Effect of EGR on Combustion and Exhaust Emissions in Supercharged Dual-Fuel Natural Gas Engine Ignited with Diesel Fuel

The combustion and exhaust emissions characteristics of a supercharged dual-fuel natural gas engine with a single cylinder were analyzed. We focused on EGR (Exhaust Gas Recirculation) to achieve higher thermal efficiency and lower exhaust emissions. The combustion of diesel fuel (gas oil) as ignition sources was visualized using a high-speed video camera from the bottom of a quartz piston. The luminous intensity and flame decreased as the EGR rate increased. Furthermore, the ignition delay became longer due to the EGR. Characteristics of the combustion and exhaust emissions were investigated with changing EGR rates under supercharged conditions. The indicated mean effective pressure and thermal efficiency decreased with increasing EGR rate. In addition, NO_x emissions decreased due to the EGR. In this study two-stage combustion was observed. When two-stage combustion occurred, it was supposed that auto-ignition of compressed natural gas and air mixture occurred during the flame development. However, knocking did not occur. Two-stage combustion occurred under the condition of low EGR rate and advanced injection timing. The indicated mean effective pressure and thermal efficiency increased, during two-stage combustion; however, NO_x emissions also increased by a large amount compared to normal combustion. High indicated mean effective pressure, high thermal efficiency, and low NO_x emissions were achieved just before the occurrence of two-stage combustion by changing EGR rate and injection timing of diesel fuel.