In an ideal, pollution-free world, everyone would be driving electric and hydrogen fuel cell cars. Audi knows society isn't quite there yet, however, so it has a stopgap if you still get around using conventional vehicles. The automaker recently produced its first batch of "e-diesel," a synthetic diesel based solely on carbon dioxide and water -- readily available chemicals that are far nicer than sulfur and other typical diesel elements. It's even relatively eco-friendly to make, as you use electrolysis to break down the water and merge it with the CO2 in a high-efficiency synthesis process.
The resulting liquid runs in existing cars, and you can mix it with fossil-based diesel if necessary. Just don't expect to find e-diesel at the pump for a while. Audi's experimental plant will only make about 793 gallons of the stuff in the months ahead, so you likely won't get to use it unless energy companies welcome it with open arms.