A Study Of Matching Optimization And Control Strategy Between Two-stage Turbocharging System For Diesel Engine

Turbocharging technology is now widely used in diesel vehicle, because it can improve automobile’s emission performance, increase engine power and pressure ratio, also it helpful to reduce engine fuel consumption and noise. With the growing application of diesel technology, people found the endless charm of diesel engine, high torque, long life-time, low fuel consumption and emissions, diesel engine has become the most realistic and more reliable method to solve the energy and emission problems. Due to the intrinsic characteristics of traditional single-stage turbocharger, it is hard to meet the requirements of diesel engine in both high and low speed conditions. However, two-stage turbocharging system can make exhaust energy recovered in lower stage, which lost at high-pressure stage, so its total efficiency is higher than single-stage engine. Besides, its work scope is expanded, and it can improve the engine torque and the transient response characteristics at low speed by the smaller low-stage turbocharger. Although the two-stage turbocharged engine has a charming application prospect, the matching of two-stage turborcharing system is important but difficult.At first, this paper uses one-dimensional simulation software GT-Power to build a model of a heavy single-stage turbocharging diesel engine and calibrated by the experiment data, which made the model meet the requirement of accuracy.And then it designed a two-stage tandem turbocharged system for the engine, and the two-stage turbocharging model was built based on GT-power, matched the two-stage turbocharging system on the basis of the model.The matched method as follows. It selected engine speed 1100r/min and 2200r/min as the high and low stage turbocharger matching points in external characteristic condition, at low speed, by-pass valve of high-stage compressor is closed, on the contrary, opened at high speed. According to the engine target performance it calculated the circulation characteristics of supercharger at all levels roughly, and then choose some high and low-pressure superchargers from the library of turborcharger, and the turbocharger MAP were entered into the model for calculation, this paper finally chose three group of pressurization system for the engine with good comprehensive performance.In the process of matching and optimization, this article studied the control strategy of two-stage turbocharging system and the distribution principle of low and high-stage pressure ratio. It obtained the boundary speed at which the intake bypass valve is open, it also analyzed the influence of pressure ratio distributeon on engine performance and determined the optimal pressure ratio at external engine speed. Finally, this paper analyzed steady and transient performance under fixed speed with variable load of two-stage turbocharged engine,The results show that torque performance of the two-stage turbocharging engine at low speed can reach target valve and brake specific fuel consumption at 1400r/min-2200r/min speed is good. Due to high-pressure turbocharger matched is not ideal, the supercharger efficiency is low, only about 70%, fuel consumption at some low speed are higher than the limit, and low-pressure turbocharger efficiency are above 75%, so they are appropriate. The exhaust temperature before high-pressure turbines is less than 950 k. If steady state performance of engine is regarded as the evaluation principle, the first scheme is the best match and the transient performance of the second scheme is the best, for its torque response time is shortened about 8%than others.Comparing to transient performance between the original engine and the two-stage turbocharged engine matched excellently, we will find that the later is able to improve engine’s transient response at low speed and when it is at 1300r/min speed, the rising time of supercharging pressure is reduced by 6.5%, air-fuel ratio rising to the same value is shortened by 8.7%, the response time of torque value rising to 1000N·m is shortened by 5.3% and the response characteristics of turbine also improve greatly, about 25%.In conclusion, the matched two-stage turbocharged engine can reach the basic performance requirements and this method is a guide to the matching of turbocharger with the engine in practical application.