Simulation Research On Compression Ignition Combustion Of The Dual-injection Diesel Engine

In this paper,the physicochemical properties of coal-based methanol and FT diesel are fully utilized,and the characteristics of combustion and emission of dual-injection diesel engine are explored.Firstly,the appropriate mathematical model and calculation model are selected in the AVL-FIRE simulation software.Then the combustion chamber structure and the initial swirl ratio are changed to optimize the gas flow in the engine cylinder,so that the mixture gas can more easily reach the ideal state of ignition.On this basis,the optimal methanol injection timing at 1200r/min and 1800r/min is researched,and the optimal methanol energy ratio and maximum methanol enery ratio at 1800r/min medium and low load are studied.The changes of fuel injection pressure,injection cone angle and compression ratio are used to investigate the effects of combustion characteristics and emission characteristics of dual-injection diesel engines.The results show:1)The swirl formed in the reentrant chamber is stronger than the straight chamber and the open chamber.Its turbulence lasts for a long time.The cylinder pressure can reach 7.23 MPa.The constant volume ratio is optimal.The pressure gradient in the cylinder is evenly distributed.The SOOT and CO emissions are lower,but the NOx emissions are slightly higher.As the initial swirl ratio increases,the airflow movement in the cylinder becomes more and more intense,which is beneficial to improve the fuel-air mixing rate and the burning rate.However,when the swirl ratio is too large,the fuel is displaced in the circumferential direction too much,and the adjacent fuel bundles overlap.It will lead to combustion deterioration.The optimal initial swirl ratio is SR=2.2)The optimal methanol injection timing under the two conditions of 1200r/min low load and 1800r/min medium load are 130℃A BTDC and 55℃A BTDC respectively.Except for NOx emissions,other harmful emissions both have been improved.The optimum methanol energy ratio at 1800r/min is 50%.The maximum methanol energy ratio is limited by the mechanical load and emissions of the engine,and it should not exceed 70%.3)With the increase of fuel injection pressure,the penetration distance of the spray increases,the atomization degree of the fuel is improved,the peak of the heat release rate increases,the starting point of combustion advances,the SOOT and CO emissions decrease,and the NOx increases slightly.The injection cone angle has a great influence on the equivalence ratio in the cylinder.When the injection cone angle is small,the fuel is close to the bottom of the cylinder and gathers at the bottom of the cylinder.It is not conducive to oil and gas mixing.When the injection cone angle is too large,more fuel will enter the clearance volume.It results in uneven oil and gas mixing and increases SOOT and CO emissions.The compression ratio has the same effect on the dual-injection diesel engine as the traditional diesel engine.With the improvement of the compression ratio,the starting performance is improved,the fuel mixture can be fully burned,the brake mean effective pressure is increased,and the brake specific fuel consumption is reduced.The compression ignition combustion mode of dual-injection diesel engine has a good development prospect.It can effectively improve its combustion and emission characteristics by changing engine structural parameters and operating parameters.