Many factors figure in fuel-economy calculation for electric vehicles

  • 06-Jul-2009 11:47 EDT

Window sticker numbers for the Mini-e are based on EPA test procedures and conversions for electric cars.

What will fuel-economy numbers look like on the window stickers of electric vehicles coming to the U.S. market next year? They will include "petroleum-equivalent" miles per gallon, but you won't see the 367 mpg cited in a recent Nissan presentation for its forthcoming 100-mi (161-km) range all-electric car.

It's true that the 367 mpg figure is based on the U.S. Dept. of Energy procedure for deriving petroleum-equivalent fuel economy, and it could be the number that will be used in part to calculate Nissan's overall CAFE (corporate average fuel economy). However, CAFE and window sticker numbers are two different things.

CAFE numbers are calculated differently for different vehicle types.

For conventionally fueled vehicles (including non-plug-in hybrids), they are derived from U.S. EPA dynamometer emissions tests, following the city and highway cycles, plus any legally mandated adjustments such as additions for flex-fuel capability.

For plug-in hybrids, it has yet to be determined how CAFE numbers will be calculated.

For all-electrics, the CAFE calculation begins with a gasoline-equivalent energy content factor in electrical terms: 12,307.3 W·h/gal, which equals approximately 41,994.3 Btu. The 12,307.3 W·h/gal figure is based on multiplying factors developed for fossil-fuel electricity efficiency (0.328) and electricity transmission efficiency (0.924), as well as the energy content of a gallon of gasoline (33,705 W·h/gal, or approximately 115,006 Btu). The result is divided by a factor for petroleum refining and distribution efficiency (0.830). Note: In preparing this report, AEI discovered an error in the equation as it appears in the Federal Register No. 36987. The equation as given here is correct.

The DOE also applies a "doesn't use petroleum" incentive adjustment (analogous to the CAFE bonus for flex-fuel). The adjustment, which also allows for some changes in "well-to-tank" efficiencies, is a 6.6667 multiplier, raising 12,307.3 to 82,049 W·h/gal for an electric vehicle with no petroleum-fired heating or other accessory. If an electric car has a single petroleum-fired accessory, such as a heater, the 82,049 W·h/gal equivalency factor drops to 73,844 W·h/gal, and the CAFE number drops accordingly.

The electric vehicle is dynamometer-tested on the city and highway cycles, producing electric power consumption numbers in W·h/mi. The numbers then are weighted 55% urban and 45% highway for an average. Nissan has not disclosed its electric car test numbers, but if the Nissan-measured weighted average is 223.57 W·h/mi, dividing 82,049 W·h/gal by 223.57 W·h/mi would equal 367 mpg.

Regarding window stickers, the one for Mini's Mini-e is a possible example of what will appear on all-electric cars, as BMW (parent company of Mini) applied an approach developed with EPA for this piece of consumer information.

For all automobile window stickers since MY2008, the EPA requires that dynamometer testing for fuel economy and emissions be based on five driving cycles: city, highway, high-speed/acceleration, air-conditioning, and cold-temperature. Through MY2010, manufacturers of petroleum-fueled cars also can choose to use only city and highway cycle testing and apply EPA-specified correction factors (which produce a reduction of about 12% for city and 8% for highway numbers) to bring the sticker numbers closer to "real world." If manufacturers do five-cycle testing (required for MY2011 onward), they can apply a complex group of EPA formulas that produce city and highway numbers "adjusted" by actual results from the dynamometer tests that replicate high-speed, A/C, and cold-temperature cycles.

For plug-in hybrids, it has yet to be determined how window sticker numbers will be calculated.

A five-cycle method for electric cars, to develop W·h/gal numbers, does not exist at this time, so automakers instead can apply an EPA alternative, a correction factor that caps at 30% (reduction) to city/highway, even past 2010. There is no "special fuel incentive" for window stickers on electric cars. The manufacturers use the petroleum energy-content-equivalent of 33,705 W·h/gal, then dyno-test for W·h/mi and apply the 30% reduction to city and highway. It is likely to produce electric-car window sticker numbers around the 100-mpg mark.

The Mini-e numbers actually originated with dynamometer testing by CARB (California Air Resources Board), which uses a different method than the EPA, including a petroleum energy-content-equivalent of 32,600 W·h/gal (for California reformulated gasoline). Because BMW wished to ship Mini-e cars from the port, it worked with EPA to develop a sticker format, and EPA testing produced kW·h/mi numbers of 33 and 36 for 100 mi. Dividing by 100 to produce per-mile figures yields 0.33 kW·h/mi-city and 0.36 kWh/mi-highway, including the 30% correction factor. Dividing the mpg equivalent of 33.7 kW·h/gal (based on 33,705 W·h/gal) by 0.33 kW·h/mi-city equals 102 mpg. Dividing 33.7 by 0.36 kW·h/mi-highway equals 94 mpg.

The numbers also were weighted (55 city/45 highway) to produce a combined mark of 0.34 kW·h/mi, rounded up. Dividing 33.7 kW·h/gal by 0.34 kW·h/mi equals 99 mpg combined, but raw numbers result in a rounding down to 98 mpg.

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