| For the demands for high efficiency and clean combustion, new combustion concepts like HCCI and PCCI are proposed. However, due to the difficulties controlling combustion phasing and heat release rate, the operation range is limited. In order to address these issues, University of Wisconsin proposed the concept of RCCI(Reactivity Controlled Compression Ignition),which has attracted much attention. The precondition to operate RCCI combustion is to understand the combustion mechanism, main factors and their variation characteristics. In this research, reduced chemical kinetic model is combined with 3D-CFD model to simulate the combustion process of gasoline-diesel RCCI and the effects of stratified reactivity to the combustion heat release process is studied, which become the fundamental for the stable control of RCCI combustion mode.In the first part, a reduced n-heptane/iso-octane chemical kinetic mechanism model has been developed. The 0D chemical kinetics software is used to reduce the latest detailed n-heptane and iso-octane reaction mechanism. Temperature sensitivity and production rate is analyzed to find a path way from the C7/C8 hydrocarbon molecular to the C3 class species. C0-C3 common mechanism is added and a initial mechanism is formed. Then kinetic parameters are modified according to the ignition delay sensitivity analysis results. A reduced mechanism consists of 73 species and 273 reactions for the primary reference fuel is proposed. The mechanism is validated with experimental data over the range of temperature from 650 to 1300 K, equivalence ratio from 0.5 to 2.0.In the second part, the reduced mechanism proposed is combined with 3D-CFD model to simulate the RCCI combustion process at the operation condition of 1300 rpm and 9.7bar IMEP. The result shows that the reaction process for n-heptane and iso-octane are different. Just prior to the high-temperature reaction(-5°CA), mole fraction of n-heptane decreased to 1/5 of the initial value while nearly 50% of iso-octane hasn’t been consumed. Also, the distribution evolution process of iso-octane, CH2 O and CO show that the reaction region moves from the outer region to the inner region of the piston bowl. Fuels react step by step from the outer region to the inner region to form CH2 O and CO, and they are further oxidized to form CO2. When the high temperature reaction is nearly ended(7°CA), little CO exists in the region of cylinder center line and near the liner. This staged reaction is the result of the spatial gradient distribution of n-heptane. The staged combustion from n-heptane to iso-octane extends the combustion duration and the rate of heat release is reduced.In the third part, a comparison study is made between RCCI and other two pre-mixed CI combustion modes. The result shows, while HCCI and CI combustion with only equivalence ratio stratified work out the combustion duration and PPRR are 5.97°CA,6.53°CA and 3.88 MPa/°CA,1.89MPa/°CA, respectively, the combustion duration of RCCI combustion is the longest as 8.44°CA and the peak pressure rise rate(PPRR) is the lowest as 0.91MPa/°CA. From the species, equivalence ratio and temperature distribution prior to low temperature reaction it is found that comparing with the stratified temperature and equivalence ratio, stratified reactivity of fuels has larger effects on the control of heat release rate and combustion phasing. |
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