I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
1336
controls to indicate an advantage in the use of the synthetic emulsifiers investigated in this study in preparing GR-S polymers. The results of investigations carried out in the rubber industry concurrently with those described in this paper suggest the following applications of the more satisfactory emulsifiers for producing and compounding latices.
Vol. 45, No. 6
LITERATURE CITED
(1) Carr, C.
W.,Kolthoff, I. M., Meehan, E. J., and Williams,
(2)
D. E., J . Polymer Sci., 5 , 201-6 (1950). Danforth, J. D. (to Universal Oil Products Co.’, U. S. Patent
(3)
Feldon, A I . , AlcKennon, F. L., and Lawrence, R.Y,, IKD. EXG.
2,466,212 (April 5, 1949).
CHCX.,
44, 1662-4 (1962).
Harkins, W.D., J. Polymer Sci., 5, 217-51 (1950). ( 5 ) XcKennon, F. L., and Lawrence, R. V. (to United States of rimerica represented by Secretary of Agriculture), U. S. Patent 2,465,901 (March 3 9 , 1949). (6) Schoeller, C., and Wittwer, 51. (to I. G. Farbenindustrie X.,G.), (4)
The excellent chemical, physical, and light stability afforded by the nonionic emulsifiers recommends their use in coating latices. A number of these emulsifiers are now being sold commercially for such purposes. Of particular interest in the paper coating and paint application is the indication that nonionic emulsifiers produce a butadiene-styrene latex of relatively large particle size. Such particle size control might contribute greatly to obtaining better fluidity in a high-solids latex. These agents can also be used to advantage for the inhibition of excessive foaming, freeze-thaw coagulation, instability to mechanical agitation, and instability to addition of compounding chemicals. Such application can also be made to latex in compounded form such as emulsion paints. The method developed for the selection of “tailor-made” emulsifiers for use in butadiene-styrene copolymerizations can be extended and adapted to other emulsion polymerization and bead polymerization systems, including copolymers based on acrylonitrile or vinyl chloride. The broad range of properties available in the individual nonionic, anionic, and cationic emulsifiers and in mixtures of these materials should assist the polymerization research chemist greatly in developing new emulsion copolymerization products and improving those already in use.
IEid., 1,970,578 (Aug. 21, 1934).
(7)
Schulse, W ,A., Tucker, C. >I., and Crouch, W. W.,Ixn. ENG.
(8) (9)
Staudinger, J. J. P., Chemistra and Industry, 1948, 563-8. Steindorff, A . , Balle, G., Horst, K., and Michel, R. (to Gcneral Aniline and Film Corp.), E. 6. Patent 2,213,477 (Sept. 3,
(10) (1 1 )
Weidlein, E. R., Jr., Ckem. Bng. Yews, 24, 771-4 (1946). Wolthan, H., and Becker, W. (to Jasco, Inc.), U. S. Patent
(12)
Znicker, B. PI.G. (to B. F. Goodrich Co.), Ibid., (Oct. 16, 1946).
CHEM.,41, 1699-1603 (1949).
1940). 2,222,967 (Kov. 26, 1 9 4 1 ) . 2,386,764
RECEIVED for review Kovember 4 , 1959. ACCEPTED March 16, 1953. Presented before the Diviaion of Rubber Chemistry, AMERICAE; CmmIcAL SOCIETY, a t Buffalo, N. Y . , October 29,1952. The laboratory and pilot plant work reported herein was carried out under the sponsorship of the Office of Synthetic Rubber, Reconstruction Finance Corp., in connection with the government synthetic rubber program.
Mechanism. of Combustion Chamber
Deposit Formation with Leaded Fuels W. E. NEWBY AND L. F. DUMOIVT Jackson Laboratory and Petroleum Laboratory, Organic Chemicals Department, E . I . d u Pont de Nemours & Co., Inc., Wilmington, Del.
T
HE formation of deposits on the combustion chamber walls
during normal operation of Otto-cycle engines increases the tendency of fuels to knock. Some of the factors responsible for this effect have been described ( 3 , 4,6), but very little information is available about the reaction mechanisms which result in the accumulation of these deposits from fuels containing tetraethyllead. Because such information would give a better understanding of ways to prevent or reduce deposit formation, a study of these reactions x a s undertaken a t the D u Pont laboratories.
experiments conducted in a single-cylinder engine used as a “reaction vessel” to confirm laboratory predictions. The lead salts in deposit8 have been found by x-ray diffraction techniques to consist of simple and complex compounds of lead chloride, lead bromide, lead sulfate, and lead oxide. Those identified a6 deposit constituents are listed in Table I. The relatively volatile lead halides are present because organic halogen compounds are used with tetraethyllead to promote elimination of lead compounds from hot engine surfaces. Lead sulfate is found, as all commercial gasolines contain some organic sulfur compounds.
SOURCES OF DEPOSITS
Practically all the gasolines used today contain tetraethyllead t o improve their octane number or their resistance to knock. Although almost all of the tetraethyllead decomposition products are removed from the combustion chamber with the exhaust gases, a small amount remains on the combustion chamber walls in the form of inorganic lead salts. In addition to lead salts, deposits contain carbonaceous materials resulting from incomplete combustion of the fuel and lubricating oil and minor amounts of metallic and nonmetallic compounds introduced into the combustion chamber with the air, fuel, and oil, and as a result of engine wear. Any investigation aimed a t defining the mechanisms leading to the formation of such a complex structure must involve a systematic isolation and study of each of the influencing factors. In this particular work the studies were limited to investigating the reactions involved in the formation of the lead salt portion of deposits. These studies consisted of theoretical thermodynamic calculations, laboratory bench test experiments, and
CHEMICAL REACTIONS OF DEPOSIT FORMATION
The !Tide variety of lead salts in deposits is formed as a result of chemical reactions which occur in the combustion chamber after the fuel-air charge is burned. The stages in vhich reactions may influence deposit composition are: 1 . Combustion of the fuel and additives. 2 . Vapor state reactions of the gases present from combustion. 3. Condensation of the gaseous lead compounds on the chamber m-all. 4. Gas-solid reactions between solid lead salts and the gases present in the chamber. 5 . Solid state reactions between lead salts in the deposit. 6. Vaporization of the volatile lead salts.
Each of these stages was studied and the reaction sequence shown in simplified form in Figure 1 was developed after their relative importance had been established. The reactions which occur in each stage can be summarized as follows:
June 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE I. LEADCOMPOUNDS FORMED IN ENGINE DEPOSITS Compound PbClz PbBn PbSOp PbO PbBrdz(PbC1z) Y bO.PbClz 2PbO.PbClz PbO.PbBre 2PbO.PbBrz PbO.PbSOr 2PbO.PbSOn 4PbO.PbSOa
h
%
-
Melting Point, O C.
Reference
496 370 1170 888 370 to 496 524 693 497 (decomp.) 709 975 961 895
COMBUSTION of fuel containing tetraethyllead, halogen scavenging agents, and sulfur compounds produces gaseous lead oxide, hydrogen halides, and sulfur dioxide. VAPORSTATEREACTIONS of lead oxide appear to be insignificant except in the special and transitor condition when the combustion chamber walls are clean and regtively cool (ca. 200" C.). Under these special circumstances lead oxide in the gas layer adjacent to the wall can be almost completely converted to lead halides by vapor state reaction with hydrogen halide gases. CONDENSATION quenches vapor state reactions. Under most engine conditions lead oxide is unchanged by vapor state reactions; so it is the major component which condenses. However, very early in deposit accumulation with a clean combustion chamber the substance initially deposited from the gas film next to the wall may be mostly lead halides. GAS-SOLID REACTIONS which occur between solid lead oxide and the acid gases are probably the most im ortant in determining the final composition of deposits. &seous hydrogen halides can convert solid lead oxide to the corresponding lead halide a t any deposit surface temperature. Sulfur oxides become effective in converting solid lead oxide to lead sulfate only when the deposit surface temperature exceeds 300' C. TEL ( A S MOTOR MIX IN GASOLINE) COMBUSTION
GAS-SOLID
1 1 PbO ( H C I , H B r , S 0 2 , 0 2 , & 0 )
4
SOLID STATE
Figure 1. Chemical Reactions of Deposit Formation
SOLIDSTATEREACTIONS produce lead oxysulfates and lead oxyhalides from free lead oxide initially deposited and lead sulfate or lead halides produced from gas-solid reactions. A number of solid state replacement reactions can also occur between the lead oxy salts and simple lead salts. The lead oxide in lead oxy salts can also participate in gas-solid reactions. VAPORIZATION may remove the relatively volatile lead chloride and lead bromide from hot locations such as the exhaust valve head or thick deposit surfaces. The reactions that occur after the condensation step are usually the most significant in deposit formation. These reactions involve competition for the free solid lead oxide by gases such as sulfur oxides and hydrogen halides and by solid lead sulfate and lead halides. EXPERIMENTAL AND THEORETICAL RESULTS
DISTRIBUTION OF LEADSALTS IN DEPOSITS.Preliminary studies indicated that the chemical composition of deposits could change with deposit accumulation time after the fashion illustrated in Figure 2 (4). These data were developed in a laboratory single-cylinder engine operated continuously under mediumduty conditions with a commercial fuel containing 1.5 ml. of tetraethyllead per gallon and a commercial heavy-duty lubricant.
1337
(The tetraethyllead mixture used in automotive fuels is designated as Motor Mix and contains 1 mole of ethylene dichloride and 0.5 mole of ethylene dibromide per mole of tetraethyllead.) Seven tests were conducted, varying in duration from 30 to 30@ hours. All of the combustien chamber deposits were removed after each test and subjected to both x-ray diffraction analysis and chemical analyses. Although x-ray patterns give a more accurate picture of the molecular structure of deposits than d o chemical analyses, they are difficult t o interpret quantitatively. Therefore, the chemical analyses were used to give a better indication of trends in deposit composition.
r;
- 0
50
100
150
200
250
300
DEPOSIT ACCUMULATION-HOURS
Figure 2.
Change in Deposit Chemical Compositioii with Accumulation Time
As shown in Figure 2, the deposits initially formed on the clean metal surfaces were composed predominantly of lead halides. As more deposit was formed during longer periods of operation, the composition changed to decreasing amounts of lead halides and increasing quantities of lead oxide and lead sulfate. These data indicate that the rather volatile lead halides of low melting point are concentrated in the cool portions of a deposit near the metal surface. The relatively nonvolatile compounds of high melting point, lead oxide and lead sulfate, are found mainly in the hottest regions of a deposit near the surface exposed to the combustion space. The fact that deposits are usually stratified in this manner has been confirmed in other engine experiments by sectioning deposita into layers and determining the chemical composition of each layer as a function of its distance from the combustion chamber wall, as shown in Figure 3. Regardless of the mechanism by which this stratification occurs, the motivating cause appears to be a gradual increase in deposit surface temperature as the result of a deposit thermal insulating effect. This thermal insulating effect may become great enough to prevent the relatively volatile lead halides from forming on the surface of thick deposits (4). The reactions responsible for this gradual shift from lead salts of low melting point to those of high melting point occur in the six stages described previously. Each step in the reaction sequence is considered separately in the following sections. COMBUSTION.Lead oxide, hydrogen chloride, hydrogen bromide, sulfur oxides, water, oxygen, carbon dioxide, carbon monoxide, nitrogen, and minor amounts of other compounds are present in the hot gases from the combustion of leaded gasoline. The available evidence indicates that lead is present as gaseous lead oxide following combustion of leaded fuel. Many investigators agree that lead oxide is produced by decomposition of tetraethyllead even before combustion (2, 6, l a ) . When a sulfurfree fuel containing only tetraethyllead is burned, lead oxide is essentially the only lead salt found on the combustion chamber walls, presumably having been produced in that form in the
1338
INDUSTRIAL A N D ENGINEERING CHEMISTRY
vapor state. Basic lead carbonates have also been found in very cool locations under these acid-free conditions, but they are much too unstable to have existed in the vapor state. It is extremely unlikely that metallic lead, rather than lead oside, is the species present in the vapor state after combustion. Although metallic lead has been reported occasionally as a minor deposit constituent, especially on spark plugs, its presence has been attributed to reactions of solid lead salts under unusual conditions (16).
Vol. 45, No. 6
in the early stages of deposit accumulation. Furthermore, it is known from laboratory experiments that if solid lead sulfate had been formed it could not have been destroyed under engine operating conditions. Therefore, it appears that lead sulfate was not deposited from the vapor state when the gases near the combustion chamber wall were as cool as possible, before a significant insulating layer of deposit had been formed. Khen deposits are present, causing a higher gas temperature near the wall, the vapor phase equilibrium between lead oxide and lead sulfate favors lead oxide even more, since thermal dissociation of lead sulfate is more likely.
PbSO
20
0 a
r
t
.4
0 100
300
500
700
GAS TEMPERATURE-OC.
Figure 5. Vapor State Equilibria for Halogens during Exhaust Stroke Calculations based on combustion of unleaded gasoline containing 0.0154 mole of ethylene dihalide per gallon
Similarly, the species of lead compound in the gas phase changes rapidly with temperature, as shown in Figure 6. If vapor state equilibrium is reached a t 500' or 600" C., most of the lead will be present as lead chloride or lead bromide. Above 900' C. it will exist largely as lead oxide. The actual temperatures which are encountered in the gas space may exceed 2000' C. during combustion. During the exhaust stroke it has been estimated that the gas temperatures
400
600
800
1000
GAS TEMPERATURE-"C
Figure 6.
Vapor State Equilibria for Lead Salts during Exhaust Stroke Calculations based on data in Table I11
The calculations discussed thus far were for the exhaust stroke at a total pressure of I atmosphere. The equilibria were also calculated for the early part of the expansion stroke, assuming a pressure of 20 atmospheres. This twentyfold increase in pressure was equivalent to about a 100' C. temperature decrease for the reactions shown in Figures 5 and 6. I n view of the large changes in temperature of combustion chamber gases, the pressure effect may be considered minor, so the foregoing conclusions will apply to any part of the engine cycle between combustion and the end of the exhaust stroke. GAS FILMAND DEPOSITSURFACE TEMPERATURE. From the preceding section it is apparent that lead oxide would not be changed by vapor state reactions in the very hot combustion gases. The gas space which is of greatest interest in deposit formation] however, is not the bulk of the gas space in the chamber, but the gas layer which is next to the deposit surface, The temperatures in this gas layer must be considered, since any material that diffuses to the surface must cross the temperature gradient of the gas film. In the combustion chamber of an engine very large temperature gradients exist between the gases and the walls. The temperature gradient is greatest a t the wall, as a result of the poor thermal conductance of the relatively stagnant film of gas which forms the laminar gas layer. The resistance offered by the metal walls and coolant film to heat transfer is so small that if it were not for this laminar gas film the conversion of heat energy t o work would be practically impossible because of the tremendous heat loss to the engine coolant (16). For a given set of combustion conditions the temperature gradient through the laminar gas layer can be considered t o be controlled by the wall temperature or, if deposits are present,
1340
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
by the deposit surface temperature. For example, the gas film temperature over an exhaust valve opwating a t a metal temperature of 800" C. will be considerably higher than that above a piston top operating a t a metal temperature of about 200" C. By the same token, the gas film over a thick, highly insulating deposit is substantially hotter than that above a thin deposit.
f
30
DEPOSIT ACCUMULATION -HOURS
Figure 7. Change in Deposit Weight with 4ccunulation Time Same engine experiments as for data i n Figure 2
Vol. 45, No. 6
is based on the thermodynamics of the vapor state reactions. However, it is supported indirectly by engine data. Figure 7 shows the change in deposit weight with accumulation time for the series of experiments in which the deposit compositions given in Figure 2 were developed ( 4 ) . These experiments were teiminated before the usual limiting level of deposit growth v a s reached. The weight of deposits formed was proportional to the duration of time the engine was operated. In other words, despite the fact that the chemical structure of the deposits changed drastically between 30 and 300 hours, the rate a t which these deposits formed remained constant. As the proportion of each element entering the combustion chamber remained constant. the proportion of each lead salt if formed in the vapor state could also be expected to remain constant. As the deposit surface temperature became too high to permit lead halides to be laid down, the rate of deposit formation should have decreased in proportion to the relative amount of lead halides formed in the vapor state. The fact that the rate of deposit formation remains the same despite the changes in composition and surface temperature indicates that the lead salts found in deposits map not have been formed originally in the vapor state. These data alone would make one suspect that all the lead initially deposited was in a single form and that the form was a high melting lead compound. This was confirmed further in the course of an engine program using radioactive tetraethyllead as a tracer. The complete results of these studies will be published a t a later date. Tetraethyllead prepared from a radioactive lead isotope, radium D, was introduced with the fuel into an engine containing normal deposits. It mas found from autoradiographs that new deposits formed preferentially on thick, hence hot, deposit surfaces, indicating that a high melting lead compound v ~ a sinitially de-
I t is difficult to determine these temperatures, but their magnitude can be estimated indirectly. Generally, no simple lead halides are found on the surface of deposits which have reached "equilibrium" thickness. This is confirmed by the data given in Figures 2 and 3. In experiments in which the formation of lead halides was accentuated by using excess halogens in the fuel, the 7 deposit surface appeared t o have been molten although the severity of the engine operating conditions was moderate. These 0 ALL REACTIONS POSSIBLE facts indicate that the deposit temperature eventually exceeds GAS-SOLID REACTIONS ALONE the melting point of the lead halides. Frequently the surface of * 5 thick deposits contains no lead oxyhalides, an indication that I even their decomposition temperatures, shown in Table I, were exceeded. It is apparent from this information that deposit c 4 s? surface temperatures, a t sonie time during the cycle, may reach w 600" to 700" C. even a t combustion chamber locations normally 3 3 t considered "cool." The temperature of the gas film during the 0 exhaust stroke will be intermediate between the surface tempera2 2 ture (ea. 200" to 700" C.) and the bulk gas temperature (ea. 900" to 1500" C.). I Therefore, it would appear that the gas film temperatures under most conditions are so high that lead oxide rather than lead 0 halides would result from vapor state reactions in this film. Only Figure 8. Significance of Gas-Solid Reactions to Cool in the special case when the combustion chamber walls are clean Engine Surface Deposit Formation and cool will the gas film temperatures be low enough to favor the Vapor atate reactions eliminated i n alternating fuel engine experiments formation of lead halides by vapor state reaction. In a sense. the VBPOI' COXDENSATION. state equilibria illustrated in Figure 6 are quenched in the condensation step. The TABLE Iv. REACTIONS O F GASESWITH LEAD S.4LTJQ temperature a t which this "quenching" Product b , a t Temperature of: occurs is the gas film temperature or Lead Salt Gas 300' C. 5000 c. B500 c. PbO PbClz PbCI? PbClz higher if equilibrium is not maintained. PhO HC' HBr PbBrz + PbO PhBrz The material which is first deposited on a SO: j air PbO PhSOa PbSO; PbO cool clean surface may be lead halides. PhO. PbC1z HC1 PbClz PbSOl ....... PhO.PhC11 HBr PbBrz.PhCl2 ... .. .. However, after an insulating layer of dePbO.PbCln Ron t air PbO.PhClz PbSO, iPbClz , , . , , ., posits has formed, the solid material PhO.PhSOr HC1 PbC12 + PhSOi ,..... PbCI? + Pb904 initially present following condensation is PbBr? i- PhSOa t PbO.PhSO6 .,,,., , .. . . . , PbO.PhSO4 HBr PhO.PbSO* Son I. air PbO.PbSO4 PbSO4 ....... lead oxide, because only lead oxide can PhClrC HzO + air PbCIz PhCh PbClz (IFC1 evolved) exist in the gases over a hot deposit Hn0 + air PhBrn PbBrn PbRr? (Rr2 evolved) PbBrgC PbSOaC H20 + air PbSOI PbSOl PbROi surface. a Gas passed over 1.0 gram of heated salt for 30 minutes. This conclusion that lead oxide is the only b Products idensified by x-ray diffraction. 0 N o reaction observed between PhCIz, PbBrl, or PbSOr and acid gases. material deposited, except in the special condition when the metal wall is clean,
1
$
June 1953
x
INDUSTRIAL AND ENGINEERING CHEMISTRY
posited. The information developed in the previous sections shows that this lead compound is lead oxide. As deposits are composed of many different lead salts, further chemical changes must take place after deposition. GAS-SOLID REACTIONS.The solid lead oxide initially deposited is subject to further attack by the gases present in the combustion chamber. To determine what reactions are possible, a series of experiments was made in which gases such as hydrogen halides and sulfur oxides were passed over heated samples of lead salts. The products of these reactions were identified by x-ray diffraction and are given in Table IV. It was found that lead oxide, either free or present as lead oxyhalide or lead oxysulfate, would react rapidly with hydrogen chloride a t all temperatures studied, from room temperature to 650' C., to form lead chloride. Hydrogen bromide appeared only slightly less active in forming lead bromide. A mixture of sulfur dioxide and air did not react appreciably with lead oxide or lead oxy salts below 300' C. but formed lead sulfate readily a t 500' C. and higher. Lead chloride, lead bromide, and lead sulfate were not affected by any gas-solid reactions which could be expected to occur in an engine deposit. A very slight conversion of lead chloride or lead bromide to lead oxide was obtained by hydrolysis at 650" C. However, because of their low melting points, free lead halides would be completely vaporized and could not be present in a deposit a t this temperature. An extensive engine program was carried out to demonstrate the role of these reactions in the formation of engine deposits. T o isolate and study gas-solid reactions in an engine, experiments were devised in which the possibility of vapor state reactions was eliminated by introducing the fuel additives into the combustion chamber separately. To avoid complicating the results by the presence of a carbonaceous residue, the fuel used was iso-octane and the oil was an ester-type lubricant. These experiments consisted of operating a single-cylinder engine for 50 hours on two separate fuels, one containing tetraethyllead without halogens a t a concentration of 3.0 ml. per gallon and the other the additive being investigated. These fuels were alternated every 2 minutes, so that each fuel was used a total of 25 hours. For example, to investigate the formation of lead chloride by gas-solid reactions the two fuels used were (1) iso-octane plus 3.0 ml. of tetraethyllead per gallon and (2) iso-octane plus a stoichiometric theory of chlorine (based on the lead in tetraethyllead) as ethylene dichloride. In such an experiment lead chloride could be formed only by a reaction between solid lead oxide on the combustion chamber wall and the hydrogen halide in the gas. Similar experiments were conducted to investigate the formation of lead bromide (one-half stoichiometric theory of bromine as ethylene dibromide) and lead sulfate (0.05 weight % ' sulfur as disulfide oil) by gas-solid reactions. The weight and composition of deposits formed in the alternating fuel tests were compared with those of deposits formed by combined vapor state and gas-solid reactions in 25-hour tests in which both tetraethyllead and the additive in question were used simultaneously in the fuel, as is usually the case. From two to four experiments were conducted for each set of conditions. The weight of deposits formed on the combustion chamber surfaces by combined vapor state and gas-solid reactions and by gas-solid reactions alone is shown in Figures 8 and 9. These data show that, regardless of whether vapor state reactions were possible or not, the amounts of deposit formed on both the cool and hot engine surfaces were identical within limits of experimental error. The cool surface deposits given in Figure 8 include those from the cylinder head, block, piston top, and intake valve top. The exhaqst valve top, which is the hot surface referred to in Figure 9, is the surface on which the halogens are known to reduce deposit formation. The halogens were just as effective when allowed to scavenge by gas-solid reactions alone as when vapor state reactions were also possible.
TABLEV.
1341
COMPOSITION OF DEPOSITS FORME~D BY VAPOR STATE A N D GAS-SOLID REACTIONS
3.0 ml. of T E L per gallon TEL A. T E L and additive togethera T E L + A. T E L alternated with additive6 Deposit Composition, Weight Per Cent Cool Surfaces Hot Surfaces PbO PbO TEL +none 81 96 T E L + none 89 99 PbSOa PbSOa TEL + S C 23 65 42 54 TELjS 20 74 60 35 PbClz PbClz TEL Cld 21 61 69 24 T E L + GI 10 66 47 34 PbBrz PbBn TEL + B r * 48 43 77 18 T E L 4 Br 50 40 64 23 a 25-hour engine test. b 50-hour engine test with fuels alternated each 2 minutes. 0 0.06 weizht % as disulfide oil. d l.O-stoi&on;tric theory of ethylcnc dicliloride. e 0.5 stoichiometric theory of ethylene dibromide.
+
+
05
0 OA
ALL REACTIONS POSSIBLE
GAS-SOLID REACTIONS ALONE
$
3$ a G
W
3 go2
n 0 W
0.1
0
Figure 9. Significance of Gas-Solid Reactions to Hot Exhaust \.alve Deposit Formation Vapor state reactions eliminated in alternating fuel engine exprrimcnts
Moreover, the composition of deposits was not altered significantly by eliminating vapor state reactions, as is shown by the data in Table V. The similarity in weight and composition between deposits formed by gas-solid reactions alone and by combined gas-solid and vapor state reactions indicates that the formation of deposits on most engine surfaces is not influenced by vapor state reactions. SOLIDSTATEREACTIONS. All of the lead oxide which is initially deposited is not converted to simple lead salts by reaction with gases. It may also participate in solid state addition reactions with lead halide or lead sulfate to form the lead oxyhalides or lead oxysulfates listed in Table I. Because minimum temperatures exist for the formation of each of these complex salts, they are not found until the deposit surface temperature increases with deposit accumulation to the necessary minimum reaction temperature. Deposits on exhaust valve tops and the surface of thick deposits are composed almost entirely of complex lead oxy salts. The formation of lead oxy salts in engine deposits by solid ttate reactions has been discussed at length in a recent paper by Lamb and Niebylski (9). These authors found that the solid state reactions follow Tammann's rule approximately-that is, the temperature required for the reaction to occur is about one half the melting point (in degrees Kelvin) of the product. In addition, solid state replacement reactions may occur between lead oxy salts and simple lead salts. For instance, lead oxybromide and lead sulfate may react in a heated deposit to form lead oxysulfate and lead bromide, thus releasing free lead
INDUSTRIAL AND ENGINEERING CHEMISTRY
1342
bromide which may be scavenged by volatilization. The formation of lead oxyhalides seriously inhibits scavenging by converting a relatively volatile compound to a less volatile one. The net effect of the competition for solid lead oxide, both by the acid gases in gas-solid reactions and by the simple lead salts in solid state reactions, is that free lead oxide is rarely found in an engine deposit. The pure acid gases can also react with the lead oxide in lead oxy salts, but apparently in an engine this reaction does not occur as readily as the attack on free lead oxide. VAPORIZATION.From the engine deposit data of Figure 9 it is evident that the halogens are effective in reducing deposit weights on hot exhaust valve surfaces. The surface temperature required for vaporization of lead halides may be estimated from the vapor pressure of the lead salts. COOL ENGINE
SURFACE
NORMAL DEPOSIT
VERY HOT DEPOSIT
SURFACE
SURFACE
Vol. 45, No, 6
gas film near a clean combustion chamber surface is cool. Under these special conditions it may be converted by hydrogen halides to gaseous lead halide. Solid lead oxide which condenses has a short life as a deposit constituent. It is attacked by acid gases to form simple lead salts or reacts with the simple lead salts in solid state reactions to form complex lead oxy salts. The course of the reactions is dependent on the temperature of the combustion chamber walls. The reaction sequence is summarized in Figure 10 for three different temperature conditions. The compounds eventually found in the deposit under the particular conditions specified are shown in boxes. When the walls are clean and relatively cool (ea. 200” C.), the reactions result in lead halide deposit formation. ilfter deposit growth has produced an insulating layer, the surfaces are hotter and a variety of higher melting compounds are formed, When areas of the combustion chamber such as the exhaust valve and spark plug become very hot, the deposit composition changes so that only very high melting compounds are found. ACKKOWLEDGNENT
The authors are grateful to J. 1,. Hyde, J. R. Lacher, 13. P. Lander], H. K. Livingston, and F. TV. Tober for their contributions to the work reported in this paper. LITERATURE CITED
Figure 10. Summary of Deposit Formation Reactions as Influenced by Surface Temperature
The partial pressure of lead compounds present from the stoichiometric combustion of fuel containing 3.0 ml. of tetraethyllead mm. a t a total pressure of 1 atmosper gallon would be 6 X phere. The vapor pressure of pure lead bromide, calculated from the free energy of sublimation, is 6 X 10-3 mm. a t about 370’ C. With pure lead chloride this vapor pressure is reached at a temperature of about 420” C. Thus, no further increase in deposit weight would be observed on a deposit surface hotter than these temperatures if the follon-ing events occurred: deposition of lead oxide, complete conversion of solid lead oxide to lead halide, and vaporization of lead halide. Actually, this ideal sequence of events cannot occur and a higher surface temperature is required before lead salt deposition is exactly balanced by vaporization. Among the factors preventing this sequence are the formation of lead sulfate as Kell as lead halide from the lead oxide initially deposited and the conversion of lead halide to a much less volatile lead oxyhalide by solid state reaction. SUMMARY
The chemical reactions which occur in the course of lead salt deposit formation have been elucidated. Gaseous lead oxide, produced from tetraethyllead, does not undergo vapor state reactions except in the special case when the
(1) Calingaert, G., Lamb, F. W., and Meyer, F., J . Am. Chem. SOL,
71,3709 (1949). (2) Chamberlain, G. H. K., and Walsh, -4.D., Inst. intern. chiin. Solvay, “Le Mecanisme de l’Oxydat,ion,” 1951. (3) Cornelius, W., and Caplan, J. D., S A E Trans.,6,488 (1952). (4) Dumont, L. F., Ibid., 5, 565 (1951). (5) Egerton, A. C., and Jain, B. D., Fuel, 31, 62 (1952). (6) Hughes, E. C., Darling, S. M., Bartleson, J. D., and Kliirgel, A. R., Jr., IND. ENG.CHEM.,43,2841 (1961). ( 7 ) Kelley, K. K., U. S. Bur. Mines, Bull. 383 (1935); 476 (1949); 477 (1950). (8) Knowles, L. M., J . Chem. Ph,ys., 19, 1128 (1961). (9) Lamb, F. W., and Kiebylski, L. M., Anal. C‘hem., 23, 1388 (1951). (10) Lander, J. J., J . EZectPochem. Soc., 95, 174 (1949). (11) Natl. Bur. Standards, “Selected Values of Chemical Thcrinodynamic Properties,” Series I11 (1947-51). (12) Retailliau, E. R., Ricards, H. A , Jr., and Jones, &I. C . K S A E Trans.,4,438 (1950). (13) Rossini, F. D., “Selected Values of Properties of Hydrocarbons,” Natl. Bur. Standards, Circ. 461 (1947). (14) Ruer, R., Z. anorg. u.allgem. Chem., 49,385 (1906). (15) Taylor, C . F., and Taylor, E. S., “Internal Combustion Engine,” pp. 80, 128, 139, Scranton, Pa., International Textbook Co., 1948. (16) Tust, V . ’ E . , and Droegemueller, E. *4.,SAE Journal, 60, 37 (1952). (17) van der Zijden, PI.J., van Hinte, J.E., and van den Ende, J. C., J . Inst. Petroleum, 36, 581 (1950). RECEIVED for review September 2 7 , 1932. ~ C E P T E D February 7, 1953. Presented before the Division of Petroleum Chemistry a t the 122nd Meeting of the AMERICAN C H E ~ I I C SOCIETY, AL Atlantic City, N. 5.
