A numerical study on the swirl-guided combustion and piston heat transfer characteristics in OP2S diesel engine

The opposed-piston two-stroke (OP2S) engine demonstrates remarkable application potential owing to its high-power density. However, it faces challenges of elevated piston thermal loads due to unique structural configuration. Therefore, reducing the thermal load while maintaining high thermal efficiency is crucial for improving its reliability. In this study, a Converge simulation was conducted to reveal the coupling mechanism between combustion and heat transfer. The results indicate that the prolongation of the post-combustion phase, caused by localized rich-mixture, is a key factor influencing the thermal-work conversion efficiency in OP2S diesel engines. The near-wall flame temperature determines the heat transfer. Reducing the rich-mixture zone in the space position of combustion chamber leads to an increase in thermal efficiency while decreasing the wall heat transfer. After fuel impingement, the momentum component along the wall governs the spatial distribution of rich-mixture. As the outer throat diameter of swirl combustion chamber increases from 31 mm to 35 mm, the tangential momentum of the spray along the inner chamber wall decreases after impacting the ridge, and the proportion of rich-mixture decreases by 1.3 %. Consequently, the post-combustion duration shortens by 6.7 °CA, and the indicated thermal efficiency (ITE) increases by 0.53 %. Meanwhile, the high-temperature flame area decreases, and the cumulative heat transfer (CHT) to the exhaust-side piston is reduced by 2.5 %. However, with a larger 37 mm throat diameter, the proportion of rich-mixture zones near the wall and crevice increases by 9.3 %, the post-combustion duration extends by 37 °CA, the ITE decreases by 4.7 %, and CHT reduces by 4 %.