INDUSTRIAL AND ENGINEERING CHEMISTRY
1744
LITERATURE C I T E D
(1) Bash, F. E., and Harsch, J. W., A m . Soc. 7 ’ & i n ~ Materinls Proc., 29, Part 11, 506-19 (1929). (2) BBnard, J., BulE. SOC. chim. Fmnce, 13, 511-21 (1946). (3) Caplan, D., and Cohen, &I., J . Metals, 4, 1057-65 (1952). (4) Chipman, J., Trans. Am. Soc. Metals, 22, 385-435 (1934). (5) Dushman, S., “Scientific Foundations of Vacuum Technique,” New York, John Wiley & Sons, 1949. (6) Gulbransen, E. A., J . A p p l . Phys., 16, 718-24 (1945). (7) Gulbransen, E. A,, Rev. Sci. Instr., 18, 546-50 (1947). (8) Gulbransen, E. A , and Andrew, K. F., J . Electrochem. Soc., 99, 402-6 (1952). (9) Gulbransen, E. A . , and Andrew, K. F., I b i d . , to be published. (10) Gulbransen, E. A , , and Hickman, J. W., Trans. Am. Inst. Mining Met. Engrs., 171, 306-31 (1947); T.P. 2068. (11) Gulbransen, E. A., Phelps, R. T., and Langer, A , IND.EKC. CHEM.,ANAL.ED., 17, 646-52 (1945). (12) Gulbransen, E. A., Wysong, W. S., and Andrew, K. F., Trans. A m . Inst. Mining M e t . Engrs., 180, 565-78 (1949); T.P. 2438.
(13) Hall, F. P., and Insley, H., “Phase Diagrams for Ceramists,” Columbus, Ohio, Am. Ceram. SOC.,1947. (14) Hauffe, K., and Pfeiffer, H., quoted by K. Hauffe, Wiss. 2 . Universitiit Greifswald, Jahrgang I, Mathematisch-naturwissenschaftliche Reihe, Nr. 1 (1951/52). (15) Hessenbruch, W., “Metalle und Legierungen fur hohe Temperaturen,” Berlin, Julius Springer, 1940.
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(16) Hickman, J. W., and Gulbransen, E. A , , Trans. Am. Pmt. Mini n g Met. E n g ~ s .180, , 519-33 (1949); T.P. 2372. (17) Holler, H. D., Trans. Electrochem. Soc., 92, 91-7 (1947). (18) Iitaka, I., and Miyake, S., Xature, 137,457 (1935). (19) Johnston, H. L., and Marshall, A. L., J . A m . Chem. Soc., 62, 1382-90 (1940). (20) Kelley, K. K., U. S.Bur. Mines, Bull. 383 (1935). (21) Lustman, B., Trans. A m . Inst. Mining Met. Engrs., 188,995-6 (1950). (22) Picard, R. G., J . A p p l . Phys., 15,678-84 (1944). (23) Quarrell, A. G., Nature, 145,821-2 (1940). (24) Scheil, E., and Kiwit, K., Arch. Eisenhiittenw., 9, 405-16 (1935-6). (25) Spciser, R., Johnston, H. L., and Blackburn, P., J . A m . Chem. SOC.,72, 4142-3 (1950). (26) Sully, 9.H., J . S C ~Inst?., . 22, 244-5 (1945). (27) Thompson, M. de Kay, “Total and Free Energies of Formation of the Oxides of Thirty-Two Metals,” New York, Electrochemical Society, 1942. (28) Wagner, C., J . Electrochem. Soc.. 99, 369-80 (1952). (29) Wagner, C.: and Zimens, K., Acta Chem. Scand., 1, 547-65 (1947). (30) Wyckoff, R. W. G., “Crystal Structures,” Kew York, Interscience Publishers, 1951. RECEIVED for review March 6, 1953. ACCEPTED X a y 7, 1933. Presented before the Division of Physical and Inorganic Chemistry at t h e 123rd Meeting of the AMERICANCHEMICALSOCIETY,Lo8 Angeles, Calif. Scientific Paper 1719, Westinghouse Research Laboratories.
Tetraethvl Radiolead Studies of J Combustion Chamber Deposit Formation H . P. LANDERL AND B. YI. STURGIS Jackson Laboratory and Petroleum Laboratory, Organic Chemicals Department, E. Z, d u Pont de Nemours & Co., Inc., Wilmington, Del.
D
EPOSITS which form in the combustion chambers of automotive and aircraft engines have been shown to cause certain undesirable effects. The elimination of these deposits, which are a combination of residues from the fuel and oil, may be possible if the mechanism of their formation and scavenging is understood more completely. Some of the physical aspects of deposit formation have been studied by Dumont ( I ) , who found t h a t lead salt deposits accumulate at a nearly constant rate under fixed engine operating conditions until a certain thickness is reached, at which time flaking sets in and maintains the total deposits a t a fairly constant weight. Flaking is believed to be the result of thermal stresses developed within the deposits as they grow thicker. More recently, Newby and Dumont ( 2 ) have discussed the chemistry of deposit formation from leaded fuels and have demonstrated t h a t reactions between solid deposits and reactive compounds in the hot combustion gases are of great importance in controlling deposit formation and composition. I n spite of the new knowledge made available by theee studies, many details of the mechanism of deposit formation and scavenging are still unknown. The study of combustion chamber deposit formation is very difficult since no direct method is known for following the formation a t the time it is occurring. The appearance of the deposits at any time during their formation only suggests the processes which have occurred. The use of tetraethyl radiolead in tracer amounts in the fuel provides a new procedure for such a study. The exact location and distributioii of the radioactive
lead salts which this additive deposits within the combustion chamber can be detected by means of x-ray film. It has been possible by this technique to observe the formation and removal of deposits resulting from tetraethyllead during a relatively short time a t any stage of the deposit growth. The importance of the temperature and physical state of a surface in influencing deposit laydown has been indicated. The efficiency of halogen scavenging also has been demonstrated and a previously unrecognized type of scavenging mechanism observed. RADIEMD AS RADIOLEAD TRACER. The radioactive lead used in this investigation was radium D, one of the decomposition products of the uranium series. The sequential decomposition of radium D occurs predominantly by the following path. RaD
p (0.026 m.e.v.)
-
22 years half life Pb;:O
-
RaE
p (1.17 m.e.v.) 5.0 days half life Biz10 83
RaF
(5.3 m.e.v.) 140 days half life-
RaG
01
p0;:o
Pbg;“ inactive
The medium energy beta particles from radium E are the only radiations in this series which can be detected easily either with a Geiger counter or with photographic film. Radium E, very
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INDUSTRIAL AND ENGINEERING CHEMISTRY
conveniently, reaches secular equilibrium in this system after about 30 days. At this time the ratio of radium D t o radium E becomes essentially constant and remains so for about 90 days. An analysis for radium E and a knowledge of the age of the sample is sufficient t o calculate the amount of radium D present. Figure 1 shows t h a t radiation equilibrium was reached in about 30 days
?
R
I
I
t
0
20
10
I
30
I
I
so
40
I
0
TIME, DAYS
Figure 1.
Secular Equilibrium of Radium E
Typical engine deposits from fuel leaded with tetraethyl radiolead
in three typical engine deposits formed while the engine was operated on fuel containing tetraethyl radiolead. These curves indicate that radium D was the only radioactive species deposited. EXPERIMENTAL
PREPARATION OF TETRAETHYL RADIOLEAD.Radium D used in these experiments was obtained as the chloride in lead chloride a t a concentration of 2 millicuries per 100 grams. The radioactive lead chloride was converted t o tetraethyl radiolead (tetraethyl radium D) by a Grignard-type reaction with ethyl chloride. When this reaction was carried out in a bomb under autogenous pressure with less than 0.5 mole of ether per mole of magnesium, yields as high as 65%, based on lead, were obtained. 4CzHEC1 3Mg Pb*Clz +(C2H,),Pb* 3MgClz This is a marked improvement over the yields obtained by the more conventional Grignard reaction which is carried out in refluxing ether where the theoretical yield of tetraethyllead is only 50%, based on lead.
+
+
1145
lead. When appreciably larger or smaller amounts of ether were used lower yields were obtained. ACCUMULATING AND DETECTING RADIOACTIVEDEPOSITS. The engine used in this program was a Lauson Model H-2 liquid-cooled oil test engine. The fuel, unless otherwise indicated, was a commercial gasoline base stock (100% catalytically cracked, containing less than 0.05% sulfur) of high enough quality t o ensure knock-free operation. Depending upon the specific experiment, the gasoline was leaded with either of the two following compositions: 1. Motor Mix, a commercial product for use in automotive fuels composed of 1 mole of ethylene dichloride, 0.5 mole of ethylene dibromide, and 1 mole of tetraethyllead 2. Straight tetraethyllead, with no halogen scavenging agents When tetraethyl radiolead was used it replaced the tetraethyllead in one of these compositions. The deposits were accumulated by operating the engine on leaded fuel under steady medium-duty conditions. When the desired stage of deposit formation was reached, the fuel was changed t o one containing an identical amount of tetraethyl radiolead, and the test continued for a short time. After completion of the test the radioactive deposits were located autoradiographically. The autoradiographs were made by placing in contact with the deposits a sheet of D u Pont x-ray film (Type 508), which was contained in a photographically opaque paper envelope to protect i t from light. A plaster of Paris mold of the combustion chamber surfaces held the film in good contact with the deposits. An exposure time of 5 hours was sufficient for the level of radioactivity found in the deposits and was used in all the tests. I n the autoradiographs illustrated, the darker areas represent greater concentrations of radioactive deposits than do the lighter areas, and thus indicate the relative concentrations of radioactive lead throughout the combustion chamber. I n the photographic reproduction of these autoradiographs considerable detail has been lost, and the exact intensity of the shading may be somewhat different from the original autoradiograph. When comparing the photographs and autoradiographs, i t should be remembered that the photographs show the location of the deposits which have accumulated throughout the test, while the autoradiographs show only the deposit which was formed during the relatively short time that tetraethyl radiolead was being iqtroduced into the engine. The change in the angular position of the valves in the photographs was due to the use of valve rotators.
+
24typical preparation consisted of agitating 0.144 mole lead chloride containing 2 millicuries of radium D chloride per 100 grams, 0.48 mole magnesium, 3.07 moles ethyl chloride, and0.131 mole diethyl ether in a bomb at 80' C . for 4 hours. After cooling, the reaction mass was hydrolyzed. The excess ethyl chloride and ether were removed by distillation and the tetraethyl radiolead separated by steam distillation. A yield of 0.09 mole was obtained, which is equivalent t o 62y0 of theory, based on
Photograph
Autoradiograph*
Figure 2. Deposit Formation with Tetraethyllead in the Absence of Halogens Run 101 hours with 3 ml. tetraethyllead/&., tetraethyl radiolead/gal.
* Black = radioactive deposits
0.5 hour with 3 ml.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 8
deposits to cooler locations, so that the most radioactive portion of the piston top is now that area farthest removed from the valves, and the least active area in the combustion chamber is the exhaust valve top. The observation that new deposits form a t a greater rate on thick deposits than on thin deposits has not, to the authors’ knodedge, previously been reported. It would be extremely difficult, if not impossible, to demonstrate this behavior Jvithout the use of a radioactive tracer element. A comparison of the photographs in Figures 3,A and A B c Autoradiograph * 3,B clearly shows that while it may be Photographs possible to the amount Of new F i g u r e 3. Deposit Formation w i t h T e t r a e t h y l l e a d in the Presence of Halogens deposit on the scraped portion of the A . Run 80 hours with 3 ml. tetraethyllead (Motor Mix)/gal. Portions of deposits removed piston top, the estimation of the amount from block, piston, and valve tops B, C. Same as A plus 1 hour with 3 ml. tetraethyl radiolead (Motor Mix)/gal. of new deposit on the remainder of the * Black = radioactive deposits piston top would be very difficult. The photographs show no apparent change FORMATION OF NEW DEPOSIT in deposit on the unscraped areas, but do shoiy that a dusty layer has been formed on the scraped poition of the piston top. The Combustion chamber deposits usually build up t o a certain autoradiograph indicates that this dusty layer was considerably thickness, then start to flake. Under ordinary operating condiless than the deposit Irhich formed on some of the other parts of tions the total deposit then removed by the Aalring process and the piston top. other scavenging mechanisms is approximately equal in amount While only the autoradiographs for the piston top. block, and t o the new deposit being formed, and the deposit is said to be in valves are shoLm, autoradiographs of the deposits on the head “equilibrium.” The deposits vary in thickness from one region of the engine are completely in agreement mith them. In these of the combustion chamber to another, depending partly on the autoradiographs, in general, the definition of the radioactive amount of flaking that has occurred a t each location. The deposits was less sharp since the irregular shape of the head made tendency of new deposits to form on the various flaked and unit difficult to hold the x-raj- film in r l o s e contact n.ith the deposits flaked surfaces in a combustion chamber which had reached a t all points. deposit equilibrium m-as investigated. The autoradiographs of the deposits on the block around the The location of new deposits xyas observed in combustion chamvalves lack definition because the valves were raised soniewhat bers in which deposit equilibrium had been reached on fuels above the level of the block, making it tiiffcult for the film t o be containing either straight tetraethyllead or Motor Nix. In the held in contact a i t h these deposits. first experiment the engine was operated 101 hours on a commercial fuel containing 3 ml. straight tetraethyllead per gallon. SCAVEKGIKG WITH HALOGENS This was followed by 1/2-hour operation on the same fuel conHalogens in the form of ethylene dichloride and ethylene ditaining 3 ml. straight tetraethyl radiolead per gallon. Figure 2 bromide are ordinarily used with tetraethyllead to reduce the shows a photograph and autoradiograph of the piston top, amount of deposit in the hotter portions of the combustion chamblock, and valve tops a t the completion of this test. The radiober by the formation of lead halides which are too volatile t o active deposits-hence, the most recently formed deposits-are remain on these hot surfaces. It has been proposed that forconcentrated on the thicker deposit surfaces. The exhaust valve mation of the lead halides on hot combustion chamber and detop (the valve top farthest from the piston top), which is coated with the thickest deposits, is the most radioactive area in the combustion chamber. The portion of the piston top farthest removed from the valves has practically no deposit and is almost completely nonradioactive. -4similar test was conducted using a commercial fuel containing 3 ml. tetraethyllead as Motor Mix per gallon. After 80 hours of qperation, one half of the valve top deposits were removed and also t h a t from a strip across the center of the piston top and across the block just below the exhaust valve. The residual deposits are shown in Figure 3 4 . The test was continued for 1 additional hour on the same fuel containing 3 ml. tetraethyl radiolead per gallon in the form of Motor Mix. B C A Figure 3,B is a photograph and 3,C an autoradioAutoradiographs* graph taken a t the end of the experiment. Figure 3,C shows t h a t the new deposits again F i g u r e 4. Deposit Scavenging b y Halogens formed t o a greater extent on the thicker deposit A . Run 152 hours with 3 ml. tetraethyllead/gal., 1 hour w-ith 3 ml. tetraethy radiolead/gal. Portions of the deposits were removed surfaces than on the thinner deposit or bare B. Same as A plus 5 hours on iso-octane C. Same as B plus 5 hours on iso-octane containing halogens equivalent t o 3 ml. metal surfaces. The presence of halogen scavtetraethyllead (Motor Mix)/gal. No tetraethyllead was present enging agents changes the location of the thickest * Black = radioactive deposits ~
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1141
posit surfaces, and their subsequent scavenging, results entirely from the reaction of the hydrogen halide gases on previously formed lead oxide deposits ( I ) . This belief is supported by the following experiment which demonstrated t h a t halogens in the fuel can scavenge previously formed lead salt deposits. A relatively heavy deposit with a thin layer of radioactive deposit on the surface was formed by operating the engine for 152 hours on fuel containing 3 ml. straight tetraethyllead per gallon, followed by 1 hour on A B C fuel containing 3 ml. straight tetraAutoradiograph* Photograph Autoradiograph* ethyl radiolead, all in the absence of Figure 5. Deposit Migration One Of the piston top’ A . Run 80 hours with 3 d. tetraethyllead (Motor Mix)/gal., 1 hour with 3 ml. tetraethyl radiolead (Motor Mix)/gal. One half of deposits removed and new valves installed block, and valves was then scraped B, C. Same as A PIUS 5 hours on iso-octane free of deposits. ~i~~~~4 , shows ~ an * Black = radioactive deposits autoradiograph of the deposits at this point. To show the effect of engine of small, irregularly distributed points of activity, giving the imoperation in the absence of fuel additives, the test was continued pression t h a t these areas had been bombarded with small fragfor 5 hours on iso-octane fuel with no additives. A very minor ments of deposit. There is some indication in Figure 5,C t h a t amount of flaking occurred around the edges of the valves, and these fragments migrated, predominantly from the end gas resome of the deposits on the portion of the piston top nearest the gion of the piston top. This could, a t least partially, account for exhaust valve also flaked. These changes can be seen in the the irregular deposit usually found in this area of the combustion autoradiograph in Figure 4,B. chamber. The effect of the halogens was observed by operating the engine This deposit migration occurred by a process which has been for an additional 5 hours on iso-octane containing a n amount of designated “flecking.” Flecking involves the removal of small ethylene dichloride and ethylene dibromide equivalent t o t h a t pieces, probably less than a millimeter in diameter, from the surpresent in fuel containing 3 ml. tetraethyllead as Motor Mix per face of the deposit; these flecks become molten when they enter gallon. No tetraethyllead was present in this fuel. I n the autothe hot working fluid. When these molten flecks strike another radiograph in Figure 4,C, a very appreciable reduction in the part of the combustion chamber there is a strong tendency for intensity of radioactivity is observed, particularly on the valves. them t o adhere. Flecking differs from flaking in t h a t flaking reHowever, even the deposits on the relatively cooler piston top sults from the rupture of bonds deep down in the deposit, resulting show a marked decrease in activity. in the separation of a relatively large piece of deposit. The cavity This scavenging of the deposits by gas-solid reactions is even which this flaking creates is easily visible. Flecking, on the other more striking if the age of the deposits is considered. It was hand, results from the rupture of cohesive bonds near the surshown in Figure 1 that during the first 25 or 30 days after the face of the deposit, leaving a relatively inconspicuous surface formation of the deposits, the concentration of radium E was irregularity. Flecking starts when only a very thin layer of increasing. Since radium E is the species which exposes the film, deposit is present and occurs continually; flaking is a sporadic the degree of exposure should increase as the deposits age. The process occurring only after relatively thick deposits have been deposits in Figure 4,A were 4 days old and Figure 4,B shows the formed. The fact that flecks were found to have deposited same deposits after 6 days. In the areas where no flaking ocon the exhaust valve head indicates t h a t many flecks probably curred, Figure 4,B does show an increase in the intensity of radiapassed out of the combustion chamber through the exhaust port. tion. The deposit in Figure 4,C was 11days old. If the halogens Thus, there are three natural scavenging mechanisms-volatilieahad removed none of the deposits the intensity of radiations in tion, flaking, and flecking-which can operate to remove deposits Figure 4,C would have been greater than in 4,B. Hence, the from combustion chambers. effect of halogens as scavenging agents is somewhat greater than t h a t shown in Figure 4. MECHANISM OF DEPOSIT FORMATION
DEPOSIT MIGRATION
The migration of deposits within the combustion chamber had been suspected and was clearly demonstrated by the following experiment. The engine was operated for 80 hours under medium-duty conditions using fuel containing 3 ml. tetraethyllead as Motor Mix per gallon. These deposits were coated with a layer of radioactive lead salts by continuing the test for an additional hour on fuel to which 3 ml. of tetraethyl radiolead as Motor Mix per gallon had been added. After removing half of the deposits on the piston top and block and installing new valves, autoradiograph A in Figure 5 was obtained. An additional 5 hours of operation on iso-octane free of lead in any form produced t h e migration of the deposits shown in Figure 5,B and C. New radioactive deposits began t o develop in the end gas region (that region farthest removed from the valves) of the piston top and also on the valve tops, particularly the exhaust valve. All the new deDosits which formed on the scraped areas consisted
It is impossible at the present time t o describe the complete mechanism of deposit formation; however, the evidence presented in this paper suggests t h a t the temperature of the deposit surfaces is one of the more important factors which determine the rate of deposit formation. Since such factors as the velocity of the gases over the deposit surfaces and the percentage of the total charge contacting the deposit surfaces are believed t o be relatively constant over any one surface, but t o vary greatly from one region t o another, each of the various regions in the combustion chamber (piston top, block, valves, etc.) must be considered separately. I n t h e absence of halogens in the fuel, t h e lead salt deposits accumulated t o a greater extent on the Bhick deposit surfaces than on the thin deposit surfaces. This is demonstrated on all the surfaces shown in Figure 2. The main factor which causes this difference in deposit thickness is undoubtedly temperature, and the critical temperature which regulates the formation of deposits is t h a t of
1748
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No 8
the surface exposed t o the lead salt vapors and acidic gases. As the deposit thickness increases, the temperature of the surface of the deposit must increase also as a result of the insulating effect of the deposit; that is, the deposits reduce the heat loss t o the coolant. Since it was found t h a t the deposits grow most readily on the thicker and, hence, hotter deposit surfaces, and since all adjacent deposit surfaces are exposed t o essentially the same lead salt vapor, which is chiefly lead oxide (W),the physical state of the surface must be the deciding facto< in determining how much deposit will accumulate. This is probably due t o the fact t h a t as the deposit surfaces become hotter they become more molten and therefore can more readily trap newly condensed lead oxide. The temperature of the surface of the deposits on the exhaust vnlve is high enough to melt or sinter the deposits. The degree to which the deposits are sintered is reduced in the regions somewhat removed from the exhaust valve. I n the end gas region 01 the piston top, sintering is almost negligible, for the deposits tend to be chalky or dusty. The same principles apply when halogens are present in the fucl along with the lead, except that now the scavenging effect ol the relatively volatile lead halides is superimposed on the adhesive effect of the sintered deposit surfaces. The halogen acids, formed from the halogen additives in the fuel, can react with the lead oxide immediately after it condenses on thecombustionchamber ivalls or deposit surfaces. The lead halides which form are much lower melting than the lead oxide and sulfate, which are formed in the absence of the halogens. Thus, sintering of these deposits can orcur in the cooler portions of the combustion chamber and deposits will form there more rapidly. Engines operated on fuels containing Slotor Mix form appreciable deposits in the cooler region of the piston top which is farthest removed from the spark plug and exhaust valve. T h e autoradiograph for such a run (Figuie 3 ) Q h o w deposits forming most readily in the end gas region. If the deposit surface temperatuies in the end gas region are high enough t o sinter the l e d halides, very severely sintered or even molten depoiits should he found on the piston top near the exhaust valve. By the reasoning applied above, this should result in even more rapid build-up of deposits when halogens are present. However, this does not occur. The deposits in general get thinner toward the exhaust valve; the autoradiographs show also t h a t new deposit has less tendency to foim. I n fact, the region where the least amount of new deposit forms is on the exhaust valve itself. Chemical and x-ray analyses shoiv that less halide is found in the deposits as they get hotter and that the exhaust valve deposits are completely free of halides. This is the result of volatilization of the lead halides. The halides are formed but their life on these hot surfaces is very short, the net result being a reduction in the amount of lead salt deposits in the hotter regions of the combustion chamber.
give the valves a speckled appearance. The deposits on the valve in Figure 3, C were formed while the engine was operated on fuel containing tetraethyl radiolead as Motor Mix, while those in Figure 5, C were formed while the engine was run on iso-octane. I n both cases large amounts of previously formed deposits were present throughout the combustion chamber. In the latter case the only way that the radioactive deposits could get to the exhaust valve was by migration from some other part of the combustion chamber. Since the autoradiographs show the same type of deposit formation, it is reasonable to assume that the deposits in both cases were formed through migration. The lack of radioactivity in Figure 3, C, except for these small patches, indicates that as rapidly as new deposit was formed on the exhaust valte it reacted with halogen acids to produce the lead halides which were scavenged through volatilization. It has been calculated that during each engine cycle only enough lead is introduced into the combustion chamber to cover less than one half of the wall surfaces with a monomolecular layer of lead salts. Such thin layers of deposit can probably be scavenged completely from the hot evhaust valve top by the halogens. On the other hand, when the exhaubt valve is bombarded by flecks from other parts of the combustion chamber, the scavenging reactions are too slow to permit ~ o m p l e t escavenging of one fleck before another is deposited. Therefore, if it were not for flecking the halogens would probably krep the hotter portions of the combustion chamber clean.
FORMATION OF EXHAUST V i L V E DEPOSITS
LITERATURE CITED
These experiments with tetraethyl radiolead have shown that the halogens are very efficient scavengers for exhaust valve deposits. h close examination of the autoradiographs of the exhaust valves in Figures 3,C and 5,C shows that the deposits in both cases are essentially the same. The general background is light; no radioactivity is present except for small patches which
(1) Dumont, L. F.,S.A.E. Q u a ~ t .Trans., 5 , 565 (1951). ( 2 ) Newby, W. E., and Dumont, 1,. l?., IND.END.CHICM.,45, 1336
SUXIMARY
A study of the formation and scavengingof combustion chamber deposits using radioactive lead as a tracer element has shown that new deposits form at a more rapid rate on the thick deposit surfaces than on the thin deposit or bare metal surfaces. I t is believed that this is due t o the differences in temperature of these surfaces, which produces a sintered surface condition on thick deposits conducive t o adhesion of fresh deposit. It also has been shown t h a t established deposits can be scavenged by halogens in the fuel through gas-solid reactions to form volatile lead halides. This scavenging action of the halogens greatly reduces the formation of new deposits on exhaust valves. The depoeits have been found t o transfer from one location t o another tvithin the combustion chamber through a process which has been termed "flecking." This migration process appears t o be responsible for the formation of almost all of the exhaust valve deposits when halogens are prewnt in the fuel. ACKSOWLEDGMENT
The authors are grateful to G. €1.Daniels and H. K. Livingston for their contributions to the work reported in this paper.
(1953). RECEIVED for review bIarcli 2 6 , 1953.
ACCEPTED May 15, 19S3. Presented before the Division of Petroleum Chemistry at"the 123rd Meeting of the AMERICASCHEMICAL SOCIETY,Los Angeles, Calif.
