Research Of EGR Control Strategy On A Turbocharged Diesel Engine Under Transient Operation

Diesel engine is an important segment of internal combustion engine industry.The ownership of diesel engine in the world is in the forefront in the machinery.So the development of diesel engine faces double pressure of energy saving and emission reduction.Simultaneously,the diesel engine is also one of the most potential products in energy saving and emission reduction aspects.In order to achieve the European ? diesel engine and its higher emission regulations,the joint use of high pressure common rail system and emission aftertreatment system is an inevitable trend.However,due to the problems of inlet hysteresis and poor hot atmosphere in the transient process,the overall emission levels are still high.In order to solve the problem of transient emission deterioration on turbocharged diesel engine,based on the typical transient process(constant speed increasing torque,1650 r/min,10%-100% load,5s transient time),the research optimized its performance by combining the two-stage turbocharging system,EGR system and high pressure common rail system.Also,the purpose of the research is to realizing the trade-off of transient smoke and NOx emissions and BSFC(the smoke peak lower than 10%,the deterioration of BSFC less than 10%).Among them,the first part of the work established the transient measurement and control platform based on CAN bus.Based on simulink programming and dSPACE platform to achieve transient throttle signal control,EGR valve control,EGR rate calculation,NOx,temperature and pressure signal acquisition.The conclusions are as follows:1.EGR rate is calculated by the oxygen density of inlet and exhaust,and through the pressure signal correction for rapid and accurate EGR rate.2.The CAN bus way reduces wiring harness layout on the measurement and control platform,improves the reliability of measurement and control system.The parameters can be collected fastly,synchronically and accurately.3.The EGR valve changes from fully open to close about 0.2s under B50 condition(1650 r/min,50% load).It is about 0.3s on the contrary.Change the delay time of EGR rate around 0.3 s.The response time of inlet pressure,exhaust oxygen concentration,NOx and smoke are within 0.5s.Its delay is mainly caused by EGR valve action delay,combustion deterioration,sensor arrangement.In general,all of these can meet the test requirements.The second part starts with the open loop and closed loop control of the EGR valve.The feasibility of trace control EGR is studyed.The conclusions are as follows:1.One of the main problems of transient condition is high emission of NOx(full load is close to 900 ppm).In addition,due to the lag of intake air(combustion deterioration),the pressure before the turbine(pumping losses increase),the deterioration of BSFC is seriously.But thanks to the increase air inflow of two-stage turbocharging system(compared with the single-stage turbocharger),the smoke peak is 5.9%.2.Based on steady state optimization EGR valve opening of open-loop control will cause transient EGR rate overshoot,combustion deterioration,extinction smoke exploded.When the EGR valve opening keeps constant,the maximum allowable EGR valve opening is 10%.Otherwise,the smoke explosion appears.Aiming at different stages of the transient process,the paper puts forward to an EGR valve control strategy “keep open at early,keep close at medium,keep open at late”(keep 10% opening before 1.5s,keep close from 1.5s to 4.0s,keep 10% opening after 4.0s).The strategy can realize the trade-off of smoke,NOx and fuel consumption(the biggest drop of NOx emission is 33.9%,the increase of smoke peak is 16.9%,and the largest decline of BSFC is up to 13.0%).3.In order to realize the trace control EGR,the closed-loop EGR valve control strategy has the very good application prospect.But feedback variable selection is crucial.The feedback variables should response the current state of the engine system,and response rapidity,accurately,reliably and conveniently.Choosing the exhaust oxygen concentration as feedback variables,combined with the use of modified transient control MAP,NOx emission peak(548 ppm)drops of 21.7%,the smoke peak is 15.8%,and the BSFC keeps the same as the original machine.Based on the closed-loop EGR valve control strategy,the third part studys the influence of injection pressure and injection timing on transient performance.The main conclusions are as follows:1.The influence of injection pressure adjustment on apparent parameters(air inflow,air fuel ratio,exhaust oxygen concentration)is minimal.At small load,increasing the injection pressure can improve emission performance,but it increases fuel consumption.At large load,increasing the injection pressure can obviously improve the combustion process and cut smoke emissions.Under "60% load" strategy,the smoke peak drops 31.8%(from 17.3% to 11.8%),and the NOx emission peak is changeless,slightly increased only in heavy load area(17.2%),the biggest drop of BSFC is 7.9%.2.The influence of injection timing adjustment on the transient performance and emissions is significantly.Injection delay promotes burning phase backward-shift,combustion thermal efficiency drop,but can promote the late oxidation of smoke.When the main injection timing delay 6°CA,the biggest drop of NOx emission is 38.5%,the smoke peak is only 3.8%,and the increase of BSFC is 15.9%.3.Reasonable control the load range of fuel injection parameters can give full play to the injection parameters(injection pressure,the main injection timing)advantage,so as to optimize the overall performance.Finally,choose “60% load” strategy(increase rail pressure 20 MPa)combining “60%-90% load” strategy(delay 6°CA),the smoke peak is 9.6%,the NOx emission peak is 597 ppm and the biggest increase of BSFC is only 9.6%.