Study On Optimization Of Heat Release Process Of Diluted Gasoline Hybrid Combustion Based On DME Micro-flame Ignition

High diluted low temperature combustion technology of gasoline engine has the advantages of high thermal efficiency and low NOx emission,which has become a research hotspot in the field of internal combustion engine in recent years.However,under the condition of high dilution,the burning velocity of gasoline engine decreases at low load,and the knock intensifies at medium and high load,which limits the further improvement of the diluted combustion economy of gasoline engine.This research intended to use DME micro-ignition combined with spark ignition technology to optimize the combustion process.By controlling the distribution of DME in cylinder to realize the flexible control of burning velocity and heat release process,and solve the problems of insufficient combustion speed and excessive PRR.The research work was based on one-dimensional simulation and single-cylinder gasoline engine.GT-power is used to study various heat release rate shapes and selected the most suitable heat release characteristics for engine test.By adjusting the ignition strategy of the engine in the experiment,the heat release rate are close to the ideal target,so as to further improve the economy.Firstly,this research summarized the various heat release rate shaped of low temperature combustion gasoline engine,and simplified the design of four heat release characteristics.Based on GT-power,the four characteristics were simulated and studied.By comparing the parameters of thermal efficiency,pressure rise rate and maximum pressure,the optimal heat release process was explored.Secondly,Under the condition of high exhaust gas dilution and low load,the slower burning velocity limits the economic improvement.The simulation results showed that improving the burning velocity of single peak heat release had the highest indicative efficiency under different heat release characteristics.By early injection of DME to improve mixture activity,the single peak heat release process was optimized by controlling spark timing.In view of the difficulty of ignition in dilution of high exhaust gas,a three-stage heat release process was adopted to stabilize the pre-ignition by late injection of DEM.Because the three-stage heat release process has flame propagation stage,its duration is longer than that of single-peak heat release process.Therefore,single peak could achieve the highest indicated thermal efficiency and can be further increased by 3.88%compared with three-stage.Finally,Under the condition of medium load with dilution of exhaust gas,the problem of excessive pressure rise rate restricting economic improvement should be considered in diluted combustion.The simulation results show that the single peak and three-stage heat release had high comprehensive evaluation results.Therefore,the single peak was studied by early injection of DME and spark timing.Because the increase of COV-_MPRR in the test caused the knock intensive,which limited the heat release rate to approach the simulated shape.At this time,the two injection strategy of DME,taking into account the advantages of early intensified ignition and late spontaneous ignition,realized the optimization of COV-imep and COV-_MPRR by cooperating with ignition.The experimental heat release rate can approximate the simulated heat release rate.Therefore,the maximum indicated thermal efficiency is obtained by using three-stage heat release at the medium load of exhaust gas dilution.Compared with PFI combustion and single peak heat release,the economy is improved by 6.9%and 1.7%respectively.