Simulation Study Of The Gas Exchange Process Of Two-Stroke Engine Based On Multi-Objective Optimization

The gas exchange loss in two-stroke aero piston engine is one of the important research directions,especially for the"port-port"type backflow scavenging structure.Although direct injection technology can better control the injection timing and avoid the short-circuit loss of fuel,the mixing of fresh air and exhaust gas in the gas exchange process makes the exhaust gas unable to be discharged and the fresh charge is insufficient,which affects the gas exchange effect.Therefore,it is necessary to design a reasonable gas exchange system structure,effectively use the dynamic characteristics of scavenging and exhaust,improve the charging efficiency,and then improve the engine gas exchange effect,power performance and economic performance.The gas exchange process of two-stroke aero piston engine directly affects the performance of the engine.In this paper,taking a two-stroke aero piston engine as an example,the one-dimensional simulation model of GT-power is established,and the calculation area grids of engine scavenge passage,exhaust passage and cylinder are established by using AVL-fire software.The accuracy of the model is verified by setting the solver,which provides the basis for the Optimization Research of the engine gas exchange process.By analyzing the influence of the structural parameters of the scavenging and exhaust system on the pressure fluctuation of the engine duct and the gas exchange process,including the length and diameter of the scavenging duct,the length and diameter of the exhaust duct,the scavenging timing and the exhaust timing,the variation rules of the structural parameters are obtained.The optimal Latin hypercube sampling method is used to design the test sample points,and the sensitivity analysis of the structural parameters of the intake and exhaust system is carried out.The results show that the key parameter affecting the engine ventilation effect and performance is the length of the duct,and the necessity of multi-objective optimization is analyzed.Because the performance optimization of two-stroke aero piston engine is a multi-objective nonlinear optimization problem,this paper proposes to use NSGA-Ⅱalgorithm to optimize the process and performance of the engine.The optimal Latin hypercube sampling method is used to carry out the experimental design,and the sample matrix of the length of the duct is obtained.According to the sample points,an approximate model is established to optimize and predict the scavenging efficiency and power.The results show that the fitting accuracy of the model is high and the overall prediction is good.Then,the multi-objective optimization of the engine at 5600rpm,100%throttle opening and8500rpm,100%throttle opening was carried out,and the optimization scheme was determined according to the optimization calculation results of the structure parameters of the scavenging exhaust system.The optimization effect of scavenging efficiency was obvious in the optimal design results of Pareto front,which increased from 0.798 to 0.841 at 5600rpm,and from 0.764 to 0.80 at 8500rpm.At the same time,the effect of improving engine economy is also better,the fuel consumption rate is reduced by 22.08 g·k W^-1·h^-1and 18.38 g·k W^-1·h^-1respectively,while the engine power optimization effect is limited,only 4.39%and 4.01%.Next,the optimization platform is used to optimize the different altitude operating points of the aero piston engine（speed is 5600r/min,100%throttle opening）,so as to calculate the optimal duct structure scheme.Comparing the simulation results before and after optimization,it can be found that the engine performance optimization using the optimization platform can achieve good optimization effect.