Numerical Simulation Of Circulating Fluctuation And Blended With Dissociated Methanol Gas In SI Engine

As energy problems become tense and regulations become increasingly stringent around the world,the vehicle exhaust emissions put forward greater challenges.It is found that the cyclic variation of engine under steady condition has great influence on the performance of engines.Therefore,it is very important to study the engine cycle variation.Based on the LES(Large-Eddy Simulation)method,three-dimensional numerical simulation is carried out on a 4-stroke ignition engine by using the CONVERGE model of3D simulation software.The cylinder pressure curve and partial flow field distribution of some working conditions are obtained.At the same time,The accuracy of the simulation is verified by comparing the cylinder pressure curve with the experimental results.Then,the influence of spark plug position,different mixing ratio,different excess air coefficient and different ignition advance angle on engine performance and the comparison of different large-eddy simulation methods with RANS model are analyzed in detail.The simulation results show that in the case of LES calculations,in addition to being able to obtain ordered large-scale vortices,a large number of irregular small vortices can be seen in the cylinder;the RANS calculation of the cylinder flow field shows a certain degree of order,and does not see To a large number of small vortex structures exist.The calculation also found that when using RANS calculations,the difference between the flow fields was significantly smaller from the second cycle.In particular,when the piston is running to-250°CA and-200°CA,the velocity field of the third and fourth cycles is similar and the extremes of velocity tend to be stable.In the case of LES calculation,there is almost no law from the comparison of four cyclic velocity fields.The flow field at the same time of four cycles shows a random state.When the piston runs to the bottom dead point,are also randomly distributed in the cylinder,the similarity of the flow field is very small.When the molar ratio of blended with dissociated methanol gas is 1,the pressure,temperature and heat release rate increase with the increase of the mixing ratio,and the emission of NO_x is also increasing.When the mixing ratio is constant,with the increase of excess air coefficient,the cylinder pressure,temperature and heat release rate are decreasing,NO_x emission is also reduced significantly.Under the same conditions,the peak values of cylinder pressure,cylinder temperature and heat release rate increase with the advance of the ignition advance angle,and the peaks corresponding to the peak value are kept ahead of time.The NOx emission tends to increase with the ignition timing.The position of the spark plug has a great influence on the combustion in the cylinder,and the peak pressure and temperature peak of the spark plug position are increasing and decreasing.The peak angle also shows the same trend.The NO_x emission increases first and then decreases The When the position than the original machine reduced 3mm,the high pressure than the original machine,but also to ensure that the NOx emissions remain unchanged.