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The exhaust emissions of off-road and utility engines have recently come under increasingly thorough scrutiny and are now becoming the subject of federal regulations. While the most straightforward emissions guidelines relate to steady-state engine performance, it is well known that duty cycles of many small engines have a transient content and that its significance can vary strongly from application to application. Hence, it is important to examine how measured emissions change when the transient content of a test cycle is varied, and what kinds of steady-state and transient test cycles might realistically imitate operational conditions. These questions have been addressed in an experimental study in which several small two- and four-stroke engines have been tested under steady state and transient cycles. The same tests were also carried out when these engines had been adjusted to operate at leaner air-fuel ratios, as might be required by forthcoming regulations.

A comprehensive cycle analysis has been developed for four-stroke spark-ignited engines from which the indicated performance of a single cylinder engine was computed with a reasonable degree of accuracy. The step-wise cycle calculations were made using a digital computer. This analysis took into account mixture composition, dissociation, combustion chamber shape (including spark plug location), flame propagation, heat transfer, piston motion, engine speed, spark advance, manifold pressure and temperature, and exhaust pressure. A correlation between the calculated and experimental performance is reported for one engine at a particular operating point. The calculated pressure-time diagram was in good agreement with the experimental one in many respects. The calculated peak pressure was 10 per cent lower and the thermal efficiency 0.8 per cent higher than the measured values. Thus this calculational procedure represents a significant improvement over constant volume cycle approximations.