Simulation Study On Mechanisms Of Natural Gas /diesel Reactivity-controlled Compression-ignition Combustion

Ideal Homogeneous Charge Compression Ignition(HCCI) is the best technology for high efficiency and low pollution combustion engine, but the intrinsic defects of the uncontrolled starting point and the combustion rate seriously restricted the play of the advantages of the HCCI, Reactivity Controlled Compression Ignition(RCCI) has the huge potential of solving this problem. In the process of the realization of HCCI engine all condition range expansion by use of diesel and natural gas RCCI combustion mode, it is necessary to understand the working mechanisms of natural gas/diesel RCCI, in order to establish the theoretical foundation of RCCI robust control.Aiming at the above mentioned problems, the 3D simulation method is used to study RCCI work mechanisms. First, the reduced zero dimensional mechanism of natural gas/diesel RCCI was generated in this article. Second, the reduced mechanism is coupled with 3D process of flow, heat transfer and spcies transport of natural gas/diesel RCCI. The spatial distribution and change of fuel molecule, intermediate group and NOx concentration by the different Crank Shaft Angle in natural gas/diesel RCCI process are analyzed.(1)The final reduced mechanism of n-heptane contains 114 kinds of components, 455 steps of reactions. Compared with detail mechanism, the species decreases by 82.6%, the reactions decreases by 83.9%. Compared with LLNL mechanism results, Ignition delay, CO and C2H2 calculation results are lower than the errors. Meanwhile the reduced mechanism contains less species and reactions, so computational efficiency would improve. It demonstrates the final reduced mechanism of n-heptane can be used to next step.(2)The final reduced mechanism of methane contains 20 kinds of components, 70 steps reactions. Compared with detail mechanism, the species decreases by 62.2%, the reactions decreases by 78.4%. Compared with GRI3.0 mechanism results, Ignition delay, CO and NO calculation results are lower than the errors. Meanwhile the reduced mechanism contains less species and reactions, so computational efficiency would improve. It demonstrates the final reduced mechanism of CH4 can be used to next step.(3)The final reduced mechanisms of n-heptane and methane are merged, the resulting gas/diesel RCCI reduced mechanism contains 118 species and 479 reactions. Validated by the three dimensional test, synthetic RCCI reduced mechanism can meet the demand of guiding natural gas/diesel RCCI test. At the same time, because of small size of mechanism, it can be used by low configuration computer with high efficiency. This mechanism can be used as a basic tool in the process of analyzing natural gas/diesel RCCI oxidation. The process of generating reduced mechanism would provide references for reduced mechanism of other different reactivity fuel.The second part of this article, the RCCI reduced mechanism is coupled with three-dimensional CFD software, under the condition of the IMEP equal to 13.5 bar, the processes of the combustion, NOx emission and the concentration of chemical kinetics mechanism of the species are analyzed. The results are as follows,(1) The CA10 is at 8 ° ATDC, the CA50 is at 13.9 ° ATDC, the CA90 is at 26 ° ATDC, the corresponding combustion duration is 18 ° CA, it shows that the RCCI model is able to control heat release rate. The high pressure rise rate is about 4 bar/deg which is less than 10bar/deg maximum pressure rise rate limit. And the highest average temperature in the cylinder is 1639 K, the less NOx emissions will be generated. It embodies RCCI belongs to low temperature combustion.(2) The consumption of n-heptane is ahead of methane. N-heptane has been slightly consumed at- 12 ° ATDC. At 3 ° ATDC, n-heptane consumption rate has quickened significantly. And methane has been slightly consumed at- 3 ° ATDC, the methane has obvious fast consumption at 8 ° ATDC. Simulation results show that the fuel oxidation temperature of hign reactivity fuel is lower than low reactivity fuel. the bigger discrepancy of the two fuel reactivity has, the more ideal RCCI performance may be.(3) There is 9 ° CA delay between the methane initial consumption time- 3 ° ATDC and n-heptane initial consumption time- 12 °ATDC. There is 5 ° CA delay between methane rapidly consumption time 8 ° ATDC and n-heptane rapidly consumption time 3 °ATDC. This shows that the high reactivity fuel will be first oxidation,the low reactivity fuel begins to consume after a period of time heating effect, it demonstrates the RCCI model can achieve " successively ignition ", thus controllable combustion(starting point) phase and heat release rate can be controlled.(4)Oxidation reactions begin with n-heptane`s high mixing ratio areas, spread to low mixing ratio area, it embodies reactivity layering and " successively ignition " in the concept of RCCI. At the same time, ignition delay period of natural gas is longer than diesel, so its activity is lower than diesel, in the region of the diesel high mixing ratio, when the heat generated by the diesel oxidation decomposition is accumulated to a certain extent, oxidation reactions of low reactivity methane begin to happen, which demonstrates that RCCI model can control heat release rate.