INDUSTRIAL AND ENGINEERING CHEMISTRY
2844
(12) Johnson, J. R.,Snyder, H. R., and Van Campen, M. G., Jr.. J . Am. Chem.Soc., 60, 115 (1938). (13) King, R. O., Can. J . Research, 25F, 326-41 (1947). (14) Krause, E., and Kitsche, R., Ber., 54B, 2784 (1921). (15) Lamb, F. IT., and Kiebylski, L. AX.. paper presented before the 16th midyear meeting of the A4mericanPetroleum Institute, Tulsa, Okla., April 30-Nay 3, 1951. (16) Laubengayer, 8.TI‘., Cornell University, private communica-
tion. (17j Marek, L. F., “Catalytic Oxidation,” Twelfth Catalysis Report,
Sational Research Council, pp. 159, 162, Sew York, John Wiley & Sons, Ino., 1940. (18) Mellor, J. W.,“A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. 5 , 1). 10G, London, Longmans, Green and Co., 1940.
Vol. 43, No. 12
(19) Mittasch, A., Wilfroth, E., and Bala, 0 . (to Badische Anilin- und
Soda-Fabrik), U. S.Patent 1,487,020 (March 18, 1924). (20) RIoller, J. A,, and hloir, 1%.L., S.R.E. J o w n a l . 46, No. 6 , 250, (June 1940). (21) Pope, J. C., Dykstra, F. J , and Edgar, G., J . Am. Chon. Soc., 51, 1875 (1929). (22) Scatteraood, A., LIilIer, IV. H., and Gammon, J.. Jr.. Ibid.. 67, 2150 -( 1945). (23) Stewart, J P., and Story, B. TI’., paper presented before the Woi Id Automotive Engineering Congiess of the Society of Automotive Engineers, Ne%. York, May 23, 1939 (24) Tiimble, H. M., and Bottenbeig, K C., Proc. Am. PefrOh7fL Inst., I l l , 21, 85 (1940). (25) Webster, S H., and Dennis, I, hI J Am. C h m . SOC.,55, 3233 (1933). RECEIVBD May 4,1951.
PRECOMBUSTION REACTIONS IN AN ENGINE Thermodynamic Analysis
OF
Pressure Developed during Preflarne Period
CLEVELAND WALCUTT
AND
ELLIS
B. RlFKlN
E t h y l Corp., Research Laboratories, Detroit, Mich.
T h e reactions of air-fuel mixtures in an engine prior to combustion were investigated in an effort to understand further the mechanism of the knocking process. The data, taken in an engine operated without spark ignition, showed that as much as 27% of the heat of combustion was released prior to actual flammation of the charge; for most of the fuels tested this heat amounted to 10 to 15%, while one fuel gave no evidence’of prccombustion reaction. It appears that resistance to lcnoclc cannot be related directly to the heat liberated w-hen a fuel undergoes precombustion reactions. Some evidence indicates that the antiknock effectiveness of tetraethyllead depends on the presence of intermediate products formed during the precombustion reaction. Application of these results to the spark-ignited engine shows that there may be irnportant effects on power output as a result of precombustion reactions.
XOCK in a spark-ignition engine is generally believed to be
K
related to the existence of preflame reactions taking place in the unburned portion of the charge before it autoignites. Numerous investigators have explored the problem, but a wide gap still exists between fundamental investigations and the practical problem of engine knock. Tizard and Pye (8) found that autoignition takes place only after a definite time lag, while Taylor and Taylor (6) pointed out the dependence of the ignition temperature on the time-temperature path prior t o ignition. These factors indicated that the elimination of knock depends on the consumption of the entire charge by the flame front before autoignition of the unburned portion can occur. After discovery of the 2-stage oxidation of fuels a t relatively low temperatures and pressures, inariy investigators attempted t o relate this phenomenon to the knock process. The discovery by Levedahl and Sargent ( 3 ) that the rate of pressure rise in an autoigniting cycle does not increase steadilv but passes through a maximum and a minimum prior to autoignition suggested that a useful study of precombustion reactions might be made in an engine. Since in an autoigniting engine
the elinlination of an advancing flame front following spark ignition puts the whole combustion chamber charge in somewhat the same condition as the last portion of the charge to burn in a spark-ignited engine, it appeared that conclusions based on work with an autoigniting engine Jvould be of value in understanding the knock phenomenon.
3OO’F: INLET MIXTURE
0
20
40
60
80
100
OCTANE NUMBER
Figure 1. Compression Ratio Requirement for Autoignition and Standard Knock Using Primary Reference Fuel Blends
Further substantiation of this concept may be seen in Figure 1, which s h o w that for both the autoigniting engine used in this research and the Motor Method engine operated at standard knock intensity, a specific relationship exists b e h e e n cornpression ratio and octane number of n-heptane-iso-octane blends. The higher compression ratio required for autoignition with a given fuel reflects the absence of the flame front; the latter has the effect of a second piston in the spark-ignited engine. Because a thermochemical interpretation of thc reactions preceding combustion can be inade more readily using pressure data from an autoigniting engine than from a spark-ignited engine, this investigation via8 undertaken using the apparatus and techniques described in succeeding sections.
December 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY EXPERIMENTAL
TEST EQUIPMENT AND ADJUSTMENTS. A photograph of the equipment used for this study is shown in Figure 2. The console to the right of the engine contains the electronic control apparatus and oscilloscope for use with the pressure transducer in the engine combustion chamber. Figure 3 is a diagrammatic representation of the apparatus, showing the electronic units in their proper relationship.
Figure 2.
2845
approximately the same engine temperatures existed as for the test fuel up t o the time in the engine cycle a t which the pressure rose because of reactions. The last part of the film, obtained while the engine was being motored, was used for pressure calibration. Typical records for one cycle from each of the first and second parts of the film are shown in Figure 4. PRFCISION OF D A T ~ .The close similarity, determined by superposition, between pressure-time records of consecutive cycles before the start of autoignition indicated that the precision was within the limits set by the width of the trace on the stripfilm record. An absolute scale of pressure was established through use of a datum a t the minimum pressure during the suction stroke. This value was found by means of a balanced pressure unit to be 12 rf 0.5 pounds per square inch absolute. Pressure calibration data above the datum line were obtained by comparing the vertical deflections of the oscilloscope beam with compression pressures a t varying compression ratios. These showed a precision of about 3%. It is felt that the accuracy of the pressure measurements was about 5%. Based on the marks on the stripfilm placed 0.01 second apart, the precision of crankangle timing reference points was 2%.
Engine and Auxiliary Apparatus
The experimental work was oonducted on an ASTM variable-compression, knock-test engine which was operated at 900 f 5 r.p.m., a cooling-jacket temperature of 205' f 4 ' F., and a fuel-air ratio of 0.074 f 0.002. For engine conditions approximating the Research Method of knock testing, an intake air temperature of 150' i 2' F. was used; for conditions approximating the Motor Method, the engine was operated a t 300' f 5" F. inlet mixture temperature. For each test fuel or fuel-additive combination used in this research (Table I), the compression ratio was adjusted until the cycle-to-cycle autoignition was steady and any reduction in compression ratio caused marked variations in the intensity of autoignition. In this paper this critical compression ratio is defined as the ratio required for incipient autoignition. Throughout the work, whether the engine was autoigniting or not, the spark ignition was maintained a t 20 degrees after top center. This retarded ignition timing caused no interference with the precombustion phenomenon, but if for any particular cycle the charge was not autoignited, the spark fired the charge and maintained nearly constant cycle-to-cycle conditions-among others, amount of residual exhaust gas, cylinder wall temperature, and fuel-air ratio. A 2000-pound-per-s uare-inch Li (2) pressure element and amplifier unit was use3 in this investigation. The output from the amplifier was fed into one circuit of a dual-beam Du Mont oscilloscope, Model No. 279. The other beam of the oscilloscope was used to record pulses from crankshaft reference points. A Fairchild continuous strip-film camera provided a time axis and recorded the oscilloscope traces. Thus, the film showed the pressure-time function in rectangular coordinates as well as the timing reference marks for consecutive engine cycles. TESTPROCEDURE.After the engine had been running long enough to establish steady-state conditions, the test fuel was supplied to the engine, the fuel-air ratio set, and the compression ratio adjusted as previously described. With everything in readiness film was run through the camera for about 6 seconds. During the first 3 seconds a record of a series of cycles was obtained while the engine was autoigniting on the test fuel, The fuel supply was then uickly switched from the test fuel to triptane containing 6.0 my. of tetraethyllead er gallon, and during the next 2 seconds records were obtainelfor additional engine cycles. Finally the fuel supply was shut off, and during the last second records were obtained while the engine was being motored. The first part of the film provided information in regard to the reactions taking place in the combustion chamber during compression and autoignition of the test fuel. The second part of the film showed the pressure-time record when there were no reactions during compression and no autoignition; hon-ever,
SUPPLY
I
I I
1
-_____-/ I U
RESSURE
VARIABLE COMPRESSION RATIO ENGINE
u Figure 3.
i
PULSE SHAPING AMPLIFIER
1
.
GENERATOR
u
Equipment Used for Investigation of Precombustion , Reactions
Tuble
1.
Description of Test Fuels
rue1
Fuel" 1 45% Diisobutylene 55% (42% iso-octane 58% n-heptane) 2 20% Diisobutylene 80% (65% iso-octane 35% n-heptane) 3 51 5 Toluene 48.5% (50% iso-octane 4- 50% 26ik!tane) oluene 75% (72% iso-octane 28% nh&tane) 5 25% Diisobutylene 75 n-heptane 6 50% Diisobutylene 50% n-heptane 7 7503 Diisobutylene 26% n-heptane 8 n-geptane 6.0ml. tetraethyllead (motor-mix) per gallon 40% n-heptane) 14.85 grams 9 (60" Iso-octane n-gutyl nitrate per gallon 40% n-heptane) 14.85 grams 10 (607 Iso-octane 3.0 ml. (motor-mix) tetraethyln-gutyl nitrate lead per gallon
NO.
+ +
+
+
+ +
+
+ + +
+ ++
++ 20% 15% n-heptane n-heptane + 407 n heptane 40% Iso-octane + 60% n:heptane 100% %-Heptane
11 85 , Iso-octane 1 2 so$ Iso-octane 13 60 Iso-octane
14 15
+ +
+ +
Ootane Number Research Mot; Method Method 90
80
85
80
90
80
85 47 78 97
80 46
72 82
57
54
29
40
68
74 85 80 60 40 0
86
80 60 40 0
6.0 ml. tetraethyllead (motor-mix) per 16 Triptane 1100 >loo gallon a Fuel com ositions given in per cent by volume. Iso-octane and nheptane were ifSTM Grade obtained from Phillips Petroleum Go. Toluene was of Technical Grade obtained from the same company. Diisobutylene was a mixture of 2,4,4-trirnethylwntene-l. and 2,4,4-trimetbylpentene-2 obtained from Esso Standard 011 Co. and desi nated as SR-10 reference fuel. Triptane (trimethylbutane) of Technlcal Graje was supplied by the General Motors Research Laboratories.
2846
INDUSTRIAL AND ENGINEERING CHEMISTRY TREATMENT O F EXPERIMENTAL DATA
dw
Also,
The following assumptions wcre made in this derivation:
Vol. 43, No. 12
= pdv
(4)
Substituting Equations 2, 3, and 4 into Equation I , and collecting terms,
1. The fuel-air mixture obeys the perfect gas lans. The pressure rise associated with the precombustion reaction is the result of heat liberation by the reacting mateiial and is not significantly affected by an increase in the number of molecules as a result of the reaction. This assumption is justifiable on the basis that the fuel comprises only about 2 mole 70 of the charge, and even complete combustion of this furl would result only in a 5% rise in the total number of molecules present. Since 2.
Assuming cc constant for the temprrature and pressure iange considered, and replacing? by C,,
Thus when limits of integration corresponding t o points I and 2 in t'he cycle are put into Equation 5 , q will represent the net heat absorbed by the working fluid between these points. In terms of the actual reacting fuel-air mixture, y represents the difference between the heat of reaction and the heat losses to the engine walls. For a nonreacting fuel, p is a measure of heat leak alone. In evaluating p for the various runs, it was necessary first to convert the pressure-time curves to a pres~ure-volumebasis. The values of J p d v and JLidp were obtained by graphical integration, using the pressure and volume corresponding to -40 crankshaft degrees (40 degrees before t,op dead center) as the lower limit of integration and a series of pressure-volume values over the period of precombustion for the upper limit. Values of C , were calculated for each fuel-air mixture at temperatures estimated from the relation
Figure 4. A. B.
- = (-:)I' where k , the polytropic TI coefficient, was assumed to have a value of 1.25. The tabulations of Souders, hIatthews, and Hurd ( 5 )were used for the heat capacities of the individual components. When these heats had been evaluated for the nonreacting mixture of leaded triptane and air a t various compression ratios and
Strip Film Records for Fuels
Chemical activity before auloignilion N o chemical activity prior to spark ignition at
+PO
degrees B
t40D
the pressure effects due to preconlbustion reactions were much higher percentages of the base pressure obtained with a nonreacting fuel described later, the validity of this assumption is clear. 3. The liberation of heat through chemical reaction prior to combustion has the same thermodynamic effect as though an equivalent amount of heat were produced in another way and transferred to a nonreacting fuel-aii, mixture. 4. The compression and expansion of the engine charge is thermodynamically reversible before combustion occurs. 5 . Over the time range between the beginning and end of the precombustion reaction, the heat capacity of the fuel-air mixture was assumed to be constant. This umption was made to simplify the calculations arid is base8 on the fact that the relative change in absolute temperature over that time interval (calculated for adiabatic conditions) \vas almays one fifth as great as the relative change in pressure, even n-hen the pressure doubled over the interval. In the Torst case this meant a variation of about 250" F. and a corresponding variation of about 7% in the heat capacity of the charge in this interval. The resulting error in the reaction heat (uncorrected for heat leak to the walls of the combustion chamber) was then as high as 10 calories, but the calculation of the heat leak ?vas subject to the same error, and the effect on the final reaction heat was about I or 2 calories for the charge.
I
-33 -20 -IO
CRANKSHAFT DEGREES
0
+IO
t70 t 3 0
CRANKSHAFT DEGREES D
400 W
0
p1300
8-+zoo g 0
+IO0
z
0
t
o
8
By the first law of thermodynamics, for a differential compression or expansion, dp = de
+ du
(1)
For an ideal gas, de
=
c,dT
Also, since pv = nRT,
pdv and
dT
=
+ vdp = nRdT pdv/nR f vdplnR
(3)
-100 -40
-3 -20 -10
0
+I0 t20
CRANKSHAFT DEGREES
Figure 5.
+M
-'WW
-10 D +I0 +XI A0 CRANKSHAFT DEGREES
-XI -XI
Heat Generated by Precombustion Reactions
Basis of heal calculation%: -40 crankshaft degrees A. Fuel: n-heptane plus 6.0 ml. tetraethyllead per gallon Compression ratio: 7.35 Intake air: 150' F. B. Fuel: n-heptane plus 6.0 ml. tolraethyllead per gallon Cornpression ratio: 6 . 2 5 Intake mixture: 300' F. C. Fuel: 60% iso-octane, 40% n-heptane Compresslon ratio: 6.55 Intake mixture: 300' F. D. Fuel: 7 5 % diisobutylene, 2 5 % n-heptane Compression ratio: 10.45 Intake mixture: 300' F.
December 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
therefore over a range of temperatures, it was possible to plot a graph of rate of heat leak versus maximum temperature of compression, with the thought in mind that q for this mixture corresponded to heat leak alone. This rate of heat leak varied in almost linear fashion from 1100 calbries per second a t 500" F. t o 4300 calories per second a t 800" F. These temperatures were obtained as described above and must be regarded as approximate. (The heat leak calculated from data taken with fuel cutoff was in good agreement with that obtained from fuel-air mixtures. This implies that heat loss by radiatlon from the fuel prior t o combustion was unimportant and therefore accounts for the linear relationship between heat leak and mixture temperature.) It was, of course, desirable to separate the effect of heat leak from the reaction heats in the runs made with the test fuels. This was done using the heat leaks obtained from the leaded triptane curves, with the assumption that any of these fuel-air mixtures would have the same heat-leak rate as any other, provided its maximum temperature was the same. It was then possible to correct the heat-time curve for each test fuel by knowing the temperature increment a t a specific moment in the engine cycle between the test fuel and the triptane run a t the same compression ratio. The corrections amounted to 25 to 50% of the uncorrected value, and they were made in such a way that the final plots, shown in Figure 5, for the test fuels are based on the same heatleak rate as the accompanying triptane curves. Thus the latter represent the true heat leak a t the indicated compression ratio, and the heat of reaction of the test fuel is given by the difference in the ordinates of the two curves a t the time specified. At the 150" F. inlet air condition, the measured values of fuel gram-moles of fuel and and air charged per cycle were 3.2 X 1.57 x 10-2 gram-moles of dry air. The corresponding values for the 300" F. inlet mixture condition were 2.9 X 10-4 grammoles of fuel and 1.43 X 10-2 gram-moles of dry air. PRECOMBUSTION REACTIONS
.
Figure 6 shows a number of pressure-time curves which were drawn from projections of the original strip-film records, care having been taken to align the curves for the test fuel and the corresponding leaded triptane in accordance with time and pressure reference marks. No effort was made t o reproduce the high frequency vibrations which appeared on the original film record of the cycle following autoignition. Triptane with 6 ml. of tetraethyllead per gallon was used in the engine over the range of compression ratio from 4.36 t o 14.5. For this interval the pressure-time curves had the same general characteristics as did curves obtained with air alone. No evidence of autoignition was observed. For these reasons it was concluded that this fuel-air mixture underwent no precombustion reactions, and the heat effects associated with the compression and expansion of this charge were used to estimate heat leak over the entire range of compression ratios employed in this research. The pressure-time records for all test fuels used in this study except diisobutylene showed evidence of a pressure rise during compression of the charge in excess of that obtained with the leaded triptane. This pressure rise was caused by an increase in the number of molecules of the charge, or by a liberation of heat and a consequent rise in temperature, or by both. Considerations outlined earlier in this paper showed that the first of these was likely to account for only a small fraction of the observed pressure increase, and, in the interpretation of the data obtained in this research, it was assumed that the total pressure rise during the period before combustion was a result of heat liberated by one or more chemical reactions. STARTOF THE REACTION. Any chemical reaction accompanied by a relatively small heat effect would result in a correspondingly small pressure increase, and if such a pressure increment were not detectable with the apparatus used in this research, that reaction would escape detection. The exact point a t which the reaction appeared to start was related both t o the oxtent of the reaction a t
that time and to the sensitivity of the measuring equipment. For this reason it is possible that the actual start of the reaction occurred sometime prior t o the first indication obtained from the pressure record. A
400
a%" 103
0 t24 CRANKSHAFT DEGREES
0 t24 CRANKSHAFT DEGREES
c
I
Figure
D
:
I
0 t24 CRANKSHAFT DEGREES
I
I
1
0 *24 CRANKSHAFT DEGREES
6. Pressure Developed by Compression, Showing Effect of Fuel Composition and Compression Ratio A. Fuels: iso-octane-n-heptane mixtures Intake mixture: 300° F. 8. Fuel: n-heptane Intake mixture: 300' F.
C.
Fuels: iso-octane-n-heptane mixtures Intake air: 150' F.
D. Fuel: n-heptane Intake air: 150' F,
A, 8, C, D. Dotted lines) compression pressure for triptane plus 6.0 ml. tetraethyllead per gallon, ignition set at +PO degrees.
It can be deduced from the pressure-time curves that an increase in fuel octane number with the engine operating at the critical compression ratio for incipient autoignition for each fuel generally caused the observed start of the precombustion reaction to occur a t a n earlier time in the engine cycle (Figure G ) . This can be understood in the light of the following discussion: The end of the induction period (and thus the start of autoignition) must be thought of in terms of the maximum permissible temperature drop which can occur following peak pressure and still allow autoignition to take place. For the higher compression ratios a given time interval after top dead center corresponds to a greater temperature decrease than is the case with a lower compression ratio. If it is assumed that only a certain temperature drop can be tolerated in the case of both high and low compression ratios, then it is clear why autoignition must occur earlier with the higher compression ratio, and, therefore, also with the higher octane number fuel, With reference to the length of the induction period, other workers (1, 7, 8) have shown that fuels with higher octane numbers have longer induction periods. Because of the varying compression ratios used in this work, it cannot be stated with assurance that the same relation applies here, but there is some evidence to substantiate this idea ( 7 ) . If this is taken for granted, then the fact that higher octane fuels would end their induction period sooner and have them last longer clearly implies that the reaction must start earlier in the cycle. This was the situation which was observed experimentally, Addition of tetraethyllead to the hydrocarbons tested had the same qualitative effect on the time a t which the precombustion reactions started as that obtained by changing the composition' of the unleaded hydrocarbons to obtain fuels of higher octane 'number (Figures GB and 6D).
2848
INDUSTRIAL AND ENGINEERING CHEMISTRY
From a comparison of the data obtained a t the two inlet temperature conditions on the start of the precombustion reaction, it is apparent that the reaction started earlier in the cycle and a t a lower pressure when the inlet mixture was hotter The cycle-to-cycle variation in the time of starting of the precombustion reactions was less than & I crankshaft degree, but the variation in the start of autoignition was much laiger than this. At the limiting condition for incipient autoignition, the pressure a t the start of autoignition was always higher than the pressure a t the start of the precombustion reaction.
Vol. 43, No. 12
the precombustion reactions decreased with increase in the fuel octane number a t both inlet temperature conditions. Constant octane-number blends of diisobutylene-iso-octane-n-heptane also showed a decrease in the heat generated with increase in the concentration of diisobutylene. Pure diisobutylene showed no evolution of heat until autoignition occurred.
e
A l l FUELS
+ 0
1501 F: INLET AIR 300'6 INLET MIXTURE 20 40 60 BO RESEARCH OCTANE NUMBER
100
Figure 8. Total Heat Generated b Precombustion Reactions for All Fuels Sbsted
In the case of wheptane blended with tetraethyllead, the heat generated during the precombustion period was greater than foy a, blend of n-heptane-iso-octane selected to cause incipient autoignition a t approximately the same compression ratio as that used for the leaded heptane. In fact the heat generated in this particular instance was greater than that measured with any nodraded fuel used in this research, regardless of compression ratio. For nonadditive fuels, a change in inlet temperaturc rondition from 150" to 300" F. (accompanied by decrease in compression rat,io for incipient autoignition) resulted in no change in the heat of the precombustion reaction. Fuels with additives were affected by this change. In a comparison of leaded anti nonlcaded fuels, the leaded fuels showed a more marked change in dope of the heat-time curves during the course of the precombustion reaction, such that the rate of heat, liberation fell off with time. If it is assumed that tetraethyllead must be decomposed before it can act as an antiknock, then it can be expected that a certain temperaturc must be
HYDROCARBON BLENDS,
3 0 P E INLET MIXTURE
20 40 60 80 PER CENT ISOOCTANE OR OIB IN BLEND
100
Figure 7. Total Heat Generated by Precombustion Reactions for Hydrocarbon Blends with No Additives
HEATOF THE PRECOAIBUSTIOK REACTIOX.The following deductions can be made from the tabular data given in Table I1 and the heat-time curves s h o m in Figure 5 , obtained as described previously. For all test fuels except diisobutj lene an appreciable amount of heat WQS liberated prior to autoignition. (The stait of autoignition was arbitrarily defined as the time in the engi?e cycle a t which the preesure-time curve changed slope markedly just prior to autoignition. All the values for the heat of the precbonihustion reaction mentioned here correspond to this time.) 4 leaded heptane-air mixture a t Table II. Summary of Data the 150" F. intake condition Start of Start of (Figure 5.4) released 188 kg.coniIntake -Precombustion ReactionAutoignition b Total IIeatb Fuel pression Temp., Pressnrc, Time, Pressure, Time. Developed by cal. per mole, which is apF, lb./sq. in. abs. degrees lb./sq. in, dbs. degrees Precombustion, Ko.0 Ratio Kg.-Cal./Blole Droximately 27% of the heat' of combustion a t the expeii1 11 30 150 222 -15 222 +13 38 59 - 20 150 178 192 16 2 10 20 60 - 20 224 12 160 196 mental mixture ratio. Other 3 10 90 63 167 -23 214 12 150 4 10 60 fuels showed less reactivltg, 84 . ( 145 15 150 140 5 6 76 86 - 33 113 17 74 5 45 300 6 the heats liberated averaging 59 - 30 153 4- 14 112 300 6 7 76 28 - 20 325 + 5 234 lZ0 10 t o 15% of the normal heat 7 14 50 28 - 37 208 + A 99 300 10 45 7 188 - 18 192 +11 134 150 of combustion in most cases. 8 7 35 141 - 37 150 72 +I1 300 8 6 25 91 - 16 140 +15 123 150 For n-heptane-iso-octane 9 6 62 66 -32 220 10 134 160 8 96 blends the heat generated by 107 39 156 14 74 300 8 59 86 126 - 28 1E6 +l5 300 precombustion reactions inlo 10 l o 11 215 tll 147 -25 145 150 12 9 80 83 creased with increase in fuel 83 - 32 126 16 300 6 55 13 117 -32 106 17 73 14 6 36 300 octane number at the 150" F. 116 -6 120 +1Q 150 115 15 5 62 117 condition and decreased a t the 15 4 36 300 66 - 24 87 +I7 Identification llsted In Table I 300" F. condition (Figure 7 ) . b Values correspond to tlmes in engine cycle at which pressure-time culvee change slope lnalkedly just prior to For diisobutylene-n-heptane autolg~ltlon. blends the heat generated bg 0
++ + ++
++ ++
Q
December 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
reached in the engine cycle before the effectiveness of tetraethyllead can be observed. The point on the heat-time curve a t which the slope changed can then be interpreted as the point a t which the decomposition products of tetraethyllead became effective. These results are in conformity with data obtained using a mechanical method of extracting samples of unburned fuel from the combustion chamber of an engine. This work, currently under way in these laboratories, has shown that under engine operating conditions less severe than the 150" F. inlet condition, the tetraethyllead remains undecomposed up to a point slightly later than the time indicated by the change of slope a t the 150 O F. condition. In spite of the relation obtained between heat generated in the precombustion reaction and the octane number of certain specified blends, such as n-heptane-iso-octane mixtures, there is no over-all relation between the magnitude of this heat and the octane number of the fuel (Figure 8). I t has been implied by others ( 4 ) that the magnitude of the precombustion reaction is linearly related to octane number for several fuels. If such were the case, and if the heat generated in the precombustion reaction were the sole criterion for knocking, then the amounts of heat generated during the precombustion reaction for these fuels would be identical in tests made using the present technique of adjusting compression ratio for incipient autoignition. The present data clearly do not substantiate this idea. With reference to the absence of any detectable precombustion reaction in the case of diisobutylene, it may be conjectured that this behavior is the reason for the poor tetraethyllead susceptibility of this fuel. The basis for this thought is that the decomposition products of tetraethyllead might be particularly effective on the intermediate products formed by the precombustion reaction, and the absence of those products in the case of diisobutylene would prevent the tetraethyllead from affecting the course of the reaction.
2849
ACKNOWLEDGMENT
We acknowledge with thanks the assistance of J. D . RiIcCulIough, who helped with instrumentation. We are also indebted to D. H. Ewen and Irene Larson for their help with the calculations. NOMENCLATURE
Lower case letters (denoting extensive quantities) refer to the amount of the air-fuel mixture charged to the engine during one cycle, while the capital letters refer to 1mole of the mixture
Symbols p = amount of heat absorbed by mixture w = work done by mixture e = internal energy of mixture cy = heat capacity a t constant volume of mixture v = volume occupied by mixture p = absolute pressure in cylinder n = number of moles of mixture T = absolute temperature of mixture R = universal gas constant per mole of mixture LITERATURE CITED
(1) Barusoh, M. R., and Payne, J. A., paper presented before the
(2) (3) (4) (5) (6) (7)
(8)
Division of Petroleum Chemistry at the 118th Meeting of AMERICAN CHEVICAL SOCIETY, Chicago, 111. Draper, C. F., and Li,Y . T., J . Aeronaut. Sn'., 16, 593 (1949). Levedahl, W. J., and Sargent, C. W., Jr., B.S. thesis, Maseaohuaetts Institute of Technology, 1948. Pastell, D. L., S.A.E. Quart. Trans., 4, 571 (1950). Souders, Matthews, and Hurd, IND. ENG.CHEM.,41,1037 (1949). Taylor, C. F., and Taylor, E. S., J . Aeronaut. Sei., I , 135 (1934). Taylor, C. F., Taylor, E. S., Livengood, J. C., Russell, W. A., and Leary, W. A., S.A.E. Quart. Trans., 4, 232 (1950). Tizard and Pye, Phil. Mag., Ser. 6 , 44, 79 (1922); Ser. 7, 1, 1094 (1926).
RECEIVEDMay 14, 1951
DIESEL ENGINE EXHAUST PRODUCTS ERNEST W. LANDEN Caterpillar Tractor Co., Peoria,
This work was done to determine whether the exhaust condensation method is suitable for evaluating the products exhausted from Diesel engines when various fuels are burned or various types of engines are used for a given fuel. Exhaust gases from Diesel engines have been condensed in a condensing system. Results from nine fuels in one engine show that some organic liquids are exhausted at all engine loads. Three different combustion systems show variations in the exhausted material. In general, the organic liquids, both water-soluble and insoluble, showed a slight maximum at the intermediate loads. Organic acids decreased as the load increased. The. direct injection engine produced more solid carbon and the Lanova system produced more liquids as compared to the precombustion chamber engine. Comparing the organic acids exhausted to the total acids exhausted showed that the doublechamber engines were similar. Differences in fuels can be measured by this method. It may be possible to evaluate engine deposit-forming tendencies and the exhausted material of engine combustion systems by this technique.
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RODUCTS exhausted from an internal-combustion engine give some clue as to what takes place in the combustion cycle. I n fact, visual obsel-vation indicates t o some degree whether oily materials, indicated by gray-colored exhaust, or carbonaceous materials, indicated by black exhaust, are exhausted. Even colorless exhaust can at times be offensive. Exhaust spotters, or perhaps sniffers, have been busy on the West Coast tracking down offenders, especially the Diesel truck operators (8). They have also been busy in large cities where exhaust odors are Rometimes a serious problem. Other problems also occur from exhausted materials such as port plugging or build-up of material on the valve stems. Exhaust systems can be deteriorated by the acids formed by combustion. I n order to evaluate the materials exhausted from engines, condensing equipment was used to collect the liquids and solids for further analysis.
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EXPERIMENTAL EQUIPMENT
Two experimental techniques were followed. I n the fimt a precombustion chamber-type engine was used and a variety of fuels was supplied to the engine in order to note differences when fuels are burned in a single engine. In the second, three
