A high efficiency energy conversion system disclosed herein incorporates a piston assembly including a sealed cylinder for storing a working fluid and an energy conversion element attached to the piston assembly. A kinematic mechanism such as a cam lobe or a scotch yoke may be used as the energy conversion element. In one implementation, the kinematic mechanism may be configured to provide rapid piston expansion in a manner so as not to allow the expanding working fluid inside the piston to achieve thermodynamic equilibrium. In an alternate implementation, the kinematic mechanism is further adapted to generate a compression stroke in a manner to provide the working fluid inside the piston to achieve thermodynamic equilibrium conditions throughout the compression stroke.