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The present study investigates the effects of boost on combustion and emissions in a homogeneous charge compression ignition (HCCI) engine. The engine was fueled with gasoline, and internal exhaust gas re-circulation (EGR) was applied to achieve HCCI combustion. The heat release rates at variable intake pressures were controlled via application of various direct injection strategies, including injection, during both negative valve overlap (NVO) and the compression stroke. It was determined that engine operation at an elevated intake pressure is possible only if a portion of fuel is injected during exhaust compression. The heat release during NVO compensated for a temperature drop that resulted from dilution by fresh air, thereby ensuring that final compression temperature was sufficiently high for auto-ignition. However, the elevated intake pressure enhanced the NVO heat release and advanced auto-ignition. Moreover, the combustion duration was reduced at the elevated intake pressure. The combination of both effects increased the pressure rise rate (PRR) beyond any acceptable level. Fuel stratification obtained via split fuel injection and the application of a certain mass of fuel during compression stroke appeared to be an effective solution for reducing both the heat release rate and PRR. However, the engine operation with fuel stratification exhibited a trade-off between NOx and CO emissions.
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