The effect of fuel octane and dilutent on homogeneous charge compression ignition combustion

This paper presents some experimental operating and combustion properties of homogeneous charge compression ignition (HCCI) combustion. HCCI operating range, start of combustion, burn duration, indicated mean effective pressure, indicated specific emissions, and indicated specific fuel consumption are evaluated as charge dilution and octane number are varied. Primary reference fuels with octane numbers of 20, 40, and 60 are used in this study. The autoignition properties of the air-fuel mixture are varied by changing the fuel octane number, percentage of exhaust gas recirculation (EGR), and air-fuel ratio, while holding the intake temperature, engine speed, and compression ratio constant. Results show that both the start of combustion and the burn duration are sensitive to mixture dilution (excess air or EGR). The fuel octane number is not an effective method of controlling the start of combustion or the burn duration but can be used to increase the load range of the HCCI engine. Both the NOx emissions and the indicated specific fuel consumption increase as the octane number is increased due to lower dilution and higher peak temperatures. Correct amounts of dilution are critical in controlling HCCI combustion. Separating dilution into EGR and excess air, it is found that a given amount of EGR is more effective at controlling the start of combustion and the burn duration than the same amount of excess air.