Effect of Hot Exhaust Gas Recirculation on the Combustion Characteristics and Particles Emissions of a Pilot-Ignited Natural Gas Engine

Natural gas has become an attractive alternative for diesel fuel due to its higher octane number, richer reserves and lower price. It has been utilized in compression ignition engines to obtain a higher thermal efficiency compared with spark ignition engines. However, its relatively higher auto-ignition temperature increases the difficulty of compression-ignition based on present hardware devices. One optimal ignition method is that a very small quantity of diesel fuel as the only ignition resource pilot-ignites the lean natural gas-air mixture. This micro diesel pilot-ignited natural gas premixed charge compression ignition (DPING-PCCI) combustion strategy is easy to implement without major hardware modifications, and can significantly reduce the NO_x and particle mass emissions from diesel engines. Although the DPING-PCCI has so many advantages, it suffers from poor engine stability and high ultrafine particles emissions at part loads. In the present paper, the hot exhaust gas recirculation (EGR) strategy was employed in a single-cylinder DPING-PCCI engine at part loads. The aim is to investigate influences of hot EGR on combustion and emissions characteristics of the DPING-PCCI engine, especially ultrafine particles emissions. The results show that, when increasing the hot EGR rate, the ignition delay period is prolonged progressively, however the total combustion duration is shortened gradually. With the increase of hot EGR rate, the shape of particle size distribution curve appears unimodal and the peak of particle number concentration gradually shifts downwards and towards the larger particle size. The total ultrafine particle number concentration was reduced up to 57 percent. While the particle mass concentration is reduced and the coefficient of variation in indicated mean effective pressure (COV_IMEP) decreased 5 percent. Meanwhile, CO, HC and NO_x emissions are decreased simultaneously. The results indicate that the hot EGR has the merits to fulfill lower emissions and improve the engine stability.