Numerical Simulation Study On Combustion And Emission Performance Of Diesel Pilot Injected Natural Gas Engine

In recent years,with the overexploitation of petroleum resources and the intensification of environmental pollution problems,it is urgent to find clean alternative fuels.Natural gas is a good alternative fuel which has many advantages such as low emissions,low price,abundant reserves and high octane number,etc.Because diesel-ignited natural gas engines have many advantages such as good economy,low emissions,high compression ratio etc.,they have been widely used at home and abroad.Researches on diesel-ignited natural gas engines in China started late,and many technical problems have not been solved now.Therefore,it is significance to carry out research on diesel-ignited natural gas engines.Based on the bench test data and geometric parameters of diesel-ignited natural gas engine,a three-dimensional simulation model of CFD（Computed fluid dynamics）was established and the validity of the model was verified in this topic.Based on the verified CFD simulation model,the effects of intake retardation angle,excess air coefficient（λ）and natural gas energy substitution ratio on combustion and emissions of dual-fuel engine were studied.The main research contents and results of this topic are as follows:（1）Under full load conditions,when the intake retardation angle was advanced by 10°CA,the peak values of cylinder pressure and heat release rate are higher than the original angle and the CO concentration was reduced significantly,while the NOx concentration was almost not change at all.The concentration of CH₄ decreased first and then increased at 1600r/min,mainly reason was the intensify airflow in the cylinder.（2）Under partial load conditions,asλincreases,the indicated work and indicated thermal efficiency increase,combustion performance of the dual-fuel engine has been improved.While in the full load conditions,in the process of increasingλ,the indicated power and the indicated thermal efficiency appear to increase first and then decrease and the maximum value is obtained atλ=1.3.In addition,with the increase ofλ,the concentration of NOx increases first and then decreases with the influence of oxygen concentration;the concentration of CO decreases rapidly;the concentration of CH₄ showed a trend of decreasing first and then increasing.Whenλ>1.3,the concentration of CH₄ increased.（3）As the natural gas replacement ratio increases,both the peak value of cylinder pressure and heat release rate reduced.That is because less diesel fuel is injected,which reduces the initial heat release rate and prolongs the retardation period.The duration of combustion increase first and then decrease with the increase of the substitution ratio,and reached the maximum at the 60%substitution ratio.The duration of combustion reduced when the replacement ratio excess 60%because the flame propagation speed increased.In the process of increasing the substitution ratio from 50%to 90%,the concentration of NOx and CH₄ increased due to the increase of the temperature in the cylinder,the concentration of natural gas and the concentration of oxygen;the concentration of CO increases first and then decreases,reaching a maximum at 60%substitution ratio.When the substitution ratio is greater than 60%,the increase in the in-cylinder temperature is good for the oxidation of CO to CO₂,and the CO concentration decreases.The research content of this paper reveals the combustion and emission characteristics of diesel-ignited natural gas engines under three parameters:intake retardation angle,excess air ratio and natural gas energy substitution ratio.It provides a theory guide for optimizing the combustion and emission performance of diesel-ignited natural gas engines.