Cowl flaps are fitted to larger piston engine cowlings to allow excess, hot, cooling air to escape. They are normally open for climb, closed in cruise and opened on landing.
Cowl gills are used on large air cooled radial engines and appear as a ring at the back of the cowling. Again normally open on the ground closed for cruise and in trail for take-off. If left open for take-of there can be a big performance penalty. These engines can also have separate oil cooler shutters to control oil temperature.