INFLUENCE OF TEMPERATURE, FUEL AND OIL ON CARBON DEPOSITION 260031

High operating-temperature, the use of the more volatile fuels and a lean air-fuel mixture and the use of lubricating oils of relatively high volatility which contain little carbon-residue all tend to reduce the deposition of carbon in an internal-combustion engine, as indicated by the experimental study reported in this paper.

Believing that heat, fuel and oil are the most important factors influencing carbon formation and deposition, the experimenters adopted the method controlling closely the other conditions of operation of a specially designed single-cylinder test-engine and varying the operating temperature and the fuel and oil, allowing the carbon deposit to build up in the normal way during the test periods of 15 and 36-hr.

The test engine and control apparatus and the test procedure are described. Room temperature was held constant throughout each run, oil and cylinder-head temperatures were controlled to within 6 deg. fahr., the air-intake temperature was kept constant within 1 deg. fahr., the engine speed was measured in revolutions per minute, and the rate of fuel intake was held constant at a constant throttle-setting, giving a uniform air-fuel ratio as determined by exhaust-gas analysis. As confirmation of the accuracy of control, it was observed that the power output was constant within an average of 1 per cent.

After conclusion of a run, the engine was taken down and the carbon deposits from the piston and from the cylinder-head were recovered separately and weighed. Seven different liquid fuels and six different lubricating oils were used in the various runs.

From data obtained it appears that the carbon values are fairly constant until the head temperature rises somewhat above 400 deg. fahr., after which there is a drop in them as the heat is increased. Deposits formed at the lower temperatures are much more asphaltic, softer and less adherent than those formed at the higher temperatures. With evaporative or steam-cooling, the head temperature was as low as or lower than the lowest with water-cooling, and carbon deposition was almost at the same rate as with water-cooling at the same temperature.

No marked change in quantity of carbon is noted with different fuels through the series of commercial gasolines, but excessively high carbon-values were obtained with the heavy cleaners' naphtha and kerosene, due, it is believed, to poor carburetion and incomplete oxidation and to asphaltization of sprayed liquid. Benzol blend gave slightly more deposit than motor fuel, but the deposit was softer and more soot-like. Natural gas gave only 5 per cent less deposit than commercial gasoline, which is interpreted to mean that in a 12-to-1 mixture a good average grade of gasoline plays a very small part in the deposition of carbon.

It runs simulating ordinary cool-weather operation of automobiles, with a 10-to-1 ratio of air and ordinary automobile gasoline, lubricating oils distilled from Pennsylvania and Mid-Continent crudes gave considerably higher carbon-deposits than oils derived from Gulf Coastal crudes. The different oils suffered dilution in the crankcase to the same extent with the same fuel and operating conditions.

Under conditions simulating summer operation, with a mixture-ratio of 12 to 1, the carbon deposit with Gulf Coastal oil was only half of that with Pennsylvania oil, while that from Mid-Continent oil was midway between. Extraction figures from the Gulf oils show that they contain less oily matter than the less-volatile oils. Carbon from them was, in general, dry, powdery and friable, while that from Pennsylvania oils was very hard and adherent over the hotter areas and sticky and asphaltic over the cooler areas of the combustion-chamber. Popular belief that there is a connection between so-called heat-resisting properties of an oil and its desirability as a lubricant is probably erroneous; the oil that will leave the least residue upon evaporation from the metal surfaces will give the least trouble from carbon deposition.

Suggestions offered for avoidance of carbon trouble are that (a) oil consumption be kept at the minimum consistent with a fair margin of safety, (b) excessive and irregular cooling be avoided, (c) the fuel-air mixture be kept on the lean side of smooth operation, and (d) the oil that is the more volatile among those of the same viscosity probably will be the more satisfactory.