Abstract:The main supercharger of the traditional fixed-turbine sequential supercharging system was replaced with a variable geometry turbocharger(VGT), and transformation of the VGT-sequential turbo charging(STC) system and the design of the opening control device were completed. Through experiments, the effects of opening degrees of 0%, 10%, 25%, 40%, 55%, 70%, 85% and 100% on the performance of STC VGT turbocharged diesel engines running at single/twin turbocharger were studied. After that, a multi-objective gray decision model was established, and the optimal VGT opening corresponding to each research load of single/twin turbocharger was calculated through subjective and objective weighting methods, and the single/twin turbocharger of the STC-VGT system was determined according to the principle of optimal fuel economy. The switching point was 50% load, and the switching opening was 0%. Finally, the performance of the STC-VGT system was evaluated. The results show that when the STC VGT system is running at the only main supercharger, compared to the original machine, the power performance is significantly improved, NOx emissions increase, and soot emissions decrease. At the same time, as the load increases, the fuel consumption rate decreases first and then increases. When running at twin turbochargers, and the VGT opening is greater than 70%, the overall power performance is lower than that of the original machine. The maximum drop is about 1.16MPa. NOx emissions are reduced while soot emissions are increased. And the fuel consumption rate is overall higher than the original machine. When the system is operated with the best VGT opening of each load, the fuel economy is better than the original machine as a whole. And when the load is in the range of 10%~80% , it is also better than the traditional fixed-turbine successive supercharging. At the same time, the soot emission performance is better than the original engine and the traditional fixed-turbine successive supercharging, but the NOx emissions are the opposite. Therefore, the power, economy and soot emissions of diesel engines using STC-VGT systems have been effectively improved, but the NOx emission performance has declined. |