Combustion Chamber Deposit Flaking and Startability Problems in Three Different Engines

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 R_w, 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. For individual cylinders, there is a sharp transition to a high leakage rate and failure to start if (T1-T2) exceeds about 55 microns or if (T2/T1) decreases below 0.5. A simple probabilistic model has been developed to assess the impact of flaking. Comparison of the results of this model with experimental results suggests that there is a very high probability of loose flakes getting trapped on the valve seat and the probability of the consequent failure to start increases very sharply above a threshold amount of flakes. Hence the best approach to ensure against failure to start is to make sure that deposit flaking is minimised.