An engine includes an engine block comprising a cylinder, an intake and exhaust port, and two linearly opposing pistons reciprocatingly mounted relative to two opposing crankshafts. A pair of piston sleeves are reciprocatingly mounted in the cylinder around each piston and connected relative to their respective crankshafts. Each piston sleeve includes a slotted port in communication with either the intake or exhaust port. A pair of sleeve couplers are pivotably connected to their respective piston sleeves and eccentrically rotatable relative to their respective crankshafts. A pair of eccentric inserts include an outside circumferential surface concentrically offset from an inside circumferential surface aperture. Each inside circumferential surface aperture is pivotable about its respective crankshaft. Each outside circumferential surface is rotatable relative to its respective sleeve coupler. A pair of phase couplers are helically moveable about their respective crankshafts and are also pivotably fixed and slidable relative to their respective eccentric inserts.