The Effect Of Diesel Spray Characteristics After Impingement On Combustion Processes In A Diesel Engine

Fuel impingement in the combustion chamber of a diesel engine has an important influence on mixture formation and combustion processes.The effect of fuel injection pressure,ambient pressure,injector cone angle,impinging distance between nozzle hole and piston head,and combustion chamber configuration on spray characteristics after diesel impingement on combustion chamber wall were investigated through schlieren photography with a high-speed digital camera.Then,the models to simulate fuel impingement and combustion processes in a diesel engine was established by means of the CONVERGE code and validated.The effect of combustion chamber structures and injection strategies on combustion processes was simulated.The experimental results about the spray characteristics after diesel impingement show that,the increase of injector pressure and the decrease of ambient pressure increase the distance spray moves along the piston head and the pit area.The change in the magnitude of fuel injection and ambient pressure can alter the fuel distribution on the combustion chamber wall.The injector with a larger cone angle can promote the mixing of fuel and air after diesel impingement while the possibility of fuel impingement on the bottom of cylinder head and cylinder liner increases.Short impinging distance betwwen the wall and nozzle hole results in increased spray droplet collision and bounding in the pit area while longer impinging distance between the wall and nozzle hole hinders the interaction between fuel and air in the combustion chamber center.In addition,moderate increase in the reentrant diameter can promote the mixing of fuel and air,and hence decrease wall wetting in the combustion chamber.The chamfer betwwen piston head and the reentrant chamber can improve the fuel atomization in the cylinder.Suitable change of the convexity angle in the chamber bottom can control the spray spreading velocity in the combustion chamber center after diesel impingement.The simulation results show that,at the fuel injection quantity of 47.6 mg in a cycle and 2200 rpm,moderately increase reentrant diameter in the chamber can increase the air flow velocity in the pit area of combustion chamber and promote the mixing of fuel and air.But larger reentrant diameter in the chamber results in the decrease in the strength of squish flow in the combustion chamber and the reduction of mixture motion.Properly increase in the convexity angle can enhance the turbulence intensity in the center zone of combustion chamber,and promote maximum heat release rate.But larger convexity angle decreases the velocity of air flow in pit area and hinder rapid heat release during the combustion.At 1200 rpm,2200 rpm and 3200 rpm,the differences of in-cylinder pressure and heat release rate curves among different chambers are decreased with the increase of engine speed.The combustion chamber with larger reentrant diameter and convexity angle are robost at different speed and results in advanced heat release rate.For the same combustion chamber,the timing for main injection has obviously effect on the combustion phasing of the diesel engine with equivalence ratio in the range of 0.6~1.0,and further affects combustion processes and the amplitude of indicated mean effective pressure.The mass ratio of pre-injection to the total fuel in a cycle mainly changes the mass distribution of the mixture with equivalence ratio in the range of 0.6~1.0 formed in the pre-mixed phase,and affects the heat release rate in the diesel engine.