Effects Of Water Direct Injection On Knock,Combustion And Emission Of GDI Engines

Supercharged downsizing technology,formed by the mutual coordination of in-cylinder direct injection technology and turbocharging technology,is the most effective ways to improve fuel economy and reduce emissions of gasoline engines.However,the use of this technology greatly increases the heat load and pressure in the cylinder,resulting in knocking combustion phenomenon,which affects the application and development of supercharged downsizing technology in practice and is the main factor limiting the performance improvement of GDI engine.The author proposed a numerical simulation method to study the feasibility of in-cylinder water injection technology to reduce knocking trend and improve combustion emission performance of GDI engine.Firstly,the knocking combustion of a GDI engine is induced by increasing the ignition advance angle to determine the auto-ignition position of the end mixture.Afterwards,the distribution of water droplets in cylinder at different injection times and their effects on combustion emission performance are investigated.Finally,the initial parameters of water injection are modified,the effects of water injection rate on knocking and combustion emissions of GDI engine are investigated at a given injection time.In order to better suppress knocking by direct water injection in cylinder,it is necessary to determine the location of knocking.The numerical simulation model of a GDI engine is validated to ensure the accuracy of the numerical simulation model.Under high speed and high load conditions,the numerical simulation of knocking is realized by increasing ignition advance angle and the location of knocking is determined.The results show that the auto-ignition of the end mixture will cause a sharp increase in the flow field,with the maximum flow rate exceeding 150 m/s.Flame isolation can be observed in the vicinity of the cylinder wall when the explosion is caused by the auto-ignition of the end mixture.The most drastic change of pressure at monitoring points is in the areas of No.1,No.4,and the peak pressure at No.4surveillance points exceeds 23 MPa,which indicates that these four areas are the most vulnerable areas to knocking.In order to reduce knocking better,it is also necessary to determine that water droplets can be distributed in the area where knocking occurs most intensely.Therefore,the influence of the distribution of water droplets in the cylinder on combustion and emission performance is studied when the injection time is between-120°CA and-20°CA.The results show that:The most uniform distribution of water droplets in the cylinder is at 80°CA.Different injection time has no effect on the mixing of fuel and air.Direct injection of water in cylinder can not improve the mixing of fuel and air.When the injection time is-80°CA,the pressure change in the area of No.4 is the smallest,so it is considered that the injection time has the potential to reduce knocking.When the injection time is-80°CA,the initial spray parameters are corrected because of the“wet wall”phenomenon in the distribution of water droplets.The effects of 0-25%water injection rate on knocking reduction,combustion and emission performance are investigated.The results show that the distribution of water droplets is more uniform and the knocking suppression effect is better after correction.With the increase of water injection,the distribution area of water droplets in the cylinder increases gradually.When water injection rate exceeds 20%,the water droplets basically distribute in the whole cylinder area.With the increase of water injection rate,the knocking intensity decreases gradually,the water injection rate is 25%,and the knocking intensity decreases to 0.286 MPa.For the cyclic work,with the increase of water injection,the cyclic work decreases gradually.When water injection rate is 15%,the cyclic work is still higher than the experimental work of 37.63J.For emission analysis,when water injection rate is between 5%and 25%,with the increase of it,the NO_X emission decreases gradually,and the NO_X emission is slightly lower than that of 5%without water injection.Soot emission increases with the increase of water injection when the water injection rate is 5%-25%and it is lower than that of non-water injection when the water injection rate is 5%.CO emission without water injection is higher than that of in-cylinder direct water injection,and in-cylinder direct water injection can reduce it.