Automatic transmission and frictional engagement element

An automatic transmission comprising a frictional engagement element,

• the frictional engagement element including:
• a first piston and a second piston each having a first face and a second face opposite to each other in an axial direction, the first and second pistons being movable in the axial direction;
• a plurality of friction plates disposed near the second faces of the first and second pistons;
• an engaging hydraulic chamber defined by the first faces of the first and second pistons, the engaging hydraulic chamber being configured to be subject to a hydraulic pressure to move the first and second pistons in a direction of pressing the friction plates;
• an opposite hydraulic chamber defined by the second faces of the first and second pistons, the opposite hydraulic chamber being configured to be subject to a hydraulic pressure to move the first and second pistons in a direction of releasing the friction plates;
• a first urging member for urging the first piston in the direction of releasing the friction plates;
• and
• a second urging member for urging the second piston in the direction of releasing the friction plates with an urging force larger than the urging force of the first urging member,
• wherein one of the first and second pistons has a communicating hole for connecting the engaging hydraulic chamber with the opposite hydraulic chamber and the other of the first and second pistons has a valve part for closing the communicating hole, and
• the first and second urging members have different urging forces so that a difference in travel distance between the first and second pistons in motion due to the different urging forces of the first and second urging members causes the valve part to open the communicating hole.