Research On Key Injection Parameters And Knock Problems Of In-cylinder Direct Injection Hydrogen Engine

The rapid growth of traditional fuel vehicles has aggravated the energy crisis and environmental pollution.Finding clean and renewable energy is one of the effective measures to solve these two problems.Hydrogen is a carbon free energy source that has received widespread attention due to its clean combustion characteristics and has become a strategic energy source that many countries focus on developing.Applying hydrogen to engines can achieve zero carbon emissions for vehicles,which is one of the key measures for China to achieve the "dual carbon" goal in the transportation sector.At present,the thermal efficiency and knock issues of hydrogen engines restrict their promotion and application.Therefore,this article uses numerical simulation methods to study the combustion process of an in cylinder direct injection hydrogen engine,mainly exploring the effects of injection parameters and knock on its thermal efficiency.In order to improve the thermal efficiency and reduce emissions of hydrogen engines,the effects of four key injection parameters,namely nozzle diameter,injection angle,injection pressure,and injection timing,on the combustion and emission characteristics of hydrogen engines were investigated.The results indicate that:(1)Appropriate injection parameters help to form an ideal layered mixture of hydrogen and air,enabling hydrogen engines to achieve higher indicated thermal efficiency(ITE)and lower NOx emissions.(2)Increasing the nozzle diameter and injection pressure can increase the turbulent kinetic energy in the cylinder and promote the uniform mixing of hydrogen and air.However,when the nozzle diameter increases to3 mm and the injection pressure increases to 6MPa,the promoting effect of turbulent kinetic energy on the uniformity of the mixed gas decreases due to the increase in hydrogen mass flow rate,and the uniformity of the mixed gas actually decreases.(3)An appropriate injection angle can not only improve the uniformity of the mixture but also reduce NOx emissions;It can also reduce the heat transfer loss caused by the contact between the hydrogen jet and the cylinder wall,and improve the ITE of the engine.As the injection angle increases,ITE first increases and then decreases.When the injection angle is50°,ITE is the highest.(4)Delaying the injection timing shortens the diffusion and mixing time of hydrogen gas,which will reduce the uniformity of the mixture.However,when the injection timing is delayed by 100 °CA before the top dead center(BTDC),strong turbulence is formed due to the collision between the hydrogen jet and the concave pit at the top of the piston,which improves the uniformity of the mixture,increases ITE,and significantly reduces NOx emissions;When the injection timing is 80 °CA BTDC.Due to the injection timing is too late,some concentrated mixture will accumulate on the cylinder wall,resulting in increased heat transfer loss,decreased ITE,decreased mixture uniformity,and increased NOx emissions.Aiming at the knock problem that restricts the improvement of thermal efficiency in hydrogen engines,the effects of three key parameters,ignition time,compression ratio,and equivalence ratio,on knock were studied.The results showed that:(1)With the advance of ignition time,the knock intensity showed a continuous increasing trend,with ITE first increasing and then decreasing.When the ignition time was 14 °CA BTDC,ITE was the highest.(2)Increasing the compression ratio is beneficial for improving the engine’s ITE,but it is limited by knock.When the ignition time is 12 °CA BTDC,the compression ratio increases from 12.5 to 13.0,and the knock intensity first increases and then decreases,with ITE continuously increasing.However,when the compression ratio increases to 13.2,the knock intensity increases again,and ITE no longer increases.(3)Under lean combustion conditions,using a lower equivalence ratio is beneficial for improving ITE.When the equivalence ratio is 0.6,the detonation intensity is 1.51 MPa,close to super knock,and the ITE is 44.41% at this time;The equivalence ratio decreased from 0.6 to 0.4,and the knock intensity gradually decreased to 0.29 MPa,approaching slight knock.The ITE continued to increase,reaching a maximum of 45.88%.The research results provide a theoretical basis for suppressing hydrogen engine knock and improving its thermal efficiency.