Abstract: A power device or mechanism embodying cyclic compression and expansion of compressible fluids, such as various gases, in a unique manner hereinafter referred to as the "Zachery" cycle. The power device, as disclosed, includes a chamber, such as a cylinder, and movable components, such as opposed pistons, associated with the chamber for varying the volume of the chamber and varying the pressure of gases therein with the movable components being mechanically connected to crankshafts or other mechanisms to enable the highest pressures obtained during the compression-expansion cycle to occur at or near the maximum lever arm of a crankshaft or other mechanism thereby generating the maximum torque possible from the gas pressure available. The power device also exerts its maximum force when the pressure within the chamber is at a maximum.