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A field problem associated with flakes of combustion chamber deposits getting trapped on the exhaust valve seat and causing starting problems has appeared recently. Four fuels have been tested in three different car models using a deposit flaking road test procedure. For each piston top, flaking can be characterised using T1 and T2, the mean deposit thickness on the piston crown before and after flaking respectively. A new measure of deposit flaking, ΔT, the mean of (T1-T2) averaged over all cylinders has been introduced and its variance established for the standard test using one of the models. ΔT quantifies the actual amount of deposits that have flaked and is likely to be a more relevant indicator of flaking for startability problems than Rw, the mean of the ratio of T2 to T1, used in previous work. Deposit flaking is directly related to an increase in valve leakage rates and startability problems.

There is increasing concern that small flakes of combustion chamber deposits (CCD) can break lose and get trapped between the exhaust valve and the seat resulting in difficulties in starting, rough running and increase in hydrocarbon emissions. In this paper we describe experimental observations which might explain how this flaking of CCD occurs and the factors that might be important in the phenomenon. The experiments include thirty one engine tests as well as tests done in a laboratory rig and show that some CCD flake when they are exposed to water; indeed water is far more effective in bringing this about than gasoline or other organic solvents. The hydrophilicity of the deposit surface which determines the penetration of water and the inherent susceptibility of the relevant deposit layer to inter-act with water are both important. Consequently there are large differences between deposits produced by different fuels and additives in terms of their susceptibility to flake.