December 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
reached in the engine cycle before the effectiveness of tetraethyllead can be observed. The point on the heat-time curve a t which the slope changed can then be interpreted as the point a t which the decomposition products of tetraethyllead became effective. These results are in conformity with data obtained using a mechanical method of extracting samples of unburned fuel from the combustion chamber of an engine. This work, currently under way in these laboratories, has shown that under engine operating conditions less severe than the 150" F. inlet condition, the tetraethyllead remains undecomposed up to a point slightly later than the time indicated by the change of slope a t the 150 O F. condition. In spite of the relation obtained between heat generated in the precombustion reaction and the octane number of certain specified blends, such as n-heptane-iso-octane mixtures, there is no over-all relation between the magnitude of this heat and the octane number of the fuel (Figure 8). I t has been implied by others ( 4 ) that the magnitude of the precombustion reaction is linearly related to octane number for several fuels. If such were the case, and if the heat generated in the precombustion reaction were the sole criterion for knocking, then the amounts of heat generated during the precombustion reaction for these fuels would be identical in tests made using the present technique of adjusting compression ratio for incipient autoignition. The present data clearly do not substantiate this idea. With reference to the absence of any detectable precombustion reaction in the case of diisobutylene, it may be conjectured that this behavior is the reason for the poor tetraethyllead susceptibility of this fuel. The basis for this thought is that the decomposition products of tetraethyllead might be particularly effective on the intermediate products formed by the precombustion reaction, and the absence of those products in the case of diisobutylene would prevent the tetraethyllead from affecting the course of the reaction.
2849
ACKNOWLEDGMENT
We acknowledge with thanks the assistance of J. D . RiIcCulIough, who helped with instrumentation. We are also indebted to D. H. Ewen and Irene Larson for their help with the calculations. NOMENCLATURE
Lower case letters (denoting extensive quantities) refer to the amount of the air-fuel mixture charged to the engine during one cycle, while the capital letters refer to 1mole of the mixture
Symbols p = amount of heat absorbed by mixture w = work done by mixture e = internal energy of mixture cy = heat capacity a t constant volume of mixture v = volume occupied by mixture p = absolute pressure in cylinder n = number of moles of mixture T = absolute temperature of mixture R = universal gas constant per mole of mixture LITERATURE CITED (1) Barusoh, M. R., and Payne, J. A., paper presented before the
(2) (3) (4) (5) (6) (7)
(8)
Division of Petroleum Chemistry at the 118th Meeting of AMERICAN CHEVICAL SOCIETY, Chicago, 111. Draper, C. F., and Li,Y . T., J . Aeronaut. Sn'., 16, 593 (1949). Levedahl, W. J., and Sargent, C. W., Jr., B.S. thesis, Maseaohuaetts Institute of Technology, 1948. Pastell, D. L., S.A.E. Quart. Trans., 4, 571 (1950). Souders, Matthews, and Hurd, IND. ENG.CHEM.,41,1037 (1949). Taylor, C. F., and Taylor, E. S., J . Aeronaut. Sei., I , 135 (1934). Taylor, C. F., Taylor, E. S., Livengood, J. C., Russell, W. A., and Leary, W. A., S.A.E. Quart. Trans., 4, 232 (1950). Tizard and Pye, Phil. Mag., Ser. 6 , 44, 79 (1922); Ser. 7, 1, 1094 (1926).
RECEIVEDMay 14, 1951
DIESEL ENGINE EXHAUST PRODUCTS ERNEST W. LANDEN Caterpillar Tractor Co., Peoria,
T h i s work was done to determine whether the exhaust condensation method is suitable for evaluating the products exhausted from Diesel engines when various fuels are burned or various types of engines are used for a given fuel. Exhaust gases from Diesel engines have been condensed in a condensing system. Results from nine fuels in one engine show that some organic liquids are exhausted at all engine loads. Three different combustion systems show variations in the exhausted material. In general, the organic liquids, both water-soluble and insoluble, showed a slight maximum at the intermediate loads. Organic acids decreased as the load increased. The. direct injection engine produced more solid carbon and the Lanova system produced more liquids as compared to the precombustion chamber engine. Comparing the organic acids exhausted to the total acids exhausted showed that the doublechamber engines were similar. Differences in fuels can be measured by this method. It may be possible to evaluate engine deposit-forming tendencies and the exhausted material of engine combustion systems by this technique.
111.
RODUCTS exhausted from an internal-combustion engine give some clue as to what takes place in the combustion cycle. I n fact, visual obsel-vation indicates t o some degree whether oily materials, indicated by gray-colored exhaust, or carbonaceous materials, indicated by black exhaust, are exhausted. Even colorless exhaust can at times be offensive. Exhaust spotters, or perhaps sniffers, have been busy on the West Coast tracking down offenders, especially the Diesel truck operators (8). They have also been busy in large cities where exhaust odors are Rometimes a serious problem. Other problems also occur from exhausted materials such as port plugging or build-up of material on the valve stems. Exhaust systems can be deteriorated by the acids formed by combustion. I n order to evaluate the materials exhausted from engines, condensing equipment was used to collect the liquids and solids for further analysis.
P
EXPERIMENTAL EQUIPMENT
Two experimental techniques were followed. I n the fimt a precombustion chamber-type engine was used and a variety of fuels was supplied to the engine in order to note differences when fuels are burned in a single engine. In the second, three
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
2850
Vol. 43, No. 12
DRY ICE CHAMBER
Table
I. Specifications
of Fuel Used for the Three Different
Combustion-Type Engines Gravity, A.P.I. at 60" F. Flash, F. Viscosity, Saybolt Universal, a t 100' F. Density Sulfur, % Ash (10% bottoms), % Carbonaceous residue (10% bottoms), % Diesel index Unsulfonated residue, % Specific refraction Hydrogen-carbon ratio
Distillation, initial boiling point, O F. 10% 33 50% 0.8063 95% 0.06 E n d point None Silica column separation Pentane eluted, % 0.01 Benzene eluted, % 74.5 Alcohol eluted. % ' 95 0.3321 2.05
44.0 170
380 415 465 416 648
PRESSURE CONTROL 4ALVi7 PRESSURE
88.Y 8.7 0.4
types of combustion system& \$?re wed and a single fuel was supplied t o the engines. This gives a comparison of engine types when a single fuel is burned. Some of this material has been previously reported (6). This report consists of additional material from the data of the first technique and data from the second technique.
All three engines are successful commercial types and are considered to be highly developed 4-stroke cycle engines. The combustion systems used were: 1. Direct injection, i-cylinder, 4l/4 X 6, water cooled. 2. Precombustion chamber, 1-cylinder, 4 X 5, water cooled. 3. Lanova auxiliary chamber, 3-cylinder, 31/8 X 3 3 / 4 , mater cooled. Simple schematic diagrams of these three combustion systems are shown in Figure 1. The engines Tere started by and delivered power to a standard dynamometer.
Figure 1.
SURGE TANK
CONDENSATE O U T L E ~
Figure 2.
Exhaust Condensing System
previously reported values (4). %'hell the percentages of these two gases are plotted as a function of the fuel-air ratio, values from all normally aspirated Diesel enginrs are substantially the same for a given fuel and are shown in Figure 4. Slight variations are found when the percentages are compared for fuels of differing carbon-hydrogen ratios. The minor constituents, such as methanr, hydrogen, carbon monoxide, and nitrogen oxides, were not measured. Liquids and solids condensed and collectrd for two precombustion chamber engines have been reported (5). Figure 5 illustrates in general the amount oi n ater-insoluble organic liquids and carbonaceous solid material exhausted by a 3 3 / 4 X 5
Diagrammatic Sections of the Three Combustion Systems
i'he condensing equipment, which was attached to the exhaust each engine, consisted of a 2-cubic foot surge tank, two laige stainless steel condensers, and two small stainless steel condensers all connected in series. This was followed by a cloth-glass wool filter. A schematic dia ram of the entire condensing system is shown in Figure 2. A pfmtograph of one of the large condensers is shown in Figure 3. The condensers were constructed in such a way that the inner member took the form of a well which contained the coolant, A spiral fin was welded around the inner member, which had an external diameter to fit into the outer member. The exhaust gases then flowed in a spiral path through the four condensers. Solid carbon dioxide (dry ice) and naphtha were used as a cooling mixture in the condenser wells. The gases were cooled down to a temperature of 0" F. before being finally exhausted. An exhaust bloxer was attached to the condensing system so that atmospheric pressure could be maintained in the surge tank a t the exhaust port. Pressure regulation 11-as achieved by a valve just ahead of the blower because the blower had a constant volume displacement, and it was necessary to adjust the flow a s the condensers started clogging. A variety of fuels was used when fuel characteristics were studied and the results have been published ( 5 ) . When comparing the three combustion systems, a single fuel was used as described in Table I. df
EXPERIMENTAL RESULTS
The dry exhaust gases were analyzed only for carbon dioxide and oxygen, and these values were found t o agree closely with
Figure 3.
Inner and Outer Parts of O n e of Condensers Used Condenser is 4 feet
hish
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December 1951 Table Intake Air Temperature, Fuel O F. 105 1 106 2 106 3 ' 102 4 99 93 101 7 8 100 9 93 9 90
: 9
9 9 9
90
91 97 102
2851
II. Operating Conditions and Exhausted Material from 33/4 X 5 Engine Operating on a Variety of Fuels Exhaust Temperature,
F.
720 715 720 765 705 710 710 730 250 350 380 480 600 865
*
Fuel Consumption, Pounds per Hour 3.22 3.16 3.25 3.51 3.46 3.41 3.15 3.17 0.76 1.32 1.50 1.94 2.50 3.59
Fuel-Air Ratio 0.0475 0.0469 0.0446 0.0422 0.0473 0.0492 0.0445 0.0413 0.0121 0.0193 0,0199 0.0297 0.0374 0,0570
Water-Insoluble Or anic Liquids gxhausted per Hour, Grams 2.15 0.97 0.71
Carbon Exhausted per Hour, Grams 2.50 2.08 2.17 2.64 5.21 8.20 1.89 1.23 0.50 0.63 0.40 1.20 0.63 19.5
0.36
0.62 0.70 0.97 0.39 0.98 1.48 2.35 1.67 2.58 1.58
Inorganic Acids Exhausted per Hour Mg. N a O d Equivalent 351 403 341 344 1477 493 244 186 163
Or anic Acids Ifxhausted per Hour M g . NaOH Equivalent 155 137 165 22 91 254 125
.... 455
....
....
525 667 383 384
659
601
405 155
Organic Residue in Water Solution, Grams per Hour 0.78 1.06
0.66 0.70 2.27 1.04 0.59 0.32 0.46 . . I .
0.68 1.58 1.22 0.88
sorption column. The water collected was analyzed after filtering off the carbon. The organic liquids were placed on a silica gel column and eluted first Kith Skelly B, then with benzene, and finally with methanol. Gkelly B hexane solvent has a gravity of 0.6846 a t 60" F., boils between 150" and 158' F. and has a refractive index of 1.3835. The benzene i s of C . P . quality and thiophenefree; the methanol is of C.P. quality, absolute, and refined. This procedure was used in Experiment 1 when the exhaust condensate from the various fuels was separated. These chromatographic separations were done by E. 0. Holmes a t the Midwest Research Institute in Kansas City, Mo., and later separations were done in Caterpillar Tractor Co. laboratories. A slightly different procedure was used in Experiment 2 when the'exhausted liquids from the three engines (direct injection, precombustion chamber, and Lanova) were studied. I n this case, the Skelly B was replaced by pentane and another operator did the separation.
Figure 4. O x gen and Carbon Dioxide Content of Dry Exhaust Gas of Fuel Having Carbon-Hydrogen Ratio of 6.5
engine. Measured values on a smoke meter appear in the upper part of the figure. The value of K log Zo/Irepresents the actual amount of smoke as measured by the meter. The smoke curve follows closely the amount of carbonaceous solid material collected in the condensers. Similar observations have been reported by Elliott (Z), who reports murh higher values of I o / I caused by the carbon, At low values of fuel-air ratio, the water-insoluble organic liquids exhausted exceeded the carbonaceous solids on a weight basis. Values from a 41/a X 5112 precombustion chamber engine are shown in Figure 6. In the case of this engine, the exhausted solids exceeded the organic liquids at all values of fuelair ratio tried. These two figures show that even in two engines which are nearly alike, the exhausted materials are not in the same proportion, Engine condition, especially the oil consumption, would cause variations in the organic liquids exhausted. I n one set of experiments when the same engine was used, the fuels showed considerable variation in the amount of liquids exhausted. Here the oil consumption was expected to be fairly uniform; therefore the effects of fuel characteristics were being measured. The condensers collect most of the material by freezing it out on the fins and cold parts of the condensers. When reclaiming this material, the condenser is warmed up slightly by the room air, after pouring out the cooling mixture, and placed in a trough. Most of the ice falls off as a solid and then the condenser parts are brushed. No water is added, but the condenser is washed with benzene to dissolve any organic material which may be adhering to it. Then the water and dissolved organic material in the benzene can be separated in a separatory funnel. One benzene extraction of the water is added to the benzene washings and the solution boiled down with some refluxing while bubbling with nitrogen until it is benzene free. This mixture was analyzed as te hydrocarbon type and oxygenated compounds on a silica gel ad-
Table I1 presents data of the material exhausted from the 3a/, X 5 precombustion chamber engine when using nine different fuels. The first eight fuels, referred to earlier (6),were operated at a fixed value of brake mean effective pressure. Fuel 9 xas used in the same engine, but the load was varied to obtain a part-load curve. Fuel and air measurements were quite accurate so that the comparisons are made on a fuel-air ratio basis. Values of organic acids, water-soluble and insoluble organic com4
\
0.2
.: M
B
A
k4 0.1 /I
4 Q
*8 0.0
o.
O.O\
0.02
0.03
0.04
0.05
Fuel-Air Ratio
Figure 5.
Liquids and Solids Exhausted from an Engine per Hour and the Measured Smoke 3 a / r X 5 oneine
INDUSTRIAL AND ENGINEERING CHEMISTRY
2852
pounds exhausted in grams per hour are plotted as a function of fuel-air ratio in Figure 7. The crosses are for the first eight fuels and the dots for fuel 9. The curve is averaged through the dots because it was a single fuel and no attempt was made i o include the crosses on the curve since the characteristics of these fuels varied considerably, and these points are put o n simply for comparison and reference.
0.02
0.01
Figure 6.
0.03
outer boundary of the shaded area, may be classed as conimercial fuels. Fuels 4, 5, and 6, indicated on the plot, are special fuels, and the numbers refer to the fuels in Table 11. Fuel 4 is a mixture of normal heptane and iso-octane blended to produce a 40 cetane number fuel by the ASTM method. Fuel 5 is a, blend of n-hexadecane and or-methylnaphthalene having an ASTM cetane number of 40. Fuel 6 is a 29 cetane number highly cracked fuel. Fuel 8 is a Fischer-Tropsch fuel having a cetane number of 84. Most of the values for all the fuels fall in a line or band a t the high values of fuel-air ratio. Fuels 4,5, and 6 must pass t'hrough a period during the combustion cycle when they exist in rich mixtures, therefore producing much more carbon earlier in the combustion cycle than the other fuels. Engine adjuetment is someidiat critical for a particular fuel becauw extremely rich mixtures in almost any Diesel engine Kill cause early exhaust smoke. In this case, there exists what Boerlago and Broeze ( 1 ) term destructive combustion. The amount of fuel breakdovn can be controlled by proper mixing. Table I11 shows results from three t'ypes of engines all operating on one fuel. Inspection data on this fuel are given in Table I. Three load condit,ions were tried on each engine-idle, intermediate load, and a moderately heavy load. The direct injection and preconibustion chamber engines were run a t rated load anti speed. The Lanova-type engine had the largest displacement so it was operated at' a speed somewhat below that at which the optimum combustion was found, so that the flow of gases through
0.04
Liquids and Solids Exhausted from an Engine per Hour 4l/a
X
51/z
engine
The quantity of water-soluble organic material was determined by evaporating a given quantity of the solution; the results are shown in Figure 7. Figure 7 also shows the water-insoluble organic material. In general, the two curves are similar, showing a ,maximum in the middle values of fuel-air ratios. A wide variation of the soluble organic compounds is found in the first eight fuels. Fuel 5, which is highly aromatic, gave the most water-soluble material and the Fischer-Tropsch fuel, which has a high cetane number, gave the least. Fuel 4 is a blend of heptane and iso-octane, having a cetane number of 40. This fuel and No. 8, the Fischer-Tropsch fuel, exhausted the least quantity of waterinsoluble organic nlaterial but fuel 4 exhausted twice the quantity of water soluble organic material when compared to fuel 8. The organic acids from both of these fuels were less at this engine load than from any of the other fuels. I n general, a greater quantity of water-soluble organic material was exhausted from fuels having higher aromatic content, The organic acids from the 3a/, X 5 prechamber engine are plotted as a function of fuelair ratio in Figure 7. This plot shows that, in general, the organic acid content of the exhaust is highest a t idle and decreases as the load is increased. Data from the 4 X 5 precombustion chamber engine are plotted as a comparison and show the same trend. The amount of solids exhausted per hour is shown as a function of fuel-air ratio in Figure 8. The exhausted solids show a more consistent pattern on the plot than do the exhausted liquids. The points plotted as dots refer to fuel 9, which is the standard fuel used in this laboratory. The other fuels, falling on the ~~
Table 111.
B
C (1
b C
Brake Mean Effective Pressure, Lb./Sq. In. 0 60 80
C
C A
Precombustion chamber
0 78 43
Lanova
.6
2 .4 g .2 LL
u1
d W
5, I
.01
.OZ .03 .04 FUEL-AIR RATIO
.05
Figure 7. Materials Exhausted from 33/a X 5 Engine as a Function of Fuel-Air Ratio Dots refer to fuel 9 in Table IIand sraphs are averaged through (here; crosses refer to fuels 1 Lo 8 in Table 11; points for reference only
Operating Conditions and Exhausted Material From Direct Injection, Precombustion Chamber, and Lanova-Type Engines
B
B
ORGANIC ACIDS-GRAMS NiOH EPUNALEN
~
Operatin Con%tion Engine Type A Direct injection
-4
Vol. 43, No. 12
0 39 71
Indicated Mean Effective Pressure, Lb./Sq. In. 20 80 100
WaterInsoluble Organic Liquids Fuel- Exhausted Air per H o u r , Ratio Grams
Intake Exhaust Fuel Tempera- TemConsumpture, peraiurea, tion, Pounds F. F. per Hour 90 280 0,942 0,0090 90
90
680 BOO
2.82 4.15
0.0278 0.0416
1.26 2.63 0.62
Inormanic A;idb Carbon Exhausted Exhausted per Hour, per Hour, 3Ig. NaOII Grams Equivalent 0.82 5.75 13.3
122 144 554
Organic Organic Acids Residue Exhausted in Watcr per Hour, Solution, Mg. S a O H Grams pel Equivalent Hour 406 243 51
0.60 0.47 0.93
38
95
330
1.56
0.0148
0.97
0.87
76
437
0.42
81 116 25 64 96
95 90 90 90 90
540 840 2OOC 310 410
2.82 4.26 1.18 2.76 4.15
0,0271 0.0416 0.0116 0.0274 0.0438
1.47 0.53 4.82 6.70 9.60
0.84 1.01 0.83 0.75 0.89
113 121 280 440 524
283 79 2200 1640 746
0.17 0.33 1.42
Exhaust temperatures measured by a suitably placed tberinocouple. Sulfuric acid, Very little sulfur dioxide retained in condensers. Exhaust manifold was water cooled.
1.20
1.18
December 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
the condensing system was comparable to the other engines. Inspecting the columns labeled water-insoluble organic liquids exhausted and carbon exhausted, the direct injection engine and the precombustion chamber were found to be similar in respect to organic liquids insoluble in water. The Lanova exhausted a much greater quantity of organic liquids insoluble in water. On the other hand, the direct injection engine produced more carbon while the Lanova and precombustion chamber engines were similar. The water-soluble organic residue was high from the Lanova engine in comparison to the other two engine types. In general, the amount of organic material exhausted from the Lanova-type was high under the operating conditions. . - engine In general, the exhausted organic liquids from the engine@ referred to in Table 111 reached a maximum a t the intermediate value of load and in thisrespect, agreed with the part load curve from the 33/r X 5 engine referred to in Table 11. The quantity of organic acids exhausted was highest from all three engines at idle operation and decreased as the Figure 8. Carbon Exhausted from 33/4 X 5 load on the enEngine as a Function of Fuel-Air Ratio g i n e s w a s inVarious lualr used creased. T h e r e is one ooint in Figure 7, showing the organic acids, which indicates that at lower fuel-air ratios the amount of organic acids decreases, which fieems contrary to the above statement. This may be caused by motoring the engine because there was not enough fuel supplied to run the engine at idle, The fuel used in these three engines had a low sulfur content of 0.06%. Even so, sulfur trioxide was collected in the condensing system. The percentage of the sulfur burned and collected as sulfur trioxide varied with the engine. Only 5.8% of the sulfur burned remained in the condensing system from the precombustion chamber engine while 15.5% was collected from the direct injection. There was 25.6% of the burned sulfur collected from the Lanova-type engine. The exhaust manifold of the Lanova engine was water cooled, which may account for this value being so high because the equilibrium of sulfur trioxide and sulfur dioxide would go toward the sulfur trioxide in the cool txhaust manifold before reaching the condenper. Exhaust temperatures and conditions under which the gases are collected for the other two engines were similar, which indicate a difference in the combustion of the direct injection and the prechamber engines. Quantitative evaluation of the organic acids was obtained by measuring the total acidity and subtracting the amount of the sulfuric acid measured by barium precipitation. When the ratio of the organic acid to total acidity is plotted as a function of the indicated mean effective pressure, the direct injection engine values follow one curve and the other two engines, which have auxiliary chambers, follow another curve. These data are shown graphically in Figure 9. The ratio of inorganic acid t o total acid is also shown as a function of indicated mean effective pressure on this graph. This indicates a similarity in the combustion of the double chamber engines when compared with the direct injection engine. These engines are both known to be
Table
IV.
2853
Separation of Exhausted Hydrocarbons b y Elution from Silica G e l Column ParaffinNaphBore and StFoke thene, % 4'/2 x 5'/2 68
Engine Type Prechambera Prechambera
homstic, % 9
Oxygenated Compo,unds of High Polarity,
%
20
31/4 x 5 64 10 27 19 m Direct injection& 4'/a x 6 47 22 19 Prechamber b 4 x 5 47 15 34 Lanovab 3'/8 x Sa/, 60 17 21 % Slightly different separating techniques applied b y two operators.
--
I -
"I
high-turbulence engines in comparison to the direct injection. Exhausted organic material was analyzed by chromatographic separations on a silica gel column. They were classified as paraffin-naphthene, aromatic, and compounds of high polarity. Variations in composition observed at different conditions of load do not warrant definite conclusions. Therefore, the values for each engine were averaged for all conditions of load. Table IV gives the percentage of oils separated by elution from silica gel columns. In some of the trials, the molecular weight of the intermediate loads was higher, indicating more polymerization at this load condition. CONCLUSIONS
The technique of condensing the Diesel engine exhaust has revealed differences in the exhausted products from fuels and differences in engines using different combustion systems. There is definitely more fuel cracking in the direct injection engine because smoke values are much higher when compared to the two-chamber engines. It is possible that the lower turbulence in the direct injection engine permits richer fuel mixtures and more of the fuel is broken down to carbon and lower molecular weight gases, so there is a deficiency of heavier fuel vapor. In the two-chamber engines, there is high turbulence, the fuel vapor is more thoroughly mixed with air, and there is less crackIO0
I
I
I
I
I
I
20
40
60
80
100
I
INDICATED MEAN EFFECTIVE PRESSURE Figure 9. Ratio of Inorganic and Organic to Total A c i d Exhausted as a Function of Indicated M e a n Effective Pressure for the Three Engines
ing. Quantity of organic liquids exhausted decreases with load and the amount of organic acid collected as water-soluble acid also decreases with engine load. Quantity of carbonaceous solid usually remained small until the smoke point was reached, a t which time large quantities of carbonaceous solid were exhausted. The Lanova-type engine exhausted more liquids and the direct injection-type exhausted more carbon when compared with the precombustion chamber-type engine. . Exhausted hydrocarbons, when classified as paraffin-naphthene and aromatic, and compounds of high polarity, show variation with engine load and with engine type. However, these do not follow any definite pattern.
2854
INDUSTRIAL AND ENGINEERING CHEMISTRY ACKNOWLEDGMENT
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
(1936).
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%20,000per 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.
