The influence of fuel properties on the operational range and the thermal efficiency of premixed diesel combustion was evaluated with an ordinary diesel fuel, a primary reference fuel for cetane numbers, three primary reference fuels for octane numbers, and two normal heptane-toluene blend fuels in a single-cylinder DI diesel engine. The fuel injection timing was set at 25°CA BTDC and the maximum rate of pressure rise was maintained below 1.0 MPa/°CA when lowering the intake oxygen concentration by cooled EGR. With increasing octane numbers, the higher intake oxygen concentration can be used, resulting in higher indicated thermal efficiency due to a higher combustion efficiency. The best thermal efficiency at the optimum intake oxygen concentration with the ordinary diesel fuel is lower than with the primary reference fuels with the similar ignitability but higher volatility. The deterioration with diesel fuel is due to an increase in fuel which does not contribute to the effective combustion, as the high distillation components of diesel fuel adhere to the wall of combustion chamber. The operational range with normal heptane-toluene blend fuels shifts to higher intake oxygen concentrations, compared with primary reference fuels with the similar octane numbers, showing poorer ignition characteristics in the premixed diesel combustion.
