Automatic transmission

An automatic transmission comprising:

• an input shaft rotatably arranged relative to a transmission case and configured to be rotated with power of a driving source relative to the transmission case;
• an output member to output rotation of the input shaft using multiple gear-shifting;
• a first planetary gear system including a first sun gear, a first carrier, and a first ring gear;
• a second planetary gear system including a second sun gear, a second carrier, and a second ring gear;
• a third planetary gear system including a third sun gear, a third carrier, and a third ring gear;
• a fourth planetary gear system including a fourth sun gear, a fourth carrier, and a fourth ring gear;
• and
• an engagement mechanism;
• the first sun gear, the first carrier, and the first ring gear being defined as a first element, a second element, and a third element based on an alignment order in accordance with distances corresponding to gear ratios in a collinear diagram illustrating ratios of relative rotational speeds of the first to third elements as straight lines;
• the second sun gear, the second carrier, and the second ring gear being defined as a fourth element, a fifth element, and a sixth element based on an alignment order in accordance with distances corresponding to gear ratios in a collinear diagram illustrating ratios of relative rotational speeds of the fourth to sixth elements as straight lines;
• the third sun gear, the third carrier, and the third ring gear being defined as a seventh element, an eighth element, and a ninth element based on an alignment order in accordance with distances corresponding to gear ratios in a collinear diagram illustrating ratios of relative rotational speeds of the seventh to ninth elements as straight lines;
• the fourth sun gear, the fourth carrier, and the fourth ring gear being defined as a tenth element, an eleventh element, and a twelfth element based on an alignment order in accordance with distances corresponding to gear ratios in a collinear diagram illustrating ratios of relative rotational speeds of the tenth to twelfth elements as straight lines,
• the third element and the twelfth element being connected to each other to provide a first connected body;
• the eighth element and the fourth element being connected to each other to provide a second connected body;
• the first element and the fourth element being connected to each other to provide a third connected body;
• the sixth element and the ninth element being connected to each other to provide a fourth connected body;
• the third connected body being connected to the input shaft;
• the tenth element being connected to the output member;
• the engagement mechanism comprising:
• a first clutch configured to connect the seventh element to the third connected body;
• a second clutch configured to releasably connect the fifth element to the seventh element;
• a third clutch configured to releasably connect the second element to the fourth connected body;
• a first brake configured to releasably connect the first connected body to the transmission case;
• a second brake configured to releasably connect the seventh element to the transmission case;
• and
• a third brake configured to releasably connect the second element to the transmission case;
• and
• the multiple gear-shifting being provided by setting at least three of the first to third clutches and the first to third brakes in a connection state.