This thesis presents a control design using dual-loop exhaust gas recirculation (EGR) and variable geometry turbo charging (VGT), installed on a medium-duty, V8 diesel engine, to adjust intake temperature, pressure, and oxygen mass fraction. The dual-loop EGR system is compiled of both high pressure EGR (HPEGR) and low pressure EGR (LPEGR) paths. Turbocharging is achieved through the use of a two-stage system consisting of a fixed geometry, low pressure turbo and a variable geometry, high pressure turbo. This extensive network creates a complex air-path that necessitates the implementation of an advanced feedback control method. Groundwork for the study is a high fidelity GT-Power computational model of the diesel engine equipped with the proposed dual-loop EGR air-path, capable of simulating the one dimensional gas dynamics of the engine. The computational model allows for an identification of system dynamics that, when validated, provides a basis for the development of a multi-input multi-output (MIMO) air-path controller. Attention must be paid to the robust performance of the controller as computational system identification is inherently inaccurate, due to system nonlinearities, variable transport delay, and other unforeseen dynamics not accounted for in simulation.
The focus of the controller is to use the dual-loop EGR system in conjunction with the VGT to establish a high control authority over intake manifold temperature, pressure, and oxygen mass fraction. Each of these conditions is highly influential on such low emission combustion modes like low temperature combustion, homogeneous charge compression ignition, and pre-mixed charge compression ignition. These combustion modes have high sensitivity to engine intake conditions and high tendency of knock and misfire, which warrant a comparison of the advanced, multivariable feedback control strategy to conventional feedback control using the complex air-path. Strong benefits to using multivariable control are seen through faster response and settling times along with better disturbance rejection capabilities when maintaining desired intake conditions. A feed-forward controller for the complex air-path is also developed and explored for additional improvements in performance when coupled with the multivariable feedback controller. Minimal benefits to the coupling were seen but these could be improved upon through more accurate knowledge of system parameters.
Chapter 1 explores the benefits when operating in advanced combustion modes and potentials for expanding their operating range with the use of the proposed complex air-path system. A linear state-space representation of the air-path system is then identified in Chapter 2 using the GT-Power model which serves as the basis for developing a MIMO feedback controller. The performance of the MIMO feedback controller, and its coupling with a feed-forward controller, are then developed and validated through the GT-Power engine simulation in Chapter 3. Finally, in Chapter 4, a decentralized feedback controller is made which relies on much more basic principles of multivariable control and is compared to the MIMO feedback controller to examine the benefits of using advanced controller development strategies. Concluding remarks and future work are then given in Chapter 5.