On-road evaluation of advanced Hybrid Electric Vehicles over a wide range of ambient temperatures

In recent years, Hybrid Electric Vehicles (HEV's) have become a production viable and effective mode of efficient transportation. HEV's can provide increased fuel economy over convention technology vehicle, but these advantages can be affected dramatically by wide variations in operating temperatures. The majority of data measured for benchmarking HEV technologies is generated from ambient test cell temperatures at 22°C. To investigate cold and hot temperature affects on HEV operation and efficiency, an on-road evaluation protocol is defined and conducted over a six month study at widely varying temperatures. Two test vehicles, the 2007 Toyota Camry HEV and 2005 Ford Escape HEV, were driven on a pre-defined urban driving route in ambient temperatures ranging from-14°C to 31°C. Results from the on-road evaluation were also compared and correlated to dynamometer testing of the same drive cycle. Results from this on-road evaluation show the battery power control limits and engine operation dramatically change with temperature. These changes decrease fuel economy by more than two times at-14°C as compared to 25°C. The two vehicles control battery temperature in different manners. The Escape HEV uses the air conditioning system to provide cool air to the batteries at high temperatures and is therefore able to maintain battery temperature to less than 33°C. The Camry HEV uses cabin air to cool the batteries. The observed maximum battery temperature was 44°C.