Researchers tally real-life mileage for plug-in cars

With more plug-in electric vehicles coming to market in the next two years, experts say new ways to measure fuel efficiency are needed, including "electricity per mile."

Martin LaMonica Former Staff writer, CNET News
Martin LaMonica is a senior writer covering green tech and cutting-edge technologies. He joined CNET in 2002 to cover enterprise IT and Web development and was previously executive editor of IT publication InfoWorld.
Martin LaMonica
4 min read

If you're wondering how the familiar term "gas mileage" translates to a car that runs partially on electric batteries, you're not alone.

Industry group SAE International plans to recommend that the Environmental Protection Agency use "electricity per mile" in addition to the familiar miles-per-gallon rating for plug-in electric vehicles, according to a member of the SAE committee tasked with the job. The EPA is working on mileage ratings for plug-ins, which are poised to enter the market, and reviewing its rules for displaying fuel economy on car stickers.

Because efficiency for gas-electric hybrids is far more tricky than gasoline-only vehicles, the National Renewable Energy Laboratory recently said that it has developed a method researchers say accurately reflects real-world mileage of plug-in hybrids, which can vary greatly with driver behavior.

To get real-world mileage for a plug-in hybrid, researchers have come up with a formula to convert standard tests for a chassis dynamometer, seen here at Argonne National Laboratory, into mileage ratings. Argonne National Laboratory

Government agencies and automakers have been studying the question of mileage for gas-electric vehicles for years. But the issue rushed to the forefront in August when General Motors said that its forthcoming Chevy Volt will get 230 miles per gallon in the city and "triple-digit" combined city and highway mileage driving based on a draft of the EPA's methodology. The EPA has not verified GM's claims, as the tests have not been completed.

Within six months, an SAE committee plans to recommend to the EPA that plug-ins come with fuel-economy stickers that show both miles per gallon and electricity per mile, said Jeff Gonder, a research engineer at the National Renewable Energy Laboratory and a member of the committee.

"There are two different fuels that are being used so you need to report what the usage is for those two fuels," said Gonder. "If you combine them into one (number) artificially, you can't derive a final output like annual costs" or annual greenhouse gas emissions from a car.

Having a rating for electricity per mile allows a consumer to figure out how much it costs to run a car per mile by using the local per-kilowatt-hour electricity cost, he added.

In addition to cost per mile, there are a number of other proposals to measure fuel efficiency for electric cars. They include an electric car's range--a big limitation of all-electric vehicles--or miles per gallon equivalent based on the energy in liquid fuels and batteries.

Recalibrating your dynamometer
With multiple alternatives and a lot at stake, it's unlikely that the question over how to represent fuel efficiency on a sticker will be resolved quickly. Sedans such as the Chevy Volt, Nissan Leaf, and plug-in Toyota Prius are scheduled for release over the next two years.

But labels aren't the only problems that new auto technologies introduce. The automated tests used to measure fuel economy before vehicles are sold need to be adjusted as well, according to NREL researchers.

That's particularly important with plug-in electric hybrids--essentially the same type of vehicle as today's hybrids with bigger batteries--because actual mileage will vary significantly based on driving conditions and how often a car is recharged.

Plug-in hybrids run almost exclusively on battery power for the first 20 or 40 miles, with the battery working with the gasoline engine after that. Driving mainly off the battery will be cheaper in part because electric motors are relatively efficient. So the fuel economy for a 40-mile drive will be substantially better than when a person drives 200 miles in a plug-in hybrid, since the bulk of the driving will be fueled by the gasoline engine, Gonder explained.

To come up with a mileage rating today, cars run a course on a machine called a dynamometer--essentially a treadmill fitted for cars and trucks--and the results are converted into miles per gallon. The current conversions don't work well because plug-ins operate in two modes--the first 20 or so miles when the car runs mainly on batteries and then in the "charge sustaining" mode for longer rides, said Gonder.

To address that issue, NREL researchers devised a formula to convert plug-in hybrid car performance on dynamometers to reflect actual driving performance, he said.

"We're trying to set appropriate expectations for what vehicles will get over a long period of time," said Gonder. "We're trying to predict the average (mileage) based on how often they drive between recharging."

Researchers found that the expected results matched actual mileage of a fleet of Toyota Priuses converted to be plug-ins operated by Idaho National Laboratory. Gonder said the methodology needs to be tested with other cars, but should be able to be adjusted for different types of plug-in vehicles, including the range-extended Chevy Volt.

The data also made clear that the cost of operating a plug-in hybrid will vary significantly based on driving style and frequency of charging.

The annual fuel cost of Idaho National Labs' plug-in Priuses ranged from $987 a year--in the case of an aggressive driver who never recharges from an outlet--to $478 per year with the driver charging about every 30 miles and seeking to maximize fuel economy. The average came to $789 per year with daily charging, from the equivalent of 55 mile per gallon mileage.

Updated on October 6 at 11:30 a.m. PT with corrected credit on photo caption.