Clutch actuating apparatus controlled by accelerator

An apparatus for actuating an automatically operated clutch for use in the driveline of an automotive vehicle. Such apparatus being operative to drive the clutch to disengage when shift is being made in the transmission with the accelerator pedal kept depressed and also being operative, in response to vehicle speed, for enabling the clutch to be engaged at a moderate rate when the vehicle speed is higher than a predetermined level. The apparatus is responsive to movement of the accelerator pedal for enabling the clutch to be engaged at a rate varying with the depth to which the accelerator pedal is depressed, and also for enabling the clutch to be driven toward an engaged condition rapidly for a certain period of time slightly before the clutch is first brought into engagement.

The present invention relates in general to clutches for use in the 
drivelines of automotive vehicles and, particularly, to a clutch of the 
type which is automatically operated in response to prescribed operational 
conditions of an automotive vehicle. More particularly, the present 
invention is concerned with an apparatus for automatically actuating such 
a clutch. 
It is an object of the present invention to provide an automatic clutch 
actuating apparatus capable of engaging and disengagine an automatically 
operated clutch smoothly and at a reasonable rate responsive to prescribed 
operational conditions of the automotive vehicle. 
It is another object of the present invention to provide an automatic 
clutch actuating apparatus adapted to prevent the clutch from being 
engaged during shifting of transmission gears with the accelerator pedal 
held released and to enable the clutch to engage rapidly upon completion 
of the gear shifting. 
It is still another object of the present invention to provide an automatic 
clutch actuating apparatus responsive to vehicle speed for permitting the 
clutch to engage at a moderate rate when the vehicle is cruising at a 
speed higher than a predetermined level with the accelerator pedal kept 
released as when a braking action is to be imparted to the engine by the 
inertia of the vehicle running. 
It is still another object of the present invention to provide an automatic 
clutch actuating apparatus responsive to vehicle speed lower than a 
predetermined level for preventing the clutch from being engaged if the 
accelerator pedal is kept depressed, enabling the vehicle to be brought to 
a halt with transmission gears held in mesh and to start from the halt 
smoothly and rapidly simply by depressing the accelerator pedal. 
It is a further and another object of the present invention to provide an 
automatic clutch actuating apparatus which will enable the clutch to 
engage at a higher rate when the accelerator pedal is depressed deeper 
from the released position, irrespective of vehicle speed. 
It is a further and another object of the present invention to provide an 
automatic clutch actuating apparatus capable of driving the clutch rapidly 
during a period of time immediately before the clutch is brought into 
engagement and driving the clutch toward the fully engaged condition 
thereof at a moderate rate after the clutch has once been engaged. 
Yet, it is still another object of the invention to provide an automatic 
clutch actuating apparatus which is simple in construction, ready for 
installation in an automotive vehicle and economical to manufacture. 
In accordance with the present invention, there is provided in an 
automotive vehicle including a vacuum source, a transmission gear shift 
lever and an accelerator pedal, an apparatus for actuating an 
automatically operated clutch forming part of the driveline of the vehicle 
and continuously operable between a disengaged condition and a fully 
engaged condition through a partial torque transmission range, comprising 
(a) a vacuum chamber in communication with a vacuum source such as the 
intake manifold of the engine, (b) an air chamber communicable with the 
open air through each of first and second restricted-flow air inlet ports, 
(c) a valve chamber alternately communicable with the vacuum chamber and 
the air chamber, (d) first valve meamns having a first position isolating 
the vacuum chamber from the valve chamber and establishing communication 
between the air chamber and the valve chamber and a second position 
blocking the communication between the air and valve chambers and 
establishing communication between the vacuum and valve chambers, (e) 
second valve means having a first position allowing the first air inlet 
port to open and a second position closing the first air inlet port, (f) 
third valve means continuously movable between a first position closing 
the second air inlet port and a second position allowing the second air 
inlet to fully open, (g) first valve actuating means for moving the first 
valve means in response to predetermined conditions of the transmission 
gear shift lever and the accelerator pedal, the first valve actuating 
means being operative to move the first valve means into the first 
position thereof in response to at least one of the condition in which the 
transmission gear shift lever is free from a manipulative effort and the 
condition in which the accelerator pedal is at least partially depressed 
from the released position and being operative to move the first valve the 
conditions in which the transmission gear shift lever is being manipulated 
and simultaneously the displacement of the accelerator pedal from the 
release position is smaller than a predetermined value kept, (h) second 
valve means responsive to vehicle speed for moving the second valve means 
into the first position thereof in reponse to a vehicle speed higher than 
a predetermined level and into the second position thereof in response to 
a vehicle speed lower than the above mentioned predetermined level, (i) 
third valve actuating means responsive to movement of the accelerator 
pedal for continuously moving the third valve means between the first and 
second positions thereof as the accelerator pedal is moved between the 
released position and the fully depressed position thereof so that the 
flow of air through the second air inlet port is varied substantially 
proportionate to the depth to which the accelerator pedal is depressed 
from the released position thereof, (j) a differential-pressure assembly 
including a variable-volume chamber which is defined in part by a flexible 
diaphragm which is at least partially movable between positions 
respectively providing minimum and maximum volume conditions of the 
variable-volume chamber, and biasing means for urging the diaphragm toward 
the position providing the maximum volume condition of the variable-volume 
chamber, the variable-volume chamber being in constant communication with 
the aforesaid valve chamber so that the diaphragm is moved toward the 
position providing the minimum volume condition of the variable-volume 
chamber in the presence of intake manifold vacuum in the valve chamber 
with the first valve means in the first position thereof and toward the 
position providing the maximum volume condition of the variable-volume 
chamber in the presence of atmospheric air in the valve chamber with the 
first valve means in the period position thereof, and (k) a mechanical 
linkage operatively interconnecting the diaphragm of the 
differential-pressure assembly and the aforesaid clutch for driving the 
clutch toward the disengaged position and fully engaged positions as the 
diaphragm is moved toward the positions providing the minimum and maximum 
volume conditions, respectively, of the variable-volume chamber. 
The automatic clutch actuating apparatus thus constructed and arranged may 
further comprise a third air inlet port for providing substantially 
unrestricted communication between the air chamber and the open air 
independently of the above mentioned first and second air inlet ports, 
fourth valve means having a first position closing the third air inlet 
port and a second position establishing the communication between the air 
chamber and the open air through the third air inlet port, and fourth 
valve actuating means responsive to the movements of the diaphragm of the 
aforesaid differential-pressure assembly toward and away from the position 
providing the minimum volume condition of the variable-volume chamber, the 
fourth valve actuating means being operative to move the fourth valve 
means into the first position thereof in response to the movement of the 
diaphragm over more than a predetermined distance away from the position 
providing the minimum volume condition of the variable-volume chamber and 
into the second position thereof in response to the movement of the 
diaphragm over the above mentioned predetermined distance toward and into 
the position providing the minimum volume condition of the variable-volume 
chamber. In this instance, the fourth valve actuating means may comprise a 
solenoid operated valve actuating unit having first and second conditions 
holding the fourth valve means in the first and second positions, 
respectively, thereof, switch means electrically connected to the valve 
actuating unit, a switch actuating element projecting into the 
variable-volume chamber and engageable at one end with the diaphragm and 
at the other end with the switch means, the switch actuating element being 
movable back and forth over the above mentioned predetermined distance in 
a direction substantially identifal with the direction of movement of the 
diaphragm, the movement of the actuating element being between a first 
position disengaged from the switch means and a second position in 
actuating engagement with the switch means, the diaphragm being in 
engagement with the switch actuating element for holding the switch means 
actuated when the diaphragm is being moved over the predetermined distance 
toward and away from the position providing the minimum volume condition 
of the variable-volume chamber, the valve actuating unit being held in the 
second condition thereof when the switch means is kept actuated, and 
biasing means for urging the switch actuating means into the second 
position thereof. The above mentioned predetermined distance is such that 
will hold the diaphragm in a position operative to hold the clutch in a 
condition immediately prior to engagement in the previously mentioned 
partial torque transmission range when the diaphragm is moved away from 
the position providing the minimum voltage condition toward the position 
providing the maximum volume condition of the variable-volume chamber.

Reference will now be made to the drawings, first to FIG. 1 which 
illustrates an embodiment of the apparatus according to the present 
invention. The apparatus is intended to actuate a clutch 10 which is 
provided in the driveline of a vehicle and which is schematically shown, 
by way of example, to be a single-plate dry-disc friction clutch which 
largely comprises, as is customary, a flywheel 12, a friction disc 14, a 
release lever assembly 16 with a spring loaded pressure plate (not shown), 
and a clutch cover 18. Though not shown, the flywheel 12 and the friction 
disc 14 are connected to the engine crankshaft and the transmission drive 
shaft, respectively, and are biased to be in engagement with each other by 
the spring loaded pressure plate of the release lever assembly and thus 
couple the engine crankshaft and the transmission drive shaft together, as 
is well known in the art. When an external force is applied to the release 
lever assembly 16 in a predetermined direction which is assumed in FIG. 1 
to be rightward of the drawing, then the pressure plate releases the 
friction disc 14 from the flywheel 12 so that the engine crankshaft and 
the transmission drive shaft are uncoupled. The shown construction of the 
clutch 10 is merely for the purpose of illustration of a clutch for 
automotive use and the apparatus embodying the present invention is 
applicable to any type of mechanically operated clutch for use in the 
driveline of an automotive vehicle. The details of the constructions and 
operation of automotive clutches in general are well known in the art and 
are rather immaterial for the understanding of the present invention, no 
further description thereof will be herein required. 
In FIG. 1, furthermore, the vehicle is shown to include, as is customary in 
the art, an air-fuel mixture supply system 20 for the engine (not shown), 
a transmission gear shift lever 22, and an accelerator pedal 24. The 
mixture supply system 20, which may be a carburetor or of the 
electronically controlled fuel injection type, is shown to include a 
throttle valve 26 located upstream of an intake manifold 28 of the engine. 
A partial vacuum is developed in the intake manifold 28 when the engine is 
in operation, as is well known in the art. The transmission gear shift 
lever 22 is shown, by way of example, to be of the type mounted on a 
steering column 30 carrying a steering wheel 32 at its top but, if 
desired, the shift lever 22 may be of the floor-board mounted type. The 
accelerator pedal 24 is connected by a mechanical linkage, part of which 
is shown at 34, to the shaft of the throttle valve 26 in the mixture 
supply system 20 for controlling the flow of air-fuel mixture (in a 
carburetor type engine) or the flow of air to be delivered into the intake 
manifold 28. 
Now, the embodiment of the present invention as shown in FIG. 1 comprises a 
generally cylindrical casing 36 which is formed with a vacuum chamber 38, 
an air chamber 30 and a valve chamber 42. The vacuum, air and valve 
chambers 38, 40 and 42 are arranged in series with each other with the 
valve chamber 42 located intermediate between the vacuum and air chambers 
38 and 40. The vacuum chamber 38 is in communication with the intake 
manifold 28 of the engine through a vacuum passageway 44 and a vacuum 
inlet port 46 formed in the casing 36. The vacuum passageway 44 has 
provided at its terminal end next to the vacuum inlet port 46 a one-way 
check valve 48 which is biased to close the vacuum inlet port 46 by means 
of a preload spring 50. The one-way check valve 48 is adapted to prevent 
leakage of vacuum from the vacuum chamber 38 backwardly into the vacuum 
passageway 44 when the vacuum in the intake manifold 28 transiently 
becomes lower in absolute valve than the vacuum which has been developed 
in the vacuum chamber 38. The vacuum chamber 38 is provided and in 
constant communication with a vacuum reservoir 52 having a predetermined 
internal capacity. 
The casing 36 is formed between the vacuum and valve chamber 38 and 42 a 
stepped internal wall portion forming an annular face at the end of the 
vacuum chamber 38 adjacent to the valve chamber 42. An annular valve seat 
member 54 is seated on this annular face through a resilient seal-off 
element 54 and is elastically held against the seal-off element 56 which 
is accordingly forced against the above mentioned annular face of the 
casing 36 by means of a preload spring 58. The annular valve seat member 
54 has a circular opening through which communication is provided between 
the vacuum and valve chambers 38 and 42. 
The casing 36 has further formed therein an air passageway 60 for providing 
communication between the air chamber 40 and the valve chamber 42. The end 
of the air passageway 60 in the valve chamber 42 is defined by a valve 
seat wall portion 62 which is spaced apart from the above mentioned valve 
seat member 54 between the vacuum and valve chambers 38 and 42. A 
two-position valve 64 consists of a rigid valve disc 66 and resilient 
seal-off layers 68 and 70 fixedly attached to the opposite end faces of 
the valve disc 66. The two-position valve 64 is positioned within the 
valve chamber 42 and is movable between a first position having one 
seal-off layer 68 in contact with the valve seat member 54 between the 
vacuum and valve chambers 38 and 42 and a second position having the other 
seal-off layer 70 in contact with the valve seat wall portion 62 at the 
end of the passageway 60 between the air and valve chambers 40 and 42. 
When the valve 64 is in the first position thereof as illustrated in FIG. 
1, the vacuum chamber 38 is isolated from the valve chamber 42 by the 
valve 64 closing the opening in the valve seat member 54 and communication 
is established between the air and valve chambers 40 and 42 through the 
air passageway 60 with the valve 64 spaced apart from the valve seat wall 
portion 62 at the end of the passageway 60. When, on the other hand, the 
valve 64 is in the second position thereof, the communication between the 
air and valve seat chambers 40 and 42 is blocked by the valve 64 seated on 
the valve seat wall portion 62 and communication is established between 
the vacuum and valve chambers 38 and 42 with the valve 64 spaced apart 
from the valve seat member 54. The two-position valve 64 is biased into 
the above mentioned first position thereof by means of a preload spring 72 
which is seated at one end on the outer circumferential portion of the 
valve disc 66 and an opposite annular wall portion of the casing 36 
surrounding the air passageway 60. 
The two-position valve 64 thus arranged is actuated into the second 
position thereof by a solenoid operated valve actuating unit 74 which is 
located adjacent the end of the vacuum chamber 38 opposite to the valve 
chamber 42. The valve actuating unit 74 comprises a solenoid coil 76 with 
input and output terminal leads 78 and 78' and a solenoid core 80 
concentrically surrounded by the solenoid coil 76 and axially movable 
toward and away from the vacuum chamber 38 when the solenoid coil 76 is 
energized and deenergized, respectively. The solenoid core 80 is fixedly 
connected to or integral with a plunger 82 axially projecting into the 
vacuum chamber 38 toward the valve chamber 42 through the circular opening 
in the valve seat member 54 between the vacuum and valve chambers 38 and 
42. When the solenoid coil 76 is energized and accordingly the solenoid 
core 80 is axially moved toward the vacuum chamber 38, the plunger 82 is 
axially moved through the vacuum chamber 38 and the opening in the valve 
seat member 54 into pressing engagement with the seal-off layer 68 of the 
two-position valve 64, which is consequently forced to unseat from the 
valve seat member 54 and is brought into pressing contact through its 
seal-off layer 70 with the valve seat wall portion 62 at the end of the 
passageway 60 between the air and valve chambers 40 and 42 against the 
opposing force of the preload spring 72. When the solenoid coil 76 is 
de-energized and accordingly the solenoid core 80 is moved away from the 
vacuum chamber 38, the plunger 82 is axially moved back away from the 
valve chamber 42 and releases the two-position valve 64. As a consequence, 
the two-position valve 64 is moved from the second position thereof into 
the first position closing the opening in the valve seat member 54 by the 
biasing force of the preload spring 72. The previously mentioned preload 
spring 58 holding the valve seat member 54 into position between the 
vacuum and valve chambers 38 and 42 is shown seated at one end on the body 
of the solenoid coil 76 and at the other end on the annular valve seat 
member 54. 
The input terminal lead 78 of the solenoid coil 76 is electrically 
connected across a switch 84 to the positive terminal of a d.c. power 
source 86, while the output terminal lead 78' of the coil 76 is 
electrically connected to ground across a series combination of switches 
88 and 90 which are respectively responsive to the motions of the 
transmission gear shift lever 22 and the accelerator pedal 24. The gear 
shift lever responsive switch 88 is arranged to close when the 
transmission gear shift lever 22 is being manipulated for making gear 
shift in the transmission and to open when the shift lever 22 is released 
from the manipulative effort. On the other hand, the accelerator pedal 
responsive switch 90 is arranged to close when the accelerator pedal 24 is 
released and to open when the accelerator pedal 24 is depressed for 
acceleration of the vehicle. The switch 84 connected to the d.c. power 
source 86 is assumed to be the ignition switch. 
The air chamber 40 has first and second restricted-flow air inlet ports 92 
and 94 formed in a wall portion of the casing 36. The air inlet ports 92 
and 94 are in communication with a common air inlet passageway 96 which is 
vented from the open air through an air filter unit 98 stuffed with a 
suitable filter medium 100. The air inlet passageway 96 has provided 
therein a two-position valve 102 which is arranged to close and open the 
first air inlet port 92 under the control of a solenoid-operated valve 
actuating unit 104. The valve actuating unit 104 is composed of a solenoid 
coil 106 with input and output terminal leads 108 and 108' and a solenoid 
core 110 concentrically surrounded by the solenoid coil 106 and axially 
movable toward and away from the above mentioned first air inlet port 92 
when the solenoid coil 106 is energized and de-energized, respectively. 
The solenoid core 110 is fixedly connected to or integral with a plunger 
112 axially projecting toward the air inlet port 92. The two-position 
valve 102 is located between the plunger 112 and the outer end of the 
first air inlet port 92 and is elastically held in pressing contact with 
the leading end of the plunger 112 by means of a preload spring 114 which 
is seated at one end on the valve 102 and received in part in an annular 
groove 116 formed in the wall portion of the casing 36 which surrounds the 
first air inlet port 92 as shown. When the solenoid coil 106 remains 
deenergized so that the solenoid core 110 and accordingly the plunger 112 
are held in their respective axial positions remotest from the first air 
inlet port 92, the two-position valve 112 is spaced apart from the air 
inlet port 92 by the biasing force of the preload spring 114 and allows 
the first air inlet port 92 to open, thereby establishing communication 
between the air chamber 40 and the air inlet passageway 96 through the 
first air inlet port 92. Under these conditions, atmospheric air admitted 
into the air inlet passageway 96 by way of the air filter unit 98 is 
allowed into the air chamber 40 through the first air inlet port 92 at a 
rate dictated by the effective cross sectional area of the port 92. The 
first air inlet port 92 thus serves, in effect, as an orifice providing a 
predetermined limited flow of air therethrough. The flow of air allowed 
through the orifice, viz., the effective cross sectional area of the first 
air inlet port 92 is predetermined in such a manner that will provide 
desired torque transmission characteristics in the clutch 10, as will be 
described more clearly. When, now, the solenoid coil 106 of the valve 
actuating unit 104 is energized, the solenoid core 108 and accordingly the 
plunger 112 are axially moved toward the air inlet port 92 and causes the 
two-position valve 102 to close the air inlet port 92 against the opposing 
force of the preload spring 114, thereby shutting off the flow of air 
through the first air inlet port 92 into the air chamber 40. In the 
arrangement shown, the two-position valve 102 and the plunger 112 may be 
fixedly connected together or, as an alternative, may be formed as an 
integral member, if desired. In this instance, the preload spring 114 may 
be dispensed with. 
The input terminal lead 108 of the solenoid coil 106 is connected across 
the previously mentioned switch 84 to the positive terminal of the d.c. 
power source 86 and the output terminal lead 108' of the coil 106 is 
connected to ground across a vehicle speed responsive switch 118 which is 
schematically illustrated on a top left corner part of FIG. 1. The vehicle 
speed responsive switch 118 is arranged to be open in response to a 
vehicle speed higher than a predetermined level vo and to close in 
response to a vehicle speed lower than the predetermined level vo. When, 
thus, the vehicle is cruising at a speed higher than the level vo, the 
solenoid coil 106 is kept de-energized and as a consequence the 
two-position valve 102 is held in a position allowing the first air inlet 
port 92 to open as shown so that atmospheric air is admitted into the air 
chamber 40 through the air inlet port 92. When, however, the vehicle speed 
is lower than the predetermined level vo, the solenoid coil 106 is 
energized from the d.c. power source 86 and holds the two-position valve 
102 in a position closing the first air inlet port 92 so that the flow of 
air through the first air inlet port 92 into the air chamber 40 is 
interrupted. If desired, the vehicle speed responsive switch 118 may be 
connected in series with an accelerator pedal stroke switch 120 which is 
responsive to the movement of the accelerator pedal 24 through a suitable 
mechanical linkage 122. The accelerator pedal stroke switch 120 is adapted 
to close when the depth of the accelerator pedal depression is smaller 
than a predetermined value and to open when the depth of the accelerator 
pedal depression exceeds such a value. The reason for which the switch 120 
of this nature is provided will be explained later. 
The flow of air through the second air inlet port 94 is controlled 
continuously between zero and a predetermined maximum value as the 
accelerator pedal is moved between the released position and the fully 
depressed position thereof. This is achieved by means of a needle valve 
124 having a tapered end portion projecting into the second air inlet port 
94 from the outer end of the port. The needle valve 124 has flanges 126 
axially slidable in a bore formed in a valve guide portion 128 of the 
casing 36 and is biased by a preload spring 130 in a direction having its 
tapered end portion seated at the outer end of the air inlet port 94 and 
thus fully closing the port. The needle valve 124 is connected by a 
flexible line such as a cable 132 to the mechanical linkage 34 of the 
accelerator pedal 24. Designated by 134 is a conduit through which the 
cable 132 extends longitudinally. The needle valve 124 is, thus, biased by 
the preload spring 130 tofully close the second air inlet port 94 but is 
driven from the accelerator pedal 24 through the mechanical linkage 34 and 
the cable 132 to move away from the air inlet port 94 so that the flow of 
air through the second air inlet port 94 from the air inlet passageway 96 
into the air chamber 40 is continuously increased as the accelerator pedal 
24 is depressed deeper and as a consequence the needle valve 124 is moved 
remoter from the air inlet port 94. 
The apparatus embodying the present invention further comprises a 
differential-pressure assembly 136 which is operative on the vacuum or 
atmospheric pressure developed in the valve chamber 42 above described. 
The differential-pressure assembly 136 comprises a hollow housing 138 
internally divided by a flexible diagram 140 into a variable-volume 
chamber 142 and an atmospheric chamber 144 which is constantly open to the 
atmosphere through an opening 146 formed in the housing 138 as shown. The 
housing 138 has a wall portion 138a which is spaced apart from the 
diaphragm 140 across the variable-volume chamber 142. The diaphragm 140 is 
urged away from the inner face of the wall portion 138a of the housing 138 
by means of a preload spring 148 which is seated between the wall portion 
138a and the diaphragm 140, the variable-volume chamber 142 being thus 
biased to expand by the spring 148. The variable-volume chamber 142 is in 
constant communication with the valve chamber 42 in the previously 
described casing 36 through a passageway 150. When the valve chamber 42 is 
in communication with the vacuum chamber 38, vacuum is thus developed in 
the variable-volume chamber 142. The vacuum acts on the diaphragm 140 and 
causes the diaphragm 140 to move toward the inner face of the wall portion 
138a of the housing 138 against the opposing force of the preload spring 
148 until the diaphragm 140 reaches a position closest to the wall portion 
138a of the housing 138 and thus providing a minimum volume condition of 
the variable-volume chamber 142. When, on the other hand, the valve 
chamber 42 is in communication with the air chamber 40 in the casing 36, 
an atmospheric pressure obtains in the variable-volume chamber 142 so that 
the diaphragm 140 is moved away from the wall portion 138a of the housing 
138 by the force of the preload spring 148 until the diaphragm 140 reaches 
a position remotest from the wall portion 138a and providing a maximum 
volume condition of the variable-volume chamber 142 as shown. 
The diaphragm 140 has its central portion securely interpoed between 
retainer discs 152 and 152' and fixedly connected to a clutch actuating 
rod 154 which extends substantially perpendicularly from the diaphragm 140 
and projects out of the housing 138 through the opening 146 opposite to 
the wall portion 138a of the housing 138. A mechanical linkage 156 
comprises a lever 158 rotatable about a shaft 160 as a fulcrum. The lever 
158 is pivotally connected at one end to the clutch actuating rod 154 and 
at the other end to a pressing member 164 which is in engagement with the 
release lever assembly 16 of the automatically operated clutch 10. When 
the diaphragm 140 of the differential-pressure assembly 136 is moved 
toward the position providing the minimum volume condition in the 
variable-volume chamber 142 as previously described, the clutch actuating 
rod 154 is moved leftwardly of the drawing as indicated by an arrow D so 
that the lever 158 is rotated about the shaft 160 counter-clockwise of the 
drawing and causes the clutch 10 to disengage. When, conversely, the 
diaphragm 140 of the differential-pressure assembly 136 is moved toward 
the position providing the maximum volume condition in the variable-volume 
chamber 142, the clutch actuating rod 158 is moved rightwardly of the 
drawing as indicated by an arrow E so that the lever 158 is rotated about 
the shaft 160 clockwise of the drawing and causes the clutch 10 to engage. 
The clutch 10 is in this fashion driven to engage and disengage when the 
diaphragm 140 of the differential-pressure assembly 136 is moved between 
the positions providing the minimum and maximum volume conditions of the 
variable-volume chamber 142. 
When, in operation, the transmission gear shift lever 22 is manipulated 
from one position into another to make a shift in the transmission, then 
the shift lever responsive switch 88 closes. If, in this instance, the 
accelerator pedal 24 is released, the accelerator pedal responsive switch 
90 also closes and makes a closed circuit through the d.c. power source 86 
and the solenoid coil 76 of the valve actuating unit 74 with the ignition 
switch 84 kept closed. The solenoid coil 76 is therefore energized from 
the power source 86 and causes the solenoid core 80 to axially move toward 
the vacuum chamber 38. The plunger 82 is moved through the vacuum chamber 
38 into pressing engagement with the two-position valve 64 in the valve 
chamber 42. The two-position valve 64 is moved away from the vacuum 
chamber 38 against the opposing force of the preload spring 72 and 
unseated from the valve seat member 54 between the vacuum and valve 
chambers 38 and 42, providing communication between the vacuum and valve 
chambers 38 and 42 through the opening in the valve seat member 54. The 
two-position valve 64 is finally seated on the valve seat wall portion 62 
at the end of the passageway 60 between the air and valve chambers 40 and 
42 and hermetically isolates the valve chamber 42 from the air chamber 40. 
Under these conditions, the vacuum which has been developed in the vacuum 
chamber 38 and the vacuum reservoir 52 draws air from the variable-volume 
chamber 142 of the differential-pressure assembly 136 through the valve 
chamber 42 and the passageway 150 with the result that vacuum is developed 
in the variable-volume chamber 142. The vacuum acts on the diaphragm 140, 
which is consequently moved toward the wall portion 138a of the housing 
138 against the opposing force of the preload spring 148. This causes the 
clutch 10 to be disengaged by the action of the mechanical linkage 156 
interconnecting the clutch 10 and the disphragm 140 of the 
differential-pressure assembly 136. The vacuum communication thus provided 
between the vacuum chamber 38 and the variable-volume chamber 142 causes a 
drop in the vacuum level in the vacuum chamber 38 and the vacuum reservoir 
52. When the vacuum level in the vacuum chamber 38 is thus reduced, 
however, the check valve 48 is forced to move away from the vacuum inlet 
port 46 by the intake manifold vacuum developed in the vacuum passageway 
44 and open the vacuum inlet port 46 until the vacuum in the vacuum 
chamber 38 and the vacuum reservoir 52 restores the level of the vacuum in 
the intake manifold 28. The force of the preload spring 50 acting on the 
check valve 48 is so small as to be capable of only retaining the valve 48 
within the valve chamber in which the valve 48 is movable. 
When the shift in the transmission is complete and the gear shift lever 22 
is released from the manipulative effort which has been applied thereto, 
then the shift lever responsive switch 88 becomes open and disconnects the 
solenoid coil 76 of the valve actuating unit 74 from the power source 86 
even though the accelerator pedal switch 90 may be kept closed. The 
solenoid coil 76 is now de-energized and causes the solenoid core 80 to 
retract into the initial position thereof and accordingly the plunger 82 
to move back away from the valve chamber 42. The two-position valve 64 is 
consequently unseated from the valve seat wall portion 62 and is forced to 
seat on the valve seat member 54 between the vacuum and valve chambers 38 
and 42 by the force of the preload spring 72, thereby blocking the 
communication between the vacuum and valve chambers 38 and 42 and 
providing communication between the air and valve chambers 40 and 42 
through the passageway 60, as shown. Atmospheric air is therefore admitted 
through the air chamber 40, valve chamber 42 and passageway 60 into the 
variable-volume chamber 142 of the differential-pressure assembly 136. The 
diaphragm 140 of the assembly 136 is now moved by the preload spring 148 
away from the wall portion 138a of the housing 138 toward the position 
providing the maximum volume condition of the variable-volume chamber 142. 
This causes the clutch 10 to engage by the action of the mechanical 
linkage 156 driven by the actuating rod 154 connected to the diaphragm 
140, as previously described. 
When the diaphragm 140 of the differential-pressure assembly 136 is in the 
position providing the minimum volume condition of the variable-volume 
chamber 142, the clutch actuating rod 154 is held in an axial position 
holding the clutch 10 completely disengaged, viz., maintaining the driving 
and driven members (i.e. the flywheel 12 and friction disc 14, 
respectively) of the clutch 10 spaced apart a maximum distance from each 
other. As the diaphragm 140 is moved away from the particular position, 
the clutch actuating rod 154 is moved in the direction of arrow E and 
causes the driven member to move closer to the driving member. Because, 
however, of an allowance which is provided for the clutch 10 and the 
clutch actuating mechanism including the actuating rod 154 and the 
mechanical linkage 156 to bring the driven member into engagement with the 
driving member of the clutch 10, there is a certain amount of interval 
before engagement is produced in the clutch 10 after the diaphragm 140 has 
been initiated into motion to move away from the position providing the 
minimum volume condition of the variable-volume chamber 142. Such an 
interval is herein referred to as a clutch disengagement allowance 
displacement range or simply as an allowance displacement range of the 
clutch actuating rod 154. At the end of the allowance displacement range 
of the actuating rod 154, the driven member of the clutch is brought into 
contact with the driving member so that the clutch 10 is allowed to 
engage. The instant thus producing engagement of the clutch 10 is herein 
called a coupling point (CP) in the movement of the actuating rod 154. For 
some time after the actuting rod 154 has passed through the coupling point 
maintaining the clutch 10 engaged, the driving and driven members of the 
clutch 10 are allowed to slide on each other and are unable to transmit 
torque therebetween at a 100 percent efficiency even though the actuating 
rod 154 continues axial displacement in the direction of the arrow E. The 
range of displacement of the actuating rod 154 under such a condition is 
herein termed a partial torque transmission range. When the actuating rod 
154 is being moved within this range, the torque transmitted to the driven 
member from the driving member increases progressively as the actuating 
rod 154 is moved in the direction of the arrow E by the movement of the 
diaphragm 140 toward the position providing the maximum volume condition 
of the variable-volume chamber 142 of the differential-pressure assembly. 
As the actuating rod 154 is further moved, the revolution of the driven 
member of the clutch 10 is completely matched to the revolution of the 
driving member at the end of the partial torque transmission range so that 
the clutch 10 is fully engaged and enabled to transmit torque therethrough 
at a 100 percent efficiency. The particular instant is herein termed a 
full engagement point (FEP) in the movement of the actuating rod 154. The 
movement of the actuating rod 154 may be terminated at this full 
engagement point but, for the reason that will be understood later, it is 
preferable to provide a small allowance for the actuating rod 154 to 
further move beyond the full engagement point. The distance of allowance 
thus provided is herein referred to as a surplus displacement range of the 
actuating rod 154. The actuating rod 154 ceases to move at the end of this 
surplus displacement range when the diaphragm 140 of the 
differential-pressure assembly 136 reaches the position providing the 
maximum volume condition of the variable-volume chamber 142. When the 
diaphragm 140 is moved away from the position providing the maximum volume 
condition in the variable-volume chamber 142 toward the position to 
provide the minimum volume condition of the chamber 142, the above 
described events occur in a reverse sequence so that the torque 
transmission efficiency through the clutch 10 starts to diminish when the 
actuating rod 154 is moved in the direction of the arrow D past the above 
mentioned full engagement point and the clutch 10 is disengaged when the 
actuating rod 154 is moved beyond the coupling point thereof. The movement 
of the actuating rod 154 thus causing the clutch 10 to disengage is 
indicated by section A.sub.1 -A.sub.2 of the plot shown by full lines in 
FIG. 2. The rate of displacement of the actuating rod 154 from the point 
A.sub.1 to the point A.sub.2, viz., the grade of the section A.sub.1 
-A.sub.2 is dictated by the rate at which vacuum is established in the 
variable-volume chamber 142 of the differential-pressure assembly 136 
after the two-position valve 164 is moved to provide the communication 
between the vacuum and valve chambers 38 and 42. The section B.sub.1 
-B.sub.2 of the plot shown by broken lines in FIG. 2 indicates the 
variation of the torque transmission efficiency as caused when the 
actuating rod 154 is moved from the point A.sub.1 to the point A.sub.2. 
The diaphragm 140 is held in the position providing the minimum volume 
condition in the variable-volume chamber 142 and, accordingly, the clutch 
actuating rod 154 is held in the axial position indicated by section 
A.sub.2 -A.sub.3 in the plot of FIG. 2 when the solenoid coil 76 of the 
valve actuating unit 74 for the two-position valve 164 is kept energized. 
When, however, the transmission gear shift lever 22 is released and as a 
consequence the gear shift responsive switch 88 is closed upon completion 
of shifting in the transmission, the solenoid coil 76 of the valve 
actuating unit 74 is de-energized and allows the valve 164 to block the 
vacuum communication between the vacuum chamber 38 and the variable-volume 
chamber 142 in the differential-pressure assembly 136 through the valve 
chamber 42. Communication is now provided between the air chamber 40 and 
the valve chamber 40 in the casing 36 and through the valve chamber 42 and 
the passageway 150 between the air chamber 40 in the casing 36 and the 
variable-volume chamber 142 in the differential-pressure assembly 136. If, 
under these conditions, the vehicle is running at a speed lower than the 
previously mentioned predetermined level vo and if the accelerator pedal 
24 is left released as when the engine is to be driven by the inertia of 
the vehicle, the solenoid coil 106 of the valve actuating unit 104 for the 
two-position valve 102 associated with the first air inlet port 92 is 
energized from the power source 86 and simultaneously the needle valve 124 
connected to the accelerator pedal linkage 34 is held in a position fully 
closing the second air inlet port 94. The two-position valve 102 is 
therefore held in a position allowing the first air inlet port 92 to open 
so that atmospheric air is admitted through the first air inlet port 92 at 
a predetermined rate (which is represented by H) into the air chamber 40 
and is passed through the air chamber 40, passageway 60, valve chamber 42 
and passageway 150 into the variable-volume chamber 142 in the 
differential-pressure assembly 136. Atmospheric pressure is thus developed 
in the variable-volume chamber 142 so that the preload spring 148 in the 
chamber 142 attempts to move the diaphragm 140 away from the position 
providing the minimum volume condition in the variable-volume chamber 142. 
Because, however, of the friction in play between some movable parts of 
the clutch 10 and the associated mechanical linkage 156 and because of the 
delay involved before the pressure in the variable-volume chamber 142 
reaches a level effective, in cooperation with the biasing force of the 
preload spring 148, to initiate the diaphragm 140 into motion after time 
t.sub.1 at which the solenoid coil 106 is assumed to be energized, the 
diaphragm 140 and accordingly the clutch actuating rod 154 start to move 
at time t.sub.2 upon lapse of a certain period of time T.sub.1 after the 
time t.sub.1. As the diaphragm 140 is thus moved away from the position 
providing the minimum volume condition of the variable-volume chamber 142 
and accordingly the clutch actuating rod 154 is axially moved in the 
direction of the arrow E, the clutch 10 is engaged progressively as 
indicated by section A.sub.4 -A.sub.5 of the plot shown in FIG. 2. The 
rate of increase of the displacement of the actuating rod 154 is dictated 
by the rate H at which atmospheric air is passed through the first air 
inlet port 92 into the air chamber 40 in the casing 36. Under the 
condition in which the actuating rod 154 is thus freely travelling within 
the clutch engagement allowance displacement range A.sub.4 -A.sub.5, the 
clutch 10 is kept disengaged so that there is no torque transmitted 
through the clutch 10. When the actuating rod 154 reaches the coupling 
point CP at time t.sub.3 upon lapse of a certain period of time T.sub.2 
after time t.sub.2, the clutch 10 is brought into engagement and commences 
to transmit torque therethrough. As the clutch actuating rod 154 is 
further moved in the direction of the arrow E beyond the coupling point CP 
as indicated by section A.sub.5 -A.sub.6 of the plot shown in FIG. 2, the 
torque transmission efficiency of the clutch 10 is increased in direct 
proportion to the amount of displacement of the actuating rod 154 as 
indicated by section B.sub.3 -B.sub.4 of the plot in broken lines and 
reaches the maximum level at time t.sub.4 upon lapse of a certain period 
of time T.sub.3 after the time t.sub.3. Under the condition in which the 
actuating rod 154 is being moved in the partial torque transmission range 
A.sub.5 -A.sub.6, torque is transmitted at a limited but increasing rate 
through the clutch 10 so that a limited braking force is applied to the 
vehicle by the inertia of the vehicle driving the engine through the 
transmission gears and the clutch 10. At time t.sub.4 which is the end of 
the partial torque transmission range, the clutch actuating rod 154 
reaches the previously mentioned full engagement point FEP thereof and 
brings the clutch 10 into a fully engaged condition capable of 
transmitting torque therethrough at a 100 percent efficiency. The 
actuating rod 154 is kept driven by the diapragm 140 of the 
differential-pressure assembly 136 and is moved for a certain period of 
time T.sub.4 through the surplus displacement range beyond the full 
engagement point FEP until the rod 154 reaches the extreme position at 
time t.sub.5, as indicated by section A.sub.6 -A.sub. 7 of the plot shown 
in FIG. 2. By reason of the allowance thus provided for the actuating rod 
154 to move slightly beyond the full engagement point FEP, the clutch 10 
is guaranteed to be brought into perfectly engaged, slip-free condition. 
The above mentioned extreme position of the actuating rod 154 providing 
such a condition of the clutch 10 is reached when the diaphragm 140 of the 
differential-pressure assembly 136 is moved into the position providing 
the maximum volume condition of the variable-volume chamber 142 of the 
assembly 136. 
If the transmission gear shift lever 22 is released from a manipulative 
effort under a condition in which the vehicle speed is lower than the 
predetermined level vo, the air chamber 40 in the casing 36 is isolated 
from the open air with the solenoid coil 106 of the valve actuator 104 
kept de-energized and, as a consequence, the clutch 10 can not be engaged 
unless the accelerator pedal 24 is depressed to drive the needle valve 124 
to open the second air inlet orifice. For this reason, the vehicle can be 
brought to a halt without shifting the transmission into the neutral 
condition or continuedly applying a manipulative force to the transmission 
gear shift lever 22 until the vehicle is to be started from a halt. 
When the accelerator pedal 24 is depressed to start the vehicle from a 
halt, the needle valve 124 is driven through the cable 132 from the 
accelerator pedal and opens the second air inlet port 94. The needle valve 
124 is arranged to vary the effective open area of the port 94 in direct 
proportion to the depth to which the accelerator pedal 24 is depressed 
from the released position. Atmospheric air is therefore introduced 
through the second air inlet port 94 into the air chamber 40 at a rate 
which increases as the accelerator pedal 24 is depressed deeper from the 
released position. Atmospheric air thus admitted into the air chamber 40 
through the port 94 is directed through the passageway 60, valve chamber 
42 and passageway 150 into the variable-volume chamber 142 of the 
differential-pressure assembly 136. The clutch 10 is therefore brought 
into engagement and driven toward the fully engaged condition 
progressively at a rate proportional to the rate of flow of air through 
the second air inlet port 94, viz., to the depth to which the accelerator 
pedal 24 is depressed from the released position. FIG. 3 illustrates 
examples of the movement of the clutch actuating rod 154 producing 
engagement in the clutch 10 which is actuated in the above described 
manner, wherein it is assumed that the vehicle is being driven at a speed 
lower than the predetermined level vo to maintain the vehicle speed 
responsive switch 118 open and accordingly the first air inlet port 92 is 
closed by the valve 102. When, thus, the second air inlet port 94 is 
opened at time t.sub.1 with the accelerator pedal 24 depressed from the 
released position, atmospheric air is admitted into the variable-volume 
chamber 142 of the differential-pressure assembly 136 at a rate which is 
proportional to the amount of displacement of the accelerator pedal 24. 
For the reason previously explained, however, the diaphragm 140 of the 
assembly 136 and accordingly the clutch actuating rod 154 are held at rest 
for some time after the second air inlet port 94 is first opened. At a 
certain time interval after time t.sub.1, the clutch actuating rod 154 is 
allowed to start to move at time t.sub.2 at a rate proportional to the 
rate of flow of air through the second air inlet port 94. The rate of 
increase of the actuating rod displacement is thus augmented continuously 
as indicated by plots P.sub.1, P.sub.2 and P.sub.3 as the flow rate of air 
through the port 94 is increased from H.sub.1 to H.sub.2 and from H.sub.2 
to H.sub.3. The actuating rod 154 therefore reaches the coupling point of 
CP earlier times e.sub.1, e.sub.2 and e.sub.3 as the accelerator pedal 24 
is depressed to depths producing the air flow rates H.sub.1, H.sub.2 and 
H.sub.3 through the second air inlet port 94. 
When the vehicle is cruising at a speed higher than the predetermined level 
v, the vehicle speed responsive switch 118 is closed to energize the 
solenoid coil 106 of the valve actuating unit 104 so that not only the 
second air inlet port 94 but the first air inlet port 92 is open. Under 
these conditions, the rate of increase of the displacement of the clutch 
actuating rod 154 is further augmented as indicated by plots Q.sub.1, 
Q.sub.2 and Q.sub.3 in FIG. 4 in proportion to the total flow rate H + 
H.sub.1, H + H.sub.2 or H + H.sub.3 of air through the first air inlet 
port 92 providing a fixed air flow rate H and the second air inlet port 94 
providing an air flow rate variable through H.sub.1, H.sub.2 and H.sub.3. 
In FIG. 4, plot Q.sub.o indicates the displacement of the actuating rod 
154 as caused by air passed through the first air inlet port 92 alone with 
the second air inlet port 94 closed. The plot Q.sub.o is thus identical 
with the plot shown in full lines in FIG. 2. 
If the accelerator pedal 24 is depressed excessively when in starting the 
vehicle from a halt, there may be a risk that the engine operates at an 
excessively high speed because the clutch 10 can not be engaged at a 
proper timing by the flow of air through the second air inlet port 94 
alone with the first air inlet port 92 kept closed until the vehicle speed 
reaches the predetermined level vo. To prevent this from occurring, the 
accelerator pedal stroke switch 120 is connected in series with the 
vehicle speed responsive switch 118. The accelerator pedal stroke switch 
120 is arranged to open in response to movement of the accelerator pedal 
24 to a depth greater than a predetermined value from the released 
position and, thus, disconnects the solenoid coil 106 of the valve 
actuating unit 104 from the power source 84 when the accelerator pedal 24 
is depressed to a depth greater than the predetermined valves. If the 
accelerator pedal 24 is depressed eccessively during starting of the 
vehicle from a halt, the two-position valve 102 is moved to open the first 
air inlet port 92 with the result that the clutch 10 can be engaged 
sufficiently fast by the flows of air through both of the first and second 
air inlet ports 92 and 94 even though the vehicle may be being moved at a 
low speed and accordingly the vehicle speed responsive switch 118 may be 
kept closed. 
FIG. 5 illustrates a further improved version of the embodiment thus far 
described with reference to FIG. 1. In addition to the members and units 
of the embodiment of FIG. 1, the embodiment shown in FIG. 5 comprises 
valve means and valve actuating means adapted to further increase the rate 
at which the clutch 10 is permitted to fully engaged condition through the 
partial torque transmission range. For this purpose, the casing 36 is 
formed with a third air inlet port 166 for providing substantially 
unrestricted air communication between the air chamber 40 and the open air 
through the previously mentioned air inlet passageway 96 and the air 
filter 98. A two-position valve 168 is provided in the air chamber 40 and 
is movable between a first position to close the third air inlet port 166 
as shown and a second position to allow the port 166 to open. The valve 
168 is biased toward the first position thereof by means of a preload 
spring which is suitably disposed within the air chamber 40. To move the 
valve 166 into the second position thereof against the action of the 
preload spring 170, a solenoid operated valve actuating unit 172 is 
mounted on the casing 36 in association with the valve 168. The valve 
actuating unit 172 comprises a solenoid coil 174 with input and output 
terminal leads 176 and 176' and a solenoid core 178 which is 
concentrically surrounded by the solenoid coil 174 and which is axially 
movable toward and away from the above mentioned air inlet port 166 from 
the outside of the air chamber 40. The solenoid core 178 is fixedly 
connected to or integral with a plunger 180 projecting axially forward 
from the core 178 and thus axially movable with the core. The solenoid 
core 178 is held in an axial position remotest from the air inlet port 166 
when the solenoid coil 174 remains de-energized and is moved into an axial 
position closest to the air inlet port 166 when the solenoid coil 174 is 
energized. When the solenoid core 178 is in the position remotest from the 
air inlet port 166 with the solenoid coil 174 kept denergized, the plunger 
180 is positioned to have its leading end located slightly ahead of the 
valve 168 and, thus, allows the valve 168 to close the air inlet port 166 
under the preesure of the preload spring 170 acting on the valve. When, 
however, the solenoid core 178 is moved into the position closest to the 
air inlet port 166, the plunger 180 is brought into pressing engagement at 
its leading end with the valve 168 and moves the valve 168 from the first 
position thereof toward the second position opening the air inlet port 166 
against the opposing force of the preload spring 170, providing 
unrestricted communication between the air inlet passageway 96 and the air 
chamber 40 through the third air inlet port 166. The input terminal lead 
176 of the solenoid coil 174 is connected across the previously mentioned 
switch 84 to the d.c. power source 86 and the output terminal lead 176' of 
the coil 174 is connected to ground across a diaphragm responsive switch 
unit 182 which is mounted on the housing 138 of the differential pressure 
assembly 136. 
The housing 138 of the differential-pressure assembly 136 has formed in its 
wall portion 138a an aperture 184 which is located in alignment with the 
center axis of the clutch actuating rod 154 extending in a direction 
opposite to the aperture 184. The diaphragm responsive switch unit 182 
comprises a switch casing 186 fixedly mounted on the outer face of the 
wall portion 138a of the housing 138 in such a manner as to contain 
therewithin the above mentioned aperture 184. The casing 186 has supported 
therewithin a set of normally-open contacts 188 located at a suitable 
spacing from the outer end of the aperture 184 in the wall portion 138a. 
The contacts 188 are electrically connected between ground and the output 
terminal lead 176' of the solenoid coil 174 and are adapted to close when 
depressed. A switch actuating element 190 has a presser portion 190a 
movable between the outer end of the aperture 184 and the set of contacts 
188 and a plunger portion 190b projecting through the aperture 184 in the 
wall portion 138a of the housing 138 into the variable-volume chamber 142 
of the differential-pressure assembly 136 in alignment with the center 
axis of the clutch actuating rod 154 connected to the diaphgram 140. The 
switch actuating element 190 is urged to be spaced apart from the set of 
contacts 188 and has its presser portion 190a forced against the outer 
face of the wall portion 138a of the housing 138 and its plunger portion 
190b projecting deepest into the variable-volume chamber 142 by means of a 
preload spring 192 which is mounted within the switch casing 186, as 
shown. The switch actuating element 190 in its entirety is, thus, axially 
movable in a direction aligned with the direction of movement of the 
clutch actuating rod 154 between a first position having the presser 
portion 190a disengaged from the set of contacts 188 and the plunger 
position 190b projecting with a maximum length into the variable-volume 
chamber 142 through the aperture 184 and a second position having the 
presser portion 190a in pressing engagement with the set of contacts 188 
and the plunger portion 190b projecting with a minimum length into the 
variable-volume chamber 142 through the aperture 184. The distance of 
stroke of the switch actuating element 190 thus moved between the first 
and second positions is represented by d in FIG. 5. The preload spring 192 
biases the switch actuating element 190 toward the above described second 
position thereof. 
When, now, the transmission gear shift lever 22 is being manipulated to 
make a shift in the transmission and at the same time the accelerator 
pedal 24 is kept released, both the gear shift lever responsive switch 88 
and the accelerator pedal responsive switch 90 are closed and energize the 
solenoid coil 76 of the valve actuating unit 74 for the two-position valve 
64 in the casing 36. The solenoid core 80 and accordingly the plunger 82 
of the valve actuating unit 74 are therefore moved forward and move the 
two-position valve 64 into the position providing communication between 
the vacuum chamber 38 and the valve chamber 42 through the opening in the 
valve seat member 54 between the vacuum and valve chambers 38 and 42, as 
previously described. Vacuun is thus developed in the variable-volume 
chamber 142 of the differential-pressure assembly 136 through the valve 
chamber 42 and the passageway 150 and acts on the diaphragm 140 in the 
assembly 136. The diaphragm 140 is consequently moved toward the wall 
portion 138a of the housing 138 against the opposing force of the preload 
spring 148 and causes the clutch 10 to progressively disengaged through 
the clutch actuating rod 154 and the mechanical linkage 156. Under these 
conditions, communication between the air chamber 40 and the valve chamber 
42 is blocked by the two-position valve 64 seated on the valve seat wall 
portion 62 of the casing 36 and, simultaneously, the third air inlet port 
166 is closed by the two-position valve 168 by the action of the preload 
spring 170 and with the solenoid operated valve actuating unit 172 held 
inoperative. As the diaphragm 140 is moved closer to the inner face of the 
wall portion 138a of the housing 138, the diaphragm 140 or, more exactly, 
the inner end of the clutch actuating rod 154 as shown is brought into 
pressing contact with the leading end of the plunger portion 190a of the 
switch actuating element 190 held by the preload spring 192 in the first 
position thereof having the plunger portion 190b projecting with the 
maximum length into the variable-volume chamber 142. As the diaphragm 140 
is further moved toward the inner face of the wall portion 138, the switch 
actuating element 190 which is thus moved in its entirety toward the set 
of contacts 188 is brought into actuating engagement with the contact set 
188. The contact set 188 is now closed and energizes the solenoid coil 174 
of the valve actuating unit 172. This causes the solenoid core 178 of the 
valve actuating unit 172 to axially move toward the third air inlet port 
166 of the air chamber 40 and brings the plunger 180 into pressing 
engagement with the two-position valve 168. The two-position valve 168 is 
moved by the plunger 180 away from the third air inlet port 166 and thus 
allows the port 166 to open, thereby providing unrestricted air 
communication between the air chamer 40 and the air inlet passageway 96 
through he air inlet port 166. Atmospheric air is therefore admitted into 
the air chamber 40 through the third air inlet port 166 but is not passed 
into the variable-volume chamber 142 of the differential-pressure assembly 
136 because the passageway 60 between the air and valve chambers 40 and 42 
is closed by the two-position valve 64 with the solenoid operated valve 
actuating unit 74 kept operative. When the diaphragm 140 reaches the 
position providing the minimum volume condtion of the variable-volume 
chamber 142, the switch actuating element 190 which has been moved the 
distance d from the initial position reaches the second position having 
the plunger portion 190b projecting with the minimum length into the 
variable-volume chamber 142 through the aperture 184. The axial 
displacement of the clutch actuating rod 154 following the movement of the 
diaphragm 140 as above described is indicated by section C.sub.1 -C.sub.2 
of the plot shown in FIG. 6. 
When the shift is complete in the transmission and the transmission gear 
shift lever 22 is released from the manipulative effort which has been 
applied thereto, the gear shift responsive switch 88 becomes open as at 
time s.sub.1 and causes the solenoid coil 76 of the valve actuating unit 
74 to be de-energized. The two-position valve 64 in the valve chamber 42 
is therefore moved back by the preload spring 62 into the position 
blocking the vacuum communication between the vacuum and valve chambers 38 
and 42 and providing air communication between the air and valve chambers 
40 and 42 through the passageway 60 therebetween. Atmospheric air which 
has been directed into the air chamber 40 is now directed into the 
variable-volume chamber 142 of the differential-pressure assembly through 
the passageway 60, valve chamber 42 and passageway 150. Because, however, 
of the frictional forces in play in the clutch 10 and the mechanical 
linkage 156 and because the delay in the development of an atmospheric 
pressure in the variable-volume chamber 142, the diaphragm 140 is 
initiated into motion to move away from the position providing the minimum 
volume condition of the chamber at time s.sub.2 which is later a certain 
period of time S.sub.1 than the time s.sub.1 at which the valve 168 is 
initially moved to open the air inlet port 166. If, in this instance, the 
vehicle is crusing at a speed lower than the predetermined level vo set 
for the vehicle speed responsive switch 118 and if the accelerator pedal 
24 is kept released, atmospheric air is admitted into the air chamber 40 
through the third air inlet port 166 alone. The diaphragm 140 and 
accordingly the clutch actuating rod 154 are moved after times s.sub.2 at 
a rate dictated by the rate of flow of air through the air inlet port 166 
into the air chamber 40 as indicated by section C.sub.3 -C.sub.4 of the 
plot shown in FIG. 6. When the diaphragm 140 is moved a distance d from 
the position providing the minimum volume condition of the variable-volume 
chamber 142, the switch actuating element 190 is released from the 
pressing force of the diaphragm 140 and is forced to disengage from the 
contact set 188, which is consequently allowed to open. The solenoid coil 
174 of the valve actuating unit 172 is now de-energized and allows the 
two-position valve 168 to close the third air inlet port 166 at time 
s.sub.3 upon lapse of a certain period of time S.sub.d after the time 
s.sub.2. The time period S.sub.d is determined by the distance of stroke d 
of the switch actuating element 190. The switch unit 182 is preferably 
arranged so that the distance of stroke d is such that will enable the 
clutch actuating rod 154 to assume a position immediately prior to the 
coupling point CP thereof, as will be seen from FIG. 6. 
When the third air inlet port 166 is closed, there is no supply of air into 
the air chamber with the first and second air inlet ports 92 and 94 kept 
closed. The clutch actuating rod 154 is therefore "locked" in the position 
immediately prior to the coupling point CP thereof as indicated by line 
C.sub.4 -C.sub.4, in FIG. 6 so that the clutch 10 can not be brought into 
engagement unless the accelerator pedal 24 is depressed to open the second 
air inlet port 94. The vehicle can be held at a halt with the transmission 
maintained in meshed condition. Because, furthermore, the clutch 10 is 
locked in a condition about to engage, the clutch 10 can be engaged almost 
instantaneously when the acclerator pedal 24 is depressed to start the 
vehicle from a halt. 
If the vehicle is cruising at a speed higher than the predetermined level 
vo when the third air inlet port 166 is closed at time s.sub.3, 
atmospheric air is admitted into the air chamber 40 only through the first 
air inlet port 92 at a fixed rate H so that the clutch actuating rod 154 
is driven to move at a reduced rate dictated by the particular air flow 
rate H after time s.sub.3, as indicated by section C.sub.4 -C.sub.5 of the 
plot shown in FIG. 7. Upon lapse of a certain period of time after time 
s.sub.3, viz., period of time S.sub.2 after the time s.sub.2 at which the 
actuating rod 154 was allowed to start, the actuating rod 154 reaches the 
coupling point CP at time s.sub.4 and brings the clutch 10 into 
engagement, enabling the clutch to transmit torque at a limited rate. The 
torque thus transmitted through the clutch 10 with the actuating rod 154 
being moved within the partial torque transmission range increases at a 
fixed rate which is dictated by the air flow rate H through the first air 
inlet port 92 as will be seen from section C.sub.t -C.sub.r of the plot 
shown by broken lines in FIG. 7. In a period of time S.sub.3 after the 
time s.sub.4 at which the actuating rod 154 reached the coupling point CP, 
the actuating rod 154 reaches a full engagement position FEP at time 
s.sub.5 and enables the clutch 10 to transmit torque at a 100 percent 
efficiency. Because of the allowance provided for the actuating rod 154 to 
slightly move beyond the full engagement position FEP, the actuating rod 
154 continues to move through a surplus displacement range C.sub.6 
-C.sub.7 and ceases to move at time s.sub.6, assuring the clutch 10 to be 
engaged in a completely slip-free condition. The rate of displacement of 
the actuating rod 154 through the partial torque transmission range can be 
adjusted by varying the rate of flow H of air through the first air inlet 
port 92. 
When the acclerator pedal 24 is depressed from the released position to 
start the vehicle from a halt with the clutch 10 held in a condition bout 
to engage, the second air inlet port 94 becomes open and allows air to 
enter the air chamber 40 at a rate which increases as the accelerator 
pedal 24 is depressed deeper. The clutch actuating rod 154 is therefore 
moved and consequently the clutch 10 is brought into engagement and driven 
toward the fully engaged condition at higher rates as the accelerator 
pedal 24 is depressed deeper from the released position, as will be 
understood from plots R.sub.1, R.sub.2 and R.sub.3. If, in this instance, 
the vehicle speed is higher than the predetermined level vo so that not 
only the second air inlet port 94 but the first air inlet port 92 is open, 
the clutch 10 will be driven toward the fully engaged condition at a 
further increased rate. 
In each of the embodiments hereinbefore described with reference to FIGS. 1 
and 5, the transmission gear shift lever and accelerator pedal responsive 
switches 88 and 90 are connected in series with the solenoid coil 76 of 
the valve actuating unit 74. Thus, even when the switch 88 is closed with 
the transmission gear shift lever 22 released from a manipulative effort, 
the clutch 10 can not be driven to disengage unless the accelerator pedal 
24 is released to make the switch 90 closed. When, therefore, the 
accelerator pedal 24 is depressed with the shift lever responsive switch 
88 kept closed, the clutch 10 is engaged automatically so that the engine 
can be prevented from operating at an excessively high speed. If, however, 
the solenoid coil 76 is connected to the power source 88 only through the 
gear shift responsive switch 88, there will be a risk that the engine 
operates at an excessively high speed if the accelerator pedal 24 is 
depressed from the released position with the gear shift lever switch is 
closed and accordingly the clutch is held disengaged. 
The predetermined level vo of vehicle speed as set for the vehicle speed 
responsive switch 118 should be of a value within a relatively low range 
for the purpose of providing a relatively wide range enabling the engine 
to be braked upon by the inertia of the vehicle. For this reason, the 
switch 118 may be arranged to close in response to a vehicle speed lower 
than about 10 miles per hour, for example.