Hydraulic mechanical transmission with increased efficiency

A hydraulic mechanical transmission comprising:

• a first hydraulic unit having a first shaft;
• a second hydraulic unit connected in hydraulic fluid communication with the first hydraulic unit by high and low pressure lines, the second hydraulic unit including a second shaft and a rotating group that is coupled to the second shaft so as to rotate in unison with the second shaft about an axis defined by the second shaft, the rotating group including a cylinder block that defines a plurality of axial cylinders, the rotating group also including pistons mounted to slide within the axial cylinders, the second hydraulic unit further including a rotatable housing that is rotatable relative to the second shaft about the axis of the second shaft, the rotatable housing supporting a swashplate that opposes ends of the pistons, the rotatable housing including a swashplate mount that mounts the swashplate such that the swashplate rotates in unison with the rotatable housing, the swashplate mount also allows the swashplate to pivot relative to the rotatable housing and the axis of the second shaft to vary a stroke length of the pistons within the axial cylinders such that a displacement of the rotating group can be varied, the rotatable housing also including a commutation structure for providing fluid communication between the high and low pressure lines and the axial cylinders of the rotating group while maintaining separation between the high and low pressure lines;
• a mechanical torque transfer arrangement for transferring torque between the first shaft of the first hydraulic unit and the rotatable housing of the second hydraulic unit;
• and
• wherein in use one of the first and second hydraulic units operates as a hydraulic pump and the other of the first and second hydraulic units operates as a hydraulic motor, and wherein both of the first and second hydraulic units have variable displacement.