Study On The Influence Of Intake Port Water Injection On GDI Engine Emission Performance

In order to reduce the emission of small supercharged GDI engines and to avoid abnormal combustion phenomena such as knocking,delaying ignition time,exhaust gas recirculation and enriching mixture are often used to slow down the combustion process under heavy load conditions.But these methods will cause dynamic and economical loss.Water injection technology has the ability to reduce in-cylinder emission,improve economy and suppress knocking,which is a promising engine application technology in the future.This project built a GDI engine bench with intake port water injection function,and then used both engine bench test and numerical simulation with chemical reaction kinetics software to analyze the physical and chemical influence of the intake port water injection technology on the emission performance to explore its potential to reduce emission and improve engine performance.By changing the width of the water injection pulse,the effect of different water injection mass on the in-cylinder emission were studied.The results show that as the amount of injected water increases,the temperature in the cylinder decrease continuously and the NO_X emission decrease correspondingly.Since the participation of water in the combustion reaction can bring about the micro-explosion effect and promote the water gas reaction,so that soot particle emission and CO emission decrease with the increase of the amount of injected water.But when the injected water mass increases too much,the combustion in the cylinder begins to deteriorate,the soot emission begins to increase.With the increase of the amount of injected water,the continuous decreasing combustion temperature in the cylinder makes the quenching layer thicker,resulting in the increase of HC emission.Water injection makes the combustion in the cylinder lag behind,but a small amount of water spray can promote the mixing of mixtures.Therefore,with the increase of injected water mass,BMEP decrease continuously,and the BSFC decreases first and then increases.By adjusting the water supply pressure of the pump,the effects of different water injection pressure on the in-cylinder emission were studied.The results show that the increase of the water injection pressure can make the water vapor mix with the in-cylinder mixture better,so that the cooling effect is more obvious,and the micro-explosion effect is enhanced,and the water gas reaction is further promoted.Therefore,with the increase of water injection pressure,the NO_X emission,soot particulate emission and CO emissions are further reduced,and the HC emission is also decreased,but still higher than the original base condition.In order to avoid excessive dynamic and economical loss caused by the intake port water injection,the optimization strategy of advancing the ignition time until the detonation boundary and enlarging the air-fuel ratio of full-load conditions under the water injection condition is adopted.It is found that the economy of the engine is greatly improved,and the power loss is kept within a small range,and the basic emission can be well controlled after optimization.Through chemical reaction kinetics numerical simulation and correlation analysis,it is found that H,O,OH,HO₂,H₂ and H₂O₂ are closely related to NO_X formation,and the influence of O and OH on the production of NO_X are the strongest.After injection,water participates in the combustion reaction as one of the reactants,which reduces the reaction rate and the amount of production of each radical.Especially,the decrease of O and OH greatly affects the production rate of NO_X,thereby reducing the NO_Xemission by inhibiting chemical reactions.