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Fuel-air mixing in a direct-injection SI engine was studied to further improve full-load torque output. The fuel-injection location of DI vs. PFI results in different heat sources for fuel evaporation, hence a DI engine has been found to exhibit higher volumetric efficiency and lower knocking tendency, resulting in higher full-load torque output [1]. The ability to change injection timing of the DI engine affects heat transfer and mixture temperature, hence later injection results in lower knocking tendency. Both the higher volumetric efficiency and the lower knocking tendency can improve engine torque output. Improving volumetric efficiency requires that the fuel is injected during the intake stroke. Reducing knocking tendency, in contrast, requires that the fuel is injected late during the compression stroke. Thus, a strategy of split injection was proposed to compromise the two competing requirements and further increase direct-injection SI engine torque output.

Heat transfer from the flame kernel to the electrodes during the spark ignition process is of interest for predicting the minimum ignition energy at a given engine operating condition. Experiments conducted in a constant volume bomb at near ignition limit conditions with small and large electrode surface areas (comparable to J gap plug), coupled with a phenomenological model, show the lumped heat transfer coefficient to range from 150-200 W/m2 K during the first 3 milliseconds of the ignition period. An additional analytical approach that uses the measured time dependent kernel-electrode contact areas gives reasonable agreement with the experimentally determined heat transfer coefficient and demonstrates that the dominant mechanism is thermal conduction. Heat loss from the flame kernel is comparable to the net ignition energy for the small electrodes after 3 milliseconds while that for the large electrodes is shown to equal the net ignition energy within 800 microseconds.