PURPOSE: To utilize the output of engine efficiently by a method wherein at the hydraulic power shovel equipped with a fixed capacity pump, the engine outputs at two points where one is larger than the other are set on its characteristic curve, while its normal driving power is set within the stated larger output.
CONSTITUTION: The engine output at the two points A and B is set so as to satisfy the output EA > the output EB, also the maximum driving power of the pump 2 is set at the output EA which is larger than the other. The revolution of engine 1 is shown at the point C at no load then decreased along C→B→A where the point A is the set up point of the minimum driving power of the pump. The pump has its output character to generate less discharge flow under the high pressure whereas more discharge flow under the low pressure. Especially for the equipment working under the highly changeable load factors as the hydraulic power shovel, the above set up is enabled the control of pumping pressure and the discharge flow correspondingly to the loads, thus making a quick motion under a light load while a larger power is available under heavy load.