A continuously variable turbine includes a case assembly with a case body defining a chamber, a rotor assembly positioned in the chamber, and a pair of valve assemblies. The rotor assembly includes a ring piston and a rotor body positioned within the ring piston. The rotor body is connected to a shaft, and the rotor body rotates concentrically about an axis extending through the shaft while the ring piston rotates eccentrically about the axis. Each valve assembly is positioned outside of the ring piston relative to the rotor assembly and includes a valve body and a seal component attached to the valve body. Each seal component has a surface with a curvature that matches the outer curvature of the ring piston to form a continuous surface seal between the seal component and the ring piston as the ring piston rotates eccentrically about the axis. The position of the continuous surface seals in the chamber defining a first sub-chamber and a second sub-chamber between the surface seals. The case body includes an intake port and an exhaust port for each sub-chamber.