Research On Combustion And Emission Characteristics Of Methanol-gasoline Dual Fuel Engine Based On Compound Injection System

Exploring energy conservation and emission reduction approaches and finding alternative fuels have been the focus of the industry.As a kind of renewable energy,methanol is an ideal fuel for internal combustion engine because of its high latent heat of vaporization,high octane number,high oxygen content,fast laminar flame speed and wide lean burn limit.In order to use it flexibly and efficiently in the current spark ignition engine,a compound injection system combining methanol port injection and gasoline direct injection(MPI + GDI)is used to realize the flexible adjustment of different methanol substitution ratio and different combustion modes.Based on a direct injection optical engine adding a methanol port injection system.The spray and cylinder pressure tests were carried out to verify the CFD model.The formation,combustion and emission characteristics of the mixture in the cylinder under the condition of homogeneous equivalence ratio and stratified lean burn were studied.The results show that:(1)In the mode of MPI + GDI homogeneous equivalence ratio,with the increase of methanol substitution ratio,the turbulent kinetic energy in the cylinder decreases.At ignition time,M20 fuel mixture has the highest uniformity and M80 is the lowest.The concentrated area of the mixture is near the piston pit,and the lean area is near the squeeze surface on the opposite side of the injector.With the increase of methanol substitution ratio,the average temperature in the cylinder decreases continuously,but when the substitution ratio exceeds 60%,the phenomenon of local supercooling appears,which restrains the evaporation of methanol fuel.In the combustion process,the increase of methanol substitution ratio leads to the increase of the peak value of the pressure and instantaneous heat release rate in the cylinder,and the corresponding crankshaft angle keeps advancing.In addition,the combustion duration and delay period are shortened,CA50 is advanced,combustion isochority is higher,and the temperature in cylinder increases after ignition.As for conventional emissions,with the increase of methanol substitution ratio,the NOx emission in the cylinder increases continuously,and the opposite side of the injector produces the lowest amount;the CO emission in the cylinder decreases,but the peak CO emission increases first and then decreases;the THC emission increases,and more than 95% of the THC composition is unburned methanol.(2)In the mode of MPI + GDI stratified lean combustion,the piston pit has a great influence on the stratified effect.When the pit depth is 7mm and the diversion surface is 45 °,the concentrated mixture is the least.When the GDI injection time is 630° CA,the gasoline equivalence ratio mixture has the highest mass fraction and can concentrate near the spark plug.The mass fraction of the concentrated mixture is the least.When GDI injection pressure is 20 MPa,the area of the stoichiometric mixture near the spark plug is the largest and the concentrated mixture is the lowest.With the increase of methanol substitution ratio,the mass fraction of stoichiometric mixture near the spark plug is reduced,the coverage area is also reduced,and the thin mixture around the spark plug is enriched.The speed has little effect on the formation of stratified thin mixture,and the reasonable design of piston pit can form excellent stratified mixture.In the process of combustion,the peak pressure and peak heat release rate of methanolgasoline stratified lean combustion,gasoline stratified lean combustion,methanol homogeneous lean combustion and gasoline homogeneous lean combustion decrease in turn,the ignition delay period extends in turn,and CA50 delays in turn.In addition,the combustion duration of methanol homogeneous lean burn is shorter than gasoline stratified lean burn.With the increase of methanol substitution ratio,the peak value of cylinder pressure and the peak value of heat release rate of methanol-gasoline stratified lean combustion first increase and then decrease;the ignition delay period continues to extend;the combustion duration first shortens and then extends;CA50 first advances and then delays.Inflexion points are all at M90 fuel.As for conventional emissions,the NOx emission of methanol-gasoline stratified lean combustion is the highest,but it is still 80.8% lower than that of stoichiometric combustion.And then the order of NOx emission from high to low is gasoline stratified lean combustion,methanol homogeneous lean combustion and gasoline homogeneous lean combustion.The emission of CO and THC of methanol gasoline stratified lean combustion is the lowest.With the increase of methanol substitution ratio,the NOx emission and the peak value of CO decrease,but the THC emission increases.