A Study on Fuel Design for Controlling Ignition and Initial Combustion of Transient Fuel Sprays

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More homogeneous and leaner combustion is needed to minimize the emissions of particulates and nitric oxides from direct-injection engines. For this purpose, a fundamental study was carried out to clarify the strategy for selection and design of fuels.From the experimental results on ignition of a n-heptane spray using a constant-volume vessel, two-stage ignition was found in the case that fuel-air mixing progressed in advance of chemical reactions. In this condition, the start of an effective heat release was delayed by promoting the mixing unlike in highly heterogeneous ignition at diesel conditions.A ignition model was developed combining a stochcastic fuel-air mixing model with a Schreiber's reduced chemical kinetic scheme. Analyzing the details of the predicted results, a qualitative explanation was made on the above measured tendencies.Ignition characteristics were investigated on mixtures of two hydrocarbons with the same or different structures. Addition of i-octane to n-heptane resulted in the appearance of the feature of two-stage ignition that was never observed in pure n-heptane ignition at a typical compression temperature and pressure in naturally aspirated diesel engines. It was suggested that the fuel with lower-ignitability acted as a diluent in the reactions of another fuel component.Knowledges obtained from the experiments indicate the possibility of homogeneous lean combustion by optimizing the fuel composition and fuel-air mixing if adequate fuel components can be utilized. Although further improvement is needed for quantitatively precise predictions, the developed ignition model will be useful to obtain the strategies for control of spray ignition and combustion employing a more detailed kinetic model.