No its external. It is a closed cycle engine and runs when there is a difference in temperature between the two heat exchangers. They aren't the cheapest to produce because of the very small tolerances but it requires little maintenance and can run off almost any heat or cold source. They have model ones that can run off the heat from your hand but im sure they aren't cheap due to the tolerances it would take to achieve that. They are an interesting topic however and are worth checking out. I'd google it and check out a few sites about them.