GASOLINE COMBUSTION Effect o f Boron or Silicon Compounds E. C. HUGHES, S. M. D A R L I N G , J. D. BARTLESON,
AND
A. R. KLINGEL, JR.
The Standard O i l Co. (Ohio), Cleveland, Ohio
Continued operation of internal combustion engines results in the formation of a thin deposit of carbonaceous material and inorganic lead compounds over most of the exposed surfaces of the combustion chamber. These deposits increase to an equilibrium thickness and cause the octane requirement of the engine to increase 10 to 15 units. Full scale engine studies showed that small amounts of borates, borines, ethyl sulfate, and silicone added to leaded fuels largely repressed this octane requirement increase. These observations are correlated with laboratory studies which showed a difference in the catalytic effect between the oxides of boron and silicon and lead sulfate, and the components of the combustion zone deposits on the oxidation of n-heptane. The boron compounds and silicone do not provide a complete answer to the problem of engine deposit harm, as they were found to be ineffectivein engines already containing deposits. However, the ability of these compounds to affect the nature of these deposits offers encouragement in solving the problem in the future and lends support to a catalytic rather than thermal .conductivity theory as an explanation of the cause of octane requirement increase.
(3, 19). Also, Marek (17) reported that boric acid reduced the rate of combustion of carbon. No studies have been reported on the effect of boron in engine deposits. LABORATORY STUDIES
The techniques of Pope, Dykstra, and Edgar (21) were used to test the activity of a number of solids on the oxidation of nheptane. Glass tubing with an outside diameter of 30 mm. and an effective length of 30 cm. was used to carry the solids which were impregnated over 3/8-inch porcelain rings. The volume per cent of oxygen that reacted over these solids and the ratio of carbon monoxide to carbon dioxide found in the offgases produced were taken as the criteria for the activity or poisoning effect of these surfaces. The results are shown in Table I. The unpacked tube showed a rather constant oxidation rate over the temperature range of 316" to 427" C. Above these temperatures the rate of oxidation rapidly increased so that a t 482' C. nearly all of the oxygen in the stoichiometric mixture was consumed. There are two classes of surfaces shown. One class contains porcelain rings, iron oxide, lead oxide, lead borate (PbO-B,Oa), and lead bromide. These promoted oxidation in the temperature range of 371' to 427' C. Another group consists of lead sulfate, lead chloride, and the oxides of boron and silicon. These surfaces had little effect, or poisoned the oxidation of n-heptane a t all temperatures, The open-tube reaction and the compounds of the second group (except lead chloride) showed a high ratio of carbon monoxide to carbon dioxide in the offgases. Although the carbon monoxide and carbon dioxide ratio had not been determined for silica, experience with catalytic cracking points to a high ratio. The compounds of the first group greatly reduced this ratio. Thus, significant catalytic differences were observed between boric oxide, silicon dioxide, or lead sulfate, and many of the solids which might be found in engines.
CONTINUED operation of gasoline internal-combustion engines leads to the deposition of a solid material over the exposed surfares of the combustion zone, except those scraped by the pistons. This material is of a carbonaceous nature with nonleaded fuels, but with fuels containing tetraethyllead it contains also a subqtantial proportion (15 to 80%) of inorganic lead compounds ( 2 , 10, 20, E?). Normally these layers reach an equilibrium thickness as a result of flaking off of the deposit. These deposita are not thick enough ordinarily to cause operational difficulty, but they increase the octane requirement of the ENGINE STUDIES engine substantially (9, 10, $4). It has been commonly believed In order to determine the influence of these selected compounds that this increaEe in octane requirement was due to an insulating on the catalytic effect of engine deposits, some full scale teste effect of the deposits, although some have indicated that the dewere made. These tests were conducted in two types of engines. posits may be causing an undesirable catalytic effect on combusOne type was the modified F-4 single cylinder aviation fuel knocktion (4, IS). Some unpublished studies in this laboratory indirating engine ( 1 ) equipped with an L-head-type cylinder. This cated that deposit forming in a modified CFR F-4 knock-test engine was operated under the conditions shown in Table 11. engine was noninsulatinp;. Nevertheless the octane requirement of the engine was increased normally by the deposits and could be reduced by their removal. This observation could Table 1. Rate of Oxidation of n-Heptane best be explained by the cataChange in per cent of oxygen consumption from open tube lytic theory. Experiment No. 1 2 3 4 5 6 7 8 9 in A laboratory investigation, Catalytic Open Porcelain PbO PbBrt Pb501 PbCh Fez03 Ba03 PbO-BaOs Si02 surfacea Tube carried out to uncover coniTemp., C. pounds that would poison such - 14 - 16 260 35 -9 -23 -3 +9 -14 -14 +16 -1: n -15 316 45 -27 catalytic surfaces, showed that -6 I K A LQ 371 ' -8 427 37 +25 boron and silicon compounds 482 97 -21 had such .an effect. A literaAverage carbon monoxideture survey revealed that, boric carbon dioxide 6.65 1.05 0.47 5.54 1.22 acid modifies clay and silica ratiob 5.61 1.41 0.61 surfaces so that the oxidation a All catalytic surfaces supported on a/s-inoh porcelain rings with the exception of PbBrz which w w supported on steel. of hydrocarbons can be conb This ratio was relatively constant over the range of 20 to 70% pxygen consumption. trolled to yield formaldehyde
A
$7;
2841
-3"
INDUSTRIAL AND ENGINEERING CHEMISTRY
2842 Table
a
the engine. This indicated that the boron was part of thc dqmsit and was not actink in the vapor phase.
II. Single Cylinder Engine Test Conditions
Speed, r.p.m. Air-fuel ratio Manifold pressure, inches iilerciiry absolute Air inlet temperature, C. Jacket temperature, C. Load, B I I E P a Oil, Commercial Grade SSE Brake mean effective pressure.
Vol. 43, No. 12
1800 13..5,/1 20 66 100 27 30
Engine knock n-as observed with a Lane-Wells magnetostriction external pickup and an oscilloscope. The other engine was a standard 1942 passenger car Chevrolet engine (6). The operating cycle provided mild operating conditions compared to the F-4 test. The conditions used are shown in Table 111. Table
111.
Chevrolet Engine Test Conditions
Time, minutes Speed, r.p.m. Load, BHPa Air-fuel ratio Water outlet temperature, C. Oil sump temperature, C. (1
Idle 1 500 0
...
I
.
.
Cruise 5 2000 11.2 13,5-14.5 74
HOJRS
Figure 1,
OF CPERATION
Effect of a Boron Additive on Octane Requirement of a Chevrolet Engine
91
,..
Brake horsepower.
The octanerequirement oftheengine wasobservedat,1000r.p.m., full throttle, and 11" before-top-center spark advance. Blends of n-heptane and iso-octane were used as the comparison fuels. The gasolines were commercial products containing some catmalytically cracked gasoline and 3 ml. of tetraethyllead per gnllon, carried in Ethyl Motor Mix KO.62. This mix contains 0.5 mole of ethylene bromide and 1.0 inole of ethylene chloride per mole of tet,raethyllead. The boron compounds that were prepared and tested rrere triethyl borate (26), tris-diisopropylcarbinyl borate (%), triisobutyl borat,e (a purchased stock), n- and tert-butylborines (6, 12, 2 4 ) , and decaborane (26). The silicon conipound chosen was Silicone 500 obtained from Dow Corning. It had a boiling point of 192" C. and a viscosity of 1.5 centistokes at 25" C. The lead sulfate was prepared in situ by feeding ethyl sulfite into the gasoline. Eone of these compounds changed the knock-rating of the leaded gasolines. Kormal butylborine reduced the knock-rating of a nonleaded fuel by about 2.5 octane numbers.
A summary of 12 teste that ww carried out with the various boron compounds in the t m type!: of engines is shown in Tatilo IV. Control tests made before each additive test indicated a n increase of 10 2 units in the octaiw wquirement of the engines. The additive tests showed t,hat tmrori from five of the six (:ompounds was effective in repressing :ilarge part of the effect of the deposit in causing detonation. The only exception was the high boiling tris-diisopropylcarbinyl I ~ o r a t r (experiment 13) rhicli was ineffective. Bot8hethyl boriit,e and tmheborines allowed not over 4 octane numbers increase in the octane requirement of t h e engines in experiments 11, 15, 16, 21, and 22. Experiments 12, 128, 20, and 20A showed that' it concentration of 0.2 nil. of R boron compound was not sufficient t o give reliable cont,rol of octane requirement increase; thrrefore 3.0 ml. n-ere used in later t,ests. Butyl borine in the absence of tet,raethyllead was not effective in repressing the increased octane requirement but was effcctive
BORON COMPOUNDS
The course of a typical pair experiments is shonn in Figure 1. The control test containing no additive showed a rapid rise in the octane number required for knock-free operation. The rise in octane requirement leveled off after 80 hours of operation a t 12 units above that of the cleau engine. With the bor?n additive, in this case, the octane requirement leveled off at only 4 units above that of the clean engine. At the end of 100 hours the additive \tap discontinued, and the octane requirement gradually increased 9 more units. Since the uctane requirement increase of 9 units was not immediate. but required 60 hours to build up, this gradual increase showed that the effect of the boron conipound lingered after it was no longer being fed into
Table
IV. Effect of Boron Additives on Engine Octane Requirement
01
Experiment NO.
ripe of Engine"
Esters 11 126 12Ad 13
C F R E"-4 Chevrolet Chevrolet Chevrolet
14
C F R F-4
Borinea 15 165 17f
Additive Ethyl borate Isobutyl borate Isobutyl borate Tris-diisopropylcarbinyl borate Tris-diisopropyloarhinyl borate
19 20 208 21
Chevrolet C F R F-4 C F R F-4 C F R F-4 Chevrolet C F R F-4 C F R F-4 C F R F-4
n-Butylhorine n-Butylborine n-Butvlhorine n-Butilhorine n-Butylhorine fert-Butylhorine tert-Butylhorine Deoaborane
22e
C F R F- 4
Decaborane
188
Octane Requirement of Engine -__With AdditGe . Increase a t equia t equiInitial lihriumb Initial lihriumi.
u t Additive _W t-h o~ _Increase __ Concentration, X I . per Gallon 3.0 0.2 0.2 ==. to B content of Expt. 11 e t o B content of Expt. 11
58 69 69 78
10 8 8 12
56 75 73 72
10
58
10
61
>8
3.0 3.0 3.0
78 91 66 67 74 62 58
12 10 12 14 10
78 89 69 68 86h 60 57
59
9
57
4
91
10
90
3
3.0 3 .O 0.2
0.2 == in B content to 3 mi. horine per galIon = in B content to 3 nil. borine per gallon
18 9
2
1
II
4 4
16
11 0 0
1;
Each test started in a clean engine with exception of second half of expt. 19. b Equilibrium established by operating engine for 60 to 100 hours until octane requirements were constant over several %hour periods. C First 67 hours of additive test made a t 12.5 air-fuel ratio. d First 56 hours of additive test made under lean oruise conditions. e Engine operated a t 20-inch manifold pressure, b u t octane requirements taken a t 30 inches. f All commercial gasolines used in this work contained 3 ml. of tetraethyllead per gallon with exception of expt. 17 xhere none was used. 0 Oil Consumption raised f r o m a normal of 12 grams per hour t o 3qgrazns per l i u u r b y removing one oil ring. h Engine was not cleaned after reaching equilibrium in blank run. The engine operation continued for 48 horirs.
December 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
when both were used simultaneously, as seen in experiment 17 of Table IV. This suggests that the catalyst poison may be a compound of lead and boron. Analysis of the piston-top deposit of experiment 11, shown in Table V, indicates that while all of the boron could have been easily present as a lead compound, most of the lead was not attached to boron. Such observations may be explained by a poisoning theory.
Table
V. Analysis of Piston-Top Deposits
Lead Carbon Boron Atom ratio, boron/lead
Weight % 53.9 10.8 0.11 0.039
The results of experiment 19 showed that an active deposit could not be poisoned very rapidly. The engine of this experiment was operated until it contained an equilibrium amount of deposits of a catalytic nature, and gasoline containing a boron additive was then used to operate the engine for 48 hours more. KO reduction in octane requirement occurred, although in the experiments shown in Figure 1 it was possible to change from an inactive deposit to an active one 20 hours after the boron in the feed was discontinued. This indicated that exchange of deposits in an engine is quite slow and the remaining active deposits can hold up the octane requirement even though coated with inactive material on their surfaces. The boron compounds were nearly as effective in restraining the rise of octane requirement a t higher manifold pressures as they were a t the lower in the F-4 engine, shown in experiments 16 and 22 in Table IV. The effectiveness of the boron additive was masked by increased oil consumption. Greater oil consumption was brought about by removing one of the oil rings while assembling the engine in experiment 18 of Table IV, +nd an octane requirement increase of 11 units was observed with the additive. While the results of the engine tests can be interpreted with the theory of a poison compound of lead-boron oxide, this poisoning action would hardly be predicted from the oxidation work summarized in Table I. Here, lead borate had more of the characteristics of lead oxide than of boric oxide. However, it is known that lead and boron oxides form a wide variety of compounds other than those tested in Table I (7,18). Further research will be necessary to idehtify the exact nature of the poison produced in the engines in this work. ’
SILICON C O M P O U N D S
The volatility of boron oxide with steam provides a mechanism by which excesses of boron in deposits can be avoided. Silica-on the other hand has no chemical mechanism for its removal. I n trial engine tests with silicone, deposit difficultieswere experienced. Thus low concentrations might be necessary for actual use. The peculiarities of the thermal expansion-temperature curve of 4Pb0-Si02 (8) led to the belief that a t this concentration, such deposits might flake off more readily than conventional deposits. Such a composition prepared in the laboratory was found to be comparatively friable. The Silicone 500 (11) was tested in leaded gasoline in a Chevrolet engine in five lead-silicon ratios between 1 to 1 and 36 to 1. The ratio which was most effective was the 4 to 1 ratio which almost completely repressed the octane requirement increase for 60 hours, This is shown in Figure 2. The valves gradually fouled, however, and by 80 to 100 hours the octane requirement increased to the level of the nonadditive fuel. Analysis of the deposits in the engine showed an atom ratio of lead to silicon of 3.3 to I, but x-ray examination of the deposits failed to reveal
2843
lines of lead tetrasilicate. Nevertheless, Silicone 500 did succeed in preventing octane requirement increase which could be predicted for silica alone from the work shown in Table I. L E A D SULFATE
Lead sulfate (16), like the oxides of boron and silicon, has a different catalytic effect on oxidation of n-heptane than have compounds such as iron oxide, lead oxide, and porcelain. Since ethyl sulfite was found to have no adverse effect on the antiknock of tetraethyllead, it was added t o the gasoline in a concentration of one atom of sulfur per atom of lead. Pure tetraethyllead was used t o furnish the lead. During a period of 72 hours in the Chevrolet test there was no increase in the octane number requirement of the engine. However, mechanical failure caused by
15-
a‘
0 WHRS 0‘ 80 HRS (VALVES TOULEDI (
e
0
0 / 18
f Figure
,
,
,
24
,
I
32
40
RATIO
9.
Octane Requirement Versus Lead-Silicon Ratio
Increase
Chevrolet engine
heavy deposits on the tulip and stem of the exhaust valves made it necessary to stop the experiment. A comparable test without ethyl sulfite showed an increase of 8 octane number requirements. These observations lend much support to the catalytic-as against the thermal conductivity-theory of increase in octane requirement in engines, since much depofiit was present with the additive, and no change was observed in octane requirement during the time ethyl sulfite was in the gasoline. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance given them by M. H. Campbell, codiscoverer of some of the effects discussed in this paper, by E. Z. Awroski, R. W. Rurhans, E. B. McConnell, Jr., members of the engine testing group and the analytical group in the experimental part of this work, and by E. R. Kosman in the preparation of the manuscript. The authors also wish to thank The Standard Oil Co. (Ohio) for the release of the information LITERATURE CITED
(1) American Society for Testing Materials, Designation D 614-47T. (2) Bassett, H. N., Automobile Engr., 28, 31 (1938). (3) Berkman S., Morrell, J. C., and Egloff, G., “Catalysis,” p. 781, New York, Reinhold Publishing Gorp., 1940. (4) Boyd, Charles, Jr., Natl. Petroleum News, 41, 17 (Aug. 3, 1949). (5) Brown, H. C., J . Am. Chem. Soc., 67, 374 (1945). (6) Coordinating Research Council, Designation L-4-545. (7) Gellei., R. F., and Bunting, E. N., J . Research Natl. Bur. Stundurds, 23, 275-83 (August 1939). (8) Geller, R. F., Creamer, A. S.,and Bunting, E. N., Ibid., 13, 2 3 7 4 4 (August 1934). (9) Gibson, H., Quart. Trans., 3, No.4, 557 (1949). (10) Gruse, W. A., and Livingstone, 0. J., paper presented before the Symposium on Lubricants, Chicago Regional Meeting of the American Society for Testing Materials, March 3, 1937. (11) Hughes, E. C., and Campbell, M. H. (to Standard Oil Co. of Ohio), U. S. Patent 2,529,496 (Nov. 14, 1950).
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.
