1. Field of the Invention
The present invention relates to a shift control system of an automatic transmission for a vehicle.
2. Description of the Prior Art
An automatic transmission for a vehicle achieves a plurality of gear positions respectively providing predetermined gear ratios by the combination of a planetary gear mechanism and a plurality of engaging elements such as clutch, brake or the like, which are actuated by hydraulic pressure, and the combination of engagement and release of the engaging elements.
For example, in the automatic transmission disclosed in Japanese Patent Laid-open No. 8-200456, clutches C1, C2 and brakes B1, B2, B3 are disposed as engaging-elements in relation to three sets of planetary gear mechanisms F1, F2, F3, as shown in FIG. 5a, and gear positions of five forward speeds are obtained by the combination of engagement and release of these clutches and brakes, as shown in FIG. 5b. Here, circles indicate xe2x80x9cengagementxe2x80x9d.
As shown in the shift map of FIG. 6a, such an automatic transmission sets one of the aforementioned plurality of gear positions as a target gear position according to a throttle opening and a vehicle speed which are based on the driver""s depression of an accelerator pedal and achieves this target gear position by setting a hydraulic pressure command value so that a predetermined engagement element is engaged or released and by controlling hydraulic pressure to be applied to each engagement element. FIG. 6a shows upshift lines only.
Here, if the vehicle speed increases in the state where the depression of the accelerator pedal is kept nearly constant during running in the fourth gear and crosses the 4-5 upshift line, as indicated by the arrow Ya in FIG. 6a, for example, the target gear position is changed to the fifth gear and a 4-5 upshift is executed.
In this 4-5 upshift, the clutch C2 engaged in the fourth gear is released and the brake B3 released in the fourth gear is engaged, that is, a so-called change-over is performed. At this time, in order to prevent engagement shock of a rotating member on the engagement side and overspeed of a rotating member on the release side, a timing control to gradually reduce and increase oil pressure command values to the clutch C2 and the brake B3 is performed by respectively spending a predetermined time.
When the accelerator pedal is depressed during the aforementioned 4-5 upshift and the 5-4 downshift line is passed, as indicated by the arrow Yb in FIG. 6b, on the other hand, a target gear position is downed from the 5th gear to the 4th gear.
In this case, while the 4-5 upshift is performed according to an oil pressure command value indicated by the solid line in FIG. 7, if an oil pressure command to immediately release an increasing oil pressure and an oil pressure command to immediately restore a reducing oil pressure to an initial level are given at a time t1 when the aforementioned target gear position changed, as indicated by a broken line, timing of these two oil pressures commands is shifted, which causes the occurrence of shock and engine racing.
Alternatively, even in case of immediately switching to an oil pressure command value of the ordinary 5-4 downshift according to the change of the aforementioned target gear position, as indicated by the imaginary line in FIG. 7, a large deviation occurs between the oil pressure command value of the ordinary 5-4 downshift and an immediately preceding oil pressure command value indicated by the solid line, thereby causing the same problem.
Such a problem also arises in case where the 5-4 and 4-3 downshift lines are crossed during the 4-5 upshift as indicated by the arrow Yc in FIG. 6 and a downshift such as sets a target gear position to the third gear is performed.
As a conventional countermeasure for prevention of shock in such a downshift-control during upshift, for example, a downshift to the fourth gear is started after the current upshift is continued to reach the fifth gear even if the downshift line is passed and a target gear position is changed.
However, this countermeasure has a problem in that shift time from the start of upshift to the finish of downshift elongates.
The present invention has been made in view of the aforementioned problem, and its object is to provide a shift control system in an automatic transmission for a vehicle which allows an immediate speed change while preventing the occurrence of shock or the like in case of performing a downshift control during an upshift.
According to the first aspect of the present invention, a shift control method for automatic transmission which has two adjacent gear positions between which a change-over of a first engaging element and a second engaging element is performed, comprises the steps of: retaining oil pressure command values to the first engaging element and the second engaging element as current values, respectively, when a target gear position changes in a downshift direction during an upshift between the two adjacent gear positions; calculating downshift calculation values for a downshift between the two adjacent gear positions, which are applied to the first engaging element and the second engaging element; and setting the downshift calculation values as oil pressure command values to the first engaging element and the second engaging element after the downshift-calculation values reach the current values retained, respectively.
Therefore, oil pressure command values are smoothly transferred, and a timing for the change-over of the first engaging element and the second engaging element is properly achieved, whereby shift shock and engine racing is prevented. Moreover, since the switching from the middle gear position of upshift to downshift is promptly made, shift time is also shortened.
When the target gear position is a lower gear position than an initial gear position at a start time of the upshift, a shift-down to the lower gear position is executed after a downshift control between the two adjacent gear positions is completed.
Since an initial shift operation changes to the next shift operation after the initial shift operation is surely performed, shift shock between a plurality of gear positions can be prevented.
According to the second aspect of the present invention, a shift control system for automatic transmission which includes a first engaging element and a second engaging element which are actuated based on oil pressure commands and has a plurality of gear positions including two adjacent gear positions between which a change-over of a first engaging element and a second engaging element is performed, comprises: target gear position determining means for determining a target gear position based on a driving condition; means for calculating upshift calculation values for an upshift between the two adjacent gear positions, which are applied to the first engaging element and the second engaging element; retaining means for retaining oil pressure command values as current values of the upshift calculation values when the target gear position changes in a downshift direction during an upshift in which the upshift calculation values are set as the oil pressure command values; means for calculating downshift calculation values for a downshift between the two adjacent gear positions, which are applied to the first engaging element and the second engaging element, when the target gear position changes to a gear position in a downshift direction; and oil pressure command value switching means for setting the downshift calculation values as oil pressure command values to the first engaging element and the second engaging element when the downshift calculation values reach the current values retained by the retaining means.
Therefore, oil pressure command values are smoothly transferred, and a timing for the change-over of the first engaging element and the second engaging element is properly achieved, whereby shift shock and engine racing is prevented. Moreover, since the switching from the middle gear position of upshift to downshift is promptly made, shift time is also shortened.
It is preferred that the shift control system further comprises shift completion detecting means for detecting completion of a shift operation based on an oil pressure command value switched by the oil pressure command value switching means, and when the target gear position is a lower gear position than an initial gear position at a start time of the upshift, a downshift to the lower gear position is executed after completion of the shift operation the initial gear position is detected by the shift completion detecting means.
Thereby, shift shock between a plurality of gear positions can be prevented.
It is preferred that the shift completion detecting means detects that a gear ratio of the automatic transmission is a gear ratio of the initial gear position and determines that the shift operation is completed.
A gear ratio is easily determined, for example, by a ratio between a vehicle speed and a turbine speed of a torque converter, and the completion of a shift operation can be easily and surely detected.
It is preferred that the shift completion detecting means detects that a gear ratio of the automatic transmission is a gear ratio of the initial gear position and determines that the shift operation is completed when a predetermined time has elapsed thereafter.
Therefore, shift shock can be all the more surely prevented.