Simulation Study On Combustion Process Of High-power Diesel Engine Under The Condition Of Ultra-high-pressure Fuel Injection

Diesel engines have the advantages of high cycle thermal efficiency,good fuel economy,and reliable and durable operation,which are widely used in large machinery,heavy-duty vehicle and marine power and so forth.With the improvement of requirements of the powertrain of the heavy machinery in the fields of industrial production and national defense,as well as people’s increasing attention to energy and environmental issues,the requirements for diesel engine combustion and emission performance are also increasing.Among the present technologies,increasing fuel injection pressure is one of the main ways to improve the combustion performance of diesel engines.At present,the fuel injection gradually moves towards ultra-high pressure.However,there is a lack of research on combustion characteristics of diesel engines under the ultra-high fuel injection pressures.Therefore,it is of great practical significance and engineering application value to clarify the influence of ultra-high pressure(UHP)spray parameters on the combustion and emission characteristics of diesel engines.In this paper,the geometry model of combustion chamber is established by three-dimensional simulation software fire.According to the grid independence,the grid of combustion chamber is divided and the corresponding dynamic grid is generated.Various models are selected and the experimental cylinder pressure curve and simulation curve are compared by comparing the experimental results.The key parameters of EBU turbulent combustion model and wave model are determined and the accuracy of the model is verified.The effects of fuel injection pressure(160MPa,200 MPa,250Mpa and 300 mpa,respectively)on turbulent kinetic energy,flame development,heat release rate,ignition delay,combustion duration and average pressure and temperature in the cylinder were studied.The atomization and mixing and combustion performance of the diesel engine under the condition of ultra-high-pressure fuel injection pressure were analyzed.The results show that the ultra-high-pressure fuel injection can significantly improve the atomization quality,enhance the turbulent kinetic energy in the cylinder,promote the mixture and combustion of oil and gas,effectively shorten the combustion delay and duration,and improve the thermal efficiency and fuel economy.Then,under the condition of ultra-high-pressure fuel injection(300mpa),the influence of combustion system parameters(injection timing,angle between fuel bunches,number of injection holes,hole diameter and swirl ratio)on fuel atomization,fuel gas mixing,combustion and emission performance in cylinder is studied.At the same time,in order to find out the influence of the secondary injection on the combustion and emission performance of the diesel engine under the condition of ultra-high pressure fuel injection,in this study,the fixed interval of the two injection is 15 ° CA,and the pre injection volume is 5% and 10% of the total fuel volume respectively.Simulation analysis of the influence of 15% on combustion process.The results show that with the increase of injection timing(injection in advance),the combustion heat release will be advanced,the combustion duration will be shortened,and the delay combustion period will increase linearly;with the increase of the angle between the fuel bunches,the drop point of the fuel bunches will shift to the residual area of the combustion chamber,which can promote the formation of the combustion air mixture.With the increase of the number of nozzle holes,the average droplet size of the spray droplet decreases,and the fuel atomization is improved.The increase of swirl ratio intensifies the circumferential flow of gas in the cylinder.For a specific combustor structure,the selection of swirl ratio is not easy to be too large or too small,and there is an optimal value.When the injection pressure of the combustion system in this study is 300 mpa,the optimal thermal efficiency can be obtained when the swirl ratio is 0.5.The introduction of pre injection strategy can make the cylinder pressure change more smoothly,which is conducive to the smooth operation of the diesel engine.Finally,in order to clarify the influence of different combustion chamber shapes on the performance of diesel engine,the influence of three different combustion chamber shapes on the mixture formation,combustion heat release and performance in cylinder is studied and analyzed.The effects of the ratio of diameter to depth(5.1,7.7,4.6,6.8,5.2,6.7,respectively)of each combustor shape on the mixing and combustion performance of oil and gas were compared and analyzed.The results show that the mixture of flared combustor is the most uniform,the combustion process and flame propagation speed are faster,and the peak pressure and temperature in the cylinder can be reduced obviously by increasing the aspect ratio.Finally,taking fuel economy as the main index,the optimal combustion chamber shape is chosen as flaring combustion chamber,the diameter depth ratio is 5.2,and then the peak value of combustion pressure in the cylinder is controlled by optimizing injection timing.There are 102 pictures,12 tables and 66 references.