The EPA and CARB like zero emissions vehicles, but don't want you controlling when you decide your vehicle is zero emissions. According to Tony Posawatz, line director for the Chevy Volt program, the two agencies concerned with protecting the emissions from cars aren't happy about a plug in vehicle having a mode which forces it to burn gas.
The 2011 Chevy Volt has an all-electric range of 40 miles, more than enough for most trips on an average day. However, at 40 miles per charge there will be days when the Volt has to use its on-board gasoline-powered generator.
GM announced not too long ago that the 2011 Volt would ship with a "Mountain Mode" button, allowing a reserve of electrical charge to be sequestered away to provide extra electrical power when climbing steep hills. In "Mountain Mode", the Volt will not deplete the entire battery pack before using gasoline power.
In Europe, the 2011 Volt, called the Opel Ampera, will sport a second button called "Hold Charge" In this mode, the Volt uses the on-board generator to power the car, preserving valuable battery charge for driving in areas where zero-emissions controls are enforced and conventional gasoline cars are taxed on their tailpipe emissions.
100 miles into the trip you pull off the freeway into heavily congested city streets and switch to regular EV operation. The batteries you charged at home are still daisy-fresh, and enable you to drive around the metropolitan area with nothing other than metaphorical daises coming out of your tailpipe.
The Hold Charge mode is the exact opposite of the EV button on the 2010 Toyota Prius. Instead of forcing the car to use electrical power, it forces the car to use gasoline power.
Allowing consumers the choice of when to use electrical power and when to use gasoline power upsets the EPA and CARB, as they find it tough to produce accurate fuel economy figures for a car operating in this style. The message is that the car should choose, not the driver.
2011 Chevrolet Volt pre-production prototype, January 2010
The fear seems more borne out of litigation fear than common sense. Any car on the market today can illustrate a wide difference in fuel economy or miles per charge dependent on driver and driving style.
Many luxury and sports gasoline powered cars on the market today even have a selector switch enabling the driver to choose between a sporty, gas-guzzling mode and a more sedate, fuel conscious mode. Why shouldn't EVs too?