I
PAUL
D. HOBSONI
Department of Engineering Research, Pennsylvania State University, University Park, Pa,
Diesel Combustion Chamber .Deposit Formation Combustion chamber deposits result primarily from pyrolytic reactions
FOR
DIESELCOMBUSTION, both its similarity to the diffusion flame (72) and the importance of pyrolytic decomposition (8) have been noted, but no accepted theory exists for soot formation in the hydrocarbon diffusion flame. Carbon formed by combustion consists of a disordered array of graphitic plates (76) which may result from partial dehydrogenation and polymerization of cyclic molecules (70). Extraction of biphenyl from a benzene flame (75) and the determination of polycyclics in solid and gaseous products of a n acetylene flame (5, 74) support this. Polymerization (9) has been contested on rather doubtful theoretical grounds. Experimental
Materials used were :
The disk surface temperature was between 50" and 75" F. above the jacket water temperature, some degree of uncertainty being due to constructional differences between the normal disk and the thermocouple. The weight of deposit on the disk was very sensitive to its temperature-a reduction of 20" F. in the jacket temperature reduced carbon deposit from n-hexadecane about 50%. Barometric pressure, ambient temperature, and humidity had no observable effect on the deposits formed by n-hexadecane and by an ordinary No. 1-D fuel within the range of variation encountered. For amyl nitrate, used as a cetane number improver, the weight of deposit was constant throughout the power range a t about 0.00005 gram per 10 grams of
B*P*at 760 Mm** g O c. O F.
n-Hexadecane I-Methylnaphthalene Decahydronaphthalene' a A mixture of isomers.
288 245 185-193
Sp. Gr. at 70° F.
Visc., 70' F.,
0.78 1.00 0.88
3.5 3.3 2.9
550 487 366-380
Cp.
Cetane No. 100 0
35 b
Determined by ASTM (1) engine method.
Limited use was made, for confirmatory purposes, of n-heptane and mixtures of aromatic and saturated cyclic hydrocarbons in the Diesel fuel range. Equipment. A CFR Diesel engine was fitted with a fixed compression ratio (17 to 1) precombustion chamber head. The pintle injector was removed from its normal position to the top of the precombustion chamber and replaced by a plug with a removable sampling disk flush with the chamber wall on which solid combustion products were deposited (see diagram). Electric immersion heaters were used in the sump and cooling system to maintain constant temperatures a t all loads. A synchronous generator of known efficiency was used to load the engine, giving a constant 900 crankshaft r.p.m. Preliminary Tests. The engine was motored at 200 " to 210 " F. oil and jacket temperature for 12 minutes a t full speed with the air intake open and again with it closed. No trace of oil was detectable on the sampling disk after either run. The disk was also clean after fuel had been injected for two or three strokes without ignition. Preliminary running with a thermocouple built into the disk showed that the disk temperature was governed by the jacket water temperature at all loads, Present address, Shell Oil Co., P.O. Box 71 1, Martinez, Calif.
amyl nitrate burned both as the sole fuel and diluted with 90% isopropyl alcohol. Deposit weight was found to be a linear function of the weight of fuel burned within the range of 200 to 400 grams. When soot and carbon particles from a given fuel were examined under the electron microscope, those from a diffusion flame and those extracted from the engine were indistinguishable. All reported results are corrected to a fuel weight of 300 grams. Experimental Procedure. The engine was run on stock fuel until temperatures were steady, and the injection system was then flushed through with the test fuel. A weighed sampling disk wire scrubbed to a bright surface was inserted and the engine restarted. After a measured volume of fuel had been burned, the engine was stopped and the disk withdrawn for weighing and examination of attached deposits. Tests were made for each fuel over a range of rack settings from maximum delivery to the lowest a t which the engine would fire regularly. Jacket water was maintained a t 210" F. and sump oil a t 195' to 205' F. Any deterioration in the fuel atomization increased deposit weight. Reproducibility was in general within &5%. Results
n-Hexadecane (Cetane).
T h e de-
posit formed by n-hexadecane a t high pump delivery (113% stoichiometric fuel to air ratio and over) was a light, fluffy soot that could be brushed readily from the disk. At normal full load (65 pounds per square inch brake mean effective pressure, 60% fuel-air ratio) the deposit was tarry and more adhesive, and under idling conditions (35% fuel to air ratio) it consisted of a n even coating of dark yellow lacquer (Figure 1). Electron microscope examination showed that the deposit formed a t full rack setting consisted of carbon particles in the form of disks of very uniform diameter, about 250 A., with a marked tendency to form chains. Thickness, dy shadow casting, ranged from about 20 to 300 A. A few particles were possibly smaller, but their size could not be determined with exactitude, as they were highly translucent to the electron beam. Electron diffraction patterns indicated a somewhat disordered graphitic type structure. As the fuel rack setting was decreased, increasing quantities of resinous material appeared together with the carbon until, under idling conditions, no free carbon could be detected. No diffraction pattern could be obtained from the resinous material. Analysis of the resin was not possible with the facilities available. A check run was made using n-heptane [boiling point, 98" C. (200" F.); cetane number, 571. The weight of deposit produced by this fuel was close to the limit of sensitivity of the available balance, but it seemed to follow the same general trend as the curve obtained with n-hexadecane. The engine and diffusion flame soot was very similar to that from n-hexadecane, except that the carbon particle diameter was about 150 A. As this is near the limit of resolution of the electron microscope, it was not possible to judge uniformity of the particles. The low deposit weight is probably due to the high volatility of this fuel. i-Methylnaphthalene. This fuel would not ignite in the engine without aid. Ethyl ether vapor was therefore used as a combustion initiator, a t the rate of about 30 grams per 100 grams of 1-methylnaphthalene, Because of the mode of introduction of the ethyl ether, it would not be appropriate to make a proportional change to the fuel-air ratio, as much of the oxygen available to the diffused vapor would not have been available to inject fuel droplets. No correction has, in fact, been made, and the VOL. 50,
NO. 3
MARCH 1958
337
IHJECTQR
ra
SAMPLING
,-DISK
Cylinder head of modified CFR engine
probable error due to this and to the effect, if any, of ethyl ether on the combustion reactions is recognized. T h e ignition delay of the fuel (20' to 22' crank angle) was such that the cetane number was assumed to be still zero. Three runs were made using l-methylnaphthalene: with ethyl ether, with 1.5% amyl nitrate to raise the cetane number to 15, and with 4.0% amyl nitrate to give a cetane number of 23. The qualitative results (Figure 1) were corrected for the weight of deposit attributable to amyl nitrate combustion and show marked increase of deposit weight with increase of cetane number. To check the possibility that this was the result of some effect of amyl nitrate other than cetane number increase, repeat runs were made with a n-hexadec-
ane-1-methylnaphthalene blends a t cetane numbers of 1 5 and 23. The weight of deposit formed, corrected for the (relatively trivial) weight of deposit due to the proportion of n-hexadecane in each blend, is in reasonable agreement with the corresponding results obtained using amyl nitrate a t the same cetane number level. The fuel-air ratio has been plotted considering the total weight of both fuels injected. The appearance of the deposits to the naked eye did not vary, except as to quantity, and resembled chimney soot. Under the electron microscope the composition of the deposits was appreciably more uniform over the range of fuel-air ratios than with n-hexadecane. Both carbon particles and resinous material were visible a t all fuel-air ratios, though a tendency to increased resin formation with decreased fuel-air ratio was visible. The appearance of deposits from the higher cetane number blends could not be distinguished from those obtained with 1-methylnaphthalene and ethyl ether vapor alone a t the same nominal fuel-air ratio except for the presence of constituents traceable to the n-hexadecane or amyl nitrate present. The carbon particles formed uniform disks about 750 A. in diameter and between 600 and 900 A. thick, with the same horizontally faulted graphitic structure found in the n-hexadecane soot. A single run using a mixture of aromatic hydrocarbons in the Diesel fuel range gave a deposit generally similar to that obtained with 1-methylnaphthalene,
Figure 1. Relationship o f combustion chamber deposit weight to fuel to air ratio 7 . n-Hexadecane, cetane number = 100 2. 1 -Methylnaphthalene and ethyl ether 3. 1 -Methylnaphthalene and amyl nitrate, cetane num. ber = 15 4. 1 -Methylnaphthalene and amyl nitrate, cetane number = 23 5. 1 -Methylnaphthalene and n-hexadecane, cetane number = 23. Deposits due to 1 -methylnaphthalene 6.
Decahydronaphthalene, cetane number =I 34 7. Decahydranaphthalene raised to cetane number 70 by addition of n-hexadecane and amyl nitrate (composite curve)
except that the carbon particle sizes varied over the range 500 to 1500,4. Insufficient material was available for quantitative results. Decahydronaphthalene. Decahydronaphthalene gave a somewhat heavier deposit than 1-methylnaphthalene under comparable conditions of fuel-air ratio, a result which may be at least partially attributable to its higher cetane number. As in the case of 1-methylnaphthalene, blending with amyl nitrate and with n-hexadecane to raise the cetane number caused an increase in the weight of deposit after correction for the weight of deposit due to the added material. The corrected deposit weight points obtained with a hexadecane-decahydronaphthalene and an amyl nitrate-decahydronaphthalene blend to cetane number 70 were so close together as to be plotted as a single curve in Figure 1. The weight of deposit formed by decahydronaphthalene combustion is close to that formed by 1-methylnaphthalene a t a comparable level of cetane number. Under the microscope most of the soot particles ere of identical size, shape, and structure with those produced by 1-methylnaphthalene. Interspersed with these were smaller particles ranging from about 400 A. in diameter down to the limit of resolution of the microscope, about 100 A. Some resinous material was visible in all deposits, the quantity tending to increase with decreasing fuelair ratio. A single run using a mixture of saturated cyclic hydrocarbons in the Diesel range gave under electron microscope examination a deposit of the same type as that obtained with decahydronaphthalene, except that the larger particles ranged in size from about 700 to 1500 A. in diameter. Two size range groups were still evident.
Discussion The major experimental findings of the investigation may be summarized as: Weight of deposit from combustion increases with decreasing fuel injection per stroke up to a maximum, after which (except for n-hexadecane) it falls off rapidly. The deDosits consist Dredominantlv of free carbon particles a i high rates of fuel injection, changing progressively to a predominantly resinous composition a t low fuel rack setting. Weight of deposit formed by n-hexadecane or n-heptane is small compared to that from decahydronaphthalene or 1-methyl-naphthalene No qualitative difference was detected, by electron microscope or electron diffraction, between the solid products of combustion deposited by a given fuel in the combustion chamber and those deposited from the luminous region of a diffusion flame of the same fuel burning under ordinary ambient atmospheric conditions. Each fuel tested formed carbon parI
STOICHIOMETRIC
338
FUE./Al
R RAT1 0
INDUSTRIAL AND ENGINEERING CHEMISTRY
COMBUSTION CHAMBER DEPOSITS
From cetane combustion
ticles of graphitic type and characteristic size or size range both in the engine and in a lamp diffusion flame. Additives to increase the cetane number of the cyclic fuels tested increase weight of combustion chamber deposit formed but have no other observable effect except presence of their products of combustion. Formation a n d Composition of Deposits. T h e first two findings are borne out by the observation (3) that “Lowload operation generally is most severe in causing the formation of extensive deposits. .(consisting of) resinous varnish and lacquer.” If the formation of solid combustion residues were the result of oxygen deficiency, the reverse result would be expected, as it is under these conditions that the greatest excess of oxygen is available. I n a precombustion chamber engine of the type used, variations of injection pressure with load should not be of major importance. If it is accepted that combustion chamber deposits are primarily due to intraor intermolecular reactions in the fuel, the sharp drop in deposit formation with decahydronaphthalene and l-methylnaphthalene a t very low fuel air ratios (Figure 1) may be explained on the assumption that this represents the point a t which the heat input into the system is below that required to maintain the reaction rate a t a significantly high value. If the lower heat value of the fuel is used as a n approximate measure of the activating energy input, the peak value for both the cyclic fuels occurs a t about 200 to 300 gram-calories per stroke. I t was not possible to achieve a peak point with n-hexadecane, even with as little as 40 gram-calories of heat release per firing stroke (calculated on the same basis), but this is not surprising in view of the instability of the higher paraffins a t high temperature. The combustion temperature in the vicinity of the burning fuel droplet is of the order of 3500’ F. even a t low powers (8). The marked increase in resinous content of the deposit toward the lower fuelair ratios may be explained in two ways:
.
Mixed with resinous material from cetane combustion
In the pyrolysis of hydrocarbons both polymerization and decomposition may occur simultaneously (6). I n general, pyrolytic decomposition is a n endothermic reaction, while polymerization is exothermic; thus it may be expected that polymerization will tend to predominate with reduction in the available heat energy in the system. Resins are readily produced by the oxidation of a wide range of hydrocarbons, and there is evidence, such as the known tendency of some cracked gasolines to form “gums,” that this is particularly true of cracked hydrocarbons. The strong aldehyde odor observed in the exhaust of nearly all Diesel engines a t very low load indicates a n oxidation reaction of the type that can lead to resin formation. The marked rise in deposit weight a t the lower fuel-air ratios may be attributed to the high resin content. T h e free carbon particles formed a t high fuel-air ratio have little tendency to stick to a metallic surface, while resins are not only notoriously adhesive a t high temperatures but also have a flypaper effect on any carbon particles present. I n view of the almost total lack of basic knowledge concerning the pyrolytic and oxidation reactions of the higher hydrocarbons a t the pressures and temperatures existing in the Diesel combustion chamber, any attempt to account for the differences in the quantity and type of the deposits formed by the various hydrocarbons is necessarily speculative. However, with natural fuels (3),paraffins are less likely than aromatics to form deposits-this also corresponds with their soot-forming tendency in a diffusion flame (6). Carbon Particle Size. The observation that each fuel produced carbon particles of characteristic size may throw some indirect light on the course of the combustion reactions. T h e typical naphthalene pyrolytic reaction is the‘ liberation of hydrogen and the formation of isomeric forms of dinaphthyl (7). Further dehydrogenation of this mole-
1-Methylnaphthalene combustion
Decahydronaphthalene particles
combustion
carbon
cule leads to the formation of a graphiticstructured carbon plate of monatomic thickness. T h e carbon particles found in the combustion chamber and diffusion flame soot have been observed by electron diffraction to consist of a n agglomeration of such laminae. PARAFFINCOMBUSTION. I t has been established in the present investigation that the carbon particles resulting from hexadecane and heptane combustion are much smaller than those formed by 1methylnaphthalene or by the mixture of aromatic fuels that was studied (and may tend to be smaller as the paraffin chain size is reduced). Gaydon and Wolfhard (6) obtained carbon particles which were uniformly 100 A. in diameter and a few 500-A.-diameter particles in a smoking flame. But the latter may be agglomerations. These particles are in any case of the same order of size as those obtained from heptane combustion, which accords with the concept that the particle size is a function of th‘e length of the molecular chain. In the case of the aromatic fuel already discussed, dehydrogenation will produce carbon of graphitic ring structure in ready-made form. I n the case of a n aliphatic molecule, decomposition of C-C bonds is the typical pyrolytic reaction (4), and while many theories exist conVOL. 50, NO. 3
*
MARCH 1958
339
cerning the detailed course of the reaction (9, 70, 7 4 , it is generally agreed that the molecule is disintegrated. The ring structure of the carbon particles must then be built up from the freed carbon atoms. The total reaction time available is, however, practically the same for all fuels, so that the time available for particles to build up is less in the case of aliphatic than of aromatic fuels. It may also be postulated that the production of resins, in so far as it may be the result of polymerization, is similarly restricted. This may account for the markedly reduced deposit formation obtained from the combustion of aliphatic fuels in comparison with cyclic fuels, although this does not explain the apparent uniformity of particle size for each aliphatic fuel molecule. A tentative postulate is that the carbon atoms from each molecule remain loosely grouped during the combustion reactions, thus forming a basic “building block’ for later polymerization related to the size of the original molecule. Decahydronaphthalene Combustion.
The majority of the carbon particles resulting from the combustion of decahydronaphthalene were the same size as those resulting from 1-methylnaphthalene combustion, together with the much smaller particles typical of aliphatic molecule combustion. The particles resulting from the combustion of a mixture of saturated cyclic fuels were similarly divided into tlro size ranges. Presumably, this is the result of dehydrogenation of some of the molecules, following essentially the same path postulated for carbon formation from the aromatic fuel, while other molecules were disintegrated by the rupture of c-C bonds followed thereafter by the same mode of particle formation as the aliphatic fuels. Effect of Variation of Cetane Number. The increase in quantity of combustion chamber deposit for a given fuel, when the cetane number was raised by the addition of amyl nitrate or nhexadecane, may be explained by injection and ignition processes in the engine. Fuel entering the combustion chamber a t the beginning of injection and u p to the time of ignition is suspended in air a t a temperature of the order of 1200’ F. and a pressure of 520 to 550 pounds per square inch. When ignition occurs, most of the fuel already in the combustion chamber burns in a very short period. In the extreme case of l-methylnaphthalene, burning under zero cetane number conditions with a n injection period of 18’ crank angle and an ignition delay period of from 20” to 22’, combustion was so rapid that even under maximum x-axis amplification the oscilloscope trace disappeared completely during the pressure rise, and no horizontal displacement could be detected between the end of the trace a t ignition and its reappearance
340
f I
7,:,1--c
0
-.--i3
:ET;,!
CLiDh!
---;&E
\
f
C r r O
- y % _ _ E
a-
Figure 2. Cylinder pressure diagrams obtained with 0- and 23-cetane fuels
after combustion. Knock was correspondingly heavy. A reproduction of the cylinder diagram obtained is shown in Figure 2. IYhen the cetane number was increased to 23 by the addition of amyl nitrate, with the same injection timing and period of injection, ignition occurred about one millisecond earlier. Schvieitzer ( 7 7) established that fuel injected into a flame has an ignition delay of the order of 65% of the ignirion delay of the same fuel injected into air heated by compression in the normal manner, with a minimum delay of the order of 1 millisecond. Fuel injected after combustion has begun will therefore be maintained a t a temperature and pressure much higher than those existing before ignition, and these conditions will be maintained for a further short, bur still finite, period during the combustion of the droplet. As rhe rate of pyrolytic reaction is an exponential function of the temperature and also increases with the pressure, the quantity of solid combustion chamber deposits from a given quaniity of a specific fuel will increase with the proportion of the fuel that is injected after the beginning of combustion. As this proportion increases with reduction in ignition delay, an increase in cetane number would tend to produce an increase in combustion chamber deposits for a given fuel. Ignition delay is not a linear function of cetane number for a constant compression ratio engine. I t has been observed in both bomb (2) and engine (13) tests that the effect of cetane number increase on the reduction of ignition delay becomes progressively less with increasing cetane number. If the above explanation of the effect of cetane number on deposit formation is correct, this will lead to the observed finding that the increase in deposit formation with increase in cetane number is progressively less as the cetane number is raised. I t follows from this that the observed tendency of some Diesel engines to smoke heavily when supplied with fuel of unusually high cetane number may be due to a greater degree of pyrolytic breakdown of the fuei under these condi-
INDUSTRIAL AND ENGINEERING CHEMISTRY
tions rather than to local oxygen deficiency. The frequently reported observation (3) that high cetane numbes fuels produce less combustion chamber deposit than do low cetane number fuels may be attributed to the fact that, in the case of the natural, straight-run Diesel fuels on which most of these observations were made, high cetane number is usually a reflection of high paraffinic content. This investigation, supported by others ( 3 ) , shows that paraffins produce less deposit than other hydrocarbons,
Conclusions Combustion chamber deposits result primarily from pyrolytic reactions of the fuel molecule, which under constant operating conditions, are governed by the molecular structure of the fuel. Paraffin fuels tend to produce less deposit than those of cyclic hydrocarbons. The more objectional-,lesolid products of combustion are those formed under low engine load conditions. Increase of cetane number increases combustion chamber deposits, but this effect is minor compared with those of fuel hydrocarbon type and of operating conditions. The combustion procxss in the Diesel engine is similar to that of the same fuel in a diffusion flame under ordinary atmospheric ambient conditions. Literature Cited
(1) Am. Soc. Testing Materials, Philadelphia, Pa., “4STM Standards on Petroleum Products and Lubricants,” Method D 613-48T. (2) Coordinating Research Council. New York. - ,N. Y . . “Combustion Characteristics-Ignition Delay Bomb 1948-1950,” 1951. Coordinating Research Council, New York, N. Y., “Survey of Available Information on the Deposit Forming Characteristics of Diesel Fuels and Engines,” 1954. Ellis, C., “Chemistry of Petroleum Derivatives,” 2nd ed., p. 71, Reinhold, Kew York, 1937. Ibid., p. 244. Gaydon, A . G., \Volfhard, H. G., “Flames, Their Structure, Radiation and Temperature,” Chap. VIII, Macmit!an, New York, 1954. Hurd, C. D., Pyrolysis of Carbon Compounds,” p. 9-’, Chemical Catalog Co., New York, 1929. Meurer. J. S., S.A.E. Journal 63, 18 ( 1955’). ( 9 ) Porter, G., Armed Services Tech. Inform. Agency, AD 44761 (1955). (10) Rummel, IC., Veh, P. O., Arch. EisenhiittenzJ.14, 489 (1941). (11) Schweitzer, P. H., AutomotiDe Inds. 78, 848 (1938): (12) Ibid., 114, 66 (1936).
(13) Shoemaker: F. G., Gadebusch, H. M., S.A.E. Journal 45, 339 (1946). (14) Stehling, F. C., Frazee, J . D., Anderson, R. C., “6th International Symposium on Combustion,” p. 247, Reinhold, New York, 1957. (15) Thorp, K. T., Long, R., Garner, F. H., Fuel 30, 266 (1931). (16) Warren, B. E., J . C h m . P h y ~ .2, 551 (1934). RECEIVED for review October 3, 1956 .ACCEPTED June 24, 1957
