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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.

Ignition delays were systematically measured in a DI diesel engine under wide ranging and various engine operating conditions, including engine speeds, fuel injection pressures, intake gas temperatures, intake gas pressures, and intake oxygen concentrations changed with EGR. Empirical equations to predict the ignition delay based on the Arrhenius equation with and without the Livengood-Wu integral and multiple regression analysis of the experimental results. The simple equation assuming constant conditions during the ignition delay without the Livengood-Wu integral can accurately predict the ignition delay. However, the lack of generality has remained as the fuel injection pressure is directly included in the Arrhenius equation which should contain only the chemical parameters. To improve the generality of the equation, the ignition delay was separated into the initial physical process of the fuel spray breakup and the following chemical process. The start of the Livengood-Wu integral was set at the breakup time of liquid fuel jet assuming that the chemical reactions do not occur before the fuel spray breakup, and that the physical factors are directly involved in the physical processes and indirectly in the chemical processes. The fuel spray tip dynamics based on Wakuri’s momentum theory was introduced to express the changes in the conditions in the fuel spray during the ignition delay. The ignition delay can be accurately predicted by the equation with the Livengood-Wu integral and six parameters, including the breakup time, the mass flow rates of air and fuel at the cross section of the spray tip, the oxygen partial pressure, the engine speed, and the averaged in-cylinder gas temperature. The empirical equation predicted longer ignition delays at high ignition pressure conditions, and the accuracy was improved by performing a multiple regression analysis separately at each fuel injection pressure, suggesting unknown factors varying with the fuel injection pressures.

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