THE Conradson carbon-residue test is the generally accepted method for predicting the relative quantities of carbon an oil will deposit in an engine. This belief arises from the fact that, although publication of results of earlier researches in this field have shown that volatility of the oil is a controlling factor, it has been assumed that in all cases volatility is measured by the carbon-residue test. The results of tests conducted by the authors, covering a period of about two years, show that no such general relationship exists when the carbon-forming characteristics of a wide variety of oils are considered. This conclusion is drawn from 50-hr. tests of a large number of commercial lubricating oils in an engine operating under fairly heavy load and at moderate speed.
The authors found that the volatility of the oil is the primary factor in engine carbon-deposition, and a laboratory method was developed for indicating the total volatility of a motor oil. The temperature of the 90-per-cent-distilled point at an absolute pressure represented by 1 mm. of mercury was found to be an excellent carbonization index, because a linear relation was found between this temperature and the quantity of carbon deposited. Other data showing the effect on carbon deposition of oil consumption, load, and length of time of operation are presented.