Effects of EGR on combustion and emissions of a common-rail diesel engine based on different turbocharging systems

Experimental studies on combustion and emissions were carried on an electronic control common-rail diesel engine which matched single-stage tubocharger(WGT), two-stage turborcharger(2TC) and variable geometry turbocharger(VGT) respectively, and high pressure cooled exhaust gas recirculation(EGR) was employed to decrease NOx emission. The results show that VGT had lower differential pressure between exhaust outlet and engine intake, smaller pumping loss and better fuel economy. At high load with the same EGR rate, the peak cylinder pressure and the heat release rate over the main combustion period increased in the order of WGT, 2TC and VGT, while the combustion duration reduced in the same order. With the increase of EGR rate, a higher intake pressure of VGT led to the higher peak cylinder pressure, and the smoke and CO emission of 2TC and VGT were substantially reduced compared with those of WGT, which could dramatically improve the NOx/soot trade-off. At medium or low-load, WGT produced the highest HC emission 2TC produced the highest peak cylinder pressure but the lowest CO emission, the combustion phase of VGT was later than that of WGT and 2TC, and its smoke decreased due to the higher ratio of premixed combustion.