Operating Characteristics of a Homogeneous Charge Compression Ignition Engine with Cam Profile Switching - Simulation Study

A single zone combustion model based on a chemical kinetic solver has been combined with a one-dimension thermo/gas dynamic engine simulation code to study the operating characteristics of a V6 engine In which Homogeneous Charge Compression Ignition (HCCI) operation (also referred to as 'Controlled Auto-ignition" CAI) is enabled by a cam profile switching (CPS) system with negative valve overlap. An operational window within which HCCI combustion is possible has been identified and the limit of HCCI operating region for varied valve lift possibilities is explored. The mechanisms and potential fuel economy improvements within the HCCI envelope are studied and modelled results compared against data from similar engines. It is shown that for the best fuel economy the valve timing strategy needs to be selected very carefully, despite the engine's capability to operate at a range of valve timing combinations. The heating effect of the exhaust gases is dominant when using less than approximately 55% residual gases. For higher EGR rates, the diluting effect coupled with reducing exhaust gas temperatures makes the ignition timing retarded. It is demonstrated that for HCCI operation the selection of valve lift for the CPS system is limited between the constraints of the breathing capacity with the lower valve lift and misfires with the higher one.