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Unburnt hydrocarbon (uHC) emission levels, from three vehicles driving the ECE 15 and US FTP75 test cycles, have been measured on a crankangle timescale. High levels of uHC emissions have been linked to particular drive-cycle features and can be explained by features of the engine management strategy. The data highlight the need to pay close attention to the fuelling of every cycle if large excursions in uHC levels are to be avoided.

A computer model has been developed to investigate ways of improving the control of the mixture strength supplied to a spark ignition engine during transients and steady-state operation. The model comprises four sub-models, which calculate the gas dynamics, the response of the sensors, the fuelling logic (mass flow and speed-density) and the air-fuel ratio of the running engine. Comparisons are made between different fuelling logics and sensor responses for various transient modes (fast and slow accelerations) and for a range of engine operating conditions.