I'll tell you why you won't see any ammonia-powered vehicles anytime soon. I'll skip the net energy issues for now.
How many of you are running ammonia refrigerators or air conditioning in your house? What? no one? Why not? Ammonia is about the best refrigerant you can get for AC and refrigeration. Efficient. Inexpensive to make. Works well with basic steel and iron compressor and piping materials. All the refrigerators in the early 20th century used it and major industrial refrigerated warehouses still use it. What happened on the consumer side?
Just a few deaths from leaks, some fires, and a lot of people not-killed-but-overwhelmed by the pungent odor. That is what sent DuPont on the search for a set of new refrigerants that were almost as efficient, non-toxic, odorless, and chemically stable. They found them. Chlorinated fluorocarbons, also known as "CFC"'s. Freon 12 (generically known as "R-12") was a huge hit. It was in every refrigerator. Freon 22 also was big in the AC market. CFC's dominated the market through the 1990's, when ozone depletion became a big deal. Seems the CFC's when released to the air, would not break down until they rose to the stratosphere. There the radiation would break them and the freed Cl would react with the ozone. R-12 production was phased out. Try to buy some now for your old automotive AC unit. R-22 is being phases out. All new AC units use R-410a. The good news - CFC emissions have plummeted and the ozone layer is now slowly recovering. (If only we could sort out the CO2 situation as easily...)
Ammonia is only used at the consumer level as a refrigerant, in minute portions, for those very-special gas-fired refrigerators used in RV's. It takes an exotic ammonia-hydrogen-water blend to make the magic of refrigeration powered only by a flame. If you are technically-interested how this works, it is a special kind of "absorption" refrigeration cycle called the Platen-Munters cycle.
https://en.wikipedia.org/wiki/Absorption_refrigerator