Optimization Of Diesel Engine Transient Performance And Emission By VNT On Different Altitudes

Due to the"turbo lag"of the turbocharger,the rate of change of boost pressure and intake volume lags behind the rate of change of fuel injection mass,resulting in the deterioration of diesel engine performance and emissions under transient conditions.The China VI regulations require the altitude of stage 6a to be not higher than 1700m and stage 6b to be not higher than 2400m.At high altitude,the performance and emissions of diesel engines under transient conditions deteriorate to different degrees due to the reduction in atmospheric pressure and oxygen content.How to improve the transient response of diesel engines while reducing the emission of pollutants has been a hot topic and focus of research.Variable nozzle turbocharger（VNT）has a great influence on diesel engine boost pressure,torque response and pollutant emission by changing the nozzle ring openings,and reasonable adjustment of VNT openings is one of the effective measures to improve the transient performance of diesel engine.In order to optimize the VNT openings in the transient operating conditions of diesel engines at different altitudes and to realize the coordinated control of diesel engines and VNT under transient operating conditions,a transient VNT openings control strategy of gradually increasing the VNT openings with the increase of the cyclic injection volume（load）is proposed for high-pressure common-rail diesel engines equipped with VNT,and combined with the atmospheric pressure simulation system,the transient VNT openings control strategy is investigated through bench tests at altitudes of 0,1000,2000 and 2400 m.The effect of different VNT openings on the transient performance and emission of the diesel engine was investigated by bench tests at 0,1000,2000 and 2400 m above sea level,with constant diesel engine speed of 1600,2000 and 2600 r/min,and a transient change of load from 0%to 100%in 1 s.The results of the study can be used as a basis for the control of transient VNT openings at different altitudes.The main research contents and results are as follows:（1）A theoretical analysis of the control strategy of the VNT presents the calculation process of the boost pressure and the VNT opening.（2）In the transient process of increasing torque,as the VNT openings decreases,the intake,boost pressure and supercharger speed response time accelerates,while a gradually increasing peak is generated,and too small a VNT openings will lead to supercharger overspeed.The torque response at different atmospheric pressures tends to speed up and then slow down as the VNT openings decreases.The response of cylinder pressure in the transient process under different atmospheric pressure is accelerated with the decrease of VNT openings,and at the same time there is a small upstroke,and the peak of the upstroke gradually increases,the cylinder pressure at full load under different atmospheric pressure is basically unchanged;the change of average effective pressure appears a small increase with the decrease of VNT openings,and the average effective pressure gradually decreases with the decrease of atmospheric pressure.The peak value of PN increases with the decrease of VNT opening,and increases with the increase of altitude.The changes of NOx volume fraction at different altitudes and speeds are different.The peak value of CO₂ decreases with the decrease of VNT opening.The peak value of CO gradually increases with the decrease of VNT opening and the increase of altitude.In the transient process,the volume fraction of O₂decreases first and then increases to the peak value and finally stabilizes.With the decrease of VNT opening,the valley value and peak value gradually increase.（3）By obtaining the transient experiment results at different altitudes and speeds,the VNT openings control of the diesel engine was optimized for the full range of operating conditions at different altitudes,taking into account the transient response and emissions of the diesel engine,and the optimized VNT openings coordination control scheme for the diesel engine under transient operating conditions at different altitudes was obtained,and WHTC cycle experiments were conducted,and then the speed,torque and power of the original engine and the optimized engine were subjected to The results show that within the tolerance range,the standard deviation of actual speed to estimated speed increases slightly,and the standard deviation of torque and power to estimated speed decreases at the same time.For emissions:NOx,PM and PN cold and hot cycle brake specific emissions at different atmospheric pressures all decrease by different percentages.