A Micrographic Study of Deposit Formation Processes in a Combustion Chamber

Growing concern about the impact of combustion chamber deposits (CCD) on engine performance and exhaust emissions has renewed interest in understanding the deposit formation process in a combustion chamber. To provide a true picture of the deposit formation process, an extensive micrographic study of the deposits in a single cylinder engine has been conducted. Four retrievable deposit sampling probes were used. The sampling period for the deposits varied from 15 minutes to 20 hours to show how the deposits evolved with time. The coolant temperature was changed from 50°C to 95°C to observe the effect of surface temperature on deposit morphology. Impacts of deposit control additives on the deposit distribution and deposit morphology were also investigated.

Deposits formed in different parts of the combustion chamber differed significantly in their morphology. The differences occur mainly because of variations in surface temperature. In the case of additive-derived deposits, proximity to the intake valve also had a strong impact. There is strong evidence that the initial deposits on the chamber surface are liquid in form. The precursor for fuel-derived deposits is partially oxidized fuel in the gaseous phase. Yet, the precursor for additive-derived deposits is the additive itself in the liquid phase.