Research of fuel injection and combustion control for maximum fuel efficiency and minimum emissions in a heavy-duty diesel engine

Abstract

The simultaneously requirements of ultra-low emissions and lower operating cost are driving the technology development of heavy-duty (HD) diesel engines. In order to meet the stringent emission regulations and maximise the fuel efficiency and thus minimise the greenhouse gases (GHG), significant efforts have been made to develop highly efficient and clean diesel combustion. In addition to the challenge of the trade-off between exhaust emissions and fuel efficiency, HD diesel engines experience low exhaust gas temperature (EGT) at low-load operation, which significantly reduces the effectiveness of the exhaust aftertreatment systems. In response to these issues, variable valve actuation (VVA)-based advanced combustion control technologies have been researched to curb engine-out emissions and maximise fuel efficiency as well as improving the low-load thermal management, helping to minimise total engine operational cost (EOC). This research started with an investigation into the influence of injector nozzle designs on engine combustion, emissions, and performance. This was followed by the systematic study of Miller cycle operation over the engine speed-load map with and without exhaust gas recirculation (EGR). Experimental work was carried out in a single cylinder HD diesel engine equipped with a high pressure common rail fuel injection system, an external cooled EGR circuit, and a VVA system. The results showed that the injector geometry optimization is necessary to improve the trade-off between engine-out emissions and fuel efficiency. Special attention should be paid to the interaction of spray-bowl and spray-wall under different operating conditions. The introduction of residual gases via an intake valve re-opening during the exhaust stroke was demonstrated as an effective combustion control strategy for exhaust thermal management at the low load operation of 2.2 bar indicated mean effective pressure (IMEP). Additionally, the combined “Miller cycle + EGR + post injection” strategy was shown to beneficial for engine-out emission reduction and EGT control at 6 bar IMEP. Furthermore, the effectiveness of Miller cycle with or without EGR was examined from low to full engine loads at constant intake pressure and constant lambda conditions. A cost-benefit and the cycle-averaged results calculated over the World Harmonized Stationary Cycle (WHSC) were presented and analysed. The results showed that the “Miller cycle + EGR” strategy with a constant lambda attained the highest corrected net indicated efficiency over the WHSC by up to 8% in comparison to the baseline without EGR. Finally, the potentials of different combustion control strategies to meet the Euro VI emission regulation were assessed at different NOx aftertreatment efficiencies. Overall, this study has presented promising cost-effective emission control and fuel efficiency technologies that could be suitable for the “SCR-only” and “SCR + EGR” technical routes for the future HD diesel engines.