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The deterioration of combustion stability as lean operating limits and misfire conditions are approached has been investigated experimentally. The study has been carried out on spark ignition engines with port fuel injection and four-valves-per-cylinder. Test conditions cover fully-warm and cold operation, and ranges of air/fuel ratio, exhaust gas recirculation rates and spark timing. An approximate method of calculating gas/fuel ratio is described. This is used to show that combustion stability, characterised by the coefficient of variation of i.m.e.p., is a function of calculated gas/fuel ratio and spark timing until near to the limit of stability. A rapid deterioration in stability and the onset of weak, partial burning occurs at a gas/fuel ratio between 24:1 and 26:1 under fully-warm operating conditions, and around one gas/fuel ratio lower under cold operating conditions.

A method for improving three-way catalyst (TWC) performance by superimposing a low frequency lean air-to-fuel ratio (AFR)bias perturbation onto the standard AFR oscillations is described. This observation of Catalyst Regeneration (CatRegen) has been attributed to a reactivation of poisoned precious metal sites on the catalyst surface. Preliminary tests under steady-state conditions show that there is a gradual reduction in TWC activity for NOx after a lean-rich transition, suggesting a temporary poisoning of the active precious metal sites on the TWC under rich conditions. This deactivation can be prevented by periodically exposing the catalyst to lean exhaust gas; which has led to the development of the CatRegen system.