The invention provides the proper form for the rotary positive displacement internal combustion engine. The construction of the engines according to the invention is based on a specific form of four bar mechanism and features unique simplicity (only three moving parts, no valves or camshafts, no gears to transfer piston movement to rotary movement), compactness and strength that make the engines according to the invention similar in these and some other aspects (e.g. power density) to gas turbines.