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National Labs Work to Settle PHEV Fuel Economy Conundrum

September 28, 2009

The U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) recently joined forces with researchers from Idaho National Laboratory (INL) and Argonne National Laboratory (ANL) to take the lead in developing and testing a new method for predicting the real-world fuel and electricity consumption of plug-in hybrid electric vehicles (PHEVs).  After examining data on the only PHEV currently available in large numbers, the NREL-developed method shows promise for reasonably predicting the PHEV’s average fuel and electricity use.

Industry debate centers on the rules for estimating miles per gallon. Current rules for conventional vehicles do not work for plug-in hybrids because the vehicles run on both electricity and gasoline. 

PHEV testing is further complicated by the fact that these vehicles operate in two different modes based on the distance they are driven (initially depleting energy from the large vehicle battery, and eventually sustaining the battery charge for longer distance driving).  Consensus is building on techniques to handle these first two complications, but one question that remains is how to adjust raw certification cycle test results to best predict a PHEV’s average real-world energy use.

“Official fuel economy testing for all vehicles is conducted on chassis dynamometers, which are basically treadmills for cars and trucks,” NREL Research Engineer Jeff Gonder explained.  “One subtlety of chassis dynamometer testing is that vehicle fuel economy measurements using decades-old standard speed profiles may be overly optimistic compared to today’s average on-road fuel use.  Official methods exist to adjust the test cycle fuel economy of conventional vehicles to better estimate expected real-world fuel use, but a similar adjustment method has yet to be finalized for PHEVs.”

In an effort to objectively predict on-road energy use of a PHEV, NREL developed a method to adjust the standard test cycle results from each mode of PHEV operation.  The adjusted values are then combined into a single fuel economy prediction.  NREL has applied this technique to its PHEV analysis for several years, but until recently has not been able to validate it against data on a large number of PHEVs operating on the road.  Partnering with two other DOE laboratories provided that opportunity.

INL monitors fleet fuel use of advanced technology cars as part of DOE’s Advanced Vehicle Testing Activity, and has accumulated more than a year’s worth of data on roughly 100 PHEVs of the same design.  (Because of limited purpose-built PHEV availability, these vehicles are production hybrid vehicles modified with aftermarket PHEV conversion kits).  ANL had collected dynamometer test data on the same type of vehicle to evaluate PHEV test procedures over the standard certification cycle speed profiles.

“This analysis provided a great example of collaboration among three DOE labs,” Gonder said.  “NREL applied the adjustment technique we developed to PHEV data from ANL dynamometer testing and compared the fuel economy predictions to on-road data from INL’s large fleet evaluation effort.  After accounting for how frequently the PHEV’s in the INL-monitored fleet actually plug in, we found excellent agreement between the adjusted test cycle predictions and the actual fleet fuel and electricity use.”

While this finding is promising, the validation has so far only been possible on a single aftermarket conversion PHEV design.  It will be important to repeat the analysis once dynamometer testing and substantial on-road fleet data becomes available for different PHEV designs, particularly those with greater electric driving capability.  In the meantime, NREL plans to extend the analysis by simulating “virtual” fleets with a variety of PHEV powertrains operating on GPS driving profiles obtained from conventional vehicle travel surveys.  It should also be noted that while this process seeks to predict the average on-road fuel and electricity use from a large number of PHEVs, fuel economy will vary greatly based on how the vehicle is driven and it will be important to educate PHEV drivers on how to obtain the best results.

This work is part of a coordinated PHEV analysis effort supported by the DOE Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program.  The results of this research were presented at the recent 5th IEEE Vehicle Power and Propulsion Conference (VPPC’09) in Dearborn, Mich.  Gonder published an earlier paper discussing broader PHEV fuel economy reporting issues and serves on the SAE subcommittee (chaired by ANL) that will shortly be issuing an official recommended practice for measuring PHEV fuel economy. 

NREL is the U.S. Department of Energy's primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for DOE by the Alliance for Sustainable Energy, LLC.



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—Heather Lammers