Doo Won Kang in the Bulletin of the Atomic Scientists:
Why ammonia? An ammonia molecule is composed of one nitrogen atom and three hydrogen atoms. Ammonia can be burned in internal combustion engines with minor modifications — emitting only nitrogen and water vapor from the tailpipe, even when only low-cost emissions controls are used. Unburned ammonia and nitrogen oxides in the engine’s exhaust would be removed by a selective catalyst reduction system. Ammonia can be produced, at an affordable cost, by a catalytic reaction between nitrogen (obtained from air, which is 78 percent nitrogen) and hydrogen (obtained by splitting water molecules into hydrogen and oxygen).
Ammonia-fueled vehicles operate in much the same way as gasoline-fueled vehicles: Liquid ammonia is burned with oxygen, producing energy that is harnessed to drive the vehicle’s wheels. This familiar technology means that ammonia-fueled vehicles can generally be built and maintained in the same way as the current vehicle fleet. But unlike conventionally fueled vehicles, ammonia-powered cars would not emit carbon dioxide.
Most cars on the road can run on a mixture of 90 percent gasoline and 10 percent liquid ammonia, and could be modified to run on a mixture of up to 80 percent ammonia—at a cost of $1,000 to $5,000 per vehicle. An engine that could run entirely on ammonia is currently under development.