2826
INDUSTRIAL AND ENGINEERING CHEMISTRY
fuels. However, fuels of this type such as tetralin (ClOHl2) and naphthalene ( CloHs) have high carbon-hydrogen ratios and, according to Table I and Equation 1, high carbon deposit tendencies. They would thus tend to enhance combustion chamber deposition problems. EFFECT OF OPERATIONAL CONDITIONS.Since carbon deposition is profoundly affected by the volatility of the fuel as shown previously, it could be assumed that evaporation might be a principal factor. Thus the temperature of the air into which the fuel is injected should be important. To investigate the effect of air temperature on coking when burning a mixture of hydrocarbons, kerosene was tested with results as shown in Figure 8. Deposits decreased with increasing temperature. Examination of Equation 1and the data of Figure 8 shows that there is probably some relation between vapor pressure of the fuel and carbon deposits. Thus the second member of Equation 1 could be replaced by an expression involving the vapor pressure rather than the boiling temperature. The derivation of an all-inclusive expression for carbon in terms of vapor pressure is made difficult however by the same factors which govern the laws of evaporation of fuel droplets. Therefore, no attempt has been made to incorporate such a relationship between vapor pressure and carbon formation.
TRAETHY LLE
Vol. 43, No. 12
The effect of combustion chamber pressure on coking is as pronounced as the effect of temperature, as shown in Figure 9. This could be expected, if fuel evaporation rate is important, since the evaporation rates of fuel droplets are functions of pressure as well as temperature. A comparison between fuel factors and operational characteristics shows that changes in operation can have as much influence on carbon deposits as maximum variations in fuel composition and volatility. Thus wide variations in volatility and composition of the fuel can cause up to 2 grams difference in carbon deposits out of 3 grams total, whereas raising the temperature of the inlet air from ambient to 300 F. results in a decrease in deposits of over 0.5 gram. Raising the combustion chamber pressure from 33 5 to 52.0 pounds absolute increased the deposits by as much as 2 grams out of 3 grams. O
LITERATURE CITED
(1) Bollo, F. G., Stanly, A. L., and Cattaneo, A. G., S.A.E. Journal, 54, 56-63 (1946). ( 2 ) Johnson, C. R , paper presented a t the iiational Aviation Mecting, American Society of Mechanical Engineers, Los Angeles, 1947. R E C E I V EMay D 2 , 1651
I CARBONS
Antiknock Effectiveness WANDA 1. ZANG AND WHEELER G. LOVELL Ethyl Corp., Detroit, Mich. T h e effectiveness of tetraethyllead in suppressing lcnoclr varies greatly with different gasolines and engines. Using the data on pure hydrocarbons from American Petroleum Institute Research Project 45, it has been found that the effectivenessof the addition of tetraethyllead may be evaluated simply in terms of the potential increase in engine power. The addition of a given amount of tetraethyllead to paraffins and naphthenes results, gerierally, in a constant percentage gain in relative potential power or performance number, regardless of the clear antiknock 'level or engine operating conditions. The effectiveness of the addition of tetraethyllead to aromatics and olefins is variable, but may be related in fairly simple ways to the molecular structure and conditions of test. Such data on pure compounds may serve as a basis for estimating the potentialities for improvementsin the utility of tetraethyllead in commercial gasolines.
H E chemical mechanisms by which the addition of tetraethyllead to motor fuels so effectively suppresses knock in engines are not well known. However, an empirical evaluation of the effect of tetraethyllead in various pure hydrocarbons, under a variety of engine operating conditions, yields results of practical interest and of speculative significance. The work of the American Petroleum Institute Research Project 45, which deals with the synthesis, purification, and properties of hydrocarbons of low molecular weight ( I ) , involves the preparation and knock-testing of hydrocarbons of high purity. The knock tests are conducted in the General Motors and Ethyl
T
Corp. Research Laboratories to evaluate the potential utility of these hydrocarbons in internal-combustion engines ( 4 ) . Almost 300 pure hydrocarbons and other compounds have been engine tested in this cooperative program under as many as 29 test conditions. The data used for the study of these pure hydrocarbons have been taken from tabulated knock-test engine tables which were assembled by the American Petroleum Institute Research Project 45; these tables, which represent about 4000 separate determinations, have been published in the Eleventh Annual Report of API Research Project 45 ( 1 ) Correlations existing betreen hydrocarbon structure and knock behavior with different methods of engine testing, between the knock behavior of pure hydrocarbons and their blends, and between the chemical natures of the hydrocarbons and the effects of the addition of tetraethyllead have already been published (7). The engine characteristics of pure hydrocarbons have been correbted with molecular structure by a study of such concepts as the free radical chain mechanism, relative reaction rates of primary, secondary, and tertiary hydrogen atoms, and the activating effect of methyl groups ( 3 ) . This paper presents further correlations of the relationships which exist among hydrocarbons with regard to gains in antiknock quality attributable to the addition of tetraethyllead. M E T H O D OF M E A S U R E M E N T
In order to deal with data obtained with a variety of engines and operating conditions on a basis which results in simple relationships, use has been made throughout this paper of the performance number scale. This scale, used to express the antiknock
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1951
160
I40
I20
in this study in which the ratings of many of the pure hydrocarbons are above the 100 octane number level. 3. It avoias the misunderstandings that may arise from the fact that an octane number in the low range represents much less economic usefulness than an octane number in the high range. 4. It leads t o simple relationships when considering the effectiveness of tetraethyllead in fuels. 5. It involves no new experimental techniques, but is merely a way of expressing antiknock quality derived from customary measurements of octane numbers.
100
80
60
40
20
ENGINE TEST CONDITIONS
Naturally aspirated and supercharged engines were used to rate the hydrocarbons. The naturally aspirated engines used were the standard ASTM knock-test engines and a single-cylinder, variable-compression, valve-in-head engine having a bore of 3.25 inches and a stroke of 4.5 inches; the single-cylinder engine can operate at speeds ranging from 600 to 2000 r.p.m. Specifically, the naturally aspirated methods of test were the ASTM Research method; the ASTM Motor method; and the General Motors Critical Compression Ratio method at: R.P.M. 600
M L TEL IN ISOOCTANE
OCTANE NUMBER
Figure 1, PerFormance Number Equivalents for Octane Numbers and For Tetraethyllead in Iso-octane
quality of a fuel, was adopted by the ArmyNavy Aeronautical Board in 1942. It is based upon definite relationships with the currently used octane number scale (or milliliters of tetraethyllead in iso-octane). Performance number is the knock-limited, indicated power output, relative to that obtained with iso-octane, of an “average” engine when operated on a primary reference fuel. The performance number-octane number relationship is expressed by the equation
PN = 2800/(128
- ON)
2827
600 2000
2000
Air
Jacket Temp., ’F. 212 350 212 350
Spark Advance M a x . power Max. power Max. power Max. power
F.
Temp., 100 150 100 150
FuelAir Ratio Max. knock Max. knock Max. knock Max. knock
The supercharged engine was a single-cylinder unit of 17.6 cubic inch displacement, operated a t 900 r.p.m., and adjusted for maximum knock, and O.07and 0.10 fuel-air-ratios. A
B OCTANE NUMBER
0
50 70
90
I00
ML TEL I N I S 0 + I O t 3 0 t50
2140 2120 c
+
(1)
which was derived from experimental ArmyNavy data obtained on fuels of 70 to 100 octane number; above 100 octane number, the performance number of iso-octane plus tetraethyllead is also based on experimental data. These relationships are shown in Figure l ( 6 ) . The advantages of the performance number scale over the octane number scale as a means of expressing antiknock quality are many, the most important of which are these: 1. It expresses antiknock quality in terms of the utility of a fuel, as measured by the power and economy to be obtained from it in both naturally aspirated and supercharged engines (6). 2. It provides a continuous scale up to 100 octane number (expressed as per cent of iso-octane in n-heptane), and above 100 octane number (expressed as milliliters of tetraethyllead in iso-octane), without changing units. This is particularly desirable
20 20
40
60
80 100 120 A N - P N . CLEAR
140
160
D OCTANE W B E R 70 90 100
ML TEL IN I S 0
t10 t30 t60
0
t602 t30: w +log
I00
(L
Y
P
90
1
Y
70
43
Figure 9.
60
80 100 120 140 160 RELATIVE IMEP, CLEAR
Effect of Tetraethyllead in ParaFfin Hydrocarbons
180
$
INDUSTRIAL AND ENGINEERING CHEMISTRY
2828 OCTANE 0
50 60 70
80
ML T E L I N
NUMBER
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100
PARAFFINS
ISOOCTGNE
+I0
+20 +30
Vol. 43, No. 12
150
In general, the effect of adding tetraethyllead to paraffin hydrocarbons is to increase their antiknock level. A critical examination of the data available on some 60 paraffins, ranging from Cd through Cl0 and including almost all of the structural isomers through Cs, has yielded the following general relations. First, the gain in performance number upon the addition of a given amount of tetraethyllead t o paraffins is substantially a constant percentage, regardless of the clear antiknock levels of the compounds. This is ehonn in Figure 2 ( A ) and is true within the limits indicated graphically and with a fexv marked exceptions. In this figure, the performance number of the paraffin TTith the addition of 3 ml. of tetraethyllead per gallon is plotted against the clear performance number of the compound for the Research method of test. For orientation, the dashed line on the figure is equivalent to zero gain in performance number, so that the actual gain for any point on the figure.is the v~rticaldisplacement of that point from the line for zero response. A solid line has been drai5n on the figure, as a reference, through the defined performance number of iso-octane with 3 ml. of tetraethylA N P E R F O R M A N C E N U M B E R , CLEGR lead per gallon; it corresponds to a 47% gain Figure 3. Effect of Tetraethyllead in Naphthene Hydrocarbons in performance number for this addition of tetraethyllead. GENERAL EFFECT OF TETRAETHYLLEAD The majority of the paraffins rated by this method of test fall consistently on or about this line of reference; the few marked The nearly 300 pure hydrocarbons involved in this investigaexceptions will be discussed later. The relationship shown is an tion are among the compounds normally found within the boiling important one, since it permits the prediction of the approximate range of motor and aviation fuels. The effect obtained by adding increase in antiknock quality that may be expected of a paraffin tetraethyllead to each member of this group of hydrocarbons is variable. Usually, the addition of tetraethyllead results in a gain in the antiknock quality of the compound, but in a few hydrocarbons it increased their tendency to knock. For the relatively insensitive paraffin and naphthene hydrocarbons, the gain in performance number upon the addition of a given amount of tetraethyllead seems to be a constant percentage, regardless of clear antiknock level or engine operating conditions. However, tetraethyllead response in the aromatic and olefinic hydrocarbons does not follow this simple pattern. These compounds have been found to be very sensitive to specific changes in engine operating conditions and also to the addition of tetraethyllead. Further study of these unsaturates has shown that the effect obtained by the addition of tetraethyllead may be related to the molecular structure of the hydrocarbon in fairly simple ways. Each of these four major chemical classesparaffin, naphthene, aromatic, and olefin-have been examined separately.
----f
Figure 4.
Effect of Tetraethyllead in Aromatic Hydrocarbons RE'LATIVE I M E B . C L E A R
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1951
NO LEAD RESPONSE
a!
,
-10
N-ALKYL CARBON ATOMS ATTACHED TO BENZENE RING
I
dicates a 26% gain. The figure of 26% for 1ml. of tetraethyllead per gallon correspondsto the defined performance number of this amount of tetraethyllead in iso-octane, and consequently is in correlation with the 47% gain in performance number previously considered for 3 ml. of tetraethyllead in iso-octane. Dat8aobtained by the National Advisory Committee for Aeronautics on paraffins tested with supercharged engines are in close agreement with this (2). The supercharged data on the 17.6 engine are reported in terms of relative indicated mean effective pressure (imep). For all practical purposes, relative indicated mean effective pressure and performance number have been regarded as being equivalent in the 17.6engine at these operating conditions. The data available are not numerous a t these conditions of test, but they appear t o indicate that the effectiveness of tetraethyllead in paraffin hydrocarbons is independent of charges in mixture ratio. The compounds which appear on most of these figures as exceptions to these generalizations-compounds in which tetraethyllead is not very effective-such as 2,2,3,34etramethylpentane and 2,2,3,34etramethylhexane, seem to have several characteristics in common which are unusual for paraffin hydrocarbons. Generally, these exceptions are highly branched paraffins having high clear antiknock levels (usually greater than 100 octane number) and poor lead response. In addition, they are "sensitive" hydrocarbons-i.e., they may show a gain or loss in antiknock level, clear or with tetraethyllead, when engine operating conditions are changed.
501
-10
I
1
I
,
I
I 5 3 4 5 CARBON ATOMS ATTACHED TO BENZENE RING
2829
E
I
I
I
I
(
I
Figure 5. Effect on Tetraethyllead Response of Changes in the n-Alkyl Side Chain Attached to the Benzene Ring 11.6 engine al maximum knock or 0.07 fuel-nir ratio 1.0 ml. tetraethyllead per eallon Air 2 2 5 O F.
upon the addition of 3 ml. of tetraethyllead per gallon, if its clear antiknock level is known. Second, the gain in performance number obtained by the addition of a given quantity of tetraethyllead is the same percentage, in general, regardless of change in engine operating conditions. Figures 2 ( B ) and 2 (C) show the results obtained when these paraffis were rated by the Motor method and one of the Critical Compression Ratio method conditions (GOO r.p.m. and 212' F. jacket temperature). The other Critical Compression Ratio method conditions gave similar results. The dashed line again indicates zero gain, while the solid line is indicative of the 47% gain in performance number upon the addition of 3 ml. of tetraethyllead per gallon and is common to each engine operating condition, The signseance is that, within the limits shown by the deviations of the individual points, the increase in, antiknock quality that may be obtained by the addition of 3 ml. of tetraethyllead per gallon to a paraffin may be predicted for any method of test, if the clear antiknock quality is known. Third, the percentage gain in performance number obtained by the addition of a given amount of tetraethyllead is a constant factor regardless of changes in mixture strength. The effectiveness of the addition of 1 ml. of tetraethyllead per gallon to the paraffis, as tested in the 17.6 supercharged engine at fuel-air ratios of 0.07 or maximum knock, are illustrated in Figure 2 (D) and comparable results were obtained at a fuel-air ratio of 0.10. As before, the dashed line represents zero gain; the solid line in-
PARAFFIN LEVEL
20
17.6 ENGINE A T MAX KNOCK I O ML TEL/GAL AIR. 225 F
10
I
I
I
-
I
NAPHTHENES
The naphthenes, or cycloparaffins, have, in general, antiknock properties which do not change much with engine operating conditions. The effectivenessof tetraethyllead has been studied in some 30 naphthenes at six naturally aspirated engine operating conditions and in the 17.6 supercharged engine at maximum knock fuel-air ratio. Generally, the gain in performance number obtained by the addition of a given quantity of tetraethyllead in these compounds is a constant percentage, similar to that of the paraffis, regardless of clear antiknock level and generally independent of engine operating conditions. To show these relations more fully, data on performance number measurements of the naphthenes are plotted in Figure 3
2830
INDUSTRIAL AND ENGINEERING CHEMISTRY 150
AROMATICS
176 E N G I N E , t I M L T E L PER GALLON
125
600 R P M - 3 5 0 F JACKET, t 3 M L T E L PER G A L L O N 6 0 0 R P M - 2 1 2 F JACKET, t 3 M L T E L P E R GALLON
The aromatic hydrocarbons are an especially interesting class of conipounds because most of those within the gasoline boiling range have antiknock levels above 100 octane number. Some 20 of them, including benzene and almost all of the alkyl-substituted benzenes ranging from Cq through GO,have been engine tested, but because of their high clear antiknock levels, the operating conditions are limited at which the lead effectiveness of these compounds has been tested. The effect of the addition of tetraethyllead in suppressing knock in the aromatics is quite unlike its effect in paraffins and naphthenes, varies widely from high to negative values, and is affected by engine operating conditions. There is evidence, however, that it may be correlated with the Rtructure of the individual aromatic compound . The most comparable set of engine data available on the effect of the addition of tetraethyllead is that obtained in the 17.6 supercharged engine at maximum knock fuel-air ratio, shown in Figure 4. The data have been evaluated in terms of relative indicated mean effective pressure (again considered as equivalent to perform-
100
75
50
25
0
Figure 7.
Voi. 43, No. 12
Tetraethyllead Response of Aromatics Relative to Iso-octane
for one naturally aspirated engine operating condition, The performance number of the compound with 3 m]. of tetraethyllead per gallon is plotted against the clear performance number. The dashed line represents zero gain in performance number; the solid line is the same reference line used for the paraffins indicating a 47% gain in performance number obtained by the addition of 3 ml. of tetraethyllead per gallon. Again, the vertical displacement from the dashed line is the actual gain for any point on the figure, The other operating conditions investigated show similar results. As mentioned, the effectiveness of adding tetraethyllead to these naphthenes, a t the six naturally aspirated conditions of test, is markedly similar to the general pattern of the paraffins The plotted points fall on or about the 47% response line for these operating conditions in the same general way as found with the paraffins, with the possible exception of the Research and Motor methods, where the values obtained seem to fall slightly below this level, However, it may be concluded that, for most of the naphtbenes, the effectiveness of tetraethyllead is not unlike its effectiveness in paraffins and is not affected significantly by changes in engine operating conditions. The few compounds which deviate from this relatively constant level, such 8s te~t-butylcyclohexane and 1,1,3-trimethylcyclopropane, are exceptional, as are a few paraffins previously noted, in that they have a highly branched or compact structure, are sensitive to engine operating conditions, and have low tetraethyllead response. The effect of mixture ratio cannot be found directly because the limited amounts of pertinent data on this class of compounds were available only a t maximum knock fuel-air ratio on the 17.6 supercharged engine. However, the naphthenes being so similar to the paraffins in other respects, it might be assumed that the same mixture ratio effects would be observed. The addition of 1 ml. of tetraethyllead t o the naphthenes in this engine gives the same general percentage gain in performance number as it did in the paraffins-a 26% gain, The data on naphthenes obtained by the National Advisory Committee for Aeronautics research laboratory are in close agreement with this result (g).
E 1
1
1
/
'
Pt50
///-+30
//
i 4
'
'+PO? i
-+lo E
// W c
-
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/
-90
Figure 8.
Effect of Tetraethyllead in Olefin Hydrocarbons
a:
2
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1951
B
A OCTANE NUMBER
0 5070
90
160
ML TEL IN IS0 100
+IO
+30+50
OCTANE NUMBER
o
M L TEL IN I S 0
160
100
150
B 2
20
40
60
BO AN-PN,
100 120 GLEAQ
140
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20
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BO 100 120 A N - P N , CLEAR
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D OCTANE NUMBER
OCTANE NUMBER 0 5070 90
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1
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s' I00
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+
2831
Another general conclusion that can be drawn from the available data on the aromatics is that engine operating conditions may have a significant effect on their tetraethyllead response. The compounds .on which there are comparable data are shown in Figure 7 at three operating conditions: 600 r.p.m. and 212' F. jacket temperature; 600 r.p.m. and 350' F. jacket temperature; and 17.6 supercharged. To relate the data at different tetraethyllead concentrations and engine operating conditions on an equitable basis, the percentage gain in performance number or relative indicated mean effective pressure obtained by the addition of a given amount of tetraethyllead to aromatics has been compared with the gain obtained by the addition of a corresponding amount of tetraethyllead to iso-octane, and expressed as a percentage of the effectiveness of tetraethyllead in iso-octane. This figure demonstrates the severalfold range over which the tetraethyllead response of the aromatics may vary with change in engine test conditions.
BO
OLEFINS 50
_ _20
I
I
PO /
20
40
60
BO 100 120 AN-PN, CLEAR
140
160
40
60
l
l
1
BO 100 120 A N - P N , CLEAR
others, the addition of tetraethyllead acts as a slight knock inducer.
0 140
160
,
The olefinic hydrocarbons constitute the greatest number of compounds normally found in the gasoline boiling range and the number of existing isomers is so numeroui
600 RPM- 350 F JACKET 3 o ML TEL /GAL
- 150 F
AIR
on each figure indicates zero response, and the solid line is the paraffin reference line of 47% gain in performance number for the addition of 3 ml of tetraethyllead per gallon. The percentage gain in performance number among the olefins is obviously not a constant amount and is erratically affected by changes in engine operating conditions. Among structurally related compounds, the effectiveness of tetraethyllead is similarly inconsistent. Figure 10 shows the percentage gain in performance number, obtained by the addition of 3 ml. of tetraethyllead per gallon, for eome of these olefins from Cg through CS drawn as simplified structural formulas, rated by the 600 r.p.m. and 350' F. jacket temperature method of test. Arrows connect structurally related compounds, but no direct correlations seem applicable. Similar plots available at the other naturally aspirated and supercharged procedures show the same lack of relationship. One important relationship is the effect of the position of the double bond on tetraethyllead response, Olehs having the same molecular structure but differing in the posi-
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December 1951
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2833
The variable results obtained by the addition of tetraethyllead to olefins may be attributed to the sensitivity of the individual compounds to changes in engine operating conditions and, in addition, may be related to the molecular structure and position of the double bond. The addition of tetraethyllead to olefins having the double bond in the alpha position generally yielde the greatest percentage gain. Tetraethyllead is also most effective in olefins tested under the milder engine operating conditions, but this effectiveness seems to decrease generally with increase in clear antiknock level. LITERATURE CITED
(1) Am. Petroleum Inst. Research Project 45,
Eleventh Annual Report, “The Synthesis, Purification, and Properties of Hydrocarbons of Low Molecular Weight,” Ohio State University Research Foundation, July 1, 1948-June 30, 1949. (2) Barnett, H. C., “Lead Susceptibility of Paraffins, Cycloparaffins, and Olefins,” Natl. Advisory Comm. Aeronaut., Wartime Rept., originally issued May 1943 as Advance Restricted Rept. 3326. (3) Boord, C. E., “The Combustion of Motor A N PERFORMANCE NUMBER, CLEAR Fuels,” Ninth Annual Report, Am. Petroleum Inst. Research Project 45, Serial No. Figure 13. Effect of Engine Operating Conditions on Tetraethyllead Response 51, Ohio State University Research Foundaof Olefins tion. (4)Chem.Eng. News, 28,No. 31,2578 (July31,1950). (a) Coordinating Research Committee Rept. No. 241, “Method for tion of this measure of sensitivity. This is a plot of the Research Expressing Fuel Antiknock Ratings,” April 1949. to Motor method ratios of the performance numbers with 3 ml. (6) Hesselberg, H. E., and Lovell, W. G., S.A.E. Journal, 59, No. 4, of tetraethyllead per gallon versus the Research to Motor method 32 (1951). ratios of the clear performance numbers. Most of the points fall (7) Lovell, W. G., IND. E m . CKEM.,40,2388 (1948). above the dashed line of equal sensitivity. The significant effect of tetraethyllead is to make these olefins more sensitive. A RECEIVED April 19, 1961. similar comparison of the clear and leaded seneitivities of the 600 r.p.m. and 212’ F. jacket temperature and 600 r.p.m. and 350OF. jacket temperature methods shows the same general increase in sensitivity with tetraethyllead, but the relationship is not quite so consistent. CONCLUSIONS
The effectivenessof tetraethyllead as a knock suppressor in pure hydrocarbons, when evaluated as the percentage gain in performance number obtained by the addition of a given amount of tetraethyllead, differs among the four major chemical groups-the p a r a f i s , naphthenes, aromatics, and olehs. Tetraethyllead is generally more effective in the paraffins and naphthenes, where the gain realized from the addition of a given amount of tetraethyllead is substantially a constant percentage] independent of clear antiknock level or change in engine operating conditions. In o l e h s and aromatics] the effect of the addition of tetraethyllead varies-it may range from an effectiveness equal to or greater than in the paraffins to negative or knock-inducing levels. For aromatics, the effectiveness of tetraethyllead may be related t o the molecular structure of the compound in that the nalkyl monosubstituted benzenes have a substantially constant tetraethyllead effectiveness, about 25% better than the paraffins] whereas branching may decrease this effectiveness (in compounds above C,) t o half that of the p a r a f i s ; and in polysubstituted benzenes, the effectivenessof the addition of tetraethyllead in structurally related compounds decreases with para, meta, and ortho substihtion.
Figure 14. Effect of Tetraethyllead on the Research-Motor Sensitivity of Olefins
