The relationship between combustion chamber deposits and octane number requirement increase.

Abstract

This paper describes the relationship between combustion chamber deposits obtained from 11 monitor cars and octane number requirement increases (ORI). Before performing the 15, 000 km accumulation, combustion chamber deposits were completely removed, and three selected crankcase oils and two fuels were employed to investigate the behavior of combustion chamber deposit formation. ORI was measured at every 2, 500km accumulation on a chassis dynamometer. After the completion of 15, 000km accumulation, the engines were disassembled and deposits were carefully removed.ORI was dependent on the car used, but it appeared to be low slightly, if at all, affected by crankcase oil composition. Cars whose initial octane requirements were small had the tendency to give rise to larger ORI values.The carbon content of the deposits was found to influence the ORI values, and this would account for the low thermal conductivity and high heat capacity of the carbonaceous materials in the deposits. The thermal conductivity of the deposits formed from unleaded fuels did not differ much from that from leaded fuels, whereas the heat capacity of the former was found to be two to three times greater than that of the latter.A mechanism of deposit formation with unleaded fuel was also mentioned.Evidence for deposit removal by flaking was obtainable from careful investigation of the deposit surfaces and their cross sections by SEM and EPMA. A likely explanation was made for the deposit accumulation resulted from the use of unleaded fuels to attain the dynamics equilibrium where ORI would level off.