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WHEELER G. LOVE IAL1 Keseurch T,;mborrztories Dirision, General iMotors Corporutiun, Detroit, Mich.
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Ibi6 review and tabulation of available data on the h o c k i n g behavior of pure hj drocarboris include as its chief part the data obtained by Arnerican Petroleum Institute Research Project 45, now published for the first time outside of reports of that project which were subject eo war restriction. The data 0x1 the knocking characieristics are critically reviewed and correlations discussed among various rnethodd of test, sensith i t y , blending values, molecular structnre, arid sunreptibility to tetraethyllead.
1s the primary purpose of this report to gather together all the data that pertain to the knocking behavior in engines of hydrocai~bonsof definite structure. The da,ta presented here have been obtained from one or more uf three sources. The first is the literat'ure. A second and most extensive source is the publications of American Petroleum Institute Resca,rch Project 45. A third source of informationis the publications of the National Advisory Committee for Aeronautics. I n the effort to make the accumulated data as useful as possible, they have been critically reviewed and tabulated in a form from which all of the information on any compound can be readily obtained. This tabulation covers more than 325 hydroca,rbons and ihcludes original and derived data on the compounds both straight, and biended, with and without tetraethyllead. I n order to increase, the ut,ility of this mass OS information and make it more coherent, the more obvious correlat'ions have been developed. Relationships are shown between different methods of engine testing, between structure and knock for various chemical classes of compounds, between the knock behavior of hydrorarbons straight and in solution, and between the nature of the hydrocarbon and the extent to which tetraethyllead is effective in changing its ant>iknockvalue. T
HISTORICAL
BEFORE 1938. Interest in the knocking characteristics of pure hydrocarbons in an internal combustion engine is of comparatively recent origin. Such interest arises from the fact that isomeric hydrocarbons may have very widely different tendencies t>o knock. As a result, the utility of these isomeric forms when used in an internal combustion engine is subject to a wide variation, bec,ause the knock is an effective barrier to the use of higher compression ratios and hence t o higher thermal efficiency of an int,ernal combustion spark ignition engine. As early as 1920 Midgley (50)observed wide dift'erences in the combustion of fuels in internal combustion engines, not only among isomeric compounds containing carbon, hydrogen, and oxygen, but also in the combustion or the knocking charaoteristics of diflerent chemical classes of hydrocarbons. I n 1921 Ricardo (37) published the result,s of investigations on the highest useful compression ratio of a number of hydrocarbons and found wide differences betvieen the values of different internal combustion engine fuels in this respect. I n 1922 Midgley and Boyd (31) published data on the detonation characteristics of various mixh r e s of aromatic and paraffin hydrocarbons. 1 Present address, Research Laboratories, Ethyl Corporation, Detroit, Xioh.
Edgar ( 8 ) ,in 1927, suggested the use of n-heptane arid 2,2,4trimethylpenOane mixtures as standards for rating fucls for knock. The straight-chain paraffin hydrocarbon n-heptane was very prone to linock, while ihc branched-chain iso-octaiie was comparatively free from knock, and it was thus apparent that different isomeric heptanes an octanes would have greatly differing tendencies t o knock. I n 1929 Birch and Stansfield (3) and Nash and Howes (15,16, 32) published data on the knock rat,ing of solutions in gasoline of several different hydrocarbons. I n 1931 Lovell, Campbell, and Boyd (22, 23) published data on an extended series of pure paraffin and olefin hydrocarbons in fairly dilute solutions in gasoline, showing a number of consist,ent and regular re!at,ionships between the molecular configuration or their conventional structural formula a,nd the tendency of these fuels to knock as measured in dilute solution in gasoline. Later the determinations ol Garner, 11-ilkinson, and Kasli (12 ) on blends of the olefins and Garner and Evans (10) on naphthene and aromatic hydrocarbons mere published. Hoffman, Lang, Berlin, and Sclimidt (13, 14) presented data on blends of a considerable number of hydrocarbons of different classes. Later, Lovell, Campbell, and Boyd (24, 26) discussed a large number of blends of naphthene and aromatic hydrocarbons. Extensive information on the behavior of about 100 hydrocarbons in the pure state was published in 1934 by Lovell, Campbell, and Boyd (26). Most of t'he data in the literature pert'ain. to the knocking characteristics of the hydrocarbons when measured under different engine conditions in fairly dilute solutions up to about 20% at a maximum. Much of the information has boen expressed in terms of octane number of the solutions, and Garner, Evans, Bprake, and Broom (11) have published a compilation of the d a h available up to 1931 in terms of "blending octane number." The blending octane number of a hydrocarbon is an extrapolated value and does not necessarily bear any relation to the actual octane number of this material v,-hen tested in the pure state, but is an extremely useful concept for practical purposes of computing the relat>iveantiknock value of different hydrocarbons in solution. The data upon t'he effectiveness of tetraethyllead as a knock suppressor in known hydrocarbons are fairly extensive. In 1932 Garner, WilKinson, and Nash ( I Z j found that the effectiveness of t,etraethyllead in increasing the octane numbcr of 2070 solubions of several a-olefins in a base fuel increased with the molecular .iveight from pentene through rionene. Siniilnr results were also obtained by Garner and Evans (10) on 21 hydrocarbons, including aromatics, cyclohexanes, and cyclopentanes, also in 2070 solutions; and they concluded that the order of increasing effectiveness was aromatics, cyclohexanes, and oyclopentanes, as ineasured in these solutions. I&er, Ca.mpbel1, Signaigo, Lovell, and Boyd (6) gave data on the effect of definite additions of tetraethyllead on the critical compression ratios of about 60 pure hydrocarbons. h.P.1. HYDROCARBON RESEARCH PROJECT. I n September 1938, the American Petroleum Institute Hydrocarbon Research Project, at Ohio State University formally began operations. Its objectives included the preparation of hydrocarbons in a high state of purity and the measuremenk of their knocking chmac-
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTPY
December 1948
2389
YL TEL/OAL.
OCTANE SMITTENBERG
Figure 1.
200 I
NUMBER
ET AL. F-2 DATA
Comparison of F-2 Ratings of Paraffin Hydrocarbons I
I
I
I
I
n
C.C.R,-
LOVELL,CAMPBELL
8 BOYD DATA
Figure 2. Comparison of Critical Compression Ratio Ratings of Various Rydrocarbons
0 PARAFFINS V OLEFINS D GICLO-PARAFFINS
X
AROMATICS
--
-40 -40
0
40
BO
120
160
200
BLENDING OCTANE NUMBER- 2 5 % BLEND GARNER E T &L.OATA
AT 212'E
JACKEl
-BO
0
BO
BLENDING
FROM
160 OCTANE
ANILINE
240
320
NUMBER
EQUIVALENTS
Figure 3. Comparison of Blending Octane Numbers of Hydrocarbons
Figure 4. Comparison of Blending Octane Numbers of Hydrocarbons
teristics in engines under a wide variety of conditions (4, g 1 ) . Up to 1947, this project and its successor, American Petroleum Institute Research Project 45, had compiled the most extensive table of knock characteristics of hydrocarbons in existence, and most of the accurate information upon hydrocarbons in relation to knock is based on the data of this project, which covers over 200 compounds in a high degree of purity. All their data were published in a series of annual reports, but have not previously received wider distribution because of the restrictions required by war conditions. N.A.C.A. DATA. The National A4dvisoryCommittee for Aeronautics has, especially during the war period, done a large amount of work in synthesizing more than 40 pure hydrocarbons, many of 'them in cooperation with the National Bureau of Standards. These were tested in engines by the N.A.C.A. laboratories, and many of them by General Motors Corporation and Ethyl Corporation in engines under test conditions identical with these used for the A.P.I. compounds. The engine test data obtained by the National Advisory Committee for Aeronautics were published in the N.A.C.A. series of reports, (6, 17-19, 97-99) and an abstract of these, and also the data obtained by the other two laboratories, were printed in the A.P.I. Project 45 Annual Reports with the permission of the National Advisory Committee for Aeronautics. OTHERRECENT DATA. The considerable body of data on gaseous hydrocarbons obtained by Puckett (36) has added to the knowledge on the subject, and Petrov and co-workers (34, 35) have added a number of diolefin and acetylenic compounds to hose upon which there are engine data.
The only other known extensive compilation, that of Doss (7), contains octane numbers of many hydrocarbons. COMPARlSON OF DATA OF DlFFERENT INVESTJGATORS
A considerable number of investigators have published data on the knocking characteristics of hydrocarbons. A comparison of these data is subject to severe limitations because the relative knock characteristic of a compound is not necessarily a property of the compound, but depends upon the engine conditiom under which a comparison with another compound is made. What might be thought of as absolute values, in terms of some engine condition only, are subject to even greater variations. Furthermore, many of the reported measurements have been made in solution in some base fuel, and both the antiknock level of the base fuel and its constitution have a profound effect upon the values obtained. Unfortunately, all these limitations were not widely recognized a t the time of some of the earlier work. However, such comparisons as may reasonably be made, which cover a fair number of compounds, show a degree of agreement that tends to establish confidence in the data. U'BLENDED COMPOUNDS.A simple comparison is that of octane numbers made on pure compounds by the standardized A.S.T.M. D-357 or F-2 procedure. I n this comparison, the only certain differences would be the experimental error of the measurements and the purity of the compounds themselves. For this comparison there are the A.P.I. data, and also the data of Smittenberg et al. on the same compounds. Such a cornparison is shown graphirally in Figure 1 where the Smittenberg
I NbDU S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
2390
Vol. 40, No. 12
( 4 1 ) data are plotted against the A.P.I. data and these of Puclrctt (36) of the Bureau of Standards. I n no cage does the deviation PARAFFINS
CYCLO- PARAFFINS & OLEFINS i
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appear to be much more than about 3 octane numbers, which speaks acll for both the purity of the compounds and the accuracy of the measurements. The only other large amount of compa.rable data on pure compounds is that of Lovell, Campbell, and Boyd (26), recorded in terms of compression ratio in the same enginc and under somewhat similar conditions to the A.P.I. data a t 600 r.p.m. 212" F. A comparison is shown in Figure 2 . The deviation for some of t,he compounds appears to be fairly great, and is probably due to a combination of slightly different and less carefully controlled engine conditions and possibly also to a lower degree of purit,y in the material3 used in the earlier work. However, even when these differences are considered, there appears to be little possibility of overlooking an important' compound or missing fundamental relationships. SOLUTIONS.A comparison of measurements made in solutions is more difficult and not completely valid. The data of Garner and others on 25'3 solutions in a base gasoline in the Series 30 engine at 212" F., from which blending octane numbers were computed by those authors, (11) might be thought' t o be somewhat comparable with the blending octane numbers obtained by the A.P.I. project on 2070 solutions in 60-40 octane-heptane mixtures under the A.S.T.M. D-908 or F-1 engine conditions. A comparison is represented graphically in Figure 3, which would seem to indicate a fair degree of relationship between the two sets of data, especially when the differences in concentration, engine conditions, and base fuel are taken into consideration. Another set which might be used is the early data of TJovell, Campbell, and Boyd (%!-,$?4, 16) recorded in terms of aniline equivalents, from which equivalent blending octane numbers have been computed by Garner and eo-workers (12). This might suitably be compared with the A.P.I. blending octane numbers in 207, solutions by the F-1 (Research) procedure. Such a comparison is shown graphically in Figure 4. Although a few compounds show great deviations, the agreement seems rernarkable in view of the almost 20 years' difference in time and thc general "state of the art'' in addition to all the other cames for differences mentioned above. From such comparisons as these, there might be engcndcrcd a fair degree of confidence in general relations between compounds when deduced from any one set of data, as applied to the particular set of engine conditions used. However, comparison of rat,ings of t,wo compounds from different sets of data is necessarily valid, To be sure of an evaluation of the effect of enginc conditions upon a blended compound it is necessary to have data 011 the compound in the same base fucl, and such data are usually confined t,o those of one invcstigator or group of investigators.
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Figure 5.
Correlation of Knock Ratings with Engine Teat Conditions
A variety of engine test methods for fuels, including specification A.S.T.M. tests, have developed over a period of years; ana all methods are not equally applicable for use with fuels of an extremely wide range of relative freedom from knock. Conscquently, it is very desirable to establish correlations between different test methods to aid in the interpretation of the data and to permit approximation of the value of compounds not tested by a particular method. The engine conditions used in various available methods of test for the knocking characteristics of hydrocarbons are such that, a t least in the range below 100 octano number, the A.P.I. GOO-212" conditions correlate with the A.S.T.M. D-908 or F-1 octane number test, the A.P.I. GOO-350' with the Ethyl 17.6 supercharge tests a t lean mixture ratios, and the A.S.T.M. D-357 or F-2 octane number with the A.P.I. 2000-212 conditions. EIowevcr, the A.P.I. 600-350 O conditions, for which there are more oxton-
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
sive data, give a useful approximation to the A.S.T.M. D-357 or F-2 octane numbers. The degree of confidence with which such correlations may be used may be best estimated by inspection of the data which are shown graphically. DISCUSSION OF DATA. The most extensive body of data on the knocking characteristics of hydrocarbons is that obtained in connection with the A.P.I. project. Consequently, the matter of correlation of engine test conditions is best related to that body of data. Data related to that project were obtained over a period of years, during which time various knock-test methods and fuel specifications have been developed. But the data of that project are fairly extensive in that they relate primarily to four sets of engine conditions with speed and jacket temperature varied over a wide range to cover extremes in combustion chamber temperature conditions. I n addition, certain other data pertain to the A.S.T.M. D-908 or F-1 and A.S.T.M. D-357 or F-2 methods of standard evaluation of fuels. On a considerable number of compounds there are also data ob,tained under supercharge conditions. It would be very helpful if the data obtained under specification methods of test for gasolines could be correlated with the four general methods of test which are applicable over a much wider range of fuels and are possibly of broader and more fundamental application. Such correlations, of course, cannot be exact and are more or less fortuitous, and one can only pick sets of conditions that seem to indicate what is possibly the best general agreement for all classes. A.S.T.M.D-908 or F-1 OCTANEPJUMBERS. This method of knock testing was developed for gasolines in the 50 to 80 octane number range many years ago, and to use it for pure compounds of widely varied boiling points is possibly not justified. The method is not so useful in the range of 100 octane and above, because of lack of adaptable instrumentation and the necessity of using leaded octane as a reference fuel. A graphical representation of such a relationship between the Research and the 600-212' octane numbers is shown in Figure 5. Deviations from the exact equivalence indicated by the broken line do not appear very great in the case of the paraffins. This relationship alone cannot be used as a conclusive test of correlation because all measurements below 100 octane number are relative to mixtures of the paraffin hydrocarbons, heptane and octane, and it is known that almost all paraffin hydrocarbons behave somewhat similarly as a class in respect to their variation in knocking tendency as engine combustion conditions are varied. The data for the cycloparaffin and olefin hydrocarbons indicate a similar relationship, although there are some significant deviations, above 100 octane number. The class of diolefins, acetylenes, bicyclics, and unsaturated naphthenes and unsaturated aromatics includes those compounds which are most sensitive to changes in engine operating conditions, so that the comparison is about as decisive a one as may readily be made from the available data. Again, the deviations for some fuels of above 100 octane number appear significant. Consequently, i t seems reasonable to conclude that the 600212" data may be taken as fairly representative of tests under engine conditions which are close to the F-1 octane number, at least for fuels below 100 octane number. Such a conclusion, of course, must be qualified by the observation that no data on aromatic hydrocarbons are included, and very few data are available very much above 100 octane number, because of the inadequacy for this purpose of tho octane number scale and the leaded isooctane extension, and possibly also the method of test. A.S.T.M. D-357 OR F-2 OCTANENUMBERS.Correlations of the F-2 or -4.S.T.M. octane numbers are subject to all the limitations mentioned as relating to the F-1 octane numbers, and an additional one that correlation with any one of the four regular compression ratio method conditions is not so good. For this reason, i t is possibly best to make a comparison not with one, but with two sets of conditions and to show the comparisons graphi-
2391
PARA,FFlNS AND AROMATICS
CYCLO-PARAFFINS, CYCLO-OLEFINS AND OLEFINS
DIOLEFINS, ACETYLENES, ARYL-OLEFINS & BlCYCLlCS 6
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80
60
40
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20 OCTANE
40 NUMBER
60 F-2
Figure 6.
80
100
2 ML
4 6 TELIGAL
CONDITIONS
Correlation of Knock Ratings Engine Test Conditions b
with
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2392
voi. 40, N ~ 12 .
PARAFFINS AND AROMATICS
PARAFFINS AND AROMATICS
CY C LO - PAR A F FI N S A N D 0L EFI N S
CYCLO-PARAFFINS AND OLEFINS
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NUMBER
2 4 M L . TEL/GAL.
OCTANE
CONDITIONS
Correlation of Knock Ratings with Engine Test Conditions
ML. TELIGAL
NUMBER
17.6 MK CONDITIONS
Figure 8.
Correlation of Knock Ratings with Engine Test Conditions
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
A BICYCLIC OLEFINS v CYCLIC OLEFINS x AROMATICS
0
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C R I T I C A L COMPRESSION R A T I O A.P.I. 600- 350" TEST CONDITIONS
402
6
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RELATIVE I M E P ( I S O - O C T A N E ~ I O O ) 2 5 % SOLUTION 176 A T 0 IO AIR-FUEL RATIO
Figure 9. Correlation of Knock Ratings with Engine Test Conditions
D
.
2393
14
C.C.R.IA.P.1. 600-350" TEST CONDITIONS Figure 11. Comparison of Supercharge Rich Mixture Rating
cally, so that the extent and scattering of the data may be readily visualized. Figure 6 shows the relationship between the F-2 or A.S.T.M. octane number and the equivalent octane number obtained from data at the 2000-212' conditions. The correlation for the paraffins appears good, but some olefins show deviations. The data for the naphthenes, unsaturated naphthenes, and aliphatic olefins indicate what might be considered a reasonable unpatterned scattering. Even the relationship for some unsaturated compounds does not show very great scattering of points. It is reasonable to inquire whether some other set of conditions might not give a better correlation; and another set of conditions which seems to correlate about as closely is that at 600-350". Figure 7 shows the relationship between the A.S.T.M. D-357 or F-2 octane numbers and those obtained from the data at 600-350 ' for paraffins and aromatics, the c.ycloparaffins and aliphatic olefins, and the sensitive unsaturated compounds. I n general, i t appears that the 2000-212" conditions show a little better correlation, all things considered. However, although all the data are represented in the figures, every compound does not necessarily appear in both sets of figures. Consequently, although i t may be concluded that 2000-212' equivalent octane numbers are close to the A.S.T.M. octane numbers, for many approximate purposes i t may be preferable to use the 600-350 ' equivalent octane numbers for rough correlations, because such data include a far greater number of compounds and consequently are more useful for such approximations as may be desired. 17.6 SUPERCHARGE DATA. Another type of available data is the group of lean mixture ratings in the Ethyl 17.6 engine under supercharge conditions for about 50 different compounds. It
Figure 10. Correlation of Knock Ratings with Engine Test Conditions
would be extremely convenient if there existed some correlation between such data and one of the A.P.I. standard test conditions, because then i t would be possible t o extend the general information about other compounds to cover a much greater field. Such correlations appear helpful when a comparison is made with such data under supercharge conditions at maximum knock mixture strength and the 600-350" data, using as a basis equivalent octane numbers as measured by both methods. Such a comparison is shown graphically in Figure 8. The correlation appears surprisingly good except for some aromatic compounds, but it covers only compounds whose rating can be expressed in terms of these standard reference fuels. I n an effort to extend the correlation to compounds of much higher ratings, the actual observed engine data have been compared in Figure 9, using the critical compression ratio data at 600-350' and the relative indicated mean effective pressure in the 17.6 engine. The correlation appears helpful, especially when the very great range of knocking characteristics or the great extent of the scales used is borne in mind. Within the degree of confidence obtained from an inspection of the data here graphically represented, one may use the 600-350 O data as indicative of relative behavior of hydrocarbons under these supercharge engine test conditions a t relatively lean mixtures. A.S.T.M. D-909 OR F-4. The F-4 ratings under rich mixture conditions are of significance in airplane use, a t least as far as specifications are concerned. It would be of interest, consequently, to consider how the body of data on the behavior of pure compounds may be tied into the F-4 engine. Unfortunately, there are only a few A.S.T.M. D-909 or F-4 ratings of pure hydrocarbons, because the method is not well adapted to the use of the small amounts of fuel that are usually available as pure hydrocarbons. However, there are data from the National Advisory Committee for Aeronautics on 18 compounds obtained in 25% solution in a leaded blend of S and M reference fuels in the F-4 engine operated at 1800 r.p.m., 225' F. air, and 375" F. jacket, which may be compared with similar data on a similarly leaded blend of S reference fuel and heptane in the 17.6 engine at900r.p.rn.,225"F.air,and300'F. jacket. The representation of the data is shown in Figure 10; the correlation for this group of paraffins, olefins, and aromatics seems fairly good for this comparison made under conditions of rich mixture or a fuel-air ratio of 0.10. Consequently, within the wide limits implied by the scatter of the points in Figure 10 i t might be concluded that these F-4 ratings a t rich mixture might be related to the 17.6 ratings. RICH MIXTURE APPRECIATION.The behavior of fuels a t mixture ratios considerably richer than the theoretical has been a matter of considerable importance in airplane engines, especially
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
2394
Vol. 40, No. 12
PARAFFINS AND AROMATICS
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12. Comparison or Hich Alixture Appreciation and Sensitivity
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Sensi LiviLy of Hydrocarbons to Engine Test Conditions
Figure 14.
CYCLO-PARAFFINS AND OLEFINS
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Figure 13. Sensitivity of Hydrocarbons to Engine Test Conditions
for military use. This conics about because many superchaiged airplane engines and especially- those that operate under engine conditions usually termed severe, may have then knock-limited pox er output considerably increased by ubing rich mixtures. This increase, usually termed “rich misture appreciation,” is greater for some fuels than for others, and it would bc desirable to have some method of correlating the rich mixture ratings of hydrocarbons with other manifestations of their engine behavior. There has been a nidespread belief that fuels which are more sensitive shorv greater rich misture appreciation. It is chiefly upon this kind of assumption that attempts have been made to correlate rich mixture appreciation with other inanifcstaliom of the engine behawor of fuels. The data nhich arc available are about 20 ratings of pure hvdrocarbons made in the 17.6 engine at a fuel-an ratio of 0.10 and also ratlngs at a fuel-air ratio of 0.07 and other data on engine behavior. Because there is some correlation betneen the 17.6 engine ratings lean or a t niavimuin knock aiid the 600-350” mcasurements, the rich ratings in the 17.6 engine might be expected to correspond with the milder 600-212’ measurements. Figure 11 shows such a relation in nhich the ciitical compression ratios a t 600-212 ’ are plottad against the relative indicated mean cffective pressure of the same fuels undel rich mivture supercharge conditions, The aTTailable data on saturated compounds seem to he somewhat on one line, and the data on unsaturated compounds, olefins, and aromatics are on another. This docs not wem to be an especially satisfactory relationship, arid does not suggest much more than that many olefins and aromatics rate h i g h
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
PARAFFINS AND AROMATICS
PARAFFINS AND AROMATICS
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SENSITIVITY
The relative antiknock values of hydrocarbons change with engine conditions, and some are sensitive, or their relative value becomes less as engine temperature conditions are higher. Paraffins are the least sensitive and serve as a basis of comparison; naphthenes and aliphatic olefins are more sensitive by' amounts that increase with the antiknock value. Highly unsaturated compounds vary greatly in sensitivity. The aromatics also show great variations in sensitivity among themselves, ranging from similarity to paraffins to extreme sensitivity.
DISCCSSION OF DATA. -4sensitive fuel is one that
U
V
2395
If
AB1 600-212' CONDITIONS
Figure 15. Sensitivity of Hydrocarbons to Engine Test Conditions
relatively under the mild 600-212 O conditions than under rich mixture supercharge conditions. Other quantitative methods of correlation have been suggested, such as comparing rich mixture appreciation, or quantitatively the ratio of relative indicated mean effective pressure at 0.07 and 0.10 fuel-air ratios, with "sensitivity" or the ratio of the critical compression ratios at 600-212" and 600-350". Such a It is difficult to see any signifiI omparison is shown in Figure 12. cant relationship here. However, if we consider only hydrocarbons boiling within the aviation gasoline boiling range, there may be a fair degree of correspondence between rich mixture appreciation and sensitivity, and the 600-212 O ratings may give a rough idea of the behavior of fuels under rich mixture supercharge conditions under fairly severe operating conditions.
becomes relatively inferior to some others, with respect to freedom from knock, usually when engine conditions are varied so as to result in higher temperatures of the unburned fuel in the combustion chamber of the engine. Relative values are to be considered. Fortunately, in the early development of relative ratings, mixtures of hep4 6 8 10 12 14 tane and octane, which C C.R A PI 600-350' CONDITIONS comprise the octane num16. Sensitivity of Hydrocarbons ber scale, were used; and to Engine Test Conditions it so happens that the paraffins, as a class, are the least sensitive fuels, so that they form a convenient base line. The A.P.I. data on hydrocarbons cover tests under a wide range of engine conditions, and they consequently may be used t o advantage to show the relative sensitivities of the various hydrocarbons. At first glance, i t might be thought that because the conditions are 600 r.p.m., 212" F. jacket temperature, 600-350", 2000-212 O , and 2000-350°, i t would be possible to evaluate separately the effects of speed and jacket temperature. But speed is not a pure time effect, and a t higher speeds the heat flow and temperature conditions are different. I n engine operation in general, i t is practically impossible to deal with single variables, and keep all other factors constant. Consequently, it may be most advantageous to think of the different engine conditions used in the evaluation of the hydrocarbons as representing different extents of severity at which engines may be operated so as to make sensitive fuels appear of less potential value. The important thing for the immediate purpose is to find the general relationships among the different hydrocarbons as far as their sensitivity is concerned. I n order to see what these relationships may be, i t is possibly best to proceed with a graphical analysis, SO that the extent of deviations from more general relationships may be readily apparent.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2396
Vol. 40, No. 12
PARAFFINS AND AROMATICS
e
2
4
6
8
IO
I2
14
16
e,
CYCLO-PARAFFINS AND O L E F I N S
6
CYGLO-PARAFFINS
AND OLEFINS
. J 4
4
'? 2
U
u
I
I
I
4
6
8
IO
12
I
I
14
16
CYCLO-OLEFINS, DIOLEFINS, ACETYLENES CYC LO D IE N E SIA RY L- 0 LE FINS 8 B I CYC L I CS
2
I 100
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0
m
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0
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8
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12
14
20
2
DIOLEFINS ACETYLENES
C Y C L O D I E N ~ SA R Y L - D L E ~ I N S AND BlCYCLfCS
0
0 16
C.C.R. A P I 600-212' CONDITIONS
CYCLO-OLEFINS, DIOLEFINS, ACETYLENES CYCLODIENES, ARYL-OLEFINS & BlCYCLlCS
Figure 17. Sensitivity of Hydrocarbons to Engine Test Conditions
600-212" vs. 600-350". We may, for example, compare the relative values of fuels as measured at 800-212" under relatively mild conditions with those as measured at 600-350", or under more severe conditions. This has been done by plotting the critical compression ratios of individual compounds under one set of conditions with the corresponding values under another set of conditions, as shown in Figure 13. The line through the points representing paraffin hydrocarbons seems to represent the location of the points without very great individual deviations, and such a line, which will be repeated for reference on other comparisons, represents a line below which almost all other compounds lie. The paraffins as a class show the Ieast loss in their value as far as knock is concerned when engine temperature conditions are increased, and because the concept of sensitivity involved comparisons or relative values, it is convenient to use the paraffins as a base line of comparison. The paraffins are more consistent as a class than any others, they show less depreciation in their value with increased engine temperature conditions, and they are used as the basis of the octane number scale. The question as to whether paraffins of lorn antiknock value are less sensitive than paraffins of high antiknock value is not discussed here The cycloparaffins and olefins are shown also in Figure 13 They behave somewhat similarly t o each other, and the compounds of higher compression ratio are more sensitive than the paraffins. The sensitivity of the hydrocarbons seems to be related to their chemical class, even as does their engine behavior in other ways, such as blending relationships and the effectiveness of lead in them. The unsaturated compounds also shown in Figure 13 are also much more sensitive than the paraffins, except those of low compression ratio, and possibly those of extremely high compression ratio. The aromatics shown in Figure 13 do not group themselves so definitely as a class, except that almost all of them are more sensitive than the paraffins. Hornever, there is a great difference in sensitivity among aromatic compounds This variation among
F-l
CONDITIONS
Figure 18. Sensitivity of Hydrocarbons to Engine T e s t Conditions
different members of this class is also shown by their rich mixture appreciation and by their p c e p t i b i l i t y to tetraethyllead. 2000-212 O us. 2000-350 . Another set of comparisons may be made between the critical compression ratios a t 2000-212" and 2000-350 '. These comparisons are shown graphically in Figure 14 for the paraffins and aromatics, the cycloparaffins and olefins, and some unsaturated compounds. The generalizations or conclusions to be drawn are again similar with respect to the sensitivities of the different classes of compounds, with one exception: the aromatics, which seem t o have about the same sensitivities as the paraffin. These and other data mag be rather simply in-
2397
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1948
PARAFFINS AND
PARAFFINS AND
AROMATICS
AROMATICS
CYCLO-PARAFFINS AND OLEFINS a 200
CYCLO-PARAFFINS AND OLEFINS
w
3 160
240 W z
s :: 120
200 W
a ,
z
4 w
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I
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:: 120
BO
N
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c?
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o
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20
40
60
80
100
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4
6
CYCLO-OLEFINS, DIOLEFINS. ACETYLENES, CYCLODIENES, ARYL-OLEFINS 8 BlCYCLlCS
m
40
0
CYCLO-OLEFINS ,DIOLEFINS, ACETYLENES, CYCLODIENES, ARYL-OLEFINS & ElCYCLlCS
A D I O L E F I N S , A C E T Y L E N E S , CYCLODIENES, ~
0
20
OCTANE
40
PRYL;OLEFINSI
60
NUMBER
Figure
19.
Blending Characteristics of Hydrocarbons
terpreted by thinking of the sensitivity of the aromatics, or their depreciation with temperature conditions, as being a variable with the temperature. In other words, the aromatics, or at least the ones that are especially sensitive, appear very good under very mild condition, such as 600-212 ";increasing the severity of engine conditions depreciates them rapidly, but further increase in severity of engine conditions does not depreciate them further. This relation also appears consistently in other data. Some other compounds, however, continue to depreciate as the engine conditions become progressixely more severe. 600-212 O us. 2000-212 Additional confirmation of the sensitivity concepts is shown by the comparison of the measurements a t 600-212" as compared with 2000-212" (Figure 15). Becaus: the comparison now is with the very mild conditions of 600-212 , some of the aromatics apgear somewhat sensitive. GOO-350 O os. 2000-350 Figure 16 is based upon a similar comparison of the GOO-350 O data with 2000-350' data; there is general agreement with the other comparisons.
.
.
100
~
2
4
ML. TELIGAL F-2
Figure 20.
AND 81&LICS
80 CONDITIONS
Blending Characteristics of Hydrocarbons
Finally, it is interesting to make a 600-212' BS. 2000-350'. Comparison of the mildest conditions, G00-212', with the most severe, 2000-350 O, as shown in Figure 17. Here the severity is increased to the maximum covered by the data, and what is observed, in addition to the previous generalizations, is a sort of ceiling above which the 2000-350 O values do not g~,~irrespective of what values the compounds may have a t 600-212
.
OCTANENUMBERS,A.S.T.M. D-908 OR F-1 AND A.S.T.M. D-357 OR F-2. All the data considered so far in relation t o sensitivity have been in terms of measurements of engine variables a t incipient knock, or in terms of critical compression ratios. It is also possible to consider sensitivity from the standpoint of the conventional octane numbers, by the A.S.T.M. D-908 (F-1) and A.S.T.M. D-357 (F-2) procedures. Here, each measurement involves a direct comparison with heptane-octane, and the results are all relative to mixtures of these two paraffins. A graphical comparison may be had by plotting the two different kinds of octane numbers as determined for each compound (Figure 18). If all paraffins were exactly alike in respect to sensitivity, the two kinds of octane numbers would be expected t o be equivalent. There are some deviations from a linear relationship of equivalence and it is especially noteworthy that generally
2398
h
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 12 higher octane number, show much higher blending octane numbers in paraffins than indicated by their behavior undiluted.
DISCCSSION OF DATA. A
considerable amount of data I is available on an extensive list of compounds in 20% solution in a mixture of 60%, iso-oct,ane and 40% n-heptane by the F-1 and F-2 standard procedures. From such data, it is possible to compute a “blending octane number” or the octane number that the component would have were simple additive relations valid. These data give some insight into the behavior of hydrocarbons in dilute solution as compared wit,h thcir behavior when evaluated in the pure state. Again, it is desirable to examine the data by a graphical met,hod, so -I t,hat the individual dcviat,ions from generalizations may bo readily apparent. IFigure 19 shows thc F-1 blending octane number plotted against the F-1 oc0 tane number of the pure conipound for various compounds. The line as drawn through the circles for the parafins indicates that the paraffins have about the same blending octane number as NUMBER OF CARBON ATOMS IN MOLECULE actual octane number by this Figure 21. Paraffin Hydrocarbons procedure, or that the octane numbers of mixtures of pa,raf‘fins are approximately addiparaffins above 100 octane number by the F-1 procedure of test ti\?. There are only a few aromatics represented in thc figure are not above 100 octane number by the F-2 method, and may be because of difficulties inherent in this method of rating pure arusignificantly below. So few data are available on the aromatics, matic compounds of a high degree of freedom from knock. because of the experimental difficulty of measuring their F-1 A second generalization to be drawn from 6uch comparisons i b octane numbers, that no conclusion about this class of comthat indicated on the part of Figure 19 showing similar data for pounds seems justified. The data on the cycloparaffins and the cycloparaffins and olefins, T+ here the line drawn through thr olefins indicate considerable depreciation in octane number points representing cycloparaffins and aliphatic olefins shon s a under the more severe F-2 test conditions. The data on the other deviation from the approximate straight-line relationship bcunsaturated compounds makes them appear as fairly sensitive tween blending and straight octane numbers which seemcd to br hydrocarbons. valid for the paraffins, and is indicated by the broken line The blending octane number of both aliphatic olefins and naphBLENDING VALUES OF HYDROCARBONS thrnes may be much greater than the actual octane number, Fuel hydrocarbons are seldom used in engines as individual especially for compounds of higher value. I n fact, in the 100 compounds, but are mixed with other compounds to obtain deoctane range the blending octane numbers seem to bcar no very sirable characteristics of the mixture. Consequently, the simple relationship t o the actual octane numbers, and the imcommercial evaluation of the knork characteristics of individual provement in octane number for a 20% addition to this base compounds must take into account their mixture with a variety fuel is much greater than the actual straight octane numbcr would of other materials. However, the effectiveness of a compound indicate on a strictly additive basis. The base fuel is a paraffin; in changing the relative knock level of a mixture depends upon were the olefins and naphthenes added to a base furl of a similar the amount added, the knock level of the base fuel, and the chemical nature, the relationships might be very different. chemical constitution of the base, so that no very general corSimilar data relationships for diolefins, acetylenes, and unrelations with the behavior of pure material are possible. Parsaturated naphthenes indicate that there is no dircct corrcaffins usually exhibit blending octane numbers in paraffins about spondence between blending and actual octane numbers of thcsc in proportion to their octane numbers in the pure state. Howcompounds, and a blending octane number of 208 is posslhlo for ever, many olefins, naphthenes, and aromatics, especially of a compound whose actual octane number is a little over 7 5 .
0
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INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1948
2399
Data also reCRITICAL COMPRESSION RATIO veal similar relationships based on F-2 engine evaluations (Figure 20). These relationships are similar to those for the E'-1 conditions previously discussed. There are a few more data available on t,he arom a t i c compounds, and these hydrocarbons show deviations from the s t r a i g h t-1 i n e blending relationships, as do also the other classes of compounds. All the blending octane number measuremen tu discussed here were made using ZOy0 solution in a BO octane number oct a n e-h e p t a n e mixture. SimiFigure 22. Octanes lar blending octane numbers made in othcr base fuels would be expected to show very difsame chemical class of compounds. Differences in tendency to ferent relationships. knock among isomers mag be so great as to cover almost the whole knownrange of freedom fromknock, and may be equivalent OF STRUCTURE AND to manyfold variations in the Dotential Dower t o be obtained from The outstanding fact about the relationship of structure and a fuel in suitable engines. knock characteristics is the great effect of isomerism, even in the Empirical relationships between structural formula and tendency t o knock show that compounds most prone to knock have long unbranched carbon atom chains. Branching of the chain produces superior fuels. Rings are superior to unbranched chains, and unsaturation in a straight carbon atom chain is advantageous, especially if the double bond is near the center of the molecule. Superior fuels usually are associated with structural formulas which are compact when drawn in the conventional manner. Such generalizatioqs can be made almost quantitative, although they are empirical. The fundamental relationship is, of course, based on mechanisms and rates of oxidation of hydrocarbons under engine conditions, but very little is known about these reactions. RELATION OF STRUCTUREAND KNOCK CHARACTERISTICS. The outstanding and perhaps most important feature of the knock characteristics of hydrocarbons is the regular and consistent relationship between the molecular structure of the pure compounds and NUMBER CARBON ATOMS SIDE CHAINS their tendency to knock. Such relationships serve as readily remembered guides to find Figure 23. Cyclopentanes
OF
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INDUSTRIAL AND ENGINEERING CHEMISTRY
2400
2'
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NUMBER OF CARBON ATOMS IN Figure 2-1.
Cyclohexanes
Thr most obvious ohservai ion about the paraffin hydrocarbons, or any class of hydrocarbons, is thc very great difference between isomers. For example, the difference between n-heptane, which is the zero of the octane number scale, arid the isomeric compound 2,2,3-trimethylbutane, often called triptane, may be from 7 to 11 compression ratios, and several hundrcd per cent in the amount of knock-limited power to be obtained from them in a supercharged engine. This difference is much more than the entire octane numbcr scale. That these enormous differences do exist is the reason why the knocking characteristics of hydrocarbons are important. It is also, incidentally, the reason why the relationships be1 I 4 5 6 tween molecular structure, or isomerisni, and knock charucteris tics are SIDE CHAINS considered important. As far an, -the paraffin hydrocarbons are concerned, there are two other important facts: The value of the compound, as far as knock is conceined, decreases as the length of the chain which is unbroken by branching increases; and the value increases as the amount of branching increases. Figure 21 gives a general picture of the knocking characteristics of the paraffin hydrocarbons, where the critical compression ratio is plotted against the number of carbon atoms in the molecule, and the valucs for the individual compounds are indicated by simplified structural formulas. Some lines are drawn from one compound t o another to suggest relationships hciween structure and critical compression ratio or tendency t o knock, As far as the length of straight chain is concerned, there seems to be a fairly consistent and regular decrease in critical compression ratio of the normal paraffin hydrocarbons from methane
compounds of superior knock characteristicas, and also as inethodd of estimating the value of compounds not tested, or as a reassurance that the field is fairly well covered. The latter result is especially desirable, because of the enormous number of hydrocarbon isomers in the gasoline boiling range which are possible of existence, and because different isomers may show enormous differences in knock characteristics. I n order t o examine such relationships betffeen structure and tendency to knock, i t is most convenient to divide the hydrocarbons into ordinary chemical classes and for simplicity to consider the classes separately a t the outset. It is also necessary to have some common basis of comparison of the compounds, or Some set of engine conditions under which all the measurements were made. For the present purpose the A.P.I. data at 600-350" have been selected because more compounds are evaluated under these conditions. The choice of data may not be too important for the present considerations, however, because, as has been shown, compounds of a chemical class behave somewhat similarly in an engine when combustion conditions are changed. Consequently, i t is chiefly when classes are compared that great attention must be paid to the engine combustion conditions under which the comparison is made. What we are really dealing with here is the relationship between different types of molecular arrangement, and the mechanisms and speeds of their reactions with oxygen. However, there is so little definite information about the mechanisms of combustion of hydrocarbons under conditions such as prevail in engines that i t is at present advantageous to deal with the problem on an empirical basis. PARAFFINS. The paraffins, as a class, have been measured very completely, and they make up the most complete picture of a chemical class of hydrocarbons. Data are available on all structurally isomeric paaffins through the octanes. As shown in the tables, 65 difNUMBER ferent paraffin hydrocarbons have had their knock characteristics measured. I
Vol. 40, No. 12,
v
I
I
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OF CARBON ATOMS IN SIDE CHAINS Figure 25.
Cyclopropanes
2401
I 1
2
NUMBER OF
to heptane. This presumably extends to higher paraffins, as may be inferred from measurements necessarily made in solution; as compounds knocking more than n-heptane are so prone to knock that i t is difficult to measure them in the pure state. A somewhat similar decrease in value with increasing length of chain is shown by the 2-methyl compounds from 2-methylpropane to 2-methylheptane. I n general, the lines directed downward on the chart all correspond t o increases in length of carbon chain, and they all run in about the same direction; this indicates a remarkable degree of consistency in this effect of the progressive lengthening of the chain. The magnitude of the effect of this increase in length of chain is very great, and the effect of going even from propane of three carbon atoms to heptane of seven carbon atoms is considerably greater than the whole octane number scale.
I I 3 4 CARBON ATOMS IN SIDE CHAINS Figure 26. Aromatics
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 12 octanes have been measured, arid 2,2,3trimethylbutarie is the best. 111such work a8 has been donc on the paraffins of $1, 10, and 11 carbon cLtoms, even the structures which by analogy miglit be thought to bc of the highest value arc found to be inferior to the trimethylbut,nrie or triptane. Consequently, i t is 1 1 0 l 110w thought reasorlablo to expect cornpourids superior to these upon which data arc ZLvailable. The effect of' optical isomerism in thc: paraffin hydrocarbons is UIIknown, althougli by analogy with cis-trans isomerism, il miglit be espccted to iio very small. C Y C I . O I ~ J 3 N T AN Hk5.
O f t h e satura,t,etf cyclic hydrocarkmns, tJhecycloperitaries form one of the moat interesting classes, as far Figure 28. Branched-Chain Olefins as the relationship between structure and knock characteristics is concerned, even though the representation Contrasted Tyith this effect of increased length of chain is the of the class is not very complete. opposite effect of branching. If thc eight carbon atoms of n.. octane are rearranged to the 2,2,4-trimethylpentane structure, the effect is t o increase the value of the compound by more than r r i the entire octane number scale. Even when C-C a methyl group is added to other than a terminal position so as t o increase the number of carbon atoms in the molecule, the effect is to incrcase the critical compression ratio of the compound. This latter effect is indicated Ly the lines directed towards the upper right hand corner of the diagram. Again, there seems t o be a remarltable consistency in these particular relationships. I n order to visualize this effect of branching or centralization, reference may be made to Figure 22, in which the critical compression ratios under t n o sets of conditions, a t 600-212" and 600-350", are plotted for all the '18 structural isomeric octanes Increasing critical compression ratio accompanies the progressive branching by methyl groups, and this effect is greater as the methyl C*=CC group is located closer to the center oi the molecule. An interesting speculation would be as* to OTHER DATA the ultimate limit of the critical compression * ratio. The data available form some basis for concluding that 2,2,3-trimethylbutane is about the top; all the paraffins t h o u g h the Figure 29. Diolefins and Acetylenes
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December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
12 n
b In
II
m
I 0 IO
0
NUMBER OF CARBON ATOMS Figure 30.
Ring Closure
Most of the important relationships of the cyclopentanes are shown graphically in Figure 23, where the critical compression ratios of the cyclopentanes are plotted for various numbers of carbon atoms in the side chain. The addition of a single unbranched side chain to the cyclopentane ring decreases the critical compression ratio, and progressively so with increasing length of chain, et least as far as is known or up to n-amylcyclopentane with five carbon atoms in the side chain. The branching of a side chain in an isomeric series increases the critical compression ratio by very considerable amounts, possibly similar in magnitude to the analogous effect with the paraffins. Distribution of the carbon atoms ! of a single side chain into two separate chains or groups also increases the critical compression ratio. It is also interesting to consider the effect of ring closure, or to compare cyclopentane with n-pentane or n-pcntene, as indicated in Figure 23. Cyclopentane has a
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critical compression ratio of 7.6 as compared with 5.4 and 4.4 for 2pentene and 1-pentene, respectively, and 3.2 for n-pentane; so that i t may be concluded that the simple ring compound is greatly superior to the isomeric straight-chain olefin or corresponding paraffin. A comparison of alkyl cyclopentanes with isomeric branched olefins is not satisfactory, inasmuch as the comparison depends upon where the ring may be assumed to open to yield the straight-chain branched olefin. The effect of position isomerism of the side chains in the cyclopentanes is unknown at present. Finally, although no measured cyclopentane hydrocarbon is as good as the best paraffin in respect to freedom from knock, both classes of hydrocarbons cover almost the same range of critical compression ratios, or tendency t o knock. C Y C L O H E X A N E SThe . general picture of the relationships among the cyclohexanes is similar to that of the cyclopentanes. A graphical representation of the data is shown in Figure 24. The addition of a straight alkyl side chain progressively lowers the critical compression ratio as the number of carbon atoms in the side chain is increased. IN RING Branching of the side chain greatly increases the freedom from knock, or the critical compression ratio of isomeric cvclohexenes. Distribution of the carbon atoms of a side chain into two side chains increases the critical compression ratio.
I
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o m I Figure 31.
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I Bicyclic Compounds
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INDUSTRIAL AND ENGINEERING CHEMISTRY
2404
Vol. 40, No. 12
The effect, of ring closure or the comparison of the critical compression ratio of cyclohexane TTith the extrapolated value for nhexane, or with the n-hexenes, is somewhat similar in magnitude to that observed with the cyclopentanes ; the cyclohexane is superior, in respect to knock, to n-hexane. CYCLOPROPANES ASUDCYCLOBUTANES. There are only a relatively few data on naphthenes wit>h t.hree and four carbon a t o m in the ring. Cyclopropane is about the same a5 propene as far as knock is concerned, and inferior to propane. Dimethyl- and dicthylcyclopropanes are of the same order of magnitude in respect to their critical compressiori ratios. Figure 25 6hows the data on the cycloA P I C G R DATA A T 2000-212° A.P.I. C.C.R. D A T A A T 2000-350' propanes in a manner similar to that for the TEST CONDITIONS T E S T CONDITIONS other hydrocarbons. ARonrAmcs. The aromatic hydrocarbons form an especially interesting class, because in addit,ion to being very sensitive compounds, almost all boiling within the gasoline range arc above 100 octane number, and also because the effect of position isomerism on their knock characteristics is so great. The data on the aromatics are also fairly complete, as almost all the alkyl substituted benzenes up to ten carbon atoms have been evaluated in engines. The data on the aromatic hydrocarbons are shown graphically in Figure 26 in a manner similar to that for t'he other classes of hy2 4 6 8 10 12 14 drocarbons. C C.R - C L E A R HYDROCARBON C C R. CLEAR HYDROCARBON The effect- of the addition of a side chain t Q Figure 32. Effect of Tetraethyllead in Paraffin Hydrocarbons the benzene ring is similar to the effect observed in other cyclic hydrocarbons, in that it accompanies a progressive decrease in critical compression The effect of position isomerism in the cyclohexanes may be ratio as it.8 length is increased, although, a . ~indicated on Figure significant. I n order of decreasing freedom from knock the 0-, 26, there is some irregularit,y in the ethyl- and n-propylbenzenes. m-, and p-dimethylcyclohexanes cover a range of about one Branching of the alkyl side chain results in an increase in compression ratio. freedom from knock, well illustrated in the series n-butyl-, secCis-trans isomerism also appears to have a small but appreciable butyl-, isobutyl- and leri-butplbenzene, which shows a progressive and consistent effect on knock. The three cis-dimethylcycloincrease in critical compression rat'io. hexanes are slightly higher in freedom from knock than the corresponding trans- isomers. A.P.I. C.C.R. D A T A A T 600-212' T E S T CONDITIONS
A P I . C C R DATA A T 600-350° T E S T CONDITIONS
I
T E T R A E T H Y L L E A D CONCENTRATION- ML/GAL. RELATIVE I M E P (ISO-OCTANE. 100) C L E A R HYDROCARBON
Figure 33.
Effect of Tetraethyllead in Paraffin Hydrocarbons
Figure 34. Effect of Tetraethyllead in ISOoctanes (S-2 Reference Fuel) A.P.I. 17.6 maximum knwok supercharge tions
teat
oondi-
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1948 I
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I I ISO.OCTANE 4' 3.0 YL. T E U G A L .
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F-l TEST CONDITIONS
I IO 100 90
BO
1x1
1
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RELATIVE I.M.E.P (ISO.OCTANE- 100) 176 M.K. CONDITIONS
Figure 35. Relative Indicated Mean Effective Pressure Obtained with Paraffin Hydrocarbons
The effect of position isomerism in the benzene hydrocarbons may be very large, and sometimes so great as to obscure other relationships. For example, m and p-xylenes have critical compression ratios of 11.5; the ortho-compound, 8.3. Similarly o-methylethylbenzene is 6.6 while the meta- and para-compounds are 8.6 and 8.8, respectively. There is not much difference between meta- and para- isomers, but the ortho- compounds are very much lower. The 1,2,3-trimethylbenzeneI which is sort of a double ortho- compound, has a value of 7.9 as compared with 8.7 for 1,2,Ctrimethylbenzene and 10.6 for the 1,3,5- compound. Consequently, it is most convenient to think of orthocompounds or those hydrocarbons with alkyl substituents in the 1,2 position as being abnormal exceptions to generalizations about distribution of carbon atoms in more than one side chain. With that exception, distribution of the side chain carbon atoms into a greater number of side chains or methyl substituents results in an increase in critical compression ratio, much as with the other cyclic or naphthenic hydrocarbons. ALIPHATIC OLEFINS. The class of aliphatic olefin hydrocarbons is interesting and includes a very large number of isomers boiling within the gasoline range. There are, for example, 27 heptenes and 66 octenes. The class has not been too well explored because of the large number of isomers, the relative difficulty of preparing them in a pure state, their instability in storage, and the greater immediate wartime importance of some other classes. It is to be hoped that the present lack of information will not long persist. STRAIGHT-CHAIN OLEFINS. It is possibly simplest t o consider first the straight-chain olefins and the effect of the double bond and its position by comparison with the corresponding paraffin hydrocarbons. While there are directly comparable data on only about 12 straight-chain olefins, there are also other data not directly comparable but of a nature to confirm some of the generalizations. The data are shown graphically in Figure 27, which also shows the data on the corresponding straight-chain paraffins. First of all, there is a consistent decrease in the compression ratios of CY- and @-olefinsas the number of carbon atoms in the chain increases. Such a progressive decrease, however, does not seem to be so great as that observed with the normal paraffins. As a consequence of this difference, the olefins lighter than butene appear lower in critical compression ratio than the corresponding paraffins, while the olefins heavier than butene are all higher in critical compression ratio than the corresponding paraffin. Viewing it in another nay, the introduction of a double bond into a straight-chain olefin above butane (or into those in the gasoline boiling range) results in an increase in critical compression ratio.
F-2 TEST CONDITIONS
OCTANE NUMBER CLEAR
Figure 36,
ML TEL/GAL
HYDROCARBON
Effect of Tetraethyllead in Paraffin Hydrocarbons
The amount of such increase appears t o vary with the position of the double bond in the straight-chain olefin, and is greater the closer the double bond is t o the center of the molecule. A centralization of the double bond in the straight-chain olefins produces increased value of the compound as far as knock is concerned. BRANCHED-CHAIN OLEFINS. The introduction of a double bond into a paraffin to form a branched-chain olefin might be expected from analogy with the straight-chain olefins to result in an increase in critical compression ratio, the amount of increase depending upon the position of the double bond. The comparable data a t hand on about 15 aliphatic branched-chain olefins confirm such generalizations to a certain extent; but it appears as if a new factor were introduced in that if the antiknock value of the parent paraffin is very high, the introduction of the double bond may decrease rather than increase the antiknock value of the compound. Available comparable data on the branched-chain aliphatic olefins are shown graphically in Figure 28, together with the corresponding paraffins, and arrows to indicate the direction and amount of change in critical compression ratio in going from paraffin t o olefin. The extent of this change clearly depends upon the position of the double bond, and centralization of the double bond usually produces an isomer of higher critical compression ratio. However, olefins derived from the compounds of fairly high compression ratio such as 2,3-dimethylbutane, 2,2,3-
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INDUSTRIAL A.P.I. C.C.R. DATA A T 600-21Z0 TEST CONDITIONS
AND ENGINEERING CHEMISTRY
Vol. 40, No. 12
tion measurements, is about. the same as nheptane. UKSATURATED CYCLICCOMPOUNDS. In the simple rings (Figure 30), the introduction of one or two double bonds in cyclopentane to form cyclopentene or cyclopentadiene, or in cyclohexane to form cycloliexadiene, results in a loivering of the crit,ical compression ratios. However, benzene, which might be thought of as having three double bonds, is very greatly superior t o cyclohexane. BICYCLICHYDROCARBOKS. Data are available on 13 bicyclic compounds, as pictured in Figure 31. About the only generalization made about them is that no saturated bicyclic is bettcr than cyclohexane. The introduction of one or more double bonds into a saturat,ed bicyclic compound increascs the value of its critical compression ratio.
A.PI. C.C.R. DATA A T 600-350’ T E S T CONDITIONS
EFFECT O F LEAD ON HYDROCARBONS
.‘The effect; of the addition of tetraethyllead on the antiknocl~level of hydrocarbons is extremely variable, and the addition may increase or decrease the tendency to knock, depending upon the hydrocarbon and to a liniit,ed extent’ upon the engine operating condit,ions. Lead is most effcctive in parafins and naphthenes and shows a fairly regular behavior; in other classcs of compounds, its effect is ~Tidclgvariable. Consequently, the effectiveness of lead is best considered by dealing with the hydrocarbons by chemical classes individually. PARAFFINS
trimethylbutane, and 2,3-diniethylpentane, are lower in value than the paraffin. Consequently, the relationships among the branched-chain olefins as far as knock is concerned are not too clear; sufficiently extensive data are not yet available. DIOLEFINSAND ACETYLENES.Data on both diolefins and acetylenes are inadequate to support extensive generalizations, but the practical importance of the knock characteristics of these materials is not great. As far as is knonn, acetylene compounds may knock either more or less than the corresponding paraffin, but centralization of the triple bond results in greater freedom from knock. However, acetylene itself is very prone to knock, and its critical compression ratio is less than 3 as compared with 9.4 for ethane and 5.6 for ethylene; its equivalent octane nurnbcr is less than 50. Diolefins, as far as are knomn, are superior in knock to the corresponding paraffins, with the exception of 1,2-butadiene, and are even more superior when the two double bonds are centralized or conjugated. Figure 29 shows the comparable data on these two classes of compounds, SIZEOF CYCLICRINGS. It is of some interest to compare the knocking characteristics of the simplest cyclic compounds. This is done graphically in Figure 30, where the critical compression ratios of a number of cyclic compounds are represented. Cyclopentane has the highest value, 7.6, of the elementary rings and is much higher than n-pentane or the n-pentenes. Cyclopropane, whose critical compression ratio is 6.6, is about the same as propylene and lower than propane. If ethylene were considered as a cyclic compound, its value of 5.6 might be compared with 9.4 for ethane. Cyclohexane with a value of 4.6 is only a little higher than 1-hexene a t 3.6 and n-hexane a t 3.0. Cycloheptane, whose value is estimated at about 3.0 from solu-
The effect of the addition of t,ctraethyllead to paraffin hydrocarbons is to increase the ant~ilinocklevel. Such an increase is of a magnitude such as t o increase by a constant fraction the knocklimited efficiency OP power of the fuel when used at thc level of incipient lrnocli except for some very highly branched compounds. This fract]ion is independent, of the engine conditions, the hydrocarbon itself, and the mixture ratio. The gains in thermal efficiency by permissible changes in com-
2401
A
0 0
40
BO
120
SATURATED BICYCLIC5 I I
160
200
240
RELATIVE I.M.E.P. (ISO-OCTANE * 100) CLEAR HYDROCARBON
Figure 38. Effect of Tetraethyllead in Cycloparaffins a n d Bicyclics A.J’.I. 17.6 maximum knock superoharge teat conditions
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1948 F-I
T E S T CONDITIONS
F-2
TEST
2407
the presence of lead in the fuel. The solid lines indicate the compression ratio which 1Tould be necessary to obtain 10% greater output from the engine, or 10% greater thermal efficiency were the volumetric efficiency of the engine unchanged by increase in compression ratio. These lines for 10% power increase were computed directly from curves of indicated mean effective pressure against compression ratio. The points plotted on the graphs seem to lie fairly closely about the lines as drawn, and it is to be concluded that (within the limits of variation exhibited on the curves) the addition of 3 ml. of lead increases the potential power and efficiency of the fuel by an average of 10%. This is so, irrespective of the antiknock level of the fuel. Since this constant amount is 10% of the efficiency of the base fuel, the actual increase is much higher for the fuels that have a greater output a t their incipient knock point. It may also be concluded that (within the limits of variation shown on the curves) the effectiveness of lead measured in this way is unaffected by engine operating conditions. The gains due to the use of lead appear constant, irrespective of whether the engine conditions are mild or severe. Some compounds of exceptional behavior appear in this group of data. Lead does not appear to be very effective in 2,2,3,3tetramethylpentane, 2,2,3,3-tetramethylhexane, and some other compounds with a large number of methyl groups tested only by the supercharge engine procedure. The reason for this behavior is not known, but i t is interesting to note that such compounds usually are sensitive to mixture ratio and engine temperature conditions. SUPERCHARGE ENGINEMEASUREMENTS. The gains due to the use of lead, or the effectiveness of lead in different compounds, when measured by the supercharge method arc indicated
CONDITIONS
OCTANE N U M B E R ML.TEL/GAL CLEAR HYDROCARBON
Figure 39. Effect of Tetraethyllead in Cycloparaffins and Bicyclics
pression ratio average 10% for the addition of 3 ml. of lead per gallon. The gains in knock-limited power by the use of supercharge amount to 3070 for 1 ml. of lead. The ratio of the gain in efficiency (and power) by increased compression ratios to the gain in power by increased supercharge is about 1 to 4 for equivalent additions of tetraethyllead. The data on the effectiveness of lead in the paraffins are tabulated in the summary tables of the engine evaluation of pure hydrocarbons, but it is convenient to deal with them on a graphical basis, so that deviations from the general rules may be readily apparent. CRITICAL COarPREssION b 4 T I O . For convenience, we may use the direct engine measurements, and plot the critical compression ratio of each hydrocarbon against the critical compression ratio of the same compound to which 3 ml. of lead per gallon have been added. This has been done for the four A.P.I. data engine conditions of 600 r.p.m., 212" F. jacket; 600 r.p.m., 350" F. jacket; 2000 r.p.m., 212" F. jacket; and 2000 r.p.m., 350' F. jacket, as shown in Figure 32. The 45' lines (broken lines), represent the line of zero lead effect, or no increase in critical compression ratio due to
A.F!I.
C.C.R. DATA AT 600-212' T E S T CONDITIONS
A.F!I. C.C.R. DATA AT 2000-212° T E S T CONDITIONS
A.F!l. C.C.R. DATA AT 600-350" T E S T CONDITIONS
A.PI.
C.C.R. DATA AT 2 0 0 0 - 3 5 0 " T E S T CONDITIONS
C.C.R.-CLEAR HYDROCARBON C.C.R.-CLEAR HYDROCARBON Figure 40. Effect of Tetraethyllead in Olefin Hydrocarbons
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
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Vol. 40, No. 12
F-l TEST CONDITIONS
0 0.07 7
FUEL-AIR
0.10 FUEL-AIR 0.10
RATIO RATIO
40
40
80 I20 160 200 240 E60 RELATIVE I M E B I I S 0 -OCTANE' 100) CLEAR HYDROCARBON
Figure 41. Effect of Tetraethyllead in Olefin Hydrocarbons A.P.I. 17.6 maximum knock supercharge t e s t conditions
F-2 TEST CONDITIONS
by the A.P.I. data. on the 17.6 engine. Graphically, these are shown in Figure 33 where the indicated mean effecbive pressure at maximum knock mixture (or a t 0.07 fuel-air ratio when the maximum knock data were lacking) is plotted against the indicated mean effective pressure at knock limited power for t,he same compound with the addition of 1 ml. of lead per gallon. The broken line represents a line of zero lead effect, or no increase in power due to the addition of lead. The observed points seem to lie about a straight solid line as drawn representing a 30% increase in knock-limited power causcd by the addition of the lead. Within the variations indicated, i t may be concluded that the addition of 1 ml. of lead per gallon increases the potential power to be obtained from these paraffins by about 30%, irrespective of the antiknock level of the compound. The points indicated on the diagram by solid circles represent similar measurements made at a rich mixture or a fuel-air ratio of 0.10 by weight, and they seem uniformly distributed with rcspect to the points obtained at lean mixtures. It may also be concluded that the use of rich mixtures does not in general significantly change the effectiveness of lead in thcse compounds. COMPARISON O F
EFFECTIVESESS O F LEADBY
T W O ?dETHODS
Because the effectiveness of lead on the basis of compression ratio measurement,s does not seem to be significantly affected by engine conditions and the relative merit of different compounds seems to be the same when measured by a compression ratio method as by this supercharge method, i t would be interesting to compare the effectiveness of lead directly by both methods of utilization. The two methods of measurement upon which extensive data are available pertain to two different amounts of lead, 1 and 3 ml.
To serve as a basis of comparison, we may take the 17.6 supercharged engine experimental values for the effect,iveness of differentamounts of lead as shown in Figure 34, where the relative indicated mean effective pressure is shown for different amounts of lead in 8 reference fuel, which is substantially pure iso-octane. Similar curves have been obt,ained for other paraffins. That t,he addition of 1 ml. of lead accompanies an increase in knock limited power of about 30% is the general rule for all paraffins. The addition of 3 ml. of lead, according to Figure 6, results in an increase Qf about 45%. This may be compared with an increase of SO% in thermal efficiency by the compression rat,io method. We may see how this ratio of 45 to SO obtained from the effectiveness of lead may correspond to the ratio obtained in going from one compound t,o another. I n order to do this, the relative knock-limited indicated mean, effect,ive pressure for several compounds including n-pentane, 2-methylbutane, 3ethylpentane, 2,2-dimethylbutane, 2,2,4-trimethylpentane, and 2,2,3-trimethylbutane, obtained by the supercharge procedure is plotted in Figure 35 against the relative knock-limited indicated
OCTANE NUMBER C L E A R HYDROCARBON
Figure 42.
ML. TELIGAL.
Effect of Tetraethyllead in Olefin Hydrocarbons
mean effective pressure a t the knock-limited compression ratios for the same compounds. A line is drawn approximately through the points. On the same diagram is a broken line indicating the effect of the addition of 3 ml. of lead to iso-octane.
As the two lines are similar in slope, it may reasonably be concluded that this ratio of about -1.5 to 1 is of fairly general application, whether deduced from measurements of the effectiveness of lead or the relative value of different pure paraffin hydrocarbons. OCTAKENCMBERR~EASUREMENTS. The effectiveness of lead has also been measured in terms of octane numbers, by the F-1 and F-2 methods. I t s effectiveness by these methods may be well revealed by plotting the octane number rating of compounds mithout lead against the octane numbers of the same compounds to which 3 ml. of lead have been added. This has been done in Figure 36 for the F-1 and F-2 procedures. The points cluster about the lines similarly d r a r n on the t a o diagrams. The points plotted for values of about 100 octane number are not significant in determining the relationship, because the coordinates of milliliters of lead in iso-octane are not in simple relation to the other coordinates. It mav be concluded from data that the effectiveness of lead, as so measured, is not significantly different for determinations made by both methods, as might be expected from other data on the effectiveness of lead in paraffin hydrocarbons. However, the effectiveness of lead in increasing the octane number varies
December 1948 A.F!I.
INDUSTRIAL AND ENGINEERING CHEMISTRY C.C.R. DATA AT 600-212° T E S T CONDITIONS
A.F!I.
2409
C.C.R. DATA AT 600-350' T E S T CONDITIONS
paraffins may indicate a fair average for most of these com16 pounds, and it might be concluded that the effectiveness of lead in the naphthenes is not 14 very greatly different from its effectiveness in the paraffins, 12 snd may not be importantly influenced by engine conditions. 10 E€ o we v e r , tert-butylcyclopentane, 1,l-dimethylcyclopropane, 8 and methylcyclopentane show considerable deviations, which 6 are probably not due t o experimental error. Inasmuch as most d 0 4 of the cycloparaffins are rather U lorn in antiknock value, this 2 method of determining the exA.Pl. G.C.R. DATA AT 2000-212° act effectiveness of lead is not so T E S T CONDITIONS consistent or reliable as for the paraffins. Similar considerations pertsin to the data obtained on the naphthenes by the supercharge procedure, as represented by the data on Figure 38 where the gains due to the effect of lead are somewhat small and the range of data is not great. However,.most of the points are helow the line for 3070 increase in indicated power taken to represent the paraffins. A similar conclusion is also t o be drawn from the data on the effect of lead in the cycloparaffins obtained from octane numC.C R . - C L E A R HYDROCARBON C.C.R. -CLEAR HYDROCARBON ber measurements. Such data, Figure 43. Effect of Tetraethyllead in Aromatic Hydrocarbons represented like those of the paraffins by the F-1 and F-2 orocedures. are shown in Figgreatly with the octane number of the compound to which the ure 39. Here, the line drawn through the cycloparaffins aplead is added. Such data may be used to estimate the relative pears definitely below the line representing the paraffin behavior, potential value, with respect to engine operation, of octane and it appears that the effect of lead in the cycloparaffins is numbers in different parts of the octane number scale. definitely less than in the paraffins. CYCLOPARAFFINS
ALIPHATIC OLEFINS
The effectiveness of lead in the saturated cyclic and bicyclic hydrocarbons follows the same general pattern as in the case of the paraffins, and the fractional improvement in potential value of a fuel upon the addition of lead seems usually independent of engine conditions and antiknock level. However, the fraction is slightly less than for the paraffins, and some compounds show a much greater deviation from the average. REVIEWOF DATA. Especially because some members of this cycloparaffin class of compounds show considerable deviation from an average behavior, it is desirable to review the data in a graphical form, so that the extent of the validity of the generalizations may be readily apparent. The available data on compression ratio measurements of these compounds are plotted graphically for the four sets of A.P.I. engine conditions in Figure 37. Although all the available data are represented there, all compounds are not necessarily represented on all the plots. In general, points a t higher values of compression ratio are not numerous because relatively few naphthenes have high antiknock value. It would seem that the lines drawn on the charts for 10% increase in power, as the representations of the data for the
Data on the effectiveness of lead in the aliphatic olefins are not yet as extensive as might be desired. Comparing their pattern of behavior with that of the paraffins as a convenient standard, the effect of lead is not a constant amount, although there is no evidence of a consistent variation with engine conditions. A better understanding of the effect of k a d in the olefins must await more extensive data. The data on the aliphatic acetylenes and the cyclic monoolefins are such that these compounds may be classified with the aliphatic olefins, as far as the effect of lead in them is concerned. REVIEWOF DATA. Because the data on the olefins are not SO complete as for some other classes of hydrocarbons, and there are important deviations from the average, a graphical review of the data is of some advantage. The data shown graphically in Figure 40 represent the four sets of engine conditions used previously, and the corresponding reference lines indicate the behavior of the paraffins. I n general, the lines drawn on the chart to represent an average behavior of the olefins would seem to be somewhat similar, indicating no very significant change in the effectiveness of lead with varied engine conditions. An important characteristic of
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 40, No. 12
The data on the olefins measured in the supercharged engine (Figure 41) are not very extensive but they seem in gcneral agreement with the measurements made in terms of compression ratio. Again, the use of rich mixt,ures seems t o have no significant effect on the effectiveness of lead. Measurements of the effectiveness of lead in olefins made in terms of octane number are show1 in Figure 42 for the F-1 and F-2 conditions. Here, there is a. considerable difference between the appearance of the two charts; the ratings by the F-2 method are lower than by the F-1, a fact which is a manifestation of \That is termed the sensioivity of olefins. By the F-1 method, a number of olefins are better than 100 octane number; but by the F-2 method there are none much better than about 90 octane number. I n the inferior olefins the effectiveness of lead approximates that of the paraffins, at least according to the data of the F-1 method of test; in the superior olefins, the effectiveness ol lead becomes very small and in some compounds may become close t o zero. I n all t,he figures on the aliphatic olefins, dat.a on the cyclic mono-olefins and aliphatic acetylenes have been included where available. Their behavior with respect to susceptibility to lead seems t o be the same as for the aliphatic olefins.
A.PI. 17.6 SUPERCHARGE T E S T CONDITIONS
AROMATICS
The data on the effectiveness of lead in the aromatic hydrocarbons are not very complete. However, there wems to be a wide variation in the effectiveness of lead in the aromatics, even among isomers and among position isomers, ranging from an effect,iveness equal to that in t,he paraffins t,o a negative lead effect. ' REYIEW OF D a ~ a . The data on the aromatic hydrocarbons are not nearly so complete as on some other classes. There is only
40
120 160 200 240 280 RELATIVE I M E P (ISO-OCTANE'1001
80
CLEAR
TEST
HYDROCARBON
CONDITIONS
TEST
CONDITIONS
14
Effect of Tetraethyllead in Aromatic Hydrocarbons
Figure 44.
_I
= these lines, lionever, is that they appear roughly parallel to the line of zero lead effectiveness. In other words, the increase in the potential value of the fuel due to the addition of lead is not a constant relative amount, as was the case with the paraffins and naphthenes. The increase due to lead appears to be an amount v, hich is relatively greater in fuels of lower antiknock level than in the superior olefins. The effectiveness of lead in some of the inferior olefins approvirnates that of lead in the paraffins. I n the olefins of high antiknock quality, however, lead may be only a third or a quarter as effective as in paraffins or naphthenes of equivalent antiknock level. There is also a considerable scattering of the data, and in some olefins lead has very little effect. Most of these compounds ale highly branched olefins.
Figure 45. Effect of Tetraethyllead in Some Unsaturated Hydrocarbons
10
4 +
10
2 m
8
2. 4
2.
2
A.I?I.
C.C.R. DATA AT 2000-212° T E S T CONDITIONS
A.i?I.
G.G.R. DATA AT 2000-350O TEST
CONDITIONS
4 C.C R - C L E A R
HYDROCARBON
C.C.R - C L E A R
HYDROCARBON
'
December 1948 240
INDUSTRIAL AND ENGINEERING CHEMISTRY F-l
TEST
CONDITIONS
2411 iMISCELLANEOUS COMPOUNDS
Grouped together as n, miscellaneous group of unsaturated compounds are 160 the diolefins, aryl olefins, and unsaturated bicyclic I20 compounds. Lead is not, very effective in any uf 80 them, and in many, especially those which have cou40 jugated double bonds, lead 40 80 120 160 200 240 280 acts as a knock-inducer or RELATIVE 1.M.E R (ISO-OCTANE-100) has a negative e@&. CLEAR HYDROCARBON REVIEWOF DATA. The Figure 46. Effect of Tetraethyllead in Some Unsaturated Hydrocarbons data are presented graphically in Figure 45 to show A.P.I. 17.6 supercharge test conditione the effectiveness of lead pertaining to the scveii d i f f e r e n t conditions of a small amount of data on the pure coniengine evaluation. pounds available under the F-1 and F-2 methConsidering first the data ods of test because of the difficulty of making in terms of compression measurements with these compounds with that ratio for the conditions of procedure. For somewhat similar reasons the 600-212 O , 600-350 O , 2000cornpression ratio measurements, especially 212O, and 2000-350”, it under mild conditions, are not very complete. may be seen that there is As a result, the data represented on Figure a considerable variation iri 43 are not vcry complcte, and about the only the effectiveness of lead 111 conclusion that can reasonably be drawn is this class of compounds. that the aromatics show a wide variation in Especially noteworthy is the extent to which lead is effective in them. OCTANE NUMBER ML.TEL/GAi the fact that inany of them C L E A R HYDROCARBON In some compounds lead is as effective as in show a negative lead effect, some paraffins, in others lead is so ineffecFigure 47. Effect of Tetraethy Head in Some or in them lead acts as a Unsaturated Hydrocarbons tive that it actually increases the knock. knock-inducer. This is The most complete set of comparable data especially evident in the on the effect of lead in the aromatic hydrocase of compounds which have two dbuble bonds in the concarbons is that obtained on the 17.6 engine under supercharge jugated position. Such compounds are very sensitive to engiue conditions. Because the conclusions to be drawn from these conditions. comparisons are related to the hydrocarbons as individuals The data obtained under supercharged conditions in the 17.6 rather than as the entire class of aromatic compounds, the data engine are shown in Figure 46, and are consistent with the comare divided into two groups. The first group is that of the pression ratio data. monosubstituted aromatics, and the data are shown in Figure The data on measurements made under the F-1 and I+’-2 44,with the points labeled to indicate the compounds. methods of engine operation (Figure 47),indicate only in a rchA number of conclusions may be drawn from these data. tively few cases much positive effect of lead. First, there is a wide variation in the effectiveness of lead in these aromatic compounds, ranging all the way from an effectiveness DESCRIPTION O F TABLES as great as in the paraffins (a class of compounds in which lead is most effective) to almost negative effectiveness. Benzene is of The accompanying tables attempt to present in tabular form special ihterest in that lead is a slight knock inducer in it. A the directly measured knock ratings for hydrocarbons of definite second conclusion is that lead is as effective in n-alkyl monostructure. Excluded from these tables are values that are essubstituted benzenes as in the paraffins. Toluene, and ethyl-, timated from other properties, values that appear to be quotatioris n-propyl-, and n-butylbenzenes all lie on a line of constant relative from other literature with or without reference to the origin2Ll lead effectiveness. A third conclusion is that branching of these experimental work, and values relating to compounds or mixturr> alkyl substituents decreases the relative effectiveness of lead in of very indefinite composition or structure. the compound. tert-Butylbenzene and tert-amylbenaenes have I n many cases, the purity of the compounds is not stated. about half the relative susceptibility to lead as the corresponding The tables contain, however, the references to the original normal compounds. publications, so that they may be consulted if desired. A most The polysubstituted alkyl aromatics are also of great interest important part of the table consists of the data obtained by (Figure 44). A first conclusion to be drawn from such data is American Petroleum Institute Project 45 which were originally the great effect of position isomerism on the lead susceptibility published as reports of that project. Such data were obtained on of these compounds. For example, o-xylene shows almost zero compounds of very high purity, and the description of these com-, lead susceptibility, p-xylene has a susceptibility almost equal to pounds is referred to by the description number of the compound the paraffins, and m-xylene is intermediate. The compound included in the tables. m-diisopropylbenzene is especially interesting because it has a The units of measurement are expressed in the tables as far as negative lead susceptibility; in it are combined both a metapossible, and an attempt has been made to include the original position and also branched alkyl side chains, both of which would measurements and also to present values computed from them be expected t o tend towards making a compound in which lead in terms of the octane number scale, so as to make the data as would have low effectiveness. comparable as possible. 200
2412
INDUSTRIAL AND ENGINEERING CHEMISTRY
The engine conditions of measurement are expressed in the tables as far a5 po.sible, and for a more complete description of the engines, the original literature may be consulted. It is not possible to describe an engine in concise form accurately enough so that it may be duplicatcd arid knock ratings repeated to a high degrre of accuracy. Sometimes the most significant way to describe an engine, as far as fuels are concerned, is in terms of the relative ratings of certain lrnovvn sensitive fuels. ACKh-OWLEDGRlENT
The obtaiuing of the engine data of A.P.I. Project 45, here published for the first time outside the reports of the project, has E. Felt, F. Gillig, S.D. been the work of niariy men including -4. IIeron, and R. V.Kerley of Ethyl Corporation and J. >I. Campbell, B. +%. D'Xlleva, and J. W. O'Donnell of General Motors Corporation. Extensive data have aIso been contributed by the Xational Advisory Committee for Aeronautics. The hydrocarbons of the A.P.1. Project were mostly prepared under the direct,ion of C. E. Boord and associates at the Ohio State University Research Foundation (4). Some other compounds, also in a high state of purity, have been contributed by various organizations, including the National Advisory Committee for Aeronautics, Phillips Petroleum Company, Standard Oil Company (Indiana), Standard Oil Company (Kew Jersey), Shell Petroleum Company, Ethyl Corporation, and General Motors Corporation. The funds for the preparation of most of the A.P.I. compounds were contributed through the American Petroleum Institute and the direction of the project has been through an advisory conimittee novy composed of R. F. ;\Iarachner, chairman, L. c. Beard, F. E. Frey, TV. A. Herbst, and W.G. Lovell. Others who have served include D. P. Barnard, chairman, George Calingaert, Chstav Egloff, and W. J. Sweeney. In the preparation of this article many have cooperated and contributed importantly, including many of those mentioned above, and also N. E. Phillips, H. K. Lichtenldner, A. E. Roach, and Patricia McDonald of General Motors Corporation.
Vol. 40, No, 12
(21) Lovell. U'. G., Proc. Am. Petroleum Inst., 27, (111), 18 (1947). ( 2 2 ) Loveii, W.G., Campbell, J. >I,, arid Boyd, T. A . , IND. ENG. CHEW,23, 26-29 (1931). (23) I b i d . , 23, 555 (1931). (24) I b i d . , 25, 1107 (1933). (25) Ibid.. 26, 1105 (1834). (26) Lorell, W.G., Campbell, J. X , Signaigo, F. IC., and Boyd, T. d., I b i d . , 26,475 (1934). ( 2 7 ) SIeyer, C. L., Natl. Advisory Comm. Aeronaut., Rept. RB E6D22 (1946). (28) hIeyer. C. L., and Barnstetter, J. R., IEid.. ARR E5D16 (1945). (29) Ibid.. ARR E6C05 (1946). 1303 Midgley, T., Jr., J . Soc. Automotive Engrs., 7, 489 (1920). ENG.CIIEM., 14, 589 (311 Midgley, T., Jr., and Boyd, T.A , , IXD. (1922). (323 Nash, A. W., and Homes, D. A., Nature, 123, 276 (1929). (333 Keptune, F B., arid Trinible, H. M., Oil Gas J., 32, No. 61, 44 (1934), (34: Petrov, A. D., Verentsova, S . K., and Kokleeva, T. A , , J . Gen. Chem., (L'SSR), 11, 1096-9 (1941; ; U.O.P. Librarv BUZZ. A b s . , 15, 203 (1940). (35) Petrov, A. D., and Voronovs. A , V , Compt. rend. acad. E C ~ . USSR, 31, N o . 8 , 765 (1941); U.O.P. LibraTy Bitll. 8 3 e . , 16, 180 (1941). (38) Puckett, -4.D.. J . Research S a t l . Bur. Standards, 35,276 (1945) (37) Ricardo, H. R., Automubile En.gis., 11, 51, 92 (1921). (35) Ricardo, H. R., and Thornycroft, O., Trans. Fuel Conference, W o r l d Power Conference, London, 1928, 111, 662, (1929). (39) Schmerling, L.. Friedman, B. S.,and Ipatieff, V. N., J . Am. Chem. Soc., 62, 2446-9 (1940). (40) Schmidt. A. W,, Petroleum Z . , 28, Motorenbetrieb., 2 (March 9, 1932). (41) Saiittenborg, J., Hoog, H., Moerheek, B. FI., and Zijden, M. J. 1:. J. I n s f . Petrolmm, 26, 294-302 (1940). 142) Thornycroft, O . , and Ferguson, A , . J . Inst. Petroleum Tech., 18, 329--49 (1932). I
n.,
RECEIVED August 4 , 1947. American Petroleum Institute Research Projeat 4 5 , "Synthesis, Purification, and Properties of Hydrocarbons of Low Molecular Weight," and its predecessor project, American Petroleum Institute Hydrocarbon Research Project, have been i n continuous operation since 1938. The methods of operation whereby compounds prepared a t Ohio &ate University are tested in engines at General Motors Research Laboratories and Ethyl Corporation Research Laboratories are described in the literature.
LITERATURE CITED
Alden, 1%. C., S a i l . Petroleum News, 24, No. 3. 32-7 (1932). Bertholet, Ch., Chimie & Industria, 39, 135T (1938). Birch, 9. F., and Stansfield, R., iVutzue, 123, 490, 639 (1929). B o d , C. E., Proc. A m . Petrolewn I m t . , 27, (111) 9 (1947). Bull, A. W., slid Jones, A . JY.,Satl. Advisory c o n i . Aeronaut., R e p t . ARR E4109 (1944). Canipbell, J. M., Eignaigo, F.K., Lovell, 117. G., andBoyd, T. A,, I X D . E N G . CHEM., 27, 593-7 (1935). Doss, XI. P., "Physics1 Constants of the Principal Hydrocarbons," N e w York, Texas Co., 1943. E m . CHEM.,19, 145 (1027). Edgar, G., IXD. Garner, F. H . , J . Inst. Automobile Enors., 27, 586 (1932). Garner, F. H., and E\mis, E. B., J . Inst. Prtroleum Technol., 18, 751 (1932). Garner, I:.FI., Evans, E. B., Sprake, C. H., and Broom, W.E. J., World Petiole?im Congr. Proc., 2, 170-80 (1933). Gamer, F. H., Wilkinson, R., and Nash, A. W., J . Soc. Chem. I n d . , 51, 265T (1932). Hoffman, F., Lang, X. F., Berlin, K.. and Schmidt, A. , .'!I Brennstof-Chem., 13, 161 (1933). Ibid., 14,103 (1933). J . SOC.C'hem. Ind., 49, 16T Howes, D. .L,and Nash, A. W., (1930). Ibid., 49, 113T (1930). Jonash, E. R., Meyer, C. L., and Branstetter, J. R., Natl. Ad. visory Comm. Aeronaut., Rept. ARR E6C04 (1946). and Bull, A . W., I h i d . , ARR E4E25 (1944). Jones, A. W., Jones, A . W., Bull, -4.W., and Jonash, E. R., Ibid., ARR E6B14 (1946). Karesev, K. K . , and Khabarova, A . V., J . @en. Chem. ( U S S R ) , 10, 1041-8 (1940).
FOOTKOTES TO TABLES Material introduced into engine a s gas. Obtained by extrapolation. 125' P. air instead of 225O F. Contains no lead. e Measurements m a d e in base fuel of (87.5 % 5-4 and 12.5 % n-heptane) i 4 ml. of lead and F-4 ratings expressed a s (70 S in n-heptane) 4 ml. of lead ( I ? ) . Data on different but similar sampIe from NACA. 0 Sample from source other than A.P.I. Obtained under A.S.T.M. motor method conditions (96). With 0.5 ml. of tetraethyllead instead of 1.0 ml. j Approximate. Increase in octane number for addition of 1 ml. of lead to blend. Percentage 5-3 i n M-4 reference fuel. 20 % solution in 53-4 ( 2 7 ) . Additional data o n some of compounds listed in NACA reports, Data on sample Ill-A. Equivalent ml. of tetraethyllead in iso-octane. - Not determined. No. Number assigned to compounds refers to A.P.I. numbers that identify t h e compound a s to purity and refer only to A.P.I. data. Purity of other compounds not generally stated. NACA. Compounds prepared by or for National Advisory Committee for Aeronautics. E. Compounds prepared by Ethyl Corp., refers to data in A.P.I. columns only. P. Compounds contributed by Phillips Petroleum Co., refers to data in A.P.I. columns only. EK. Compounds obtained from Eastman Kodak Co., refers to data in A.P.I. columns only. Numbers in parentheses indicate fuel-air ratio. a
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