Effects of pilot injection timing on the combustion and emissions characteristics in a diesel engine using biodiesel-CNG dual fuel

Combustion and emissions characteristics of a compression ignition engine with a dual fuel (biodiesel-CNG) combustion system were investigated in this study. This experiment utilized a biodiesel pilot injection to ignite a main charge of compressed natural gas (CNG). The pilot injection pressure was maintained at approximately 120 MPa while the pilot injection timing was varied across the range 11-23 crank angle degrees (CAD) before top-dead-center (BTDC) to investigate the characteristics of engine performance and exhaust emissions in a single cylinder diesel engine. Results show that performance can be optimized for biodiesel-CNG dual fuel combustion (DFC) by advancing the pilot injection timing for low loads and delaying the injection timing for high loads. However, overall performance of diesel single fuel combustion (SFC) still exceeds that of biodiesel-CNG DFC. Slight cycle-to-cycle variations are observed when dual fuel is used, but remains less than 1.3% at all conditions. The combustion of biodiesel-CNG begins at a later CAD compared to that of diesel SFC due to the increase of ignition delay of the pilot fuel. The ignition delay in DFC is 1.6-4.4 CAD longer than that of the diesel SFC. Ignition delays are reduced with the increased engine load. BSEC of biodiesel-CNG DFC improves with advanced pilot injection timing at low load and with delayed pilot injection timing at full load. Smoke is decreased and NOx is increased with advanced pilot injection timing in the biodiesel-CNG DFC. Compared to the diesel SFC, however, smoke emissions are significantly reduced over the range of operating conditions and NOx emissions are also reduced except for the full load condition. DFC yields lower CO2 emissions compared to diesel SFC over all engine conditions. Biodiesel-CNG DFC results in relative high CO and HC emissions at low load conditions due to the low combustion temperature of CNG but no notable trend of HC emissions with variations of pilot injection timing were discovered.