Expansion of HCCI Operating Region by the Combination of Direct Fuel Injection, Negative Valve Overlap and Internal Fuel Reformation

A gasoline-fueled homogeneous charge compression ignition (HCCI) engine with both direct fuel injection and negative valve overlap for exhaust gas retention was examined. The fuel was injected directly into the residual in-cylinder gas during the negative valve overlap interval for the purpose of reforming it by using the high temperature resulting from exhaust gas recompression. With this injection strategy, the HCCI combustion region was expanded dramatically without any increase in NOx emissions which were seen in the case of compression stroke injection. Injection timing during the negative valve overlap was found to be an important parameter that affects the HCCI region width. The injection timing also had the most suitable value in each engine load for the best fuel consumption. From this result, A new injection strategy in which only a portion of the fuel was injected during the negative valve overlap interval, while the rest of fuel was injected in intake stroke, was proposed. With this split injection, by selecting the suitable amounts of fuel, further fuel consumption improvement was realized. The higher the engine load becomes, the less amount of fuel injected during negative valve overlap was required for the suitable HCCI combustion. Finally, the advantage of utilizing the combination of the negative valve overlap and the direct fuel injection in expanding HCCI operating region was summarized in a figure as contours of required negative valve overlap interval on engine speed and compression ratio plane. The effect of mixture stratification and the effect of fuel reformation can be utilized to reduce the required negative valve overlap interval for suitable HCCI combustion under each set of engine speed and compression ratio conditions.