A man fuels up his natural gas vehicle at a Clean Energy station in San Francisco, Calif.

AP Photo/Marcio Jose Sanchez

Much has already been said about improving fuel economy and raising general standards in the conventional cars that many of us drive. For several years in the United States, there was a debate over whether the states or the federal government should control auto regulations, but in May 2009 the Obama administration announced tough new emissions and mileage standards that will affect every vehicle on a national scale. Starting in 2012, the new standard is intended to introduce cars and trucks onto the road with an average of 35.5 miles per gallon (15 kilometers per liter). California had led the way in the meantime, enacting tough emission standards on cars since 2002 and offering drivers incentives to promote the purchase of hybrid cars instead of conventional cars in order to reduce greenhouse gas pollution [source: Broder].

But with all of this talk about making gradual changes to gasoline- and diesel-powered vehicles, what about cars that run on something else entirely? While auto industry executives in Detroit worry about how they're going to cut back on larger, heavy-duty truck models and make the switch to smaller, less powerful cars, there are other kinds of cars out there that have to go through their own battery of tests to meet certain standards.

To promote fuels other than petroleum-based ones like gas and diesel fuel, state and federal governments have offered tax incentives and other benefits to people who consider a category of cars known as alternative fuel vehicles. Whereas most of the cars you see on the road operate on fuel derived from petroleum, an alternative fuel vehicle is any car, truck or bus that operates on resources that come from something other than petroleum. There are several types around, and many cite their domestic availability and potential for renewable use as a positive factor.

So while fuel economy numbers look to be the toughest standards conventional gasoline-powered cars are going to have to meet in the coming years, what issues do alternative fuel vehicles typically have to battle in order to gain approval? Read the next page to find out.