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It’s all in the numbers: The EPA has finally figured out exactly what the testing procedures might be (the regulation hasn’t formally been adopted yet) for plug-in hybrid vehicles like the Chevy Volt. This week Chevy announced that the Volt will deliver 230 mpg. Other plug-in-hybrid manufacturers won’t be long in announcing similarly astonishing fuel-economy numbers. But where on earth does that number come from?
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The Volt’s complex drivetrain uses a 16-kWh battery pack (of which only about 8 kWh will be usable, to preserve battery life). This battery pack will be recharged overnight, a process that will take 8 to 12 hours from a 110-volt outlet, and 3 to 4 hours from a dedicated EV charging station installed in your garage, or maybe at a public station near your office. This battery pack, fully charged, has enough energy to propel a Volt for 40 miles, according to GM. When the battery pack’s state of charge is low enough, a four-cylinder sustainer motor, running on gasoline, kicks in and provides electricity to power the car’s electric motor for an additional 250 miles or so. The sustainer motor will never charge the battery. The pack can only be recharged by grid-supplied power.

Consequently, if your daily commute is 40 miles or fewer, the gasoline engine will never operate, and your fuel economy—gasoline-wise—is essentially infinite. However, when the Volt is operating in range-extender mode, when the gasoline engine is running, the actual economy is 50 mpg, according to GM. Then, after a fresh charge, you can drive for another 40 miles on the battery pack.

So where does 230 mpg come in? It’s an artifact of the way the EPA has structured the test cycle. And incidentally, the EPA has said it cannot confirm the Volt’s 230 mpg claim until it has tested the vehicle officially. The EPA test normally uses an 11-mile loop to do what is referred to as the Federal Test Procedure (FTP). Based on the results of an 18-month study done by Argonne National Labs, the EPA tests plug-in hybrids like this: Drive the vehicle without recharging, through 51 miles of FTP repeatedly. This means most of this cycle will be on battery power—in the case of the Volt, almost four full FTP tests end-to-end. The total mileage includes, in the case of the Volt, 11 miles or so with the internal combustion engine running. Figure out the total amount of gasoline consumed, and divide for the mileage. If all this seems rather arbitrary, it is. This procedure obviously rewards vehicles with larger battery pack capacity. But it’s based on studies that found that 78 percent of drivers go fewer than 40 miles per day. The upcoming Prius PHEV, with an electric-only range of approximately 20 miles, won’t look quite so compelling.

With the announcement comes some fine print. The EPA fuel economy for PHEVs, the big numbers you see on the window sticker, will also include the energy demands to recharge the battery, expressed as kWh/100 miles. So, you can also calculate the amount of electricity to go the 230 miles that burns up one gallon of gas. With the average cost of electricity nationwide at 11 cents, it works out to about 3 cents per mile. Some utilities are planning to offer substantially lower rates for off-peak charging, with Portland General Electric planning on halving rates off-peak, while Pacific Gas & Electric in San Francisco promising a differential as high as 5:1. —Mike Allen

Source: popular Mechanics. aug.13.2009