Simulation Study On First Cycle Of Diesel Engine Starting Process Based On The Target Torque

The starting condition is a special transient condition of diesel engine.The engine speed is low when starting making the intensity of air flow weak,both the temperature and injection pressure are low making the fuel atomization process difficult.These factors increase the difficulty of the fuel-air mixing process and combustion in the cylinder,which makes the economic performance and emission performance of the starting process worse.The first cycle,as the first combustion cycle of the engine starting process,is more severely affected by all above,it is extremely prone to severe incomplete combustion and misfires phenomenon,which deteriorates the economic and emission performance of the starting process,and it is very important to organizes the combustion of the first cycle to increase the combustion temperature,improve the environment in the cylinder and the combustion of subsequent cycles.In this paper,a commercial CFD software AVL-FIRE is used to establish a firstcycle simulation calculation model.The grid model of the calculation area is divided according to the structural parameters of experimental prototype.By reasonably selecting and setting initial conditions,boundary conditions,turbulence models,combustion models,spray models,etc.,the model can accurately simulate the working conditions in the first cycle of the engine.Based on this model,the simulation of the initial injection engine speed,fuel injection amount,and injection mode of the first cycle is performed.Among them,the calculation of the fuel injection amount in the first cycle is based on the fuel injection amount control strategy during the starting process based on the target torque proposed earlier,and the target acceleration curve of the starting process is optimized through simulation to calculate the optimal fuel injection amount in the first cycle.During the evaluation of the combustion performance in the first cycle,by analyzing two-dimensional curves such as cylinder pressure curve,heat release curve,CO generation curve,NO generation curve,and HC generation curve,the first cycle combustion process and combustion effect under different combustion conditions are described;by analyzing the three-dimensional physical field distribution such as temperature field,velocity field and concentration field in the cylinder,the influencing factors of the formation and combustion process of the mixed gas in the cylinder are discussed and studied;by comparing the thermal efficiency,CO emissions,NO emissions and HC emissions,the combustion process under different conditions of the first cycle combustion are optimized.Finally,through the comprehensive evaluation of the first cycle economic performance and emission performance,the optimal initial injection engine speed and optimal injection amount of the first cycle at different starting temperatures were determined,and optimal spraying scheme for the first cycle under normal temperature starting conditions was determined: For the starting process at the ambient temperature of 20°C,5°C,-10°C,the optimal injection engine speed of the first cycle were determined to 220r/min,240r/min and 200r/min.The optimal fuel injection amounts are 17.3 mg,19.1 mg and 20.2 mg Correspondingly.For normal temperature starting conditions at an ambient temperature of 20°C,it is determined that the best injection scheme for the first cycle is two injections,and the main injection time is 358 ° CA,The pre-injection volume is 3mg.