DOT or DOE wants to save billions in fuel every year by prohibiting semi rigs from idling at stops. All they need to do is get some cops to go to fuel centers and bang on cabs at 3 am.
Auxilary power units are considered too expensive for many owners so the regulations stay off the books. Here's something that might be cheaper and possibly aftermarket:
Instead of running the entire engine of large turbo diesel or gas engines bypass the cylinders with ducting that goes straight from the turbo compressor to the turbine then run the turbo part of the engine to power a generator.
You would need to install:
- bypass valves and ducting
- a combuster w/fuel line
- high speed air or electric starter
- reducer gearing to power the ac compressor and/or alternator
- possibly a regenerator
The thermo efficiency might not be too great but it'll last longer than batteries and be more reliable than a small gas engine..