Thermal Efficiency Improvement and its Mechanism at Low Load Conditions in Semi-Premixed Diesel Combustion with Twin Peak Shaped Heat Release 2019-01-1153
Semi-premixed diesel combustion with twin peak shaped heat release applying a combination of the premixed combustion after a sufficient mixing period with a suitably long ignition delay of the first injection and the spray combustion of the second injection just after the premixed combustion has the potential to establish efficient, low emissions, and low noise operation. The authors reported that at medium load conditions indicated thermal efficiencies exceeding 50% can be established with reductions in the intake oxygen concentrations and increases in the intake gas pressures. However, with the semi-premixed diesel combustion at low load conditions the indicated thermal efficiencies are lower than at medium load conditions due to the lower combustion efficiencies. Here, to increase the combustion efficiencies at low load conditions by optimizing the intake gas temperature, the thermal efficiency related parameters in the semi-premixed diesel combustion with twin peak shaped heat release were investigated in a 0.55 L single cylinder diesel engine. The results showed that the indicated thermal efficiency improved with increases in intake gas temperatures at low load conditions as improvements in combustion efficiencies were larger than the deteriorations in cooling losses. However, at higher load conditions where the combustion efficiencies were relatively high the indicated thermal efficiencies decreased with increases in the intake gas temperatures due to increases in the cooling losses. Further, the combustion characteristics of the semi-premixed diesel combustion with twin peak shaped heat release and the premixed diesel combustion with a single injection at low load conditions were analyzed with CFD simulation, showing that the CO with the semi-premixed combustion was lower than with the single combustion due to a higher temperature near the central region of the cylinder increased by the second-stage combustion.