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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.

The effect of fuel anti-knock quality on power and acceleration performance is studied in twenty-three European and Japanese cars equipped with knock sensors. The anti-knock quality of a fuel in a given car and operating condition is defined by its octane index OI = RON - KS where K is a constant for that condition and S is the sensitivity, (RON-MON), and RON and MON are the Research and Motor Octane numbers respectively. The higher the octane index, the better the antiknock quality of the fuel and the better the power and acceleration performance. K is often assumed to be 0.5 so that OI =(RON+MON)/2. However, it is found that in most cases considered here, K is negative so that for a given RON, a fuel with higher sensitivity (lower MON) has better anti-knock quality and better performance. Even when K is not negative it has a small (< 0.2) positive value so that MON contributes much less to fuel anti-knock quality than generally assumed.