I have a 20# CO2 tank that I use mainly for inflating tires when offroad, and in the past for powering nail guns for projects in the house. The advantage for nailers is portability, quiet, and no need for electricity to power a compressor. The same can be said for powering small engines at shows where there are not air lines. Would also be useful in setting up for shows like CF that turn air on only minutes before opening the doors to visitors.
I previously tried it on the big Joy engine, and it didn't work. Plenty of pressure but insufficient volume. But when I tried it on the Coventry this morning the engine ran on even less pressure than with air. Tank pressure is about 900 PSI, and my regulator is set for 100.
My tank setup cost me a little over $100 a decade ago, and a refill is about $20. 10 pound tanks are also available commercially, but the 20 lb. size is what is used in soda dispensers and are most easy to find.