Simulation On Gas Exchange Process Of Super Long Stroke Low Speed Diesel Engine And Valve Distribution Performance Optimization

The application technologies for marine diesel engines have been improved significantly over the past century,which caused more and more severe problems to the energy and the environment at the same time.Improving the fuel economy and reducing the emissions has become an important development direction of marine diesel engines.Long-stroke and superlong-stroke low-speed engines are increasingly used for marine application in recent years.The simulation modelling for super-long-stroke low-speed engines differs from that for four-stroke diesel in terms of the physical structure and the gas exchange process,whilst the gas exchange process makes the difficulty.Therefore,it’s of great significance to investigate the gas exchange process simulation and the distribution performance optimization of super-long-stroke lowspeed engines.In order to simulate the gas exchange process of super-long stroke low-speed engines,three commonly used theories for the modeling of diesel engine gas exchange process are analyzed firstly,i.e.,the complete mixing theory,the complete stratification theory and the "mixed-layered" theory.Then,three corresponding models are developed and compared in Matlab/Simulink to find the most suitable theory for modeling the gas exchange process of twostroke diesel engines.Results show that the errors of the fully mixed model and the fully stratified model are large,whilst the error of the "mixed-layered" model remains within the acceptable range if the proportion coefficient is reasonably selected.Confirming the "mixedlayered" as the most suitable theory for the gas exchange process of two-stroke diesel engines,it is applied to the modelling for a 7G80ME-C super-long-stroke low-speed diesel engine.The three models are compared and analyzed.The influence of proportional coefficient on scavenging characteristics in the "mixed-layered" model under 100% load was studied,and the proportional coefficient and simulation results of other operating conditions of diesel engine were obtained.As the hydraulic valve distribution mechanism is used for the investigated engine,the closing time of exhaust valve under 100% load was optimized as well as a singlefactor optimization for the maximum lift and scavenging timing.Finally,a three-factor orthogonal experiments were carried out to optimize the exhaust timing,scavenging timing and the maximum lift of the exhaust valve.The air exchange process model based on the "mixed-layered" theory in this study can accurately simulate the air exchange process of a marine super-long stroke uniflow scavenged two-stroke diesel engine.Besides,it provides accurate prediction for the scavenging characteristic of two-stroke diesel engines when combined with experimental data.Moreover,the air exchange efficiency can be improved when the exhaust valve closes properly in advance.The work provides a basis for the future low-speed machine modeling,which has important academic value and wide application prospects.