Method and apparatus for controlling continuously variable transmission for vehicle

Method and apparatus for controlling a continuously variable transmission operatively connecting an engine and drive wheels of a vehicle, wherein a speed ratio of the transmission is controlled such that an actual engine speed coincides with a target engine speed determined based on a currently required engine output and according to a predetermined relation between the target engine speed and the required engine output. The method and apparatus comprise a step or device for restraining a rate of variation in the speed ratio during a time span between the moment when the actual engine speed or output has exceeded a variation-restraint trigger speed or output of the engine, and the moment when the actual engine speed or output has reached the target speed or output. The trigger engine speed or output is determined based on a predetermined transmission efficiency of the transmission, and on the target engine speed or output, and is lower or smaller than the target engine speed or output.

BACKGROUND OF THE INVENTION 
The present invention relates generally to a method and an apparatus for 
controlling a speed ratio of a continuously variable transmission for a 
vehicle, and more particularly to impovements in such method and apparatus 
for increased fuel economy of the vehicle during acceleration periods. 
In the art of a continuously variable transmission for a vehicle to 
transmit a rotary motion of an engine to drive wheels with a continuously 
variable ratio, a control apparatus is known which comprises regulating 
means for controlling the speed ratio of the variable transmission (ratio 
of a rotating speed of its output shaft to a rotating speed of its input 
shaft) such that an actual speed of the engine coincides with a target 
speed thereof which is determined based on a currently required output of 
the engine and according to a predetermined relation between the target 
speed and the required output of the engine. With such a known control 
apparatus to control the speed ratio of a variable transmission, the 
actual engine speed is controlled into conformity with a target engine 
speed which is continuously or intermittently determined so that the 
specific-fuel consumption is minimum. Thus, the control apparatus permits 
the transmission and the engine to operate with a high fuel economy over 
an entire range of the vehicle speed, particularly while the vehicle is 
running in a relatively stable conditions or at a relatively constant 
speed. In a continuously variable transmission, however, it is generally 
recognized that the transmission efficiency is unavoidably reduced in a 
process of changing the speed ratio. According to a controlling method 
practiced by the known control apparatus for the variable transmission, 
the speed ratio of the transmission is rapidly varied during acceleration 
of the vehicle, because the target speed of the engine is rapidly 
increased in response to a rapid increase in the required output of the 
engine as a result of an abrupt depression of an accelerator pedal for 
acceleration. Therefore, the transmission efficiency of the variable 
transmission and consequently the fuel economy of the vehicle are reduced 
during acceleration of the vehicle. In short, the known apparatus for 
controlling a continuously or steplessly variable transmission suffers the 
problem of relatively low fuel economy. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide improved 
method and apparatus for controlling a speed ratio of a continuously 
variable transmission for a vehicle, which are capable of maintaining a 
high fuel economy of the vehicle even while the vehicle is in 
acceleration. 
According to the invention, there is provided a method of controlling a 
speed ratio of a continuously variable transmission for a vehicle for 
transmitting a rotary motion of an engine to drive wheels of the vehicle 
with a stepless speed change, by regulating the speed ratio of the 
transmission such that an actual speed of the engine coincides with a 
target speed thereof which is determined based on a currently required 
output of the engine and according to a predetermined relation between the 
target speed and the required output of the engine, said method comprising 
the steps of: (a) determining a variation-restraint trigger speed of the 
engine based on a predetermined transmission efficiency of the 
transmission and based on the determined target speed of the engine, the 
variation-restraint trigger speed being lower than the determined target 
engine speed; and (b) restraining a rate of variation in the speed ratio 
of the transmission during a time span between a first moment when the 
actual speed of the engine has exceeded the variation-restraint trigger 
speed, and a second moment when the actual speed has reached the target 
speed. 
According to the invention, there is also provided an apparatus for 
practicing the above method of the invention, including regulating means 
for controlling the speed ratio of the transmission such that an actual 
speed of the engine coincides with a target speed thereof which is 
determined based on a currently required output of the engine and 
according to a predetermined relation between the target speed and the 
required output of the engine. The apparatus comprises: (a) means for 
determining a variation-restraint trigger speed of the engine based on a 
predetermined transmission efficiency of the transmission and based on the 
determined target speed of the engine, the variation-restraint trigger 
speed being lower than the target speed of the engine; and (b) means for 
restraining a rate of variation in the speed ratio of the transmission 
during a time span between a first moment when the actual speed of the 
engine has exceeded the variation-restraint trigger speed, and a second 
moment when the actual speed has reached the target speed. 
In the controlling apparatus constructed as described above, the variation 
in the speed ratio of the variable transmission is restrained by the 
restraining means immediately after the actual engine speed has exceeded 
the variation-restraint trigger speed determined by the trigger speed 
determining means, and until the actual engine speed has reached the 
target engine speed. While the actual engine speed is elevated from the 
variation-restraint trigger speed up to the target speed during 
acceleration of the vehicle, the rotary motion of the engine is 
transmitted through the transmission with a substantially constant speed 
ratio, because the rate of variation in the speed ratio is restrained or 
inhibited during that time span. Accordingly, otherwise possible reduction 
in transmission efficiency of the transmission due to variation in the 
speed ratio is minimized, and as a result the engine is operated with a 
relatively high fuel economy. Further, since the variation-restraint 
trigger speed of the engine is determined based on a transmission 
efficiency of the transmission during acceleration of the vehicle and on 
the target engine speed, the output of the engine obtained with a 
substantially constant speed ratio of the transmission is at least 
equivalent to that which is obtained during acceleration of a vehicle 
where the transmission is controlled by a known control apparatus. This 
means that the restraint of the speed ratio variation of the transmission 
according to the invention will not impair the acceleration performance, 
or drivability of the vehicle. 
According to another aspect of the invention, there is provided a method of 
controlling a speed ratio of a continuously variable transmission for a 
vehicle for transmitting a rotary motion of an engine of the vehicle to 
drive wheels of the vehicle with a stepless speed change, by regulating 
the speed ratio of the transmission such that an actual speed of the 
engine coincides with a target speed thereof which is determined based on 
a currently required output of the engine and according to a predetermined 
relation between the target speed and the required output of the engine, 
the method comprising the steps of: (a) determining an actual output 
(horsepower or torque) of the engine based on the actual speed of the 
engine; (b) determining a target output (horsepower or torque) of the 
engine to obtain the target speed of the engine; (c) determining a 
variation-restraint trigger output of the engine based on a predetermined 
transmission efficiency of the transmission and based on the determined 
target output of the engine, the variation-restraint trigger output being 
smaller than the determined target output; and (d) restraining a rate of 
variation in the speed ratio during a time span between a first moment 
when the actual output of the engine has exceeded the variation-restraint 
trigger output, and a second moment when the actual output has reached the 
target output. 
According to the invention, there is further provided an apparatus for 
practicing the above method of the invention. The instant apparatus 
comprises: (a) first determining means for determining an actual output 
(horsepower or torque of the engine based on the actual speed of the 
engine and based on the currently required output (horsepower or torque) 
of the engine; (b) second determining means for determining a target 
output (horsepower or torque of the engine so as to obtain the target 
speed of the engine; (c) third determining means for determining a 
variation-restraint trigger output of the engine based on a transmission 
efficiency of the transmission during acceleration of the vehicle and 
based on the determined target output of the engine, the 
variation-restraint trigger output being smaller than the determined 
target output; and (d) means for restraining a rate of variation in the 
speed ratio during a time span between a first moment when the actual 
output of the engine has exceeded the variation-restraint trigger output, 
and a second moment when the actual output has reached the target output. 
In the controlling apparatus constructed according to the above aspect of 
the invention, the variation in the speed ratio of the variable 
transmission is restrained by the restraining means during a time span 
between the moment when the actual output of the engine determined by the 
first determining means has exceeded the variation-restraint trigger 
output determined by the third determining means, and the moment when the 
actual engine speed has reached the target engine output determined by the 
second determining means. Thus, the previously indicated advantages are 
provided according to this aspect of the invention. 
A further advantage of this aspect of the invention stems from the use of 
the variation-restraint trigger output to establish the timing at which 
the restraint of the speed ratio variation is initiated. In this case, the 
variation-restraint trigger output is determined based on a predetermined 
transmission efficiency of the transmission and on the determined target 
engine output. Therefore, the timing of starting the restraint of the 
speed ratio variation can be determined to assure higher drivability and 
fuel economy of the vehicle, than in the case where the engine speed is 
used to trigger the restraint of the speed ratio variation. 
In a preferred embodiment of the invention, the transmission efficiency 
used for determining the variation-restraint trigger speed or output of 
the engine is an efficiency during a rapid change in the speed ratio of 
the transmission. According to one form of the above embodiment, the 
variation-restraint trigger speed or output is obtained by multiplying the 
target engine speed or output by a value of the predetermined efficiency 
of the transmission.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the accompanying drawing showing a preferred embodiment of the 
invention, there is shown in FIG. 1 a continuously variable transmission 
14 (CVT 14) of belt-and-pulley type which is operatively connected to an 
engine 10 of a vehicle through a clutch 12. The variable transmission 14 
serves to transmit the output of the engine 10 to drive wheels (not shown) 
of the vehicle such that a speed ratio of the engine 10 and the drive 
wheels relative to each other is steplessly variable. The variable 
transmission 14 comprises: a drive or input shaft 16 coupled to the clutch 
12; a first variable-diameter pulley 18 having a variable effective 
diameter and associated with the input shaft 16; a driven or output shaft 
20; a second variable-diameter pulley 22 having a variable effective 
diameter and associated with the output shaft 20; a transmission belt 24 
connecting the first and second variable-diameter pulleys 18 and 22; and 
hydraulic cylinders 26 and 28 associated with the pulleys 18 and 22, 
respectively, to change widths of Vee-grooves of the pulleys 18, 22 for 
varying their effective diameters engaging the belt 24. The first pulley 
18 includes an axially stationary rotor 30 fixed to the input shaft 16, 
and an axially movable rotor 34 which is mounted on the input shaft 16 
axially slidably relative to the rotor 30 but not rotatable relative to 
the shaft 16. Similarly, the second pulley 22 includes an axially 
stationary rotor 32 fixed to the output shaft 20, and an axially movable 
rotor 36 mounted on the output shaft 20 slidably relative to the rotor 32 
but not rotatable relative to the shaft 20. The movable rotors 34 and 36 
are slidably moved on the respective input and output shafts 16 and 20 
with a hydraulic pressure applied to pressure chambers in the hydraulic 
cylinders 26, 28, whereby the effective diameters of the first and second 
pulleys 18, 22 engaging the belt 24 may be continuously varied. The 
hydraulic cylinder 28 is normally subject to a line pressure of a 
hydraulic system, while the hydraulic cylinder 26 is subject to a variable 
hydraulic pressure which is adjusted through a speed-ratio control valve 
38 incorporated in the hydraulic system. Thus, a balance of hydraulic 
pressures exerted on the movable rotors 34 and 36 is changed, and 
consequently the speed ratio of the output shaft 20 with respect to the 
input shaft 16 may be varied. In this connection, it is noted that the 
movable rotor 34 has a larger pressure receiving area than the movable 
rotor 36. 
The hydraulic line pressure normally applied to the hydraulic cylinder 28 
is obtained from a pressure control valve 44 which adjusts a pressure of a 
pressurized working fluid which is pumped by a pump 42 from an oil 
reservoir 40. The line pressure is fed to the hydraulic cylinder 28 and to 
the speed-ratio control valve 38 through a conduit 46. The pressure 
control valve 44 comprises a linear solenoid operable in response to a 
PRESSURE CONTROL signal SP which will be described, and further comprises 
a valving member driven by the linear solenoid. The line pressure supplied 
from the pressure control valve 44 is regulated by changing, according to 
the PRESSURE CONTROL signal SP, a relief amount of the fluid which is fed 
back to the oil reservoir 40 to relieve a part of the fluid pressure in 
the pressure control valve 44. The speed-ratio control valve 38 comprises 
a linear solenoid operable in response to a SPEED RATIO signal SS which 
will be described, and further comprises a valving member driven by the 
linear solenoid to control amounts of the working fluid which are supplied 
to or discharged from the hydraulic cylinder 26. More specifically 
described, the speed-ratio control valve 38 is connected to the hydraulic 
cylinder 26 through a conduit 48, and to the oil reservoir 40 through a 
drain conduit 50. A movement of the valving member of the speed-ratio 
control valve 38 in one direction enables the conduit 46 to communicate 
with the conduit 48, and permits a variation in area of their 
communication within the speed-ratio control valve 38, thereby adjusting 
an amount of the fluid to be supplied to the hydraulic cylinder 26. On the 
other hand, a movement of the valving member of the speed-ratio control 
valve 38 in the opposite direction enables the conduit 48 to communicate 
with the drain conduit 50, and permits a variation in area of their 
communication in the speed-ration control valve 38, thereby adjusting an 
amount of the fluid to be discharged from the hydraulic cylinder 26 back 
to the oil reservoir 40. While the valving member of the speed-ratio 
control valve 38 is placed in its neutral position at which the conduit 48 
is held in a substantially disconnected relation with both of the conduits 
6 and 50, the amount of the pressurized fluid (hydraulic pressure) within 
the hydraulic cylinder 26 is held substantially constant and consequently 
the movable rotor 34 is located at a fixed axial position on the input 
shaft 16, whereby the speed ratio of the transmission 14 is held constant. 
When the conduit 48 is brought into communication with the conduit 46 due 
to the movement of the valving member of the speed-ratio control valve 38, 
the amount of the fluid (hydraulic pressure) within the hydraulic cylinder 
26 is increased and consequently the movable rotor 34 is moved away from 
the stationary rotor 30, with a result of increasing the effective 
diameter of the first pulley 18 and decreasing that of the second pulley 
22, whereby the speed ratio of the transmission 14 is made higher. 
Conversely, when the conduit 48 is put into communication with the drain 
conduit 50, the movable rotor 34 is moved towards the stationary rotor 30, 
whereby the speed ratio is lowered. 
The engine 10 is provided with an intake manifold 51 in which is disposed a 
throttle valve 54 linked with an accelerator pedal 52. An opening angle 
.theta. of the throttle valve 54 is detected by a throttle sensor 56 which 
is attached to the throttle valve 54 as means for detecting a currently 
required output of the engine 10. The throttle sensor 56 generates a 
voltage signal, i.e., a THROTTLE signal TH which corresponds to the 
opening angle .theta. of the throttle valve 54. The THROTTLE signal TH is 
applied to an I/O (input/output) port 59 of a microcomputer 57 through an 
A/D (analog/digital) converter 58. Adjacent to the input and output shafts 
16 and 20, there are respectively disposed a first and a second rotation 
sensor 60, 62. The first rotation sensor 60 serves as engine speed 
detecting means which detects a rotating speed of the input shaft 16 and 
produces an INPUT ROTATION signal SE whose number of pulses corresponds to 
the number of revolution of the engine 10. On the other hand, the second 
rotation sensor 62 serves as vehicle speed detecting means which detects a 
rotating speed of the output shaft 20 and produces an OUTPUT ROTATION 
signal SC whose number of pulses corresponds to a running speed of the 
vehicle. These INPUT and OUTPUT ROTATION signals SE and SC are fed to an 
I/F circuit (interface circuit) 64, which converts the ROTATION signals SE 
and SC into coded signals which represent the number of pulses per unit 
time of the signals SE, SC. These coded signals are applied to the I/O 
port 59. 
The I/O port 59 is connected through a data bus line to a CPU 66 (central 
processing unit), a RAM 68 (random-access memory), and a ROM 70 (read-only 
memory). The CPU 66 is operated according to a program which is stored in 
the ROM 70 provided as one memory means, and utilizes a temporary data 
storage function of the RAM 68 which is provided as another memory means. 
The CPU 66 thus cooperated by the RAM 68 and the ROM 70, processes signals 
to be fed to the I/O port 59. Further, the CPU 66 feeds to a D/A 
(digital/analog) converter 72 and a driver circuit 74 the SPEED RATIO 
signal SS representing a speed ratio "e" and a variation rate "e" 
(=.DELTA.e/.DELTA.t) at which the speed ratio "e" is varied. The CPU 66 
further feeds to the converter 59 and the driver circuit 74 the PRESSURE 
CONTROL signal SP representing the line pressure of the hydraulic system. 
The driver circuit 74, which is a power amplifier, amplifies the SPEED 
RATIO and PRESSURE CONTROL signals SS, SP from the D/A converter 72 and 
applies the amplified SPEED RATIO and PRESSURE CONTROL signals SS, SP to 
the linear solenoids of the speed-ratio control valve 38 and the pressure 
control valve 44. 
FIG. 2 is a schematic diagram generally showing a control arrangement for 
the variable transmission 14. Target engine speed determining means 76 
determines a target engine speed Ne' (rotating speed of the input shaft 
16) based on the opening angle .theta. of the throttle valve 54 
represented by the THROTTLE signal TH, and according to a predetermined 
and stored relation between the opening angle .theta. and the target 
engine speed Ne', which relation is represented by a graph of FIG. 3. The 
target engine speed Ne' is determined such that the required horsepower of 
the engine 10 (required engine output) represented by the opening angle 
.theta. of the throttle valve 54 is obtained with a minimum specific fuel 
consumption. Means 78 determines a later described variation-restraint 
trigger speed Ne" by multiplying the target engine speed Ne' by a 
predetermined efficiency .eta. of the variable transmission 14. This 
efficiency .eta. is a value which is commonly established during a rapid 
change in the speed ratio of the transmission 14 under acceleration of the 
vehicle when the transmission 14 is controlled by a known control 
apparatus. For example, the efficiency is approx. 90%. Speed-ratio control 
enable/disable discriminating means 80 activates selectively speed-ratio 
control enabling means 82 or speed-ratio control disabling means 84. More 
specifically described, the speed-ratio control enabling means 82 is 
activated when an actual speed Ne of the engine 10 detected by the engine 
speed detecting means 86 (i.e., actual rotating speed Ni of the input 
shaft 16 detected by the first rotation sensor 60) falls within a range 
between the target engine speed Ne' and the variation-restraint trigger 
speed Ne". Alternatively, the speed-ratio control disabling means 84 is 
activated when the actual engine speed (detected engine speed) Ne is not 
within the above range. The speed-ratio control enabling means 82, when 
activated by the discriminating means 82, determines an actual speed ratio 
"e" of the variable transmission 14 based on an actual running speed V of 
the vehicle detected by the vehicle speed detecting means 88 (i.e., actual 
rotating speed No of the output shaft 20 detected by the second rotation 
sensor 62), and based on the actual engine speed Ne. Simultaneously, the 
speed-ratio control enabling means 82 determines a target speed ratio "e'" 
of the transmission 14 for coincidence of the actual engine speed Ne with 
the target engine speed Ne', and directs regulating means 90 for 
controlling the speed-ratio control valve 38 and thereby controlling the 
variable transmission 14 such that a difference between the actual speed 
ratio "e" and the targt speed ratio "e'" is zeroed. When the speed-ratio 
control disabling means 84 is activated by the discriminating means 80, 
the disabling means 84 causes the target speed ratio "e"" to be replaced 
by the actual speed ratio "e" and interrupts or inhibits a change in the 
speed ratio of the transmission 14, thereby controlling the actual engine 
speed Ne for coincidence with the target engine speed Ne' without changing 
the speed ratio "e" of the transmission 14. The speed-ratio control valve 
regulating means 90 feeds the SPEED RATIO signal SS to the speed-ratio 
control valve 38, which is operated in response to the SPEED RATIO signal 
SS to vary the speed ratio "e". 
Engine torque detecting means 92 detects an actual output torque T based on 
the opening angle .theta. of the throttle valve 54 (representing the 
currently required output of the engine 10) and on the actual engine speed 
Ne, and according to a predetermined and stored relation among the actual 
engine speed Ne, throttle valve opening angle .theta. and engine output 
torque T, which relation is represented by a graph of FIG. 4. Line 
pressure determining means 94 determines a line pressure in the conduit 46 
based on the actual output torque T of the engine 10, actual engine speed 
Ne detected by the engine speed detecting means 86 (60), and actual 
rotating speed No of the output shaft 20 detected by the vehicle speed 
detecting means 88 (62), and according to a predetermined relation among 
the above values T, Ne and No. To establish the line pressure deteremined 
by the line pressure determining means 94, regulating means 94 feeds the 
PRESSURE CONTROL signal SP to the pressure control valve 44 to regulate 
the operation of the valve 44. As a result, the line pressure is 
maintained at a minimum level necessary to prevent a torque transmission 
loss due to slip of the belt 24 on the pulleys 18, 22, whereby otherwise 
possible power loss and shortened service life of the belt 24 due to 
excessive tension thereto may be avoided. 
Referring next to a flow chart of FIG. 5, the operation of the present 
invention will be described. 
At first, step S1 is executed to store in the RAM 68 data on the opening 
angle .theta. of the throttle valve 54 and data on the actual engine speed 
Ne, according to the THROTTLE and INPUT ROTATION signals TH and SE. Step 
S1 is followed by step S2 which corresponds to the target engine speed 
determining means 76 previously indicated. In step S2, the target engine 
speed Ne' is calculated based on the opening angle .theta. and according 
to the stored relation of FIG. 3. Then, step 3 is performed to check if 
the actual engine speed Ne is lower than the target engine speed Ne'. In 
the case where the actual engine speed Ne is higher than the target engine 
speed Ne', step S3 is followed by step S7 in which a speed-ratio control 
routine 2 is executed as described later. In the case where the actual 
engine speed Ne is lower than the target engine sped Ne', step S3 is 
followed by step S4 which corresponds to the determining means 78 for 
determining the variation-restraint trigger speed Ne". In step S4, the 
variation-restraint trigger speed Ne" as shown in broken line in FIG. 3 is 
calculated by multiplying the target engine speed Ne' by an efficiency 
.eta. of the variable transmission 14, which is a common value obtained 
during a rapid change in the speed ratio "e" of the transmission 14 while 
the vehicle is in acceleration where the transmission 14 is operated under 
control of a known control apparatus, as previously described. This 
efficiency percent used in the calculation may be an average value during 
periods of acceleration of the vehicle. Subsequently, step S5 is executed 
to check if the actual engine speed Ne is smaller than the 
variation-restraint trigger speed Ne". If the checking reveals that the 
speed Ne is smaller than the variation-restraint trigger speed Ne", the 
speed-ratio control routine 2 is performed in step S7. If the speed Ne is 
not smaller than the speed Ne", that is, if the speed Ne has been raised 
to a level at which the engine 10 is able to provide, without a change in 
the speed ratio "e", an output (T.times.Ne) not less than the output 
(T.times.Ne'.times..eta.) which is obtained during acceleration of the 
vehicle if and when the transmission 14 is controlled by a known 
apparatus, step S5 is followed by step S6 wherein a speed-ratio control 
routine 1 is effected. In other words, steps S3 and S5 correspond to the 
previously described discriminating means 80 which selects the speed-ratio 
control routine 1 when the actual engine speed Ne falls within the range 
between the speeds Ne' and Ne", but selects the speed-ratio control 
routine 2 when the actual engine speed Ne does not fall within the above 
range. 
The speed-ratio control routines 1 and 2 are executed in the manner as 
illustrated in FIG. 6 and as described below. 
In the speed-ratio control routine 2, step SR1 is executed at first, 
wherein the rotating speeds Ni and No of the input and output shafts 16 
and 20 of the variable transmission 14 are calculated based on the INPUT 
and OUTPUT ROTATION signals SE and SC, repsectively. Then, step SR2 is 
executed to calculate the actual speed ratio "e" (=No/Ni) of the variable 
transmission 14 based on the calculated rotating speeds Ni and No of the 
input and output shafts 16, 20. Step SR2 is followed by step SR3 in which 
the target speed ratio "e'" (=No/Ni') is calculated, and then by step S4 
to check if the target speed ratio "e'" is lower than a minimum ratio 
"e".sub.min . If the ratio "e'" is lower than the minimum ratio 
"e".sub.min, step S5 is executed to set the target speed ratio "e'" at the 
minimum ratio "e".sub.min. If the ratio "e'" is not lower than the minimum 
ratio "e".sub.min, step SR4 is followed by step SR6 to check if the target 
speed ratio "e'" is lower than a maximum ratio "e".sub.max. If the ratio 
"e'" is not lower than the maximum ratio "e".sub.max, step SR6 is followed 
by step SR7 in which the target speed ratio "e'" is set at the maximum 
ratio "e".sub.max. If the ratio "e'" is lower than the maximum ratio 
"e".sub.max, step SR6 is followed by step SR8. 
In step SR8, an error ".DELTA.e" (="e'"-"e") is obtained by subtracting the 
actual speed ratio "e" from the target speed ratio "e'". Then, in step 
SR9, a flow control voltage Vf to zero the error ".DELTA.e" is determined 
according to the following formula (1): 
EQU Vf=K.sub.1 .multidot."e"+K.sub.2 .multidot.".DELTA.e" (1) 
where, K.sub.1 and K.sub.2 are constants. 
The SPEED RATIO signal SS representing the flow control voltage Vf is 
applied to the speed-ratio control valve 38, which supplies a controlled 
flow of the working fluid to the hydraulic cylinder 26 according to the 
SPEED RATIO signal, whereby the speed ratio "e" of the variable 
transmission 14 is controlled. Thus, steps SR1 through SR8 correspond to 
the previously indicated speed-ratio control enabling means 82, and step 
SR9 corresponds to the previously indicated regulating means 90 for 
controlling the speed-ratio control valve 38. 
Successively, the control goes to step SR10 which corresponds to the engine 
torque detecting means 92 to calculate the actual output torque T of the 
engine 10 according to the predetermined relation of FIG. 4. Step SR10 is 
followed by step SR11 corresponding to the line pressure determining means 
94 and the pressure control valve regulating means 96. That is, a pressure 
control voltage Vp to control the hydraulic line pressure is calculated 
according to the following formula (2), and the PRESSURE CONTROL signal SP 
representing the pressure control voltage Vp is applied to the pressure 
control valve 44, whereby the line pressure is controlled according to the 
PRESSURE CONTROL signal SP. 
EQU Vp=f (T, Ni, No) (2) 
In the speed-ratio control routine 1, steps SR12 and SR13 identical to 
steps SRT1 and SR2 of the control routine 2 are executed to calculate the 
rotating speeds Ni and No of the input and output shafts 16, 20, and to 
obtain the actual speed ratio "e" (=No/Ni) of the variable transmission 
14. Step SR13 is followed by step SR14 in which the target speed ratio 
"e'" is set at the actual speed ratio "e" obtained in step SR13. 
Subsequently, the previously discussed steps SR4 through SR11 are carried 
out. Since the target speed ratio "e'38 is set at the actual speed ratio 
"e" in step SR14, the error ".DELTA.e" obtained in SR8 is zero, whereby a 
variation or change in the speed ratio "e" of the transmission 14 is 
inhibited. Thus, steps SR14, SR13 and SR14 correspond to the previously 
indicated speed-ratio control disabling means 84. These steps SR12-SR14 
cooperate with the previously indicated steps S3 and S5 to constitute 
means for restraining a rate of variation in the speed ratio. In this 
connection, it is possible in step SR14 that the target speed ratio "e'" 
be set at the actual speed ratio "e" plus a small value .alpha.. In this 
case, the speed ratio "e" of the transmission 14 is gradually changed when 
the speed-ration control routine 1 is executed. 
According to the instant embodiment which has been described hitherto, a 
change in the speed ratio "e" of the variable transmission 14 is inhibited 
or interrupted during a time span between the first moment when the actual 
speed Ne of the engine 10 has exceeded the variation-restraint trigger 
speed Ne", and the second moment when the actual engine speed Ne has 
reached the target engine speed Ne', as shown by a speed ratio curve in 
solid line in FIG. 7. This is contrary to a continuous change in the speed 
ratio in a traditional arrangement, as shown in broken line of the speed 
ratio curve of in FIG. 7. The inhibition of the speed ratio change or 
variation according to the invention minimizes a reduction in transmission 
efficiency due to change in the speed ratio "e" of the transmission 14 
during acceleration of the vehicle, and thereby provides a reduction in 
specific-fuel consumption, i.e., an improvement in fuel economy. 
As described previously, the variation-restraint trigger speed Ne" is 
determined by multiplying the obtained target engine speed Ne' by a 
predetermined transmission efficiency .eta.. Hence, the acceleration 
performance or drivability of the vehicle is not influenced as experienced 
in a known arrangement. In the known arrangement, the transmission 
efficiency .eta. of the variable transmission 14 is reduced as the speed 
ratio thereof is varied during acceleration of the vehicle. The output of 
the engine while the speed ratio "e" is held constant is equal to an 
output which is obtained at an engine speed that is a product of the 
target engine speed Ne' mulfiplied by a transmission efficiency .eta.. 
This is the point to which an attention is directed in the present 
invention in determining the variation-restraint trigger speed Ne". 
In the known apparatus for controlling a variable transmission (14), after 
the actual engine speed Ne has been raised to the target engine speed Ne' 
with an abrupt depression of an accelerator pedal, the vehicle 
acceleration is effected with some shock irrespective of the operation of 
the acclerator pedal, due to an abrupt change in transmission efficiency 
.eta. of the transmission (14) upon stopping of variation in the speed 
ratio "e" and due to an inertia force of the engine 10, as indicated by a 
vehicle speed curve in broken line of FIG. 7. According to the present 
embodiment, however, the actual engine speed Ne is raised to the target 
engine speed Ne' without a change or variation in the speed ratio "e" of 
the transmission 14. As a result, otherwise possible development of a 
shock upon acceleration of the vehicle is minimized, as indicated by the 
vehicle speed curve in solid line of FIG. 7. 
Referring next to FIGS. 8 and 9, another embodiment of the invention will 
be described. In these figures, the same reference characters as used in 
the preceding figures are used to identify the corresponding elements, and 
the description of these elements is omitted. 
In FIG. 8, steps S8 through S11 are provided in place of steps S4 and S5 of 
FIG. 5 of the preceding embodiment. In step S8 of FIG. 8, the target 
torque T' is calculated based on the target engine speed Ne' obtained in 
step S2 and on the opening angle .theta. of the throttle valve 54 detected 
in step S1, and according to the predetermined relation as shown in FIG. 
4. The obtained target engine torque T' is multiplied by the target engine 
speed Ne' to obtain a target engine output Ps'. In the next step S9, a 
variation-restraint trigger output Ps" is calculated by multiplying the 
obtained target engine output Ps' by a transmission efficiency .eta. of 
the variable transmission 14 during the acceleration. Step 9 is followed 
by step S10 wherein an actual engine output Ps is calculated according to 
the predetermined relation as shown in FIG. 4. Then, step S11 is executed 
to check if the actual engine output Ps is smaller than the 
variation-restraint trigger output Ps". If the actual engine output Ps is 
smaller than the variation-restraint trigger output Ps", step S11 is 
followed by step S7 in which the speed-ratio control routine 2 is carried 
out. If the output Ps is not smaller than the output Ps", that is, if the 
speed Ne has been raised to a level at which the engine 10 is able to 
provide, without a change in the speed ratio "e", an output (T.times.Ne) 
not less than the output (T.times.Ne'.times..eta.) which is obtained 
during acceleration of the vehicle if and when the transmission 14 is 
controlled by a known apparatus, step S11 is followed by step S6 in which 
the speed-ratio control routine 1 is performed. 
FIG. 9 is a schematic block diagram showing a control arrangement according 
to this modified embodiment, wherein the above described steps S8, S9, S10 
and S11 correspond respectively to: target engine speed determining means 
100; means 102 for determining a variation-restraint trigger output Ps"; 
engine output detecting means 104; and speed-ratio control enable/disable 
discriminating means 80. 
According to the instant embodiment, a variation or change in the speed 
ratio "e" of the variable transmission 14 is inhibited during a time span 
between the first moment when the actual engine output Ps has exceeded the 
variation-restraint trigger output Ps" and the second moment when the 
actual engine output Ps has reached the target engine ouput Ps'. Thus, the 
same result as obtained in the preceding embodiment is offered by this 
modified embodiment. 
In addition, the embodiment of FIGS. 8 and 9 provides an advantage that the 
said time span during which the variation in the speed ratio "e" should be 
inhibited is determined more exactly and suitably, because 
variation-restraint trigger output Ps" of the engine is determined by 
multiplying the target engine output Ps' by a transmission efficiency 
.eta. of the transmission 14. 
While the present invention has been described in its preferred embodiments 
with reference to the accompanying drawing, it is to be understood that 
the invention is not limited thereto; but may be otherwise embodied. 
For example, the invention is applicable to other types of continuously 
variable transmission than the variable transmission 14 in connection with 
which the illustrated embodiments have been described. 
While an opening angle .theta. of the throttle valve 54 is used in the 
illustrated embodiments to detect a currently required output of the 
engine 10, it is possible that the required engine output be determined by 
detecting an amount of operation of the accelerator pedal 52, a vacuum 
pressure in the intake manifold 51, an amount of fuel injection, or other 
parameters which represent an output of the engine 10 which is currently 
required. 
While the illustrated embodiments are adapted to inhibit or interrupt a 
variation in the speed ratio "e" of the transmission 14 during the 
predetermined time span as described before, it is appreciated that the 
speed ratio "e" be varied at a very low, i.e., restrained or limited rate 
for the predetermined time span. 
As another modification, the first rotation sensor 60 for detecting a 
current or actual speed Ne of the engine 10 may be replaced by a sensor 
disposed on a distributor of the engine 10. 
As a further modification, the actual engine output Ps may be obtained in 
step S10 of FIG. 8 by detecting an actual torque of the engine with a 
suitable torque sensor, and by multiplying a detected torque value by an 
actual engine speed. 
Further, it is possible to arrange such that steps S6 of FIGS. 5 and 8 be 
executed only while the vehicle is in acceleration. 
It will be obvious that other changes and modifications of the invention 
may occur to those skilled in the art within the scope of the invention 
defined in the appended claims.