I;VDII’STRIAL AND ENGINEERIR’G CHEMISTRY
July, 1925
731
Factors Influencing Carbon Formation in Automobile Engines‘ By J. W. Orelup and 0. I. Lee COOPERATIVE
RESEARCH LABORATORIES O F T H E BOYCE & VEEDER
COMPANY AND
T
H E formation of carbon in automobile engines might be considered of little economic importance. However when the extent of loss of efficiency due to carbon is compared with the vast number of automobiles in use, it is remarkable that so little quantitative information is available on carbon deposits in the engine. As the margin of power for acceleration and climbing hills is small in a medium-priced touring car-only about 5.4 per cent of that theoretically available-slight losses of efficiency seriously impair the performance of a car.2 Carbon in an engine is like scale in a boiler. I t is one of the best heat insulators. The temperature of fuel burning in a heat-insulated cylinder soon rises above its critical temperature and thereupon breaks down in an extraordinary manner. This is manifested by knocking or detonating. Sudden large pressures are produced. To avoid the knocking the operator must run with spark retarded, thus losing power. There is an increased fuel consumption and a tendency for the engine to overheat. The car is no longer able to climb steep hills in high gear and in general lacks that desirable activity described as “pep.” Another detrimental effect of carbon formation is the intensified pitting of the valve seats by the hard particles of carbon preventing the valves from closing tightly, resulting in loss of power. Chemical Properties of “Carbon”
In order to have an adequate grasp of the factors influencing carbon depositions, their chemical characteristics were investigated. The properties in general have been found to agree with those noted by other o b s e r ~ e r s . ~ Cylinder carbon does not consist of elementary carbon, as ordinarily thought, but is for the most part complicated, highly condensed hydrocarbons of a bituminous nature, mixed or combined with mineral matter containing numerous common elements. The larger portion of the deposit consists of heavy hydrocarbons, some of them of a coke-like nature, possibly having a structure of interlinked carbon rings, and about 15 per cent of a lighter hydrocarbon similar to lubricating oil. The mineral matter varies from 6 to 25 per cent, according to the conditions of formation, and is mainly oxide of iron and other metals associated with silicon, sodium, chlorine, sulfur, and oxygen. This inorganic portion seem5 to have its origin in the finely divided metals worn off from the engine and bearings, to oxidation of the heads, in iron originally contained in the lubricating oil, and in the accumulation of road dust carrying silicates, chlorides, and sulfate (sodium). The organic portion contains small amounts of nitrogen and sulfur. Very few organic or inorganic solvents have any effect on cylinder carbon. A few of the most effective dissolve only a small amount of the lighter hydrocarbon. The best solvents found are monochlorobenzene, xylene, and cymidine, and to a lesser degree pyridine. Carbon deposits have the property in common with ordinary asphalts of forming a dark, soluble product with aqueReceived April 29, 1926 “Investigation of Lubrication of Internal Tide Water Oil Co , 1924 3 U‘aters, Bur \ f a n d o r d r , Circ 99 (1920) 1
2
COMPANY OF AMSRIC.4, SPRINGFIELD,
N. J
ous caustic soda, indicating material of an acidic nature resembling salts of weak bases. The bituminous character of the carbon deposit is probable, especially when the manner of formation is compared with that of natural asphalt. This is clear in view of the similiarity of its derivation from petroleum hydrocarbons by polymerization, free oxygen acting as a catalyst of the change. Yet the amount of oxygen absorbed is too small to explain the formation of the asphalt through oxidation. Waters3 found a slight increase in acidity due to oxygen addition during the carbonization of lubricating oils. Chemical Reaction Involved in Formation
The exact reaction that takes place in converting the petroleum into a carbon deposit is still an open question. Although not yet established, the reaction will probably run according to the metamorphic course of events: petroleum-naphthenic acids-asphaltogenic acids-asphaltenes-carbenes-carboids (cylinder carbon). Nellensteyn’s4 conclusions that bitumen is formed by the breakdown of carbon chains into very reactive fragments, followed by an oxidizing reaction, which depends, not on the oxygen concentration, but on the velocity with which the chains are broken, are interesting in this connection. Oxidation of an oil a t 200” to 275” C. results in the formation of formic and acetic acids, water, carbon monoxide, and dioxide; a t lower temperatures reactions also occur. Tests of oils showing a high carbonization on oxidation in the laboratory (Waters method) show a low actual carbonization in the engine. On the other hand, oils having a high residue on distillation (Conradson method of carbonization) show a high carbon deposit in the engine.6 Although both factors, the oxidation and the residue distillation (qometiines described as cracking), may be effective in forming the carbon it is seen that the Conradson or residue distillation is of greater importance. Waters’ found that the addition of various substances-iron oxide, alkalies, asphalt-catalyzes the formation of carbon in lubricating oils during the oxidation method. As the Conradson value is more closely connected with carbonization in the engine it was thought to be interesting to determine the effect of these catalysts on the residue distillation method. This was held to be significant since all these catalysts are normally present in an engine. The alkali is present from road dust, the iron oxide from the engine head, and the asphalt from the carbon formation. However, tests on a good grade of lubricating oil widely used for automobile engines, using the improved Conradson apparatus, showed that additions of iron oxide, alkali, “carbon,” and powdered and suspended metallic iron dust had no effect on the amount of carbon formed. Factors Governing Deposition
The task of determining the actual factors influencing carbon formation and their valuation has been greatly complicated by their number and the difficulty of controlling them. 4
Combustion Engines,”
THEC H E X I C A L
5 6
7
Chem Weekbl , 11, 42 (1924) Am Soc. Testing Materials, Standards, 1918, p 620 Parish, J A m Soc Naval Eng ,31, 45 (1920) TITSJ O U R N A L , 3, 812 (1911)
732
INDUSTRIAL AND ESGIiYEERIiVG CHEMISTRY
The Bureau of Standards, after extensive investigation has stated the difficulty of obtaining definite information on carbon formation due to the difficulty of governing or evaluating factors other than the quality of the fuel.8 The Bureau of Mines,g during a recent series of controlled tests found “the grams of carbon deposited per gallon of fuel values to be irregular, in fact, useless for drawing any definite conclusions.” From the careful work carried on by these observers, it is
plain that no ordinary difficulty exists in drawing comparative conclusions as to the relative carbon formation of fuel and oils. The present writers have found that the irregularity of results appears to reside in the inability to maintain the same conditions. Even with the utmost care no two runs, made apparently under identical conditions, show comparable results in carbon deposits. By identical conditions is meant carefully controlled laboratory tests using an electric dynamometer, maintaining constant: load, ratio of air to fuel, spark setting, speed, temperature water cooling, temperature of oil, viscosity of oil, fuel consumption, intake manifold pressure, checking exhaust gas by analysis, and reading barometer and humidity.
T’ol. 17, S o . 7
t,rucks and represent’sconditions found in average use. Specifications were as follows: four cylinder, “L” head, cylinder cast en bloc, water-cooled, circulation by pump, poppet valves, conical seat; full force feed lubrication by gear pump. DIMENSIONS Bore, 9.523 cm. (33/a inches) Stroke 12.70 cm. (5 inches). Displakement volume 3622 cc. (221 cubic inches) Clearance volume peicylinder, 334.4 cc. (20.4 cubic inches) Compression ratio, 3.7 Rosch ignition system Stromberg type M - l carburetor with 2.222 cm. (7/s inch) Venturi and No. 65 bleeder, adjustable main and idle Jets Compression Measurements Cylinder 1 2 3 4 6.132 6.132 6.096 6.027 Kg. per sq. cm. 87.2 86.7 87.2 Pounds per square inch absolute 85.7
Power and economy test indicated maximum power a t a fuel consumption of 7.076 kg. (15.6 pounds) gasoline per hour. The dynamometer was selected with a load characteristic to absorb the power output of the engine a t all speeds. The Sprague Electric Dynamometer TLX-24 satisfied the requirements. d cast iron electric resistance was used to absorb the power output of the dynamometer. The fuel supply system was installed so that the rate of fuel flow could be measured by weight with a tenth second stop watch. Cooling of the engine was maintained with water by running the outflow into a 378.5-liter (100-gallon) steel drum and taking the inlet water from the bottom. This water temperature could be controlled by the addition of either steam or water. The temperature of the oil was maintained constant by means of a copper coil in the crank case. Longdistance type thermometers measured the temperature of the inlet and outlet water and oil. A recording vacuum gage gave
Road Tests
Initial tests to determine factors affecting the deposition of carbon in the engine were made under conditions approaching those obtained in practice-that is, four new automobiles of the same make were run over the same route with identical carburetor settings, fuels, and oils, for a considerable length of time. Even the drivers were rotated to eliminate the personal factor. It was expected that these conditions would furnish comparable data as to the amounts of carbon formed in the engine. However, after running these cars a total of 19,310 km. (12,000 miles), greatly differing quantities of carbon were found. Some cars would be greatly carbonized after a 2413-km. (1500-mile) period and would then partially lose their carbon and then regain it in an irregular fashion. Dynamometer Equipment
It was then decided that these cars did not represent equal loads or sufficiently accurate speeds. A test motor of a satisfactory design was selected for its ruggedness and dependability. A Continental Type 5-4 engine was obtained, as this has the “L” type head without overhead valves and a suitable lubricating system. This type is widely used in J . SOC.AufomofiveEng , 16, 472 (1924). Fieldner and Jones, Bur I l t n e s , Repts of lnoestrgalrons 2617 (August, 1923). 8 0
Time Hours
a manifold pressure reading in inches of mercury. The speed was read with an electric tachometer and checked with a revolution counter. The viscosity of the oil was determined by a standard Saybolt viscometer. The viscosity of the oil throughout the tests was held approximately constant, by adding the required amount of fresh oil from time to time t o bring the whole up to a set viscosity (200 Saybolt a t 54.4” C.). Dynamometer Tests
Tests were carried out for different periods: 2, 4, 8, 12, 2-1, 36, and 48 hours. Here again it was found that by using constant load, oil and jacket water temperature, and fuel consumption, the amount of carbon obtained varied widely. Examination of the heads of the cars used during the road tests and of the engine connected to the dynamometer indi-
cated that wbenever oil sppeared uii tdie heads ur on tllc pistons a considerable amount, of carbon was found. Variibtions of the type of oil, type of fuel, temperature of cooling jacket, temperature of tbe sir, as ivdl as other fact.ors: were found to have an ciYect. Tlie rxsults of tlie dynarnonieter tests are sliowii on tlie charts (Series I, 11, and III), giving the amounts of carbon on the cylinder heads plotted against the time of running. Conditions of test vere the same on all tlie charts with the exception only of the precautions noted later in the case of Series IV. The irregularities of the amount of carbon deponition under constant fuel consumption, load, temperature, and mixture are plainly seen. The peaks and the rising of the curves of carbon were coincident with the observation of deposition of oil on the head in the engine. Howe>:er, the amount of oil was in no way extraordinar?-, rrpresenting only average conditions in an engine.
sity) of the oil determine tlie character of ical properties of the oil also determine to some exteiit how mueli carbon is formed after it has gained entrance to tlie combustion chamber. The controversy of asphaltic--base oils versus paraffin-base oil is still undecided in the absence of quantitative data. Some oils have a greater teridency to deposit carlioii than others. Summing up, the minditions affecting mink-case dilution which in turn may be read as an index of carbonization are as follows: (I) average temperature of cyliiider walls or indirectly, circulating water temperature; @) teinperature of air; (3) load of engine; (4) carburetor mixture (fuel air
Connection of Carbon Deposits with Crank-Case Dilution
The problem of carbon deposition thus appears to be intimately connected with the amount oi oil projec,ted into the combustion chamber, and this, in turn, is linked with the iactors affect.ing crank-case dilution. Thus, conditions causing extensive crank-case dilution nniat a t tlie same time cause spraying of oil into the combustion chamber by the breaking of the oil seal of the piston rings, tlius bringing about increased deposition of carbon. Under condit.iom Eavoring crank-case dilution or breaking of the oil seal, the piston carries down gasoline into the craiik rase, and a t the next stroke takes up oil into the combustion chamber where it is sprayed on tlie walls. Introduction of oil into the combustion chamber is especially marked duriiig the suction stroke, when vacua of from 635 t,o 130 mm. (5 to 25 inches) of mercury suck the oil from tlre cylinder walls into tlie crank case. Crank-case dilution bas rrcently been thoroughly investigated.'@ Once realizing that it is the amount of oil projected into the combustion chamber tliat is the important factor, t,lien the other factors assume an inipirrtanoe reli~tiva to tlreir influence on the first. For instance, factors i~n~rrec,ted with variations of the volatility of tlie fucl-as ( k u l a t i n g water temperature, carburetor mixtore.~--determiiie lion mnch unvaporized gasoline changes the oil seal on tlie pist,ons. It
Time
Hours
ratio); ( 5 ) t.ypc of fuel (especially in rrgard t o volatility); (0) type of lubricatiiig oil. lVit,litbis theory in view it was believed that if the amount of lubrieating oil iiitroduced into tlie combustion chamber could be controlled the amount of carbon deposibion might also be controlled. Accordingly, tests vere made running the engine under conditions favoring practically no crankcase dilution. This condition, however in practice. 1':xtreme care was taken the cylinder tightly, using specially and placing proper drainage holes in the piston, in order that no leakage wuiild take place irr the engine. Tests were made for a. period of 48 hours, wit.11threequarter load a t 1200 r. p. in., approximately equivalent to 24 miles
Figure l--Cylinder Heads w i t h Carhon Deposita
is this oil seal wbich prevents the introduction of large ainounts ai oil from tlie crank case. In addition, factors affecting the chemical and physical
per hour; 82.2" C . (180' F.) water jacket temperature; using a high-grade luhricating oil; carburetor setting rich, 20 pounds of fuel per hour fuel consumption; average exhaust
I S D C S T R I d L .4SD ESGI-ISEERISG CHEMISTRY
July. 1925
tion, sinal1 deposition, higher temperature; (1)erosion and flaking, diminishing depoqit. higher temperature. Soot, pitch, coke. graphite are descriptive only physically and miid not be taken t o indicate chemical characteristics. This cyclic change can be explained as due to a change in the state of the carbon due to a rising temperature. Since the carbon has a heat conductive power only one fiftieth that of iron, it can be seen that the greater the formation the higher the temperature. This in turn changes the soft deposit into the harder, graphitic type. This material erodes and flakes off, exposing clean surfaces and produces bettter heat transference through the head of the engine. The interesting photograph of a badly carbonized engine after running under full throttle showing diminished deposits of carbon iq a n example of this type of action12(Figures 1 and 2 ) . Summary
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Acknowledgment
The authors wish to take this occasion of acknowledging their gratitude to those who have advised and helped them in this undertaking. In particular they would like to express their indebtedness to Kaldemar T*ernet, director of research of the Boyce Cf; S'eeder Company, and Samuel Isermann. president of The Chemical Company of America and Van Dyke & Company, for their valuable advice, help, and cooperation. They also acknowledge the help given by Donald Brooks, formerly of the U.S.Bureau of Standards, a t Washington, D. C., for his management of the dynamometer laboratory a t Springfield, S.J. Additional References
The major factors of carbon deposition in an automobile engine are: (1) The amount of lubricating oil projected into the combustion chamber. This is by far the greatest factor. ( 2 ) The kind of oil used. (3) The temperature of the combustion chamber. (4) The extent of time the preceding factors have been in effect. A 3tudy of the effect of a large number of chemical wb1
stances on carbon is being carried out a t the laboratory and the results will be published in the near future.
4ulomofree i l ~ g 15,499 (1924)
4 t k i n s , Aulomobile Eng., 1919,426. Bryan, J . A m . SOL.N a v a l Eng., 27, E3 (1916). Garner, J . Insl. Pelroleurn Tech., 7, 98 (1921). Holde and Mueller, "The Examination of Ilydrocarhon Oils and of Saponifiable F a t s and Waxes," 2nd ed., 1932. John XXey & Sons, I n c . James, A m . Petroleum Inst., Bull. 4, N o . 73, p. 132 (December31, 1923). Robinson, Atloiific Lubricator, 3, 8 (February, 1920). Spielmann, "Bituminous Substances. Scientific Progress of Practical Iniportance during the Ldst Fifteen Years," 1925. D. Van Nostrand 8: Company.
Pressure-Temperature Charts for Organic Vapors' By D. S. Davis
T
15'0 years ago, Cox2 brought forth R very convenient method of plotting vapor pressure data of hydrocarbons of the paraffin series. The present paper will show that this method may be applied with equal success to alcohols, organic acids, and the series of halogen-substituted benzenes. Cox plotted pressures in pounds per square inch on a logarithmic scale as abscijsas against the temperature in degrees Fahrenheit on the ordinate scale, which was not a uniform scale. He obtained the temperature scale bv dr:irvina a
VAPOR
Figure 1-Alcohols.
PRC-JSURf
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A
straight line a t a convenient angle to the horizontal which was to represent the curve for steam. Then from a steam table he marked points on the line a t preswres corresponding to temperatures from 30" to 700" F. and drew through them horizontal lines to represent the temperature ordinates. Cox found that when the vapor pressure data of the paraffin hydrocarbons were plotted on paper constructed in this way, all the curves mere straight lines and, further, that they intersected in a common point. Thus after the curves for anv two of the series are drawn and the
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Homologous Series CnH?. -
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F i g u r e 2-Organic
Acids.
Homologous Series CnHmOz
