Research On Performance Optimization Of Natural Gas Engine Based On Early Intake Valve Closing Strategy

As an alternative fuel,natural gas has good development prospects because of its abundant reserves and low combustion pollution.Under thestrict emission standards of China VI,equivalent combustion with three-way catalytic converter has become the mainstream of natural gas engines,but equivalent combustion natural gas engines have the defects of high thermal load and low thermal efficiency.In order to improve the above defects,this paper takes a 6-cylinder four-stroke turbocharged equivalent ignition natural gas engine as the research object,and discusses the effects of different intake valve closing times on engine performance by combining simulation calculations with bench tests;in addition,the effects of ignition time,EGR rate and compression ratio on engine combustion process,economy and emissions under the early closing strategy of the intake valve are studied,which provides a reference for optimizing equivalent combustion natural gas engines.Firstly,through the simulation calculation platform,at the economic working point 1200rpm-2150N·m,the impact of the intake valve closing time on the performance of the engine equivalent combustion natural gas engine was carried out,and the study found that the early intake valve closing(EIVC)cylinder pressure peak was higher,the exothermic heat was more concentrated,and the thermal efficiency was higher;through the microscopic field,the turbulence kinetic energy in the EIVC cylinder was more concentrated,the dissipation degree was slower,and the temperature was higher at the same crankshaft angle.And methane combustion is more complete,which shows that EIVC is better than the late-closing strategy of the intake valve.On this basis,in order to give full play to the advantages of EIVC,the impact of EIVC coupling ignition time on engine performance was studied,and the study found that with the advance of the ignition moment,the peak pressure increased,and the peak phase was closer to the upper stop point,which improved the engine’s functional force and improved the indication thermal efficiency.Compared with other EVC moments,the coupling ignition moment of the engine at 60° of EIVC has the greatest improvement in engine performance,indicating a more significant increase in thermal efficiency.Then,based on the conclusion of the simulation calculation,the prototype processing was guided,and the economy and emissions of the original machine were compared with the original machine at 60°at the early closing of the intake valve through the bench test,and the study found that under different compression ratios,EIVC had economic advantages compared with the original machine at1200rpm-1000N·m and 1200rpm-1700N·m,and the economy was comparable to that of 1200rpm-2150N·m;at the same time,it was equivalent to that of1200rpm-1000N·m The emission advantages of EIVC in m working conditions are more advantageous,and the emission advantages in 1200rpm-1700N·m and1200rpm-2150N·m are not obvious.Then,based on the conclusion of the simulation calculation,the prototype processing was guided,and the engine was compared with the original engine economy and emissions at EIVC60°through the bench test,and the study found that under different compression ratios,EIVC had economic advantages compared with the original machine at 1200rpm-1000N·m and1200rpm-1700N·m,and the economy was comparable to that of 1200rpm-2150N·m;at the same time,it was equivalent to the economy of 1200rpm-1000N·m The emission advantages of EIVC in m working conditions are more advantageous,and the emission advantages in 1200rpm-1700N·m and 1200rpm-2150N·m are not obvious.Finally,the impact of ignition time,EGR rate and compression ratio on the combustion process,economy and emission of the equivalent combustion natural gas engine under EIVC under different working conditions was studied,and it was found that with the advance of the ignition time,the pressure temperature exothermic rate in the cylinder under each working condition showed an increasing trend,the combustion center of gravity was shifted forward,and the functional force was increased,except for the small and medium-sized load effective thermal efficiency increased first and then decreased,the effective thermal efficiency of the remaining working conditions increased with the advance of the ignition time,but the advance of the ignition moment would increase the emission of NOx.The increase of the EGR rate,the cylinder pressure of each working condition,the peak of the cylinder temperature and the exothermic rate all show a decreasing trend,the peak phase shows a lagging trend,the combustion stage shows an extended trend,and the cycle change increases;economically,the small and medium-sized load conditions increase slightly with the increase of the EGR rate,and the rest of the working conditions decrease with the increase of the EGR rate;the emission of NOx decreases with the increase of the EGR rate,the THC increases with the increase of the EGR rate,and the CO changes with the EGR rate is not obvious.The increase of the compression ratio can effectively improve the pressure,temperature and pressure increase rate in the engine cylinder,and improve the thermal atmosphere in the engine cylinder;at the same time,the increase of the compression ratio is conducive to ignition,reduces the cycle change in the cylinder,so that the stagnation period of each working condition is significantly shortened,the improvement can improve the thermal efficiency of the engine,and the effective thermal efficiency improvement in the small and medium-load working conditions is the most obvious;for emission performance,the improvement of the compression ratio helps to reduce the emission of CO and THC,and the emission of NOx is increased.