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The Homogeneous Charge Compression Ignition (HCCI) engine combustion uses heat energy from trapped exhaust gases enhanced by the piston compression heating to auto ignite a premixed air/gasoline mixture. As the HCCI combustion is controlled by the charge temperature, composition and pressure, it therefore, prevents the use of a direct control mechanism such as in the spark and diesel combustion. Using a large amount of trapped residual gas (TRG), is seen as one of the ways to achieve and control HCCI in a certain operating range. By varying the amount of TRG in the fresh air/fuel mixture (inside the cylinder), the charge mixture temperature, composition and pressure can be controlled and hence, the auto ignition timing and heat release rate. The controlled auto ignition (HCCI) engine concept has the potential to be highly efficient and to produce low NOx, carbon dioxide and particulate matter emissions.

The homogeneous charge compression ignition-HCCI (also to be known as controlled auto ignition-CAI) engine concept has the potential to be highly efficient and to produce low NOx, carbon dioxide and particulate matter emissions. It experiences, however, problems with cold start, running at idle and high loads that together with controlling the combustion over the entire speed/load range limits its practical application. A solution to overcome these problems is to operate the engine in ‘hybrid mode’, where the engine operates in HCCI mode at low, medium and cruising loads and switch to spark ignition (SI) mode (or diesel mode-CI) at a cold start, idle and higher loads. To operate such ‘hybrid mode’ engine, a transition between SI and HCCI and SI modes, as a result of changes in engine speed and load must be seamless in operation, whilst keeping all relevant engine and combustion parameters in an acceptable range.