Modelling Study Of Combustion And Gas Exchange In A CAI Engine With Combustion Chamber EGR

With the developing of automobile industry, the number of cars increases so sharply, that environment and energy sources are faced with the highest pressure so far. All the developing and developed countries have established more and more rigorous regulation. In order to release the pressure of energy source and meet the regulation, new style of combustion must be invented, which is more efficiency and clean. Furthermore, the alternative fuel should also be developed for the same purpers.Cotrolled auto ignition (CAI) is the combusting process of in-cylinder homogeneous charge, combining the advantages of gasoline engine and diesel engine, which have higher efficiency and lower NOx and PM emissions. At the meantime of experiment study, the study on modeling CAI engines have gained plentiful achievement on several aspects, such as combustion model, combustion mechanism and variable parameters, resently this study are focused on more and more concerns. However the problems of entire load and speed range limit the application of CAI combustion. Because CAI are controlled by chemstry kinetics, not by flame spread and turblent, in order to realise CAI combustion, the composition, pressure and temperature of the in-cylinder charge at IVC must be controlled properly. Varialbe vavle timing which is able to change the capatured hot exhaust gas shows the potential of controling CAI combustion.In this paper, AURORA module in the large-scale CHEMKIN, GT-Power and MATLAB software are applied to set up CAI combustion model. The VVT system model is established by GT-Power software, ethanol combustion model by CHEMKIN software. The vavel lift curve, heat release rate and in-cylinder composition concentration are caculated by MATLAB software.The study indicates the following conclusion:1. Ethonal CAI combusion model are applied to analyse the influence of the different valve timing including exhaust valve close timing, exhaust valve open timing, intake valve open timing, intake valve close timing. The results indicates : The EVC has a very strong impact on the engine parameters and EGR;the EVO depends greatly on the gas dynamics of the engine .The IVC has a very low impact on the engine parameters as it mainly in.uences the e.ective displacement volume and effective compression ratio.but the influence of EVO and IVC is negligible in comparision the influence of the EVC.2. With symmetrical EVC and IVO timings, for CAI combustion EGR rate can change from 40% to 68.7%.The results indicates : the internal EGR has the dual function to the CAI , one function is to heat fresh air to make it easy to fire;the other function is the heat capacity and the dilution function,which may reduce the chemical reaction speed , postpone the fire, lengthen the burning duration and depressurize elevating rate. When EGR rate is smaller than 57.6% heat function performance to be remarkable. Along with EGR rate increasing, fire beginning spot is ahead of time; When EGR rate is bigger than 57.6%, reducing the chemical reaction rate act to be remarkable role. Along with the EGR rate increasing, the fire beginning spot postpones.3. In this paper, author also analyses the influence of unsymmetrical EVC and IVO timings on cotrolling Ethonal CAI engine. The results obtained indicate that the variable valve timing strategy has a strong influence on the gas exchange process, which in turn influences the engine parameters and the cylinder charge properties, hence the control of the CAI process.4. Ethonal CAI combusion model is also applied to analyse the influence of speed on EGR, flowing the rotate speed accreting , the gas sport strengthen, hence it influences the EGR rate. This model is used to forecast the range of the speed and the load of the CAI engine.