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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hydraulic pressure control device for an
automatic transmission system, and more particularly relates to such a
hydraulic pressure control device which is particularly suitable for a
twin type automatic transmission system which incorporates first and
second transmission mechanisms arranged in series in the rotational force
transmission path.
2. Description of the Prior Art
In the prior art, there have been proposed various types of automatic
transmission systems which incorporate first and second transmission
mechanisms arranged in series in the rotational force transmission path,
i.e. with the second transmission mechanism receiving the output of the
first transmission mechanism and further transmitting it, particularly in
Japanese Patent Laying Open Publications Ser. Nos. 56-138020 (1981) and
58-128929 (1983), neither of which is it intended hereby to admit as prior
art to the present patent application except to the extent in any case
required by applicable law.
Conventionally, in such a twin type transmission system in which the first
and the second transmission mechanisms are both hydraulic type
transmissions such as gear transmission mechanisms which are set to their
various speed stages by selective supply of actuating hydraulic fluid
pressures to various friction actuating mechanisms incorporated in them
such as hydraulic clutches and hydraulic brakes, the actuating pressures
which are thus selectively supplied to both the first transmission
mechanism and also the second transmission mechanism are provided by
switching the same line pressure by appropriate control mechanisms such as
hydraulic valves. In other words, the two hydraulic fluid pressure control
devices for the first and the second transmission mechanisms are
conventionally both provided with the same line pressure from one and the
same line pressure control valve.
There is however a problem with such a twin type transmission system, in
that, in the typical case that the second transmission mechanism is
provided in the power transmission path after the first transmission
mechanism and thus receives a supply of rotational power from said first
transmission mechanism, the torque acting upon the friction engaging
mechanisms such as the hydraulic clutches and the hydraulic brakes of said
second transmission mechanism is not only determined by the load on the
vehicle engine, but also fluctuates depending on the engaged speed stages
both of the first transmission mechanism and also of the second
transmission mechanism. In other words, even if the engine load is
substantially constant, when the first transmission mechanism is set to a
relatively low speed stage, the torque on the friction engaging mechanisms
such as the hydraulic clutches and the hydraulic brakes of said second
transmission mechanism is higher than when said first transmission
mechanism is set to a relatively high speed stage. Accordingly, in the
above outlined conventional case that the supply of line hydraulic fluid
pressure for being switched for providing the actuating hydraulic fluid
pressures for the various friction engaging mechanisms of said second
transmission mechanism is the same supply as the supply of line hydraulic
fluid pressure for being switched for providing the actuating hydraulic
fluid pressures for the various friction engaging mechanisms of said first
transmission mechanism, and in the conventional case that the pressure
value of said line pressure does not vary according to engagement of the
speed stages of either of said transmission mechanisms (although it may
vary with engine load), the problem arises that the supply of line
hydraulic fluid pressure for said second transmission mechanism is
sometimes unsuitable, even if it is appropriate for said first
transmission mechanism. This can lead to slippage of said friction
engaging mechanisms, thus deteriorating the performance of the
transmission, reducing its reliability, and shortening its service life.
As a means of compensating for this problem, it has been conceived of to
appropriately increase the number of plates in the friction engaging
mechanisms of the second transmission mechanism, in order to increase the
torque transmission capability thereof. This can certainly remove all
problems relating to shortage of torque transmission capability in said
friction engaging mechanisms of the second transmission mechanism, but at
the price of increasing the size and weight and cost of said second
transmission mechanism. Such a concept is accordingly not very
economically feasible.
SUMMARY OF THE INVENTION
The inventors of the present invention have considered the various problems
detailed above, from the point of view of the desirability of providing
appropriate actuating pressures for both the first transmission mechanism
and also the second transmission mechanism of such a twin type
transmission system.
Accordingly, it is the primary object of the present invention to provide a
hydraulic pressure control device for an twin type automatic transmission
system incorporating first and second transmission mechanisms arranged in
series in the power transmission path, which avoids the problems detailed
above.
It is a further object of the present invention to provide such a hydraulic
pressure control device for such an automatic transmission system, which
prevents any shortage of actuating hydraulic fluid pressure for the
various friction engaging devices of the first and second transmission
mechanisms.
It is a further object of the present invention to provide such a hydraulic
pressure control device for such an automatic transmission system, which
always supplies appropriate and adequate actuating pressures to said
friction engaging devices of said first and second transmission
mechanisms.
It is a further object of the present invention to provide such a hydraulic
pressure control device for such an automatic transmission system, which
ensures against slippage of said friction engaging devices of said first
and second transmission mechanisms.
It is a further object of the present invention to provide such a hydraulic
pressure control device for such an automatic transmission system, which
maximizes transmission reliability.
It is a further object of the present invention to provide such a hydraulic
pressure control device for such an automatic transmission system, which
maximizes transmission service life.
It is a further object of the present invention to provide such a hydraulic
pressure control device for such an automatic transmission system, which
does not require the friction engaging devices of the second transmission
mechanism to be made as unduly large or bulky or weighty.
It is a further object of the present invention to provide such a hydraulic
pressure control device for such an automatic transmission system, which
does not require any extra plates to be provided in the friction engaging
devices of the second transmission mechanism.
It is a yet further object of the present invention to provide such a
hydraulic pressure control device for such an automatic transmission
system, which minimizes cost of component parts.
It is a yet further object of the present invention to provide such a
hydraulic pressure control device for such an automatic transmission
system, which minimizes cost of assembly.
It is a yet further object of the present invention to provide such a
hydraulic pressure control device for such an automatic transmission
system, which allows said transmission system to be made compact and light
in weight.
According to the most general aspect of the present invention, these and
other objects are attained by, for a transmission system for a vehicle
incorporating an engine, comprising, in series in the specified order in
the rotational force transmission path, a first transmission mechanism
which can be set to any one of a plurality of speed stages according to
selective supply of hydraulic fluid pressures, and a second transmission
mechanism which similarly can be set to any one of a plurality of speed
stages according to selective supply of hydraulic fluid pressures: a
hydraulic pressure transmission control device, comprising: a first
hydraulic fluid pressure control device, comprising a first line pressure
control valve for supplying said first hydraulic fluid pressure control
device with a first line pressure for being then selectively supplied by
said first hydraulic fluid pressure control device to said first
transmission mechanism as its said selective supply of hydraulic fluid
pressures; and: a second hydraulic fluid pressure control device,
comprising a second line pressure control valve for supplying said second
hydraulic fluid pressure control device with a second line pressure for
being then selectively supplied by said second hydraulic fluid pressure
control device to said second transmission mechanism as its said selective
supply of hydraulic fluid pressures.
According to such a hydraulic pressure transmission control device as
specified above, since the line pressures for the first and the second
transmission mechanisms are produced from the separate first and second
line pressure control valves, therefore they can be set appropriately for
each of said first and second transmission mechanisms without any
compromise having to be made between them, and accordingly this hydraulic
pressure control device can prevent any shortage of actuating hydraulic
fluid pressure for the various friction engaging devices of the first and
second transmission mechanisms, and can always supply appropriate and
adequate actuating pressures to said friction engaging devices of said
first and second transmission mechanisms. Thus, slippage of said friction
engaging devices of said first and second transmission mechanisms is
ensured against, and transmission reliability and transmission service
life are maximized. Further, this is done without requiring the friction
engaging devices of the second transmission mechanism to be made unduly
large or bulky or weighty, i.e. without requiring any extra plates to be
provided in the friction engaging devices of said second transmission
mechanism. Accordingly, the cost of component parts is minimized, as is
the cost of assembly, and said transmission system is allowed to be made
compact and light in weight.
According to a particular specialization of the present invention, the
above and other objects may more particularly be accomplished by such a
hydraulic pressure transmission control device as first specified above,
wherein said second line pressure control valve of said second hydraulic
fluid pressure control device comprises means for increasing the line
pressure which it supplies, the lower is the speed stage of said first
transmission mechanism which is engaged. This is an appropriate way of
altering the line pressure for the second transmission mechanism.
Alternatively or concurrently, according to another particular
specialization of the present invention, the above and other objects may
more particularly be accomplished by such a hydraulic pressure
transmission control device as thus specified above, wherein said second
line pressure control valve of said second hydraulic fluid pressure
control device comprises means for increasing the line pressure which it
supplies, the lower is the speed stage of said second transmission
mechanism which is engaged. This also can be an appropriate manner of line
pressure alteration. Finally, alternatively or concurrently, according to
another particular specialization of the present invention, the above and
other objects may more particularly be accomplished by such a hydraulic
pressure transmission control device as thus specified above, wherein said
second line pressure control valve of said second hydraulic fluid pressure
control device comprises means for increasing the line pressure which it
supplies, the higher is the value of a parameter representing the load on
said engine.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described with respect to the preferred
embodiment thereof, and with reference to the illustrative drawings
appended hereto, which however are provided for the purposes of
explanation and exemplification only, and are not intended to be
limitative of the scope of the present invention in any way, since this
scope is to be delimited solely by the accompanying claims. With relation
to the figures, spatial terms are to be understood as referring only to
the orientation on the drawing paper of the illustrations of the relevant
parts, unless otherwise specified; like reference numerals, unless
otherwise so specified, denote the same parts and gaps and spaces and so
on in the various figures; and:
FIG. 1 is a schematic skeleton diagram showing a vehicle four wheel drive
transmission system which comprises an automatic transmission device and a
two wheel drive / four wheel drive switchover device with a control
clutch, and which incorporates the preferred embodiment of the hydraulic
pressure transmission control device of the present invention;
FIG. 2 shows a four wheel drive line hydraulic fluid pressure control valve
in detailed schematic longitudinal sectional view and a main transmission
line hydraulic fluid pressure control valve and a hydraulic control
circuit for an auxiliary gear transmission mechanism both in block
diagrammatical view, all said elements being incorporated in the FIG. 1
preferred embodiment hydraulic pressure transmission control device of the
present invention;
FIG. 3 is a graph, in which throttle pressure is shown along the horizontal
axis and line pressure is shown along the vertical axis, for explanation
of the behavior of an auxiliary transmission line pressure with respect to
change of a throttle pressure, for a fixed value of a control pressure, in
the two cases of said auxiliary gear transmission mechanism being in its
speed reduction speed stage as shown by a dashed line, and being its
directly connected speed stage as shown by a solid line;
FIG. 4 is another graph, in which duty ratio is shown along the horizontal
axis and said control pressure is shown along the vertical axis, showing
an exemplary behavior of variation of said control pressure with the duty
ratio of a pulse electrical signal supplied to a coil of an
electromagnetic solenoid valve; and:
FIG. 5 is a flow chart for explaining a fragment of the flow of a program
stored in and obeyed by a micro computer incorporated in a electrical
control device which cooperates with the preferred embodiment of the
hydraulic pressure transmission control device of the present invention,
as comprised in the vehicle four wheel drive transmission system shown in
FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention will now be described with reference to the preferred
embodiment thereof, and with reference to FIGS. 1 through 5.
OVERALL TRANSMISSION SYSTEM CONSTRUCTION
FIG. 1 shows in schematic part block diagrammatical view a vehicle four
wheel drive transmission system, as well as an internal combustion engine
denoted by the reference numeral 100 of an automotive vehicle not
otherwise shown. This engine 100 is mounted longitudinally in an engine
room formed in the body of said automotive vehicle, the front of said
vehicle being to the left as seen in the figure and the rear of said
vehicle being to the right, and the aforesaid four wheel drive
transmission system comprises, in series in the specified order from the
engine 100: an automatic transmission device 1, provided as directly
coupled to and behind said engine 100 and incorporating a torque converter
2 of a per se known type whose rotational power input shaft 4 is
rotationally driven from the crank shaft of said engine 100, and a main
gear transmission mechanism 3 whose rotational power input shaft is
rotationally driven from the rotation power output shaft of said torque
converter 2; and a four wheel drive device 15, similarly provided as
directly coupled to and behind said automatic transmission device 1, with
its rotational power input shaft being rotationally driven from the
rotational power output shaft of said main gear transmission mechanism 3.
As will be seen later, this four wheel drive device 15 comprises a two
stage auxiliary gear transmission mechanism 16 and a two wheel drive /
four wheel drive (hereinafter abbreviated as "2WD/4 WD") switchover device
20.
In more detail, the rotational power produced by the internal combustion
engine 100 is transmitted through the torque converter 2, with a certain
degree of slippage and torque amplification being applied thereto as is
per se conventional, to the gear transmission mechanism 3. This gear
transmission mechanism 3 is of a per se conventional type, being settable
to provide a plurality, exemplarily four in this preferred embodiment, of
forward speed stages, i.e. values of speed reduction and torque
amplification, and one reverse speed stage between its rotational power
input member and its rotational power output member, according to control
of said gear transmission mechanism 3 provided by selective supply of
hydraulic fluid pressures from a main transmission hydraulic control
device 10. Typically, this gear transmission mechanism 3 may incorporate
various selectively engagable friction engaging mechanisms such as
clutches and brakes, supply of said hydraulic fluid pressures to selective
ones of which engages them for providing the various speed stages, and
planetary gear mechanisms or the like for providing various different
degrees of rotational power reduction and amplification gearing; the
details of these matters will not be particularly discussed herein.
The four wheel drive device 15 incorporates a planetary gear wheel type two
stage auxiliary gear transmission mechanism 16 for providing either a
directly connected speed stage or a speed reduction speed stage. This
auxiliary gear transmission mechanism 16 may, as schematically shown in
the figure, incorporate a sun gear, a ring gear, a plurality of planetary
pinions fitted between said sun gear and said ring gear and meshed with
both of them and performing planetary motion between them in a per se
known manner, and a carrier to which said planetary pinions are
rotationally mounted and which is rotationally coupled to the
aforementioned rotational power output shaft 19 of this auxiliary gear
transmission mechanism 16 so as to be rotationally driven thereby.
Selectively, either the ring gear may be rotationally connected to the
casing of this auxiliary gear transmission mechanism 16 by a brake 18, or
the carrier may be connected to the sun gear by a clutch 17; this clutch
17 and this brake 18 are selectively operated by selective supply of
hydraulic fluid pressures from a four wheel drive hydraulic control device
12, as will be described shortly. Thereby, the auxiliary gear transmission
mechanism 16 is caused to provide either a directly connected speed stage
or a speed reduction speed stage.
The rotational power output shaft of the auxiliary gear transmission
mechanism 16 is connected to the rotational power input member of the
2WD/4WD switchover device 20. This 2WD/4WD switchover device 20 has a rear
wheel drive output shaft 22 for driving the rear wheels of the vehicle,
the forward end of which is always connected to the aforesaid rotational
power input member of this 2WD/4WD switchover device 20; and the other end
of this rear wheel drive shaft 22 is connected, via a universal joint
device or the like and via a propeller shaft to a differential device
(none of these devices are particularly shown) for distributing rotational
power between the rear wheels (also not shown) of the vehicle. On the
other hand, the aforesaid rotational power input member 19 of this 2WD/4WD
switchover device 20 is also selectively rotationally connected, via a
selectively engagable hydraulic clutch 28, a drive sprocket 24 connected
to the rotational power output member of said clutch 28, an endless drive
chain 26 which picks up drive from said drive sprocket 24, and a driven
sprocket 25 which receives drive from said endless drive chain 26, to a
front wheel drive shaft 23. This front wheel drive shaft 23 is connected,
via another universal joint device or the like and a front propeller
shaft, to a differential device (none of these devices are particularly
shown) for distributing rotational power between the front wheels (also
not shown) of the vehicle. This front wheel drive shaft 23 extends
forwards along and underneath the four wheel drive device 15 and the
automatic transmission device 1 generally longitudinally to the vehicle
body. Thereby, according to control of the hydraulic clutch 28, the
2WD/4WD switchover device 20 can either supply the rotational power which
it receives via the rotational power output shaft 19 only to the rear
wheels of the vehicle, or both to the front and to the rear wheels of the
vehicle. This hydraulic clutch 28 is engaged by selective supply of
hydraulic fluid pressure from the aforementioned four wheel drive
hydraulic control device 12, and its engagement pressure and accordingly
the maximum torque value which it can transmit without slipping
substantially increase along with increase of said supplied hydraulic
fluid pressure.
The main transmission hydraulic control device 10 incorporates a main
transmission line hydraulic fluid pressure control valve 11, which is of a
per se known construction, and, as can be seen more particularly with
reference to FIG. 2 which will be explained hereinafter, receives a supply
of pressurized hydraulic fluid from a hydraulic fluid pump 41 which picks
up said hydraulic fluid from a sump 40, and bleeds a certain amount of
said pressurized hydraulic fluid back to the sump 40 so as to regulate the
pressure of the remainder thereof to a main transmission line pressure
value which is determined according to the current value of the throttle
opening of the engine 1, which is taken as representative of the current
value of the load on said engine 1. This main transmission line pressure
is transmitted by the main transmission hydraulic control device 10 to
selective combinations of the hydraulic fluid pressure friction engaging
means such as hydraulic clutches and hydraulic brakes incorporated in the
main gear transmission mechanism 3, for controlling said main gear
transmission mechanism 3 to be engaged to one or the other of its various
speed stages, according to control signals which said main transmission
hydraulic control device 10 receives from an electrical control device 30
which will be described hereinafter.
Further, the four wheel drive hydraulic control device 12 incorporates a
four wheel drive line hydraulic fluid pressure control valve 42, which can
be seen more particularly with reference to FIG. 2 which will be explained
hereinafter, and which receives supply of pressurized hydraulic fluid from
said hydraulic fluid pump 41 which picks up said hydraulic fluid from said
sump 40, and bleeds a certain amount of said pressurized hydraulic fluid
back to said sump 40 so as to regulate the pressure of the remainder
thereof to a four wheel drive line pressure value which is determined
according to the current value of the throttle opening of the engine 1,
which is taken as representative of the current value of the load on said
engine 1. This four wheel drive line pressure is transmitted by the four
wheel drive hydraulic control device 12 either to the clutch 17 but not to
the brake 18, or alternatively to the brake 18 but not to the clutch 17,
for controlling the auxiliary gear transmission mechanism 16 to be engaged
either to its directly connected speed stage or to its speed reducing
speed stage, according to control signals which this four wheel drive
hydraulic control device 12 receives from an electrical control device 30
which will be described hereinafter. In fact, the electrical control
device 30 selectively engages one or the other of said clutch 17 and said
brake 18 according to the set position of a low/high selection switch 34,
which is mounted in the passenger compartment of the vehicle so as to be
accessed by the driver to be set, and which dispatches an electrical
output signal representative of its said set position to said electrical
control device 30. Also, this four wheel drive line pressure is
transmitted by said four wheel drive hydraulic control device 12 either to
engage the hydraulic clutch 28 of the 2WD/4WD switchover device 20, or to
release said hydraulic clutch 28, according again to a control signal
which the four wheel drive hydraulic control device 12 receives from said
electrical control device 30 which will now be described.
The operations of the main transmission hydraulic control device 10 for the
main gear transmission mechanism 3 and of the four wheel drive hydraulic
control device 12 for the four wheel drive device 15 are controlled by a
electrical control device 30 for the transmission system as a whole, which
dispatches control signals (which typically may be electrical signals) to
said hydraulic control devices 10 and 12. The full structure and
functioning of this electrical control device 30 for controlling the
hydraulic control devices 10 and 12 and thereby controlling the main gear
transmission mechanism 3 and the four wheel drive device 15 will not be
described herein, since they are not directly germane to the present
invention, and since the details thereof could vary over any of a wide
variety of possibilities. This electrical control device 30 receives: a
signal representative of the current value of the throttle opening of the
internal combustion engine 100 from an engine throttle opening sensor 31
mounted to said engine 100; a signal representative of the current value
of the road speed of the vehicle (in fact a signal representing the
rotational speed of the rear wheel drive output shaft 22) from a vehicle
road speed sensor 32; a signal representative of the current set position
of a manually operated transmission range setting level (not particularly
shown) which is mounted in the passenger compartment of the vehicle so as
to be accessed by the driver to be set for setting transmission operating
range (such as "P" or parking range, "R" or reverse range, "N" or neutral
range, "D" or drive range, or "L" or low range) from a manual shift range
detection sensor 33; as previously mentioned, a signal representative of
the current set position of the manually operated low/high selection
switch 34 mounted in the passenger compartment of the vehicle; and a
signal representative of the current set position of a manually operated
4WD/2WD selection switch 35 mounted in the passenger compartment of the
vehicle. Exemplarily and typically although this is not mandatory for the
present invention, the electrical control device 30 controls the engaged
speed stage of the gear transmission mechanism 3 according to the current
values of vehicle road speed, engine throttle opening, and transmission
set range, by following certain internally stored data such as shift range
diagrams and so on. And, exemplarily and typically although this is not
mandatory for the present invention, this electrical control device 30
incorporates a microcomputer which obeys a control program, and
incorporates various A/D and D/A converters which supply data to and
output data from said microcomputer; this preferred construction for the
electrical control device 30 will be assumed in the following.
Now, a particular exemplary construction for the four wheel drive line
hydraulic fluid pressure control valve 42 incorporated in the four wheel
drive hydraulic control device 12, as utilized in this preferred
embodiment of the hydraulic pressure control device of the present
invention, will be described with reference to FIG. 2 which shows a
schematic longitudinal sectional view thereof, also shows the main
transmission line hydraulic fluid pressure control valve 11 for the main
transmission hydraulic control device 10 in block diagrammatical form, and
further shows a hydraulic control circuit 70 for controlling the auxiliary
gear transmission mechanism 16 and the 2WD/4WD switchover device 20 also
in block diagrammatical form.
In detail, therefore, the oil pump 41 picks up hydraulic fluid from the
sump 40 and compresses it, then supplying the thus compressed hydraulic
fluid to the conduit 71. The conduit 71 supplies this compressed hydraulic
fluid to the previously described (and per se conventional) main
transmission line hydraulic fluid pressure control valve 11 for the main
transmission hydraulic control device 10, and also supplies said
compressed hydraulic fluid to an input port 46 of the four wheel drive
line hydraulic fluid pressure control valve 42.
This four wheel drive line hydraulic fluid pressure control valve 42 is
structured as a spool valve which has a bore formed in a housing. In the
upper portion of said bore as seen in FIG. 2 there is slidably fitted a
pressure control valve spool element 43, while in the lower portion of
said bore as seen in FIG. 2 there is slidably fitted a line pressure
adjustment valve spool element 44; said valve spool elements 43 and 44
thus being coaxial. And between said valve spool elements 43 and 44 there
is fitted a compression coil spring 55, which thus biases said valve spool
elements 43 and 44 away from one another in the valve bore, i.e. biases
the pressure control valve spool element 43 in the upward direction as
seen in FIG. 2 and biases the line pressure adjustment valve spool element
44 in the downward direction as seen in FIG. 2.
The pressure control valve spool element 43 is the one that actually
performs the regulation of line pressure. As shown in FIG. 2, said
pressure control valve spool element 43 is formed with three lands denoted
as 90, 91, and 92 in the figure, and the portion of the valve bore in
which said pressure control valve spool element 43 slides is formed with
five ports: the previously mentioned input port 46, a drain port 47, a
control port 51, and two output ports 48 and 49. In detail, the input
pressure from the intermediate point of the conduit 71 is supplied to the
input port 46 between the lands 91 and 92 of the pressure control valve
spool element 43, and, if the land 91 is low enough in the valve bore to
clear the lower lip in the figure of the barrier portion between this
input port 46 and the drain port 47, some of said input pressure is let
past said lower lip to be drained through said drain port 47. In any
event, the pressure present around the cutaway of the pressure control
valve spool element 43 between its lands 91 and 92 is taken out from the
output port 48 via a throttling element 93, and is supplied as an output
regulated line pressure to the output conduit 72, as well as being fed
back to the control port 51, which opens to a pressure chamber defined at
the top of the four wheel drive line hydraulic fluid pressure control
valve 42 between its end and the upper end in the figure of the pressure
control valve spool element 43. Thereby, as will be understood by one of
ordinary skill in the transmission valve art based upon these explanations
and the figure, the pressure control valve spool element 43 oscillates to
and fro about an intermediate position, so as to regulate the pressure in
the output conduit 72 to an output line pressure value which is determined
by the biasing force supplied to the lower end of said pressure control
valve spool element 43, i.e. by the biasing force exerted by the
compression coil spring 55. The other output port 49 is provided for, if
the pressure control valve spool element 43 should become very far
displaced in the downwards direction in the valve bore, venting the
inputted pressure to the conduit 72 by bypassing the throttling element
93, thus avoiding that the pressure control valve spool element 43 should
be even further driven downward in the valve bore. This output line
pressure in the conduit 72 is supplied to the hydraulic control circuit 70
for controlling the auxiliary gear transmission mechanism 16 and the
2WD/4WD switchover device 20.
Now, the biasing force exerted by the compression coil spring 55, which as
above explained determines the output line pressure value which is
generated in the output conduit 72, is determined by the degree of
compression of said compression coil spring 55, which depends upon the
axial position of its lower extremity which is supported by the line
pressure adjustment valve spool element 44. This axial position of the
lower extremity of the compression coil spring 55 is in turn determined by
a balance between the upward force exerted by the line pressure adjustment
valve spool element 44 on said lower extremity of said compression coil
spring 55 and the biasing force of said compression coil spring 55.
As shown in FIG. 2, said line pressure adjustment valve spool element 44 is
formed with three lands denoted as 95, 96, and 97 in the figure, and the
portion of the valve bore in which said line pressure adjustment valve
spool element 44 slides is formed with three input ports: a throttle
pressure port 52 opening to a throttle pressure chamber defined at the
lower end in the figure of the line pressure adjustment valve spool
element 44 between its land 97 and the end of the valve bore; a control
pressure port 54 opening to a control pressure chamber defined at a lower
intermediate point of the line pressure adjustment valve spool element 44
between its land 97 and its land 96 which is larger in diameter than said
land 97; and a low/high pressure port 53 opening to a low/high pressure
chamber defined at an upper intermediate point of said line pressure
adjustment valve spool element 44 between its land 96 and its land 95
which is in turn larger in diameter than said land 96. In detail, the
throttle pressure produced from a throttle pressure valve, not shown in
the figures but of a per se known type which produces an output pressure
which varies according to the amount of depression of a throttle pedal of
the vehicle incorporating the engine 100--said throttle pedal depression
amount being taken as representative of engine load--is fed to the
throttle pressure port 52, so as to impel the line pressure adjustment
valve spool element 44 in the upward direction with a biasing force
proportional t | | |
