Method and apparatus for controlling continuously variable transmission for vehicle
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 acutal 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 a 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 engine speed, said method comprising the steps of: (a) determining a normal target speed based on a currently required output of the engine and according to a predetermined relation between the normal target speed and the required output of the engine, the normal target speed being normally used as the target engine speed to regulate the speed ratio of the transmission; (b) determining a variation-restraint trigger speed of the engine based at least on the required output of the engine and according to a predetermined relation between the trigger speed and the required output, the variation-restraint trigger speed being lower than the normal target speed of the engine; (c) checking whether the vehicle is in acceleration or not; (d) restraining a rate of variation in the speed ratio of the variable transmission during a first 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 normal target speed, while the vehicle is in acceleration; and (e) selecting the variation-restraining trigger speed as a special target speed when the actual speed of the engine has been raised to the normal target speed with the restrained rate of variation in the speed ratio of the transmission during acceleration of the vehicle, the special target speed being used as the target engine speed to regulate the speed ratio of the transmission during a second time span between said second moment and a third moment when the actual speed of the engine has been lowered to the variation-restraint trigger 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 engine speed, the apparatus comprising: (a) means for determining a normal target speed based on a currently required output of the engine and according to a predetermined relation between the normal target speed and the required output of the engine, the normal target speed being normally used as the target engine speed by said regulating means; (b) means for determining a variation-restraint trigger speed of the engine based at least one the required output of the engine and according to a predetermined relation between the trigger speed and the required output, the variation-restraint trigger speed being lower than the normal target speed of the engine; (c) means for checking whether the vehicle is in acceleration or not; (d) means for restraining a rate of variation in the speed ratio of the variable transmission during a first 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 normal target speed, while the vehicle is in acceleration; and (e) means for selecting the variation-restraint trigger speed as a special target speed when the actual speed of the engine has been raised to the normal target speed with the restrained rate of variation in the speed ratio of the transmission during acceleration of the vehicle, the special target speed being used as the target engine speed by said regulating means during a second time span between said second moment and a third moment when the actual speed of the engine has been lowered to the variation-restraint trigger speed. 
     In the present invention as described above, the variation in the speed ratio of the variable transmission during acceleration of the vehicle is restrained by the restraining means immediately after the actual engine speed has exceeded the variation-restraint trigger speed which is determined based at least on the currently required output of the engine. The restraint of variation in the speed ratio of the transmission continues until the actual engine speed has reached the normal target speed. As soon as the actual engine speed has been raised to the normal target speed, the variation-restraint trigger speed is selected as a special target speed which is used as the target engine speed, in place of the normal target speed, to regulate the transmission. That is, once the actual engine speed has been raised from the variation-restraint trigger speed up to the normal target speed with the restrained rate of variation in the speed ratio, the transmission is controlled such that the actual speed of the engine coincides with the variation-restraint trigger speed, whereby the engine speed is rapidly lowered down to the trigger speed while the speed ratio of the transmission is increased. When the engine speed has been lowered to the variation-restraint trigger speed, the transmission is again controlled such that the engine speed is raised to the normal target speed with the restraint of variation in the speed ratio of the transmission. In this manner, the transmission is controlled with the engine speed varied between the variation-restraint trigger speed (special target speed) and the normal target speed, once the engine speed has been raised to the variation-restraint trigger speed during acceleration of the vehicle. Since a change or variation in the speed ratio of the transmission is restrained periodically during acceleration of the vehicle, and the overall rate of variation in the speed ratio is held at a considerably reduced level, a decrease in transmission efficiency of the transmission due to variation in the speed ratio is minimized. Accordingly, the engine is operated with a relatively high fuel economy. 
     According to one embodiment of the method or apparatus of the invention, the determination of the variation-restraint trigger speed is based further on a running speed of the vehicle and according to a predetermined relation between the trigger speed and the running speed of the vehicle. 
     In one form of the invention, the transmission is of belt-and-pulley type, including a first variable-diameter pulley, a second variable-diameter transmission, and a transmission belt connecting the first and second variable-diameter pulleys. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other objects, features and advantages of the present invention will be better understood from reading the following description of the preferred embodiment taken in conjunction with the accompanying drawing in which: 
     FIG. 1 is a schematic illustration of one embodiment of an apparatus of the invention for controlling a continuously variable transmission for a vehicle; 
     FIGS. 2A and 2B are schematic block diagrams generally showing a control arrangement for the transmission of FIG. 1; 
     FIG. 3 is a graphical representation of a characteristic relation between an angle of a throttle valve opening, and a normal target speed of the vehicle engine; 
     FIG. 4 is a graphical representation of a characteristic relation between the throttle valve opening angle and a running speed of the vehicle; 
     FIG. 5 is a graph representing iso-horsepower curves and iso-throttle valve opening curves as a function of engine speed and engine output torque of the vehicle; 
     FIGS. 6-8 are flow charts illustrating the operation of the embodiment of FIG. 1; and 
     FIG. 9 is a timing chart showing an operational timing and changes in related variables of the embodiment of FIG. 1. 
    
    
     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 width 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 hydaulic 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 θ 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 θ 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 &#34;e&#34; and a variation rate &#34;e&#34; (=Δe/Δt) at which the speed ratio &#34;e&#34; 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 signal 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. Normal target speed determining means 76 determines a first or normal target speed Ne&#39; of the engine 10 (rotating speed of the input shaft 16) based on the opening angle θ of the throttle valve 54 represented by the THROTTLE signal TH, and according to a predetermined stored relation between the opening angle θ and the first or normal target engine speed Ne&#39;, which relation is represented by a graph of FIG. 3. The normal target engine speed Ne&#39; is determined such that the required horsepower of the engine 10 (required engine output) represented by the opening angle θ of the throttle valve 54 is obtained with a minimum specific fuel consumption. Means 78 determines a later described variation-restraint trigger speed Ne&#34; based on an angle θ of opening of the throttle valve 54 and according to a predetermined relation between the trigger speed Ne&#34; and the opening angle θ, which relation is shown in broken line in FIG. 3. The variation-restraint trigger speed Ne&#34; shown in FIG. 3 is a valve obtained by multiplying the normal target engine speed Ne&#39; by a transmission efficiency η of the transmission 14. Thus, the trigger speed Ne&#34; is lower than the normal target speed Ne&#39; of the engine 10. In this connection, it is noted that the variation-restraint trigger speed Ne&#34; may be determined based on a running speed V of the vehicle and on an opening angle θ of the throttle valve 54, and according to a predetermined stored relation of FIG. 4 between the opening angle θ  and the running speed V, with the trigger speed Ne&#34; as a parameter, which relation is determined with the transmission efficiency η taken into account. First chacking means 79 checks if the vehicle is in accleration or in deceleration. The first checking means 79 activates second checking means 80 if the vehicle is in acceleration, but activates speed-ratio control enabling means 82 if the vehicle is in deceleration. The second checking means 80 checks if the actual speed Ne of the engine 10 detected by engine speed detecting means 86 falls within a range between the normal target engine speed Ne&#39; and the variation-restraint trigger speed Ne&#34;, or not. If the actual engine speed Ne falls within the above-identified range, the second checking means 80 selects flag detecting means 83 which detects the content of a flag FN. If the actual engine speed Ne does not fall within the above range, the second checking means 80 selects flag setting means 81 which sets the flag FN in one of two states, i.e., &#34;1&#34; or &#34;0&#34; according to whether the actual engine speed Ne is higher than the normal target engine speed Ne&#39; or lower than the variation-restraint trigger speed Ne&#34;. The flag detecting means 83 detects the content of the flag FN set by the flag setting means 81 when the checking by the second checking means 80 reveals that the actual engine speed Ne is within the range between the normal target speed Ne&#39; and the variation-restraint trigger speed Ne&#34;. The flag detecting means 83 activates speed-ratio control disabling means 84 if the actual engine speed Ne detected in the last cycle of control of the transmission 14 according to this invention is lower than the variation-restraint trigger speed Ne&#34;, but activates selecting means 85 if the actual engine speed Ne in the last cycle is higher than the normal target engine speed Ne&#39;. Normally, the transmission 14 is controlled by the speed-ratio control enabling means 82 such that the actual engine speed Ne coincides with the normal (upper) target engine speed Ne&#39;. However, as soon as the actual engine speed Ne has been raised to the normal target engine speed Ne&#39; during acceleration of the vehicle, the selecting means 85 selects the variation-restraint trigger speed Ne&#34; as a second or special (lower) target speed of the engine 10 which is lower than the first or normal target engine speed Ne&#39;. As described later in more detail, once the actual engine, speed Ne has been raised to the normal target speed Ne&#39; in a manner as described later, the engine speed Ne is lowered to the variation-restraint trigger speed Ne&#34; which is utilized as the special target speed. 
     The speed-ratio control enabling means 82 determines an actual speed ratio &#34;e&#34; of the variable transmission 14 based on the actual running speed V of the vehicle detected by vehicle speed detecting means 88 (i.e., based on 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 &#34;e&#39;&#34; of the transmission 14 for coincidence of the actual engine speed Ne with the normal target engine speed Ne&#39;, 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 &#34;e&#34; and the target speed ratio &#34;e&#39;&#34; is zeroed. The disabling means 84 causes the target speed ratio &#34;e&#39;&#34; to be replaced by the actual speed ratio &#34;e&#34; 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 normal target engine speed Ne&#39; without changing the speed ratio &#34;e&#34; 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 &#34;e&#34;. 
     Engine torque detecting means 92 detects an actual output torque T based on the opening angle θ 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 θ and engine output torque T, which relation is represented by a graph of FIG. 5. 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 determined 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. 6, 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 θ of the throttle valve 54, data on the actual engine speed Ne, and data on the running speed V of the vehicle, according to the THROTTLE, INPUT ROTATION and OUTPUT ROTATION signals TH, SE and SC. Step S1 is followed by step S2 which corresponds to the normal target speed determining means 76 previously indicated. In this step S2, the normal target speed Ne&#39; of the engine 10 is calculated based on the opening angle θ and according to the stored relation of FIG. 3. Then, step 3 corresponding to the first checking means 79 is performed to check if an acceleration/deceleration flag FV is set at &#34;1&#34; or not. In the case where the vehicle is in deceleration and the flag FV is set at &#34;0&#34;, a later described speed-ratio control routine 2 in step S11 will be performed. If the vehicle is in acceleration and the flag FV is set at &#34;1&#34;, step S3 is followed by step S4. 
     The flag FV is set through execution of an interruption routine shown in FIG. 7. In this interruption routine, step SW1 is first executed to check is a timer flag FT is set at &#34;1&#34; or not. If the timer flag FT is set at &#34;1&#34;, the control goes to step SW2 wherein the running speed V of the vehicle (rotating speed No of the output shaft 20 of the transmission 14) represented by the OUTPUT ROTATION signal SC is stored in the RAM 68. It is noted here that the timer flag FT is set to &#34;1&#34; at a predetermined time interval, whereby the interruption routine is executed in a cyclic fashion. Then, step SW3 is executed to check if the current running speed V of the vehicle is lower than a running speed V o  of the vehicle which was detected in the last cycle of execution of the interruption routine. If the current running speed V is lower than the last speed V o , step SW3 is followed by steps SW4 and SW5 to store the current running speed V as the last running speed V o  for the next cycle (SW4), and to set the acceleration/deceleration flag FV to &#34;0&#34;, which means that the vehicle is in deceleration. In the case where the current running speed V of the vehile is not lower than the last speed V o , step SW3 is followed by step SW6 wherein the current speed V (stored in SW2) is stored as the last speed V o  for the next cycle. Then, the control goes to step SW7 to check if the current running speed V is lower than a predetermined relatively low level V 1  (e.g., 10 kg/h). If the current running speed V is lower than the predetermined low level V 1 , which means a partial engagement of the clutch 12, step SW7 is followed by step SW5. If the running speed V is higher than the predetermined low level V 1 , step SW7 is followed by step SW8 wherein the acceleration/deceleration flag FV is set to &#34;1&#34; which indicates that the vehicle is in acceleration. 
     Referring back to FIG. 6, the previously indicated step S4 is executed to check if the actual engine speed Ne is lower than the normal target speed Ne&#39;. If the actual engine speed Ne is not lower than the normal target speed Ne&#39;, step S5 is executed to set a flag FN to &#34;1&#34;, and the control goes to step S11. If the actual engine speed Ne is lower than the normal target speed Ne&#39;, the control goes to step S6 which corresponds to the trigger speed determining means 78. In this step S6, the variation-restraint trigger speed Ne&#34; is calculated based on the opening angle θ of the throttle valve 54 and according to the predetermined relation (represented in broken line in FIG. 3) between the opening angle θ and the normal target speed Ne&#34;. This variation-restraint trigger speed Ne&#34; is calculated by multiplying the normal target speed Ne&#39; by a transmission efficiency η of the transmission 14, which efficiency may be a common value obtained during a rapid change in the speed ratio &#34;e&#34; of the transmission 14 during acceleration of the vehicle where the transmission 14 is operated under control of a known control apparatus. This transmission efficiency η may be an average value during periods of acceleration of the vehicle, or may be exactly calculated from time to time based on a rate of variation &#34;e&#34; in the speed ratio &#34;e&#34; of the transmission 14. Then, step S6 is followed by step S7 to check if the actual engine speed Ne is lower than the variation-restraint trigger speed Ne&#34;. If the engine speed Ne is lower than the trigger speed Ne&#34;, step S8 is executed to set the flag FN to &#34;0&#34;. If the engine speed Ne is not lower than the trigger speed Ne&#34;, step S7 is followed by step S9 which corresponds to the previously indicated flag detecting means 83. In this step S9, the flag detecting means 83 checks if the content of the flag FN is &#34;1&#34; or not. Thus, the previously stated steps S5 and S8 correspond to the flag setting means 81, and the current setting of the flag FN is checked or detected in step S9 by the flag detecting means 83. It is further noted that the steps S4 and S7 select the flag setting step S5 or S8, or select the step S9 which selects one of the speed-ratio control routines 1 and 2. In other words, the steps S4 and S7 correspond to the second checking means 80 which cehcks whether the engine speed Ne falls within the range between the speeds Ne&#39; and Ne&#34;. 
     In the case where the checking in step S9 reveals that the flag FN is set at &#34;1&#34;, the control goes to step S10 which corresponds to the selecting means 85. In this step S10, the variation-restraint trigger speed Ne&#34; is selected as a special or second target speed which is used, in place of the normal target speed Ne&#39;, to control the operation of the transmission 14. Successively, step S11 is performed to initiate the execution of the speed-ratio control routine 2. If the checking in step S9 reveals that the flag FN is set at &#34;0&#34;, step S9 is followed by step S12 in which the speed-ratio control routine 1 is executed. 
     The speed-ratio control routines 1 and 2 are executed in the manner as illustrated in FIG. 8 and as described below. 
     In the speed-ratio control routine 2, step SR1 is executed at first, whrein 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, respectively. Then, step SR2 is executed to calculate the actual speed ratio &#34;e&#34; (=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 &#34;e&#39;&#34; (=No/Ni&#39;) is calculated, and then by step S4 to check if the target speed ratio &#34;e&#39;&#34; is lower than a minimum ratio &#34;e min  &#34;. If the ratio &#34;e&#39;&#34; is lower than the minimum ratio &#34;e min  &#34;, step S5 is executed to set the target speed ratio &#34;e&#39;&#34; at the minimum ratio &#34;e min  &#34;. If the ratio &#34;e&#39;&#34; is not lower than the minimum ratio &#34;e min  &#34;, step SR4 is followed by step SR6 to check if the target speed ratio &#34;e&#39;&#34; is lower than a maximum ratio &#34;e max  &#34; . If the ratio &#34;e&#39;&#34; is not lower than the maximum ratio &#34;e max  &#34;, step SR6 is followed by step SR7 in which the target speed ratio &#34;e&#39;&#34; is set at the maximum ratio &#34;e max  &#34;. If the ratio &#34;e&#39;&#34; is lower than the maximum ratio &#34;e max  &#34;, step SR6 is followed by step SR8. 
     In step SR8, an error &#34;Δe&#34; (=&#34;e&#39;&#34;-&#34;e&#34;) is obtained by subtracting the actual speed ratio &#34;e&#34; from the target speed ratio &#34;e&#39;&#34;. Then, in step SR9, a flow control voltage Vf to zero the error &#34;Δe&#34; is determined according to the following formula (1): 
     
         Vf=K.sub.1 ·&#34;e&#34;+K.sub.2 ·&#34;Δe&#34;      (1) 
    
     where, K 1  and K 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 &#34;e&#34; 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. 5. Step SR10 is followed by step SR11 corresponding to the line pressure determined 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. 
     
         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 rotatiang speeds Ni and No of the input and output shafts 16, 20, and to obtain the actual speed ratio &#34;e&#34; (=No/Ni) of the variable transmission 14. Step SR13 is followed by step SR14 in which the target speed ratio &#34;e&#39;&#34; is set at the actual speed ratio &#34;e&#34; obtained in step SR13. Subsequently, the previously discussed steps SR4 through SR11 are carried out. Since the target speed ratio &#34;e&#39;&#34; is set at the actual speed ratio &#34;e&#34; in step SR14, the error &#34;Δe&#34; obtained in SR8 is zero, whereby a variation or change in the speed ratio &#34;e&#34; 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 &#34;e&#39;&#34; be set at the actual speed ratio &#34;e&#34; plus a small value α. In this case, the speed ratio &#34;e&#34; of the transmission 14 is gradually changed when the speed-ration control routine 1 is executed. 
     According to the instant embodiment wherein the steps of operation of FIG. 6 are repeatedly executed, the engine speed Ne during acceleration of the vehicle is elevated toward the normal target speed Ne&#39;. As shown in FIG. 9, the rise of the engine speed Ne to the variation-restraint trigger speed Ne&#34; is accomplished while the speed ratio &#34;e&#34; of the transmission 14 is varied. However, after the engine speed Ne has exceeded the variation-restraint trigger speed Ne&#34;, a change in the speed ratio &#34;e&#34; of the variable transmission 14 is inhibited or interrupted until the actual engine speed Ne has reached the normal target speed Ne&#39;. That is, the speed ratio &#34;e&#34; of the transmission 14 is held constant while the engine speed Ne is raised from the trigger speed Ne&#34; to the normal target speed Ne&#39;. The actual engine speed is then controlled so as to coincide with the normal target speed Ne: As soon as the actual engine speed Ne has reached the normal target speed Ne&#39;, is switched in step S10 from the normal target speed Ne&#39; to the variation-restraint trigger speed Ne&#34; which is lower than the normal target speed Ne&#39;. As a result, the speed ratio &#34;e&#34; of the transmission 14 is rapidly changed so that the actual engine speed Ne is lowered from the normal target speed Ne&#39; to the special target speed Ne&#34;, i.e., to the variation-restraint trigger speed Ne&#34; at which the inhibition or restraint of a variation in the speed ratio &#34;e&#34; is triggered. In summary, the actual engine speed Ne during acceleration of the vehicle (during an increase in the vehicle running speed V) is repeatedly changed between the variation-restraint trigger speed Ne&#34; (special or second target speed Ne&#34;) and the normal or first target speed Ne&#39;, once the engine speed Ne has been raised to the trigger speed Ne&#34;. 
     In the above described speed cotnrol arrangement for the variable transmission 14, the change in the speed ratio &#34;e&#34; during acceleration of the vehicle is inhibited periodically, and the length of time during which the speed ratio &#34;e&#34; is changed is considerably limited. Therefore, a reduction in the transmission efficiency η due to change in the speed ratio &#34;e&#34; is minimized, and accordingly the fuel economy of the engine is maximized. Although the speed ratio &#34;e&#34; of the transmission 14 is increased in steps for limited time spans as shown in FIG. 9, a decrease in the transmission efficiency η due to this change in the speed ratio &#34;e&#34; is sufficiently compensated by an inertia force of an output shaft or other rotating components of the engine 10, because the increase in the speed ratio &#34;e&#34; causes a decrease in the engine speed Ne from the normal target speed Ne&#39; down to the lower special target speed Ne&#34;. 
     As described previously, the variation-restraint trigger speed Ne&#34; is determined with considerations given to a transmission efficiency η of the transmission 14 during acceleration of the vehicle. Hence, the acceleration performance or drivability of the vehicle is not influenced as experienced in a known arrangement. In the know arrangement, the transmission efficiency η of the 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 &#34;e&#34; is held constant is equal to an output which is obtained at an engine speed that is a product of a target engine speed (Ne&#39;) multiplied by a transmission efficiency η. This is the point to which an attention is directed in the present invention in determining the variation-restraint trigger speed Ne&#34;. 
     In the known apparatus for controlling a variable transmission (14), after the actual engine speed Ne has been raised to a target engine speed (Ne&#39;) with an abrupt depression of an accelerator pedal, the vehicle acceleration is effected with some shock irrespective of the timing of the accelerator pedal depression, due to an abrupt change in transmission efficiency η of the transmission (14) upon stopping of variation in the speed ratio &#34;e&#34; and due to an inertia force of the engine 10, as indicated by a vehicle speed curve in broken line of FIG. 9. According to the present embodiment, however, the engine speed Ne is controlled so as to coincide with the normal target speed Ne&#39; or the special target speed Ne&#34; in such manner that a change or variation in the speed ratio &#34;e&#34; of the transmission 14 is substantively restrained. As a result, the vehicle running speed is increased smoothly as shown in FIG. 9 by the vehicle speed curve V indicated in solid line, while otherwise possible generation of a shock upon acceleration of the vehicle is minimized. 
     While the present invention has been described in its preferred embodiment 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 embodiment has been described. 
     While an opening angle θ 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 &#34;e&#34; to the transmission 14 during the predetermined time span as described before, it is appreciated that the speed ratio &#34;e&#34; be varied at a very low, i.e., restrained or limited rate for the predetermined time span. 
     As another modification, the normal 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. 
     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.