Interrelated power delivery controls – including engine control – Transmission control – Transmission controlled by engine
Reexamination Certificate
2001-09-18
2003-04-29
Estremsky, Sherry (Department: 3682)
Interrelated power delivery controls, including engine control
Transmission control
Transmission controlled by engine
C477S140000
Reexamination Certificate
active
06554741
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a shift control device for an automatic transmission, and particularly to a technique for transient downshift control executed when a value of transmission input torque becomes reduced to below a torque value needed to advance a shift during downshifting with an accelerator pedal depressed (with a throttle whose opening increases) or in a power-on condition of the vehicle.
BACKGROUND ART
As is well known, automatic transmissions in automotive vehicles operate to determine a desired power flow or a power-transmission path in a power train, that is, a desired transmission range gear mode (a desired gear position) by selectively hydraulically operating friction elements such as a plurality of clutches and brake bands, and to perform shifting from a range gear mode to another range gear mode by properly switching engagement/disengagement of each of the friction elements. In determining the desired range gear mode matching to engine/vehicle operating conditions, the automatic transmission uses a throttle opening (regarded as engine load) and a vehicle speed. A shift pattern is preprogrammed, so that a lower gear is selected as the throttle opening (engine load) increases. Therefore, when the driver depresses or pushes the accelerator pedal (or throttle pedal), downshifting to a lower gear occurs. Such downshifting with the accelerator pedal depressed will be hereinafter referred to as a “drive downshift” or a “power-on downshift”. Conversely, when releasing the accelerator pedal from the depressed state, upshifting to a higher gear occurs. Such upshifting with the accelerator pedal released will be hereinafter referred to as a “coast upshift”. Usually, the previously-noted drive downshift is achieved as follows.
At the former stage of the drive downshift, the transmission input speed begins to rise by virtue of the transmission input torque owing to a drop in working-fluid pressure applied to each of friction elements to be disengaged or released (that is, owing to a drop in working-fluid pressure on the released side), and thus each of the friction elements to be disengaged or released begins to slip, with the result that an inertia phase starts. At the latter stage of the drive downshift, an electronic transmission controller determines that the inertia phase has been terminated as soon as the transmission input speed reaches an after-drive-downshift-termination synchronous speed, and then the controller completes the shifting operation by a rise in working-fluid pressure on the applied side). In this manner, a drive downshift is smoothly achieved with less shift shocks. However, if the current engine/vehicle operating conditions (including the current throttle opening) change, for example, in the event that the accelerator pedal is released from the depressed state, engine power output tends to drop. Thus, the transmission input speed itself cannot rise. Under such a condition, there is a tendency for the transmission input speed to decrease. When an effective gear ratio i (=N
t
/N
o
) of the transmission input speed (or turbine speed) N
t
to transmission output speed N
o
reaches an after-drive-downshift-termination gear ratio, the controller usually determines that the transmission input speed rises up to the previously-noted synchronous speed and thus the inertia phase has been terminated. For the reasons set forth above, in the presence of the release of the accelerator pedal after depression, there is a possibility that the effective gear ratio cannot rise intendedly, due to the undesirable decrease in transmission input speed N
t
. In this case, the inertia phase cannot terminate for a long period of time. In other words, the system has difficulty in accurately determining a timing of termination of the inertia phase. This prevents a smooth shifting operation, that is, a smooth drive downshift and a properly timed inertia-phase termination. To avoid this, Japanese Patent Provisional Publication No. 6-129528 (hereinafter is referred to as “JP6-129528”) teaches the rise in working-fluid pressure on the applied side at a first predetermined time rate of increase and the drop in working-fluid pressure on the released side at a second predetermined rate of decrease, regardless of the decision result based on the effective gear ratio (N
t
/N
o
), for forcibly advancing drive downshift in the presence of the return of the accelerator pedal from the depressed state to undepressed state during the drive downshift. This control will be hereinafter referred to as a “drive-downshift forcible termination control”. In the system disclosed in JP6-129528, an electronic transmission controller determines that the accelerator pedal is returning from the depressed state to undepressed state, when the transmission input speed tends to reduce during the drive downshift. However, there are some drawbacks, if the controller uniformly determines the presence or absence of the return of the accelerator pedal from the depressed state to undepressed state by way of detection of a drop in the transmission input speed N
t
during the drive downshift.
SUMMARY OF THE INVENTION
Assuming that a command value of the working-fluid pressure on the released side is reduced according to a preprogrammed time rate of decrease when downshifting occurs due to a stepwise increased throttle opening, there is a possibility of great shift shocks. To avoid this, Japanese Patent Provisional Publication No. 10-47468 (hereinafter is referred to as “JP10-47468”) has disclosed a downshift control device that ensures a smooth and moderate rise in the turbine speed N
t
by way of a temporary rise in the command value of the working-fluid pressure on the released side during the inertia phase. However, in the presence of negative transmission input torque fluctuations, in the presence of positive fluctuations in a friction coefficient of the friction element to be released, or in the presence of positive fluctuations in working-fluid pressure applied to the friction element to be released, there is an increased tendency the transmission input speed (turbine speed) N
t
to reduce during the inertia phase. Under such a condition, suppose the previously-described way (of JP6-129528) to determine the return of the accelerator pedal from the depressed state to undepressed state during the drive downshift is used. In such a case, although the transmission input torque value exceeds a set value but a rate of change in the transmission input speed is below a set value, the controller erroneously determines that a decrease in the rate of change in transmission input speed arises from a drop in the transmission input torque. Thus, the process of shifting down is wastefully advanced forcibly. This may result in shift shocks rather than smooth downshifting. As discussed above, as a first problem, in the system disclosed in JP6-129528, it is difficult to accurately detect or determine a transmission-input-speed drop arising from the return of the accelerator pedal from the depressed state to undepressed state during the drive downshift. As a second problem, when moderately releasing the accelerator pedal such that the transmission input speed (turbine speed N
t
) continues to rise slightly, in the system of JP6-129528 it is difficult to detect the return of the accelerator pedal from the depressed state to undepressed state during the drive downshift. In this case, it is impossible to timely initiate the drive-downshift forcible termination control disclosed in JP6-129528. Additionally, when a value of torque created by the turbine runner itself tends to reduce owing to a rise in the turbine speed without returning the accelerator pedal to its undepressed state, the turbine speed has already been risen, but a time rate of increase in the turbine speed is excessively low. In such a case, it is impossible to timely initiate the drive-downshift forcible termination control disclosed in JP6-129528. As a third problem, under a particular condition that there is a great difference between
Estremsky Sherry
Jatco Transtechnology Ltd.
Pang Roger
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