An engine includes an engine shaft configured to rotate and cause one or more pistons to reciprocate within a cylinder chamber along an axis, each piston having a first piston part and piston stem to move in unison with or separately from a second piston part to define piston strokes for different thermal functions of the engine. The engine further includes a piston lever having a first end coupled to a movable fulcrum point and a second end coupled at a copy point to the piston stem, an actuation mechanism configured to move the piston lever and thereby the copy point, and a guide apparatus configured to dictate movement of the copy point in a direction substantially parallel to the cylinder axis.