1. Field of the Invention
The invention relates to an automatic transmission for automotive vehicles having multiple-ratios wherein ratio changes between at least one pair of ratios is a swap-shift.
2. Background Art
Swap-shift transmissions for automotive vehicle powertrains are disclosed in prior art U.S. Pat. Nos. 6,292,731; 5,722,519; 5,553,694; 5,758,302; 6,370,463; and 6,577,939. Each of these patents discloses a control system for a multiple-ratio automatic transmission for automotive vehicle powertrains. The transmission includes first and second gearsets arranged in series so that the torque output element of the first gearset delivers torque to the torque input element of the second gearset. The first gearset is an overdrive gearset or an auxiliary gearset. The second gearset is a multiple-ratio gearset, which is referred to as the main gearset. In the case of the prior art patents identified above, the multiple-ratio gearset is a so-called Simpson gearset, which comprises a first planetary gear unit and a second planetary gear unit with a common sun gear.
The torque input element of the first gearset is connected to the turbine of a hydrokinetic torque converter driven by an engine in the powertrain. The torque output element of the second gearset is connected to vehicle traction wheels through a differential-and-axle assembly in known fashion.
During acceleration of the vehicle, as the engine delivers power to the traction wheels, the overall transmission ratio can progress from an initial high torque multiplication ratio to a low torque multiplication ratio with ratio steps between the high ratio and the low ratio.
The transmission is characterized by a so-called swap-shift during upshifts from a second overall forward driving gear ratio to a third overall forward driving gear ratio and from the second overall forward driving gear ratio to a fifth overall forward driving gear ratio. Conversely, a swap-shift occurs during downshifts from the third overall gear ratio to the second overall gear ratio and from the fifth overall gear ratio to the second overall gear ratio.
During a swap-upshift from the second overall gear ratio to the third overall gear ratio, the overdrive gearset must be downshifted while the Simpson gearset is upshifted, the shifts in the gearsets being synchronized or coordinated during the overall shift progression. Conversely, a swap-downshift from the third overall gear ratio to the second overall gear ratio requires a simultaneous upshift of the overdrive gearset and a downshift of the Simpson gearset in a synchronized fashion.
Ratio changes are controlled by a pressure operated friction clutch or brake for each gearset. In the case of a 2-3 swap-upshift from the second overall gear ratio to the third overall gear ratio, a reaction brake on the overdrive gearset must be released as a reaction brake for the Simpson gearset is applied. Conversely, on a 3-2 swap-downshift, a reaction brake for the Simpson gearset must be released in synchronism with the application of a friction brake for the overdrive gearset.
A swap-upshift from the second overall gear ratio to the fifth overall gear ratio requires release of a reaction brake for the overdrive gearset in synchronism with engagement of a friction clutch for the Simpson gearset. The friction clutch for the Simpson gearset drivably connects together two gear elements of the Simpson gearset so that the Simpson gearset gear ratio, following the upshift, is unity.
In the transmission of the present disclosure, the overdrive gearset is a simple planetary gear unit with two gear ratios: a low ratio, which is unity, and a high ratio, which is an overdrive. The Simpson gearset is characterized by three forward drive gear ratios (as well as a reverse ratio). The first forward drive gear ratio has high torque multiplication, and the third forward drive gear ratio is unity. The second gear ratio is approximately midway in the torque ratio progression from the highest torque multiplication ratio to the lowest torque multiplication ratio.
The overdrive gearset, in combination with the three Simpson gearset gear ratios, is capable of producing an overall ratio range of six gear ratios, since each of the three Simpson gearset gear ratios can be combined with each of the two overdrive gearset gear ratios.
Precise synchronization is required to achieve acceptable shift quality during swap-upshifts and swap-downshifts. This synchronization should be maintained throughout the life of the transmission, notwithstanding the presence of wear of the torque transfer friction elements and changes in environmental conditions, such as temperature, lubricant viscosity changes and changes in coefficients of friction for the clutches and brakes.
Even small errors in synchronization of the gear ratio changes for the overdrive gearset and the Simpson gearset, as the two gearsets are upshifted and downshifted during a swap-shift, will have a significant adverse effect on the overall shift quality. One of these adverse effects is referred to as a ratio “flare” condition. Another is referred to as a ratio “overshoot” condition. These conditions, as well as other error conditions, can cause significant, perceptible torque disturbances at the torque output shaft for the transmission.
A ratio “flare” occurs when the overdrive gearset begins its gear ratio progression during a swap-shift before the Simpson gearset begins its gear ratio progression. A ratio “overshoot” occurs if the Simpson gearset shift progression ends before slipping of the friction element for the overdrive gearset is completed.