HYBRID WORKING MACHINE

Three pressure sensors 40, 41 and 42 are provided to detect the delivery pressure of a hydraulic pump 2, the output pressure of an engine revolution speed detecting valve, and the output pressure of a differential pressure reducing valve 11 generating the differential pressure between the delivery pressure of the hydraulic pump and a maximum load pressure as an absolute pressure that is then output. A vehicle controller 64 calculates virtually the displacement of the hydraulic pump 2 by use of the detected pressures and an equation of motion about a swash plate of the hydraulic pump 2, calculates the power need of the hydraulic pump 2 and the output of the engine 1 using these values, and switches a motor 60 between powering control and generation control in accordance with the result of a comparison between the pump power need and the engine output. In this manner, there is provided a hybrid working machine equipped with a hybrid system that can be mounted easily even on a small-size working machine and is capable of estimating the displacement of the hydraulic pump without using sensors to detect the tilting angle of the swash plate of the hydraulic pump.