Here is a more recent old design of a RPA combustion engine.

Below is my first RPA design with gears and a different kind of rotating valves:

The rotors are able to transport considerable heat on their surfaces from one working chamber into another, or from inside a working chamber directly into the outer air. This effect can be used to construct heat pumps, fridges and air conditioners.

A special form of the rotary piston array machine is the following heat motor. The clue is that heat is being transported on rotor surfaces from the flames outside the rotors into the working chambers between the rotors, where it causes the enclosed working gas to expand. The heat of the working gas is being reused afterwards in the flame.