Improvements in thermal efficiency of premixed diesel combustion with optimization of combustion-related parameters and fuel volatilities

Abstract

The influence of combustion-related parameters and fuel volatility on the premixed diesel combustion was experimentally investigated in a modern direct injection diesel engine, and the evaporation process of the fuel spray in the combustion chamber was analyzed with computational fluid dynamics simulation. By optimizing the fuel injection timing and the intake oxygen content, ultralow NOx and smokeless premixed diesel combustion with high thermal efficiency and acceptable levels of CO and total hydrocarbon emissions is possible with both diesel fuel and normal heptane. The optimum fuel injection timing for the indicated thermal efficiency is obtained when the fuel spray does not enter the squish area, maintaining the 50% heat release crank angle at around top dead center. The indicated thermal efficiencies reach the maximum at around 12% intake oxygen concentration. The indicated thermal efficiency in the premixed diesel combustion with normal heptane is slightly higher than with diesel fuel and is very similar to the conventional diesel combustion in a wide indicated mean effective pressure range of below 0.8 MPa. The indicated thermal efficiency decreases when advancing injection timings mainly due to the deterioration in the combustion efficiency when the fuel is injected to the outside of the piston cavity. The degree of decrease in the indicated thermal efficiency with advancing injection timings is more significant with diesel fuel than with normal heptane due to the wall wetting.