Study Of Optimized Matching Of Two-stage Turbocharging And EGR System Of Heavy-duty Diesel Engine

Internal combustion engine as the main power plant of modern car hasbeen widely used in the whole world, in promoting the social and economicdevelopment at the same time also make great pressure on energy and theenvironment. As countries around the world gradually tightened emissionsregulations and fuel consumption standard, diesel engine, with theadvantage of high thermal efficiency, low specific fuel consumption andCO2emissions, favored by consumers and producers of internal combustionengine, its technology is towards the goal of zero emission, low fuelconsumption, high power density higher. In order to improve thecomprehensive performance of diesel engine, two-stage turbocharging(2TC)system will be the key of achieving high pressure even ultra-high pressure.At the same time, it will allow more fuel injection to increase enginepower and reduce the engine fuel consumption and exhaust smoke effectivelybecause of the increasing amount of fresh charge. The distribution ofenergy of high and low pressure turbine through adjusting the exhaustbypass valve, is beneficial to optimizing the whole working range ofdiesel engine. In addition,2TC system can increase the oxygenconcentration in cylinder and provide good conditions for introducing EGRto reduce the emission of NOx.The six-cylinder, heavy-load, in-line diesel engine was studied andthe EGR system was built and improved in this paper. The combustion andemission performance of single-stage turbocharging(TC) and2TC werecompared in experiments. Besides, the effect of EGR on performance,combustion and emission of2TC diesel engine was also studied by changing EGR rate. The optimum EGR control strategies were established to achievehigh performance and low emissions. The results turn out that2TC canimprove engine’s power performance under low speed and full loadcondition. It can contribute to improve fuel economy under low and mediumspeed full load conditions. With engine’s speed increase, the2TC dieselengine can reach higher air-fuel ratio compared with TC engine, thusrealizing relatively low fuel economy in these zones. The2TC cansignificantly reduce brake specific emissions of CO and PM, and HCemissions can decrease44%. Due to2TC engine can increase intake air andair-fuel ratio by a large scale, higher EGR rate can be achieved by EGRsystem. With EGR rate increase, the explosion pressure of diesel enginein cylinder decrease obviously, and the peak value of heat release andignition delay period remains unchanged. In terms of emissions, the sootof2TC engine increases slightly with EGR rate increase, and the reductionspace of NOxemissions is higher. Under c50load condition, afteroptimizing the EGR rate is established at20%, the NOxemissions reduce70%, the increase scale of soot emissions is slightly, the HC emissionsremain almost unchanged and the CO emissions are in acceptable range.On the basis of experimental research, this paper applied the GT-Power engine simulation software for2TC heavy-duty engine with highpressure EGR for further optimization matching research work. Simulationresults show that through the optimization of exhaust bypass valve controlstrategy, redistribute exhaust energy of high and low-pressure turbineunder various operating conditions, this can reduce fuel consumption andincrease torque of engine under medium and low speed working conditions,which is conducive to enhance the comprehensive performance of dieselengine. Through the inter-stage cooling system for low pressure stageoutlet air for cooling, can improve the working efficiency of the highpressure stage compressor economy and power performance of the diesel engine, and the effect of inter-stage cooling system on economicalefficiency of2TC diesel engine under the high load condition is moresignificant than that under the low load condition.