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Analysis of pressure traces from within the cylinder of IC engines is a long established technique, particularly in automotive applications. This approach allows burn rate data to be calculated from the shape of the pressure traces, providing direct combustion information to development engineers. With the proliferation of high-powered and low-cost computers, recording of pressure traces and analysis to give burn rates are now becoming standard measurements. However, this is still a complex technique, which is very open to error and prone to misinterpretation of results. This is particularly relevant for two-stroke engines where cyclic variations can be high and traces can be difficult to analyze. This paper considers the standard techniques available for pressure trace analysis, highlighting the areas for problems and outlining good practice for reliable and accurate measurement.

The two-stroke engine, by its nature is very dependent on the unsteady gas dynamics within an exhaust system. This is demonstrated by the tuning effects on two-stroke engines, which have been well documented. In consideration of current emissions legislation, a two-stroke engine can be fitted with a catalytic converter for the outboard, utility or automotive markets. The catalytic substrate represents a major obstruction to the flow of exhaust gas, which hinders the progression of the main exhausted pulse, and in turn effects the scavenging of the cylinder and ultimately the performance of the engine. Within this investigation, a 400 cc direct injection two-stroke engine was used with various catalysts positioned at different distances from the exhaust manifold. Comparison tests were performed between a fully lit off catalyst and a non-operational bare substrate.

This paper details the investigation of the properties of inlet gases and shows how they affect the flow patterns immediately in front of the catalyst and the subsequent loss of efficiency. A thorough analysis of the flow distribution at the inlet of the catalyst enabled the effective catalyst diameter to be calculated. Subsequent calculations were then carried out to determine the loss of catalyst function through flow maldistribution. Experimental work involved flowing engine proportioned amounts of air through canisters of a fixed geometric profile containing a catalyst. Inlet cones of angles 10°, 15° and 45° were flowed to estimate the effect of the cone design on the velocity distributions at the face of the catalyst. Simple geometric profiles were investigated to allow a thorough understanding of the mechanism of flow to be comprehended and its affect on catalyst conversion to be analysed.