Abstract:
The torque capacity of a torque transmitting device incorporated in a power transmission system between an output shaft of a hydrodynamic power transmission and a drive shaft of a vehicle is so controlled as to provide a predetermined rotation speed difference between the input and output shafts of the hydrodynamic power transmission when the vehicle is in a state of standstill, thereby reducing creeping of the vehicle in the standstill state and also improving the response of the power transmission system at the time of vehicle starting. When the vehicle is to be started, the torque capacity is feedback-controlled so as to change the rotation speed of the output shaft of the hydrodynamic power transmission at a predetermined changing rate, thereby ensuring smooth starting without any appreciable shock.

Description:
FIELD OF THE INVENTION 
     This invention relates to a power transmission including a hydrodynamic power transmission, and more particularly to improvements in an automatic transmission for a vehicle. 
     BACKGROUND OF THE INVENTION 
     A vehicle equipped with an automatic transmission of the type including a hydrodynamic power transmission, such as a torque converter or a fluid coupling, frequently encounters a problem called &#34;creeping phenomenon&#34; which permits gradual movement of the vehicle from its standstill state. To obviate this problem, an automatic transmission has been proposed which is automatically placed in its neutral mode when the vehicle is in a state of standstill and the accelerator pedal has not been depressed. As soon as depression of the accelerator pedal is detected, frictional engaging devices such as brakes and clutches are selectively engaged to attain power transmission. 
     Vibration of the engine and fuel consumption of the engine in a standstill state of the vehicle can be reduced by employment of the automatic transmission of the above construction. However, there occurs a substantial shock at the time of vehicle starting due to abrupt engagement of the frictional engaging devices. 
     This problem can be obviated by gradually engaging the frictional engaging devices at the time of vehicle starting. However, such gradual engagement of the frictional engaging devices leads to further problems in that establishment of the power transmission system at the time of vehicle starting time is delayed, this resulting in degradation of the starting acceleration characteristic. Also, a rise in the engine rotation speed can impart excessive load to the frictional engaging devices and result in burnout of the frictional engaging devices. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to provide a control system for a power transmission of a vehicle, which can reduce the undesirable tendency of the vehicle to creep from a standstill state of the vehicle. 
     According to the present invention, a control system is provided for controlling a torque transmitting device interposed between an output shaft of a hydrodynamic power transmission and a drive shaft of a vehicle. The torque capacity of the torque transmittig device is so controlled as to provide a predetermined difference between the rotation speeds of the input and output shafts of the hydrodynamic power transmission when the vehicle is in a state of standstill. The torque transmitting device is placed in a partially engaged state by the control so that the undesirable creeping can be reduced, and the response of the power transmission system at the time of vehicle starting can be improved. 
     Also, according to the present invention, the predetermined rotation speed difference between the input and output shafts of the hydrodynamic power transmission is changed in dependence on the rotation speed of the input shaft of the hydrodynamic power transmission (the output shaft of the engine), so as to prevent burnout of the torque transmitting device, and, also to prevent an undesirable increase in the tendency of the vehicle to creep, due to an increase in the amount of transmitted torque resulting from an increase in the rotation speed of the engine, such as when the engine is in its cold state in a standstill state of the vehicle. 
     Further, according to the present invention, the torque capacity of the torque transmitting device is feedback-controlled when the vehicle is to be started, so that the changing rate of the rotation speed of the output shaft of the hydrodynamic power transmission is maintained at a determined rate, thereby alleviating the shock occurring at the time of vehicle starting and ensuring smooth vehicle starting. 
    
    
     BRIEF DESCRITION OF THE DRAWINGS 
     FIG. 1 is a skelton diagram showing schematically the structure of a prior art automatic power transmission for a vehicle to which the present invention is applied. 
     FIG. 2 is a circuit diagram of a prior art hydraulic circuit comprising part of the control sytem controlling the automatic transmission. 
     FIG. 3 is a flow chart showing the steps of control processing in a preferred embodiment of the present invention applied to the automatic transmission. 
     FIGS. 4(a) and 4(b) are graphs illustrating the manner of control according to the flow chart shown in FIG. 3. 
    
    
     DESCRIPTION OF THE PRIOR ART EMBODIMENT 
     Before describing the present invention in detail, an example of an automatic transmission to which the present invention is applied will be briefly described with reference to FIG. 1 which shows schematically the structure of such an automatic transmission. 
     Referring now to FIG. 1, a prime mover or an internal combustion engine 11 providing a power source for driving a vehicle is integrally coupled at its output shaft to a pump 14 of a torque converter 13 through an input shaft 12 of the torque converter 13. The torque converter 13 includes the pump 14, a turbine 15, a stator 16 and a one-way clutch 17. The stator 16 is coupled to a casing 18 through the one-way clutch 17. The stator 16 is so arranged that it can rotate in the same direction as the input shaft 12 but is not permitted to rotate in the reverse direction by the function of the one-way clutch 17. 
     The torque transmitted to the turbine 15 is then transmitted by an output shaft 19 of the torque converter 13 to a speed change gear assembly which is provided behind the torque converter 13 and which obtains four forward speeds and one reverse speed. 
     The speed change gear assembly includes three clutches 20, 21, 22, two brakes 23, 24 and one ravigneaux type planetary gear set 25. The planetary gear set 25 includes a ring gear 26, a long pinion gear 27, a short pinion gear 28, a front sun gear 29, a rear sun gear 30, and a carrier 31. The carrier 31 rotatably supports the two pinion gears 27 and 28 and is also rotatable around its own axis. The ring gear 26 is coupled to an output shaft 32 of the transmission, and the front sun gear 29 is coupled to the converter output shaft 19 through a kickdown drum 33 and the front clutch 20. The rear sun gear 30 is coupled to the converter output shaft 19 through the rear clutch 21. The carrier 31 is coupled to the casing 18 through the low reverse brake 24 and is also coupled to the converter output shaft 19 through the 4th speed clutch 22 provided at the rear end of the speed change gear assembly. The kickdown drum 33 can be integrally coupled to the casing 18 by the kickdown brake 23. The torque transmitted through the planetary gear set 25 is transmitted from an output gear 34 fixed to the transmission output shaft 32 to a drive shaft of the vehicle via a transfer gear, a final reduction gear, a differential gear unit, etc. all of which are not shown. 
     The frictional engaging devices or the individual clutches 20, 21, 22 and brakes 23, 24 described above are hydraulic devices including engaging piston units, servo units or the like respectively and are selectively actuated through a hydraulic pressure control system by a hydraulic pressure generated at an oil pump (not shown) which is coupled to the pump 14. Combinations of the operations of the individual clutches and brakes achieve the speed ratios of four forward speeds and one reverse speed as shown in Table 1. In the table, the symbol o indicates that the corresponding clutch or brake is engaged. 
     
                       TABLE 1______________________________________Clutch,  Speed ratiobrake    1st       2nd    3rd      4th R______________________________________Clutch 20                 o            oClutch 21    o         o      oClutch 22                          oBrake 23           o               oBrake 24 o                             o______________________________________ 
    
     As is commonly known, all the clutches and brakes are released in the neutral position of the speed change gear assembly in the automatic transmission. 
     FIG. 2 shows part of the hydraulic pressure control system which achieves the speed ratios described above. The rear clutch 21 and the low reverse brake 24 are engaged to achieve the 1st speed ratio, and a hydraulic pressure chamber (not shown) of the low reverse brake 24 communicates with a 1-2 shift value 35 through an oil passage 36. The 1-2 shift value 35 is changed over for communicating an oil passage 38 with the oil passage 36 or another oil passage 35b under control of a signal hydraulic pressure supplied from a shift control valve (not shown) by way of an oil passage 35a. When a manual valye (not shown) known per se is changed over by a selector lever (not shown) to a position (referred to hereinafter as an R range) at which the reverse speed ratio is obtained by the speed change gear assembly, line pressure is supplied from the manual valve to the low reverse brake 24 by way of an oil passage 35c. The line pressure is also supplied from the manual valve by way of an oil passage 39 to a hydraulic pressure control valve 37 communicating with the oil passage 38. The hydraulic control valve 37 regulates the line pressure to a desired hydraulic pressure depending on a control hydraulic pressure in an oil passage 40 and supplies such a regulated hydraulic pressure to the oil passage 38. The line pressure is supplied from the manual valve to the oil passage 39 when the manual valve is shifted by the selector lever to positions (a D range, a 2 range, an L range, etc. all of which will be referred to hereinafter as the D range for the purpose of simplicity) where the forward speed ratios are obtained by the speed change gear assembly. A hydraulic pressure supplied from a source of low hydraulic pressure (not shown) is suitably controlled by an electromagntic valve 43 duty-controlled by an electronic control device 42, so that the control hydraulic pressure in the oil passage 40 is controlled to provide any desired hydraulic pressure corresponding to the duty ratio. 
     Such a hydraulic pressure control system for an automatic transmission is described in detail in the present applicants U.S. Pat. Nos. 4,538,482 and 4,513,639. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the embodiment of the present invention, the hydraulic pressure supplied to the low reverse brake 24 is suitably controlled to correspondingly regulate the torque capacity of the brake 24, so as to attain the desired reduction of creeping when the vehicle is in a state of standstill with its selector lever positioned in the D range, and also to ensure satisfactory response of the power transmission system at the time of vehicle starting. For this purpose, electrical signals from a load sensor 44, an input-shaft rotation speed sensor 45, an output-shaft rotation speed sensor 46, a vehicle speed sensor 47, a selecter lever position sensor or an inhibitor switch 48 and an accelerator-pedal depression sensor or a pedal switch 49 are applied to the electronic control device 42. The load sensor 44 detects the load of the internal combustion engine 11 by measuring the operating parameters of the engine 11 such as, for example, the opening degree of the throttle valve or the displacement of the control rack of the fuel injection pump. The input-shaft rotation speed sensor 45 detects the rotation speed of the engine 11 by known means so as to detect the rotation speed N e  of the converter input shaft 12 which rotation speed is the same as that of the engine 11. The output-shaft rotation speed sensor 46 shown in FIG. 1 detects the rotation speed N t  of the converter output shaft 19. The vehicle speed sensor 47 shown in FIG. 1 detects the rotation speed of the gear-assembly output shaft 32 so as to sense the traveling speed N o  of the vehicle. The inhibitor switch 48 is a position sensor which detects the position of the known selector lever which can take any one of the positions P, R, N, D, 2, L, etc. for changing over the manual valve, hence, the position of the manual valve. The pedal switch 49 detects whether or not the accelerator pedal is released, that is, whether or not the accelerator pedal is depressed, so as to detect whether or not the driver intends to start the vehicle. 
     How the control system controls the power transmission system in the standstill and starting states of the vehicle will now be described with reference to a flow chart shown in FIG. 3. 
     The program shown in FIG. 3 starts in response to the starting of the engine 11. After the program starts, the electronic control device 42 judges as to whether or not the vehicle is in its standstill state, that is, whether or not Nhd o=0, in the step 50 on the basis of the electrical output signal from the vehicle speed sensor 47. When the result of judgment in the step 50 is &#34;NO&#34;, execution of this program is ceased, and a usual shift control program (not shown) is executed. When, on the other hand, the result of judgment in the step 50 is &#34;YES&#34;, judgment is made in the step 51 as to whether or not the selecter leer is in a position other than a position (usually, the N or P range) at which the speed change gear assembly is placed substantially in its neutral position. In other words, judgment is made as to whether or not the selecter lever is positioned in the D or R range. The above judgment is based on the signal applied from the inhibitor switch 48. When the result of judgment in the step 51 is &#34;NO&#34;, the usual shift control program is executed as in the above case. When, on the other hand, the result of judgment in the step 51 is &#34;YES&#34;, judgment is made in the next step 52 as to whether or not the accelerator pedal is depressed, that is, whether or not the driver intends to start the vehicle. The above judgment is based on the output signal of the pedal switch 49. 
     When the result of judgment in the step 52 is &#34;NO&#34;, that is, when the electronic control device 42 judges that the accelerator pedal is not depressed and, hence, the driver does not intend to start the vehicle, the rotation speed N e  of the input shaft 12 of the torque converter 13 is detected in the step 53 on the basis of the output signal of the input-shaft rotation speed sensor 45, and then, the rotation speed N t  of the output shaft 19 of the torque converter 13 is detected in the step 54 on the basis of the output signal of the output-shaft rotation speed sensor 46. In the next step 55, the speed difference ΔN=N e  -N t  is computed. 
     In the next step 56, a desired rotation-speed difference ΔN&#39; (for example, ΔN&#39;=150 rpm) which is a objective value of the computed rotation-speed difference ΔN is set, and, in the step 57, the difference δ(=ΔN&#39;-ΔN) between the desired rotation-speed difference ΔN&#39; and the actual rotation-speed difference ΔN computed in the step 55 is computed. In the step 58, judgment is made as to whether the difference δ is positive (δ&gt;0) or not. When the result of judgment in the step 58 proves that δ is positive, processing for decreasing the duty ratio of current supplied to the electromagnetic valve 43 to increase the value of hydraulic pressure supplied to the low reverse brake 24 thereby increasing the engaging force, hence, the torque capacity of the low reverse brake 24, is executed in the step 59. When, on the other hand, the result of judgment in the step 58 proves that δ is negative, processing for increasing the duty ratio of current supplied to the electromagnetic valve 43 thereby decreasing the torque capacity of the low reverse brake 24 is executed in the step 60. 
     In the step 58, whether δ=0 or not may also be judged. In such a case, the manner of processing may be such that the current torque capacity of the low reverse brake 24 is maintained when δ=0. 
     After execution of the step 59 or 60, the program returns to the step 50 again, and the steps 50 to 59 or 60 are repeated unless the driver&#39;s intension to start the vehicle is detected under the circumstance in which the selector lever is positioned in the D or R range in the standstill state of the vehicle. As a result, the torque capacity of the low reverse brake 24 is regulated so that the actual rotation speed difference ΔN between the input and output shafts 12 and 19 of the torque converter 13 converges to or coincides with the desired rotation-speed difference ΔN&#39;, and the low reverse brake 24 is maintained substantially in a partially engaged state thereby preventing or reducing the undesirable creeping. In the meantime, the line pressure, which is not decreased, is supplied to the rear clutch 21 or front clutch 20 which is to be engaged to achieve the 1st speed ratio or the reverse speed ratio, so that the clutch 21 or 22 is completely engaged. 
     In a standstill state of the vehicle, the rotation speed difference between the input and output shafts 12 and 19 of the torque converter 13 is preferably as small as possible. This is because all the energy transmitted to the low reverse brake 24 via the torque converter 13 is turned into heat due to the partially engaged (slipping) state of the low reverse brake 24, and, when the rotation speed difference ΔN is large, greater energy is transmitted to increase the amount of generated heat until finally burnout of the low reverse brake 24 may occur. Especially, burnout of the low reverse brake 24 will occur more easily, and the force causing the creeping will increase, if the rotation speed difference ΔN is selected to be the same as that desired under the normally idling condition of the engine when the idling rotation speed of the engine is higher than the normal value as when the engine is in its cold state or when the engine operates with an increased load due to, for example, continuous operation of the compressor of the cooler. This is because, the energy transmitted via the torque converter 13 increases in proportion to the second power of the rotation speed of the input shaft 12 of the torque converter 13 with the increase in that rotation speed. 
     However, if the rotation speed difference ΔN is set at an excessively small value when the idling rotation speed is low, the torque transmitted via the torque converter 13 is also decreased to such an extent that the torque is now absorbed by the resistances including the resistance of oil against the torque-converter output shaft due to agitation of oil and the dragging resistances of the other frictional engaging devices, resulting in impossibility of maintaining the low reverse brake 24 in its partially engaged state. 
     Therefore, it is necessary to set the rotation speed difference ΔN at a relatively large value at a low idling rotation speed of the engine 11 and to set the rotation speed difference ΔN at a relatively small value at a high idling rotation speed of the engine 11. 
     To meet the above requirement, the desired rotation-speed difference ΔN&#39; determined in the step 56 is selected to become smaller with an increase in the rotation speed of the input shaft 12 of the torque converter 13. Thus, the actual rotation-speed difference ΔN is also controlled to become smaller with the increase in the rotation speed of the input shaft 12 of the torque converter 13. That is, the value of ΔN is so controlled as to decrease the torque transmitted via the torque converter 13. 
     The value of the desired rotation-speed difference ΔN&#39; may be computed on the basis of the output signal of the input-shaft rotation speed sensor 45. Various values of ΔN&#39; corresponding to various rotation speeds of the input shaft 12 of the torque converter 13 may be stored in a memory to be read out as required. The effect essentially the same as that described above is obtained by a modification in which the value of the desired rotation-speed difference ΔN&#39; is changed depending on the rotation speed N t  of the output shaft 19 of the torque converter 13 in lieu of the rotation speed N e  of the torque-converter input shaft 12. 
     When, on the other hand, the result of judgment in the step 52 is &#34;YES&#34;, that is, when the result of judgment proves that the accelerator pedal is depressed by the driver who intends to start the vehicle, the rotation speeds N e  and N t  of the input shaft 12 and output shaft 19 of the torque converter 13 are detected in the step 61 on the basis of the output signals from the input-shaft rotation speed sensor 45 and output-shaft rotation speed sensor 46 respectively. In the next step 62, the changing rate N t  of the rotation speed N t  of the torque-converter output shaft 19 is computed. Then, in the step 63, a reference changing rate N t  &#39;, which is a reference of the changing rate N t , of the rotation speed is set on the basis of the output signals from the load sensor 44, input-shaft rotation speed sensor 45 and output-shaft rotation speed sensor 46. Then, in the step 64, judgment is made as to whether the result of subtraction of the actual changing rate N t  from the reference changing rate N.sub. t &#39; is positive (N t  &#39;-N t  &gt;0) or not. When the result of judgment in the step 64 is &#34;YES&#34;, that is, when the result of subtraction is positive, processing for decreasing the engaging force, hence, the torque capacity of the low reverse brake 24 by increasing the duty ratio of current supplied to the electromagnetic valve 43 thereby decreasing the value of hydraulic pressure supplied to the low reverse brake 24, is executed in the step 65. When, on the other hand, the result of judgment in the step 64 is &#34;NO&#34;, that is, when the result of subtraction is negative, processing for increasing the torque capacity of the low reverse brake 24 by decreasing the duty ratio of current supplied to the electromagnetic valve 43, is executed in the step 66. 
     As in the case of the step 64, the manner of processing may be such that the current torque capacity of the low reverse brake 24 is continuously maintained when the result of judgment in the step 64 proves that N t  &#39;=N t  =0. The values of the changing rates N t  and N t  &#39; are negative. Therefore, the relation N t  &#39;-N t  &gt;0 indicates that the absolute value of N t  is larger than that of N t  &#39;. That is, this relation indicates that the rotation speed of the output shaft 19 of the torque converter 13 decreases rapidly with a changing rate which is larger than the reference changing rate in absolute value. 
     After execution of the step 65 or 66, the traveling speed N o  of the vehicle is detected in the step 67 on the basis of the output signal of the vehicle speed sensor 47. Then, in the step 68, judgment is made as to whether or not the ratio between the speed N o  of the vehicle and the rotation speed N t  of the torque-converter output shaft 19 has attained a value corresponding to the 1st or reverse speed ratio to be achieved at the time of vehicle starting, thereby detecting whether the low reverse brake 24 has been completely engaged to establish the desired speed ratio or has not been completely engaged yet. When the result of judgment in the step 68 is &#34;NO&#34;, that is, when the low reverse brake 24 has not been completely engaged yet, the steps 51, 52 and 61 to 68 are repeated until the low reverse brake 24 is completely engaged. That is, the torque capacity of the low reverse brake 24 is feedback-controlled so that the actual changing rate N t  coincides with or follows the reference changing rate N t  &#39;. As soon as the complete engagement of the low reverse brake 24 is attained, that is, when the result of judgment in the step 68 is now &#34;YES&#34;, execution of the program is ended, and the usual shift control program is executed. 
     By the execution of the above manner of control, the torque-converter output shaft 19 rotates at a rotation speed N t  lower from that N e  of the torque-converter input shaft 12 by the desired rotation-speed difference ΔN&#39; which is set at, for example, 150 rpm, before time t 1  at which the vehicle is in a state of standstill, and the driver&#39;s intension to start the vehicle is not detected although the selecter lever is positioned in the D or R range, as shown in FIG. 4(a). Commonly, the rotation speed difference ΔN between the input and output shafts 12 and 19 of the torque converter 13 is very small or less than 50 rpm when the speed change gear assembly is in its complete neutral position. Also, when the vehicle is in a state of standstill with a speed ratio achieved in the automatic transmission, the rotation speed of the torque-converter output shaft 19 is 0 rpm. Therefore, the rotation speed difference ΔN between the torque-converter input and output shafts 12 and 19 is equal to the rotation speed (600-700 rpm) of the engine at that time. Therefore, when the torque capacity of the low reverse brake 24 is so regulated as to maintain the rotation speed difference of about 150 rpm between the torque-converter input and output shafts 12 and 19 in a standstill state of the vehicle as described above, the low reverse brake 24 is placed in a partially engaged state after its piston has stroked a dead stroke, and a very slight creeping torque only is transmitted to the output shaft 32 of the speed change gear assembly. 
     As a result, such a very slight creeping torque is transmitted to the gear-assembly output shaft 32 in the standstill state in which the accelerator pedal is released, as shown in FIG. 4(b). Therefore, the tendency of creeping of the vehicle on a flat road can be reduced or prevented, and, also, the tendency of backword movement of the vehicle on a gentle ascent can be prevented. Further, there is utterly no danger of burnout of the low reverse brake 24 since the energy loss due to slip of the low reverse brake 24 under the above condition is very slight. Furthermore, since the low reverse brake 24 is maintained in the partially engaged state after its piston has stroked the dead stroke, any further wasteful stroking of the piston of the brake 24 would not occur when the hydraulic pressure supplied to the brake 24 is increased to place the brake 24 in a completely engaged state when the driver starts the vehicle. Therefore, the hydraulic pressure supplied to the low reverse brake 24 can be easily controlled to ensure an excellent response to the actuation for starting the vehicle. 
     Then, when the driver&#39;s intension to start the vehicle is detected at time t 1 , the reference changing rate N t  &#39; of the rotation speed of the torque-converter output shaft 19 is set so that the rotation speed changes along the solid curve shown in FIG. 4(a). And the hydraulic pressure supplied to the low reverse brake 24 is feedback-controlled until the low reverse brake 24 is completely engaged, that is, until the 1st or reverse speed ratio is achieved at time t 2 . As a result of this feedback control, the changing rate N t  of the actual rotation speed N t  of the torque-converter output shaft 19 coincides with or follows the reference changing rate N t  &#39;. Thus, an abrupt fluctuation of the torque of the output shaft 32 of the speed change gear assembly can be prevented to ensure smooth starting of the vehicle. 
     In the aforementioned embodiment of the present invention, the torque capacity of the low reverse brake 24 engaged for achieving the 1st or reverse speed ratio is controlled so as to reduce or prevent creeping of the standstill vehicle regardless of whether the selecter lever is positioned in the D range or R range. However, the mode of judgment in the step 51 may be modified so that the program cannot be executed when the selecter lever is positioned in one of the D and R ranges. 
     The low reverse brake 24 is fixed at one of its frictional elements to the casing 18, and the engaging force can be easily regulated by hydraulic pressure. Therefore, this low reverse brake 24 is a torque transmitting device most suitable as the object of control according to the present invention. However, the torque transmitting device which is the object of control according to the present invention is in no way limited to the low reverse brake 24, and it is apparent that the object of control may be the rear clutch 21 or the front clutch 20 engaged for achieving the 1st speed ratio or the reverse speed ratio. In such a modification, the line pressure not decreased is supplied to the low reverse brake 24, unlike the aforementioned embodiment. 
     Further, when the automatic transmission is of the type in which a speed ratio other than the 1st or rear speed ratio, for example, the 2nd speed ratio is achieved in the standstill state of the vehicle, the torque transmitting device which is the object of the control may be either the rear clutch 21 or the kickdown brake 23. 
     The present invention has been described with reference to its embodiment to a power transmission apparatus for a vehicle which includes a torque converter and a speed change gear assembly achieving a plurality of speed ratios by being actuated by a plurality of hydraulically-operated frictional engaging devices. However, the present invention is in no way limited to this embodiment to such a specific power transmission apparatus and is also equally effectively applicable to, for example, a power transmission apparatus for a vehicle which includes a hydrodynamic power transmission, a continuously variable transmission, and a gear assembly incorporated in a power transmission system, disposed in a front stage or a rear stage of the continuously variable transmission, for changing over between forward and rearward speed ratios by being actuated by engageable torque transmitting devices. 
     Also, the torque transmitting devices may be any suitable ones as far as the torque capacity can be controlled during engagement. Therefore, electromagnetic powder clutches or the like may be employed in place of the hydraulically-operated frictional engaging devices.