A comparative study of directly injected, spark ignition engine performance and emissions with natural gas, gasoline and charge dilution

Abstract

This paper presents an experimental study of a 4-cylinder, downsized and boosted, spark ignition engine fuelled by either directly injected compressed natural gas (DI CNG) or gasoline (GDI). Three different charge preparation strategies are investigated for both fuels: stoichiometric engine operation without external dilution, stoichiometric operation with external exhaust gas recirculation (EGR) and lean burn.The performance of the optimized DI CNG engine is first shown to be comparable to that of the baseline GDI engine at stoichiometric conditions, exhibiting the potential of matching the peak torque at lower engine speeds with substantial fuel economy benefits. The impact of charge dilution on DI CNG engine performance is then examined with external EGR and lean burn. Between these two strategies, lean burn demonstrates higher brake thermal efficiency than that of stoichiometric operation with external EGR at equivalent dilution levels. Similar tests are performed with GDI fuelling and the results are compared to those of DI CNG, showing that a charge dilution strategy’s influence on engine performance is fuel specific. The engine-out total CO2 equivalent emissions of a DI CNG engine utilizing all three modes of operation are then assessed and compared to those of stoichiometric GDI operation over a range of engine operating conditions. DI CNG operation consistently demonstrates lower total CO2 equivalent emissions than GDI, though the final choice of DI CNG combustion strategy requires consideration of the aftertreatment system.