Effects Of EGR Coupling Two-stage Turbocharger On HD Diesel Engine Performance And Emissions

Development and implementation of emissions regulations and fuel consumptionregulations proposed more stringent requirements for diesel vehicle. Achievingnear-zero emissions, low fuel consumption, high power density is the future directionof diesel technology. For heavy-duty diesel engines, with conventional single-stageturbocharger cannot satisfy the need for enhanced boost pressure. In recent years,enhanced supercharging diesel engine is a research focus. The results show that, Highand adjustable pressure ratio is a necessary means to achieve the Euro V emissionstandards. Because of its high pressure ratio and flexible adjustability, two-stageturbocharger is full of potential，which can provide ample inlet conditions for EGR toreduce NOXemissions. Therefore, the study of two-stage turbocharger coupled EGRsystem has great realistic significance.Firstly, two-stage turbocharged engine model was established using GT-Power,the results were calibrated using experimental data, and the opening degree of exhaustbypass valve of high pressure stage is optimized. On this basis, high-pressure,low-pressure and inter-pressure EGR loops simulation models were established andeffects of different implementations of EGR on engine performance were analyzed toprovide a theoretical basis for bench EGR. Then established one-stage and two-stageturbocharged diesel engine test platform with the high-pressure EGR to study theEGR coupling influence on engine performance and emissions. And single-stageturbocharged engine was selected for the comparison benchmark to analyze thepotential of EGR coupling two-stage turbocharger to improve engine performance andreduce emissions.The results show that, two-stage turbocharger system can improve low-speedtorque and fuel economy in low speed heavy load conditions. The engine performancecan be further improved by optimizing the opening degree of the exhaust bypass valve.The optimized test results show that engine smoke and PM quality emissions aresignificantly reduced, the particular ESC weighted average emissions are76%lowerthan before. The number of particles are also significantly reduced, especially thenumber of accumulation mode particles, while NOXemissions increased10%-20%compared with the single-stage turbocharged engine.Three different implementations of EGR have different effects to the engineperformance at different operating conditions. Pressure difference between theexhaust pipe and intake pipe was significantly greater than the single-stageturbocharger, thus can to achieve a greater high-pressure EGR rate. And with theincreasing of EGR rate, single-stage turbocharged diesel engine performance andemissions deteriorates more obviously. While the two-stage turbocharger system canmaintain a good power and economy performance in a wider range of EGR rate witha better trade-off between NOXand smoke and BSFC. Compared with the single-stageturbocharger, in the same EGR rate, accumulation mode particulate emissions fromtwo-stage turbocharged engine was significantly lower, but the number of nucleateparticles is larger.