Expert: Don’t Dumb Down Window Stickers for Plug-in Cars

By · August 09, 2010

Mike Duoba testing a Tesla

Mike Duoba, chief engineer at Argonne’s Advanced Powertrain Research Facility, looks on as Geoff Amann, senior technician, completes a driving cycle with the all-electric Tesla Roadster at the Lab’s two-wheel dynamometer laboratory.

For more than a half-century, consumers have associated vehicle fuel consumption with a single number: the federal government's Miles Per Gallon rating. When the window sticker reads “50 mpg,” consumers know it’s an amazing fuel-sipper. And when it reads “10 mpg,” you know it’s a gas-guzzler. But in a world where drivers charge cars up with electricity instead of pumping up with petroleum, MPG is meaningless.

Okay, but what number can replace the MPG window sticker for a plug-in hybrid or electric car?

After three years of looking into the question, task force committees organized by the Society of Automotive Engineers are no closer to a new metric to replace MPG. That’s because, according to Mike Duoba, a research engineer at Argonne National Lab who serves as chair for the SAE’s J1711 and J1634 committees, “There is no final answer.”

Duoba speaks about the challenge in philosophical terms. “You just can’t explain to a one-dimensional man what a two-dimensional painting looks like,” Duoba told me.

Magical Thinking

The SAE test protocols will not produce a single number for an individual vehicle. Those three years of hard work, instead, produced a complex test protocol to yield an entire set of data. SAE's J1711 is the recommended industry practice for measuring emissions and fuel economy for hybrids and plug-in hybrids, while SAE J1634 is the test procedure for electric vehicle energy consumption and range.

Each car is tested on five different test cycles, including urban and highway routes of different lengths, and cycles for cold temperature, heavy loads, and using air-conditioning. In addition, results for plug-in hybrids with varying degrees of all-electric range and battery sizes are weighted based on large Department of Transportation surveys of average U.S. driving patterns.

The responsibility for transforming all that data into a meaningful number or set of numbers is falling on the Environmental Protection Agency. With the first plug-in cars—the Chevy Volt and Nissan LEAF—only months away, the EPA is running out of time to figure out what will be printed on the window stickers.

Some observers suggest that the EPA use some kind of electric MPG proxy—a fabricated term called “miles per gallon equivalent” or “MPGe.” Duoba doesn’t like that idea for multiple reasons. After years of working on the challenge, Duoba believes that “trying to boil the information down to one number is only going to mislead people. There’s no magical way to turn electricity into gasoline.”

Getting High

There are at least two other reasons not to dumb down plug-in car efficiency to a single answer. First, the numbers are going to be too high to make any sense. Last year, G.M. advertised that the Chevy Volt would achieve 230 miles per gallon, while Nissan said the all-electric LEAF would get 367 mpg. “Any time you go over 100 miles per gallon, depending on how you’re calculating things, you’re going to really confuse people,” Duoba said. “The difference between 200 miles per gallon and 300 miles per gallon can fit into a thimble. As the numbers get higher, the amount of fuel we’re talking about is tiny.”

Duoba also worries about “blatant gaming or unintentional gaming of the system by carmakers.” He believes that a carmaker could design a car to turn on its engine only once, for a second of two, during a specific test cycle. “You average that out, and it will be 900 miles per gallon. That sounds like a really impressive number because miles-per-gallon is upside down. MPG is an incorrect metric because consumption is what really matters.”

Raising the Bar

While Duoba believes that a direct MPG or MPGe number doesn’t make sense, he acknowledges that consumers understand that a vehicle with a certain MPG has a certain level of impact on an owner’s wallet, the environment, or the nation’s dependence on foreign oil.

For that reason, Duoba believes that the results of the SAE protocols could produce a table to indicate how a plug-in hybrid or electric car would fare in terms of fuel cost, petroleum consumption or CO2—compared to an internal combustion engine.

Wouldn’t that require consumers to think deeper about energy consumption?

Yes, and that’s perfectly fine, according to Duoba. “Don’t dumb it down too much. I’ve heard people say the window sticker has to be understandable by a fifth grader. Well, fifth graders don’t drive,” Duoba told me. “We need to place the bar a little higher, folks. If you’re an early adopter, you can have more than one thought in your head at once.”

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