Influence of sub-freezing conditions on fuel consumption and emissions from two plug-in hybrid electric vehicles

Dynamometer testing was conducted on a Hymotion Prius PHEV at several ambient temperatures ranging from 22°C to-18°C. Fuel consumption, electrical energy consumption and emissions were measured and several key vehicle parameters were recorded. This dynamometer testing was conducted at Environment Canada's ERMS cold chamber test facility. Testing included urban (UDDS) and highway (HWFET) cycles over the full charge depletion range of the vehicle. For comparison with the dynamometer testing, a Hymotion Prius PHEV and a Hymotion Escape PHEV were tested on-road at an ambient temperature of-5°C. This on-road testing was conducted over a prescribed urban driving route that has several similarities to the UDDS cycle. An additional on-road study was conducted in an attempt to determine the impact of battery temperature on fuel consumption of PHEVs. This involved testing at sub-freezing temperature while regulating battery temperature to a normal operating temperature of 15°C. The results showed a dramatic increase in fuel consumption (up to three times) in charge depletion operation at sub-freezing temperatures. A large percentage of this increased fuel consumption can be attributed to engine warm-up which takes roughly 10 minutes to reach steady state operating temperature. The battery warm up period is considerably longer as well (60 minutes) which results in the majority of the additional fuel consumption over the entire charge depletion operation. These results included the temperature limitations of the production NiMH battery since both the Hymotion Prius and the Hymotion Escape utilize a dual battery configuration (production NiMH battery and additional Li-ion battery). Advanced Li-ion technology batteries are more robust than NiMH at very low temperature operation which could reduce this effect of increased fuel consumption for PHEVs.