Numerical Simulation About Effect Of REGR On HCCI Combustion Of Dimethyl Ether

In order to meet increasingly stringent emission regulations and alleviate the oilcrisis, advanced controlling technology and suitable alternative fuel of internalcombustion engine are needed, which have become popular research field in the wholeworld. In recent years, Homogeneous Charge Compression Ignition (HCCI) combustionas a new combustion mode has become a worldwide research focus by its high thermalefficiency and low emissions. Furthermore, Dimethyl (DME) Ether as a sort of dieselalternative fuel causes the attention of researchers due to its excellent ignitionperformance. But DME HCCI combustion mode has some disadvantages, such as thedifficulties in controlling the ignition time and the combustion rate, narrow operationscope and higher HC and CO emissions. In order to solve these problems in DMEHCCI combustion, a variety of technologies have been proposed. Reformed ExhaustGas Recirculation (REGR) is a combination of Exhaust Gas Recirculation (EGR) andon-board fuel reforming technology, merging the advantage of them. This paper firstanalyzed the DME reforming process in perspective of thermodynamics, and thenstudied REGR effects on DME HCCI combustion process and emission using CFDcoupling chemical kinetics model.In this paper, first of all, the thermodynamic analysis of DME steam reformingfocused on the reforming system thermal efficiency and the equilibrium concentrationof components under different temperatures, pressures and steam-to-DME ratio. Theresults show that the reformed gas has a higher low heat value degree up to15.45%thanDME; the DME has a high conversion rate in the temperature of473K andsteam-to-DME ratio larger than2, and the equilibrium concentration is close to zero.Dimethyl Reformed Gas (DRG) mainly consists of H2, CO and CO2. The hydrogencontent is the highest with the biggest concentration of72℅; At high temperature andlow steam-to-DME ratio, carbon in DME mainly generates CO. While at lowtemperature, high water ether ratio, it mainly generates CO2. Pressure has little effect onreaction equilibrium concentration.Then, the zero-dimensional single zone model based on detailed chemical kineticmechanism of DME was used to analyze the influence of EGR, DRG and REGR oncombustion process and the role of every component of DRG was also analyzed indetail by chemical kinetics. The results show that EGR reduce the heat release rate and pressure rise rate, reduce the cylinder temperature and pressure, reduce the engineindicated work, delay ignition time at the same time. DRG increase the heat release rateand pressure rise rate leading the raise of cylinder temperature and pressure, improvethe engine indicated work, and delay the ignition moment. H2in DRG plays a importantrole in the ignition time delay, which mainly cause by it compete with DME for OH,suppressing the dehydrogenation exothermic reaction of DME in low temperature stagereaction that reduces the heat of reaction at low temperature, and eventually led to theignition time delay. REGR combine the advantage of EGR and DRG, through adjustingthe proportion between two can effectively control the ignition time of DME on HCCIcombustion, and broaden the engine load range to a certain extent.Finally, the NOXformation mechanism was added in the detailed chemical kineticsmodel of dimethyl ether to study the REGR on HCCI combustion process and emissioncharacteristics of dimethyl ether by using three-dimensional CFD model with thecoupling calculation. The results show that HCCI combustion temperature distributionis not uniform in the phases of low temperature and high temperature reaction stage, buttend to be more uniform after low temperature and high temperature reaction stage.Low temperature reaction first occurs in the vicinity of the cylinder wall and the bottomof the piston bowl, REGR has no effect on regional start time and location of lowtemperature reaction. High temperature reaction started in the center of the bowl region,REGR delay the start moment of high temperature reaction. DRG can reduce CO andHC emissions, but increase the emissions of NOX, while EGR increased CO and HCemissions, but reduces the emission of NOX. Therefore, using REGR technology caneffectively control the engine emissions by adjusting the DRG and the EGR ratio.