Patent ID: 7048515

Claim:
A hydraulic drive system for a working machine comprising: an engine; a fuel injection control unit for controlling an amount of fuel injected into said engine such that an output torque characteristic of the engine in at least a part of a governor region where the fuel injection amount is adjustable comprises an isochronous characteristic, a reverse drooping characteristic, or a combined isochronous characteristic and reverse drooping characteristic; a variable displacement hydraulic pump driven by said engine; and a plurality of hydraulic actuators driven by a hydraulic fluid delivered from said hydraulic pump, wherein said hydraulic drive system comprises pump absorption torque control means for controlling a displacement of said hydraulic pump such that, when a delivery pressure of said hydraulic pump exceeds a first predetermined pressure (P 1 ), an absorption torque of said hydraulic pump does not exceed a maximum output torque of said engine in said governor region; and flow rate compensation control means for controlling the displacement of said hydraulic pump such that, when the delivery pressure of said hydraulic pump is not higher than the first predetermined pressure (P 1 ) and said output torque of the engine is within a range of said governor, the displacement of said hydraulic pump is increased as the delivery pressure of said hydraulic pump lowers from a second predetermined pressure (P 1 ) and the output torque of said engine is reduced.