Combustion Qualities of Diesel Fuel - Industrial & Engineering

Combustion Qualities of Diesel Fuel. G. D. Boerlage, J. J. Broeze. Ind. Eng. Chem. , 1936, 28 (10), pp 1229–1234. DOI: 10.1021/ie50322a025. Publicat...
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by volume, and about 37 per cent total sugars. The following is a typical formula for grapefruit: Sucrose, grams Corn sugar, grams 164-proof citrus spirits, cc. Distilled water, cc. Naringin. gram Grapefruit oil (cold-pressed), cc. Certified yellon dye

paratus and processes to yield the products discuqced here, as well as a number of by-products.

Literature Cited


888 1510 1165 0.90


Su5oient quantity

The oil and nari,lgill nere dissolved in the spirits by violent and corn agitation and then added to the sirup Of wear. The mixture \vas allowed to stand a t about 27” to 30’ C. (80” to 85” F.)for :iweek and then filtered, using a filter aid. Aging was carried out in glass. Tangerine, lime, lemon, and orange cordials were made in a similar manner by the use of the cold-pressed and distilled oils from the peel of the particular fruit. Figure 7 shows in schematic form the arrangement of ap-


(1) Congress of U. S. (74th), Public S o . 401 (H. R. 8870), pp. 12-14, See. 11 and ff. (2) Cruess, W. V., Calif. Agr. Expt. Sta., BULL244, 157-70 (1914). (3) Gary, W. Y . , Fla. Dept. Agr., Chem. Lab. Div., p. 17, March. 1 Q25 _---.

(4) Hill, H. P.. Fruit Products J . , 14, 138-42, 156 (1935) (5) J o s h , M.A., and sfarsh, G. L , I b i d , 1 3 , 3 0 7 (1934). (6) McNair, J. B , “Citrus Products,” Pt. 1, pp. 138-9, 1926. (7) U. S. Dept. Agr , Service and Regulatory Announcement, Food and Drun S o . 2. Rev. 3. D. 18. June. 1932. (8) U.S. Treasury Dept., Bur.’Prohibition Reg. 7, May, 1930, Sec. 610 (Act of Feb. 24, 1919: 40 Stat. 1057). (9) VonLoesecke, H. W., Cztrus Ind., 15, S o . 7 , 8-9, 20-1 (1934); Proc. Fla. State Hort. SOC.,1934, 85-90. (io) Von Loesecke, H. w., F L ~Grower, . 43, 5-6 (1935). RBCEIVED

July 11, 1936.

Department of igriculturs, Food Kesearch 131-

vision Contribution 293.

Combustion Qualities of Diesel Fuel

0 . I). BOERLAGE AND J. J. BROEZE N. V. de Bataafsche Petroleum Maatschappij, The Hague, Holland

DEAL conibustion in a C. I. (compression ignition) engine means instantaneous and complete combustion of every particle of fuel as soon as i t is injected (Figure 1, I). I n practice, combustion lags behind the theoretical curve at two points: ( a ) Mainly because of insufficient reaction velocity during the pre5ame or ignition period (phase 1, Figure 1, 11); this lag, however, is overcome by the rapid spread of flame (phase 2) so that even fuel injected later (phase 3) burns with great rapidity; the reaction velocity is meanwhile greatly increased by the high temperature. Mainly because of inefficient mixing of fuel and air; this lag(b) increases towards the end of phases 2 and 3.

Combustion is not finished when fuel injection stops; it continues as long as fuel particles find oxygen. This is called “after-burning” (phase 4); i t lowers the efficiency and performance of t h e engine because the expansion ratio is decreasing a t the same time. As the fuel quantity per cycle is increased, after-burning increases and results in incomplete combustion a t the end of the expansion stroke, although air is still present. The imperfection of the mixture is due to two causes: defective micromixture (insufficient atomization and evaporation) and defective macromixture (insufficient distribution) (2, 3). As the fuel is injected in the liquid phase, there is insufficient evaporation during the ignition period. The latter is represented as follows (Figure 2) : (a) Heating of the droplets, partial evaporation, and rapid subse uent heating of the vapors to air temperature (600’ to 800’ by d’irect contact. (b) Development of heat from reaction of the vapors, which causes locally increased temperatures (beginning with one or more points, where the vapor concentration and other conditions are most favorable), until flame temperature is attained which spreads rapidly.


Period a may be called the “physical delay” (endothermic), and period b the “chemical delay” (exothermic); actually the two overlap. For normal light Diesel fuels the chemical

character governs the total ignition period, showing that the physical delay is short; heavy fuels such as oil residues have relatively lower cetene numbers than the gas oil fractions from their crudes (Table I). Evaporation before the flame is formed depends mainly on the temperature of the compressed air and of the walls and on the nature of the spray. When starting cold, i t may be insufficient in many types of engine; sometimes an excess of fuel may be helpful (the equivalent of choking in carburetor engines)

The ignition period consists of a physical and a chemical delay. The former becomes important with heavy fuels, the latter is normally predominant. After-burning is mainly due to uneven distribution and to slow evaporation of fuel deposits on combustion chamber walls : the most favorable mixing conditions are, at best, a compromise between these two. The fuel influences the mixing process by its viscosity, its volatility, and its ignition quality; the optimum value of each property varies with the engine type. A better criterion for volatility is needed. Combustion in C. I. engines is mostly of the destructive type: under certain conditions there is evidence that it may be partially an oxidation process-for example, according to the hydroxylation theory.

Evaporation of atomized fuel after the flame is formed is governed mainly by the high temp e r a t u r e s of the $1 f l a m e (1500’ to 7 VI 2300’ C.) a n d i s kn rapid enough to ensure proper combustion. Often i t is too rapid, and local accumulations of vapor may resultLe., a bad macroFIQVRE 1. ACTUALPRESSURE mixture. DIActRAM Serio u s trouble m a y a r i s e when liquid fuel is deposited on the combustion chamber walls, since such fuel has to be evaporated under adverse conditions. The quantity of fuel thus deposited (a fault of the macromixture unless it is done intentionally as in the case of a “vaporizer” chamber, Figure 13), its volatility, the temperature of the wall and the gases, and the amount of swirl determine the rate a t which such fuel will be taken up again in the combustion process. The range over which these factors may spread is easily appreciated when a light gas oil burned a t full load in a hot combustion chamber using a strong swirl is compared with a residual fuel burned a t no load in a cold engine without swirl. Under the latter conditions blue smoke, lubricating-oil dilution (or thickening) by unburnt fuel, and carbonization are to be expected; but even when these are avoided, a large amount of invisible after-burning may occur and affect the performance. This puts a limit inter alia to the grade of fuel on which an engine may operate to satisfaction. Some of the flame photographs in Figure 5 of a report of the National Advisory Committee for Aeronautics (8) show the after-burning of fuel deposited at the end of jet travel (particularly for air-fuel ratios of 34.3 and 50.5).



OaaOil Fractions


60 58-60

44 44






VOL. 28, NO. 10





Crude Oil E F


Gas Oil Fractions 44

30-33 27

Residue 39 24c 15

Crude oil.

Incomplete distribution is caused primarily by the uneven character of the spray. The influence of fuel viscosity on spray formation has been studied elsewhere (‘7); high viscosity accentuates a tendency of the spray to penetrate too far, thus depositing fuel on the opposite wall. Low viscosity tends to improve atomization, with a corresponding lack in penetration leading t o local accumulations of fuel. Both faults may occur and may even coincide when a spray is illsuited to the combustion chamber. Air movement may correct these faults to some extent although, if overdone, an air swirl may cause extra deposits by centrifuging. Evaporation may also correct distribution, especially in the case of fuel deposits, but too fast evaporation in the spray itself tends to “fix” its irregularities. This happens particularly a t the moment when the spray ignites; this moment and therefore the ignition quality of the fuel are important in engine efficiency. Apparently, therefore, the total efficiency of distribution depends on a number of factors, such as engine design and operating conditions, on the one hand, and fuel characteristics

on the other. A compromise may be reached when various mixture faults balance one another and cause the smallest total amount of after-burning. This compromise does not always coincide with the type of fuel for which the engine was designed.

Influence of Fuel Characteristics on the Mixture The three fuel properties that influence the mixing procesr have now been discussed: VOLATILITY. This property bas been considered in general and more particularly for fuel deposits: With heavy fuels the physical part of the ignition period is often important. VIBCOSITY. This property influences the character of the spray and therefore the distribution and the amount of fuel deposited on walls: IGKITION QUALITY. This is the main factor during the ignition period; at the end of this period temperatures jump from 600800” to 1500-2300” C., which influences distribution via the resulting evaporation.

Other fuel properties might be mentioned here, such as the carbon-hydrogen ratio which determines the theoretical fuelair ratio and the specific gravity which influences spray energy but these properties vary only over a small range in comparison with t h e FLAME

ity, low boiling range, and high ignition quality tend to make a spray “softer,” l e s s penetrating, and more e a s i l y evaporated, tending ‘%TART OF INJLCTION TIME generally t o wards- local FIGURE2. TEMPERATURES DURING over-concentraIcthTTION tions of vapor. Conversely, high viscosity, high boiling range, and low ignition quality tend to make a spray “harder,” more penetrating, and more liable to cause fuel deposits; the occurrence of the latter greatly increases the effect of the high boiling range.

Need of Volatility Scale for Diesel Fuels One difficulty in discussing these effects is due to the lack of a volatility scale. The A. S. T. M. distillation can be used only up t o about 350” C., when cracking begins in many fuels. For Diesel fuels the distillation curve is best utilized in accordance with kerosene practice; the 65 per cent distilled point is taken which is close to the dew point and therefore represents a measure for total evaporation. Neither initial nor tail fractions have been very illuminating. With heavier fuels a distillation test has to be made under vacuum or with the aid of steam in order to avoid decomposition. On the other hand, it is doubtful whether such fuels evaporate in the engine without decomposition. Their volatility may be in practice a combination of physical and chemical properties, and it may be useful to distinguish between “physical” and “chemical” volatility. The only tests which a t the moment give an indication of these combined properties are carbonizing tests; these, however, show only the amount of residue left. The best known is probably the Conradson carbon test which combines physical and chemical volatility to some extent, since it allows products which evaporate below cracking temperatures to escape. The other components of the fuel will crack and carbonize to a certain ex-



tent, depending on their nature. The 10 authors have found a fair agree8 ment between cons u m p t i o n measurements and 2 6 carbon deposits in the engine and t h e Conradson V number in many 2 cases. The percentage of carbon residue 65% A STM in this test is not FIGURE3. VISCOSITY AT 50” C. us. an indication of VOLATILITY “unburnable” products; in fact, with a suitable arrangement no carbon will be formed in the engine. The authors are experimenting Rith a solid injection engine of 30 horsepower, in which fuels u p to 12 per cent Conradson carbon re,idue have been burned without the formation of either carbon in the engine or a trace of smoke a t a B. M. E. P. (brake mean effective pressure) of 100 pounds per square inch. The engine was developed on the lines of attacking the fuel deposits. Yet, in general, a fuel with a high Conradson residue is more difficult to burn efficiently and cleanly than one with a lower residue. Special engine design is necessary for the best utilization of such fuels, and hot combustion chamber walls are a primary condition. It is extremely unsatisfactory that the lack of a suitable test forces one t o base the volatility of light fuels on the A. S. T. M. distillation and that of heavy fuels on the Conradson t e d , since neither test is of 220 any use in the other case. Some c o m m o n b a s i s Yhould be found. Modif +200 fied c a r b o n tests and 9 d evaporation t e s t s d e s< 180 vised by certain engine dg makers point to the fact z$ that this need is being cI $ul 4 kg6 BRAKE 8 1 0 1 2 felt. FORCL The s o - c a l l e d hard4. FUEL CONSUMPTIOS asphalt content-i. e., FIGURN CURVEBFOR ENGINEA the percentage of fuel inYoluble in aromatic-free petroleum spirit or diethyl ether-is not a satisfactory indication of low-volatility components. The carbon test agree> far better with practical results. 12




Engine Experiments on Light Fuels For the fuels used in these experiments there appeared to be a close relation (Figure 3) between the viscosity a t 50’ C . and the 65 per cent point of the A. S. T. M. distillation curve; the two i d u e n c e s could not be separated. Fuel consumption curves on a typical direct-injection highspeed engine are reproduced in Figure 4, where i t is shown that the B. M. E. P. for just visible exhaust may differ considerably with the fuel used (P = Q). Keeping the load constant a t the best value obtainable, Q, and using various fuels, a graph was obtained (Figure 5) which shows t h a t minimum fuel consumption coincides with a definite combination of volatility and ignition quality; i t corresponds apparently with optimum distribution. The difference in the amount of after-burning for fuels I and I1 is shown in Figure 6; the most efficient fuel (11) was m n i n g rather rough and was certainly not the fuel for which the engine was designed. At a lower load, R (Figure 4), where fuel consumptions were


almost equal over a wide range of fuels and the exhaust gases were equally clear, the more efficient fuel, 11, was found to produce less soot in the lubricating oil (as expressed in gramb per brake horsepower per hour) than fuels of higher cetene number (Figure 7 ) . A paradox i- thu. obtained which will be referred to later; a more paraffinic fuel, -uch a> fuel I, may posqibly have a greater tendency to caube v i o t in a D i e 4 engine than a more aromatic one, iucli as fuel 11. An important conclusion is as followvs: A low-\-iscobity, volatile fuel of high cetene number by no meanu guaranteebetter results as to power or economy, although it is good for Ytartingand smooth running. On the contrary, it iq often the lower cetene fuel t h a t will make for higher perforniance. The example quoted confirms the theory that evaporation may be too fast in a C. I. engine. Figure 8 is similar to Figure 5 for a different type of engine. In this case, the best result a t full load is obtained on kerobenes of varying cetene number, again a type of fuel for which the engine was not designed. Further, an increasing boiling range is partly counteracted in its effects by a n increasing cetene number. Probably in this engine too much fuel reaches a cold wall. At low loads when the walls are colder, the ignition period+ longer, and the air excess many times greater, the risk of untimely evaporation and local overrichness is small; what must he feared mainly under these conditions is fuel deposit, leading to blue smoke, acrid odors, and in extreme cases, lubricating oil dilution and carbonization. Figure 9 gives a typical example, ihowing consumption and blue smoke as a function of the same variables as before on t h e engine of Figure 5. A given fuel may behave quite differently in different engines and in one and the same engine under different conditionq: therefore a compromise has to be made in practice.

Engine Experiments on Heavy Fuels Taking the Conradson carbon residue as an indication of volatility, a better separation of the factors volatility and viscosity could be arrived at, showing that, contrary to an opinion generally held, volatility is of more importance than viscosity. This can be explained by the facts that (a) the influence of viscosity on spray is not very important unless a wide range is employed, and (b) far greater amounts of fuel are deposited on comlni-tion chamber walls than iq usually believed.



280 320 360 65 % A . S . W .


Figure 10 shows fuel consumptions at full load plotted against Conradson carbon residue, viscosity, and hard



asphalt content in a low-speed direct-injection engine. The correlation with the Conradson carbon is the best; a t the same time no influence is felt up to a certain minimum. This can be compared with the minimum in Figure 5 where a 1o we r b o i 1 i n g TO C r a n g e gives no m R -BURNING improvement Or even the reverse. Ignition quality has no influence, the delays being very short relaI tive to the inFU j e c t i o n period. FIGURE 6. PRESSURE DIAGRAMSFOR Whatever relaLOAD.IT POINTP (FIGURE 4)ATD ENGISEA tion exists between the resultin Figure 10 and viscosity is due rather to the relation between viscosity and Conradson carbon of the fuels tested; preheating. which affects viscosity, causes little difference in this engine. Figures 11and 12 show some examples where the Conradson carbon residue is related to fuel consumption and carbonieation in a n idling engine. I n this engine the spray, which is directed towards the lower half of the combustion chamber (Figure 13), is fairly "hard," so that the influence of viscosity on the amount of fuel deposited in the lower half of the chamber is apparent from the amount of carbon found there. Thi. influence is seen particularly in Figure 12 which gives results of tests on two series of fuels with widely different viscosity-Conradson relations. These fuels were blend., respectively, of gas oil and a residue, and of the same gas oil and the same residue deasphaltized; the latter blends had higher i-iscosities for equal Conradson residues (Table 11). The fact that they contained no hard asphalt illustrates the fallacy of the hard asphalt content as a guide to the carbonizing tendency.

Here large amounts of residue are found even in the exhaust pipe. The fuel is approximately the same as that in Figure 12 a t the extreme right. The influence of air temperature predominates over those of viscosity (fuel temperature) and spray characteristics (nozzle size), even though some of the tests were macle ~ v i t hextremely high viscosities.

Chemical Considerations of After-Burning and Unburned Products Clieinically t n o factor; are of intere.t. In the firqt place, the velocity of reaction i+of actual importance only during the ignition period; later 011the combuition velocity is practically no longer dependent on the chemical reaction velocity hut on the phyqical factor.. controlling the mixture formation. I n the second place, the kind of reactions is of great importance in view of the nature of the product.. to be expected when combustion is incomplete. It is: useful to separate the reactions into those occurring during the ignition period and those occurring after the flame has been formed, since it is by no means to be exI 1 pected that they are 20 30 40 50 60 ofthesamekind. As CLTLNE VALUE OF THE FULL to the preflame reacFIGURE 7 . SOOT IN LGBRICATING OIL tions whose .. FOR LOAD AT POINT Q (FIGURE 4) determinetheignition AND EXGINEA delay, evidence ha. been brought forward for the formation of peroxides and for partial decomposition. This controversy has not, perhaps, been concluded, but further reference to it would go beyond the scope of this paper. The engineer is interested only in the result-namely, the ignition quality of the fuel as measurable from the delay period


Gas Oil




100 80 60 40 20


40 60 80


1005 75"

25 33

674 a

Viscosity (50° C.) Centistokes 500 65 19 7 6 3.6 96 25 18





10.6 8,s 6.4 4.2 2.1 3.5 2.6 2.4


5.1 3.8 2.6

1.3 0.0 0.0 0.0

Deasphaltized residue.



1 2 3 4 5 6

(Cooling water temperature. 60' C.) Unburned Fuel and Carbon In combustion Sprayer Air Fuel Fuel chamIn Dimensions Temp. Temp. Viscosity ber exhaust CentiAim. OC. "C. stokes Gram Grams 240 75 500 20 60 0.25 X 4 100 45 60 240 90 0.25 X 4 200 45 90 15 10,000 0.25 X 4 240 65 380 25 60 0.10 X 4 35 240 15 90 60 0.10 X 4 35 60 90 15 10,000 0.10 X 4 ~~


Hard Asphalt


T'OL. 28, NO. 10



fz 40 d 50



5 u








360 65%



The combustion chamber of Figure 13 resembles H. R. Ricardo's designs and is the one referred to earlier with respect t o clean burning of heavy residue. I n a more normal combustion chamber the amounts of unburned fuel and carbon after running a t no load are far greater (Table 111).






1.7 ,

0 2 4 6 8 1 0 1 2 CONRADSON CARBON 2 00

100 -I ul
















J 400










centiitokes a t 75' C )


tu 5 tl 0 0

$$ L




(Kumbers in parentheses represent visco3ities i n oentistokes a t ,500 C.)

Comparing the delay period TI ith those of reference fuels he find. that to every fuel can be ascribed a number (cetene number) which indicates the relative ignition quality. As to the reactions during the flame period, although i t has been shown that on the whole their velocity is so high as not to impede the process, they are still of great interest from the point af view of the products (of incomplete combustion due to different kinds of after-burning. The various kinds of carbonization, of odors, and of smoke from the exhaust that a Dieqel engine is capable of producing can be better explained with reference to the different types of reaction. Combustion of hydrocarbons may be, in principle, a direct oxidation (Bone and Wheeler's theory of hydroxylation, 4) or a decomposition followed by oxidation of the destruction products (AufhSuser's theory of destructive combustion, I ) ; in practice there will be a race between the two processes (6). According to Haslam and Russell (6) the conditions are more favorable for hydroxylation when the fuel has been well vaporized and mixed with air before it is burned; the flame is then blue and has no tendency to soot. When this flame is chilled, however, there is a tendency to stop this process a t an intermediate stage, leaving products such as aldehydes and organic acids which cause acrid exhaust odors. The conditions are more favorable for destructive combustion when the fuel is exposed very suddenly and in a badly vaporized condition to flame temperature; the overheated fuel particles decompose rapidly before they can find oxygen. The flame shows yellow radiation because of the glowing carbon and has a tendency to form soot either when i t is chilled or when the mixture is too rich. Experience shows, as is to be expected, t h a t the latter


process is followed largely in the C. I. engine. Flame photographs (8) and the tendency to soot. which limits the pon-er output,,strengthen this opinion. Yet the aldehyde odor from cold engines, especially a t low loads, and varnish deposits (found during conditions of lon- load, of misfiring, or of escessive turbulence, 51, which consist of organic acids and polymerized aldehydes, both lead to the conclusion that a t least part of the combustion has the character of hydroxylation. The circumstances under which it takes place make t,he authors doubt, however, that hydroxylation could ever be the sole combustion process for C. I. engines. Its advantage would be the avoidance of soot so that' a greater overload could be carried for short times, and less contamination of the lubricating oil might result. The three typical products of incomplete combustion, then, are aldehydes and varnish (both by chilled hydroxylation), soot from overrich mixtures, and deposition on the walls of the combustion chamber by chilled destructive combustion. Further products of incomplete combustion occur, owing to the wetting of combustion chamber walls. Especially a t lower loads and lower volability, some fuel niay be left unevaporated and, according to circumstances, may color the exhaust blue or gray, dilute the lubricating oil, or, if the fuel is heavy, be carbonized. The latter form of "carbon" is entirely different from the soot mentioned previously; it is a product of evaporation, decomposibion, and oxidation of liquid fuel, ranging from a thick oily substance to sticky asphalt and hard, often glittering, carbon. The latter is formed (at the spot where i t is found) from deposited fuel when this has a tendency to carbonize, as measured, for instance, in a carbonizing test (Conradson). The soot from destructive com-

Low load, cold walls Any Ovcdoad." underuenetration t w

little swirl, high load Unlmmsd fuel (blue-ErriLv

Low load, o d d w d s . overpene-


tration, too muoh swirl

Lubricating oil dilution .4sphaltic or osrbonrtooow deposits XTouzle onrbon

hustion i- a product formed in mid-air, so to speak, from gasified fuel and is theref o r e Eoft a n d Aiiffy bii: never oily or asphaltic. I: is found wherever it iiiay qettle out from the gwcs 07 where the A:une Ir, eliilleri against a cold wall. \Vhsi is fouiid in a more solid form on the top lands of p i s t o n s or behind pistoti rings may be aii acaumulation of soot with deconiposition prorlueti of lubrioatiiig oil. Among the produets O S carbonization of liquid fuel. t h e carbon formed oii nozzles is u a r t i c u l a r l Y objectionable b e c a u s e i t interferes with the spray (Figure 14). I t occurs iilieii fuel* possessing a tendency to carbonize or containirig some kinds of ash are used; it can be combated by eouliiig the surface of the nozzle below temperatures at which carlx,nimtion takes place. This requires intensive cooliirg of tlie nuzzle piece itself; the same procedure for avoiding carbonizatioii carinot he applied to the rest of the combustion chamber, since it involves drastic chilling of the reaction, which in the latter ease would leave a large quantity of fuel entirely unhurued. The reverse method which is applied to the chamber wallsi. e., heating to the extent. necessary for evaporating the fuelis, on the other hand, practically precluded for nozzles on account of the fact that carbonization would then take place inside the nozzle. The products of incomplete combustion may he classified according to the kinds of after-burning with which they are connected and according to the engine conditions with which they are most likely to occur. It must be remembered that these conditions are strongly iduenoed by design; actually in a badly designed engine there is no difficulty in producing most of them at the same time. Even so, Table IV must not he regarded as a n infallible guide. The appearance of soot and blue fumes have thus been considered from the chemical standpoint. A h a 1 word may be

Low load. odd walls, oveipene-

tratirrn Nozzle not properly cooled

d e v o t e d t o t h e seeming paradox that paraffinic fuels may soot more than more aromatic fuels. It will now b e r e c o g n i z e d t h a t the l a t t , e r , a p a r t from their better distribution during longer delay, which results i n l e s s o v e r r i c h patches, may have a hotter chance of more hydroxylative burning. It must be emphasized that in the cace illustrated i n F i g u r e 6 the ignition period was of the order of o n e - t h i r d of theinjection p e r i o d f o r the paraffinic fuel but lasted about as long as the injection period in the case of the aromatic fuel. I n a low-speedengine where delays &re, say, from 5 tu 15 per cent of the injection period, and the mixture conditions therefore more nearly equal, the chance would he for the aromatie fuel to produce il,