Influence of Fresh Charge Preparation and Composition on Auto-Ignition Delays and Combustion Development in an Optical HCCI Direct Injection Diesel Engine 2003-01-3174

This article investigates the fuel/air mixture homogeneity that may be achieved at part/low load using a conventional common rail Diesel fuel injector. The experiments are carried out with an optical, single cylinder, small bore Diesel engine equipped with a 6 hole, narrow angle, common rail injector and a bowl shaped piston. While the rate of combustion is essentially controlled in HCCI using very diluted mixtures achieved with Exhaust Gas Recirculation (EGR), the influence of mixture homogeneity on the onset of auto-ignition is also investigated here. Mixing between air and fuel is performed using several multiple injection strategies. Mixture homogeneity corresponding to these strategies is assessed using 2D Laser Induced Exciplex Fluorescence images. Fuel images indicate that the process is never fully homogeneous. Auto-ignition and combustion are also monitored using direct light emission observation. A cool flame stage could be observed, followed by a hot flame and soot kernels radiation spots related to fuel/air mixture inhomogeneities. Finally, the effect of EGR rate and NO content of EGR on combustion phasing were investigated using OD thermodynamical analysis of cylinder pressure recordings. Auto-ignition delays were mainly affected by EGR rate, but some effects of fuel/air mixture homogeneity and NO content of the fresh charge could clearly be highlighted.