Effects of high- and low-pressure EGR on performance and emissions of natural gas engines

Heavy-duty gas engines show significant advantages in reducing carbon emissions. However, heavy-duty gas engines still suffer knocking issues when operating under high-load conditions. This paper investigated the role of combined high- and low-pressure EGR in improving stoichiometric combustion in a six-cylinder 14.5 L heavy-duty gas engine. The cylinder pressure-derived combustion characteristics and intake-exhaust boundary parameters were performed to analyze the gas consumption tendency at various EGR rates and proportions. The results show that under low-speed high-load conditions, the EGR rate is enhanced significantly through low-pressure EGR circuit. The thermal efficiency is significantly improved by 3.1% from the original 41.97% with an increment of EGR rate. The advantages of a tumble combustion system at high EGR rates are mainly reflected in the reduction of cycle variation and ignition delay. At the same EGR rate, the proportion of high- and low-pressure EGR has little effect on performance because of the increased pumping loss with the higher low-pressure EGR ratio. Under the rated power condition, the results show that only very low low-pressure EGR can be added based on the high-pressure EGR because of the engine misfire. The thermal efficiency is improved by 0.6% compared to the original calibration of 40.98%. Therefore, the combined high- and low-pressure EGR has the potential to mitigate the knocking of stoichiometric combustion in heavy-duty gas engines.