This is a air compressor off a CAT engine. My piont of doing this was to so the iner works for a compressor and to understand more fully how it works. Come to find out that is pretty simple in design.
It has two pistons inside witch operate pretty much the same way the postions in a regualer engine do. The gear, that you see on the left, is driven by an other gear straight off the main shaft the vehicle's engine. The compressor's gear is on a crankshaft, in witch the piston are attached to. Obviously the strokes are different. The compressor just has a intake and a exhaust, (witch is the air being compressed into the air system.
This is basically the cylinder head of the compressor. There are four thin pieces of metal that cover up holes. As the pistons move up down the thin pieces of metal will let air, pervent air from escaping, or compressed the air.