Sticker shock

: Image by gmeurope via Flickr

Ignore the 230 miles-per-gallon claims being touted for GM’s plug-in hybrid.

SO, THE Chevrolet Volt—the plug-in hybrid car on which General Motors is pinning its resurrection—will get a staggering 230 miles per American gallon (or 97 kilometres per litre) in stop-go city driving? Well, er, no. That is just marketing hype, albeit prose that is permissible under the draft guidelines for “extended range” electric vehicles that America’s Environmental Protection Agency (EPA) is currently working on. When it is launched, late in 2010, motorists who have bought the dream will awaken to find their shiny new Chevy Volts get around 50mpg in traffic.

True, 50mpg is still pretty good for a roomy five-seater. Though slightly smaller and a good deal cheaper than the $40,000 Volt, Toyota’s latest Prius already boasts 50mpg for combined motorway and city driving in the United States—and considerably more in Japan, where the method of calculating fuel consumption is different. And that is the problem. There is no easy way to compare the efficiency of an electric vehicle—plug-in hybrid or pure electric—with a fossil-fuelled equivalent.

Comparing the performance of self-charging hybrids like the Toyota Prius and the Ford Fusion is easy. As far as their fuel bills are concerned, the motorist pays only for the bit that comes from the pump. The regenerative energy captured during braking—which is used to charge a hybrid’s battery—is, in effect, free. Like any other fossil-fuelled car, a self-charging hybrid’s official fuel consumption is simply the number of miles it can eke from a gallon of petrol while being tested on the EPA’s cycles that mimic driving on city routes and faster roads.

Not so the hybrid’s plug-in siblings. The Volt, for instance, has enough electrical storage to travel 40 miles (64 kilometres) on battery power alone. GM reckons that is enough to enable three out of four Americans to travel to work and back on a single charge. But if owners were to restrict their motoring to trips of less than 40 miles between charges from the grid, the Volt would give them, not the claimed 230mpg, but an almost infinite number of miles per gallon. Such quirks make a mockery of the EPA’s tests.

The EPA has yet to reveal its draft methodology for testing the Volt and other plug-ins. It clearly takes into account some contribution from the vehicle’s petrol-powered generator. It also seems to factor in the cost of the electricity (a national average of 11 cents per kilowatt-hour is quoted) used to recharge the battery. But how 230mpg can be claimed for the Volt is difficult to fathom.

Start with what is known about the vehicle’s traction system. GM reckons the platform needs only 25 kilowatt-hours of energy to cover 100 miles of city driving. At 11 cents a kilowatt-hour, that would cost $2.75 for 100 miles. If petrol costs $3 a gallon (as regular does today), then the Volt would be able to travel 109 miles for the same price as that of a gallon of petrol.

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What if the owner does 50 miles between charges instead of 40 miles? Then the Volt’s 1.4 litre petrol-engine has to kick in to cover the extra ten miles—not to drive the wheels directly, like the Prius’s engine does, but to recharge the battery, which then feeds juice to the electric motor, which, in turn, drives the wheels.

If that were an efficient way of delivering torque to the wheels, all cars would have electric transmission systems instead of mechanical ones. They don’t, for good reason. So expect no more than 20mpg for a car the size and weight of the Volt when running under petrol power. In that case, the fuel used for a 50-mile journey would be half a gallon and the efficiency would be 100mpg. (Alternatively, you could take the figures for the price of motoring given above, and calculate an efficiency of 58mpg.) Go 60 miles and it would drop to 60mpg using the first measure, or 44mpg using the second.