A kinetic energy recovery system (KERS) is provided. The KERS (1) comprises a first speed-up gear arrangement (12) having an input (10) connectable to a vehicle powertrain. The KERS further comprises a hydraulic variator made up of first and second bent axis motors (20,22) fluidly connected to one another, wherein at least the first motor (20) is a variable displacement motor, and the first motor (20) is connected to an output of the first speed-up gear arrangement (12). A second speed-up gear arrangement (34) has an input connected to the second motor (22). At least one flywheel (52) is connected to an output of the second speed-up gear arrangement (34), where the at least one flywheel is located in a vacuum within at least one flywheel chamber (58).