operating at I1 to 1 compression ratio
-4nother incidental observation involves the possible application of dual-fuel combustion techniques in determining the fraction of air used for scavenging a 2-stroke-cycle engine. If gas is added t o the intake of such an engine in concentration greater than the lower limit of flammability, the gas present during cornbustion will react completely with oxygen. Any unburned gas in the exhaust must have passed through the engine with the Scavenging air. Therefore, the fraction of gas unburned is a meaaure of the fraction of air wed for scavenging. Further work is required to demonstrate the soundness of thip ohaervation. ACKNOWLEDGMENT
The writers are indebted t o George €I. Hindman for his assistance in the test work.
R. L., “Status of Ihal-I.’uel Eiigiue Dcuelopincnt,” paper presented at 9.1E .innual .\Icetirig, Detroit, hIich., Jan. 10-14, 1949. ( 2 ) Eoyer, 11. L., “Summary of Cuwcnt Developments in 1,arge Diesel and Gas Engines,” paper presented at World I’umcr Conference, London, England, July 1950. (3) Conn, E. L., Beadle, R. H., and Schauer, G. ii., “Two-C~-cle Dual-Fuel Diesel Engine with Automatic Fuel Conversion.” paper presented before Oil & Gas Power Division ASAII?, Chicago, Ill., April 25 to 29, 1949. (4) Elliott, bf. A,, S.A.E. Trans., 3, 489-512 (1949). ( 5 ) Elliott, AI, -i., and Borger, L. B., ISD. ENS. CHEX.,34, 10G3 7 1 (1942). ( G j Elliott, AI. -4..and Davis, 11. F., S.A.E. Trans., 4, 330-44 (1950). ( 7 ) I,:lliott, &I. A,, Holtz, J. C., Bergor, L. B., and Schronk. H . M., U. S.Bur. Mines, Rept. Incest. 3584 (1941). (8) Landen, E. I?-.,S.A.E. J o z ~ m a 254, , 270 (1946). (9) Mason, IT., and Wheeler, K. V., J . Chem. Soc., 113,45 (1918). , E., and Plena, F., J . Gnc?bcZeucht.,57, 941 (1914). (1 j Boyer.
RECEIVED ,June 26, 1951.