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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control system for an automatic
transmission and, in particular, to a control system for an automatic
transmission which works as an engine brake effectively by holding a lower
shift gear stage.
2. Description of Related Art
Japanese Laid-Open Publication Patent No. 2-38747 discloses a control
system for an automatic transmission which can work as an engine brake
effectively by holding lower shift gear stage thereof when a driver
requires a reduced vehicle speed. Namely, in this conventional control
system, a 3-4 upshift line is extended to 75 km/h where the throttle valve
opening is near zero, for example, as shown in a shift line map in FIG. 2.
Further, when the driver closes the throttle valve opening at the third
shift gear stage in order to reduce a vehicle speed as shown by a symbol
.beta. in FIG. 2, and then the driving condition goes across the 3-4
upshift line, the upshift operation to the fourth shift gear stage is
delayed for a predetermined time. Thereafter, when the driving condition
goes into the extended zone of the lower shift gear stage before the
predetermined has passed, the upshift operation to the fourth shift gear
stage is prohibited and the third shift gear stage is held so that the
engine brake works effectively.
However, in the conventional control system for the automatic transmission,
when the throttle valve opening is closed a little and the opening needs
to be held as shown by a symbol .alpha. in FIG. 2, the upshift operation
to the fourth shift gear stage is not carried out immediately after the
driving condition goes across the 3-4 upshift line and, then, the upshift
operation is delayed for the predetermined time. As a result, the driver
feels uncomfortableness in the upshift operation of the automatic
transmission.
On the other hand, in the conventional control system for the automatic
transmission, when the throttle valve opening is fully closed, the engine
brake works effectively by holding the lower shift gear stage. Thereafter,
when the driver operates an accelerator pedal, the holding of the third
shift gear stage is released. However, if the driving condition goes
across the 3-4 upshift line from the extended zone of the third shift gear
stage, the upshift operation to the fourth shift gear stage is carried out
immediately after the driver operates the accelerator pedal. As a result,
the driver feels uncomfortably under this situation.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a control
system for an automatic transmission which does not make a driver feel
uncomfortableness of the shift operation.
It is another object of the present invention to provide a control system
for an automatic transmission which works an engine brake effectively when
the throttle valve opening is closed and thereafter the driving condition
goes across a predetermined upshift line.
It is still another object of the present invention to provide a control
system for an automatic transmission which can work an engine brake
effectively when the throttle valve opening is rapidly closed when the
vehicle is running at the higher shift gear stage.
It is yet another object of the present invention to provide a control
system for an automatic transmission which work an engine brake when the
throttle valve opening is fully closed and the driver operates a brake
pedal.
It is still another object of the present invention to provide a control
system for an automatic transmission which does not make a driver feel
uncomfortableness when holding of the lower shift gear stage is released.
These and other objects are achieved, according to the present invention,
by providing a control system for an automatic transmission comprising
shift means for shifting respective shift gear stages, a shift line map
including upshift lines and downshift lines, means for detecting a shift
timing at which a shift operation should be carried out based on an engine
load on the shift line map and a possibility is estimated that a driving
condition will return back to the previous shift gear stage, engine load
detecting means for detecting a stable condition of the engine load, and
control means for controlling the shift means based on the shift line map
after the engine load detecting means detects the stable condition of the
engine load at the shift timing detected by the shift timing detecting
means.
Preferably, the engine load detecting means detects the stable condition of
the engine load by employing a pseudo engine load signal which is changed
when a difference between a actual engine load signal and an slow rate
engine load signal, made based on the actual engine load signal, becomes
less than a predetermined value. The control means controls the shift
means based on the pseudo engine load signal.
Preferably, the shift line map is provided with a zone extended to a higher
vehicle speed side in a lower shift gear stage near no engine load, and
said shift timing detecting means detects the shift timing when the
driving condition goes across the extended zone based on a change of the
engine load.
In a feature of the present invention, the control means controls the shift
means so that the lower shift gear stage is held when a no load signal of
the engine is input thereto.
In another feature of the present invention, the shift line map includes
divided driving zones different from the upshift and downshift lines, and
the control system further comprises means for detecting when a driving
condition moves from the extended zone of the lower shift gear stage to
another zone except the extended zone on the shift line map, and second
control means for switching a shift operation based on the upshift and
downshift lines to a shift operation based on the driving zones and
holding the lower shift gear stage when the driving condition is detected
by the driving condition detecting means.
The above and other objects and features of the present invention will be
apparent from the following description and by taking reference with
accompanying drawings employed for preferred embodiments of the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a schematic view of an automatic transmission to which a control
system in accordance with the present invention can be applied;
FIG. 2 is a shift line map in accordance with the present invention;
FIG. 3 is a graphical representation showing a pseudo throttle valve
opening signal;
FIG. 4 is a shift control circuit in accordance with an embodiment of the
present invention;
FIG. 5 is a graphical representation showing divided driving zones for
releasing an automatic engine brake;
FIG. 6 is a time chart showing a shift operation when the throttle valve
opening is fully closed;
FIG. 7 is a time chart showing a shift operation when the throttle valve
opening is closed a little;
FIG. 8 is a time chart showing a shift operation when the throttle valve
opening is rapidly closed;
FIG. 9 is a time chart showing a shift operation when the driver operates a
brake pedal; and
FIG. 10 is a time chart showing a shift operation and a lockup operation
when the driving condition goes into a lockup control zone.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be explained with reference to the preferred
embodiments and the drawings.
FIG. 1 is a schematic view of an engine, an automatic transmission, a
controller and the like of a motor vehicle. Referring to FIG. 1, there is
provided an engine 1 which includes an intake passage la and a throttle
valve 1b for adjusting an intake air amount. In the rear side of the
engine 1 there is disposed an automatic transmission 2 which includes a
torque converter 3 with a lockup clutch 3a and a shift gear mechanism 4
for establishing four shift gear stages in the forward direction and one
speed stage in the reverse direction. The automatic transmission 2 further
includes a shift means 7 which consists of three solenoid valves 5 for
shift control operation and a solenoid valve 6 for control operation of
the lockup clutch engaging force. To the rear side of the shift gear
mechanism 4 is connected a propeller shaft 8 through which an engine power
is transmitted to right and left driven wheels.
There is further provided a controller 10 which includes a throttle opening
sensor 11 for detecting an opening of the throttle valve 1b of the engine
1, a vehicle speed sensor 12 for detecting a vehicle speed, a shift gear
stage sensor 13 for detecting a shift gear stage selected in the automatic
transmission 2, an idle switch 14 which detects a fully closed condition
of the throttle valve 1b and thereafter is turned on, an engine speed
sensor 15 for detecting an engine rotation number, and a brake switch 16
which detects a brake operation by the driver and thereafter is turned on.
Signals from those sensors 11-17 are input to the controller 10 which
carries out a shift operation of the automatic transmission and a lockup
control of the lockup clutch 3a of the converter 3 by controlling the
solenoid valves 5 and 6.
FIG. 2 is a graphical representation showing a shift line map. Referring to
FIG. 2, the shift line map is stored in advance in the controller 10. In
the shift map, there is shown a vehicle speed, a throttle opening,
respective upshift lines for 1-2, 2-3 and 3-4 shifts shown by solid lines,
and respective downshift lines for 3-4, 2-3 and 1-2 showed by dashed
lines. In the 3-4 upshift line, the lower shift gear stage extends to the
vehicle speed of 75 km/h at the engine no load region or the region where
the throttle opening is almost zero. As a result, as shown by symbol
.beta. in FIG. 2, when the throttle is closed fully at the lower shift
gear stage and then the driving condition moves to the above mentioned
extended lower shift gear stage, the shift gear stage is held at the third
shift gear stage so that the engine brake is operated effectively. This
operation is hereinafter called an automatic engine braking function.
Referring to FIG. 3, the controller 10 generates a slow rate throttle valve
opening signal TVOb (shown by a dashed line) which is prepared so as to be
slower than an actual throttle valve opening signal TVOa (shown by a solid
line) sent from the throttle valve opening sensor 11. The controller 10
further generates a pseudo throttle opening signal TVO' (shown by a chain
line) which is updated at a time (shown by A) when a difference between
the signal TVOa and the signal TVOb becomes less than a predetermined
value (shown by k) after the actual throttle valve opening signal TVOa
starts to change. That is, when the stable condition of the engine load is
detected, the throttle opening is updated by the pseudo throttle valve
opening signal TVO'. Therefore, the controller 10 also works as a load
stable condition detecting means 17. The load stable condition detecting
means 17 may detect the load stable condition by employing a rate of the
throttle valve opening.
FIG. 4 shows a shift control circuit in accordance with the present
invention.
Referring to FIG. 4, the controller 10 includes a shift control circuit
which has a 3-4 upshift waiting circuit 20, an automatic engine brake
circuit 30 and a release circuit 40 for the automatic engine brake.
The 3-4 upshift waiting circuit 20 is provided with an OR gate 21 and an
AND gate 22. The OR gate 21 outputs "1" when either one of the pseudo
throttle valve openings TVO'(i) nat the present time and TVO'(i-1) at the
previous time locates in a zone except the fourth shift gear stage in the
forward direction in the shift line map shown in FIG. 2. The AND gate 22
outputs "1" or outputs a signal for delaying an upshift operation to the
fourth shift gear stage, when the vehicle is running in the speed zone of
less than 75 km/h where the automatic engine braking function is required,
the shift gear stage locates in the fourth shift gear stage in the shift
line map based on the actual throttle valve opening TVOa, and the AND gate
22 receives the output signal "1" from the OR gate 21. In other
conditions the AND gate 22 outputs "0" or does not output the signal for
delaying the upshift operation to fourth shift gear stage. The AND gate 22
is controlled to release the output "1" after a predetermined time has
passed.
Thus, the 3-4 upshift waiting circuit 20 works as a shift timing detection
means 23. That is, the shift timing detection means 23 detects an upshift
timing to the fourth shift gear going across the 3-4 upshift line in FIG.
2 according to the changes of the throttle valve opening, and then
estimates that there is a possibility that the upshift timing moves into
the extended region of the 3-4 upshift line based on the fully closed
operation of the throttle valve.
The automatic engine brake circuit 30 is provided with first, second, third
and fourth AND gates 31-34, an OR gate 35 and a holding circuit or
flip-flop circuit 36. The first AND gate 31 outputs "1" when receiving the
upshift waiting signal or the output "1" from the AND gate 22 and a signal
representing an ON signal of the idle switch 14. The second AND gate 32
outputs "1" when receiving a signal representing the ON signal of the idle
switch 14 at the time of the fourth shift gear stage being selected under
the condition that the vehicle is running in the speed zone of less than
75 km/h required the automatic engine braking function, namely when the
basic conditions of the automatic engine braking function are established.
The third AND gate 33 outputs "1" when the difference between the slow
rate throttle valve opening signal TVOb and the actual throttle valve
opening signal TVOa is greater than a predetermined value X (TVOb-TVOa=X)
and the second AND gate 32 outputs "1", namely when the throttle valve
opening is rapidly changed to the closing direction. Further, the fourth
AND gate 34 outputs "1" when the second AND gate 32 outputs "1", the brake
switch 16 is turned on, and a difference between the vehicle speed V(i-1)
at the previous time and the vehicle speed V(i) at the present time is
less than a predetermined value f(V) which is a function of the vehicle
speed. Moreover, the OR gate 35 receives the output "1" from the first,
third or fourth AND gate 31, 33 or 34, and the holding circuit 36 receives
the output "1" from the OR gate 35 at the S terminal and output "1" or a
third shift gear stage holding signal at the Q terminal.
Thus, when the 3-4 upshift waiting circuit 20 detects the 3-4 upshift
timing going across the 3-4 upshift line according to the actual throttle
valve opening by employing the OR gate 21 and the automatic engine braking
circuit 30, the waiting circuit 20 stops the output "1" (the output of the
signal for delaying the upshift operation to the fourth shift stage) from
the AND gate 22 and outputs "0" after the pseudo engine load signal TVO'
of the load stable condition detecting means 17 is changed, namely the
engine stable condition is detected. Thereafter, the solenoid valves 5, 6
are controlled so as to shift the shift gear stages based on the shift
line map in FIG. 2. Concurrently, when the vehicle is running at the
fourth shift gear stage in the speed zone of less than 75km/h and the
throttle valve opening is fully closed, and the closing rate of the
throttle valve (the difference between TVOb and TVOa) is equal to or
greater than the predetermined value X, or the driver operates the brake
pedal under the condition that the negative accelerating speed
(V(i-1)-V(i)) is equal to or less than the predetermined value f(V), the
holding circuit 36 outputs the third shift gear stage holding signal
(output "1"). Therefore, the automatic engine braking circuit 30 works as
a control means 37.
The release determining circuit 40 for the automatic engine brake has a
function of determining the release of the holding the third shift gear
stage controlled by the automatic engine braking circuit 30. The release
is determined based on driving zones which are divided by the throttle
valve opening and the vehicle speed as shown in FIG. 5.
Referring to FIG. 5, a 3rd zone indicated by the numeral 3 is a driving
zone where the driving condition goes across the extended region of the
3-4 upshift line and the vehicle can cruise at a constant speed at the
third shift gear stage. Namely, the 3rd driving zone locates on the zone
whose vehicle speed is less than a third shift gear stage cruising speed
line indicated by a chain line corresponding a first predetermined engine
load.
A 2nd zone indicated by the numeral 2 is a driving zone where the driving
condition goes across the 3-4 upshift line and is greater than the first
predetermined engine load. The 2nd zone includes a zone indicated by the
numeral 2' which is a driving zone enclosed by a fourth shift gear stage
cruising speed line indicated by a chain double-dashed line corresponding
to a second predetermined engine load, the third speed stage cruising
speed line, the 3-4 upshift line and the 4-3 downshift line.
A 1st zone indicated by the numeral 1 and slanting lines is a driving zone
including two driving zones. One driving zone is a zone whose engine load
is greater than the fourth shift gear stage cruising speed line or the
second predetermined engine load and whose vehicle speed is less than the
3-4 upshift line. Another driving zone is a zone whose vehicle speed is
less than the 4-3 downshift line.
The controller 10 also works as a driving condition detecting means 39
which detects that the driving condition moves from the 3rd zone to the
zones except the 3rd zone, based on the respective outputs of the throttle
valve opening sensor 11 and the vehicle speed sensor 12.
Referring back to FIG. 4, the release determining circuit 40 also works as
a second control means which switches the shift operation based on the
shift line map in FIG. 2 to the shift operation based on the divided
driving zones in FIG. 5 when the controller 10 detects that the driving
condition moves from the 3rd driving zone to the driving zones except the
3rd driving zone.
The release determining circuit 40 includes a flip-flop circuit or holding
circuit 41 and a first AND gate 42. The holding circuit 41 holds the third
speed stage holding signal from the automatic engine braking circuit 30.
The first AND gate 42 generates a release signal for releasing the third
shift gear stage holding signal only when receiving the third shift gear
stage holding signal from the holding circuit 41. To the first AND gate
42, input is an inverting signal of the output representing that the
present driving condition is in the 3rd zone. As a result, the AND gate 42
prohibits the output of the release signal when the driving condition is
in the 3rd zone.
The release determining circuit 40 for the automatic engine brake further
includes an OR gate 43, second and third AND gates 44, 45 and a holding
circuit 46. The OR gate 43 has input the signal representing that the
present driving condition is in the 1st driving zone, and outputs the
signal to the first AND gate 42 so that the AND gate 42 can output the
release signal. The second AND gate 44 outputs "1" when both of the
driving condition at the previous time and the driving condition at two
times before are in the 1st driving zone in FIG. 5. The holding circuit 46
holds the output of the second AND gate 44. The third AND gate 45 outputs
"1" when it receives the output "1" of the holding circuit 46 and a
difference between the pseudo throttle valve openings previous time and at
the two times before (TVO'(i-2)(TVO'(i-1)-) is equal to or greater than a
predetermined small value Y near zero. The latter condition means that the
vehicle speed is reduced a little in order to move into a stable driving
condition after the driver operates an accelerator pedal. The first AND
gate 42 receives the output "1" through the OR gate 43 so as to be able to
generate the release signal.
The OR gate 43 receives an inverting signal of an output representing that
the present driving condition is in the 3rd driving zone in FIG. 5 through
a timer 47 after a predetermined time, and receives an output representing
that the present vehicle speed V(i) is equal to or greater than a total
value of a vehicle speed value V(MEMO) stored at a time of the driving
condition going out from the 3rd driving zone and a predetermined small
value Z (V(MEMO)+Z). Thus, the OR gate 43 generates the release signal
when the driving condition goes outside from the 3rd driving zone and the
predetermined time has passed or the vehicle speed has increased.
The release determining circuit 40 further includes a fourth AND gate 48
which outputs "1" or a fourth shift gear stage upshift signal when
receiving the release signal from the first AND gate 42, an output
representing that there is a third shift gear stage holding signal at the
previous time, an output representing that there is no third speed stage
holding signal at the present time, and an output representing that the
present driving condition is in the 2nd driving zone in FIG. 5. Namely,
the release determining circuit 40 outputs "0" or holds the third shift
gear stage when the driving condition is in the 3rd and 1st driving zones
in FIG. 5 while the release signal is being output. On the other hand, the
circuit 40 outputs "1" or the fourth speed stage upshift signal when the
driving condition is in the 2nd driving zone in FIG. 5 while the release
signal is being output.
The release determining circuit 40 still further includes a fifth AND gate
49 which outputs "1" or a lockup signal when receiving an output from a
timer 50 delaying a predetermined time, an output which represents the
present driving condition is in a lockup zone in FIG. 2, and the output
"1" or the fourth shift gear stage upshift signal from the fourth AND gate
48. Namely, the fifth AND gate 49 outputs the lockup signal after the
upshift operation to the fourth shift gear stage is carried out and the
predetermined time has passed. As a result, the lockup clutch 3a is
engaged by the operation of the solenoid valve 6.
Next, there will be described embodiments of the present invention in
respective driving conditions.
Referring to FIG. 6, in accordance with an embodiment of the present
invention, when the throttle valve opening is fully closed by a driver
where the vehicle speed is less than 75 km/h and the shift gear stage is
in the third shift gear stage as shown by the symbol .beta. in FIG. 2, the
AND gate 22 of the 3-4 upshift waiting circuit 20 outputs "1" or the
fourth shift gear stage upshift holding signal at a time of B when the
actual throttle valve opening TVOa goes across the 3-4 upshift line, by
which the upshift operation to the fourth shift gear stage is forced to
wait or be held. Thereafter, when the idle switch is turned on by the
actual throttle valve opening TV0a being fully closed, the first AND gate
31 of the automatic engine brake circuit 30 outputs "1" and the Q terminal
of the holding circuit 36 outputs "1" or the third shift gear stage
holding signal so that the automatic transmission 2 holds the third shift
gear stage without upshifting operation to the fourth shift gear stage. As
a result, the engine brake is operated and the vehicle speed is reduced
effectively. On the other hand, when the difference between the actual
throttle valve opening TVOa and the slow rate throttle valve opening TVOb
becomes less than the predetermined value k, the OR gate 21 of the 3-4
upshift waiting circuit 20 outputs "0" and then the AND gate 22 stops to
output the fourth shift gear stage upshift waiting signal. However, the
fourth shift gear stage upshift waiting signal is held by the holding
circuit 36 and therefore the upshift operation to the fourth shift gear
stage is prevented.
Referring to FIG. 7, in accordance with another embodiment of the present
invention, when the throttle valve opening is closed a little by a driver
and the driving condition goes across the 3-4 upshift line as shown by the
symbol .alpha., the AND gate 22 of the 3-4 upshift waiting circuit 20
outputs "1" or the fourth shift gear stage upshift holding signal at a
time of B. At this moment, since the idle switch is not turned on, the
first AND gate 31 of the automatic engine brake circuit 30 does not output
"1" and therefore the holding circuit 36 does not output the third shift
gear stage holding signal. In this embodiment, when the difference between
the actual throttle valve opening TVOa and the slow rate throttle valve
opening TVOb becomes the predetermined value k, the fourth shift gear
stage upshift waiting signal is stopped to be output and then the upshift
operation to the fourth shift gear stage is carried out. Further, since
the actual throttle valve opening TVOa is changed a little and therefore
the difference between the actual throttle valve opening TVOa and the slow
rate throttle valve opening TVOb becomes less than the predetermined value
k in a short time, the output term of the fourth shift gear stage upshift
waiting signal is relatively short. As a result, when the driver makes the
throttle valve opening close a little, the upshift operation to the fourth
shift gear stage is carried out in relatively short time and therefore the
driver does not feel uncomfortableness.
Referring to FIG. 8, in accordance with another embodiment of the present
invention, when the throttle valve opening is fully closed by a driver
under the condition that the vehicle speed is less than 75 km/h and the
vehicle is running at the fourth shift gear stage in the forward
direction, the second AND gate 32 of the automatic engine braking circuit
30 outputs "1". In this embodiment of the present invention, when the
difference between the actual throttle valve opening TVOa and the slow
rate throttle valve opening TVOb becomes equal to or greater than the
predetermined value X or the throttle valve opening is rapidly closed by
the driver, the third AND gate 33 outputs "1" and then the Q terminal of
the holding circuit 36 outputs "1" or the third shift gear stage holding
signal so that the automatic transmission 2 is downshifted from the fourth
shift gear stage to the third shift gear stage. That is, the downshift
operation from the fourth shift gear stage to the third shift gear stage
is carried out only when the driver requires the throttle valve opening to
be rapidly closed in order to reduce the vehicle speed. As a result, the
effective downshift control operation can be carried out when the driver
wants to reduce the vehicle speed. Further, since the downshift operation
is not carried out in all situations by the automatic engine braking
function, the number of the shifting operations can be limited
effectively.
Referring to FIG. 9, in accordance with another embodiment of the present
invention, when the throttle valve opening is fully closed at the fourth
shift gear stage by a driver under the condition that the vehicle speed is
less than 75km/h and the vehicle is running down on a slope, the second
AND gate 32 of the automatic engine braking circuit 30 outputs "1". In
this embodiment of the present invention, when the difference between the
vehicle speed V(i-1) at the previous time and the vehicle speed V(i) at
the present time is less than the predetermined value f(V) or when the
negative acceleration of the vehicle on the slope is small, and the driver
operates the brake pedal and then the brake switch is turned on, the
fourth AND gate 34 of the automatic engine braking circuit 30 outputs "1"
and then the Q terminal of the holding circuit 36 outputs "1" or the third
shift gear stage holding signal so that the automatic transmission 2 is
downshifted from the fourth shift gear stage to the third shift gear
stage. That is, the downshift operation from the fourth shift gear stage
to the third shift gear stage is carried out only when the throttle valve
opening is fully closed and the driver operates the brake pedal so as to
reduce the vehicle speed much more on the down slope. As a result, the
effective downshift control operation can be carried out when the driver
wants to reduce the vehicle speed on the down slope. Further, since the
downshift operation is not carried out in all situations by the automatic
engine braking function when the throttle valve opening is fully closed,
the number of the shifting operation can be limited effectively.
Referring back to FIGS. 4 and 5, on the contrary, when the driver operates
the accelerator pedal after the driving condition moves into the extended
zone of the third shift gear stage of the 3-4 upshift line, the driving
condition moves into another zone where the vehicle can not run at a low
speed at the third shift gear stage. Under this situation, in accordance
with another embodiment of the present invention, since the first AND gate
42 in the release determining circuit 40 does not output the releasing
signal based on the inverting signal of the zone indicated by the numeral
3, the third shift gear stage is held. As a result, uncomfortable feelings
caused by the upshift operation to the fourth speed stage can be prevented
under this situation.
Further, when the driving condition moves into the 2nd zone and then the
reduced amount of the pseudo throttle valve opening TVO' becomes greater,
a predetermined small value Y or the TVO' becomes stable, the third AND
gate 45 outputs "1" and then the first AND gate 42 outputs the release
signal. Thereafter the fourth AND gate 48 outputs the fourth speed stage
upshift signal. As a result, since the shift gear stage is upshifted from
the third shift gear stage to the fourth shift gear stage after the
throttle valve opening becomes stable, the driver does not feel
uncomfortableness caused by the shift operation immediately after the
driver operates the accelerator pedal.
Referring to FIG. 10, when the driving condition moves into the lockup
driving zone in FIG. 2, the upshift operation to the fourth shift gear
stage is carried out after the throttle valve opening becomes stable as
described above. In accordance with another embodiment of the present
invention, the timer 50 outputs "1" when the upshift operation is carried
out and then a predetermined time has passed. At this moment, the fifth
AND gate 49 outputs the lockup signal so as to engage the lockup clutch
3a. As a result, since the lockup clutch 3a is engaged when the shift
operation to the fourth shift gear stage is carried out and then the
predetermined time has passed, shift shocks can be prevented effectively.
Referring back to FIGS. 4 and 5, in accordance with another embodiment of
the present invention, when the driving condition moves into the 1st
driving zone, the third AND gate 45 outputs "1", the first AND gate 42
outputs the release signal, and the fourth AND gate 48 does not output the
fourth shift gear stage upshift signal. As a result, the shift stage is
held at the third shift gear stage, and therefore good acceleration
ability can be obtained. Further, when the driving condition goes across
the 3-4 upshift line thereafter, the shift stage is upshifted to the
fourth shift gear stage based on the basic shift line in FIG. 2, the
driver does not feel uncomfortableness caused by the upshift operation.
Moreover, since the boundary between the 2nd driving zone and the 3rd
driving zone i | | |
