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The maximum thermal efficiency of gasoline engine has been improving and recently the maximum of 40% has been achieved. In this study, the potential of further improvement on engine thermal efficiency over 40% was investigated. The effects of engine parameters on the engine thermal efficiency were evaluated while the optimization of parameters was implemented. Parameters tested in this study were compression ratio, tumble ratio, twin spark configuration, EGR rate, In/Ex cam shaft duration and component friction. Effects of each parameter on fuel consumption reduction were discussed with experimental results. For the engine optimization, compression ratio was found to be 14, at which the best BSFC without knock and combustion phasing retardation near sweet spot area was showed. Highly diluted combustion was applied with high EGR rate up to 35% for the knock mitigation.

Lots of design concepts for intake ports of SI engines have been introduced due to their significant effects on in-cylinder flow pattern and volumetric efficiency. However, those efforts have been made usually based on port experts' experience, which means that systematic analyses are not well established. As an attempt for improving the situation, an investigation has been performed to analyze experimental results and correlate them with port design parameters in this study. Twenty-eight SI engines, which cover a wide range of displacement volume from 1,000cc to 4,500cc, were tested to evaluate port Cf (Flow Coefficient). Except two small engines, these are four-valve engines with a pent-roof combustion chamber. The test results were analyzed with different port shapes, which were defined by several port design parameters. The parameters were chosen to be easily measured in the real intake ports and then re-defined for further analyses by appropriate non-dimensional arrangements.