A general hydraulic driving apparatus provided in a working machine such as a hydraulic excavator includes an engine, a hydraulic pump driven by the engine to discharge a hydraulic fluid, and a hydraulic actuator supplied with hydraulic fluid to be actuated. Regarding an engine rotation speed, which is indicated by the number of rotations of the engine per unit time, a throttle lever is provided in a cab to be operated by an operator, and the engine rotation speed is controlled based on a target engine rotation speed corresponding to the amount of operation applied to the throttle lever.
However, in this technique, there can be a case where the rotation speed of the engine specified by use of the throttle lever is not coincident with the actuation speed of the hydraulic actuator desired by the operator. For example, even in the case where the operator does not require a high working speed for the hydraulic actuator, specifically, even in the case of a small amount of operation applied to the control lever for working by the operator, the engine rotation speed can be kept high if the amount of the operation applied to the throttle lever is large. This significantly hinders fuel efficiency from improvement.
As a technique for improving the fuel efficiency of the engine in the working machine as described above, known is an apparatus described in Japanese Patent No. 4812843. The apparatus includes a variable displacement hydraulic pump, pump-displacement-volume detection means for detecting the pump displacement volume of the variable displacement hydraulic pump, engine-rotation-speed command means for specifying the engine rotation speed, and setting means for setting a target engine rotation speed. The setting means sets a first target rotation speed in accordance with a command value specified by the engine rotation speed command means, and further a second target rotation speed lower than the first target rotation speed, controlling engine rotation speed based on the second target rotation speed of the engine. Improvement of the fuel efficiency of the engine is thereby encouraged.
Moreover, at the point in time when the pump displacement volume detected by the pump-displacement-volume detection means has increased to a first predetermined pump displacement volume or higher during an operation based on the second target rotation speed, the setting means changes the target rotation speed of the engine from the second target rotation speed to a third target rotation speed, which is higher than the second target rotation speed and equal to or lower than the first target rotation speed, to control the engine rotation speed. Thus secured is a pump discharge amount for operation where high-speed driving is required.
However, in the apparatus described in Japanese Patent No. 4812843, where the pump displacement volume of the hydraulic pump is controlled in accordance with the amount of operation of the control lever or a load on the hydraulic pump, it is impossible to sufficiently utilize the capability of the hydraulic pump for improvement of the fuel efficiency of the engine. On the other hand, there exists a requirement for performing control equivalent to the conventional control of the pump displacement volume, namely, positive control including increasing and decreasing the pump displacement volume based on the amount of operation of a control lever, i.e., an operation member for speed specification, and horsepower control including increasing and decreasing the pump displacement volume based on the load on the hydraulic pump so as to properly maintain the horsepower of the engine.