Effects of Bore-to-Stroke Ratio on the Efficiency and Knock Characteristics in a Single-cylinder GDI Engine 2019-01-1138

As global regulations on fuel economy and CO2 emission are getting stringent, development of high-efficiency SI engine is now more urgent than ever before. Along with advanced techniques on friction reduction, automakers endeavor to decrease bore-to-stroke ratio (B/S) to reduce the heat loss and enhance the fuel conversion efficiency. In this study, the effect of B/S was investigated in aspects of efficiency and knock characteristics using a single-cylinder 0.5L LIVC (late intake valve closing) GDI engine. Three B/S ratio (0.68, 0.83 and 1.00) were tested under the same mechanical compression ratio of 12:1. The head tumble ratio was maintained at the same level to solely investigate the effect of geometrical change of B/S. In addition, as the engine was equipped with dual CVVT system, the optimization on valve timings was conducted to fully exploit the potential of each geometry. As the result, lower bore-to-stroke ratio apparently showed the higher knock resistance not only because of faster burn speed but also due to advantage of low unburned gas temperature under the same valve timing in high load condition. However, it was shown that advantage on shorter burn duration did not substantially increase as B/S was additionally decreased after 0.83. In overall operating condition, the best efficiency with an optimized valve timing was achieved in B/S 0.83 as the increased heat transfer loss was observed paradoxically in lower B/S.