Abstract: A housing defining a flow passage therethrough including inlet and outlet ends as well as a central portion intermediate the inlet and outlet ends is provided. Positive displacement gas pump structure in the form of a roots-type compressor is operatively associated with the inlet end of the passage for pumping air under pressure therethrough and into the central portion between the inlet and outlet ends which defines a combustion chamber. The roots-type compressor or pump also defines an obstruction to the reverse flow of gas under pressure from the combustion chamber outwardly of the inlet end of the passage. A positive displacement gas motor in the form of a roots-type fluid motor is operatively associated with the outlet end of the passage for developing rotary torque in response to elevated gas pressures within the combustion chamber and the passage of gas under elevated pressures from the combustion chamber, through the roots-type motor and out the outlet of the passage.