Characterisation of the soot formation processes in a high pressure combusting Diesel fuel spray

As part of an ongoing investigation, the influence of In Cylinder Pressure (ICP) and fuel injection pressure on the soot formation processes in a diesel fuel spray were studied. The work was performed using a rapid compression machine at ambient conditions representative of a modern High Speed Direct Injection diesel engine, and with fuel injection more representative of full load. Future tests will aim to consider the effects of pilot injections and EGR rates. The qualitative soot concentration was determined using the Laser Induced Incandescence (LII) technique both spatially and temporally at a range of test conditions. Peak soot concentration values were determined, from which a good correlation between soot concentration and injection pressure was observed. The peak soot concentration was found to correlate well with the velocity of the injected fuel jet. Charge air pressure was observed to have minimal effect on the peak soot concentration indicating insensitivity to ignition delay and spray break-up length. Injection pressure was also observed to strongly influence the early soot formation process. Soot was found to form earlier closer to the injector at high injection pressures. It was proposed that air-fuel mixing promoted by better atomisation of the spray at high injection pressures results in early pyrolysis of the fuel and the formation of soot.

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