Numerical Simulation On Combustion System Optimization For High Efficiency Heavy Duty Diesel Engine

As energy and environmental issues become prominent,improving the thermal efficiency of diesel engines and reducing their pollutant emissions has become a top priority for researchers.Increasing the compression ratio of the diesel engine is an effective method to improve the thermal efficiency of the diesel engine,but at the same time it will also induce a series of problems such as the increase of the peak pressure.With the continuous development of diesel engine technology,the strengthen of the engine block and parts are continuously improving,and the peak in-cylinder pressure is also continuously increasing,which implies that increasing compression ratio has become a feasible method to improve the thermal efficiency of diesel engines.In this paper,a three-dimensional simulation model of diesel engine was setup based on the numerical simulation software CONVERGE.Firstly,the influence of combustion chamber geometric compression ratio on the combustion process and emissions of diesel engine was studied.On this basis,the potential of the coupling of compression ratio and excess air coefficient to improve the thermal efficiency of diesel engine was explored.The results show that increasing the compression ratio can effectively improve thermal efficiency.However,under the condition of low excess air coefficient condition,excessively high compression ratio will prolong the combustion duration and reduce the thermal efficiency.Increasing the excess air fuel ratio has significant effect on combustion and thermal efficiency of diesel engine.The combination of high compression ratio and excess air fuel ratio can effectively improve thermal efficiency.When the excess air fuel ratio is low,it has a great influence on thermal efficiency,thus increasing excess air fuel ratio should be one of the main measures to improve thermal efficiency.However,as the excess air coefficient is further increased to about 2.3,its effects on combustion and thermal efficiency gradually diminished.At this time,higher compression ratio should be adopted to improve the thermal efficiency.With peak pressure constrains of 20MPa,25MPa and 30MPa,the compression ratios corresponding to the highest indicated thermal efficiency are 17.5,20.5 and 23.5,with indicated thermal efficiency improvement of 2.27%,3.92% and 5.11%,respectively.Based on the above research,a combustion chamber with compression ratio of 20.5 was selected,and the effects of combustion chamber structure and fuel injection system parameters were studied and optimized to explore the potential for thermal efficiency improvement with peak pressure constraint of 25MPa.The results show that the combustion chamber geometry affects the spatial distribution of the mixture,and by optimizing the combustion chamber,the combustion duration can be effectively shortened,and the thermal efficiency can also be improved;keeping the fuel injection flow rate the same,increasing the number of injection holes and reducing the diameter of the injection holes have little effect on thermal efficiency,but can effectively reduce the soot emission;higher injection pressure can significantly improve the mixing process,the combustion duration can be shorten,and consequently higher thermal efficiency can be obtained,together with lower soot emission,however,NO_x increases.By adjusting the start injection time to-6°CAATDC,soot emission can be reduced while maintaining high thermal efficiency,but at the same time NO_x emission increase.With 25MPa peak pressure constraint,the indicated thermal efficiency can be improved by 0.55% compared to original configuration through combustion system optimization.Compared with the original engine,the indicated thermal efficiency increased from 42.80% to 47.27% through combustion system optimization,with an increase of 4.47%.The soot reduced to 29% compared with original engine,and the NO_x increased to 220%compared with original engine.In conclusion,the present work has studied the influence of compression ratio and excess air coefficient on combustion and thermal efficiency under different peak pressure conditions,then combustion system optimization was conducted with a peak pressure limit of 25MPa,and it was found that the indicated thermal efficiency can be significant improved with the current approach.Therefore,the current study should have important guiding significance and engineering application value for the development of combustion system of high efficiency diesel engine.