From a prototype car that runs on compressed air to a converted, electric Mini Cooper, the third annual AltCar Expo and Conference that begins in Southern California on Friday will allow automakers to show off a wide range of cars, trucks, buses and motorcycles fueled by electrical power, natural gas, fuel cell and biofuels.

The two-day conference offers the public a chance to learn about alternative-fuel vehicles and test-drive some of them at the Santa Monica Civic Auditorium.

People who couldn't make it to General Motors' unveiling of its plug-in hybrid Chevy Volt in Detroit last week, for example, could get a glimpse of it at the AltCar Expo.

GM has high hopes for the Volt, which comes with a plug to allow drivers to charge its batteries for increased fuel economy (see Chevy Volt Cleared for 2010 Production). The batteries charge the electric motor, and – unlike regular hybrids available today, which are propelled by both their motors and engines – only the motor propels the Volt.

The carmaker said Friday it has reached a preliminary agreement with U.S. regulators to certify the Volt as the country's first car to get 100 miles per gallon, according to Bloomberg. The carmaker on Thursday said it would spend $370 million to build a factory in Flint, Mich., to make Volt engines.

Several major automakers, including Toyota, Nissan and Chrysler, have announced plans to launch plug-in hybrids or all-electric cars within the next two years (see Chrysler Eyes 2010 for Launch of One of Three Electric Cars).

The AltCar Expo provides an opportunity for small private companies to show off cars that already are available to consumers today. Some of them have been converted from existing, gasoline-powered cars into electric models. Others are selling kits to convert regular hybrid cars into plug-in hybrids, which can be recharged by plugging into standard electrical outlets at home (see Plug In, Move On: A Hybrid Future).

The conference also presents panel discussions about the future of the public transit systems, different alternative-fuel car technologies and AB32, a new California law with comprehensive measures to reduce greenhouse-gas emissions (see California Offers Plan to Clear the Air).

Click through this photo slideshow to see some of the cars at the expo: