Numerical Investigation And Experimental Study On The Air System Optimization Of A Heady Duty Diesel Engine

The increasingly stringent emissions regulation has posed more requests on the development of engine technologies,how to achieve simultaneous reduction of particle matter(PM)and nitrogen oxides(NO_x)emission while maintaining high thermal efficiency is the major challenge that diesel engines are facing with.Engine air system optimization,mainly the turbocharging and exhaust gas recirculation(EGR)system optimization,is one of the key approaches to achieve high efficiency and clean combustion.In the current study,experimental and numerical studies have been conducted to explore the optimization strategy of the dual-loop EGR with variable twoturbocharging system.Firstly,the effect of primary parameters in the two-stage turbocharging system on boost pressure and pumping losses were investigated using a 0-D thermodynamic model.The results showed that the turbocharger efficiency,inter-stage cooling and pressure ratio distribution(PRD)between the two compressors are the main factors that affect the engine boost pressure and pumping losses.Then,a dual-loop EGR system model was developed using the GT-Power package to analyses the effect of both the high pressure EGR(HP EGR)and low pressure EGR(LP EGR)on engine performance and to explore the optimization strategy of the dual-loop EGR system.Results showed that,the HP EGR could reduce the pumping losses at all engine operating conditions,however,accompany with lower intake manifold pressure and gross indicated thermal efficiency(ITEg);While the LP EGR has little effect on intake pressure and pumping losses,therefore,it is preferred to apply HP EGR at low EGR rate conditions to reduce the pumping losses and then to use the LP EGR at high EGR rate conditions to maintain the ITEg.A variable two-stage turbocharging system,which is composed of a HP stage variable geometry turbocharger(VGT)and a LP stage fixed geometry turbocharger(FGT),was evaluated and matched for a 8.4L heavy duty diesel engine base on the 0-D thermodynamic model and the 1-D GT-Power model.Improvement in brake specific fuel consumption(BSFC)(3-4g/kW-h)can be obtained by installing an inter-stage cooler between the two compressors at full load conditions.This can be mainly attributed to the reasons that,since the inlet temperature of HP stage compressor can be reduced by the inter-stage cooler thus less driven work of the compressor is needed,which resulted in higher intake pressure and lower pumping losses.Then,experiments were conducted to investigate the effect of different coupling strategies between the HP EGR,LP EGR and the variable two-stage turbocharging system on engine performance.The results showed that the dual-loop EGR strategy was capable of achieving better fuel efficiency and lower emissions at wider low-NO_x emission range.However,the optimized VGT closure position for the dual-loop EGR strategy at certain engine operating condition was mainly restricted by the NO_x emission target.At high NO_x emission conditions,wide VGT opening was effective in both pumping losses and BSFC reduction.At low NO_x emission conditions,the pumping losses can be reduced by HP EGR system,thus narrower VGT opening was favorable for higher intake pressure and ITEg.The ESC test results showed that,much better fuel efficiency and lower soot emission at comparable or lower NO_x emission can be achieved through engine air system opmization.