Study On The Influence Of Port Water Injection On Combustion And Knock Of GDI Engines

Gasoline direct injection engines(GDI)can not only promote the formation of the in-cylinder mixture and increase the thermal efficiency of combustion,but also reduce the emission of particulate matter and nitrogen oxides.However,with the increase in the compression ratio and the load of the GDI engine,knocking is very easy to occur.As a simple and effective method,water injection can not only effectively suppress the knocking combustion of the engine but also reduce the emission of pollutants.Because the higher latent heat of vaporization of water can reduce the combustion temperature in the engine cylinder,at the same time,a series of combustion chemical reactions that water participates in can effectively suppress the sudden change of the knock mechanism.In this paper,numerical simulation is used to explore the inhibition effect of different water injection strategies of intake port water injection on GDI engine knocking.On the basis of the knocking condition of a certain type of in-cylinder direct injection gasoline engine,the combustion reaction of different water injection strategies(water injector position,water injection temperature,water injection timing)on the end mixture region around the cylinder is analyzed from the perspective of chemical kinetics process and the influence mechanism of HCO radical generation,thus revealing the influence mechanism of different water spray strategies on knocking.The effects of different water injection strategies on in-cylinder combustion and emissions are analyzed from the physical aspect.In this study,the geometric model of the GDI gasoline engine is first established by3 D simulation software,and then the appropriate numerical sub-model and fuel mechanism are selected.Validation,to determine the feasibility of the selected model.Finally,the compression ratio was increased to 10.3,and the ignition timing was adjusted to-14℃A to induce knocking of the engine.Under the above-mentioned knocking conditions,six different sprinkler installation methods are designed according to the distance and angle from the intake valve.The position of the water sprayer is parallel to the intake port(90°)and closer to the intake valves(0.07 mm and 0.1 mm),which can effectively suppress knocking.Because the atomization effect of water and fuel is better,the equivalence ratio distribution is more uniform,the combustion in the cylinder is sufficient and uniform,and there is no pressure fluctuation in the cylinder,nor a sudden change of HCO radicals.The circulating power of this sprinkler arrangement is higher than that without water spraying,and the closer it is to the intake valve,the greater the circulating power.The farther away,CO emissions are the lowest.Selecting the installation method of the water sprayer with a distance of 0.07 mm from the center of the cylinder and a direction of 90°,and setting the water spray temperature of 298 K,338K,and 393 K.The atomization effect of water and fuel is inversely proportional to the water spray temperature,and the knocking intensity increases with the increase of the water spray temperature,so the 298 K water spray temperature can completely suppress the knocking and can effectively discharge soot and CO.In the above sprinkler arrangement,the water spray temperature is set to 298 K,and the suppression effect of different water spray timings with a gradient of 20 ℃A on knocking is explored from-370 ℃A to-290 ℃A.The results show that if the water injection time is too early or too late,the flame propagation speed will be accelerated,but if the water injection time is too late,it will not only deteriorate the atomization of fuel in the cylinder and the mixing effect of oil and gas,but will eventually fail to suppress the occurrence of knocking,and will also increase soot and CO.emissions.When the water spraying time is set from-370 ℃A(Crank angle)to-330 ℃A,the flame propagation speed increases gradually,but no knocking occurs.When the water injection time is set at-310 ℃A,a large number of HCO radicals are generated on the cylinder wall surface,resulting in concentrated heat release and enhanced knocking tendency.When the water spraying time is-290 ℃A,the flame spreads the fastest,and it is on the verge of detonation.The cycle power at 350 ℃A with water spraying was2.4% higher than that without water spraying.The CO emission at-370 ℃A with water spraying was 33.7% lower than without.