Control system for hydraulic tandem pump in motor vehicle

A control system for a hydraulic tandem pump assembly including a primary pump for supplying fluid under pressure to a power-assisted steering device and a secondary pump for supplying fluid under pressure to a hydraulic motor of an engine cooling fan, which pumps are mounted on a common drive shaft for rotation therewith. The control system includes an electrically operated flow quantity control valve disposed within a communication passage between the secondary pump and the hydraulic motor to bypass fluid under pressure discharged from the secondary pump into an inlet passage connecting a fluid reservoir to the secondary pump, a temperature sensor arranged to detect an ambient temperature of a prime engine of the vehicle for producing therefrom an output signal indicative of the ambient temperature, and an electric control apparatus connected to the sensor to control an electric current applied to the control valve in response to the output signal from the sensor in such a manner that the control valve is conditioned to bypass a large part or the entirety of fluid under pressure discharged from the secondary pump into the inlet passage when the ambient temperature of the engine is below a predetermined value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a hydraulic tandem pump assembly adapted 
for use in motor vehicles, and more particularly to a control system for a 
hydraulic tandem pump assembly of the type which is adapted to supply 
fluid under pressure to a power-assisted steering device and to a 
hydraulic motor of an engine cooling fan. 
2. Discussion of the Background 
A conventional hydraulic tandem pump assembly of this kind includes a 
primary pump for the power-assisted steering device and a secondary pump 
for the hydraulic motor of the engine cooling fan, which pumps are mounted 
on a common drive shaft in drive connection to an output shaft of a prime 
engine of the vehicle. In operation, both the pumps are simultaneously 
driven by rotation of the common drive shaft to supply fluid under 
pressure therefrom to the power-assisted steering device and to the 
hydraulic motor, even in a cold season as well as in a warm season. In the 
cold season, the viscosity of hydraulic fluid increases to cause an 
increase of fluid resistance in pipe lines of the pumps. This results in 
an increase of internal pressure in the pumps, causing power loss of the 
prime engine. As the secondary pump has a large displacement capacity for 
supplying a sufficient amount of fluid under pressure to the hydraulic 
motor of the engine cooling fan, the power loss of the prime engine 
becomes large in operation of the hydraulic motor. When the power 
assisted-steering device is operated to steer the vehicle without driving, 
the pressure in the primary pump increases to render the entirety of the 
tandem pump assembly in a heavy loaded condition. In such a condition, 
there is a fear of causing jam of the pumps. 
SUMMARY OF THE INVENTION 
It is, therefore, a primary object of the present invention to provide a 
control system for the hydraulic tandem pump assembly capable of 
eliminating power loss of the prime engine caused by operation of the 
hydraulic motor of the engine cooling fan at a low temperature. 
A secondary object of the present invention is to provide a control system 
for the hydraulic tandem pump assembly, having the above-described 
characteristics, capable of decreasing the load acting on the secondary 
pump in operation of the power-assisted steering device. 
According to the present invention, the primary object is attained by 
providing a control system for the hydraulic tandem pump assembly which 
comprises an electrically operated flow quantity control valve disposed 
within a communication passage between the secondary pump and the 
hydraulic motor to bypass fluid under pressure discharged from the 
secondary pump to an inlet passage connecting a fluid reservoir to the 
secondary pump, a temperature sensor arranged to detect an ambient 
temperature of a prime engine of the vehicle for producing therefrom an 
output signal indicative of the ambient temperature, and an electric 
control apparatus connected to the temperature sensor to control an 
electric current applied to the flow quantity control valve in response to 
the output signal from the temperature sensor in such a manner that the 
flow quantity control valve is conditioned to bypass a large part or the 
entirety of fluid under pressure discharged from the secondary pump to the 
inlet passage when the ambient temperature of the engine is below a 
predetermined value and conditioned to decrease the bypass flow quantity 
of fluid in accordance with the rise of the ambient temperature of the 
engine higher than the predetermined value. 
To attain the secondary object, the control system may be modified to 
include a pressure sensor arranged to detect hydraulic pressure applied to 
the power-assisted steering device from the primary pump for producing an 
output signal therefrom when subjected to the hydraulic pressure in excess 
of a predetermined value. In the modified control system, the electric 
control apparatus is further arranged to control the electric current 
applied to the flow quantity control valve in response to the output 
signal from the pressure sensor in such a manner that the flow quantity 
control valve is conditioned to bypass a large part or the entirety of 
fluid under pressure discharged from the secondary pump to the inlet 
passage in operation of the power-assisted steering device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, a hydraulic tandem pump assembly shown in 
FIG. 1 includes a primary pump 11 for supplying fluid under pressure to a 
power-assisted steering device 30 and a secondary pump 12 for supplying 
fluid under pressure to a hydraulic motor 14 of an engine cooling fan 15. 
The pumps 11 and 12 are mounted on a common drive shaft 13 in drive 
connection to a prime engine 10 of a motor vehicle to be driven 
simultaneously in operation of the engine 10. 
The primary pump 11 is in the form of a vane pump which has an inlet port 
connected to a fluid reservoir 20 through an inlet passage and an outlet 
port connected to the power-assisted steering device 30 through a flow 
quantity control valve 32 and a fixed throttle 33. The power-assisted 
steering device 30 is arranged to be operated by a steering wheel 31 
through a steering shaft 31a. In operation, fluid under pressure is 
supplied to the power-assisted steering device 30 under control of the 
flow quantity control valve 32 and returned to the fluid reservoir 20. The 
quantity of fluid under pressure discharged from the primary pump 11 
increases in accordance with an increase of rotational speed of the prime 
engine 10. The flow quantity control valve 32 cooperates with the fixed 
throttle 33 to bypass an excessive quantity of fluid to the inlet passage 
of pump 11 so as to maintain the supply quantity of fluid under pressure 
to the power-assisted steering device 30 in a predetermined amount. A 
relief valve 34 is arranged to release an excessive hydraulic pressure 
applied to the power-assisted steering device 30 at the downstream of 
throttle 33. 
The secondary pump 12 is also in the form of a vane pump which has an inlet 
port connected to the fluid reservoir 20 through an inlet passage 21 and 
an outlet port connected to the hydraulic motor 14 through passages 22 and 
22a. The hydraulic motor 14 is connected to the fluid reservoir 20 through 
an oil cooler 26. Disposed within the passages 22 and 22a is an 
electrically operated flow quantity control valve assembly 40 which is 
activated under control of an electric control apparatus 60 as will be 
described in detail later. The engine cooling fan 15 is arranged behind a 
radiator 16 of the water cooling type and is drivingly connected to the 
hydraulic motor 14. A relief valve 25 is connected in parallel with the 
hydraulic motor 14 to bypass therethrough an excessive quantity of fluid 
under pressure to the fluid reservoir 20 thereby to define a maximum 
rotational number of motor 14. The passage 22a is connected to the inlet 
passage 21 through a relief valve 24 which is arranged to release an 
excessive hydraulic pressure at the downstream of the secondary pump 12. 
As shown clearly in FIG. 2, the flow quantity control valve assembly 40 
includes an electrically operated throttle valve 50 for controlling the 
opening degree of a throttle element 55 between the passages 22 and 22a 
and a flow control valve 44 associated with the throttle valve 50 to be 
operated in accordance with a difference in pressure between the passages 
22 and 22a. The throttle valve 50 includes a support member 51 threaded 
into a housing 12a of the tandem pump assembly in a fluid-tight manner, a 
movable spool 52 in a sleeve fixed to the support member 51, a valve rod 
53 carried by the movable spool 52 for movement therewith, and a solenoid 
winding 56 arranged in surrounding relationship with the movable spool 52 
and mounted in place on the sleeve. The movable spool 52 is biased by a 
coil spring 54 rightwards in the figure to normally open the throttle 
element 55. When the solenoid winding 56 is energized by an electric 
current applied thereto, the movable spool 52 is moved leftwards against 
the coil spring 54 in accordance with the applied electric current so that 
the valve rod 53 approaches the throttle element 55 to decrease the 
opening degree of the same. 
The flow control valve 44 includes a stepped valve spool 46 which is 
axially movably disposed within a bore 45 in the pump housing 12a and 
exposed at opposite ends thereof to the pressures in passages 22 and 22a. 
The valve spool 46 is biased by a compression coil spring 47 toward the 
throttle element 55 for abutment therewith to normally close fluid 
communication between the passages 22 and 22a and between the discharge 
passage 22 and a bypass passage 23. The bore 45 is closed by a plug 48 
which is retained in place in the pump housing 12a to receive the coil 
spring 47 thereon. When applied with fluid under pressure from the 
secondary pump 12 through the discharge passage 22, the valve spool 46 is 
moved by the difference in pressure between passages 22 and 22a against 
the coil spring 47 to open the throttle element 55 for fluid communication 
between the passages 22 and 22a. In such operation, the discharge passage 
22 is communicated with the bypass passage 23 in accordance with the axial 
movement of valve spool 46 to bypass therethrough a portion of the fluid 
under pressure into the inlet passage 21. 
The bypass flow quantity of fluid into the inlet passage 21 decreases in a 
condition where the throttle valve 50 is fully opened and increases in 
accordance with a decrease of the opening degree of throttle valve 50. 
When the throttle valve 50 is fully closed, the entirety of fluid under 
pressure from discharge passage 22 flows into the inlet passage 21 through 
bypass passage 23. The pressure loss of fluid flowing into the inlet 
passage 21 can be minimized by appropriate determination of the opening 
area of throttle element 55 and the biasing force of spring 47. The bypass 
passage 23 is formed in the pump housing 12a in such a manner as to open 
into the inlet passage 21 at an appropriate angle. This is useful to 
supercharge the fluid sucked into the inlet passage 21 from reservoir 20 
to enhance the suction efficiency of the secondary pump 12. 
As shown in FIG. 1, the electric control apparatus 60 is connected at its 
input terminals to a fluid temperature sensor 61 disposed within the fluid 
reservoir 20 to detect the temperature of hydraulic fluid and to a water 
temperature sensor 62 disposed within the radiator 16 to detect the 
temperature of cooling water and is connected at its output terminal to 
the solenoid winding 56 of throttle valve 50. In this embodiment, the 
fluid temperature T detected by sensor 61 represents an ambient 
temperature of prime engine 10, and the cooling water temperature t 
detected by sensor 62 represents a loaded condition of prime engine 10. 
The control apparatus 60 is arranged to control the electric current i 
applied to the solenoid winding 56 in dependence upon the fluid 
temperature T and cooling water temperature t thereby to control the flow 
control valve 44. Namely, the control apparatus 60 is arranged to maintain 
the electric current i at a predetermined maximum level iA when the fluid 
temperature T is below a predetermined valve T.sub.o (for instance, 85 C), 
as shown by a broken line I.sub.1 in FIG. 3 and to control the electric 
current i in accordance with the cooling water temperature t when the 
fluid temperature T exceeds the predetermined T.sub.o, as shown by a solid 
line I.sub.2 in FIG. 3. 
Under such control of the control apparatus 60, the electric current i is 
maintained at the maximum level iA when the cooling water temperature t is 
lower than or equal to a value t.sub.1, decreased in accordance with rise 
of the cooling water temperature t higher than the value t.sub.1 and 
maintained at a minimum level iB when the cooling water temperature t is 
higher than a value t.sub.2. In a practical embodiment, it is preferable 
that the values t.sub.1 and t.sub.2 are determined as 65 C and 95 C, 
respectively. As a result, the opening degree of throttle valve 50 is 
maintained at a minimum value pA when the fluid temperature T is lower 
than the predetermine value T.sub.o, as shown by a broken line P.sub.1 in 
FIG. 4. When the fluid temperature T exceeds the predetermined value 
T.sub.o, the opening degree of throttle valve 50 is maintained at the 
minimum value pA when the cooling water temperature t is lower than the 
value t.sub.1, increased in accordance with rise of the cooling water 
temperature t higher than the value t.sub.1 and maintained as a maximum 
value pB when the cooling water temperature t is higher than the value 
t.sub.2. 
When the opening degree of throttle valve 50 is maintained at the minimum 
value pA, the flow control valve 44 acts to return a large part of 
discharged fluid from pump 12 into the inlet passage 21 through bypass 
passage 23. In such a condition, as shown by a broken line A in FIG. 5, 
the supply quantity Q of fluid under pressure to the hydraulic motor 14 is 
gradually increased in accordance with an increase of the rotational speed 
of drive shaft 13. When the opening degree of throttle valve 50 is 
maintained at the maximum value pB, the flow quantity control valve 44 
acts to decrease the quantity of discharged fluid flowing into the inlet 
passage 21 through bypass passage 23. In such a condition, as shown by a 
solid line B.sub.0 in FIG. 5, the supply quantity Q of fluid under 
pressure to the hydraulic motor 14 is rapidly increased in accordance with 
an increase of the rotational speed of drive shaft 13. 
When the supply quantity Q of fluid under pressure to the hydraulic motor 
14 reaches a predetermined value Q.sub.o, the relief valve 25 acts to 
bypass an excessive quantity of fluid therethrough to the fluid reservoir 
20. When the opening degree of throttle valve 50 is between the minimum 
and maximum values pA and pB, the supply quantity D of fluid under 
pressure to the hydraulic motor 14 is increased in accordance with an 
increase of the rotational speed of drive shaft 13, as shown by sold lines 
B.sub.1, B.sub.2, B.sub.3 and B.sub.4 in FIG. 5. Thus, the hydraulic motor 
14 is driven at a rotational speed proportional to the quantity Q of fluid 
under pressure supplied thereto to rotate the cooling fan 15 for cooling 
the water in radiator 16. 
As is understood from the above description, when the ambient temperature 
of prime engine 10 is below the predetermined value T.sub.o to cause an 
increase of viscosity of the hydraulic fluid, the opening degree of 
throttle valve 50 is maintained at the minimum value pA so that the flow 
control valve 44 acts to bypass a large part of discharged fluid from pump 
12 into the inlet passage 21 through bypass passage 23. As a result, only 
the remaining part of discharged fluid is supplied to the hydraulic motor 
14 through passage 22a and returns to the fluid reservoir 20 through the 
oil cooler 26. This is effective to decrease loss of energy caused by 
fluid resistance in the pipe line between the passage 22a and the fluid 
reservoir 20 and to decrease the load acting on the secondary pump 12. 
When the viscosity of fluid increases at a low temperature, there is a 
fear of causing cavitation in the secondary pump 12 due to an increase of 
fluid resistance in the inlet passage 21. In such a condition, the 
discharged fluid from the secondary pump 12 is returned to the inlet 
passage 21 through bypass passage 23 to decrease the quantity of fluid 
sucked from the fluid reservoir 20. This is effective to eliminate 
cavitation in the secondary pump 12 caused by insufficient suction of 
fluid from the fluid reservoir 20. When the fluid temperature T rises 
higher than the predetermined value T.sub.o, the fluid under pressure 
discharged from the secondary pump 12 is supplied to the hydraulic motor 
14 in accordance with the cooling water temperature t detected by sensor 
62. Thus, the hydraulic motor 14 is driven by the fluid under pressure to 
rotate the cooling fan 15 at a speed proportional to rise of the cooling 
water temperature t. 
Although in the control system, the throttle valve 50 has been arranged to 
remain a slight space for fluid communication in its fully closed 
position, it may be arranged to eliminate the slight space so as to render 
the supply quantity of fluid under pressure to the hydraulic motor 14 
substantially zero. 
In FIG. 6 there is illustrated a modification of the control system, 
wherein the electric control apparatus 60 is connected to a pressure 
sensor 63 which is arranged to detect hydraulic pressure in the inlet 
passage of the power-assisted steering device 30 for producing an output 
signal therefrom when subjected to the hydraulic pressure in excess of a 
predetermined value. In this modification, the electric control apparatus 
60 is arranged to fully close the throttle valve 50 in response to the 
output signal from pressure sensor 63 so as to bypass the entirety of 
fluid under pressure discharged from the secondary pump 12 into the inlet 
passage 21 through bypass passage 23. With such arrangement of the control 
apparatus 60, the supply quantity of fluid under pressure to the hydraulic 
motor 14 is made substantially zero under control of the throttle valve 50 
as shown by a broken line A in FIG. 7. This is effective to decrease the 
load acting on the secondary pump 12 when the power-assisted steering 
device 30 is operated to steer the vehicle without driving. As a result, a 
peak value of load acting on the tandem pump is temporarily decreased in 
operation of the power-assisted steering device 30 without causing any 
problem in cooling of the engine. 
In another modification of the control system, the pressure sensor 63 may 
be replaced with a steering angle sensor 64 which is arranged to detect 
operating condition of the power assisted steering device 30 for producing 
an output signal therefrom when the steering wheel 31 is steered in a 
predetermined angle. In this modification, the electric control apparatus 
60 is arranged to fully close the throttle valve 50 in response to the 
output signal from steering angle sensor 64 so as to bypass the entirety 
of fluid under pressure discharged from the secondary pump 12 into the 
inlet passage 21 through bypass passage 23. Furthermore, the electric 
control apparatus 60 may be connected to a speed sensor 65 which is 
arranged to detect the rotational speed of engine 10 (or drive shaft 13) 
for producing therefrom an output signal indicative of the rotational 
speed of engine 10. In this modification, the electric control apparatus 
60 is arranged to increase the supply quantity of fluid under pressure to 
the hydraulic motor 14 in accordance with an increase of the rotational 
speed of engine 10 during operation of the power-assisted steering device 
30, as shown by a dot and dash line C in FIG. 7. 
Having now fully set forth both structure and operation of preferred 
embodiments of the concept underlying the present invention, various other 
embodiments as well as certain variations and modifications of the 
embodiments herein shown and described will obviously occur to those 
skilled in the art upon becoming familiar with the underlying concept. It 
is to be understood, therefore, that within the scope of the appended 
claims, the invention may be practiced otherwise than as specifically set 
forth herein.