Thermal efficiency improvement and its mechanism at low load conditions in semi-premixed diesel combustion with twin peak shaped heat release

Semi-premixed diesel combustion with a twin peak shaped heat release with the two-stage fuel injection (twin combustion) has the potential to establish efficient, low emission, and low noise operation. However, with twin combustion at low loads the indicated thermal efficiencies are poorer than at medium loads due to the lower combustion efficiencies. In this report, to increase the combustion efficiencies at low loads, the thermal efficiency related parameters were investigated in a 0.55 L single cylinder diesel engine. The results show that the indicated thermal efficiency improves with increases in the intake gas temperatures at low loads. However, at the higher loads where the combustion efficiencies are somewhat higher the indicated thermal efficiencies decrease with increases in the intake gas temperatures due to increases in the cooling losses. At the low load condition below 300 kPa IMEP, the indicated thermal efficiency is higher and the combustion noise is lower in the twin combustion than in the single premixed combustion. Further, the combustion characteristics of twin and single premixed diesel combustion at low loads were analyzed with CFD simulation, showing that in the twin combustion, re-oxidation of the CO is promoted by the second-stage combustion and the indicated thermal efficiencies are higher than in the single premixed combustion due to higher combustion efficiencies. Both the heat flux value and the high heat flux area in the twin combustion are smaller than in the single premixed combustion due to decreases in the quantities of burned gas near the combustion chamber wall due to the separating of the fuel injections.