1. Field of the Invention.
This invention relates to the field of continually variable transmissions that produce stepless speed ratio changes. More particularly, the invention pertains to such transmissions having fixed ratio and variable ratio drive mechanisms, which regenerate torque and extend the range of the transmission speed ratio.
2. Description of the Prior Art
Automatic transmissions for automotive use that employ a belt drive variable speed ratio mechanism usually include a torque converter located in the driveline between the power source and a variable drive mechanism, or between a forward and reverse gear set and the variable drive mechanism. At the lower speed ratios, the torque converter produces torque multiplication to assist in accelerating the vehicle from rest. At higher speed ratios, the torque converter is locked up to produce a mechanical connection between its input and output, rather than the conventional hydrodynamic connection, to avoid the losses inherent in its operation. At the higher speed ratios, the belt drive mechanism alone produces the torque multiplication and speed reduction between the power source and drive wheels.
It is preferred that the added expense of a torque converter and the inherent hydrodynamic losses associated with its operation be avoided in a continually variable transmission by using, in place of the torque converter, a fluid coupling, which like a torque converter, can attenuate driveline vibrations and avoid operating harshness. However, an inherent disadvantage in merely substituting a fluid coupling for a torque converter is the loss of the torque multiplication that the torque converter can provide.
A compound planetary gearset can be operated with a variable belt driven mechanism and a fixed ratio drive mechanism in a torque regeneration mode to produce torque multiplication at lower speed ratios when its sun gear and ring gear are driven and a planet carrier is used as the output of the gearset.
A compound planetary gearset can be operated also in a geared neutral condition where the speed of the ring gear is the same multiple of the speed of its sun gear as the diameter of the ring gear is of the diameter of the sun gear. The speed ratio of the gearset is increased from the geared neutral speed ratio when the sun gear is driven at a lower multiple of the ring gear speed than the ratio of the diameters of the sun gear and ring gear. An increase in the speed of the sun gear in relation to the speed of the ring gear from the geared neutral relation between these speeds produces a speed ratio that passes through zero and becomes negative. The gearset can be operated in this manner to produce reverse drive and forward drive. However, in certain conditions near the zero speed ratio condition, the torques carried by the mechanisms that drive the gearset inputs can rise to enormous levels, in excess of their ability to carry the load.