Study On Hot-EGR Application For Exhaust Thermal Management Technology

Now EU4 emission legislation has been executed since 2015 all over the country,but many cities require the new trucks and buses to meet the EU5 and EU6 emission legislation,so diesel exhaust thermal management technologies are used more and more widely,but more and more problems are exposed in the application.For example,the test cycles of legislation are different from the real operation conditions,the points of the Europe Transient Cycle have higher speed and higher load than the on-road engine,so the SCR after-treatment plays an unimportant role on the engine,it even has nothing to do with the reduction of NOx.So WHTC is used for city bus which is focused on the lower speed and lower load area and it is consistent with real operation conditions.But the exhaust temperature of the engine is lower in this area,which can not meet the requirement of SCR system.The conversion efficiency of SCR is lower with low temperature,so how to improve the exhaust temperature is important for WHTC.This article is aim to do research on the different exhaust thermal management methods and develop control strategy to meet the requirement of EU VI.Firstly,several typical exhaust thermal management technologies is compared and analysis with the influence of exhaust temperature,NOx emission,FSN and BSFC.The hot-EGR technology can not only improve the exhaust temperature,but also reduce the NOx emission and with seldom deterioration of BSFC and FSN.So the hot-EGR is chosen for the engine to the next step.Secondly,closed-loop control strategy is designed,analysis and verified for the accurate control for hot-EGR system.The result shows that,the time of response lower with HFM,but the line tube is needed for the entrance and exit and the precision is bad for small mass flow.The precision of Lambda sensor is also bad in the low load area of engine map,which has higher air fuel ratio,and the response of signal is slowly which can not meet the requirement.Venturi system is designed depend on the exhaust mass flow,differential pressure and flow fluency,and simulation is done to confirm the best performance for all the parts.Compared to the HFM and lambda sensor,the precision of measurement of Venturi system is higher and the response time is also meet the requirement,so Venturi is used for the closed-loop for hot-EGR technology.Lastly,experiment is done with the strategy and Venturi system which is finished and evaluated before,and the results of experiment are compared with the intake throttle valve.It shows that,hot-EGR technology can not only improve the engine exhaust temperature,but also reduce NOx emission,the BSFC of engine is much lower than intake throttle valve,but the FSN is deteriorated un-conspicuous which is acceptable for the emission.Then WHTC is verified with specific calibration of database,the exhaust temperature is improved and the precision of fresh air mass flow is also meet the after-treatment.In the low load area of engine map,hot-EGR can reduce the NOx emission,which will fetch up the low efficiency of SCR.