Effect Of Timing Of Gas Distribution And Injection On Methane Emission Of Natural Gas Engine

With the shortage of fossil resources and serious environmental pollution,it is urgent to find new clean and renewable fuels with abundant reserves.Because of its abundant reserves and clean combustion,natural gas is widely used as alternative fuel of fossil energy,and becomes a variety of marine engine fuel.However,the research shows that the greenhouse effect of methane is 25 times of that of CO2.Methane emission will also lead to the low economy and combustion efficiency of natural gas engine.China’s emission regulations strictly limit the methane emission of marine natural gas engine,but at present,it is difficult to meet the requirements of marine natural gas engine.Therefore,it is urgent to study how to reduce the methane emission of marine natural gas engine.The methane emission comes from the residual methane in the air inlet and the unburned methane in the cylinder.The timing of gas distribution and injection will directly affect the residual methane in the intake port,the amount of methane entering the cylinder and the gas distribution in the cylinder,and then affect the combustion and the amount of unburned methane in the cylinder,and finally affect the methane emission.The intake multi-point injection natural gas engine has a good response,the methane emission is more serious under low load condition,and the weight of methane emission is larger under high load condition.The three-dimensional simulation method can obtain the detailed information of the complex changes of the gas in the cylinder during the engine working process from the micro point of view.Therefore,this paper uses AVL fire three-dimensional simulation software to study the influence of gas distribution and injection timing on methane emission of multi-point injection natural gas engine under 1000r/min,10% load and 1000r/min,100% load conditions,and makes quantitative analysis on the two parts of methane emission sources,i.e.residual methane in the inlet and unburned methane in the cylinder.The results show that the methane discharged at 1000r/min and 10% load is unburned methane in the cylinder;when the intake valve is closed later than 573 ° CA,the equivalence ratio near the ignition position is slightly reduced,and the methane emission increases;when the intake valve is closed later than 440 ° CA,the methane emission in the cylinder is relatively thick on the right side,which is not conducive to ignition and flame propagation on the left side,and the methane emission increases;about 88% of the unburned methane in the cylinder is discharged out of the cylinder,Methane emission is 1.22g/k W·h-18.7g/k W·h.Under 1000r/min and 100% load conditions,96% of the methane emission comes from the unburned methane in the cylinder;when 563 ° CA to 593 ° CA closes the intake valve,the methane distribution in the cylinder is basically the same,and the methane emission is the same;when the air is injected later than 400 ° CA,the methane distribution on the right side is relatively thick,which is not conducive to ignition and left flame propagation,but the methane emission is the same;about 95% of the unburned methane in the cylinder is discharged,Methane emission is 2.48g/k W·h-3.35g/k W·h.To sum up,most of the methane emission comes from the unburned methane in the cylinder.The reason is that the combustion condition at the clearance cylinder is poor.The solution is that the combustion is well organized in the combustion chamber.Although the residual methane in the air inlet will not be discharged basically,it will lead to the fact that the methane actually involved in combustion cannot be accurately controlled,resulting in the increase of cycle variation.Reducing methane emissions is conducive to improving gas efficiency and reducing CO emissions,but it will lead to an increase in NOx emissions.