Engine brake control for automatic transmission

An engine brake control comprises a pressure reduction valve which effects pressure reduction on hydraulic fluid pressure supplied to an engine brake friction element for activation thereof by discharging hydraulic fluid from the engine brake friction element. The engine brake friction element is to be activated to effect engine brake during operation with each of various speeds. The pressure reduction valve is operative to effect the pressure reduction when a first predetermined drive range (D range) is selected, while it is rendered inoperative to prevent the pressure reduction when a second predetermined drive range (II range) is selected.

COPENDING RELATED APPLICATIONS 
Reference should be made to the following copending U.S. applications which 
have been assigned to the assignee of the present application. 
U.S. application Ser. No. 885,136, filed July 14, 1986 claiming priority of 
Japanese Patent Application No. 60-171866 filed on Aug. 6, 1985: 
U.S. application Ser. No. 885,135 filed July 14, 1986 claiming priority of 
Japanese Patent Application No. 60-154244 filed on July 15, 1985: 
U.S. application Ser. No. 890,370, filed July 29, 1986 claiming priority 
of Japanese Patent Application No. 60-166647 filed on July 30, 1985: 
U.S. application Ser. No. 893,243, now U.S. Pat. No. 4,680,992 claiming 
priority of Japanese Patent Applications No. 60-171154 filed on Aug. 5, 
1985, No. 60-171865 filed on Aug. 6, 1985, No. 60-171869 filed on Aug. 6, 
1985, and No. 60-197078 filed on Sept. 6, 1985; 
U.S. application Ser. No. 905,078 filed Sept. 9, 1986 claiming priority of 
Japanese Patent Applications No. 60-199318 filed on Sept. 11, 1985, and 
No. 60-199319 filed on Sept. 11, 1985. 
BACKGROUND OF THE INVENTION 
The present invention relates to an engine brake control in an automatic 
transmission. 
Automatic transmissions effect a shift between various speeds having 
different gear ratios by a change-over in the power delivery path after 
selective actuation of one or more kinds of friction elements. It is the 
commonly employed practice to provide a one-way clutch which serves as a 
reaction member against a rotary member to establish a power delivery 
path. 
With this one-way clutch, if the rotary member is subject to reverse 
torque, the one-way clutch is released to interrupt the transmission of 
this reverse torque. However, since the one-way clutch interrupts the 
transmission of reverse torque, no effective engine braking results. 
Thus, a known automatic transmission is provided with a friction element 
which is arranged in parallel to a one-way clutch such that the friction 
element is activated to effect engine braking when a driver places a 
manual selector valve at a predetermined drive range position. 
In the case of known automatic transmission of the THM-700 type 
manufactured by General Motors Corporation in the United States, a single 
friction element is commonly used in effecting engine braking during 
running with each of a plurality of speeds and the required capacity of 
this friction element for engine braking operation during running with the 
highest speed is considerably small as compared to the required capacity 
for engine braking operation during running with a speed lower than this 
highest speed. 
In order to tailor the capacity of the engine brake friction element with 
the required capacity for running with each speed, it has been the 
conventional practice to use normal line pressure as the working hydraulic 
fluid pressure for activating the friction element when engine braking is 
to be effected during running with the highest speed and boost the line 
pressure when engine braking is to be effected during running with a speed 
other than the highest speed. 
However, since the line pressure is used as the working hydraulic fluid 
pressure for activating friction elements other than the engine brake 
friction element, it has been impossible to set the line pressure at a low 
level which would provide the engine brake friction element with the 
required capacity for effecting engine braking operation during running 
with the highest speed. Thus, there has arisen a problem that when engine 
braking is to be effected during running with the highest speed by 
activation of the engine brake friction element with the normal line 
pressure, substantial shocks take place upon effecting the engine brake 
operation because the engine brake friction element is provided with an 
overabundance in capacity. 
An object of the present invention is to solve the above mentioned problem 
encountered in the conventional engine brake control in automotive 
automatic transmission. 
More particularly, an object of the present invention is to provide an 
engine brake control wherein a shift to engine brake operation is made 
without substantial shocks. 
SUMMARY OF THE INVENTION 
An engine brake control according to the present invention is provided in 
an automatic transmission for an automative vehicle having an engine. The 
automatic transmission includes an engine brake friction element which is 
to be activated to effect engine brake during operation with each of 
various speeds, a capacity required by the engine brake friction element 
to effect engine brake during operation with the highest one of the 
various speeds being smaller than a capacity required by the engine brake 
friction element to effect engine brake during operation with another of 
the various speeds. The engine brake control comprises: 
means for causing the automatic transmission to shift to the highest one of 
the various speeds to effect engine brake during operation with the 
highest one of the various speeds by hydraulically activating the engine 
brake friction element when a first predetermined drive range is selected 
and causing the automatic transmission to shift to the another of the 
various speeds to effect engine brake during operation with the another of 
the various speeds by hydraulically activating the engine brake friction 
element when a second predetermined drive range is selected; 
means for effecting pressure reduction on hydraulic fluid pressure supplied 
to the engine brake friction element for activation thereof by discharging 
hydraulic fluid from said engine brake friction element; and 
means for rendering said pressure reduction effecting means inoperative to 
prevent discharging of hydraulic fluid from said engine brake friction 
element when said second predetermined drive range is selected, whereby 
hydraulic fluid pressure which is not subject to the pressure reduction is 
supplied to the engine brake friction element for activation thereof when 
said second predetermined drive range is selected. 
Specifically, the present invention provides an engine brake control 
wherein, in order to adjust the capacity of an engine brake friction 
element to a capacity required by the engine brake friction element for 
effecting engine brake during operation with various speeds, rather than 
using a line pressure for activation of the engine brake friction element 
during running with the highest one of the various speeds, a pressure 
reduction is effected on the line pressure to provide a hydraulic fluid 
pressure having a predetermined low value upon causing the automatic 
transmission to shift to the highest one of the various speeds to effect 
engine brake during running with the highest speed, while such pressure 
reduction is not effected to allow the line pressure to activate the 
engine brake friction element during operation with another of the various 
speeds. A pressure reduction valve is fluidly disposed in a circuit for 
activating the engine brake friction element. The pressure reduction valve 
is operative to discharge hydraulic fluid from the circuit via a drain 
port to effect the pressure reduction. The drain port is operative during 
operation with the highest speed, but it is supplied with a hydraulic 
fluid pressure that is generated when the automatic transmission is to 
shift to the another of the various speeds to effect engine brake during 
operation with the another of the various speeds. The pressure reduction 
by the pressure reduction valve is prevented when the drain port is 
supplied with the hydraulic fluid pressure.

DESCRIPTION OF THE EMBODIMENT 
Referring to FIG. 1, the transmission illustrated herein is described in 
copending U.S. application Ser. No. 885,136 filed on July 14, 1986 
claiming priority of Japanese Patent Application No. 60-171866 filed on 
Aug. 6, 1985. This U.S. application is assigned to the same assignee of 
the present application, and it has been incorporated by reference in its 
entirety. Referring to FIG. 2, friction elements which are to be activated 
or engaged are denoted by the reference character o, while friction 
elements which are not to be activated or released are denoted by the 
reference character x. FIG. 2 shows the pattern of engagement and 
disengagement of various friction elements in different speeds of the 
transmission shown in FIG. 1. 
Referring to FIG. 3, there is diagrammatically shown, as an engine brake 
friction element, an overrun clutch OR/C which is arranged in parallel to 
a forward one-way clutch FO/C in the automatic transmission shown in FIG. 
1 and which is adapted to effect engine braking when it is activated or 
engaged. Also shown is a band brake B/B, as a shifting friction element. 
As will be understood from FIG. 2, the band brake B/B is activated or 
engaged to effect an upshift from the 3rd speed to the 4th speed, while it 
is released to effect a downshift from the 4th speed to the 3rd speed. 
Referring to FIG. 3, there are diagrammatically shown, as a shifting 
friction element, the above mentioned band brake B/B which is activated or 
engaged to establish the 4th speed in cooperation with activation of a 
front clutch F/C and a high clutch H/C as will be understood from FIG. 2, 
and as an engine brake friction element, the above mentioned overrun 
clutch OR/C which is activated or engaged for engine brake operation with 
any one of the 1st to 3rd speeds as will be understood from FIG. 2. For 
shifting from the 4th speed to the 3rd speed, the shifting friction 
element B/B is deactivated. The capacity required by the engine brake 
friction element OR/C varies with a speed ratio with which engine braking 
is effected such that the capacity required during engine brake with the 
3rd speed is smaller than the capacity required during engine brake with 
the other two speed ratios including the 2nd and 1st speeds. 
A manual selector valve 10 includes a spool 10a which is manually movable 
to P range when a driver wishes to park an automotive vehicle, R range 
when he/she wishes to drive the vehicle in the reverse direction, N range 
when he/she wishes to establish the neutral in the automatic transmission, 
D range when he/she wishes automatic shift in the transmission during the 
forward drive, II range when he/she wishes an engine brake to be effected 
with the 2nd speed, and I range when he/she wishes an engine brake to be 
effected with the 1st speed. In each of the above mentioned ranges, a line 
pressure P.sub.L from a line pressure circuit 12 is allowed to output from 
the corresponding one of the output ports 10R, 10D, 10II, and 10I in 
accordance with the pattern shown by the following table. 
______________________________________ 
Range 
Port R R N D II I 
______________________________________ 
10R o 
10D o o o 
10II o o 
10I o 
______________________________________ 
In the above table, the reference character "o" denotes the particular port 
which is supplied with the line pressure from the circuit 12. The other 
ports which are not denoted by the reference character "o" are connected 
to the drainage. 
A shift valve 14 comprises a spool 14a and a spring 14b biasing the spool 
14a to a spring set downshift position as illustrated by the right half 
thereof as viewed in FIG. 3. The spool 14a defines within the valve bore a 
chamber 14e adapted to be supplied with a shift pressure P.sub.S. The 
spool 14a is movable against the spring 14b responsive to the hydraulic 
pressure within the chamber 14e to an upshift position as illustrated by 
left half thereof as viewed in FIG. 3 when the shift pressure P.sub.S is 
supplied to the chamber 14e. In the downshift position, the spool 14a 
allows an outlet port 14c to communicate with a drain port 14d, 
discharging hydraulic fluid from a circuit 36 connected to the outlet port 
14c. In the upshift position, the spool 14a allows the outlet port 14c to 
communicate with an inlet port 14f, supplying hydraulic fluid from a 
circuit 34 connected to the inlet port 14f to the circuit 36 via the 
outlet port 14c. 
The circuit 34 extends from the port 10D of the manual selector valve 10 to 
an inlet port 16f of an engine brake control valve 16. The engine brake 
control valve 16 comprises a spool 16a and a spring 16b biasing the spool 
16a to a spring set lower position as illustrated by the left half thereof 
as viewed in FIG. 3. The spool 16a defines within the valve bore a chamber 
16i adapted to be supplied with a hydraulic fluid pressure which builds up 
under the control of a solenoid 42. The spool 16a is movable against the 
spring 16b responsive to the hydraulic pressure within the chamber 16e to 
an upper position as illustrated by the right half thereof as viewed in 
FIG. 3 when the hydraulic fluid pressure is supplied to the chamber 16i. 
In the lower position, the spool 16a allows a first outlet port 16c to 
communicate with a second inlet port 16d which a circuit 32 extending from 
the port 10II is connected to, a second outlet port 16e to communicate 
with the first inlet port 16f, a third outlet port 16g to communicate with 
a drain port 16h. To the third outlet port 16g is connected a circuit 30 
leading to the shifting friction element B/B. In the upper position, the 
spool 16a allows the first outlet port 16c to communicate with a drain 
port 16j, the second outlet port 16e to communicate with a drain port 16k, 
and the third outlet port 16g to communicate with a third inlet port 16l 
which the circuit 36 extending from the outlet port 14c of the shift valve 
14 is connected to. The second outlet port 16e is connected to the engine 
brake friction element OR/C via hydraulic circuits 20 and 22 between which 
a pressure reduction valve 18 is fluidly disposed. 
The pressure reduction valve 18 comprises a pressure regulating spool 18a 
and a spring 18b biasing the spool 18a to a lower position illustrated by 
the left half thereof as viewed in FIG. 3. The spool 18a defines within 
the valve bore a feedback chamber 18c connected to the circuit 22 via an 
orifice 26 and movable responsive to hydraulic pressure within the chamber 
18c against the spring 18b to an equilibrium position as illustrated by 
the right half thereof as viewed in FIG. 3. In the equilibrium position as 
illustrated by the right half thereof as viewed in FIG. 3, the spool 18a 
closes both ports 18d and 18e. To the first inlet port 18d is connected 
the circuit 20 leading from the second outlet port 16a of the engine brake 
control valve 16. To the second inlet port 18e is connected a circuit 28 
extending from the first outlet port 16c of the engine brake control valve 
16. Since the circuit 28 communicates with the drain port 16 j when the 
spool 16a of the engine brake control valve 16 stays in the upper position 
as illustrated by the right half thereof as viewed in FIG. 3, the port 18e 
serves also as a drain port under this condition. Under a condition where 
the spool 16a of the engine brake control valve 16 assumes the lower 
position as illustrated by the left half thereof as viewed in FIG. 3, the 
port 18e still serves as the drain port as long as the circuit 32 is 
drained when the spool 10a of the manual selector valve 10 is placed at D 
range as illustrated in FIG. 3. Reference is also made to the preceding 
table. This port 18e does not serve as the drain port any more if, under 
this condition, the spool 10a of the manual selector valve 10 is placed at 
II or I range because the line pressure P.sub.L is supplied to the circuit 
28 through the circuit 32. There is arranged between the ports 18d and 18e 
an outlet port 18f which the circuit 22 leading to the engine brake 
friction element OR/C is connected to. The circuits 20 and 22 are 
connected to each other by a check valve 24 in a short circuit manner 
bypassing the pressure reduction valve 18. 
Referring again to the engine brake control valve 16, the chamber 16i 
thereof is connected to a circuit 38 branching off from the circuit 34. 
Within this circuit 34, an orifice 40 is disposed. A solenoid 42 is 
provided at a portion of the branch circuit 38 between the orifice and the 
chamber 16i. The solenoid 42 includes a coil 42d, a plunger 42b, and a 
spring 42a biasing the plunger 42b to a spring set position as illustrated 
by the left half thereof where fluid communication between the circuit 38 
and a drain port 42c is blocked to prevent discharge of hydraulic fluid 
from the circuit 38 via the drain port 42c. When the coil 42d is 
energized, the plunger 42b is urged for upward movement against the spring 
42a to open the fluid communication between the circuit 38 and the drain 
port 42c, allowing the discharge of hydraulic fluic from the circuit 388 
via the drain port 42c. 
ON/OFF (Energization/Deenergization) of the coil 42d is controlled by an 
engine brake switch 44 which may be manually operated by a driver. The 
engine brake switch 44 is operatively connected to the manual selector 
valve 10 to be closed when the spool 10a is placed at II range of I range. 
The coil 14a is connected to a battery 46 via the switch 44. 
The operation is hereinafter described. 
When a driver places the spool 10a of the manual valve 10 at D range as 
illustrated in FIG. 3 and leaves the engine brake switch 44 open as 
illustrated in FIG. 3, reflecting the driver's intention that he/she does 
not wish engine braking to be effected, the line pressure P.sub.L is 
supplied to the circuits 34 and 38 via the port 10D. Since the plunger 42b 
assumes the position as illustrated by the left half thereof as viewed in 
FIG. 3 where the communication between the circuit 38 and the drain port 
42c is blocked, the line pressure P.sub.L reaches the chamber 16i through 
the circuit 38, urging the spool 16a of the engine brake control valve 16 
to the position as illustrated by the right half thereof as viewed in FIG. 
3. In this position of the spool 16a, the line pressure P.sub.L having 
reached the port 16f through the circuit 34 is blocked and the circuit 20 
is allowed to communicate with the drain port 16k. Since the circuit 20 is 
not pressurized, no hydraulic fluid pressure is supplied to the engine 
brake friction element OR/C, leaving it deactivated. 
The line pressure P.sub.L is supplied to the port 14f of the shift valve 
14, too. If, under this condition, the shift pressure P.sub.S supplied to 
the chamber 14e is low to allow the spool 14a to assume the downshift 
position as illustrated by the right half thereof as viewed in FIG. 3, the 
port 14f is blocked and the circuit 36 is allowed to communicate with the 
drain port 14d. This causes the circuit 30 communicating with the circuit 
36 via the ports 16l and 16g to be depressurized, leaving the shifting 
friction element B/B deactivated. As a result, the 4th speed is not 
established. Although not shown in FIG. 3, there is a hydraulic valvular 
network communicating with the port 10D for receiving the line pressure 
P.sub.L and distributing this pressure to selected one/ones of friction 
elements which cooperate with each other go establish any desired one of 
the 1st, 2nd and 3rd speeds (see FIG. 2). Thus, the 1st, 2nd or 3rd speed 
is established under this condition. 
As the shift pressure P.sub.S supplied to the chamber 14e rises, the spool 
14a of the shift valve 14 is urged upward to assume the upshift position 
as illustrated by the left thereof, as viewed in FIG. 3. In this position 
of the spool 14a, the drain port 14d is blocked and the port 14e is 
allowed to communicate with the port 14f supplied with the line pressure 
P.sub.L. This causes the line pressure P.sub.L to reach the shifting 
friction element B/B via the circuits 36 and 30, activating the shifting 
friction element B/B. Under this condition, the engagement of the shifting 
friction element B/B causes establishment of the 4th speed in cooperation 
with engagement of other two friction elements F/C and H/C (see FIG. 2). 
In this embodiment, the 4th speed provides an overdrive. 
When the driver wishes engine braking and causes the engine brake switch 44 
to be closed, the coil 42d is energized to urge the plunger 42b to assume 
the position as illustrated by the right half thereof where the drain port 
42c is allowed to communicate with the circuit 38 downstream of the 
orifice 40. This causes the hydraulic fluid to be discharged from the 
chamber 16i, depressurizing same. As a result, the spool 16a of the engine 
brake control valve 16 is allowed to assume the lower position as 
illustrated by the left half thereof where the circuit 30 is allowed to 
communicate with the drain port 16h, deactivating the shifting friction 
element B/B, while the circuit 20 is allowed to communicate with the 
circuit 34 via the ports 16e and 16f, causing the line pressure P.sub.L to 
be supplied from the circuit 34 to the circuits 20 and 22, activating the 
engine brake frictrion element OR/C. As a result, the automatic 
transmission shifts down to the 3rd speed and effects engine braking 
operation. 
In this lower position of the spool 16a of the engine control valve 16, the 
circuit 28 is allowed to communicate via the ports 16c and 16d with the 
circuit 32 which in turn communicates with the port 10II of the manual 
selector valve 10. Since the port 10II is a drain port when the spool 10a 
of the manual selector valve 10 is placed at the D range, the port 18e of 
the pressure reduction valve 18 is open to the atmosphere at the port 
10II, thus serving as a drain port of the pressure reduction valve 18. As 
a result, the pressure reduction valve 18 operates to effect pressure 
reduction on the line pressure P.sub.L supplied to the port 18d to provide 
a reduced pressure at the port 18f that is used for activating the engine 
brake friction element OR/C. Initially the spool 18a of the pressure 
reduction valve 18 stays in the lower position as illustrated by the left 
half thereof as viewed in FIG. 3 so that an increase in hydraulic fluid 
pressure at the port 18d is directly transmitted to he port 18f, causing 
the hydraulic fluid pressure within the circuit 22 to increase. This 
increase in the hydraulic fluid pressure within the circuit 22 is fed back 
via the orifice 26 to the chamber 18c, urging the spool 18a upwards 
against the bias of the spring 18b. The fluid hydraulic pressure within 
the circuit 22 keeps on increasing until the spool 18a of the pressure 
reduction valve 18 reaches the equilibrium state position as illustrated 
by the right half thereof where the force with which the spool 18a is 
urged against the spring 12b balances with the bias force of the spring 
18b. A further increase in hydraulic fluid pressure urging the spool 18a 
beyond the equilibrium state position causes the hydraulic fluid to be 
discharged from the circuit 22 toward the drain port 18e, thus suppressing 
such hydraulic pressure increase. As a result, the hydraulic fluid 
pressure supplied to the engine brake friction element OR/C through the 
circuit 22 results from reducing the line pressure P.sub.L to a level 
corresponding to the bias force of the spring 18b. With this reduced 
hydraulic pressure, the engine brake friction element OR/C is activated or 
engaged without any substantial shocks, thus suppressing a so-called 
engine brake shock. This is attribute to the fact that the capacity of the 
engine brake friction element OR/C is adjusted to the required capacity 
thereof under this condition. 
In the case where the driver wishes engine braking during running with the 
2nd speed or the 1st speed, what the driver has to do is to place the 
spool 10a of the manual selector valve 10 at II range or I range. This 
results in closing of the engine brake switch 44, thus causing the 
shifting friction element B/B to be deactivated and the engine brake 
friction element OR/C to be activated. As will be understood from the 
preceding table, the line pressure P.sub.L is supplied to the ports 10II 
and 10I when the spool 10a is placed at II range, while it is supplied to 
the port 10I when the spool 10a is placed at I range. The line pressure 
P.sub.L from the port 10I is supplied to an appropriate valvular network, 
not shown, to cause the transmission to establish the 2nd speed state, 
while the line pressure from he port 10I is supplied to an appropriate 
valvular network, not shown, to cause the transmission to establish the 
1st speed. Thus, there is provided engine braking during running with the 
2nd speed when the spool 10a is placed at II range, while there is 
provided engine brake during running with the 1st speed when the spool 10a 
is placed at I range. 
During operation with the II range or I range selected, the line pressure 
appearing at the port 10II is supplied through the circuit 32, ports 16d, 
16c, the circuit 28 to the port 18e of the pressure reduction valve 18. 
Under this condition, since the port 18e does not serve as the drain port, 
the pressure reduction valve 18 terminates the pressure reduction which 
has been previously described. Thus, the line pressure P.sub.L is supplied 
to the engine brake friction element OR/C via the circuit 22. The pressure 
reduction valve 18 operates as follows. Even if the spool 18a of the 
pressure reduction valve 18 is urged upwards against the spring 18b beyond 
the position as illustrated by the right half thereof to uncover the port 
18e, the hydraulic fluid is not discharged from the port 18e but the 
hydraulic fluid is supplied from this port 18e to the circuit 22, causing 
the hydraulic pressure therein to the level as high as the line pressure 
P.sub.L. Thus, the capacity of the engine brake friction element OR/C 
during engine brake running with the 1st speed or the 2nd speed becomes 
larger than the capacity thereof during engine brake running with the 3rd 
speed. As a result, the occurrence in slip during engine braking running 
with the 1st speed or 2nd speed is prevented. 
The hydraulic fluid is discharged from the engine brake friction element 
OR/C to the drain port 16k to deactivate or release the engine brake 
friction element OR/C upon driver's placing the spool 10a of the manual 
valve 10 at D range or opening the engine brake switch 44 is opened to 
terminate engine braking running. During this transition, the hydraulic 
fluid is quickly discharged from the engine brake friction element OR/C to 
the drain port 16k via the one-way check valve 24 bypassing the pressure 
reduction valve 18, thus assuring a quick release from engine braking 
operation.