A hybrid hydraulic drive system for a vehicle comprises a prime mover, a
transmission connecting the prime mover to drive wheels, a fluid energy
storage accumulator, and a reversible hydraulic machine in fluid
communication with the energy storage accumulator and drivingly coupled
to the prime mover upstream of the transmission. The hybrid hydraulic
drive system is arranged such that in a retarding mode the reversible
hydraulic machine retards the drive wheels of the vehicle by pumping
fluid into the accumulator. In a driving mode the reversible hydraulic
machine supplies a supplementary power to the drive wheels of the vehicle
using the pressurized fluid from the accumulator to assist propulsion of
the vehicle. In a neutral mode the reversible hydraulic machine is
disconnected from the prime mover to render the reversible hydraulic
machine substantially inoperative to exert any significant driving or
retarding influence on the drive wheels.