A pump system that pressurizes a fluid includes a shaft, a first rotor and a second rotor that is selectively driven by the shaft. Relative rotation between the first rotor and the second rotor generates variable sized pockets therebetween to pressurize the fluid. A clutch regulates a degree of coupling of the rotor to the shaft between a decoupled state and a coupled state to regulate the relative rotation between the first and second rotors.