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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.

Tests were performed in an actual engine with swirl and without to understand charge swirl effect for an 1.5l, 4-cyl and 4-stroke SI engine. Swirling inlet flow resulted in the improvement of the engine performance at engine speed of up to 4,000rpm, however, it was reversed at higher engine speed, compared with that of ordinary non-swirling case. Additionally, a single shot visualized engine driven by a rapid compression expansion machine was built separately to inspect the swirl effect on combustion process. In the visualization study, stoichiometric LPG -air mixture was used at three different levels of swirl strength, where the equivalent engine speed was set at 1500, 2500 and 3500 rpm and taken by shadowgraph technique with a high speed camera. A visualized tests supported the combustion rate increasing phenomena excited by swirl by observations on equivalent flame radii and, also, by energy release fractions and cyclic variations.