Many of the very very large diesel engines from early on, and perhaps still today, use a second independent small engine to run an air compressor just as you describe.
A few years ago I had opporunity to visit the engine room of a tug boat. Two big Diesels to power the boat plus a couple more smaller ones, one of which drove an air compressor. This was maybe five years ago, and the occasion was the owner showing me his new installation as he had repowered a 50 or 60 year-old tug.
My knowledge of Diesels of any size is limited, but clearly in marine applications using smaller motors to support larger ones was still being done that recently. I'd say the smaller motors on the tug may have been similar in size to the small truck motors being discussed here.
1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more... It's true. I really don't do anything but browse the Internet looking for trouble...
