The Coupling Of VNT And EGR To The Performance And Emissions Of Vehicle Diesel Engines Under Different Atmospheric Pressures

The low atmospheric pressure in plateau area has a lot of adverse effects on diesel engines,such as reduced power,deteriorated economy,increased some pollutants and decreased reliability.In the past,in allusion to diesel engines operating in plateau area,the research emphasis was to solve the reliability and performance recovery.With the offficial relasese of the "Limits and measurement methods for emissions from light-duty vehicles(China 6)",this emission regulation has stipulated that light-duty vehicles running at different altitudes must meet the corresponding emission limits.Thererfore,the pollutant emissions of internal combustion engines operating at different altitudes has been paid more attention and concern.It is difficult for the traditional fixed geometry turbocharger to guarantee the desired intake air mass flow rate of diesel engines operating at different altitudes.Meanwhile,it will also lead to a serious problem that boost pressure is higher than exhaust manifold pressure.By using exhaust gas recirculation(EGR),this problem will become more prominent.However,employing a variable nozzle turbocharger(VNT)can effectively alleviate the above problem.In order to obtain good performance of diesel engines running at different altitudes,a research about the effect of VNT and EGR coupling on the performance and emissions of a vehicle diesel engine at differentc atmospheric pressure was proposed.An in-depth theoretical analysis and systematic experimental study was conducted.The research work manily focuses on the following asepects.(1)The effects of VNT on the performance of the high pressure common rail diesel engine running at different atmospheric pressures,such as power,economy,combustion characteristics and emission performance were studied systematically.The results showed that VNT can improve the diesel engine power performance and fuel economy at different atmospheric pressures.In addition,the improvement amplitude of fuel economy was greater with an increasing altitude.In the plateau area,in order to prevent compressor surge and overspeed phenomena,too small VNT nozzle opening should not be used at the engine low and high speed modes.Brake specific Nitric oxides(NOx)emission increased with an increasing of VNT nozzle opening or a decreasing of atmospheric pressure at engine low and medium load conditions,and it decreased with an increasing of VNT nozzle opening or a decreasing of atmospheric pressure at engine high and full load conditions.In low altitude area,a smaller VNT nozzle opening may caused a higher brake specific carbon monoxide(CO)emission at small and medium load conditions.At different load conditions in plateau region,smoke emission showed a different variation trend with the change of VNT nozzle opening.(2)Based on the EGR driving method by using VNT,the effects of EGR based on VNT on diesel engine power performance,fuel economy,combustion and emissions were studied at different atmospheric pressures by experiment,at the same time,the law of vary with atmospheric pressure was also analysed.Experimental results showed that with an increasing of atmospheric pressure,the torlerence of EGR rate increases.Based on the EGR driving method by adjusting VNT nozzle opening,the variation trends of intake air mass flow rate and air-to-fuel ratio with the EGR rate were related to the choosed range of VNT nozzle opening.The effect of EGR on the performance of diesel engine was mainly reflected in the change of air-to-fuel ratio.At different atmospheric pressures,brake specific NOx emission decreased with an increasing of EGR rate.At the same EGR rate,with an increasing of atmospheric pressure,brake specific NOx emission increased and CO brake specific emission decreased.In plateau area,it can simultaneously reduce NOx and somke emission by the EGR driving method based on VNT at high speed and high load conditions.(3)Based on the response surface method,the influence of VNT and EGR coupling at different atmospheric pressure on key performance parameters of diesel engine were analyzed by the form of response surface diagrams.The results showed that when the VNT and EGR coupled,the power performance and fuel economy demonstrated different trends at different operating modes.At low and medium engine speeds,air-to-fuel ratio decreased with an increasing of VNT nozzle opening when EGR valve was closed or the opening was small.However,air-to-fuel ratio showed the opposite trend with the change of VNT nozzle opening when EGR valve opening was lagre.At high engine speed,no matter how the change of EGR valve opening,air-to-fuel ratio always decreased with an increasing of VNT nozzle opening.When EGR valve was closed or the opening was small,with an increasing of VNT nozzle opening,brake specific NOx emission decreased and brake specific CO emission increased.When EGR valve opening was large,with an increasing of VNT nozzle opening,brake specific NOx emission increased,brake specific CO emission decreased when engine speed was low and medium,but brake specific CO emission showed the opposite trend when engine speed was high.(4)In order to solve the coupling problem between VNT and EGR,to alleviate the trade-off inherent relationship of NOx and smoke,and to compromise the contradiction between fuel economy and emission performance a high pressure common rail diesel engine running in the plateau area,the calibration variables which had significant effects on diesel engine performance were choosen,the multi-objective optimization of fuel economy and emission performance were conducted at the whole engine operating range by using support vector machine(SVM)to model the calibration models and genetic algorithm(GA)to optimize,meanwhile,the multi-objective optimization was also under some constrains,such as keeping the original engine power performance unchanged and satisfying durability constraints and minimum smoke limit.The results showed that with the optimized calibration variable MAP,the diesel engine maintained the power performance of original manchine,and also it satisfied the durability contraints and the minmum smoke limit.Compared with the original engine,the fuel economy had a little improvement,brake specific NOx emission decreased by 52.95%without significant deteriorated smoke emission at the chosed 36 optimized opints.This multi-objective optimization method can balance the contradiction of diesel engine operating at high altitude,such as the decline of power performance,the deterioration of fuel economy,the poor reliability and the high emission of some pollutants.