Study on the influence of combustion chamber geometry on the performance of an opposed-piston two-stroke diesel engine

Abstract

The opposed-piston two-stroke (OP2S) diesel engine is characterized by its high power density, low heat transfer losses, and excellent balance performance. In this study, CFD simulation methods were employed to investigate the effects of various key parameters of the original combustion chamber and different shapes on engine combustion performance. The results indicate that due to the large area of the original combustion chamber, the swirl flow cannot effectively disperse the fuel spray, making it difficult for the side-mounted three-hole injector to ensure coverage across the entire circular combustion chamber area. This leads to issues such as low equivalence ratios on both sides of the cylinder and limited adjustability of piston depth in the original combustion chamber. Adjustments to key geometric parameters did not significantly improve the overall engine performance. To address the shortcomings of the original combustion chamber, two new combustion chamber configurations were designed in this study. The results show that removing the redundant low equivalence ratio regions on both sides of the original combustion chamber and increasing the piston depth can achieve better fuel spray mixing and combustion effects. Although the annular combustion chamber and semi-ellipsoidal combustion chamber did not achieve satisfactory scavenging performance, they provided superior combustion characteristics by enhancing fuel spray mixing, concentrating high-temperature regions in the center of the cylinder, and reducing heat transfer energy losses, thereby effectively improving the indicated mean effective pressure (IMEP). The removal of the ω-shaped structure in the central region of the annular combustion chamber facilitated the generation of in-cylinder tumble flow, thereby improving combustion performance and increasing the combustion rate. Compared to the original combustion chamber, the semi-ellipsoidal combustion chamber achieved a 7.3% increase in peak cylinder pressure and a 15.1% improvement in IMEP.