Study On The Matching Method And Optimal Control Strategy Of Twin Turbocharging System

Turbocharging technique is an important method to achieve diesel engine goals that high power, downsizing and low fuel consumption. Turbocharger and diesel engine are different in flow characteristic, so there is contradiction between turbocharger and diesel engine in the steady condition. Otherwise turbo-lag effect causes conflict with turbocharger and diesel engine in the transient control. Twin turbocharging system which consist of sequential turbocharging(ST) and regulated two stage turbocharging(R2S) system is one of techniques to alleviate the above contradictions. But this technique brings the following new problems: Iteration method and trial-and-error method which are used in typical turbocharger matching is not appropriate for twin turbocharging because of huge calculated amount; the control strategies of the turbocharging system in transient process are more difficult because the number of control variables increase. The matching method that the variables of the turbocharging system are calculated by analytical method is studied to solve the matching problem and the control strategies in switching and transient process are optimized by optimization algorithm to improve the transient control problem in this dissertation.The number of variables for twin turbocharging system is high, so the variable analytical method is proposed to solve twin turbocharging matching method. The thermodynamics equations of combined operation with turbocharging system and diesel engine are established and the relation between matching variables of turbocharging system and performance parameters of diesel engine.are conducted. For ST matching, the calculated method for the number of ST phase, feasible range and optimal value of equivalent areas of turbine nozzle(EATN) is proposed. So the optimized matching problem for ST system is converted to the minimum problem with nonlinear constraints. For R2 S system matching, the thermodynamics equations of R2 S system are established by the typical matching laws and the generalized calculation method of these equations are conducted. The variable analytical method is used to design ST and R2 S system for D6114 diesel engine, and the matching results are proved by GT-Power. The simulation results show that: the turbocharging systems meet the diesel design requirements well; the operation points of turbochargers are all in high efficient area; the predicted switching boundary for ST by variable analytical method is closed to the one by GT-Power, and the relative error is 2.5%; The relative error of the diesel engine performance parameters for R2 S by variable analytical method and GT-Power is lower than 6%.The valves control strategies in ST switching process are studied by simulation. The mean value model which is used to study on the valve control strategy in ST switching process is established in Matlab/Simulink. The model is calibrated and validated by the experiment results.The reasonable valves’ s action sequences are acquired by analysis and calculation. The compressor open timing is at the time when pressure before and after the valve is equal and closed timing is at the time when the compressore mass flow rate reaches zero. The control strategies in ST switching process with different environments are studied, and the correction of the ST switching strategies in high-land environment is also predicted. The results show that: with the same engine speed and fuel mass flow, the open delay time increases as environment pressure decreases or environment temperature increases; the closed delay time decreases as environment pressure and temperature decrease.The optimum valve control strategies in ST switching process and the optimum valve and fuel combined control strategies of twin turbocharing in acceleration and loading process are studied with simulation procedures. The optimum valve control in the ST switching process is simplified to the problem to get the optimum delay time between the valves open or close. Then this problem is solved by subsection gradient method. The optimum combined control for twin turbocharging in accelation and loading process is transfer to the corresponding mathmactics problem and is solved by sequential quadratic programming(SQP). The ST optimum combined control in acceleration and loading process is: the fuel mass flow reaches high level in the initial phase to get quick engine response and decreases when the target variables are close to or reach the target value; the cut-off turbine closes immediately and the cut-in turbine opens when the boost pressure, engine speed and torque rise. The R2 S optimum combined control in acceleration and loading process is: the bypass valve of high pressure stage turbine closes immediately and opens when engine parameters reach target value.Control strategies of twin turbocharging system during switching, acceleration and loading processes are investigated on the test bench. The steady and transient experiment data is used to calibrate the simulation models. Effects of different valve open and closed timings on the performances of the diesel engine and turbocharger system during ST switching process are studied, and the simulation optimum control strategies are validated. Effects of different valves and fuel control strategies for twin turbocharging on the transient performances of diesel engine and turbochargers during acceleration and loading process, and the experiment results are used to validate the simulation optimum control strategies.