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The Environmental Protection Agency’s fuel economy measurement system fails to factor in several metrics that could shed light on the fuel efficiency of electric and plug-in hybrid vehicles, according to Mike Duoba, Vehicle Systems Research Engineer at Argonne National Laboratory.

The EPA bears responsibility for generating information on vehicle fuel economy for all new cars and light trucks. This data then informs the Department of Energy’s yearly Fuel Economy Guide, as well as the Department of Transportation’s Corporate Average Fuel Economy (CAFE) standards and the Internal Revenue Service’s Gas Guzzler tax.

Fuel economy data for all vehicles is provided in miles per gallon. For some alternative fuel vehicles, that makes sense, but in the case of electric vehicles and plug-in hybrids, it can ignore considerations that might factor into a consumer’s cost-benefit analysis, according to Duoba.

“For CNG [compressed natural gas], and maybe ethanol, as well, there are some merits to saying ‘here’s the mpg equivalent’,” Duoba told Breaking Energy. “What they’ve done for electric cars is to extend that same process, but it’s a little bit different when you compare thermal energy and fuel.”

Looking strictly at thermodynamics, the amount of energy in a gallon of gasoline is equivalent to 33.7 kilowatt hours of electricity. But expressing EV fuel economy in miles per gallon “really only works when you’re comparing EV to EV,” said Duoba.

A more comprehensive, consumer-focused system of comparing vehicle fuel efficiency might weigh factors such as cost per mile, oil displacement, and CO2 emissions, Duoba said.

“If you’re just talking about cost, then maybe cents per mile would be one way to do it,” Duoba said. “And then there’s also petroleum displacement – electricity is almost 100% petroleum-free, so it’s kind of a ‘divide by zero’ issue.”

“One selling point of an EV is really that you’re not using petroleum, so whatever the fuel source is, it’s domestic,” Duoba said.

And many consumers might put a high premium on the emissions-reduction potential of an EV. “That would require some analysis of where you live, how clean your grid is,” Duoba said, noting that emissions-reduction benefits will be greater if power is generated from renewables or natural gas than from coal. “It’s possible that there are areas where a hybrid can do better than an electric car in CO2 emissions depending on the grid’s baseload.”

Muddying the Waters

There are other challenges to making accurate fuel efficiency comparisons between combustion engine vehicles and EVs or plug-in hybrids.

The fuel efficiency of a plug-in hybrid, in miles per gallon terms, can vary widely depending on driving style. “When you get to a plug-in hybrid, things get really muddy,” Duoba said. “The ratio of electricity to gasoline can vary by a lot,” Duoba said.

“If you drive more aggressively, you have a greater contribution from the engine,” he said. “Your expected fuel consumption could be all over the place, depending on how you drive.”

Another obstacle is energy lost in the process of conversion of a fuel – coal, natural gas, biomass, etc – to electricity. The EPA’s fuel economy figures do not account for conversion efficiency at the power plant.

“An EV label will typically say 90-100 mpg, but those numbers would drop down to around 35-40 mpg if you include the electric power plant efficiency, and that would depend somewhat on the feedstock,” Duoba said. “Electric motors can be over 95% efficient,” but “there are unavoidable thermodynamic losses when you convert fossil fuels into some kind of usable mechanical energy.”