Effects of Bore-to-Stroke Ratio on the Efficiency and Knock Characteristics in a Single-Cylinder GDI Engine

As a result of stringent global regulations on fuel economy and CO2 emissions, the development of high-efficiency SI engines is more urgent now than ever before. Along with advanced techniques in friction reduction, many researchers endeavor to decrease the B/S (bore-to-stroke) ratio from 1.0 (square) to a certain value, which is expected to reduce the heat loss and enhance the burning rate of SI engines. In this study, the effects of B/S ratios were investigated in aspects of efficiency and knock characteristics using a single-cylinder LIVC (late intake valve closing) GDI (gasoline direct injection) engine. Three B/S ratios (0.68, 0.83 and 1.00) were tested under the same mechanical compression ratio of 12:1 and the same displacement volume of 0.5 L. The head tumble ratio was maintained at the same level to solely investigate the effects of geometrical changes caused by variations in the B/S ratio. In addition, because the engine was equipped with a dual CVVT (continuous variable valve timing) system, the valve timings were optimized to fully exploit the potential of each geometry. As a result, lower bore-to-stroke ratios apparently exhibited higher knock resistances not only because of faster burn speeds but also because of the advantage of low unburned gas temperatures under the same valve timing in high-load conditions. However, it was shown that the advantage of a shorter burn duration did not substantially increase as the B/S ratio was decreased below 0.83. Among the tested operating conditions, especially in part load condition, the best efficiency under optimized valve timing was achieved at a B/S ratio of 0.83 because of the paradoxical increase in heat transfer loss observed in lower B/S ratios.