Numerical Simulation Study On Early Injection Wall Wetting And Its Combustion Process In IC Engine

As the traditional spray/wall impingment model often has large prediction errors in simulating the combustion process of new engines and new combustion modes,the estimation of wall-wetting process is insufficient,resulting in excessive premixed combustion ratio,in-cylinder heat release rate and cylinder pressure,their curves are higher than the experimental measurement values.Therefore,the topic of this research is focused on the spray/wall collision process in the direct injection engines.It started with the simulation of micro-scale droplet/wall collision process,and then a spray/wall impingment model suitable for spray/wall impinging combustion process in modern IC engines was constructed.The main implementation steps of this study are as follows: Firstly,the numerical simulation study of the micro-scale droplet/wall collision process is carried out.According to the numerical study of the micro-droplet impacting process,the spray/wall impingement model is constructed,and then this model is verified by multiple experimental cases.Finally,the constructed spray/wall impingement model was applied to analyze the wall-wetting combustion process in the cases of diesel engine applying early injection strategy.The final achievements of this study are as follows: A spray/wall impingement model suitable for the wall-wetting combustion process in modern internal combustion engines is constructed,which is based on the micron-diameter droplet collision process;At the same time,the residual liquid film formed after the spray hits the wall is calculated independently,the superimposition and fusion process between the micro liquid film is calculated to amend the spreading area and thickness of the wall film.This wall film submodel is also embed into the spray/wall impingement model to calculate the liquid film shape and thickness distribution independent of the mesh scale,it can also obtain more accurate oil film morphology and oil film thickness on sub-grid scale.Finally the spray/wall impingment model makes a more accurate prediction of the droplet distribution and oil film distribution on the near wall region,which can improve the simulation accuracy of the spray/wall impinging combustion process and the near-wall pollutant generation rate in the internal combustion engine.The numerical simulation of micro-droplet hitting the wall found that it is more difficult to splash and spread after the micro droplets hit the wall compared to the millimeter droplets,and the influence of liquid viscosity on the wall collision process is greater than the surface tension;The incident angle,wall temperature and wall wetness are the key factors in the process of droplet impacting the wall.Specifically,during the process of micro-droplet hitting the wall,the surface tension of the liquid has little effect on the amount of splash and the spreading of the liquid film,it only slightly affects the number of splashed secondary droplets.The dynamic viscosity of the liquid significantly affects the amount of splash and the spreading radius of the deposited wall film;the incident velocity of the liquid droplet is the key factor determining whether the splash occurs after the liquid hits the wall.The critical Weber number of the splash is greater than the droplet breakage in free space;the existence of thin liquid film on the wall will promote splash occurring,the thickness of the liquid film on wall increases,the mass of splashed droplet increases,and even exceeds the mass of the incident droplet.However,the increase in the thickness of the liquid film also increases the diameter of the splashed droplet and decreases the velocity;The roughness of the wall has little influence on the millimeter diameter droplet impacting process,but the large roughness wall has a great influence on the micro-scale droplet collision process;The three-phase contact angle has no obvious influence on the droplet collision process,but it has certain influence on the final stable shape of the wall film;The deposit effect of continuous droplets colliding with the wall shoud be considerated in constructing the spray/wall impingment model.This paper uses the developed new spray/wall impingement model to simulate the combustion process in engines which applying early injection strategy.It is found that there is a local over-concentration zone near the wall after the spray hits the wall.When the fuel is concentrated near the wall,its ignition position is close to the wall surface,possibly on the wall surface.A large amount of soot is generated nearby,and the amount of soot is determined by both the wall-wetting amount and the fuel film evaporation rate.Specifically,in different injection advanced angle conditions,the main reason for the soot emission forming a “BUMP” zone at SOI=-55°CA ATDC case is that the spray collides with the inner wall of the cylinder.The increase in injection pressure does not reduce the soot emissions under the wall-wetting combution conditions.The increase of intake pressure does not have a significant impact on the soot emission trend,and the increase in intake air temperature shortens the ignition delay period,so that the soot emission curve shifts correspondingly with the SOI timing,but the “BUMP” region still exists and its peak value didn’t change significantly.The increase in coolant temperature can significantly increase soot emissions during wallwetting combustion.EGR can reduce the soot emission in the wall-wetting combustion process,and if implement EGR under the condition of ensuring the same air-fuel ratio,its effect on reducing the soot emission is obviously strengthened,so the soot emission in wall-wetting combustion process with EGR is affect by the combination of the airfuel ratio and the ignition delay period.In addition,the gasoline/diesel blended fuel or dual injection modes can both significantly reduce soot emissions and substantially eliminate the “BUMP” zone of soot.The dual injection mode can reduce the quality of in-cylinder injection fuel mass and directly reduces or even eliminates wall-wetting phenomenon.