TransForum Vol. 9, No. 1

Real-World Temperature Effects on Plug-in Hybrid Electric Vehicles

Engine surface temperature collected during a soak time sensitivity study using Argonne’s Modular Automotive Technology Testbed. Pictures quantify cooling that occurs in powertrains during engine-off conditions. Data collected by Henning Lohse-Busch.

Forrest Jehlik and his colleagues at Argonne’s Transportation Technology R&D Center are studying the effects of temperature on powertrains in plug-in hybrid electric vehicles (PHEVs) to discover the thermal effects on fuel efficiency.

Testing PHEVs over standardized and real-world drive cycles shows relatively large differences between ambient cold starts and hot starts. While testing various PHEVs’ efficiencies, losses of 25-40% efficiency were observed when starting the engines at 20°C and allowing them to achieve optimal hot operating temperatures. Understanding the effect temperature variations have on overall fuel economy numbers due to soak times is pertinent in defining proper industry measurement standards. (Soak time is defined as the duration of time in which the vehicle’s engine is not operating and which precedes a successful vehicle start.)

Combined with methods of predicting the engine thermal state from its initial temperature, these researchers have
been able to accurately predict the fuel efficiency of a PHEV drive cycle from a cold engine start-up to its fully operational temperature.

Experiments

Studies using surface methodology techniques were performed on a number of vehicles of varying architecture.

Prius Study Results

In one study, Jehlik and his colleagues analyzed the total fuel efficiency loss as a function of engine temperature using a 2007 Toyota Prius Hymotion PHEV conversion. Results of their analysis showed that

• As expected, engine efficiency significantly increases with increasing engine temperature.


• An initial cold start enrichment spike accounts for a 3% fuel efficiency decrease. Much greater losses follow cold start enrichment.


• In the range of 25-60˚ C, each 5˚ C increase in engine temperature decreases fuel consumption by 1.9-3.2%. Fuel consumption decreases as temperature increases, eventually reaching a projected minimum at approximately 85˚ C. For the Prius, ten-minute soak times may result in ~5˚ C variations in temperature as a function of powertrain temperature.


• Losses dues to electric components, rolling and transaxles are minimal as compared to engine and transmission thermal losses.


• The projected optimal engine temperature is ~25% more efficient than a 22˚ C ambient cold start. Cold starts at lower temperatures would lead to greater efficiency losses.

It is the ultimate goal to integrate this methodology into vehicle systems modeling and better quantify the thermal
effects on vehicle fuel consumption. Ultimately, the results from this methodology, and following studies, may result in more efficient engineered thermal management systems for powertrains of PHEVs. This would result in significant fuel consumption reductions.

A complete copy of this conference paper can be obtained by ordering it from SAE. Select “Technical Papers,” and then “Systems and Components.”

This research was supported by DOE’s Office of Vehicle Technologies under the direction of Lee Slezak.

May 2009