Q: Context: The IW engine is a game engine developed by Infinity Ward, Treyarch, and Sledgehammer Games for the Call of Duty series. The engine was originally based on id Tech 3 as its core, since the engine itself is proprietary with inclusion of GtkRadiant by id Software. It has been used by Infinity Ward, Treyarch, Raven Software and Sledgehammer Games.

Question: what engine does call of duty run on? Let's give stream of consciousness first:
The relevant sentence in the passage is: The IW engine is a game engine developed by Infinity Ward, Treyarch, and Sledgehammer Games for the Call of Duty series. Therefore, the final answer is The IW engine.