Numerical Simulation Study On Mixture Formation And Performance Optimization Of In-Cylinder Direct Injection Rotary Engine

In recent years,with the in-depth promotion of energy conservation and emission reduction,the rotary engine has become a good choice for the range extender of electric vehicles with the advantages of small volume,light weight and high power density.However,since the advent of the rotary engine,it has always been plagued by the problems of high fuel consumption and high emission.The in-cylinder direct-injection technology not only helps to improve the thermal efficiency of the rotary engine,but also takes into account the fuel economy and emission indicators to a certain extent,which has a good research significance to solve the problems of high fuel consumption and high emission of the rotary engine.In this thesis,the working process of rotary engine was numerically simulated with the help of three-dimensional numerical simulation software,the performance differences of direct injection and port injection under different injection modes and ignition modes were compared.Moreover,the effects of direct injection position,injection angle and injection pressure on the mixture formation and combustion process of the rotary engine were investigated,and an optimized in-cylinder direct injection strategy was proposed.Firstly,the CFD model of the gasoline rotary engine was established based on the structural parameters of the gasoline rotary engine,and the geometric model was divided into several regions.The chemical reaction mechanism,turbulence,combustion and other calculation models were selected to simulate the working process of the rotary engine.According to the experimental data,three-dimensional model calibration of the in-cylinder pressure curve and the heat release rate curve of target speed and load was carried out,and the mesh independence of the model was verified.Finally,the spray shape and penetration distance of the direct injection gasoline spray were calibrated by the constant volume combustion bomb simulation model.Under the condition that the engine speed was 2500 rpm,the intake pressure was50 k Pa and the equivalence ratio was 1,by comparing the effects of different spark plug positions and quantities on the combustion and emission characteristics of the rotary engine under the two injection modes of in-cylinder direct injection and port injection,it was found that in-cylinder direct injection can make the fuel stratification in the combustion chamber.Higher in-cylinder peak pressure can be obtained at the upper and middle direct injection positions,and the local equivalence ratio of unburned area at the rear of the combustion chamber was lower.Using the dual spark plug ignition mode,the peak in-cylinder pressure can be higher than that using only the front spark plug and rear spark plug ignition.Comparing the indicated thermal efficiency of different injection and ignition modes,it can be seen that the indicated thermal efficiency corresponding to the dual spark plug ignition mode was the highest.Through the investigation of mixture formation,flame propagation and emission generation of the rotary engine,it was found that as the injection position approached the top dead center,the oil film attachment area in the combustion chamber changed from the rotor surface to the middle cylinder block,and the uneven distribution of fuel in the combustion chamber gradually decreases at the ignition timing.At the same injection position,with the increase of injection angle,the fuel enrichment area gradually appeared in the middle and front of the combustion chamber,and the local equivalence ratio at the rear of the combustion chamber decreased.During the combustion process,the in-cylinder pressure corresponding to the upper injection position increased rapidly,and the corresponding cylinder peak pressure was also higher than the original pressure.The in-cylinder inhomogeneity index of fuel corresponding to the upper injection position is the lowest and the indicated thermal efficiency is the highest.In the optimization of injection pressure at the upper injection position,it was found that with the increase of injection pressure,the fuel enrichment area gradually moved to the front of the combustion chamber,and the in-cylinder inhomogeneity index of fuel showed a decreasing trend.Within the injection pressure range of 10-20 MPa,the in-cylinder inhomogeneity index of fuel corresponding to 0° injection angle was the lowest,and the local equivalence ratio at the rear of the combustion chamber corresponding to 25° injection angle was the lowest.Higher injection pressure helps to obtain a higher in-cylinder peak pressure,which was most significant at 15 MPa and18 MPa injection pressure.It was found that the in-cylinder peak pressure corresponding to 0° injection angle at the upper injection position and 15 MPa injection pressure was 12.39% higher than that original pressure,and the indicated thermal efficiency can reach 24.99%.Moreover,under this injection strategy,the generation of soot was the least and the generation of CO was significantly reduced.