Methods adopted and results obtained in an investigation of the formation of carbon in an internal-combustion engine and its influence on the performance of the engine, as carried out by a series of tests, are described by the author. It was found that special methods of controlling the character and quantity of lubricating oil that reached the combustion-chamber were necessary to obtain concordant results on successive tests.
Four factors are believed to control the formation of carbon in an engine, namely, (a) quantity of oil that reaches the combustion-chamber, (b) quality of the oil, (c) rate of breakdown of the oil in the chamber, and (d) time. Secondary factors are important only insofar as they influence these primary factors.
Certain substances are shown to accelerate or retard the rate of formation of carbon. Carbon is shown to increase materially the indicated thermal efficiency of an engine operating under conditions such that no detonation or preignition occurs. The increase of efficiency is found to be proportional to the weight of the carbon deposit.