Study On The Lean Burn Performance Of MPI+GDI Engine

Improving the thermal efficiency and reducing emissions of internal combustion engines,and achieving clean and efficient combustion of internal combustion engines have become the focus of the industry.Methanol has excellent performance and is a recognised alternative fuel for automobiles.This paper is based on an optical in-cylinder direct injection engine,retrofitted with an inlet tract injection system to form a combined injection system of methanol inlet tract injection and gasoline in-cylinder injection(MPI+GDI).A simulation model was established and verified by cylinder pressure and spray tests to study the combustion and emission characteristics of lean burn under both stratified and homogeneous conditions.The results show that.(1)In the stratified lean burn mode,(1)The higher the excess air ratio,the slower the flame propagation,the later the peak of the exothermic rate,the lower the peak in-cylinder pressure and the later the moment of occurrence,the more the optimal ignition timing interval shifts forward and the indicated thermal efficiency increases,reaching 41.98% at an excess air ratio of 1.5 for M90 fuel;in terms of emissions,CO and NOx emissions decrease,while unburned hydrocarbon(THC)emissions first decrease and then increase,with an excess air ratio of 1.4 emissions are relatively low.(2)Methanol substitution ratio increases,flame propagation speed accelerates,centre of gravity of combustion shifts forward,peak in-cylinder pressure rises and advances,optimal ignition timing interval is delayed,the difference between M80 fuel and M90 is small,methanol blending significantly expands the lean burn limit of MPI+GDI dual-fuel engines;in terms of emissions,CO and carbon soot emissions decrease,NOx emissions increase,THC emissions first fall and then rise.(3)The speed increases,the incylinder combustion characteristics improve first and then deteriorate,the best combustion is at 1600r/min.The increase in turbulence energy promotes the formation of stratified mixtures,but too much turbulence energy causes the airflow to blow out the flame propagation and slow down the spread of fire core.With M80 fuel,increasing the ignition energy helps to increase cylinder pressure,but higher ignition energy is not better and has little effect when the ignition energy is greater than 0.2 J.(2)In homogeneous lean burn mode,(1)The methanol substitution ratio increases,the centre of gravity of combustion moves forward,the peak in-cylinder pressure and the peak exothermic rate become larger,the difference is smaller when the methanol substitution ratio is higher than 80%;in terms of emissions,CO and carbon soot emissions decrease,NOx emissions increase,NOx emissions are similar when the methanol replacement ratio is greater than 80%,THC and non-conventional emissions formaldehyde and methanol show an increasing trend.(2)As the excess air ratio increases,the peak of the heat release rate and incylinder pressure decreases,the centre of gravity of combustion is pushed back and the combustion isotropy becomes worse.It can be seen that for MPI+GDI engines,homogeneous lean burn is better than stratified lean burn when the methanol substitution ratio is lower;when the methanol substitution ratio increases,stratified lean burn improves.The homogeneous lean burn emission advantage is on CO and carbon soot,and the stratified lean burn emission advantage is on NOx.For THC emission,homogeneous lean burn with low excess air ratio is better,and stratified lean burn with high excess air ratio is better.