I
CLIFTON N, SECHRIST and HAROLD H. HAMMEN American Oil Co., Texas City, Tex.
Radiotracer Studies of Engine Deposit Formation )An excellent example of tracers used at background level by novel techniques in the field
THE
-
EFFECT of combustion chamber deposits on the performance of internal combustion engines is becoming increasingly serious as the trend to higher compression ratios continues. Both the fuel and lubricant contribute to deposit formation, but clear-cut distinctions between the deposits arising from the fuel and those due to the lubricant have often been dificult. A test program was set up to study the deposit-forming characteristics of unleaded fuels, fuel components, and/or different lubricants. I n part of this work, fuel components tagged with carbon-I4 have been used to establish the source of deposits. thereby distinguishing between deposits resulting from the fuel and deposits from the lubricant.
Equipment and Procedure Equipment. The equipment used in the single cylinder laboratory studies consists basically of a Cooperative Fuel Research (CFR) motor engine. The standard CFR overhead-valve cylinder and piston were used. This head is probably more typical of current automotive engines than the L-head modification used in the past by many investigators. The standard carburetor was replaced with a Marvel-Schebler Model VH-14 carburetor and a magnetically controlled throttle for cycling operation. Octane requirement measurements on the engine were determined a t trace knock levels with a Phillips Model 501 detonation meter. Engine Test Procedure. The test procedure is a n adaptation of the cycling procedure of the Ethyl Corp. (4). Cycling operations have been used exclusively, as this type of operation approximates road test data more closely than constant speed operation. Typical test conditions are shown in Table I. The engine is alternately operated for 150 seconds under full load and 100 seconds under idle conditions. The air to fuel ratios were determined by analysis of the exhaust gases. The 48-hour test period was set as standard because test
data have shown that this is a minimum time in which reproducible deposit levels are obtained. Reproducibility of f10% of the total weight of deposits, a t 9501, confidence limits, has been obtained. The test engine was operated a t 900 r.p.m. for 4 hours on a paraffinic reference fuel for break-in a t the start of each run. Then, an octane requirement check was made on the engine using primary reference fuels. If the octane requirement was within 1 3 units of the normal value, the test was started with the test fuel, using cycling operations. At the completion of a test period, the octane requirement was again determined. Collection of Deposits. At the end of a test, the engine was disassembled. So that deposits on the parts were not unduly disturbed, only a minimum amount of disassembly was performed a t the unit. Preliminary washings were made on each part with a minimum amount of isooctane to remove all oil and loose deposits. The washings of all parts were filtered and weighed. After each part had been washed, it was then scraped \ free of deposits. As carbon-I4 is a low energy beta emitter and the activity level was extremely low, no special shielding precautions were necessary. The test area was, however, isolated to control any possible low level contamination. Radioassay Procedure. The specific activity of all engine deposits formed using carbon-14-tagged materials was determined by the method developed a t the Oak Ridge National Laboratories (6). The apparatus used was modified by Burr (2). The steps involved a Van Slyke combustion of a 20- to 30-mg. sample, sweeping the carbon dioxide formed into a n ionization chamber, and determination of the ionization current
Table 1. Test Conditions Compression ratio Spark advance, degrees before top center Air :fuel ratio Full load, 900 r.p.m. Idle, 600 r.p.m. Full load brake horsepower Air intake temperature, O F. Oil temperature, O F. Cooling water temperature, F. Run length, hours
7 .5 to 1 3 13.0 11.0
4.0 110-15
155-60 175-80 48
using the Applied Physics Corp., Pasadena, Calif., vibrating reed electrometer (Model 30). Because of the high cost of carbon-I &tagged hydrocarbons and the large volumes of fuels needed, the specific activities of samples were in the range of 10“ mc. per mg. of carbon. The tagged hydrocarbons used in this work were obtained from Tracerlab, Inc., Waltham, Mass. Ionization currents were determined by the “rate of drift” method using a pen recorder. Corrections were made for discontinuities caused by alpha contamination. The net rate of drift was 1 to 2 times that due to background. The precision of specific activity determinations a t this low level was about &lo%. The radioassay procedure was checked frequently by oxidizing a sample of benzoic acid-C-14 with a specific activity of 1.08 X 10-8 mc. per mg. secured from Tracerlab, Inc. The completeness of the Van Slyke wet combustion of the engine deposits was confirmed by comparison with a Tracerlab analysis using a conventional combustion furnace.
Results Initial work was directed toward finding a lubricant which would give very low deposit weights and octane requirement increase, so that deposits formed from tagged fuel components would constitute a significant proportion of total deposits. Other investigators (7, 3, 5 ) reported that synthetic lubricants of the diester type contribute very little to deposits when used with iso-octane. In this work the synthetic lubricant of the polyalkylene glycol type, used in conjunction with a paraffinic fuel, gave very low deposit weights and little or no octane requirement increase. Because the weight of deposits is not a true indication of their harmful effects, octane requirement increase data were obtained. The results of early work on lubricants are shown in Table 11. When using a paraffinic fuel in conjunction with an uncompounded petroleum-base oil, 0.732 gram of deposits was formed. With the same fuel and a premium grade petroleum oil, deposit weight was 0.430 gram. The paraffinic fuel and the synthetic polyglycol type lubricant gave deposit weights 350/, of those formed with the premium oil and only 20% of those with uncompounded oil. From these data, VOL. 50, NO. 3
MARCH 1958
341
Table II.
Deposit-Forming Tendencies of Lubricants with Paraffinic Fuel"
Lubricant
Run 1
Run 2
Run 3
Uncompounded SAE 30 petroleum
Premium SAE 30 petroleum
Synthetic
0.732 1.3
0,430 0
Total weight of deposits, gram Octane requirement increase a
0.150 0
Blend of 61% i-Cs, 15% n-G, 24% i-cs.
Table Ill.
Summary of Radioactive Engine Deposit Tests
Run 4
Run 5
Run 6
Run 7
25% Toluene in
Fuel
Toluene
para5nic base
Lubricant Total weight of deposits, grams Octane requirement increase
Synthetic 1.017 12.8
Synthetic 0.317 +4.5
+
Radioactivity, Fuel Total deposits Cylinder deposits Piston crown deposits
5.0
the synthetic lubricant was only slightly, if at all, involved in the formation of deposits. T h e conclusion that the synthetic oil contributed very little to the deposit formation was further checked with run 4 made with pure toluene, plus radiotracer amounts of ring-labeled toluene1-(2-14, as the fuel, Table 111. If the synthetic lubricant contributes nothing to the deposit, the radioactivity of the deposit, p,er weight of carbon-Le., specific activity-should be the same as that of the fuel. If the oil contributes significantly to the formation of deposits, however, the radioactivity of the deposits should be substantially lower than that of the fuel. The radioactivity of the total deposits actually formed was of the same order of activity as the fuel. An additional test was made to determine the increase in deposits resulting from addition of an aromatic to the paraffinic base fuel and to see if it was due to the aromatic alone or interaction of the aromatic with the base fuel. I n run 5, 25% toluene (containing ring-labeled toluene-C-14) was blended into the paraffinic base fuel. Using the synthetic lubricant, addition of toluene increased in deposit weight, approximately twice that for the paraffinic fuel alone. The specific activity of the total deposits was twice that of the fuel. If all of the deposits in this run had been formed from toluene, its radioactivity would have been four times that of the fuel, as toluene comprised 25% of the fuel. Assuming the same deposit formation characteristics from the base fuel and lubricant as previously determined, the radioactivity would be reduced to approximately twice that of the
342
Benzene Uncompounded Synthetic SAE 30 petroleum 1.140 2.151 f5.0 f7.5
Conclusions
Mo./Mg. of Carbon
6.5 6.8
7.5
Benzene
8.5 16.7
16.4 12.2
6.4 5.7 6.0 4.5
11.5 5.7 6.6 3.8
fuel, because the bbse fuel and lubricant contributed almost one half of the total deposit weight. The contribution of the synthetic Iubricant to deposit formation is a fairly constant and small amount, and the increased weight of deposit obtained with 25% toluene was derived almost completely from the toluene. R u n 6 was made using the synthetic lubricant and benzene, plus radioactive benzene-C-24, as fuel. If essentially all of the deposits were derived from the fuel, the radioactivity of the deposits should be the same as the fuel. Analysis showed it to be about 90%. A final run, No. 7 , was made using benzene, containing radioactive benzene, as fuel and a n uncompounded SAE 30 petroleum oil as lubricant. The run with the petroleum-base lubricant gave 1.011 grams more deposit than the run with the synthetic lubricant. If this difference is truly the effect of the petroleum lubricant, it should be possible to predict the specific activity of the deposits. In this instance, approximately one half of the deposits came from the oil; therefore, the activity of the deposits should be one half that of the fuel. The data show this to be the case. Therefore, the over-all increase in weight of deposits for this run as compared with the corresponding run with the synthetic lubricant is a reflection of the deposit-forming tendencies of the petroleum base lubricant. I n all the radiotracer tests, the deposits on the piston top were less radioactive than those on the cylinder head and upper cylinder walls (Table 111). The larger contribution of the oil would be expected as a result of flat piston surface and the higher temperatures
INDUSTRIAL A N D ENGINEERING CHEMISTRY
encountered. The radiotracer results confirm this expectation. Approximately 43'30 of the cylinder and wall deposits and 67% of the piston crown deposits came from the lubricant in run 7 while approximately 670 of the cylinder and wall deposits and 30% of the piston crown deposits came from the lubricant in run 6. This work definitely shoxs that with a polyglycol synthetic oil, the engine deposits are formed predominantly from the fuel (benzene or toluene), and that this particular synthetic motor oil is involved to only a minor extent in the production of deposits. The deposits formed from the synthetic lubricant are deposited almost exclusively on the piston crown.
The use of C-14-tagged fuel components in engine testing makes it possible: 1. T o distinguish between the contribution of the fuel and lubricant in the formation of deposits. 2. T o determine the deposit-forming tendencies of specific fuel components, 3. T o determine the location, in the combustion chamber, where the deposits resulting from either the fuel and/ or lubricant are laid down. This technique could be extended by labeling different positions in the same molecule and comparing the activities of deposits formed using each as fuel. The contribution of gasoline and lubricant additives to deposit formation and the location of these deposits can be determined by using tagged additives and applying the same technique.
Ac knowledg ment The authors gratefully acknowledge the suggestions and assistance of G. D. Walters and R. J. Lee in carrying out this work.
Literature Cited (1) Bartleson, J. D., Hughes, E. C., IND. ENC. CHEW4 5 , 1501 (1953). ( 2 ) Burr, J. G., Jr., Anal. Chem. 26, 1395 (1954). (3) Corzilius, M. W., Diggs, D. R., Hoffman, R. A , , 21st Midyear Meeting, American Petroleum Institute, Montreal, Quebec, Can., May 13, 1956. (4) Gibson, H. J., Hall, C. A., Hirschler, D. -4., S.A.E. Trans. 61, 361 (1953). (5) Mikita, J. J., Bottoney, W. E., 41st Annual Meeting, Western Petroleum Refiners Assoc., March 23-5, 1953. (6) Neville, 0. K., J . Am. Chem. SOG.70, 3499 (1948). RECEIVED for review April 5, 1957 ACCEPTED July 15, 1957 Petroleum Division, Southwide Chemical Conference, Memphis, Tenn., December 6, 1956.
