Yeah, at first it seems a bit strange/dangerous/scary/whatever... But apparently (as I've read up more on the concept) its not even a remotely new idea.
The concept has been around since sometime shortly after the advent of the locomotive, and has been attempted and even extremely successful in a few uses. The X-15 for example used it as a primary fuel during development because it didn't cause carbon related combustion chamber fouling and such.
There is apparently more than one company trying to get the idea off the ground for cars, each with slight variations on the same basic premise. They all seem to have a common problem: Which method is best for making ignition easier. Ammonia has decent energy density, but has a little bit of resistance when it comes to starting the burn.
The approach in the link above uses the vehicle's original fuel at certain times (mostly starting, warmup, and idling) to address this issue. Another utilizes a small percentage of raw hydrogen (~5%) mixed in with the ammonia to do the same.
Apparently, the modifications required for the latter are a bit simpler than the hydrocarbon method. With that, everything in the fuel delivery system is essentially doubled; one set of injectors for the ammonia, the other for the fossil. Not exactly ideal.
The hydrogen method is a little simpler, and has even been considered to utilize electrolysis on the fly (much like the crack pot > 100% nuts), but because the amount required is so low, the hopes are that the overall efficiency will still be high enough when cracking water is involved that it may still be practical.
Of course, this energy does have to come from somewhere, and so part of the energy in the ammonia would be used up generating the hydrogen. But because its a small amount and is essentially used as a catalyst, some of the energy used to crack the hydrogen is returned to the system when it returns to the engine.
Two comments came up in IRC about this whole idea that both need addressing:
First, this is NOT a get-something-for-nothing scheme. Its analogous to the use of an alternator to return the losses incurred in cranking the engine to start it. Nothing more.
The other, which carries not only a lot of merit but is definitely a valid concern - anhydrous ammonia is one of the key ingredients in a certain DIY pharmaceutical operation that is a great concern in most (if not all) locations in the world. So, simply because of that, it might never go anywhere, no pun intended.
It also carries with it a set of challenges for storage. But any fuel does. Where there's significant energy stored, there are going to be respect issues that must be addressed. Gasoline is poisonous, volatile, and will ignite readily and rather violently. Hydrogen has a significant storage issue, primarily getting enough of it in a reasonably sized container to make it worthwhile. Unfortunately, this involves all kinds of little tweaks to the containment vessel, one of which is dealing with pressures in the 10k PSI range. Then theres the whole massive bang thing to deal with.
Ammonia is no joke either. Corrosive, toxic, environmentally a challenge (primarily aquatic life), and of course is saught after by the darwin award ceremony attendees.
BUT - It does have a few big advantages over its fossil cousin - There are a few ways to make it, and at least one doesn't involve carbon. So not only does burning it not produce the so-called "greenhouse" gasses (sorry, I'm not exactly a tree hugger), producing it doesn't either, and best of all, the sole source of energy input can be RE. Also, the hydrogen mix version doesn't require much modding to the engine itself, so an additional bonus there. Storage can be accomplished at reasonable pressures with only a couple container design considerations, primarily materials used for the construction.
Exhaust is no more of a concern than the hydrogen-only counterpart either. The nitrogen and oxygen come from the air, the hydrogen comes from water, and upon combustion, there is literally no trace of anything except what went in. Nitrogen (atmosphere's primary constituent) and water (eventually just ends up where it came from), and the energy that makes the wheels go round? Ultimately, from the big white hot fusion ball that we all see a fair percentage of the time
Now, it might not entirely be PRACTICAL to generate it that way, simply stating that its physically possible.
Food for thought.