A variable displacement pump includes a control valve formed with a high-pressure chamber for introducing therein high-pressure fluid on the upstream side of a metering orifice, a pressure chamber for introducing therein pressure fluid on the downstream side of the metering orifice, a low-pressure chamber arranged between the high-pressure and pressure chambers for introducing therein low-pressure fluid, and a communication passage for providing fluid communication between one of the high-pressure and low-pressure chambers and a first fluid-pressure chamber of the pump. First and second recessed grooves are formed in the outer-peripheral surface of a spool of the control valve to provide fluid communication between the low-pressure and high-pressure chambers through the communication passage of the control valve when the spool carries out selective switching between the high-pressure and low-pressure chambers to supply fluid to the communication passage, thus restraining abrupt pressure rise in the first fluid-pressure chamber.

 
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< Pump unit with multiple operation modes

> Fluid machine operable in both pump mode and motor mode and waste heat recovering system having the same

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