Quantitative analysis of heat transfer characteristics and advantages in opposed-piston 2-stroke diesel engines

Abstract

Opposed-piston 2-stroke diesel engines (OP2S) have lower combustion chamber surface-area-to-volume ratio, resulting in lower transfer loss but also excessive surface temperature. Investigation of combustion parameters on heat transfer, which provide a chance in improving combustion efficiency and solving surface overheating issue in OP2S at the same time, is yet to be undertaken. In this paper CFD simulation by CONVERGE software has been carried out to study heat transfer advantages in OP2S. Effect of initial swirl ratio and injection phase on combustion efficiency and heat transfer of combustion chamber surface area is investigated. Results show that OP2S has obtained peak combustion efficiency of 0.97, which is 0.02 higher than that of conventional diesel engine (4S). Also, combustion efficiency loss due to undesirable swirl ratio is only 20 % of 4S. Lowest heat transfer ratio achieved by OP2S is 0.045 of fuel energy, which is 34 % lower than that of 4s. However, heat transfer ratio of cylinder wall in OP2S is at least twice of that in 4S, and increase significantly with swirl. Unable to utilize squish, OP2S would suffer from at most 0.1 lower combustion efficiency compared to 4S when injection is retarded to TDC. Heat transfer ratio in OP2S is less sensitive to variation of injection phase, and stays at least 0.01 lower than 4S for all injection phase studied. In conclusion, OP2S achieved higher combustion efficiency while significantly cutting heat transfer loss compared to 4S at optimized swirl ratio and injection phase, proving its potential as a high-efficiency power source. Thermal load issue on cylinder wall in OP2S can be fixed by lowering swirl ratio. Reducing heat transfer loss in OP2S with delayed injection is undesirable, however, since combustion efficiency would suffer.