NH3CAR is a company that is developing new technology and manufacturing conversion kits for cars to run on ammonia. Based on research done at the University of Michigan's Mechanical Engineering Department, the NH3CAR conversions are automotive dual fuel (gasoline and anhydrous ammonia) systems that run on both fuels simultaneously. The NH3CAR technology provides additional NH3 fuel to the engine depending on the engine's operating conditions.
Although NH3CAR guards their technology closely, my guess is that they have developed a fuel system controller that measures the operating parameters of the engine and fumigates NH3 into the airhorn of the throttle body. Because EFI system continuously measures the exhaust oxygen to control the amount of gasoline injected, the additional NH3 would allow the engine to use less gasoline. Such a system would likely be reliable and fail-safe.
At this time, NH3CAR has developed a Chevrolet S-10 prototype vehicle. To prove the operation of their technology, Shawn Grannell and Casey Stack drove the pickup truck from Detroit, MI to San Francisco, CA, starting on July 31, 2007 and arriving sometime around mid-August. They apparently only refuelled once with NH3 along the entire trip because the truck was able to carry three NH3 cylinders in its bed.
NH3CAR expects to have commercially available automotive ammonia conversions available for around $1000. For more information, you please visit the NH3CAR contact page.