HYDROCARBONS IN THE DIESEL BOILING RANGE R. W. HURN
H. M. SMITH
AND
Bureau of Mines, Petroleum Experiment Station, Bartlesville,
Increased campetition for the petroleum cuts most suitable for use as straight-run Diesel fuel has resulted in the need for more widespread utilization of cracked stocks and other less desirable fuels in Diesel service. Relatively little is known regarding the combustion characteristics of sych fuels, but if they are to be economically and intelligently utilized, these properties must be determined or methods must be developed for predicting ignition and combustion characteristics. To gain fundamental information relating to this problem. the Bureau of Mines has initiated a program to study the combustion Characteristics of pure hydrocarbons and separated fuel fractions which, in mixture, comprise the Diesel fuels.
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Data have been obtained which show the manner i n which simple changes in hydrocarbon structure affect autoignition lag of fuels. The normal paraffins, C7 through CIS, the corresponding or-olefins, and several naphthenic types have been investigated. The data reported are intended to form the framework for a more comprehensive cataloging of the ignition and combustion characteristics of the principal hydrocarbons found in Diesel fuels. Study of these characteristics and of the manner in which each hydrocarbon type influences the combustion behavior of any mixture in which it may occur should yield a better basic understanding of t h e relationships between fuel composition and its combustion behavior under diverse conditions of utilization.
ture. Any systematic study of structure and combustion phenomHE course of combustion in any heat engine is controlled by ena xould require investigation of many hydrocarbons, most of two general factors. The first factor includes mechanical which are or would be available only in .mall quantities. This and physical influences that control the introduction of fuel fact precludes use of a conventional engine in the investigations. into the system, evaporation of the fuel, and its final mixing. The A Combustion bomb ansivers some of the requirements and in addisecond is a chemical factor that dictates the combustion course tion provides a flexibility of operation unequaled by any convenonce the fuel has been admitted and brought into intimate contional engine. For these reasons, a constant-volume combustion tact with its reactants under reacting conditions. Until quite bomb was selected by the Bureau of 1Iines t o be used in its investirecently in Diesel-engine and Diesel-fuels research, major emgation of these chemical factors which have become so important. phasis was placed upon the involved physical factors, and the As early as 1922 European investigatorF (8) had used a form of greater effort had been expanded upon improving the mechanics combustion bomb, and later, valuable contiibutions aere made to of autoignition combustion systems. Consideration of the chemthese studies by Selden (6) in this country and by Michailova and ical factor was given little attention because the virgin stock Neuman ( 4 ) in Russia. Petrov ( 6 ) utilized the equipment of fuels commonly used appeared to carry the process forxvard satishlichailova and Neuman to study many Diesel-range hydrofactorily after the chemical factor was in control. carbons, and his measurements constitute the only ignitionNow, however, the picture is reversed. At the same time that quality data on many compounds of interest in Diesel-fuels rethe engine manufacturer seems to be to the point of diminishing search; therefore, his data have been used aa a background for returns in improvement of the mechanical and physical factors, much of the bureau’s contemporarr work. the fuel supply of favorable stocks has been threatened with deIn 1948 the Bureau pletion by competing of Mines began active interests. Therefore, FUEL development of a conconsiderable attention stant-volume combusis being focused on the , I CURVE tion bomb for Dieselrelationships of DieselHIGH IGNITION fuels studies. About fuel composition and m z QUALITY-75CETANE t w o y e a r s were reengine performance, CURVE 89 w *9v quired to develop the and especially on the npparatus and to refine e f f e c t of chemical u 3 the technique. By 5 9 structure on the comMEDIUM IGNITION m midyear of 1950 the bustion process; thus m z ‘ OUALITY- SOCETANE W 4 data obtained were reE : c h e m i c a l considera001 sec. a w 0 producible enough to t i o n s h a v e become Lo W O 4 - 2 meet the experimental pre-eminent. r e q u i r e m e n t s . The Theoretically, Diesel LOW IGNITION ignition characteristics fuels probably consist OUALITY - 2 5 CETANE cbf Eeveral typical Dieof thousands of differsel fuels ( 3 ) have been ent combinations of -TIMETIMEoutlined by this equiph y d r o ca r bo n strucNORMAL PRESSURE HIGH PRESSURE ment and the utility of tures, with each strucSENSl T I V I T Y SENSITIVITY the combustion bomb ture contributing its as a test tool has been degree of importance Figure 1. Constant Volume Combustion Records established. t o the combustion picSecondary reference fuels 500 pounds per square inch, 1000° F.
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APPARATUS
The apparatus used in these experiments has been described (3). Essentially it consists of a cylindrical combustion bomb, which has an internal diameter of 21/4 inches and an internal
length of 31/* inches, a fuel-injection apparatus, and instrumentation accessories to portray gra hically injection of fuel into the pressurized and heated and the resultant pressure history during combustion. The injection apparatus permits injection of a single shot of a metered quantity of fuel and the entire equipment allows tests of fuels m e r wide ranges of bomb pressure and temperature.
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Figure 1 shows dual oscillograph traces swept to the same horizontal time-displacement scale. One trace relates to the velocity of the injector pintle and the other to the pressure existing within the bomb a t any instant. Timing markers are superimposed on the injection trace to allow quantitative interpretation of time intervals. These typical records show that injection of fuel into the bomb causes a decrease of pressure resulting from cooling of the initial bomb charge (usually air) by the injected fuel. After an interval of time the fuel autoignites, and the resultant combustion is shown as a pressure increase. As the pressure record seldom indicates a discrete point a t which ignition may be said to occur, the ignition lag must be measured to a n arbitrary point. Thus far it has been impractical to determine the point a t which there is first evidence of a pressure rise or indication of the start of ignition. Therefore, in these data the ignition lag has been arbitrarily selected to mean that time interval between the beginning of injection and the time a t which the original charge pressure of the bomb is restored by release of heat in the combustion process. For the majority of cases of normal combustion, this interpretation of ignition lag will be a reasonable indication of the elapsed time between the beginning of injection and the beginning of rapid combustion. lGNlTlON CHARACTERISTICS OF
FUELS
FUEL COMPONEWTS. If the combustion and ignition characteristics of a fuel could be related to those characteristics of its component parts, the behavior of fuels could be predicted or explained with greater accuracy. In following this thought, an earlier report ( 8 ) has presented data on three fuels to show how the ignition characteristics of their component hydrocarbon types influence behavior of the whole fuels. The fuels were: a predominantly paraffinic gas oil, prepared from Bradford, Pa., crude oil; a gas oil with a high content of dicyclic naphthenes prepared from Hastings, Tex., crude oil; and a gas oil having a high content of dicyclic aromatics originating from Conroe, Tex., crude oil. Data obtained on the whole fuels and on the gel-separated components are reproduced in Figure 2; properties of the fuels are given in Table I. Both the aromatic and paraffin-naphthene components of the
Table
eoMe Figure 2.
TEMPERATURE, OF.
Influence of Hydrocarbon Type
Bradford fuel are superior within their respective classes with regard to ease of ignition. The whole fuel is also shown to be superior in this respect. In contrast, the quality of the paraffinnaphthene portion of the Hastings fuel is degraded by the presence of dicyclic naphthenes, and the quality of the aromatic portion of the Conroe stock is impaired by its dicyclic aromatics. As a consequence, the whole fuels are similar in ignition characteristics. However, the influence of the dicyclic aromatics in the Conroe fuel is suhh as to give it a higher temperature sensitivity a t low temperatures. Qualitatively, the ignition characteristics of these fuels may be explained on the basis of the characteristics of their components.
I. Properties of Gas Oils and Silica Gel-Extracted Fractions of Gas Oils from Selected Crude Oils
Hastings Bradford Conroe ParaffinGas ParaffinGas Paraffinnaphthene Aromatics oil naphthene Aromatics oil naphthene Aromatics Gas oil, volume % 100.0 72.1 27.9 100.0 83.0 17.0 100.0 67.0 33.0 Gravity (6Oo/6O0 F.) 0 API 32.7 38.1 22.6 42.8 46.0 44.3 32.8 35.0 21.5 Aniline point, F. 143.2 168.8 140.9 Bromine number 3.5 8.0 3.5 Dicyclic hydrocarbons, volume ’36 (est.) 17 20 0 0 0 45 Sulfur, weight % 0.09 0.01 0.25 0.06 0.01 0.26 0.05 0.02 0.09 52.1 Cetane number 45.2 57.8 70.0 48.8 69.5 ASTM distillation 10’7 recovered, F. 427 399 412 50% recovered F. 516 500 504 90% recovered: O F. 624 617 597 End point, a F. 660 653 636
Gas oil
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In order t o utilize more fully the possibilities of this method of evaluating fuels, there is a great need for more complete determination of the ignition characteristics of different hydrocarbon types. It appears that the efficient approach t o the problem is a systematic investigation of pure hydrocarbons of the types that represent the bulk of the material in Diesel fuels. PUREHYDROCARBONS. A number of the simpler compounds in or near the Diesel boiling range were tested a t temperatures ranging from 850" to 1000" F. and pressures varying from 300 to 650 pounds per square inch gage. These compounds included n-heptane and its homologs having even-numbered carbon atom from CSto C18, the corresponding alpha olefins from CBt o Cis, and five of the simpler naphthenes of from 7 t o 12 carbon atoms.
fuel. Ita behavior under autoigniting conditions appears to be anomalous and further study is necessary. With the addition of a methyl group t o cyclohexane, fairly good results can be obtained with the bomb, but some peculiar behavior is still evident. More specifically, it appears that the ignition lags of cyclohexane and t o some extent methylcyclohexane are very uncertain, being extremely short a t one time, and a t others extremely long, although apparently the same conditions obtain. When these cyclohexanea
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Figure 3.
I I I 900 950 BOMB TEMPERATURE. O F .
Temperature Sensitivity
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OF Paraffinic Types
Data obtained in tests of these fuels are arranged first (Figures 3 and 4) t o show the influence of molecular size and of hydrocarbon type on ignition lag sensitivity to changes in bomb temperature. These compounds, with some of their properties, are listed in Table 11. Curves for the paraffinic types are given in Figure 3, which shows that the values of the n-paraffins lie in a relatively narrow band with n-octadecane registering the 10x3 est lag. These values become progressively higher as the chain is shortened. It is significant that for the range tested, all of the nparaffins showed a relatively flat temperature-sensitivity curve. The effect of unsaturation in the paraffin chain is to lower the ignition quality and increase temperature sensitivity of the fuel. Xot only does unsaturation lessen ignition quality, but the lessening effect becomes progressively more severe as the chain is shortened. Whereas 1-octadecene registers temperature lags only moderately longer than its corresponding a-paraffin, 1-octene shows from 50 t o almost 100% greater lag than its corresponding n-paraffin. Comparable data t o show the temperature sensitivities of the naphthenic types are presented in Figure 4. For purposes of comparison the curve for 1-octene is also shown in this figure. Included are data for the methyl-, ethyl-, and butylcyclohexanes and the two-ring naphthenes, dicyclohexyl and Decalin. Although it was intended to include cyclohexane in these tests, it was found impossible t o obtain reoroducible results ~1ith this
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Slreishl chain paraffins and a-olefins Bomb pressure, 300 pounds per square inch
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900
Figure
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950
BOMB TEMPERATURE,
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4. Temperature Sensitivity of Naphthenic Types Bomb pressure, 300 pounds per square inch
show extremely long lags, there appears to be a gradual release of heat until the point is reached where rapid combustion takes place. Even if the data for methylcyclohexane at the lower temperatures may be questioned, they are included for comparison with those for the other fuels. Figure 4 shows that butylcyclohexane exhibits the best ignition quality of the series tested. Shortening of the alkyl chain to ethylcyclohexane results in a fuel with comparable temperature sensitivities but having increased lags at all points. Still further shortening of the alkyl chain to methylcyclohexane results in a material that has an appreciably higher temperature sensitivity and introduces the aforementioned anomalous behavior at the lower tempei atures. Dicycloheuyl, with its separated rings, appears to be about equal in ignition quality to cyclohexane with the butyl group attached. The position of Decalin in this series is interesting in that it exhibits the same high temperature sensitivity as methplcyclohexane without any of its anomalous behavior. Another interesting comparison of these t v o hydrocarbons concerns cetane number, Whereas the ignition characteristics shown on Figure 4 are roughly the same for the two, Decalin has an engine cetane rating of approximately 42, compared with 20 for methylcyclohexane. The same kind of erratic combustion with meth~-lcyclohexaneis noticed in the engine as in the bomb. Figures 3 and 4 indicate the general influence of hydrocarbon
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1951 Table
II. Pure Hydrocarbons Tested Boiling Point, ' F. Cetane
and loss of two carbons to form Decalin, there is a further great loss of ignition quality, as shown by the high values for Decalin. Grouping of the data obtained under one set of bomb conditions for the three hydrocarbon types gives a n illuminating comparison of the effect of structural changes on ignition lag (Figure 8). Comparatively, the effect on ignition lag of addition of carbon atoms or -CH2 groups to a n-paraffin chain is small. However, as the alkyl chain is lengthened on the cyclohexane ring, ignition characteristics of the long-chain cyclohexanes a p proach the ignition characteristics of the n-paraffins. This would follow because, as the chain is lengthened on the naphthene ring, the ring itself contributes proportionately less to the characteristics of the compound.
Number
n-Paraffins Heptane Octane Nonane Decane Dodecane Tetradecane Hexadecane (cetane) Octadecane
209 259 304 345 421 489 549 604
Olefins 1-Octene 1-Decene I-Dodecene 1 -Tetradecene I-Hexadecene 1-Octadecene
252 340 415 261 at 15 mm. 525 356 at 16 mm.
40 5 60 2 71.3 82 7 84.2 90.0
Naphthenes Methylc>clohexane Ethylcyclohexane Butylcyclohexane Dicyclohexyl Decalin
214 270 357 462 367
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56.3 63.8 76.9 87.6 96 1 100.0 102.6
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type on ignition characteristics but do not indicate how changes of structure within given types affect these characteristics. Figures 5 through 8 present data to show this effect. Figure 5 illustrates the often-observed trend that lengthening the chain in the n-paraffi series results in decreased ignition lag for the fuel. These changes appear to be smoothly progressive, with the greatest effect being produced by the addition of --CH2 groups to the short chains. Much the same trend is seen in Figure 6, which presents data for the alpha olefins. However, as mentioned previously, the effect of unsaturation on ignition lag is more pronounced with the shorter chains.
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IO I1 12 13 14 15 16 17 NUMBER CARBON ATOMS IN STRUCTURE
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Figure 6.
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Ignition Characteristics of a-Olefins
Bomb pressure, 300 pounds per square inch
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NUMBER
Figure 5.
CARBON ATOMS IN STRUCTURE
Ignition Characteristics of n-Paraffins
Although these data seem to show that, for a given number of carbon atoms, an unsaturated bond has a more deleterious effect on ignitibility than the presence of a saturated ring structure, they are not complete enough to establish the fact. However, they show that the double-ring structures are much more resistant to ignition than the open-chain structures, and it is indicated that the fused ring is still more resistant, time-wise, to combustion oxidation. I n considering volatility, it appears that the cyclic rings and olefinic bonds have a much greater effect in increasing ignition lags in highly volatile compounds as compared to their effect in material of lower volatility. Some of the effects of structure on ignition lag are summarized in Table I11 for hydrocarbons having 10 to 12 carbon atoms, commonly considered as covering the initial boiling range of a Diesel fuel.
Bomb pressure, 300 pounds per square inch
Table 111.
The effect of structural changes in straight-chain types appears to be relatively simple and easily predictable. Unfortunately, this clarity does not extend to the naphthenes. Data for the cyclic compounds tested are shown in Figure 7, in which ignition lag is related to carbon content of the compounds. As previously pointed out, lengthening of the alkyl chain attached to cyclohexane decreases the ignition lag, and references to the other curves will indicate that the effect of these additions to the cyclohexane ring ia quite marked in comparison to the effect of similar additions to a n-paraffin group and, to a lesser extent, to the a olefins. Although no hexylcyclohexane was available for test, its ignition lag was estimated and is plotted in Figure 7. If, from this point on the curve, it is postulated that the 6-carbon hexyl chain is closed to form dicyclohexyl, then the ignition lag is increased appreciably. With fusion of the two dicyclohexyl rings
Effect of Structural Changes on Ignition Lag
300 pounds per square inch, 900' F. Decrease or Increase in Ignition Lag, RIilliseconds Structural Change Addition of -CHn to decane -0 4 -1.1 Additjon of -CHa to dscene Addition of -CHI to butylcyclohexane -1 5 Ring closure hexylc clohexane t o dicyclohexyl +4.2 Ring fusion,.dicycloKexy1 t o Decalin +7.1 +4.1 Double bond, decane to decene
Although the foregoing data on the hydrocarbon types do not present any new concepts concerning the effect of hydrocarbon structure, some additional data are presented on temperature sensitivities of the different types which, it is believed, will be U8eful. In addition, these data are intended to form the basis for an
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obtained. However, the same correlation is not obtained for all types of fuels. This is logical when it is considered that fuels have different temperature and pressure sensitivities and that engine determinations are made under pressure-temperature conditions that are varied with each increment in cetane number. Conversely, these test conditions remained constant for the bomb determinations considered for correlation. It is recognized that adjustment of bomb conditions for different levels of fuel-ignition quality might yield better correlation, but this in itself would largely defeat the fundamental value of bomb data-more significant interpretation of combustion pheno mena An engine cetane number should be recognized as being a measure of a fuel's ignition characteristics under only one particular set of conditions, and it is not necessarily representative of that fuel's ignition characteristics under other operating conditions. This does not deny the usefulness of cetane number as an indication of a fuel's ignition quality for certain purposes but does recognize a limitation so as to avoid confusion in interpretation of its meaning.
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8 9 IO I1 12 13 14 15 &UMBER CARBON ATOMS I N STRUCTURE
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Figuie 7 . Ignition Characteristics of Naphthenes Bomb piessure, 300 pounds per square inch
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.expanded systematic investigation of the ignition characteristics o f those hydrocarbon types and pure compounds of greatest interest in the study oi Diesel fuels.
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UTILIZATION OF B O M B DATA
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IGA-ITIOK LAG-CET INE NUMBERCORRELATION,Because cetane number nas thus far been one of the principal criteria for
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expressing the ignition quality of Diesel fuels, it is believed that some consideration siiould be given to the question of whether or not bomb data may ' ) ecoirelated with engine cetane number. To answer this question, the data obtained in these and other reported tests are Bhov,n in Figure 9, which presents the measured ignition lags as a function of the cetane number of the fuels tested. This figure shows that. for any given series of like fuels of progressively changmg cetane number, a smooth correlation may be
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20
30
40
50
60
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CETANE NUMBER
Figure 9.
Cetane Number-ignition Lag Correlation for Various Fuel Types Bomb pressure 300 pounds per square inch Bomb temperahe, 1000° F.
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Figure
I1 12 13 14 15 16 C A R B O N ATOMS IN STRUCTURE
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8. Comparative ignition Characteristics of Different Hydrocarbon Types Bomb pressure, 300 pounds per square inch Bomb temperature, 900' F.
IGNITION LAGT O DETERMINE FUNDA?JENT-4L PROPERTIES OF FUELS,As the adaptation of Diesel engines spreads from heavy stationary units to high speed railway, truck, and portable-power units, and as the conditions of operation of these units are broadened to include operation in the tropics and in the arctic zones, there is an ever increasing necessity for some method of expressing ignition and combustion characteristics of fuels in terms of fundamental units. Blthough it would be gratifying if a single index could satisfy this need, it appears improbable that any unitary value will be found adequate. However, one approach on the fundamental side may be found in establishing curves of ignition characteristics for a fuel to cover wide ranges of operating pressures and temperatures. It may not be practical to apply such a system directly to fuels to be used in the field, but it may be found practical to establish these ignition characteristics for certain prototype fuels and for hydrocarbons of certain structure. Characterization of a commercial fuel might then be made by comparison with prototype fuels or by direct estimation from its hydrocarbontype content.
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fields of application of this type of equipment lies in the observation of unusual combustion behavior. This is made possible by the fact that extremely long ignition lags may be observed in the bomb, whereas, because of the quenching of slow reactions by the expansion stroke of the piston, only comparatively short lags can be observed in an engine. This characteristic of the bomb has made it possible to make interesting studies of fuel evaporation and combustion behavior in artificial atmospheres. It has been possible also to observe rather unusual combustion phenomena as illustrated in Figure 10. This figure shows combustion records for a paraffin-naphthene fuel, for an aromatic extract, and for secondary amylbenzene. Although comparable conditions were not used for the records shown in this figure, it does illustrate the very different patterns of pressure development that may be observed in the combustion of fuels. A study of the combufition records of the two aromatic fuels indicates that, for these fuels, combustion prqceeded in two distinct phases. I n the case of secondary amylbeneene, it appears almost as if a primary reaction had gone to completion and that a secondary reaction occurred after a considerable interval of time following completion of the first reaction. Further observation along these lines and further systematic studies of the ignition and combustion characteristics of the various hydrocarbon types should lead to better understanding of the way in which molecular size and molecular structure affect the course of combustion of fuels. It is also hoped that additional light will be shed on the fundamental nature of the combustion mechanisms.
A
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ACKNOWLEDGMENT
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Particular acknowledgment is made to K. J. Hughes and J. 0. Chase of the Bartlesville Station, whose untiring efforts and cooperative interests made possible the collection of these data. Acknowledgments are also due to the Coordinating Research Council and its Diesel fuels division, whose financial support and technical guidance in related projects have given invaluable stimulus t o this work. LITERATURE CITED
(1) Elliott. M. A.,“Diesel Fuel Oils,” Am. SOC.Mech Engrs. publication, pp. 57-118, 1948. (2) Hurn, R. W., and Hughes, K. J., paper presented to the annual
Y Figure 10.
Variations in Combustion Pressure Development
Timing marks, I millisecond
A.
B. C.
Typical normal combustion Paraffin-naphthene portion of gar-oil from Conroe, Tex. Bomb pressure, 400 pounds per square inch Bomb temperature 859’ F. Staged combustion( Aromatic extract of gas-oil hom Conroe, Tex. Bamb temperature 975 pounds per square inch Bomb tempetatur; 1050O F. Staged combustlo; Secondary amylbmrene Bomb pressure 300 pounds per square inch Bomb temperahe, 950’ F.
meeting of the Society of Automotive Engineers, Detroit, 1951. R. W., and Hughes, K. J., Proc. Am Petroleum Inst., 30M,227-37 (1950). (4) Michailova, M. N.,and Neuman, 11.1. B., Natl. Adcisory Comm.
(3) Hurn,
Aeronaut., Tech. Mem. 813 (1936).
(5) Petrov, A. D.,“Dependence of the Antiknock Properties and Pour Points of Diesel Fuel Hydrocarbons upon Their Structure,” translation 649, U.O.P. Survey of Foreign Petroleum Literature, 1946. (6) Selden, R. F., Natl. Advisory Comm. Aeronaut., Tech. Rept. 617 (1938). (7) Seminoff, N.,“Chemical Kinetics and Chain Reactions,” Oxford, England, Oxford University Press, 1935. (8) Tiaard, H.T., and Pye, D. R., Phil. Mag., 44, No. 259,79, 121 (July 1922).
Another possibility that will be investigated is the use of Seminoff’s chain-reaction theory (7) in relating ignition lag and ignition temperatures and pressures. Elliott has given an excellent review and bibliography (I) of the data available in application of this theory. For the present, it is enough to say that, on the basis of derivations from chain-reaction theory and by introduction of three constants that depend upon a given fuel, the ignition delay of that fuel can be related to the fundamental conditions of pressure and temperature. If these fuel constants can be used to obtain an expression of chemical ignition delay, it may be said that they represent fundamental properties of the fuel. It remains to be determined if such relationships can be established and the range of conditions over which they may be said to apply. UNUSUAL COMBUSTION BEHAVIOR.One of the more interesting
RECEIVED August 6,1961.
