Combustion Improvement and Exhaust Emissions_Characteristics in a Direct Injection Natural Gas Engine by Throttling and Exhaust Gas Recirculation

A natural gas direct injection test engine equipped with a newly developed natural gas injector was built. High total hydrocarbon (THC) emission at part-load and high NOx emission at high-load remain as problems for direct injection natural gas engines. THC reduction and combustion improvement by throttling and NOx reduction by EGR were investigated. The following results were obtained: (1) the combustion at light and medium load conditions is improved by throttling. It is possible to improve the thermal efficiency at light-load in spite of the pumping loss by throttling. THC emissions are greatly decreased in this condition; (2) a large NOx reduction can be obtained without combustion deterioration by appropriate EGR at high-load conditions; and (3) it is possible to decrease both THC and NOx emissions by both throttling and EGR at part-load conditions.