Effect of intake air temperature and common-rail pressure on ethanol combustion in a single-cylinder light-duty diesel engine

Abstract

Gasoline compression ignition (GCI) engines have a great potential to achieve the simultaneous reduction of smoke and nitrogen oxides (NOx) emissions via partially premixed combustion (PPC) using low cetane number fuel for the extended pre-combustion mixing time. The premixed combustion realised in high compression ratio engines also improves the engine efficiency with previous studies often reporting 50% brake efficiency or higher. The ability to control the combustion phasing by the fuel injection timing differentiates this new regime from widely investigated homogenous charge compression ignition (HCCI) or its variants making it a practical alternative to conventional gasoline or diesel combustion. In this study, ethanol produced from biomass has been selected as a GCI fuel, considering its higher octane number (i.e. lower cetane number), evaporative cooling and oxygen contents than gasoline, all of which could further improve the GCI combustion. The ethanol-fuelled GCI, or in short ECI, was investigated in a single-cylinder automotive-size diesel engine connected to an Eddy Current (EC) dynamometer. The focus is the engine start-up conditions and the influence of intake air temperature and common-rail pressure on ECI combustion. From the experiments, it is found that the engine can be successfully started by ECI combustion using a conventional start motor at low engine speed of 1000rpm when the intake air temperature is higher than 60°C. For higher engine speed of 2000rpm and stable operations, however, a double injection strategy and increased intake air temperature of 80°C are required suggesting the important role of wall wetting on ECI combustion. From the intake air temperature variations up to 100°C, it is observed that both the peak in-cylinder pressure and heat release rate increase, leading to the improved engine efficiency. The measured engine-out emissions of unburnt hydrocarbon and carbon monoxide also show a decreasing trend with increasing intake air temperature, likely due to the reduced wall wetting. The smoke and NOx emissions of ECI combustion are much lower than those of a conventional diesel, regardless of the intake air temperature. The common-rail pressure variations at fixed brake power conditions show that the friction loss increases with increasing common-rail pressure, leading to the increased brake specific fuel consumption. This suggests that the common-rail pressure for ECI applications should remain low for the efficiency gain. The engine-out emissions also exhibit an increasing trend with increasing common-rail pressure although the smoke and NOx levels are always lower than that of a conventional diesel. Compared to the diesel reference case, the optimised engine operating conditions of this study achieves 50% higher fuel conversion efficiency, 5% lower brake specific fuel consumption and 27% lower NOx emissions while smoke emissions are kept at a negligible level.