In this paper, a common rail diesel research engine was converted to operate in dual-fuel mode and extensive experiments were conducted to investigate the effects of natural gas injection timing on the combustion and emissions performance under different pilot injection pressure and timing at low load conditions. The presented results include the cylinder pressure, heat release rate (HRR), ignition delay, combustion duration and brake thermal efficiency, as well as CO, HC and NOx emissions at different natural gas injection timing under pilot injection pressure (46 and 72MPa) and pilot injection timing (−8° and −17° ATDC) operation conditions at low load (BMEP=0.24MPa). The results indicated that retarded natural gas injection timing can achieve a stratified-like air–fuel mixture in cylinder under the different pilot injection conditions, which provided a method to improve the combustion performance and exhaust emissions at low load. Moreover, under higher pilot injection pressure (72MPa) conditions, better combustion performance, such as shorter ignition delay and combustion duration, higher brake thermal efficiency, were achieved; however, the exhaust emissions significantly increased compared with those under lower pilot injection pressure (46MPa). On the other hand, under the advanced pilot injection timing (−17° ATDC), the combustion performance was radically better, THC and CO emissions were lower but the NOx emissions were significantly higher compared with those under the regular pilot injection timing (−8° ATDC). This is attributed to faster flame propagation speed, better combustion phasing and higher volumetric efficiency. Consequently, employing appropriate natural gas injection timing accompanied with reasonable pilot injection parameters is critical to further improve combustion performance and exhaust emissions of a dual-fuel engine at low loads.
