Clarification and Control of Ignition Mode Transition by Addition of Turbulence

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This study aims at clarification of combustion phenomena and obtaining control strategies for direct-injection Premixed Charge Compression Ignition(DI-PCCI) combustion to realize low NOx(nitrogen oxides) and PM(particulate matters) emissions with controllable ignition and heat release process. To this end, we attempted to utilize intense turbulence, which would force spray ignition into two-stage ignition mode found in ignition of homogeneous fuel-air mixture.To obtain fundamental understanding of ignition process in DI-PCCI, effects of ambient and injection conditions were investigated for n-heptane spray combustion in a constant volume vessel. Ambient pressure and temperature were selected so as to simulate PCCI-like ignition and heat release process. Changing oxygen concentration and injection conditions, homogeneity in mixture concentration was varied at the hot flame initiation. A total gas sampling system was constructed to measure NOx concentration during heat release. A spray ignition and combustion model was developed based on stochastic turbulent mixing model with a simplified n-heptane oxidation scheme to understand the relation between mixture formation and chemical reaction progress.Weak mixing with larger nozzle orifices and lower injection pressure provides earlier onset of hot flame, while intense mixing with smaller orifices and higher injection pressure gives delayed hot flame and reduced heat release rate. Experiments with total gas sampling technique reveals that quite low NOx production connects to the intense mixing case with delayed hot flame. Analysis of histories of equivalence ratio and temperature in fuel-air mixture with the aid of a ignition and combustion model suggests that delayed ignition and reduced heat release rate are given in the case that entire mixture falls into lean region where the reaction speed decreases with decreasing equivalence ratio.