Nox. reduction from a heavy-duty diesel engine with direct injection of natural gas and cooled exhaust gas recirculation

Abstract

A heavy-duty ISX diesel engine has been commissioned for single-cylinder operation fuelled with pilot diesel ignited natural gas injected directly into the cylinder. The stock ISX engine was modified by replacing the diesel fuelling system with a high-pressure natural gas system, replacing the turbocharger with an independently controlled supercharger and installing a variable-flow exhaust gas recirculation (EGR) system. A study of the impact of cooled EGR on engine performance and gaseous emissions was carried out. Various engine speeds, loads and injection timings were tested over a range of EGR fractions. A preliminary study of the effect of EGR ‘type’—supplemental or replacement—was also carried out. The results indicate that the NOx emissions varied linearly with the intake oxygen mass fraction (representative of the EGR fraction) until NOx emissions reached 20 per cent of their non-EGR levels. Further NOx reductions were achieved with higher EGR fractions, but the rate of reduction was significantly reduced. The NOx reductions were found to be independent of engine speed and load. An overall activation energy for NOx formation was determined by correlating the NOx reductions with a representative flame temperature. The emissions of combustion by-products, including carbon monoxide (CO) and unburned total hydrocarbons (THC) increased significantly at higher EGR fractions. The engine performance and efficiency were not significantly affected except at very high EGR fractions.