High-load Expansion by Varying Effective Compression Ratio Using Variable Valve Duration System under Dual-fuel Premixed Compression Ignition

In this study, a high-load expansion strategy was investigated by varying the effective compression ratio (CR) using the variable valve duration (VVD) method during combustion in a gasoline/diesel dual-fuel premixed compression ignition engine. The effective CR was varied from 15.0 to 10.6 via the retardation of the intake valve closing timing (IVC) at 1,500 rpm. With respect to optimization, the diesel injection timing, fuel ratio, and exhaust gas recirculation (EGR) rate were adjusted for each optimized condition. The limitations were the maximum pressure rise rate (below 10 bar/deg) and in-cylinder pressure (below 150 bar) with NOx < 40 ppm and soot < 0.2 FSN. The results emphasized that when the effective CR was 12.7, the maximum load increased by 10.5 % (gIMEP from 13.3 to 14.7 bar) compared with that at a CR of 15.0. Although the decrease in CR resulted in a gross thermal efficiency loss, the difference was negligible (from 45.9 to 45.5 %). The main reason for high-load expansion was the increase in intake air volume under the same intake pressure upon lowering the CR and the margin for the maximum in-cylinder pressure. From this result, it can be concluded that the Miller cycle is useful for the expansion of high load ranges in premixed combustion systems.