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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.

A highly efficient two stroke engine was designed for a small vehicle to race in a fuel economy competition. The engine must power a lightweight prototype vehicle with a total mass of 80 kg on a closed circuit. This paper describes the approach taken to the design of this engine. An uniflow two stroke engine assisted by crankcase supercharging was chosen to provide the best trapping efficiency and reduce fresh mixture short circuiting. This design allows an increased transfer port area and improved swirl control inside the cylinder. The engine has two exhaust valves at the head, various cylinder transfer ports and a crankcase reed valve. To further reduce brake specific fuel consumption a fully digital electronic fuel injection system and two spark plug ignition are used. The injection is a semi-direct system, spraying the fuel into the cylinder at approximately two thirds of its height. The first prototype uses no air assistance for the injection.

When the exhaust valve of a conventional spark ignition engine opens at the end of the expansion stroke, a large quantity of high pressure exhaust gas is freed to the atmosphere, without using its availability. An engine that could use this lost energy should have a better efficiency. The equations for an over-expanded cycle (Miller cycle) are developed in this paper, together with equations for the Otto cycle, diesel cycle and dual cycle, all at part load, so they can be compared. Furthermore, indicated cycle thermodynamical comparisons of a S.I. engine at part load (Otto cycle at half load), a S.I. engine at WOT (with half displacement) and two over-expanded S.I. engines (with different compression strokes) are examined and compared, with the aim of extending the referred theoretical cycle comparisons.