Control device for a continuously variable transmission for motor vehicles

A continuously variable transmission for a motor vehicle has a closed hydraulic circuit disposed between a hydraulic pump and a hydraulic motor with the hydraulic motor being of the variable displacement type. The hydraulic pump is driven by the engine and the hydraulic motor is driven by the hydraulic pressure from the hydraulic pump for driving the road wheels. A pair of bypass passages are connected between the two oil passages of the closed hydraulic circuit between the hydraulic pump and hydraulic motor and a clutch valve is disposed in each bypass passage for adjusting the opening of that bypass passage. The opening of the bypass passages corresponding to the amount of operation of the clutch valves when the transmission receives driving forces from the road wheels during deceleration is controlled so as to be greater than the opening of the bypass passages when the transmission is driven by the engine to drive the road wheels.

The present invention relates to a control device for a continuously 
variable vehicle transmission of the type employing a hydraulic pump and a 
hydraulic motor, and, in particular, for controlling the hydraulic clutch 
mechanism of such a transmission. 
Japanese Laid-Open Patent Publication No. 59-95722 discloses a continuously 
variable transmission comprising a hydraulic pump and a hydraulic motor 
which are interconnected in a closed circuit, in which one or both of the 
hydraulic pump and the hydraulic motor are of a variable displacement 
type, and a bypass passage capable of providing communication between the 
high pressure and low pressure oil passages of the closed circuit which 
are disposed between the hydraulic pump and the hydraulic motor, with the 
opening in the bypass passage being controlled by a restrictor or clutch 
valve for controlling the transmission. When the motor vehicle 
incorporating the continuously variable transmission is stopped suddenly 
and then is to be started again quickly within a short time, the 
transmission ratio of the transmission should be increased quickly in 
order to obtain driving forces large enough to get the motor vehicle 
started, even if the transmission ratio was small before the motor vehicle 
was stopped suddenly. Therefore, it is necessary to change the 
transmission ratio from a small value to a large value during the short 
interval after the motor vehicle is stopped and before it is started. The 
present applicant previously has proposed an arrangement for meeting the 
above requirement by operating the transmission more quickly than normal 
when it is detected that the clutch is shifted into a shutoff position, as 
disclosed in Japanese Laid-Open Patent Publication No. 61-49202. 
With the previously proposed arrangement, however, when the motor vehicle 
is being stopped by its idle inertia-dependent movement, since the 
transmission ratio is abruptly increased while the clutch is partly 
engaged, braking of the vehicle (so-called "engine braking") which is not 
intended by the driver is applied and results in an uncomfortable change 
in vehicle speed. 
It is an object of the present invention to provide a hydraulic control 
device for a hydraulically operated continuously variable vehicle 
transmission which will solve the above problems. 
In order to achieve the aforesaid object, a control device according to the 
present invention includes a bypass passage connected between the two oil 
passages of the closed hydraulic circuit which extends between the 
hydraulic pump and the hydraulic motor, and a clutch valve disposed in the 
bypass passage for adjusting the opening of the bypass passage, the 
arrangement being such that the opening of the bypass passage 
corresponding to the amount of operation of the clutch valve when the 
continuously variable transmission receives driving forces from the 
vehicle's road wheels during deceleration is controlled so as to be 
greater than the opening of the bypass passage with respect to the amount 
of operation of the clutch valve when the continuously variable 
transmission is driven by driving forces from the engine to drive the road 
wheels during acceleration. 
With the control device of the foregoing construction, during stopping of 
the motor vehicle on its idle inertia-dependent movement, i.e., engine 
braking, since the opening of the bypass passage with respect to the 
amount of operation of the clutch valve is larger than the opening of the 
bypass passage when starting the motor vehicle, the engine braking is not 
abruptly and intensively applied and the motor vehicle can smoothly be 
stopped even if the transmission ratio is suddenly increased while the 
clutch is in partly engaged condition.

FIG. 1 shows a hydraulic circuit of a hydraulically operated continuously 
variable transmission T according to the present invention, incorporated 
in a motor vehicle. The continuously variable transmission T has a 
hydraulic pump 2 of the fixed displacement type which is driven by an 
engine E, and a hydraulic motor 4 of the variable displacement type having 
a drive shaft 3 for driving the road wheels W of the vehicle through a 
forward/reverse mode selector device 20. The hydraulic pump 2 and the 
hydraulic motor 4 are connected in a closed hydraulic circuit 5 having a 
first oil passage 5h communicating between the outlet port of the pump 2 
and the inlet port of the motor 4 and a second oil passage 5l 
communicating between the inlet port of the pump 2 and the outlet port of 
the motor 4. Between the two oil passages 5h and 5l, there are two bypass 
passages 6a, 6b having respective first and second clutch valves 7a, 7b 
comprising variable restrictors serving as clutches. The bypass passage 6b 
with the second clutch valve 7b has a deceleration check valve 8 for 
allowing oil to flow only from the second oil passage 5l to the first oil 
passage 5h. 
A charging pump 10 that is actuatable by the engine E has an outlet port 
connected to the oil passages 5h, 5l through check valve 11 and two check 
valves 9 and 9. Working oil pumped from an oil sump 12 and regulated in 
pressure by a relief valve 13 is supplied to the closed hydraulic circuit 
5 in order to compensate for any oil shortage in the closed circuit 5. 
An output shaft 28 coupled to the road wheels W extends parallel to the 
drive shaft 3 of the hydraulic motor 4, with the forward/reverse mode 
selector device 20 disposed between the shafts 3, 28. The forward/reverse 
mode selector device 20 comprises first and second driver gears 21, 22 
spaced axially from each other, a first driven gear 23 rotatably supported 
on the output shaft 28 and meshing with the first driver gear 21, a second 
driver gear 25 meshing with the second driver gear 22 through an 
intermediate gear 24 and rotatably supported on the output shaft 28, a 
clutch hob 26 fixed to the output shaft 28 between the first and second 
driven gears 23, 25, and a sleeve 27 axially slidable for selectively 
connecting clutch gears 23a, 25a on sides of the driven gears 23, 25 to 
the clutch hub 26. When the sleeve 27 is slid to the left in FIG. 1 to 
connect the clutch gear 23a of the first driven gear 23 to the clutch hub 
26, as shown, the output shaft 28 is rotated in a direction opposite to 
the direction of rotation of the drive shaft 3 for rotating the road 
wheels W in a forward direction in response to operation of the 
continuously variable transmission T. When the sleeve 27 is slid to the 
right in FIG. 1 to connect the clutch gear 25a of the second driven gear 
23 to the clutch hub 26, the output shaft 28 is rotated in the same 
direction as the direction of rotation of the drive shaft 3 for rotating 
the road wheels W in a reverse direction. 
The clutch valves 7a, 7b are actuated by a servocylinder 30, whereas the 
forward/reverse mode selector device 20 is operated by a hydraulic 
cylinder 40. The displacement of the hydraulic motor 4 is controlled by a 
hydraulic cylinder 50. The operation of these cylinders 30, 40, 50 is 
controlled by a control unit CU to which there are connected a first 
detector means S1 for detecting the speed of rotation of the engine E as 
an indication of the power output of the engine E and a second detector 
means S2 for detecting the opening of the throttle valve of the engine E 
as an indication of the driver's intention of acceleration and 
deceleration. The control unit CU controls the servocylinder 30 and the 
hydraulic cylinder 40 in response to input signals from the first and 
second detector means S1, S2 and operation of a manual selector lever 16, 
and also controls the hydraulic cylinder 50 in response to input signals 
from the detector means S1, S2, operation of the manual selector lever 16, 
and operating conditions of the clutch valves 7a, 7b. 
The first detector means S1 comprises, for example, a hydraulic governor 
responsive to the rotation of the input shaft 1, and has an input port 90 
connected to an oil passage 101 which can be supplied with a hydraulic 
pressure Pl discharged from the charging pump 10. The first detector means 
S1 also has an output port 91 for issuing a governor pressure Pg 
proportional to the speed of rotation of the engine E. The governor 
pressure Pg is selected to be lower than the pressure Pl discharged from 
the charging pump 10 (Pg&lt;Pl). 
The second detector means S2 comprises, for example, a throttle valve 
opening/hydraulic pressure converter having an output port 93 for issuing 
a throttle pressure Pt proportional to operation of a throttle valve 
opening/closing device 15. The second detector means S2 has an input port 
94 connected to the oil passage 101. The throttle pressure Pt is selected 
to be lower than the pump pressure Pl (Pt&lt;Pl). 
The clutch valves 7a, 7b may be identical and, as shown in FIGS. 2a through 
2c, the clutch valves 7a, 7b are formed by a cylindrical fixed member 71 
and a cylindrical rotatable member 72 rotatably fitted in the fixed member 
71. The rotatable member 72 defines therein a valve chamber 75 which is 
held in communication with the first oil passage 5h at all times. As shown 
in FIG. 3 which is a cross section taken along line III--III of FIG. 2(a), 
the rotatable member 72 has a pair of axially spaced valve holes 74a, 74b 
defined in a side wall thereof. The fixed member 71 has a pair of bypass 
holes 73a, 73b defined in a side wall thereof in corresponding relation to 
the valve holes 74a, 74b, respectively. The holes 73a, 74a constitute the 
clutch valve 7a, whereas the holes 73b, 74b constitute the clutch valve 
7b. The bypass hole 73a is in communication with the second oil passage 5l 
through the bypass passage 6a at all times. The bypass hole 73b is held in 
communication with the second oil passage 5l via the bypass passage 6b 
which has the deceleration check valve 8. 
The valve holes 74a, 74b and the bypass holes 73a, 73b are defined such 
that the holes 73a, 74a register with each other and the holes 73b, 74b 
register with each other when the rotatable member 72 is in a prescribed 
angular position. In this prescribed angular position, the first and 
second oil passages 5h, 5l communicate with each other bidirectionally 
through the first clutch valve 7a, and unidirectionally through the second 
clutch valve 7b with the deceleration check valve 8 allowing only an oil 
flow from the second oil passage 5l to the first oil passage 5h. 
The rotatable member 72 has a radially outwardly projecting arm 76 which is 
coupled to the servocylinder 30 through a link 31. The link 31 has an end 
connected to the arm 76 through a pin 31a parallel to the axis of the 
rotatable member 72, and the other end to the servocylinder 30 through a 
pin 31b parallel to the pin 31a. 
In response to operation of the servocylinder 30, the rotatable member 72 
is angularly moved in an angular range of substantially 90.degree. by the 
link 31 for continuously varying the opening of the bypass passages 6a, 6b 
from a fully open condition to a fully closed condition. The servocylinder 
30 comprises a cylinder 32, a piston 35 slidably disposed in the cylinder 
32 and dividing the interior of the cylinder 32 into a head chamber 33 and 
a rod chamber 34, a piston rod 36 integrally joined to the piston 35 and 
extending hermetically and movably through the end wall of the cylinder 32 
facing the rod chamber 34, and a spring 37 disposed in the rod chamber 34 
for normally urging the piston 36 into the head chamber 33. 
The link 31 is coupled to the distal end of the piston rod 36 through the 
pin 31b. When the piston 36 is moved a maximum stroke to the right, the 
clutch valves 7a, 7b are fully opened to cut off power transmission 
through the transmission T. When the piston 35 is moved to the left 
against the resiliency of the spring 37, the valve holes 74a, 74b are 
slightly shifted in position off the bypass holes 73a, 73b to reduce the 
opening of the bypass passages 6a, 6b, resulting in a partly engaged 
clutch condition, as shown in FIG. 2b. By moving the piston 35 a maximum 
stroke to the left against the force of the spring 37, the valve holes 
74a, 74b are fully shifted out of registry with the bypass holes 73a, 73b 
to close the clutch valves 7a, 7b, whereupon power can be fully 
transmitted by the transmission T, as shown in FIG. 2c. 
The relationship between the amount of rotation of the rotatable member 72 
caused by the operation of the servocylinder 30, i.e., the amount of 
operation of the clutch valves 7a, 7b, and the opening of the bypass 
passages 6a, 6b will be described with reference to FIG. 6. When the 
hydraulic pressure in the first oil passage 5h is higher than the 
hydraulic pressure in the second oil passage 5l to cause working oil to 
flow from the first oil passage 5h to the second oil passage 5l, the 
bypass passage 6b remains closed by the check valve 8 even if the second 
clutch valve 7b is open, and hence only the opening of the first clutch 
valve 7a may be considered. If the clutch valves 7a, 7b are angularly 
shifted from the fully closed position (FIG. 2c) to the fully open 
position (FIG. 2a), then the opening of the bypass passages 6a, 6b with 
respect to the amount of operation of the clutch valves 7a, 7b varies 
along a.fwdarw.b.fwdarw.d.fwdarw.e as indicated by the solid line. 
When the hydraulic pressure in the second oil passage 5l is higher than the 
hydraulic pressure in the first oil passage 5h, as during deceleration of 
the vehicle, to cause working oil to flow from the second oil passage 5l 
to the first oil passage 5h, the bypass passage 6b is also opened. Upon 
shifting of the clutch valves 7a, 7b from the fully closed position to the 
fully open position, the opening of the bypass passages 6a, 6b vary along 
a.fwdarw.b.fwdarw.c.fwdarw.e as indicated by the dotted line. It follows 
that the opening of the second clutch valve 7b is selected to be of a 
range represented by the hatched area in FIG. 6. 
The hydraulic cylinder 40 comprises a cylinder 41, a piston 44 slidably 
disposed in the cylinder 41 and dividing the interior space of the 
cylinder 41 into a head chamber 42 and a rod chamber 43, a piston rod 45 
integrally joined to the piston 44 and extending hermetically and movably 
through the end wall of the cylinder 41 facing the rod chamber 43, and a 
spring 46 disposed in the chamber 43 for normally urging the piston 44 
into the head chamber 42. 
A connector 47 is fixed to the distal end of the piston rod 45 and also to 
the sleeve 27. In response to operation of the piston 44 and the piston 
rod 45, therefore, the sleeve 27 is moved to operate the forward/reverse 
mode selector device 20. More specifically, when the piston 44 and the 
piston rod 45 are moved to the lefthand limit position, a forward gear 
train is established in the forward/reverse mode selector device 20. When 
the piston 44 and the piston rod 45 are moved to the righthand limit 
position, a reverse gear train is established in the forward/reverse mode 
selector device 20. The head chamber 42 is connected to an oil passage 
102, whereas the rod chamber 43 is connected to an oil passage 103. 
As shown in FIG. 4, the hydraulic motor 4 comprises a variable displacement 
axial piston motor comprising an angular array of pistons 4b slidably 
fitted in a cylinder block 4a coupled to the drive shaft 3 and disposed 
around the axis of rotation of the drive shaft 3, and a swash plate 4c 
inclined at a variable angle .theta. for controlling the reciprocating 
stroke of the pistons 4b. Cylinder chambers 4d defined in the cylinder 
block 4a and housing those pistons 4b which are on an expansion stroke are 
held in communication with the higher-pressure oil passage 5h, and 
cylinder chambers 4e defined in the cylinder block 4a and housing those 
pistons 4b which are on a contraction stroke are held in communication 
with the lower-pressure oil passage 5l. 
In the hydraulic motor 4 of the above structure, as is known in the art, 
higher-pressure oil discharged from the fixed displacement hydraulic pump 
2 is forced into the cylinder chambers 4d, and lower-pressure oil is 
discharged from the cylinder chamber 4e and returns to the hydraulic pump 
2, during which time the cylinder block 4a and the drive shaft 3 are 
rotated about their own axes under a reactive torque which the pistons 4b 
on the expansion stroke receive from the swash plate 4c. 
The displacement of the hydraulic motor 4 is determined by the stroke of 
the pistons 4b. Therefore, the transmission ratio R of the transmission T 
can be continuously controlled from a maximum value to a maximum value by 
varying the angle .theta. of inclination of the swash plate 4c from a 
maximum indicated by the solid line to a minimum indicated by the 
two-dot-and-dash line. The transmission ratio R is expressed by the 
following equation: 
##EQU1## 
The swash plate 4c has one end connected to one end of a swing link 56 by a 
pin 56b, and the other end of the swing link 56 is connected to the 
hydraulic cylinder 50 by a pin 56a parallel to the pin 56b. 
The hydraulic cylinder 50 comprises a cylinder 51, a piston 54 slidably 
disposed in the cylinder 51 and dividing the interior space thereof into a 
head chamber 52 and a rod chamber 53, and a piston rod 55 integrally 
joined to the piston 54 and extending hermetically and movably through the 
end wall of the cylinder 51 facing the rod chamber 53. The other end of 
the swing link 56 is connected to the distal end of the piston rod 55 by 
the pin 56a. When the piston 54 is moved a maximum stroke to the right (as 
shown in solid lines), the angle .theta. of the swash plate 4c is 
maximized and so are the displacement of the hydraulic motor 4 and the 
transmission ratio R. When the piston 54 is moved a maximum stroke to the 
left (as indicated by the two-dot-and-dash line), the angle .theta. of the 
swash plate 4c is minimized and so are the displacement of the hydraulic 
motor 4 and the transmission ratio R. 
Referring back to FIG. 1, the control unit CU has a pair of pilot valves 
80, 60, a pair of directional control valves 38, 49, and a manual valve 
48. 
The pilot valve 80 comprises a sleeve 81 disclosed between oil passages 
104, 105 communicating respectively with the head chamber 33 and the rod 
chamber 34 of the servocylinder 30, an oil supply passage 106 
communicating with the outlet port of the charging pump 10, an oil release 
passage 107 communicating with the oil sump 12, and a spool 82 relatively 
movably disposed in the sleeve 81. The pilot valve 80 has ports 83a, 83b 
communicating with the oil passages 104, 105, respectively, and ports 84a, 
84b communicating with the oil supply passage 106 and the oil release 
passage 107, respectively. The sleeve 81 is connected to the piston rod 36 
of the servocylinder 30 through a link 85 so that movement of the 
servocylinder 30 can be fed back to the pilot valve 80. 
The spool 82 is movable relative to the sleeve 81 between three selectable 
positions, i.e., a lefthand position in which the spool 82 communicates 
the ports 83b, 84a and the ports 83a, 84b, a neutral position in which the 
spool 82 cuts off communication between the ports 83a, 83b and the ports 
84a, 84b, and a righthand position in which the spool 82 communicates the 
ports 83a, 84a and the ports 83b, 84b. A spring 86a is held against the 
lefthand end of the spool 82 for normally urging the spool 82 to move in a 
rightward direction, and another spring 86b is held against the righthand 
end of the spool 82 for normally urging the spool 82 to move in a leftward 
direction. The pilot valve 80 also has a switching port 87a for applying a 
hydraulic pressure to the lefthand end of the spool 82 and a switching 
port 87b for applying a hydraulic pressure to the righthand end of the 
spool 82. 
A force F1 acting on the lefthand end of the spool 82 of the pilot valve 80 
is the sum of a biasing force F11 of the spring 86a and a hydraulic force 
F12 acting on the lefthand end of the spool 82 (F1=F11+F12), whereas a 
force F2 acting on the righthand end of the spool 82 is the sum of a 
biasing force F21 of the spring 86b and a hydraulic force F22 acting or 
the righthand end of the spool 82 (F2=F21+F22). The spool 82 is moved in 
response to a change in the equilibrium condition of the forces F1, F2. 
For example, if F1&lt;F2, the spool 82 is moved to the left to reach the 
righthand position for introducing the hydraulic pressure Pl discharged 
from the charging pump 10 into the head chamber 33 of the servocylinder 
30, and the hydraulic pressure in the rod chamber 34 is released into the 
oil sump 12. The piston 35 and the piston rod 36 are now moved to the left 
thereby operating the clutch valves 7a, 7b in the closing direction. 
Upon movement of the spool 82 to the left, the biasing force F11 of the 
spring 86a is increased, and the biasing force F21 of the spring 86b is 
reduced. When F1=F2, the spool 82 is stopped. The sleeve 81 is moved to 
the left by the link 85 in response to the movement of the piston rod 36 
to the left. Therefore, when the sleeve 81 and the spool 82 reach the 
neutral position, the flow of working oil between the ports 83a, 84a, 83b, 
84b is cut off, and the leftward movement of the piston rod 36 is stopped 
and so is the operation of the clutch valves 7a, 7b. The sleeve 81 is also 
stopped in unison with the piston rod 36. 
When F1&gt;F2, the spool 82 is moved to the right into the lefthand position 
with respect to the sleeve 81. The hydraulic pressure Pl discharged from 
the charging pump 10 is introduced into the rod chamber 34 of the 
servocylinder 30, and the hydraulic pressure in the head chamber 33 is 
released into the oil sump 12. Therefore, the piston 35 and the piston rod 
36 are moved to the right, whereby the clutch valves 7a, 7b are operated 
in the opening direction. 
With the spool 82 moved to the right, the biasing force F21 of the spring 
86b is increased and the biasing force F11 of the spring 86a is reduced 
until F1=F2 whereupon the rightward movement of the spool 82 is stopped. 
At this time, the sleeve 81 is also moved to the right with the rightward 
movement of the piston rod 36. When the sleeve 81 and the spool 82 are 
relatively in the neutral position, the oil supply into the rod chamber 34 
is stopped to stop the rightward movement of the piston rod 36, whereupon 
the operation of the clutch valves 7a, 7b is also stopped. The rightward 
movement of the sleeve 81 is also stopped in unison with the piston rod 
36. 
The aforesaid mechanism is a general servomechanism by which the piston rod 
36 can be moved in response to movement of the spool 82 for adjusting the 
opening of the clutch valves 7a, 7b, i.e., the opening of the bypass 
passages 6a, 6b. 
The directional control valve 38 comprises a three-port two-position 
directional control valve interposed between an oil passage 108 connected 
to the outlet port of the charging pump 10, an oil passage 109 connected 
to the output port 93 of the second detector means S2, and a pilot oil 
passage 121 connected to the switching port 87a of the pilot valve 80. The 
directional control valve 38 is movable between a lefthand position in 
which it communicates the oil passage 108 with the pilot oil passage 121 
and a righthand position in which it communicates the oil passage 109 with 
the pilot oil passage 121. The directional control valve 38 assumes the 
righthand position when the hydraulic pressure discharged from the 
charging pump 10 is introduced into a pilot oil passage 122 branched from 
the oil passage 101 and coupled to the righthand end of the valve 38. 
The switching port 87b of the pilot valve 80 is in communication with the 
output port 91 of the first detector means S1 through a pilot oil passage 
123. 
When the hydraulic pressure Pl discharged from the charging pump 10 is 
supplied to the oil passage 101, the hydraulic pressure Pl is also 
introduced into the pilot oil passage 122 thereby to shift the directional 
control valve 38 into the righthand position, whereupon the throttle 
pressure Pt is applied to the switching port 87a of the pilot valve 80. 
The other switching port 87b is supplied with the governor pressure Pg 
from the first detector means S1. The pilot valve 80 is operated to open 
or close the clutch valves 7a, 7b until the forces F1, F2 including the 
hydraulic forces F12, F22 are balanced or put in equilibrium. 
When the pump pressure Pl is not supplied to the oil passage 101, i.e., 
when the hydraulic pressure in the oil passage 101 is zero, the 
directional control valve 38 takes the lefthand position to apply the pump 
pressure Pl to the switching port 87a of the pilot valve 80. Since the 
hydraulic pressure in the input port 90 is zero, the governor pressure Pg 
from the first detector means S1 is also zero, and no hydraulic pressure 
acts on the righthand end of the spool 82. Inasmuch as the pilot valve 80 
is arranged such that the force F1 tending to move the spool 82 to the 
right is greater than the force F2 tending to move the spool 82 to the 
left (F1&gt;F2) at this time, the spool 82 is moved to the righthand limit 
position for reliably fully opening the clutch valves 7a, 7b. 
The manual valve 48 comprises a six-port three-position directional control 
valve interposed between the oil passages 101, 102, 103, a pair of oil 
passages 110, 111 communicating with the oil supply passage 106, and an 
oil release passage 112 communicating with the oil sump 12. The manual 
valve 48 can manually be moved selectively between three positions, i.e., 
a forward position, a neutral position, and a reverse position. More 
specifically, in response to operation of the manual selector lever 16, 
the manual valve 48 can selectively assume the forward position F 
(lefthand position), the neutral position N, and the reverse position R. 
In the forward position F, with the valve spool shifted to the right as 
viewed in FIG. 1, the manual valve 48 provides communication between the 
oil passages 110, 101, the oil passages 111, 103, and the oil passage 102 
and the oil release passage 112. In the neutral position N (as shown), all 
of the oil passages 101, 102, 103 communicate with the oil release passage 
112. In the reverse position R, the manual valve 48 provides communication 
between the oil passages 110, 101, the oil passages 111, 102, and the oil 
passage 103 and the oil release passage 112. 
The pilot valve 60 comprises a four-port restriction directional control 
valve interposed between an oil passage 113 communicating with the head 
chamber 52 of the hydraulic cylinder 50, an oil passage 114 communicating 
with the rod chamber 53 thereof, an oil passage 115 branched from the oil 
supply passage 106 for introducing the hydraulic pressure Pl from the 
charging pump 10, and an oil release passage 116 connected to the oil pump 
12. 
The pilot valve 60 has ports 61a, 61b communicating respectively with the 
oil passages 113, 114, ports 62a, 62b communicating respectively with the 
oil passage 115 and the oil release passage 116, and a spool 63. The spool 
63 is selectively movable between a lefthand position in which the spool 
63 communicates the ports 61a, 62b and the ports 61b, 62a, a neutral 
position in which communication between all of the ports 61a, 61b, 62a, 
62b is cut off, and a righthand position in which the spool 63 
communicates the ports 61a, 62a and the ports 61b, 62b, while having an 
intermediate position in which the restriction of the pilot valve 60 is 
continuously variable. A spring 64a is held against the lefthand end of 
the spool 63 for normally urging the spool 63 to the right, and a spring 
64b is held against the righthand end of the spool 63 for normally urging 
the spool 63 to the left. The pilot valve 60 also has a switching port 65a 
for applying a hydraulic pressure to the lefthand end of the spool 63, and 
a switching port 65b for applying a hydraulic pressure to the righthand 
end of the spool 63. The switching port 65a is connected to a pilot oil 
passage 124, whereas the other switching port 65b is connected to a pilot 
oil passage 125 branched from the pilot oil passage 123 which is coupled 
to the output port 91 of the first detector means S1. 
A force F3 acting on the lefthand end of the spool 63 of the pilot valve 60 
is the sum of a biasing force F31 of the spring 64a and a hydraulic force 
F32 acting on the lefthand end of the spool 63 (F3=F31+F32), whereas a 
force F4 acting on the righthand end of the spool 63 is the sum of a 
biasing force F41 of the spring 64b and a hydraulic force F42 acting on 
the righthand end of the spool 63 (F4=F41+F42). The spool 63 is moved in 
response to a change in the equilibrium condition of the forces F3, F4. 
As illustrated in FIGS. 1 and 2a through 2c, the directional control valve 
49 is a three-port two-position directional control valve interposed 
between an oil passage 117 branched from the oil passage 106, an oil 
passage 118 connected to the output port 93 of the second detector means 
S2, and the pilot oil passage 124 connected to the switching port 65a of 
the pilot valve 60. The directional control valve 49 is selectively 
movable between a lefthand position in which the oil passage 117 
communicates with the pilot oil passage 124, i.e., a spool 49b is moved to 
the right in a sleeve 49a, and a righthand position in which the oil 
passage 118 communicates with the pilot oil passage 124, i.e., the spool 
49b is moved to the left in the sleeve 49a. 
The spool 49b is connected to one end of a link 49c the other end of which 
is coupled to the piston rod 36 of the servocylinder 30. When the 
servocylinder 30 is operated to open the clutch valves 7a, 7b for cutting 
off power to the transmission, the directional control valve 49 is shifted 
to the lefthand position to communicate the oil passage 117 with the pilot 
oil passage 124. The directional control valve 49 continues to be in the 
lefthand position until the clutch valves 7a, 7b are moved in the closing 
direction to a certain opening which is smaller than the opening under the 
partly engaged clutch condition. When the servocylinder 30 is operated to 
close the clutch valves 7a, 7b to an opening smaller than the aforesaid 
certain opening, the directional control valve 49 is shifted into the 
righthand position in which the oil passage 118 communicates with the 
pilot oil passage 124. Thus, under the power transmitting condition in 
which the opening of the clutch valves 7a, 7b is smaller than the 
foregoing certain opening, the throttle pressure Pt from the second 
detector means S2 is applied to the switching port 65a of the pilot valve 
60. Under the power cutoff condition in which the opening of the clutch 
valves 7a, 7b exceeds the certain opening, the hydraulic pressure Pl from 
the charging pump 10 acts on the switching port 65a. 
When F3&lt;F4 on the pilot valve 60, the spool 63 is moved to the left to 
increase the biasing force F31 of the spring 64a and reduce the biasing 
force F41 of the spring 64b. When F3=F4, the leftward movement of the 
spool 63 is stopped. At this time, working oil in an amount commensurate 
with the opening between the ports 61a, 62a and between the ports 61b, 62b 
is introduced into the head chamber 52 of the hydraulic cylinder 50, and 
discharged from the rod chamber 53 thereof, for thereby moving the piston 
54 and the piston rod 55 to the left. When F3&gt;F4, the spool 63 is moved to 
the right to increase the biasing force F41 of the spring 64b and reduce 
the biasing force F31 of the spring 64a. When the condition F3=F4 is 
reached, the rightward movement of the spool 63 ceases. At this time, 
working oil in an amount commensurate with the opening between the ports 
61a, 62b and between the ports 61b, 62a is discharged from the head 
chamber 52 of the hydraulic cylinder 50, and introduced into the rod 
chamber 53 thereof, for thereby moving the piston 54 and the piston rod 55 
to the right. The pressure distribution between the head chamber 52 and 
the rod chamber 53 is determined by the degree of restriction by the pilot 
valve 60. The piston 54 and the piston rod 55 is operated at a speed 
dependent on the pressure difference between the head chamber 52 and the 
rod chamber 53 for thereby varying the displacement of the hydraulic motor 
4. 
Assuming that the areas of the opposite ends of the spool 63 of the pilot 
valve 60 are the same, the following equations are established: 
EQU F32=Pt.times.S or F32=Pl.times.S, 
EQU F42=Pg.times.S. 
Since Pl&gt;Pt, Pl&gt;Pg, the following inequalities are established at all 
times: 
EQU Pl.times.S&gt;Pt.times.S 
EQU Pl.times.S&gt;Pg.times.S. 
Operation of this embodiment will be described below. Before the engine E 
is started, the charging pump 10 is at rest, and the hydraulic pressure Pl 
discharged thereby is zero. Therefore, the governor pressure Pg and the 
throttle pressure Pt are also zero. At this time, the position of the 
spool 82 in the pilot valve 80 is determined by the set loads or biases of 
the springs 86a, 86b, which are determined such that the spool 82 is moved 
in the direction to open the clutch valves 7a, 7b, into the lefthand 
position. The piston 38 and the piston rod 36 of the servocylinder 30 are 
moved to the right under the bias of the spring 37 into the position to 
open the clutch valves 7a, 7b, and the sleeve 81 of the pilot valve 80 is 
also moved to the right through the link 85. 
Now, it is assumed that the manual valve 48 is in the neutral position N 
after the engine E has started. The oil passages 101, 102, 103 communicate 
with the oil sump 12, and hence the hydraulic pressure in the head chamber 
42 and the rod chamber 43 of the hydraulic cylinder 40 is released. 
Therefore, the piston 44 of the hydraulic cylinder 40 is pushed to the 
lefthand limit position under the resiliency of the spring 46, and the 
sleeve 27 is also in the lefthand limit position. The forward/reverse 
selector mode device 20 is thus in the forward position. Since the 
hydraulic pressure in the oil passage 101 is zero, the directional control 
valve 38 is in the lefthand position. 
The hydrulic force F12 on the pilot valve 80 is of a valve corresponding to 
the pump pressure Pl, the hydraulic force F22 is zero since Pg=0 with the 
oil passage 101 being connected to the oil sump 12, and F1&gt;F2 at all 
times. Therefore, the spool 82 is in the lefthand position. The piston 35 
of the servocylinder 30 is also stopped in the righthand limit position, 
so that the clutch valves 7a, 7b are fully open. 
In this condition, because working oil discharged from the hydraulic pump 2 
dependent on the speed of rotation of the engine E flows through the 
bypass passage 6a, the hydraulic motor 4 is not actuated, and hence no 
power is transmitted to the output shaft 28, so that the road wheels W 
remain at rest. When the engine speed is increased by depressing the 
accelerator pedal of the throttle valve opening/closing device 15, at this 
time, the spool 82 of the pilot valve 80 is not moved to the left because 
F22=0 (Pg=0). Thus, the clutch valves 7a, 7b remain fully open 
irrespective of the throttle valve opening and the speed of rotation of 
the engine E. 
Since the clutch valves 7a, 7b are fully open, the directional control 
valve 49 is in the lefthand position, and F3=F31+F32, F4=F41 on the pilot 
valve 60 because the governor pressure Pg from the first detector means S1 
is 0. The set loads of the springs 64a, 64b are determined so that F3&gt;F4 
under this condition. The spool 63 is thus in the lefthand position to 
introduce the hydraulic pressure Pl discharged from the charging pump 10 
into the rod chamber 53 of the hydraulic cylinder 50 and also to release 
the hydraulic pressure from the head chamber 52 to the oil sump 12. 
Consequently, the piston 54 and the piston rod 55 of the hydraulic 
cylinder 50 are moved to the righthand limit position to maximize the 
angle .theta. of the swash plate 4c of the hydraulic motor 4. The 
transmission ratio R in the neutral position N is thus at maximum. 
When the manual selector lever 16 is operated to shift the manual valve 48 
from the neutral position N to the forward position F, whereupon the 
manual valve 48 provides communication between the oil passages 110, 101, 
the oil passages 111, 103, and the oil passage 102 and the oil release 
passage 112. Therefore, the pump pressure Pl is supplied to the rod 
chamber 43 of the hydraulic cylinder 40, and the head chamber 42 remains 
released, with the piston 44 remaining pushed to the lefthand limit 
position. Therefore, the forward/reverse mode selector device 20 remains 
in the forward position. Also, the oil passage 101 is supplied with the 
pump pressure Pl. 
When the pump pressure Pl is supplied from the oil passage 101 to the pilot 
oil passage 122, the directional control valve 38 is shifted to the 
righthand position for thereby supplying the throttle pressure Pt from the 
second detector means S2 to the switching port 87a of the pilot valve 80. 
The governor pressure Pg from the first detector means S1 is supplied via 
the pilot oil passage 123 to the switching port 87b of the pilot valve 80. 
Therefore, the pilot valve 80 is operated to open or close the clutch 
valves 7a, 7b until equilibrium is reached between the throttle pressure 
Pt and the governor pressure Pg. 
If the above operation is carried out in an engine idling condition in 
which the motor vehicle is at rest and the throttle valve opening/closing 
device 15 is not operated, then the first detector means S1 produces the 
governor pressure Pg corresponding to the idling speed of the engine E, 
and the second detector means S2 generates the throttle pressure Pt 
corresponding to the throttle valve opening of zero. By arranging the 
pilot valve 80 such that the condition F1=F2 is reached when the spool 81 
thereof is moved a predetermined distance from the righthand limit 
position, the piston rod 36 of the servocylinder 30 is stopped after it 
has moved to the left by the predetermined distance. The clutch valves 7a, 
7b are now in a partly engaged clutch condition. 
In the engine idling condition, the governor pressure Pg corresponding to 
the idling speed acts on the switching port 65b of the pilot valve 60, 
whereas the sump pressure Pl acts on the the switching port 65a thereof. 
The spool 63 then goes to the lefthand position by determining the biasing 
forces F31, F41 of the springs 64a, 64b so that F3&gt;F4 at all times. 
Therefore, the hydraulic pressure in the head chamber 52 of the hydraulic 
cylinder 50 is released into the oil sump 12, and the pump pressure Pl 
acts in the rod chamber 53. The piston 54 and the piston rod 55 remain 
shifted to the righthand limit position, the angle .theta. of the swash 
plate 4c is at maximum, and the transmission ratio R remains maximum. 
When the accelerator pedal is depressed to get the vehicle started from the 
engine idling condition, the throttle valve is opened to increase the 
throttle pressure Pt, whereupon the force F12 on the pilot valve 80 is 
increased, and as a result the force F1 becomes greater. Since the speed 
of rotation of the engine E goes higher, the output or governor pressure 
Pg from the first detector means S1 is also increased and so is F2 on the 
pilot valve 80. When the condition F2&gt;F1 is reached, the spool 82 is moved 
to the left to introduce the pump pressure Pl into the head chamber 33 and 
release the hydraulic pressure from the rod chamber 34 into the oil sump 
12. The piston 35 and the piston rod 36 of the servocylinder 30 are now 
moved to the left for progressively closing the clutch valves 7a, 7b 
thereby to enable the hydraulic pump 2 to start actuating the hydraulic 
motor 4. 
Until the forces F1, F2 are balanced, the spool 82 is further moved to the 
left until finally the spool 82 is stopped at the lefthand limit position. 
The clutch valves 7a, 7b are fully closed to enable the hydraulic pump 2 
to exert its full power to actuate the hydraulic motor 4. Since the 
forward/reverse mode selector device 20 is in the forward position at this 
time, the motor vehicle is started in the forward direction in response to 
rotation of the hydraulic motor 4. 
At this time, the hydraulic pressure in the first oil passage 5h is higher 
than the hydraulic pressure in the second oil passage 5l. Therefore, the 
bypass passage 6b is closed by the check valve 8, and working oil only 
flows through the first clutch valve 7a. The opening of the bypass 
passages 6a, 6b with respect to the amount of operation of the clutch 
valve in the closing direction varies along e.fwdarw.d.fwdarw.b.fwdarw.a 
as indicated by the solid line in FIG. 6. 
Upon a shift of the clutch valves 7a, 7b from the position of FIG. 2b to 
the position of FIG. 2c at this time, the directional control valve 49 is 
displaced to the righthand position to allow the throttle pressure Pt to 
act on the switching port 65a of the pilot valve 60, and F3 increases with 
the hydraulic force F32. The pilot valve 60 is arranged such that F3&gt;F4 
under this condition. The transmission ratio R is maintained at maximum, 
as during engine idling, as indicated by a curve A or a curve E in FIG. 5. 
As the engine speed goes much higher, the governor pressure Pg is 
increased, and so is the force F4 on the pilot valve 60. When F3&gt;F4 or 
F3=F4, the spool 63 is in the lefthand position or the neutral position, 
and the piston 54 and the piston rod 55 of the hydraulic cylinder 50 
remain in the righthand limit position, with the transmission ratio R 
being maximum. However, the vehicle speed increases as indicated by a 
curve B or a curve F in FIG. 5 in response to an increase in the engine 
speed. 
When the condition F3&lt;F4 is reached as a result of a further increase in 
the engine speed, the spool 63 of the pilot valve 60 is moved to the left 
into the righthand position in which the pump pressure Pl is supplied to 
the head chamber 52 of the hydraulic cylinder 50 and the hydraulic 
pressure is released from the rod chamber 53 into the oil sump 12. 
Therefore, the piston 54 and the piston rod 55 is moved to the left to 
angularly move the swash plate 4c in a direction to reduce the angle 
.theta., i.e., to reduce the transmission ratio for thereby increasing the 
vehicle speed. Inasmuch as the engine load is increased at this time to 
control an increase in the engine speed, the engine speed is maintained 
substantially at a constant level. This condition is indicated by a curve 
C or a curve G in FIG. 5. If the accelerator pedal is continuously 
depressed even after the transmission ratio R is reduced to its minimum 
value, the engine speed is increased until the engine power output and the 
engine load are brought into equilibrium, and so is the vehicle speed. The 
motor vehicle runs under a steady condition when the engine power output 
and the engine load are balanced. This is represented by a curve D or a 
curve H in FIG. 5. 
The relationship between the engine speed and the vehicle speed from a 
start to a steady running condition when the vehicle is started slowly 
with the accelerator pedal depressed to a small depth is different from 
the relationship when the vehicle is started suddenly with the accelerator 
pedal depressed to a large depth. When the vehicle is started slowly, the 
relationship between the engine speed and the vehicle speed varies along 
the lines A.fwdarw.B.fwdarw.C.fwdarw.D.fwdarw.H as shown in FIG. 5. When 
the vehicle is started suddenly, the relationship varies along the lines 
E.fwdarw.F.fwdarw.G.fwdarw.H as shown in FIG. 5. 
As is apparent from FIG. 5, the engine speed when the transmission is 
operated for a vehicle speed change is selected dependent on the amount of 
depression of the accelerator pedal, and the transmission is controlled so 
that the engine speed is made constant. 
When releasing the accelerator pedal in order to stop the motor vehicle 
from a steady running condition, the throttle pressure Pt is lowered to a 
value corresponding to the throttle valve opening of zero. The condition 
F3&lt;F4 is reached on the pilot valve 60, and the transmission ratio R is 
minimized. The hydraulic motor 4 is driven through the shaft 3 from the 
road wheels by the load inertia created by the vehicle speed, and the 
hydraulic pump 2 is rotated according to the pressure discharged by the 
motor 4. At the same time, the engine E is coupled to the hydraulic pump 2 
and is rotated thereby, whereupon engine braking is applied to decelerate 
the motor vehicle. 
The engine speed and the vehicle speed at this time vary along the lines 
H.fwdarw.I.fwdarw.I.fwdarw.J.fwdarw.K.fwdarw.L as shown in FIG. 5. The 
condition in which engine braking is exerted to decelerate the motor 
vehicle while the transmission ratio R remains minimum is indicated by 
H.fwdarw.D.fwdarw.I in FIG. 5. As the engine speed is lowered past the 
intersection of lines I and J, the condition F3&gt;F4 is reached for the 
pilot valve 60, and the transmission ratio R is increased. At this time, 
the load on the engine braking is increased to prevent the engine speed 
from dropping, so that the engine speed becomes substantially constant as 
indicated by line J in FIG. 5. When the transmission ratio T reaches the 
maximum level, the engine speed drops with the vehicle speed as indicated 
by line K in FIG. 5. Upon a further reduction in the engine speed, the 
condition F1&gt;F2 is reached on the pilot valve 80 and the clutch valves 7a, 
7b are operated in the opening direction, and the engine speed and the 
vehicle speed are varied as represented by line L in FIG. 5. At this time, 
the bypass passages 6a, 6b are opened quickly along 
a.fwdarw.b.fwdarw.c.fwdarw.d.fwdarw.e including the dotted line b.fwdarw.c 
in FIG. 6 through the opening of the check valve 8. Therefore, no 
intensive engine braking is applied during the final stages of 
deceleration, and the vehicle can be stopped smoothly. The power 
transmission between the hydraulic pump 2 and the hydraulic motor 4 is now 
cut off to allow stopping of the motor vehicle. At this time, the engine E 
is idling, and the clutch valves 7a, 7b are operated from the position of 
FIG. 2c to the position of FIG. 2b, causing the directional control valve 
49 to reach the lefthand position. Therefore, the pump pressure Pl is 
applied to the lefthand end of the spool 63 of the pilot valve 60 to keep 
the transmission ratio R at its maximum level. 
It is assumed then that the motor vehicle is abruptly stopped from a 
running condition while the manual valve 48 is in the forward position F. 
In this case, the clutch valves 7a, 7b start opening before the operation 
of the transmission for a vehicle speed change is completed, because 
whereas the transmission is operated slowly to achieve a smooth vehicle 
speed change, clutch operation which is of high response is started 
earlier. 
More specifically, as illustrated in FIG. 5, when the accelerator pedal is 
released and the motor vehicle is abruptly braked, the second detector 
means S2 produces a low throttle pressure Pt corresponding to the throttle 
valve opening of zero and the condition F3&lt;F4 is reached on the pilot 
valve 60. The piston 54 of the hydraulic cylinder 50 is moved to the left 
to minimize the transmission ratio R. Then, engine braking is applied and 
an ordinary brake is operated to lower the vehicle speed quickly along 
H.fwdarw.D.fwdarw.I in FIG. 5. Upon this reduction in the engine speed, 
F3&gt;F4 on the pilot valve 60, and the transmission ratio R is gradually 
increased with the reduction in the engine speed as indicated by the line 
M in FIG. 5. The engine speed is lowered without an increase in the load 
on the engine braking. 
As the engine speed further drops, F1&gt;F2 on the pilot valve 80, and the 
clutch valves 7a, 7b are operated in the opening direction. In response to 
such opening of the clutch valves 7a, 7b, the directional control valve 49 
is shifted to the lefthand position, applying the pump pressure P to the 
lefthand end of the spool 63 of the pilot valve 60 to quickly move the 
spool 63 to the right. The piston 54 of the hydraulic cylinder 50 is also 
moved to the right into the righthand limit position. When the motor 
vehicle is stopped, the transmission ratio R is quickly increased to the 
maximum value. This is indicated by the line N in FIG. 5. At this time, 
since the opening of the bypass passages 6a, 6b with respect to the amount 
of opening operation of the clutch valves 7a, 7b is sharply increased as 
indicated by a.fwdarw.b.fwdarw.c.fwdarw.d.fwdarw.e along the dashed 
inclined line in FIG. 6, no intensive engine braking is applied even if 
the transmission ratio R is increased as described above, and hence the 
motor vehicle can smoothly be stopped. 
If the manual valve 48 is shifted from the forward position F to the 
neutral position N after the motor vehicle is stopped, the hydraulic 
pressure in the chambers 42, 43 of the hydraulic cylinder 40 is released, 
and the piston 44 remains biased into the lefthand limit position under 
the resiliency of the spring 46. Therefore, the forward/reverse mode 
selector device 20 is continuously held in the forward position F. Because 
the hydraulic pressure in the oil passage 101 is zero, the clutch valves 
7a, 7b are fully open. 
When the engine is idling while the manual valve 48 is in the forward 
position, the transmission ratio R is maximum. If the manual valve 48 is 
shifted into the neutral position N in this condition, the transmission 
ratio R remains maximum. Therefore, when the manual valve 48 is shifted 
from the neutral position N to the forward position F to get the motor 
vehicle started, a maximum transmission ratio required to start the motor 
vehicle can immediately be obtained, and the motor vehicle can smoothly be 
started. 
By shifting the manual valve 48 from the neutral position N to the reverse 
position R while the motor vehicle is at rest, the manual valve 48 
provides communication between the oil passages 110, 101, the oil passages 
111, 102, and the oil passage 103 and the oil release passage 112. The 
head chamber 42 of the hydraulic cylinder 40 is supplied with the pump 
pressure Pl, and the hydraulic pressure in the rod chamber 43 is released. 
Thus, the piston 44 is moved to the right, and the sleeve 27 of the 
forward/reverse mode selector device 20 is moved to the right out of 
engagement with the forward clutch gear 23a into engagement with the 
reverse clutch gear 25a. 
Since the oil passage 101 is supplied with the pump pressure Pl, the clutch 
valves 7a, 7b are operated until the throttle pressure Pt and the governor 
pressure Pg are balanced. When the engine is idling, the partly engaged 
clutch condition is achieved. At this time, the hydraulic cylinder 50 
operates in the same manner as when the manual valve 48 is shifted from 
the neutral position N to the forward position F, and the transmission 
ratio R becomes maximum. 
In order to start the motor vehicle backwards, the throttle valve 
opening/closing device 15 is operated to open the throttle valve. As with 
the operation to move the motor vehicle forwardly as described above, the 
clutch valves 7a, 7b are gradually closed (the clutch valve 7b is 
effectively closed from the outset because of the check valve 8) to start 
actuating the hydraulic motor 4 to start the motor vehicle backwards. The 
pilot valve 60 operates in the same manner for effecting the same 
transmission control as when the motor vehicle is moved forwardly. 
When the motor vehicle is stopped while the manual valve 48 is in the 
reverse position R, the forward/reverse mode selector device 20 is kept in 
the reverse position R whereas the clutch valves 7a, 7b and the swash 
plate 4c operate in the same way as when the motor vehicle is stopped 
while running forwardly. When the manual valve 48 is shifted from the 
reverse position R to the neutral position N after the motor vehicle is 
stopped, both of the chambers 42, 43 of the hydraulic cylinder 40 are 
brought into communication with the oil sump 12. The piston 44 is 
therefore moved from the righthand limit position to the lefthand limit 
position, so that the forward/reverse mode selector device 20 is released 
from the reverse position. At this time, since the hydraulic pressure in 
the oil passage 101 is zero and the clutch valves 7a, 7b are fully open, 
the clutch gear 25a and the sleeve 27 can smoothly disengage from each 
other. The transmission ratio R is maximum when the manual valve 48 is in 
the reverse position R and the engine is idling as when the manual valve 
48 is shifted from the forward position F to the neutral position N. 
According to another embodiment of the present invention, the control unit 
CU may be constructed of a microcomputer or the like. More specifically, 
the engine speed, the throttle valve opening, the shifted position of the 
manual valve, and the selected position of the forward/reverse mode 
selector device 20 are electrically detected, and the detected data items 
are applied to a microcomputer which is programmed to process them to 
produce the same mechanical movements as described above in the aforesaid 
embodiment by operating a driver in place of the hydraulic cylinder 50. 
The hydraulic cylinder 50 which is a hydraulic actuator may be replaced 
with an electric actuator such as a stepping motor, a linear stepping 
motor, a DC motor, or an AC motor dependent on the construction of the 
control unit CU. The pilot valve 60 and the manual valve 48 may comprise 
an electro-hydraulic servovalve or a solenoid-operated proportional 
pressure control valve. 
The present invention is not limited to the hydraulically operated, 
continuously variable transmission for motor vehicles comprising the fixed 
displacement hydraulic sump 2 and the variable displacement hydraulic 
motor 4. The principles of the present invention are applicable to a 
transmission comprising a variable displacement hydraulic pump and a fixed 
displacement hydraulic motor, or a transmission comprising a variable 
displacement hydraulic pump and a variable displacement hydraulic motor. 
While the forward/reverse mode selector device 20 is of the dog clutch type 
in the foregoing embodiment, it may comprise a wet-type clutch or the 
like. While the pressure P discharged from the charging pump 10 is 
employed as the working pressure for operating the hydraulic cylinder 50 
in the above embodiment, a high hydraulic pressure in the closed hydraulic 
circuit 5 may be utilized as the working pressure. 
The detection of the throttle valve opening or the accelerator pedal 
depression as an indication of the driver's intention of acceleration and 
deceleration may be replaced with the detection of the vacuum developed in 
the intake pipe of the engine, and the amount of fuel supplied to the 
engine, and the engine torque may be detected instead of the engine speed 
as an indication of the engine power output. 
With the present invention, as described above, since the opening of the 
bypass passages controlled by the clutch valves disclosed in the bypass 
passages which are connected between the two oil passages of the closed 
hydraulic circuit is quickly increased when the transmission is driven by 
driving forces from the road wheels at the time of decelerating the motor 
vehicle, intensive engine braking is prevented from being applied to the 
motor vehicle even if the transmission ratio is abruptly increased during 
stopping of the motor vehicle by idle inertia-dependent movement. 
Therefore, the vehicle can be stopped smoothly with an improved feeling to 
the driver over previous systems.