Research On Optimization And Performance Improvement Of Diesel Two-stage Supercharging System In High Altitude Environment

Plateau area wide,the performance of internal combustion engine is significantly reduced when running at high altitude low pressure condition.How to optimize collaborative control of intake supercharging and EGR is to improve the modern diesel engine’s adaptability at high altitude condition and the key to achieve the requirements of the EU6 emission regulations.In this paper,in order to improve diesel engine plateau adaptation and reduce emissions,based on the two-stage turbocharging system(RTST)of common rail diesel engine,to study on combustion process and control strategy.Based on the common rail high turbocharged heavy-duty diesel engine as the research model,launched the original machine performance test.The research shows:based on the technology of SCR EU IV of the original machine,the single-stage turbocharged diesel engine performance and fuel economy is better,Soot is lower,but the EGR introduced ability is poor,so using EGR need to rematch supercharging system.Author using one-dimensional thermodynamic simulation software GT-Power,through the collaborative calibration of combustion model,emission model,injector model and supercharger model,to build a predictive and adaptive wide one-dimensional thermodynamic simulation model.Based on CFD software CONVERGE coupling simplified multicomponent diesel dynamics mechanism,build the 3D CFD simulation model.By using numerical simulation method,study on the matching optimization and altitude performance improvement strategies of the plateau diesel engine two-stage turbocharging system coupling EGR.Based on the traditional two-stage turbocharging system optimization matching and coupling EGR influence on diesel engine working process,the research show:on the external characteristic(800rpm to 2200rpm),by reducing the mass flow rate of compressor or turbine or,widen the compressor pressure ratio or turbine expansion ratio(scheme one to five),its high efficient area widen and pressurization system power capability enhancement,diesel engine intake flow increase,PMEP is reduced,the working condition of high-low speed performance and fuel economy improve significantly.Low speed conditions,with the increasing of EGR rate,diesel engine two-stage turbocharging system compressor pressure ratio and efficiency reduce,inlet flow rate to decrease.Compared to the rest of four scheme of two-stage supercharging,the diesel engine matching the fifth supercharging can significantly weaken the influence of EGR on engine power,BSFC and the Soot emissions;High speed conditions,when the EGR rate remained at 20%,with the bypass flow increasing of two-stage turbocharging system,high/low stage compressor pressure ratio and efficiency reduce,diesel engine inlet flow rate and PMEP is reduced,the torque and thermal efficiency is firstly increased and then lowered.When the bypass flow rate is high,compared with the rest of the charging scheme,the pressure ratio and efficiency of fifth scheme increased significantly,inlet flow rate increases,effective thermal efficiency is higher.Based on matching different traditional two-stage turbocharging system of diesel engine variable altitude working characteristic,the research show:at low speed condition and with the increasing of altitude(0-4 km),diesel engine high/low stage compressor efficiency and pressure ratio decrease,the inlet flow rate reduce,the PMEP,heat transfer loss and exhaustion loss increase,the performance and fuel economy become poor,Soot specific emissions increase.Under different altitude condition,the high/low stage compressor efficiency of the fourth and fifth scheme is significantly higher than the rest of the schemes,which can improve the diesel engine high-altitude adaptability.Therefore,the comprehensive performance of two-stage diesel engine of the fifth scheme is the best.Under the condition of high altitude of 2km,based on the regulated two-stage turbocharging system(RTST)matching optimization of diesel engine,the research show:reducing the reduced mass-flow,widen the compression ratio and expansion ratio,the high efficient area widen,while inlet flow rate is equal,the compression ratio is reduced,which can promote EGR introduced ability.Under the condition of high EGR rate,compared with the traditional two-stage turbocharging system(the fifth scheme),diesel engine matching RTST can significantly reduce Soot spcific emissions,and improve the Trade-off relations between NOx-Soot.Based on the working process of RTST diesel engine under variable altitude(0～4km),the research show:with the opening of the variable high-pressure stage turbine and altitude increase,the inlet flow rate and PMEP of diesel decrease simultaneously.Under low speed condition,the intake flow rate is more sensitive for the opening of the blade,while under high speed condition,the intake flow rate is more sensitive to the altitude.Meanwhile,the PMEP decrease with the opening of the blade increase.Under certain altitude condition,the diesel engine torque and thermal efficiency at low speed condition increase firstly then decrease with the opening of the blade increasing,and at high speed condition both of them increase remarkable.When the opening of the blade is large,the diesel engine torque and thermal efficiency is less effected by altitude.With the altitude increasing,engine exothermic duration prolong and heat transfer loss increase.With the opening of blade increasing,the heat transfer at low speed condition decrease firstly then increase gradually.While at high speed condition,the heat transfer loss decrease firstly for the residual high temperature exhaust gas in the cylinder decrease then increase slowly.With the opening of the blade and altitude increasing,the exhaust loss and friction loss of engine decrease,Soot specific emission increase,NOx specific emission at low speed condition decrease quickly.while at high speed condition is less effected by altitude and at low altitude condition is more effected by the opening of the blade.Based on the opening of the blade of the RTST diesel engine high stage VGT cooperation with EGR ratio control,the research show:engine matching RTST can all achieve high EGR rate under variable speed condition(above 60%).With the opening of the blade decrease,the EGR ratio increase quickly(the EGR ratio increase quickly at low and middle speed condition),the efficiency of high/low stage and turbine decrease simultaneously(high pressure stage decrease more quickly).Meanwhile,PMEP increase,effective thermal efficiency,heat transfer loss and friction loss decrease simultaneously,exhaust loss increase.The rule of high speed condition is much remarkable.Besides,under different speed condition,with the EGR ratio increasing,NOx specific emission decrease quickly,BSFC increase gradually,Soot specific emission increase firstly then decrease,the comrehensive performance of RTST engine achieve the best.Based on the effect of EGR coupling post injection strategy on RTST engine working process at high altitude,the research show:with the interval of main-post injection and fuel post-injection quantity increase,the efficiency of high/low stage of turbocharging improve simultaneously,intake flowrate increase while the PMEP increase.When the interval of main-post injection is small,the post injection can improve the power performance,fuel economy and effective thermal efficiency of engine and decrease the NOx specific emission.But when the interval of main-post injection and post injection ratio increase,the performance and thermal efficiency of diesel engine decrease gradually,exhaust loss increase,heat transfer loss decrease,Soot specific emission increase greatly,while the exhaust temperature could increase which beneficial to improve the post-processing system purification efficiency.Therefore,to achieve the efficient in-engine/out-engine purification,the optimal control of post-injection strategy coupling EGR should considering the key factor such exhaust temperature,NOx and Soot,etc.