Idea for a emissions free car

The equipment needed for one of these already exists

•  The vehicle's power plant would run on ammonia.  Ammonia has no carbon.  The ammonia would be in aqueous solution to make it safer to handle.  Nothing new here.
• This would require a "de-watering" device to be installed on the car.  Such things exist.
• The power plant could be a high efficiency Stirling Engine, which would eliminate any nitrogen emissions.  Since the Stirling Engine is more efficient than an internal combustion engine, it would take less fuel.  Nothing new here either.
• It would be paired with a battery electric system, and Stirling Engine would provide range extension.  Nothing new in any of this.

There may be holes in this idea, so it may not be practical.  But as far as the tech goes, all these things exist and can be put into operation now.

Would it eliminate all carbon emissions?  No, not without some additional hardware, but that exists too.

Why isn't it done?    That's what you call the 64k dollar question, kemosabe.