September, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
proper procedures in studying the size distribution of mill products should yield more accurate evaluations of grinding operations.
Acknowledgment The author is indebted to L. T. Work for his assistance in this study.
Literature Cited (1) Andreasen, A. H. M., et al., Kolloid-Z., 82, 37 (1938). (2) Andreasen, A. H. M., et al., Trans. Ceram. Soc., Wedgewood Bicentenary, 29, 239 (1930). ( 3 ) A. 9. T. M. Standards, Vol. 111, p. 562, Tentative Method D408-37T (1939).
1065
(4) Ibid., p. 566, Tentative Method D409-37T. (5) Ibid., p. 647, Tentative Method D445-39T (6) A. S. T. M. Standards Supplement, Vol. Ii p. 267, Tentative Method C115-41T (1941). (7) Farrant, J. C., Trans. Inst. Chem. Engrs. (London), 13, 10 (1931). (8) Frisch, M., and Foster, A. C.,Proc. A m . Soc. Testing Materials, 37, 11, 441 (1937). Gross, J., U. 5. Bur. Mines, Bull. 402 (1938). Metz, G . F., Bull. A m . Ceram. Soc., 16,461 (1937). Romer, J. B., Proc. A m . SOC.Testing Materials, 41, 1152 (1941). Schweyer, H. E., Rock Products (to be published). Schweyer, H. E., and Work, L. T., A. 8. T. M. Symposium on New Methods for Particle Size Determination in the Subsieve Range, 1941. (14) Work, L. T., Dissertation, Columbia Univ., 1928.
COMBUSTION IN DIESEL ENGINES Effect of Adding Gaseous Combustibles to the Intake Air MARTIN A. ELLIOTT AND L. B. BERGER Central Experiment Station,
U. S. Bureau of Mines, Pittsburgh, Penna.
In a study of the effects on exhaust gas composition of adding natural gas to the intake air of two commercial Diesel engines, information was gained incidentally on combustion phenomena in this type of engine. The results furnish considerable data on the mechanism of combustion in the Diesel engine and suggest the value of adding combustible gases to the intake as an experimental technique for studies in
HE Bureau of Mines recently conducted extensive studies relating to the composition of the exhaust gas of Diesel engines (1, 6, 6). These studies were initiated as part of a program to evaluate the hazards that may attend the use of these engines as prime movers for haulage in mines and other underground operations. Among the experimental conditions chosen was the addition of natural gas (mainly methane and ethane) to the intake air of the engine to simulate conditions that would exist if Diesel engines were operated in coal mines or in other situations where methane or similar hydrocarbons may be liberated from the strata and contaminate the underground atmosphere (6). The objective of this phase of the study was to determine the effects on exhaust gas composition of operating in such atmospheres and the resultant bearing of these effects on ventilation requirements. Incidental to this primary objective, considerable information was obtained on the combustion process in Diesel engines as a result of seeking explanations for certain effects observed when natural gas was added to the intake. This suggested the possibility that the addition of combustible gases to the intake air of an engine might offer a valuable new technique for studying certain aspects of combustion in the engine. Accordingly, this paper discusses the information
T
this field. Evidence is presented indicating the importance of an effect analogous to the lower limit of flammability in relation to combustion in the Diesel engine. The existence of overlean regions in which flame does not propagate is discussed, and evidence is offered indicating that such regions probably are the source of carbon monoxide and aldehydes in the exhaust gas at fuel: air ratios less than the chemically correct value.
obtained when natural gas was added to the intake. These results are presented to. illustrate the potentialities of this technique, inasmuch as detailed studies in this field were outside the scope of the Bureau of Mines investigation. COMBUSTION IN DIESEL ENGINES The fundamental operating principle of Diesel or compression-ignition engines may be reviewed briefly as follows : Air is drawn into the cylinder and compressed, and near the end of the compression stroke, fuel oil is injected into the combustion chamber under high pressure and in a state of fine subdivision. The temperature of the compressed air is such that autoignition of the fuel occurs and combustion ensues. Although every effort is made to inject the fuel in a fine spray and to attain uniform mixing with the air, these reactants remain far from being brought together in a state of molecular intimacy, and the ensuing reactions may be considered as starting in a heterogeneous mixture (2, 8,8). An outline of the combustion process in Diesel engines was presented by Boerlage and Broeze (2, 8) who stated that combustion is characterized by two processes that go on side by side: (a) direct oxidation of the hydrocarbon fuel through a series of reactions in which intermediate, partly oxidized
1066
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 34, No. 9
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FUEL AIR RATIO POUND OF FUEL PER f>OLND OF AIR
OF COMPOSITION OF EXHAUST GASTO FUEL:AIRRATIO FIGURE 1. RELATION
compounds are formed, and (b) thermal decomposition of the fuel followed by combustion of the destruction products. Chilling of the direct-oxidation reactions will result in the presence of carbon monoxide and aldehydes in the exhaust. If the reactions occuring in destructive combustion are chilled, soot will be present also. Under overrich conditions carbon monoxide and hydrogen are formed by either process, Overrich conditions exist in some parts of the combustion space, even when the total quantity of air present is greater than that necessary for complete combustion of the fuel. Such regions are said to be locally overrich and occur because of the difficulty of obtaining a uniform mixture of fuel and air in the time available and under the heterogeneous conditions attending the injection of liquid fuel. The presence in the exhaust gas of any product of incomplete combustion when the quantity of air present is more than enough for complete combustion indicates either (a) chilling of combustion reactions or (b) the existence of other conditions unfavorable to complete reaction with oxygen. Consequently, evaluation of the products of incomplete combustion furnishes information on the nature of the combustion process in Diesel engines.
obtaining the experimental data are not included in this paper, since the test equipment and procedure have already been described (1, 5, 6). Two cominercial four-stroke-cycle Diesel engines were used and are designated as engines A and B. Figure 1 presents the relation of composition of exhaust gas t o fue1:air ratio. The curves show all the significant constituents except aldehydes and oxides of nitrogen, whose variations may be presented to better advantage separately. Water vapor formed by combustion is not included; for the
CONSTITUENTS OF EXHAUST GAS FROM ENGINES OPERATED IN NORMAL AIR
Although the purpose of this paper is to discuss phenomena observed when air containing natural gas was supplied to the engine intake, it is desirable first to illustrate exhaust gas composition under different operating conditions when normal air was supplied to the intake. Methods used in
FUEL AIR RATIO, POUND OF FUEL PER POUND OF AIR
OF COKCENTRATION OF OXIDESOF FIGURE 3. RELATION EXHAUST GASTO FUEL:AIR RATIO
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September, 1942
INDUSTRIAL A N D ENGINEERING CHEMISTRY
purposes of this investigation exhaust gas composition was calculated on the dry basis. Fuel :air ratio is used as a basis for comparison because it is a fundamental performance factor independent of engine speed. Furthermore, in the normal operating range of a Diesel engine, power output varies directly with fue1:air ratio at a given speed because under such conditions the quantity of air drawn into the cylinder is essentially constant and power output is regulated by the quantity of oil injected. Figure 1 includes performance a t fuel :air.ratios greater. than the chemically correct mixture of air and fuel-in other words, under conditions such,., that theuxewassnot enough ’L air- €or complekz combustion:*large.. increases in .carbon monoxide :weFe40bsewed, and the exhaust gases contained appreciable quantities of free carbon (not shown in Figure 1) and hydrogen and methane. I n actual practice, however, the operation of Diesel engines is usually limited to a range of fue1:air ratios less than that representing the chemically correct mixture. Figure 1 shows that the carbon dioxide and oxygen contents of the exhaust bear an essentially linear relation to fue1:air ratio, as might be expected from stoichiometric considerations if complete combustion is assumed throughout the operating range for which Diesel engines usually are adjusted. In the case of the minor constituents in this range-namely, Garbon monoxide, aldehydes, and nitrogen oxides-a different situation exists. Figure 2 shows carbon monoxide concentration plotted on a logarithmic scale to illustrate more clearly the variations of this constituent under different operating conditions. Figure 3 gives the relation of concentration of nitrogen oxides to fue1:air ratio under selected conditions. It is evident that engine design and speed play a significant part in the production of these compounds. Figure 4 shows the trend of aldehyde concentration with fuel :air ratio. Because aldehyde concentration was low under these test conditions and because considerable variation occurred under comparable conditions, possibly through analytical difficulty in determining these low concentrations, the relation shown is termed a “trend” since the data a t different speeds were grouped in selected ranges of fuel :air ratios and averaged. Many of the relations shown in Figures 1 to 4 were affected considerably when natural gas was added to the intake. The following discussion presents exp l a n a t i o n s for these effects and shows that they are c o n s i s t e n t with the assumption of the coe x i s t e n c e of direct oxidation and destructive combustion react i o n s as mentioned by Boerlage and Broeze (2,5). Evidence is also presented pointing out the significance in the combustion p r o c e s s of a n effect analogous t o t h e lower limit of flamFIGURE 4. TRENDOF ALDEHYDE CONCENTRATION IN EXHAUSTGAS WITH mability. Inthis FUEL:AIRRATIO connection, a t -
0
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1067
0.07
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FIGURE5. RELATION BETWEEN CARBON MONOXIDE IN ExHAUST GAS,RATIOOF REACTING FUELTO AIR, AND NATURAL IN TESTS OF ENGINE B AT 600 R. P. M. GASIN INTAKE
tention is directed to the importance of “local overleanness” as a factor to be considered. COMPOSITION OF EXHAUST GAS WITH NATURAL GAS ADDED TO INTAKE AIR
Carbon Monoxide and Aldehydes. Figure 5 shows the relation of carbon monoxide in the exhaust to ratio of reacting fuel to air when different concentrations of natural gas were added to the intake air. The term “ratio of reacting fuel to air” is used in this instance because under some conditions part of the natural gas passed through the engine unburned and therefore cannot be considered as fuel consumed by the engine. Figure 5 shows that a t the higher concentrations of natural gas in the intake the relation between concentration of carbon monoxide and ratio of reacting fuel to air appeared to differ from that observed either when the natural gas concentrations were lower or when the intake consisted of normal air. Consideration of these relations shows that the concentration of carbon monoxide must be zero a t a fue1:air ratio of zero which, of course, signifies normal air. Consequently, when the intake consists of normal air, the concentration of mrbon monoxide in the exhaust must pass through a maximum a t some ratio of reacting fuel to air between 0.01 and 0.0 pound per pound. This can be demonstrated graphically by showing actual variations in relation to the maximum possible concentration of carbon monoxide computed for different fuel :air ratios by assuming that all carbon in the oil reacts to form carbon monoxide and all hydrogen to form water (6). Figure 6 presents such results and emphasizes the basic similarity between the relation for normal air and for an intake consisting of normal air and natural gas in the range of concentrations studied. In Figure 6 fue1:air ratio is on a logarithmic scale to illustrate more clearly variations at low ratios. Variations a t fue1:air ratios less than 0.01 are quali-
1068
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TWEEN
Vol. 34, No. 9
hydes decrease, it seems reasonable that carbon monoxide comes from incomplete combustion in locally overrich regions that have greater opportunity of being formed as the chemically correct mixture is approached from the lean side. Thus the relation of carbon monoxide to fue1:air ratio may be considered as the combined effect of carbon monoxide produced by incomplete direct oxidation reactions and of carbon monoxide from combustion in overrich regions. Figure 8 shows qualitatively the manner in which carbon monoxide from either source varies with fuel :air ratio. Curve A indicates how carbon monoxide from combustion in overrich regions probably varies with fuel:air ratio, and curve B shows the probable variation of carbon monoxide from incomplete direct oxidation reactions. Curve C is the composite relation observed in actual tests. At low fue1:air ratios the chances of forming locally overrich regions are minimized; therefore, nearly all the carbon monoxide comes from incomplete direct oxidation, and curves B and C are nearly coincident in this range. At fue1:air ratios greater than the chemically correct ratio, curves A and C are almost coincident because nearly all the carbon monoxide comes from combustion under overrich conditions. The increase with fue1:air ratio of carbon monoxide from combustion in locally overrich regions may be explained by the greater probability of forming such regions as the chemically correct fue1:air ratio is approached from the lean side. However, variation with fuel :air ratio of carbon monoxide from incomplete direct oxidation is affected by so many factors that an explanation cannot be formulated so readily. It is of interest, nevertheless, to mention some of the more important considerations.
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tative and have theoretical rather than practical interest, since Diesel engines normally do not operate at fue1:air ratios in this range. Relations shown a t ratios greater than 0.09 are also qualitative. If similar considerations are applied to the variations of concentration of aldehydes with ratio of reacting fuel to air (Figure 7), the characteristics of the relation are the same when natural gas was present in the intake as when the intake consisted of normal air. If these relations observed for aldehydes are compared with those for carbon monoxide, certain similarities are apparent. For example, a t a given concentration of natural gas in the intake, the variation of aldehyde concentration with fue1:air ratio has the same general characteristics as the variation of carbon monoxide with reacting fue1:air ratio up to that ratio a t which carbon monoxide concentration is a minimum. After this minimum is reached, carbon monoxide concentration increases rapidly with increase in fuel :air ratio, whereas concentration of aldehydes decreases under similar conditions, These observations are significant in relation to the probable source of carbon monoxide in the exhaust of Diesel engines. Carbon monoxide is a n intermediate product in the direct oxidation of hydrocarbons and is also a final product of combustion under overrich conditions, but aldehydes are formed only in direct oxidation. Consequently, in that range of fuel :air ratios in which variations of carbon monoxide have the same general characteristics as the variation of aldehydes with fuel: air ratio, the carbon monoxide appears to originate chiefly from incomplete reactions occurring in the direct oxidation process. However, in the range of fue1:air ratios in which carbon monoxide increases with fuel :air ratio and alde-
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One of the important factors in relation to carbon monoxide and aldehydes from direct oxidation reactions is the possible existence of “locally overlean regions” within the combustion space. The locally overlean region has not been mentioned specifically, although it is the antithesis of the locally overrich region so frequently referred to in discussions of combustion in Diesel engines. Locally overlean regions are those in which concentration of fuel is such that rapid reaction does not take place and flame propagation does not occur at the existing temperature and pressure. I n locally overlean regions, slow combustion occurs by direct oxidation, and unless the temperature of a n overlean region is raised by coming in contact with a region of intense combustion or by receiving
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1942
the tendency to form locally overrich regions even though the total quantity of air present is greater than that required for complete combustion. It would be expected that any improvement in the distribution of fuel throughout the combustion space would be reflected in a shift of the fue1:air ratio a t which this minimum occurs toward the chemically correct ratio. It is not surprising, therefore, that such a shift occurred in the tests in which natural gas was added to the intake (Figure 5 ) , because the mixture of natural gas with the intake air in effect was equivalent to providing better distribution of combustible throughout the combustion space.
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Nitrogen Oxides. I n connection with the shift in fuel: air ratio a t which minimum concentration of carbon monoxide occurred, it may be noted that a shift in similar direction was also observed in the fue1:air ratio at which the maximum concentration of nitrogen oxides occurred (Figure 9). Before discussing this point, i t should be stated that the observed relation between fuel :air ratio and concentration of nitrogen oxides in the exhaust gas would be predicted from thermodynamic considerations for the reaction,
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additional fuel, flammation will not occur in that region and combustion will not be complete within the time available. Therefore, the products of direct oxidation will appear in the exhaust gases. The manner in which carbon monoxide and aldehydes from incomplete direct oxidation vary with fuel :air ratio is conditioned by such factors as (a) the extent to which locally overlean regions are formed, (b) the extent of chilling of regions in which direct oxidation reactions occur, (c) the concentration of fuel, ( d ) variables affecting reaction velocity and mechanism, and ( e ) the time available for reaction. The chances of forming locally overlean regions and of chilling such regions are greater as the fue1:air ratio decreases; therefore, if these were the only considerations, a decrease in fue1:air ratio would be expected t o cause an increase in carbon monoxide from direct oxidation reactions, and the results corroborate this for a certain range of fue1:air ratios (Figures 5 and 7). On the other hand, reduction in fue1:air ratio could reduce the concentration of carbon monoxide and aldehydes from direct oxidation reactions as indicated at the lower fuel :air ratios (Figures 5 and 7) for two reasons: because the fuel concentration is lower, and because the heat that can be liberated (and therefore the temperature in a given region) is. lower and may decrease reaction velocity. It appears that the combined effects of all the factors mentioned in the foregoing discussion could explain the observed variation with fue1:air ratio of carbon monoxide and aldehydes from direct oxidation reactions. The minimum concentration of carbon monoxide in the normal operating range of the engines tested occurred at a fuel :air ratio somewhat lower than the chemically correct value. As mentioned previously, this minimum is an isdication of the point at which carbon monoxide from overrich regions begins to predominate and, therefore, is a measure of
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which indicate that the formation of nitric oxide is favored by increases in either temperature or oxygen concentration. Accordingly, at relatively low fuel :air ratios when the concentration of oxygen is comparatively high, an increase in fue1:air ratio causes an increase in the concentration of nitrogen oxides as a result of the increased temperatures in the combustion space. I n contrast with this at somewhat higher fue1:air ratios, an increase in fue1:air ratio causes a decrease in the concentration of nitrogen oxides as a result of the decrease in oxygen concentration which becomes increasingly important a t the comparatively low average oxygen concentrations existing a t the higher fue1:air ratios. Figure 9 shows that maximum concentration of nitrogen oxides occurred a t a fue1:air ratio on the lean side, and that the effect of temperature predominated a t lower fuel :air ratios whereas
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1070
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 34, No. 9
natural gas increased, the shift toward the chemically correct value of the fue1:air ratio corresponding to maximum concentration of nitrogen oxides is conditioned by factors similar to those responsible for the shift in the fue1:air ratio corresponding to minimum carbon monoxide. Both of these inflection points are therefore a measure of the average concentration of oxygen existing in regions of intense combustion, The decrease in nitrogen oxides caused by the addition of natural gas a t fue1:air ratios less than 0.03 pound per pound probably occurred because of the slightly lower temperature existing in regions of intense combustion as a result of greater local excess air. From the foregoing it is evident that concentration of nitrogen oxides depends on the fue1:air ratio and appears to be closely associated with fuel distribution in the combustion space. If better fuel distribution is obtained due to increased turbulence as engine speed increases a t a given fuel :air ratio, then temperature in regions of intense combustion will be lower a t the higher speeds, and a decrease in concentration of nitrogen oxides will be expected a t those fue1:air ratios where the effect of temperature predominates in the formation of oxides of nitrogen. At fue1:air ratios in the range where the effect of oxygen concentration predominates, an increase in speed will be expected to increase concentration of nitrogen oxides because of better distribution of fuel and increase of oxygen in regions of intense combustion. This effect of engine speed is shown clearly by the results of tests presented in Figure 3.
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FIGURE 10. VARIATIOXOF PROPORTIOX OF NATURAL GASREACTING WITH OXYGE~Y TO RATIOOF REACTIXG FUELTO AIR
the effect of oxygen concentration predominated a t higher fuel :air ratios. Figure 9 shows that, a t the higher concentrations of natural gas in the intake, marked increases occurred in the concentration of nitrogen oxides a t ratios of reacting fuel to air greater than approximately 0.03 pound per pound. Inasmuch as the average oxygen concentration is always approximately the same for a given fue1:air ratio, the foregoing results would seem to indicate that, in regions where nitric oxide was formed, the concentration of oxygen was on the average greater a t the higher concentrations of natural gas and, furthermore, must have been different from the average concentration for the entire combustion space. This presents further evidence of heterogeneity during combustion. 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 to the intake a t a given 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 offers further evidence to support the view that the tendency toward the formation of locally overrich regions was reduced when natural gas was present because, as stated previously, the mixture of natural gas with intake air was in effect equivalent to providing better distribution of that portion of the combustible throughout the combustion space. If we assume that the point of maximum concentration of nitrogen oxides indicates approximately the same average local concentration of oxygen in regions of intense combustion, it follows from the foregoing that this local average oxygen concentration could occur a t a higher fue1:air ratio (lower over-all average oxygen concentration) when natural gas was present. I n other words, as the concentration of
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All points are from average curves of Figure 10.
Natural Gas Unburned. The presence of significant concentrations of hydrocarbons in the exhaust gas when the intake contained natural gas indicated that some of the natural gas passed through the engine unburned, even though enough oxygen for complete combustion was present in the combustion space. There was definite evidence that these unburned hydrocarbons came from the natural gas and not from the oil. As the concentration of natural gas in the intake generally was considerably less than the lower limit of flammability,
September, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
complete reaction of natural gas and air would not be expected unless the distribution of liquid fuel was such that flame propagated throughout the entire combustion space (as in the case of complete combustion in a stoichiometric mixture or combustion of homogeneous rich mixtures). As might be expected, Figure 10 shows that substantially all of the natural gas reacted with oxygen at fue1:air ratios greater than the chemically correct value. However, a t fuel :air ratios less than that value, the proport,ion reacting depended principally on the fuel: air ratio. The failure of some of the natural gas to react with the oxygen present indicates that relatively cool regions existed within the combustion space. The existence within the combustion space of regions not traversed by flame was demonstrated by photographs of the progress of combustion in compression-ignition engines (8). Therefore the foregoing discussion is consistent with the observations of others, and the incomplete reaction of natural gas and oxygen furnishes a new type of evidence corroborating these observations and presents further evidence of the heterogeneous character of combustion in Diesel engines. Average Fuel :Air Ratio in Regions of Intense Combustion. If i t is assumed t h a t the proportion of natural gas reacting with oxygen is a measure of the proportion of total air present in regions of intense combustion, it is possible to calculate the average fue1:air ratio in regions of intense combustion ( 5 ) . The results of such calculation are shown in Figure 11, using data from the average curves of Figure 10. Figure 11 shows that, as the over-all ratio of reacting fuel to air decreased to low values, the average fue1:air ratio in regions of intense combustion approached a limiting value ranging from approximately 0.016 to 0.024 pound per pound. This suggests that in Diesel engines intense combustion occurs only in those regions in which the Concentration of fuel at least equals some minimum value analogous to the lower limit of flammability a t the temperature and pressure within the combustion space. It is interesting, therefore, to estimate the lower flammable limits of mixtures of oil and natural gas under conditions within the engine and to compare these values with the limiting values indicated in Figure 11. Such estimates were made (6) of the lower limits of flammability at 500" C. of methane and of an assumed higher hydrocarbon similar to Diesel fuel containing 86 per cent carbon and 14 per cent hydrogen. The estimate is based upon the assumption that the proportion of combustible in a lower limit mixture equals 0.57 times the proportion of combustible in a stoichiometric mixture. This factor satisfactorily represents experimental data for a variety of hydrocarbons (7). No data were available for estimating the effect of pressure, which is small at normal temperatures (4). The results of these estimates indicate that the fue1:air ratio in lower limit mixtures of methane and the higher hydrocarbons would range between 0.021 and 0.023 pound per pound. The fairly close agreement of these estimated limits with those indicated by Figure 11 appears to be rather strong evidence of the significance of an effect analogous to the lower flammable limit in relation to the combustion process in the Diesel engine. It must be pointed out that precision of data upon which Figure 11 is based might be improved and the results should be extended to fue1:air ratios less than 0.01, but such refinements were beyond the scope of the present investigation. One observation that may be made regarding the significance of the lower flammable limit in relation to combustion in Diesel engines is that, as every effort is made to obtain complete Combustion a t high fuel :air ratios, Diesel engines are designed to provide uniform distribution of fuel throughout the combustion space, and this may lead to losses from
1071
incompletely burned fuel a t low fuel :air ratios because of the formation of overlean regions. Although the present investigation was limited in scope in so far as studies of combustion were concerned, the foregoing results indicate that the addition of combustible gases to the intake in concentrations less than the lower limit of flammability may be a valuable experimental technique in studying combustion in Diesel engines and, possibly, in obtaining data on the flammable limits of certain combustibles under conditions existing in internal combustion engines. I n utilizing this technique, consideration should be given to the addition of such combustibles as hydrogen and carbon monoxide and to the use of other additives. SUMMARY
The results furnish evidence indicating that intense combustion in Diesel engines probably occurs only in regions in which the local concentration of fuel at least equals a certain minimum value analogous to the lower limit of flammability. Regions in which the concentration of fuel is less than this minimum value are designated as overlean because flammation does not occur in them, although direct oxidation reactions are taking place. The importance of locally overlean regions as the source of carbon monoxide and aldehydes in the exhaust gas at fue1:air ratios less than the chemically correct value is pointed out in discussing the variation of the concentration of these constituents with fue1:air ratio. The analysis of data on the concentration of nitrogen oxides in the exhaust at different operating conditions disclosed the possible relation of certain of the observed effects to the conditions existing in regions of intense combustion. Most of the information on combustion was made available as a result of seeking explanations for effects observed when natural gas was added to the intake. Accordingly, this suggests the possibility that the addition of combustible gases to the intake air of a Diesel engine might offer a valuable new technique for studying certain aspects of combustion in engines of this type. ACKNOWLEDGMENT
The authors are indebted to A. C. Fieldner, D. Harrington, and Wilbert J. Huff of the U. S. Bureau of Mines, who inaugurated and provided facilities for studies of Diesel engines. Acknowledgment is made to J. C. Holtz and H. H. Schrenk for helpful comments and criticisms, and to H. A. Watson, A. P. Rowles, R. L. Beatty, R. E. Kennedy, and C. K. White, who participated in this investigation. Thanks are also due H. M. Cooper, under whose supervision fuel samples were analyzed in the coal-analysis laboratory of the Bureau of Mines, and to the National Bureau of Standards for cooperation in furnishing additional data on fuel properties. LITERATURE CITED (1) Berger, L. B., Elliott, M. A., Holtz, J. C., and Schrenk, H. H . , U. S. Bur. Mines, Rept. Investigations 3541 (1940). (2) Boerlage, G . D., and Broeze, J. J., Chem. Rev., 22, 61-85 (1938). (3) Boerlage, G. D, and Broeze, J. J., IND. ENG.CHEW,28, 1229-34 (1936). (4) Coward, H. F.,and Jones, G. W., U. S. Bur. Mines, Bull. 279, 45 (1938). (5) Elliott, M. A,, Holtz, J. C., Berger, L. B., and Schrenk, H. H., U. S. Bur. Mines, Rept. Investigations 3584 (1941). (6) Holtz, J. C., Berger, L. B., Elliott, M. A,, and Schrenk, H . H., Ibid., 3508 (1940). (7) Jones, G. W., Chem. Rev., 22, 1-26 (1938). (8) Rothrock, A. M., and Waldron, C. D., Natl. Advisory Comrn. Aeronaut., Rept. 545 (1935). PR~ENTE before D the Division of Gas end Fuel Chemistry at the 103rd of tha AMERICAN CHEMICAL SOCIEITY. MernDhis, Tenn. Published Meetinn .. by permiasion of the Direator, U. S Bureau of Mines.
