"Audi and Sunfire claim that the process to make e-diesel is 70 percent efficient. They estimate that once production is ramped up, the cost to consumers in Europe would be comparable to traditional diesel, at 1.00 to 1.50 euro per liter (approximately $4.00 to $6.00 per gallon)."

http://blog.caranddriver.com/powering-your-car-with-water-au...

(Where does that heat come from? Many possibilities, but currently from fossil fuels.)

First, the researchers heat up steam to very high temperatures to break it down into hydrogen and oxygen. This process requires temperatures of over 800 degrees Celsius (1,472 Fahrenheit) and is powered by green energy such as solar or wind power.