Ultracapacitor assisted powertrains: Modeling, control, sizing, and the impact on fuel economy
2008 American Control Conference
This thesis investigates possible fuel economy gains attainable by a combination of high-power density ultracapacitors (also called supercapacitors) and an induction motor integrated into a conventional vehicle powertrain for power assistance. Periods of quick acceleration require a much higher power output from an automobile than what is encountered under more typical driving conditions. A simple kinetic energy calculation can show that accelerating a 2000 kg vehicle (roughly the size of a
... y the size of a Ford Explorer SUV) from 0 to 60 mph in 10 seconds requires almost 70 kW of power, in addition to the power needed to overcome road and air drag forces. Situations such as these consume a disproportionately high amount of fuel, and have a negative impact on the fuel economy of the vehicle. In conventional powertrains, the engine is typically sized much larger than is needed for steady-state operation, in order to meet these spikes in power demand. A larger engine is more expensive to manufacture and to operate. Such rapid transients in power may be better handled by the use of high power density ultracapacitors which represent the latest trend in electrostatic energy storage systems. While the total energy an ultracapacitor can store is typically ten times less than a battery of the same size, the ultracapacitor is capable of releasing or storing energy roughly ten times faster. The potential of this relatively new technology to assist the combustion engine during brief demand spikes, and to capture kinetic energy through regenerative braking, is the subject of this study. A mild parallel hybrid powertrain is considered in which an ultracapacitor-supplied motor assists the engine during periods of high power demand, and the ultracapacitor may be recharged by the engine during periods of low demand, and through regenerative braking. A detailed simulation model of the powertrain is created to evaluate the fuel economy of the vehicle. The fuel economy gains are strongly dependent on how well the power split decision is made, that is the decision of how to distribute the power demand between the engine and the electric motor at each instant in time. To this end two forms of implementable control are designed to determine the power split between the engine and motor. A rule-based controller, which can ii be quickly tuned and implemented, is applied for more exploratory simulations. Simplicity and expedience in both tuning and implementation make this method useful for testing the impact of different component combinations on fuel economy. After a suitable combination of engine, motor, and ultracapacitor sizes has been determined, an optimization-based power management strategy is created which shows a better overall performance. Various component sizing and control strategies tested consistently indicate a potential for 10 to 15 percent improvement in fuel economy in city driving with the proposed mild hybrid powertrain. This order of improvement to fuel economy was confirmed by deterministic dynamic programming (DDP) which finds the best possible fuel economy. iii Acknowledgments I would like to thank Dr. Ardalan Vahidi for his help in the completion of my written works, for valuable knowledge and advice on the subject of my study, and for making my stay here at Clemson possible through research funding over the last two years. My graduate education would not have been able to progress without his help.