PHEV Energy Management Strategies at Cold Temperatures with Battery Temperature Rise and Engine Efficiency Improvement Considerations

Abstract

Limited battery power and poor engine efficiency at cold temperature results in low plug in hybrid vehicle (PHEV) fuel economy and high emissions. Quick rise of battery temperature is not only important to mitigate lithium plating and thus preserve battery life, but also to increase the battery power limits so as to fully achieve fuel economy savings expected from a PHEV. Likewise, it is also important to raise the engine temperature so as to improve engine efficiency (therefore vehicle fuel economy) and to reduce emissions. One method of increasing the temperature of either component is to maximize their usage at cold temperatures thus increasing cumulative heat generating losses. Since both components supply energy to meet road load demand, maximizing the usage of one component would necessarily mean low usage and slow temperature rise of the other component. Thus, a natural trade-off exists between battery and engine warm-up. This paper compares energy management strategies for a power-split PHEV for their ability to warm –up the battery and the engine, and ultimately the resulting fuel economy. The engine model predicts engine fuel rate as a function of engine utilization history and starting temperature, apart from speed and torque. The battery temperature rise model is a function of battery utilization. Engine and battery utilization is varied by changing the control parameter - wheel power demand at which the engine turns ON. The paper analyses the sensitivity of fuel and electrical energy consumption to engine and battery temperature rise, for different driving distances and driver aggressivenes