Numerical Simulation Study On Mixture Formation And Combustion Process Of Off-Road Diesel Engine

Off-road diesel engines are widely used in construction machine,agriculture machine and other engineering machines.In recent years,the output of off-road diesel engine occupies half of the total output of diesel engine.However,off-road engine emission regulation has drawn less attention than automotive diesel engine,although they have compared emission amount.For off-road engine,it has high demand on reliability,while its cost is strictly controlled.Therefore,it is necessary to investigate the in-cylinder fuel-air mixing and optimize the combustion process of off-road engine.The present work aims to reduce emission of an off-road diesel engine for meeting requirement of IV-stage emission regulation,so both one and three dimensional computational fluid dynamic(CFD)were carried out to find appropriate strategy,including the fuel injection strategy and matching techniques of injection and incylinder flow.First,a comprehensive optimization of injection pressure and injection timing at eight conditions were carried out on GT-power platform.Additionally,intake valve timing was optimized based on orthogonal design method.Final,fuel consumption and soot emission can be reduced by 14.7% and by 70.9% respectively.For further investigate the effect of spray parameters with swirl introduced on engine performance,three-dimensional CFD were employed on Converge platform to simulate engine combustion,including injection pressure,spray angle,multiple injection and swirl.At single injection condition,appropriate swirl intensity(1.5)may improve the fuel-air mixing process,thereby reducing fuel consumption and emissions of soot.In addition,proper spray angle(140°)helps to make use of fresh air in combustion chamber and squeeze area,promoting fuel-air mixing and optimizing combustion.Moreover,optimization was obtained through increasing injection pressure and enlarging swirl ratio.With multi-injection strategy,by appropriate adjustment,pre-injected fuel pre-consume mixture and release heat,leading to promotion of the mixture formation and combustion of main injection.But too much pre-injected fuel leads to the overlapping of spray area between pre-injection and maininjection.Under optimized injection time interval,squish flow driven pre-injected fuel to enter the chamber and overlapping could be alleviated by using optimized squish and swirl flow.Consequently,larger power and lower emission were obtained with deterioration of NOx emission.At last,late-injection strategy was studied,it is found that in-cylinder combustion temperature could be enhanced using appropriate postinjection fuel and in-cylinder flow was promoted by post-spray,resulting in reduction of soot emission due to the faster soot oxidation rate and the same level of NOx emission.