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Experiments were performed with a 4-stroke, natural gas fueled SI engine to investigate the effects of several parameters on engine performance under lean operating condition. A favorable effect of charge swirl on stable lean burn operation was observed at a conventional compression ratio. There was an optimum EGR rate which gave a substantial reduction in NOx emissions with minor penalties in efficiency and UHC emissions. Marginal improvement was noticed with lean operations in a long spark duration ignition system. The flame jet ignition system displayed noticeable capability in extending the lean limit. In addition, shadowgraph visualization tests were performed for combustion diagnostic purposes.

In order to find the dynamic TDC a microwave sensor was installed on the head and a proximity probe on the wall of 4-cyl SI engines. The dynamic TDC determined by the microwave system exhibited fairly consistent results having less than 0.05 degrees in standard deviation and remaining nearly constant throughout the speed range of 1,000-2,500 rpm. There was 0.1-0.2 degrees difference between the dynamic TDC measured by the microwave sensor and that measured by the proximity probe; the microwave result was believed to be more reliable. The difference seemed to originate from the built-in system errors on the part of proximity apparatus caused by insufficient sensitivity and availability of data points and piston tilting motion. Both flush mounted and spark plug type pressure transducers were used to measure the motoring peak pressure.