The Future of Cars: The Realities of Alternative Fuels : Rolling Stone

Algae
Who's Doing It: Sapphire Energy, San Diego
How It Works: Hydrocarbons get refined from algae-based "Green Crude," then extracted to run on a plug-in electric Toyota Prius hybrid, nicknamed the Algaeus.
Claims: 147 mpg in the city, 52 mpg on highways
Reality: In September, the Algaeus took a 10-day trip from San Francisco to New York City. Sapphire plans to produce 1 million gallons of algae fuel by 2011, at a cost of $60 to $80 a barrel.

Air
Who's Doing It: The Scuderi Group, West Springfield, Massachusetts
How It Works: A special "Air-Hybrid Engine" uses oxygen to supplement the combustion process.

Claims: 50 percent increase in gas mileage for standard cars
Reality: First air-hybrid engines due Q3 2010.

Water
Who's Doing It:
Genepax, Japan
How It Works: A "Water Energy System" extracts hydrogen and oxygen from water, then converts it to electricity in a fuel cell.

Claims: One quart of water per 20 miles
Reality: Scientists and bloggers have debunked the car as "rubbish."

Urine
Who's Doing It: Gerardine Botte, Ohio University
How It Works: Hydrogen is extracted from pee using nickel-based electrodes, then converted to electricity inside fuel cells.
Claims: 90 mpg
Reality: Initial research with synthetic urine has given way to the real thing, but a working pee-mobile has yet to run.

Hemp
Who's Doing It: Hemp Car
How It Works: Hemp is charcoalized or bio-chemically composted, then converted into gasoline like other biomass materials.
Claims: 21 mpg
Reality: A hemp car completed a 13,000-mile cross-country trek, and Lotus is now using hemp-based materials in its new Eco Elise sports car.

Manure
Who's Doing It: Western Washington University Vehicle Research Institute
How It Works: Cow manure is transformed into biomethane fuel.
Claims: Researchers at WWU say there's enough cow poop in their area to fuel every vehicle in the county
Reality: The university just received a $500,000 grant from the U.S. Department of Energy to increase the Biomethane for Transportation project.