The Analysis Of Heat Transfer Characteristics Of Diesel Engine Under Transient Conditions

The internal combustion engine,as the main power device of the automobile,releases energy through the combustion of fuel in the cylinder to output mechanical work to the outside.With the emergence of energy crisis and energy security issues,environmental pollution is becoming more and more serious,and emission regulations and standards are refined and strict year after year,the energy saving and emission reduction of internal combustion engines have also become problems that need to be solved urgently.Most of the working conditions of internal combustion engines in practical applications are transient working conditions(working conditions in which the speed or torque changes with time);under transient working conditions,due to the hysteresis effect of the turbocharger intake,the fuel-air ratio is uneven,which makes the combustion process deteriorates of the internal cylinder,and emissions and combustion parameters deteriorate.The heat transfer process of a diesel engine has an important influence on combustion and emissions.By studying the heat transfer characteristics of the transient process,the energy transfer law,heat transfer flow path and cylinder wall temperature distribution characteristics of the transient operating conditions can be revealed.The influence mechanism of characteristics provides a theoretical basis for optimizing and improving transient performance.In this paper,a combination of experiment and simulation are used for research and analysis.The 3.8L turbocharged,inter-cooled,high-pressure common rail diesel engine is used as the original engine test object.A simulation research platform is established through the simulation software GT-Power to research transient and steady-state operating conditions which incloud the difference in heat transfer performance and the influence of boundary parameters on the heat transfer characteristics of transient operating conditions.The paper selects the intermediate speed(1921r/min)with load of10%,25%,50%,75% and 90% as the steady-state operating conditions of the study;And selects the intermediate speed(1921r/min),the load that is linearly loaded from10% to the transient process of 100% as the transient working condition of the research.Compare and analyze the performance parameters,combustion parameters and heat transfer parameters(heat transfer flow rate,heat transfer distribution and heat transfer process)under steady state and transient conditions,and reveal the difference between the steady and transient states.By changing the boundary parameters(intake air temperature and coolant temperature),the influence of different boundary parameters on transient heat transfer characteristics is studied,the influence mechanism of boundary conditions on transient heat transfer characteristics is clarified,and the mechanism of boundary conditions is enriched.The research indicates:1.The main factor for performance degradation in transient conditions is the reduction of intake air caused by the hysteresis effect of the turbocharger,which worsens the combustion and degrades the performance of the diesel engine.Torque and thermal efficiency under transient conditions deteriorate under varying degrees.The peak cylinder pressure and the peak heat release rate under steady-state operating conditions are both greater than those under transient operating conditions;Due to the lag of the intake air,The combustion speed slowed down under the transient operating conditions,and the combustion duration is greater than the steady-state operating conditions,and CA90 lags significantly behind the steady state conditions.2.The proportion of transient heat transfer and heat transfer under each load is greater than that of steady-state conditions.As the load increases,the heat transfer gap between the steady and transient conditions will further increase;reducing the combustion duration of transient conditions is an effective measure to reduce heat transfer losses and to improve transient performance.The heat transfer process of a diesel engine is divided into "thermodynamics","structure" and "cooling" stages.The transient average wall temperature is greater than the steady state.The maximum temperature area of fire deck is in the nose bridge area between the two exhaust valves.Due to the difference in the combustion process caused by the intake lag effect,the heat transfer flow curve shows different laws.The heat transfer characteristics of the pre-injection section will affect the combustion characteristics of the pre-injection section,resulting in a difference in the combustion characteristics of the main injection section,which changes the heat transfer characteristics of the main injection section.The difference between steady-state and transient heat transfer performance is finally characteristics reflect in the cylinder wall temperature.3.During the transient loading process,the intake air temperature increases,the maximum pressure in the cylinder is slightly reduced,the maximum temperature rises,and the heat transfer increases in the cylinder,which increases the temperature of the cylinder wall,but the heat transferred to the coolant from the boundary of the cylinder wall also increases at the same time,The two reason have a mutual restriction on the wall temperature.High intake air temperature can effectively increase the thermal atmosphere in the cylinder and improve the combustion characteristics of the preinjection section.However,due to the high intake air temperature corresponding to high heat transfer loss,the peak heat release rate of the main injection section decreases and the combustion duration increases.As the coolant temperature increases,the maximum pressure and maximum temperature in the cylinder are slightly increased,the heat transfer and the proportion of heat transfer in the cylinder are reduced,and the heat transferred to the coolant from the boundary of the cylinder wall decreases rapidly,which increases the temperature of the cylinder wall and promotes fuel evaporation and atomization conducived to the combustion of the pre-injection section,thereby promoting the combustion of the mixed gas in the main injection section.High coolant temperature corresponds to low heat transfer loss,which is beneficial to improve transient performance.4.Increasing the intake air temperature affects the heat transfer process in the cylinder by changing the combustion conditions in the cylinder.The difference in heat transfer process is reflected in the wall temperature characteristics;increasing the coolant temperature is by changing the heat transfer capacity of the coolant,thereby changing the wall temperature characteristics affecting the heat transfer process in the cylinder,thereby changing the combustion process in the cylinder.Reasonably use intake air temperature and coolant temperature to improve the internal and external heat transfer characteristics of the internal combustion engine,thereby improving the transient performance of the internal combustion engine.