Combustion Characteristics Of Diesel Ignition Natural Gas Engine Based On In-cylinder Activity Control

Natural gas is a low-carbon fuel,and realizing the efficient and clean utilization of natural gas in internal combustion engines can alleviate the increasingly severe energy shortage and environmental pollution problems.In this paper,the diesel pilot natural gas engine is the main research body,uses the single cylinder engine test in combination with the three-dimensional combustion simulation and the method of chemical dynamics analysis to study the effects of fuel injection quantity,ignition diesel injection strategy and intake pressure,EGR synergistic coupling on the evolution process of active groups in cylinder,chemical reaction path,combustion characteristics,thermal efficiency and pollutant emission under different working conditions.The results provide a reference for efficient and clean combustion of diesel ignited natural gas engine.In this paper,under the condition of engine speed of 1000rpm and IMEPg of 6bar,the influence of fuel injection amount on the formation and consumption process of active groups in cylinder and combustion stability was investigated,and the minimum fuel injection amount for stable combustion was obtained.The results show that the mass of active groups generated in the cylinder increases with the increase of fuel injection amount,the activity of the mixture increases,and the combustion is more stable,and the ITEg increased.However,high fuel injection amount leads to incomplete premixing of diesel oil,and increasing fuel injection amount has little effect on the combustion process of natural gas.Therefore,the minimum fuel injection quantity under this working condition is 8mg/cycle,which can ensure the stable combustion of the mixture.Based on the minimum fuel injection amount,the influence of ignition diesel injection strategy on combustion and emission characteristics was further studied,and the influence mechanism was analyzed in combination with the evolution process of active groups in the cylinder and chemical reaction path.The results show that the ignition phase can be effectively controlled by injection of ignition diesel near TDC,but the active groups of combustion reaction are mainly generated by diesel pyrolysis,which results in a slow combustion rate.The injection of pilot diesel in the early stage of the compression stroke can provide a large number of active groups,especially to promote the rapid conversion of methyl(CH3)to formaldehyde(CH2O),accelerate the formation rate of CH2O and H2O2,and greatly increase the combustion reaction rate.However,this strategy is difficult to control the ignition phase,and too many active groups cause the overall combustion to occur before the compression top dead center,the negative compression work increases,and the ITEg is not significantly improved.Therefore,this paper proposes a two-injection strategy for pilot diesel.The early injection of diesel provides active radicals,and the late injection of diesel controls the ignition phase to achieve efficient and clean combustion.When the engine speed is 1000rpm,the intake pressure is 1.1bar,and the IMEPg is 6bar,the strategy of piloting diesel with two injections(SOI1:-60℃A,SOI2:-4℃A ATDC)can achieve a ITEg of 49.34%.Under heavy load,the effects of intake pressure and EGR coupled injection timing on in-cylinder activity and combustion performance were studied.The research shows that the starting point of mixture combustion is mainly controlled by the injection timing,and the intake pressure and EGR have little effect on the starting point of combustion.The higher intake pressure inhibits the evaporative atomization process of diesel,resulting in a smaller fire nucleus and slow flame propagation speed when igniting diesel,but it promotes the initial oxidation reaction of CH4 and improves the overall activity in the cylinder.Due to the smaller amount of pilot fuel,higher thermal efficiency and emissions can be achieved with higher intake pressure than with relatively lower intake pressure and earlier injection timing.The addition of EGR inhibits the oxidation reaction of the fuel,which greatly reduces the quality of the generated active groups,resulting in a decrease in the concentration of active groups in the cylinder,a decrease in the combustion heat release rate,and an increase in the incomplete combustion loss.Appropriate EGR rate is conducive to large load expansion.Combining the effects of intake pressure and EGR coupled injection timing on in-cylinder activity and combustion characteristics,high intake pressure,low EGR and advanced injection timing can achieve high thermal efficiency and low THC and CO emissions,but higher NOx emissions.Under the condition of engine speed of 1000rpm and IMEPg of 17bar,using intake pressure of 3.4bar,40%EGR,and fuel injection timing of-4℃A ATDC,the ITEg can reach 50.46%.