Modeling and Optimal Control for Dynamic Driving of Hybridized Vehicles with Turbocharged Diesel Engines

Abstract: Reducing the fuel consumption of today's vehicle fleet is of great importance due to the environmental impact of using fossil-based fuels. The turbocharged compression ignition (CI) engine is widely used for trucks. The CI engine efficiency is dependent on the operating point, in terms of rotational speed and load. The selection of load point can be controlled by selecting suitable gears, but remains a challenging task during dynamic driving, due to the turbocharger dynamics which introduces a lag in the system. Electric turbocharger technologies can improve the engine response time, but developing efficient control strategies can be challenging. Due to turbocharger lag, all conditions that are reachable in stationary operation for the turbocharged CI engine are not always reachable during dynamic events, for example after an up-shift where the engine speed and torque demand changes rapidly.  In this work the fuel saving potential of electric turbocharging for a heavy-duty truck performing a long-haulage driving mission is investigated.  An electric turbocharger control strategy is proposed and evaluated. The results show that the fuel consumption can be reduced using the electric turbocharger, when comparing to a conventional turbocharged CI truck performing a long-haulage driving mission.A turbocharged CI engine model suitable for optimal control of transient behavior is developed. Sub-models are validated using data describing the components, and the model suitability for optimal control is shown with a tip-in example. To increase the model accuracy, the torque model is extended with a further dependence on the air-fuel ratio and operating point dependent losses. The complete engine model is parameterized for a set of stationary load points. The model is validated using data from a dynamic engine test, where it is shown that both the stationary and dynamic features in the data is represented well by the model. The developed engine model is used as a foundation in an optimal control problem setup to solve fuel optimal accelerations including gear changes. The setup is used to investigate the impact of driveshaft flexibility on the optimal control results, when compared to a stiff driveshaft model. Apart from a slight increase in fuel consumption, the driveshaft flexibility is shown to have minor effects on the fuel optimal control signals, in terms of general torque output and gear shift characteristics. The hybrid electric vehicle (HEV) technology can potentially reduce the consumption of diesel fuel, but how to design and control the system, consisting of several degrees of freedom remains a challenging task. Energy optimal accelerations of a CI parallel HEV with electric turbocharger is investigated using the optimal control problem setup. The results show that the electric turbocharger is used when the electrical energy cost is high, and the usage of the crank shaft motor is increasing with decreasing electric energy cost. To summarize, the developed models and problem setups enable investigations of different powertrain configurations and optimal control of these. One conclusion is that the energy savings using an electric turbocharger and crank shaft motor during accelerations are significant.

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