Effect of operating condition on particulate matter and nitrogen oxides emissions from a heavy-duty direct injection natural gas engine using cooled exhaust gas recirculation

Abstract

Abstract

Two methods for reducing nitrogen oxides (NO_X) emissions from direct injection, compression ignition, heavy-duty engines are exhaust gas recirculation (EGR) and the high-pressure direct injection of natural gas. Tests combining these two techniques were carried out on a single-cylinder research engine (SCRE) based on a modified heavy-duty automotive engine. No attempt was made to optimize the engine's combustion chamber or the injector geometry for EGR operation. The SCRE's independent charge-air system allowed for more controlled testing over a wider range of test variables than can be carried out by a standard engine. These tests investigated the effects of cooled EGR on particulate matter (PM) and NO_X emissions while varying the injection timing, engine speed, equivalence ratio and intake manifold pressure. The results suggested that, with EGR, higher equivalence ratios reduced power-specific NO_X but increased PM emissions. Increasing the charge mass at a constant EGR fraction resulted in significant reductions in PM, at the cost of slightly increased NO_X By advancing the injection timing at high EGR fractions, PM emissions and fuel efficiency were improved, with only a slight increase in NO_X emissions compared to the more retarded injection timings. The engine speed influenced the amount of EGR that could be recirculated, with lower speeds resulting in higher achievable EGR fractions. These results suggest that EGR fractions in excess of 20 per cent can achieve NO_X reductions beyond 75 per cent, without causing unacceptable increases in PM emissions or significant reductions in fuel efficiency.