Automatic transmissions often use planetary gear sets through which torque can be split to achieve a high overall torque ratio. Depending on the number of planetary gear sets and the number of selectively engagable torque-transmitting mechanisms, a high number of speed ratios with a pleasing ratio progression can be achieved.
Layshaft transmissions, also referred to as countershaft transmissions, utilize parallel layshafts with gears intermeshing to selectively distribute torque between the layshafts as synchronizers are engaged to connect the gears with the respective layshafts. Layshaft transmissions may offer a lower number of shifting elements and a more compact arrangement than an automatic transmission with planetary gear sets. The suitability of an automatic transmission versus a layshaft transmission for a given application typically involves a tradeoff between the efficiency and compactness of a layshaft transmission with the high torque capacity of a planetary transmission. Current front-wheel drive automatic transmissions are typically six-speed transmissions that have three planetary gear sets. There is difficulty in achieving more forward speed ratios than six in front wheel-drive applications due to transmission axial length restrictions. For rear wheel-drive applications, eight speed transmissions are known, but typically require four planetary gear sets. The axial length required to package four planetary gear sets may be too large for an on-axis passenger vehicle (i.e., a vehicle having a transmission with the transmission input member aligned with the transmission output member).