Abstract:
Disclosed is a transmission comprising a first power transfer path for transferring an output of an engine to a vehicle driving shaft through a planetary gear connected to a motor, a second power transfer path for transferring the output of the engine to the vehicle driving shaft through gears and, and a power transfer switch which switches over the first and second power transfer paths from one to the other. In a hybrid vehicle wherein an engine, a motor and a generator are connected to a planetary gear, a follow-up loss caused by the generator is avoided in the case where the engine stops and the vehicle travels with the motor alone. In high-speed running, it is avoided that an electric energy for stopping the rotation of the generator is consumed. Further, torque assist by the generator is not restricted by constraints of the planetary gear.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a transmission composed of a motor, a differential mechanism and a power coupling mechanism, as well as a vehicle using the same. 
     As a drive system capable of attaining a low power consumption of an engine there is known a hybrid vehicle which utilizes a driving force of a motor. 
     As to the hybrid vehicle, there has been proposed a system which uses two motors and one planetary gear. For example, in Japanese Patent Laid Open No. Hei 7-135701 there is described a method in which a control is made by a generator so that a driving force of an engine is inputted to a planetary gear and so that the vehicle is driven by a driving force obtained from an output shaft of the planetary gear. 
     In the above method, out of three constituent gears of the planetary gear, other gears than a gear connected to a shaft of the engine are controlled on their speed so as to stop the gear connected to the engine shaft, thereby realizing a vehicular running with a motor alone. 
     In high-speed running, a shaft of the generator is fixed electrically and the driving force of the engine is transmitted to a vehicle driving shaft through the other gears in the planetary gear than the gear connected to the engine shaft. 
     In the above method, however, there occurs a loss due to follow-up rotation of the generator during vehicular running with the motor, resulting in consumption of the driving motor output. 
     In high-speed running, moreover, an electric energy for stopping the rotation of the generator is consumed. 
     Further, torque assist by the generator is restricted due to a constraint based on the planetary gear. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished for eliminating the above-mentioned drawbacks and it is an object of the invention to attain a high degree of efficiency by diminishing a loss and an electrical loss both caused by follow-up rotation of a motor in a vehicle having an engine and the motor and make it possible to effect torque assist by a generator irrespective of the vehicle speed. 
     The above object is achieved by a transmission comprising; a first power transfer path for transferring an output of an internal combustion engine to a vehicle driving shaft through a differential mechanism in which power of an electric rotary machine is transferred to one of rotary elements; a second power transfer path for transferring the output of the internal combustion engine to the vehicle driving shaft through gears; and a power transfer switching means for switching over the first and second power transfer paths from one to the other. 
     The above object is achieved also a vehicle having an internal combustion engine and an electric rotary machine, the vehicle comprising: a vehicle driving shaft to which are fixed at least a high speed gear and a low speed gear; a planetary gear having at least three rotary elements, of which a first rotary element is connected to the low speed gear and a second rotary element is connected to a rotating shaft of the electric rotary machine; and a dog clutch having at least three rotary elements, of which a first rotary element is connected to a rotating shaft of the internal combustion engine, a second rotary element is connected to the high speed gear, and a third rotary element is connected to a third rotary element of the planetary gear; the dog clutch having a mechanism for selectively connecting the first rotary element thereof to the second or the third rotary element thereof and for neutralizing the first rotary element relative to the second and third rotary elements. 
     Further, the above object is achieved by a vehicle having an internal combustion engine and a motor generator, the vehicle comprising: a first power transfer path for transferring an output of the internal combustion engine to a vehicle driving shaft through a differential mechanism in which power of the motor generator is transferred to one of rotary elements; a second power transfer path for transferring the output of the internal combustion engine to the vehicle driving shaft through gears; and a dog clutch for switching over from one to another among a first mode which selects the first power transfer path, a second mode which selects the second power transfer path, and a neutral mode which separates the internal combustion engine from the first and second power transfer paths. 
     Further, the above object is achieved by a hybrid vehicle comprising: an internal combustion engine; a plurality of electric rotary machines; a first drive path through which an output obtained by the addition of an output of the internal combustion engine and an output of the first electric rotary machine is transferred to a vehicle driving shaft; a second drive path through which an output obtained by subtracting one of an output of the internal combustion engine and an output of the second electric rotary machine from the other is transferred to the vehicle driving shaft; and means for selecting either the first or the second drive path. 
     Further, the above object is achieved by a control unit provided in a hybrid vehicle, the hybrid vehicle comprising: an internal combustion engine; an electric rotary machine; a vehicle driving shaft to which are fixed at least a high speed gear and a low speed gear; a planetary gear having at least three rotary elements, of which a first rotary element is connected to the low speed gear and a second rotary element is connected to a rotating shaft of a motor generator; and a dog clutch having at least three rotary elements, of which a first rotary element is connected to a rotating shaft of the internal combustion engine, a second rotary element is connected to the high speed gear, and a third rotary element is connected to a third rotary element of the planetary gear; the dog clutch having a mechanism for selectively connecting the first rotary element thereof to the second or the third rotary element thereof and for neutralizing the first rotary element relative to the second and third rotary elements, wherein the number of rotation of the third rotary element in the dog clutch is controlled in accordance with a detected number of rotation of the vehicle driving shaft and by controlling the number of rotation of the electric rotary machine, and the first and third rotary elements in the dog clutch are connected together upon substantial coincidence in the number of rotation of the two. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a system configuration of a hybrid vehicle according to an embodiment of the present invention; 
     FIG. 2 is a flow chart in a motor travel mode in the system configuration illustrated in FIG. 1; 
     FIG. 3 is a flow chart associated with start-up of an engine in the system configuration illustrated in FIG. 1; 
     FIG. 4 illustrates operations of components at the time of start-up of the engine in the system configuration shown in FIG. 1; 
     FIG. 5 is a flow chart in an electric speed change mode in the system configuration illustrated in FIG. 1; 
     FIG. 6 is a flow chart in an OD mode in the system configuration illustrated in FIG. 1; 
     FIG. 7 is a time chart in the system configuration illustrated in FIG. 1; 
     FIG. 8 illustrates a system configuration of a hybrid vehicle according to another embodiment of the present invention; 
     FIG. 9 illustrates a system configuration of a hybrid vehicle according to a further embodiment of the present invention; 
     FIG. 10 illustrates a system configuration of a hybrid vehicle according to a still further embodiment of the present invention; 
     FIG. 11 illustrates a system configuration of a hybrid vehicle according to a still further embodiment of the present invention; 
     FIG. 12 illustrates a system configuration of a hybrid vehicle according to a still further embodiment of the present invention; 
     FIG. 13 illustrates a system configuration of a hybrid vehicle according to a still further embodiment of the present invention; 
     FIG. 14 illustrates a system configuration of a hybrid vehicle according to a still further embodiment of the present invention; 
     FIG. 15 illustrates a system configuration of a hybrid vehicle according to a still further embodiment of the present invention; 
     FIG. 16 illustrates a system configuration of a vehicle driving system according to a still further embodiment of the present invention; and 
     FIG. 17 illustrates a system configuration of a hybrid vehicle according to a still further embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described hereinunder with reference to the accompanying drawings. 
     FIG. 1 illustrates a vehicle embodying the present invention with a transmission according to the invention mounted thereon. In the same figure, an engine  11  is an internal combustion engine. The internal combustion engine indicates an engine in which combustion gas is a working fluid. Examples thereof include a reciprocating engine, a rotary engine, a gas turbine, and a jet engine. The engine used in this embodiment is a reciprocating engine. 
     Motors A 12  and B 13 , when an electric energy is given thereto, release a kinetic energy, while when a kinetic energy is given thereto, they convert it into an electric energy. Wheels  14  are connected to a vehicle driving shaft  15 . 
     A high speed engine-side gear  16  is a meshing gear and a high speed vehicle-side gear  17  is also a meshing gear and is in mesh with the gear  16 . The high speed vehicle-side gear  17  is fixed onto the vehicle driving shaft  15 . 
     A planetary gear  18  having a differential function is made up of a sun gear  18   s , a planetary carrier  18   p , and a ring gear  18   r . A low speed vehicle-side gear  19  is a meshing gear and is in mesh with the ring gear  18   r  in the planetary gear  18 . The low speed vehicle-side gear  19  is fixed onto the vehicle driving gear  15 . 
     A dog clutch  20  has a function of connecting an engine output shaft  21  to the high speed engine-side gear  16  or to the planetary carrier  18   p  in the planetary gear  18  or neutralizing the same shaft. The dog clutch  20 , which corresponds generally to a clutch, is constituted by a gear train With the dog clutch, components to be coupled can be coupled using a small power if both components are equal in the number of rotation. 
     The dog clutch  20  is composed of an engine-side transfer element  20   c  connected to the engine output shaft, a high speed-side transfer element  20   h  connected to the high speed engine-side gear  16 , and a low speed-side transfer element  20   l  connected to the planetary carrier  18   p . The engine-side transfer element  20   c  is disposed between the high speed-side transfer element  20   h  and the low speed-side transfer element  20   l . The high speed engine-side gear  16  and the high speed-side transfer element  20   h  are connected with each other through a hollow pipe, with the engine output shaft  21  extending through the hollow pipe. The engine output shaft  21  can be shortened by disposing the engine-side transfer element  20   e  between the high speed-side transfer element  20   h  and the low speed-side transfer element  20   l . The overall length of the dog clutch  20  is set small so that the dog clutch can be mounted more easily. 
     The motor A 12  is connected to the vehicle driving shaft  15 , while the motor B 13  is connected to the sun gear  18   s  of the planetary gear  18 . A motor control unit  22 , which controls the motors A 12  and B 13 , is supplied with energy from a battery  23 . The clamping device  24  has a function of stopping the rotation of the low speed-side transfer element  20   l  and restraining the same element. Generally, a band brake or a multiple-disc clutch is used as the clamping device, but in a hybrid vehicle not having any special oil pressure source it is desirable to use a clamping device which is driven with an electric energy, using a motor for example. 
     A state-of-connection detecting device  25  judges a state of connection of a power transfer switching means. A number-of-rotation detecting device  26  detects the number of rotation of the low speed-side transfer element  20   l  or of the planetary carrier  18   p  in the planetary gear  18 . The number of rotation of the planetary carrier  18   p  can also be obtained from the number of rotation of the motor A 12  and that of the motor B 13 , A clamping state detecting device  27  judges the state of the clamping device  24 . 
     The transmission according to the configuration illustrated in FIG. 1 has the following features. 
     Firstly, there can be realized a vehicular running at a high efficiency and a low fuel consumption. 
     In the case where the vehicle is driven with the motor A 12  alone, loss can be diminished during vehicular running by maintaining the engine-side transfer element  20   e  in a neutral state. The torque of the motor A 12  is transmitted to the planetary gear  18  via the gear  19 , but a loss caused by follow-up rotation of the motor B 13  is nearly zero. 
     By keeping the engine-side transfer element  20   e  neutral, the inertia of the planetary carrier  18   p  in the planetary gear  18  comes to be based on gear only, whereas the inertial of the sun gear  18   s  is large because the motor B 13  is connected thereto. In the planetary gear  18 , therefore, the planetary carrier  18   p  of a low resistance based on inertia is turned due to torque balance, while the sun gear  18   s  remains stopped. Since the motor B 13  is off, a loss caused by follow-up rotation of the motor B 13  with rotation of the motor A 12  is nearly zero and the torque of the motor A 12  can be transmitted efficiently to the wheels  14 . Besides, an electrical loss is also nearly zero because it is not necessary to provide an electric current to the motor B 13 . 
     Where the vehicle is driven by the output of the engine, a vehicular running at a reduced loss can also be effected by keeping the engine-side transfer element  20   e  coupled with the high speed-side transfer element  20   h.    
     Since the engine-side transfer element  20   e  is in a coupled state with the high speed-side transfer element  20   h , the driving force of the engine  11  is transferred efficiently to the wheels  14  via a pair of gears which are the high speed engine-side gear  16  and the high speed vehicle-side gear  17 . 
     Since the vehicle speed is high and the load is large, the engine  11  can be used in a highly efficient region. Moreover, since the low speed-side transfer element  20   l  is in a no-load state, the motor B 13  is off and a follow-up loss and an electrical loss are nearly zero. 
     Secondly, the accelerating performance can be improved. 
     In a motor travel mode wherein the vehicle is driven by the motor A 12 , when a torque higher than an allowable torque of the motor A 12  is required, the torque of the motor B 13  can be transmitted to the vehicle driving shaft  15  by fixing the low speed-side transfer element  20   l  with use of the clamping device  24 . 
     Likewise, with the engine  11  in operation, if a torque larger than the sum of an allowable torque of the engine  11  and that of the motor A 12  is required, the torque of the motor B 13  can be transmitted to the vehicle driving shaft  15  by fixing the low speed-side transfer element  20   l  with use of the clamping device  24 . 
     Since the motor B 13  is connected to the sun gear  18   s  in the planetary gear  18 , the torque generated by the motor B 13  is amplified and transmitted to the vehicle. Therefore, a motor of a small capacity can be selected as the motor B 13 . Besides, since torque assist can be done by the motor B 13 , it is also possible to use a motor of a small capacity as the motor A 12 . 
     Thirdly, it is possible to effect a highly efficient power generation. 
     Since the motors A 12  and B 13  can assist torque each independently, it is, conversely, also possible to absorb torque and generate power in an independent manner. 
     When the capacity of the battery  23  is extremely low, the low speed-side transfer element  20   l  is fixed by the clamping device  24  and power generation is performed by both motors A 12  and B 13 . At this time, the engine  11  is forced to operate at a high load, and since a highly efficient region is present on a high load side of the engine, there is attained a high overall efficiency in power generation. Moreover, a large regenerative braking torque can be obtained because it is possible to effect a regenerative braking simultaneously with two motors. Thus, there is attained a high energy recovery efficiency. 
     Fourthly, the vehicle can travel even in the event of failure of a motor. 
     In the event of failure of the motor A 12 , thus affording no output, the planetary carrier  18   p  in the planetary gear  18  is fixed by the clamping device  24 , allowing the vehicle to travel with the torque of the motor B 13 . 
     For starting the engine  11  in a stopped state of the vehicle, the engine-side transfer element  20   e  is connected to the high speed-side transfer element  20   h  and the torque of the motor B 13  is transmitted to a crank shaft of the engine via the high speed gears  16  and  17  to effect cranking. Cranking can also be performed by connecting the engine-side transfer element  20   e  to the low speed-side transfer element  20   l  and disengaging the clamping device  24 . 
     In the event of failure of the motor B 13 , affording not output, the engine-side transfer element  20   e  is made neutral and the vehicle is allowed to travel with the torque of the motor A 12 . At this time, the engine-side transfer element  20   e  is connected to the high speed-side transfer element  20   h  and the torque of the motor A 12  is transmitted to the crank shaft of the engine via the high speed gears  16  and  17  to effect cranking, thereby causing the engine  11  to start operating. 
     If no output is obtained due to failure of both motors A 12  and B 13 , the engine-side transfer element  20   c  is made neutral if the vehicle is running, whereby the vehicle can be stopped safely without stopping the engine  11 . 
     The following description is now provided about a basic processing method for controlling the engine  11  and the motors A 12  and B 13  in each operation mode. 
     FIG. 2 is an explanatory diagram relating to a motor travel mode, which mode is selected for start-up and low-speed travel. 
     In step  41 , a most efficient mode is selected on the basis of operating conditions and a residual battery capacity. 
     In step  42 , it is made sure that the motor travel mode has been selected. The mode identifying operation in step  42  may be omitted, but it can be utilized as an indication to the driver of the vehicle. 
     In step  43 , the engine-side transfer element  20   e  is set neutral upon making sure that the selected mode is the motor travel mode. This identifying operation can be detected by the state-of-connection detecting device  25 . 
     In step  44 , the state of the engine  11  is checked If the engine is found to be off, the processing flow shifts to step  47 . On the other hand, if the engine  11  is in operation, the flow shifts to step  45 , in which the supply of fuel is stopped. The engine is in a no-load condition because the engine-side transfer element  20   e  is neutral. If the supply of fuel is topped, the operation of the engine stops due to its own friction and a compressing work thereof. 
     By stopping the supply of fuel to the engine  11  simultaneously with neutralizing the engine-side transfer element  20   e , it is possible to prevent blow-up of the engine and vibration caused by variation in torque which is attributable to misfire. The occurrence of vibration and blow-up of the engine can be prevented also by gradually decreasing the supply of fuel up to the time just before neutralizing the engine-side transfer element  20   e  during operation of the engine  11 . 
     In step  46 , the stall of the engine is confirmed and the processing flow shifts to step  47 , in which a check is made to see if the required torque exceeds the allowable torque of the motor A 12 . 
     If torque assist is not needed, the flow shifts to step  48 , in which the vehicle travels with the motor A 12  alone. At this time, in the planetary gear  18 , the sun gear  18   s  becomes fixed due to torque balance between gears and a follow-up loss of the motor B 13  and an electrical loss are nearly zero. 
     On the other hand, if torque assist is needed, the flow shifts to step  49 , in which the speed of he motor B 13  is controlled to make zero the rotational speed of the engine-side transfer element  20   e.    
     In step  50 , the low speed-side transfer element  20   l  is fixed by the clamping device  24 . This operation can be confirmed by the clamping state detecting device  27 . By fixing the low speed-side transfer element  20   l , the torque of the motor B 13  is transmitted while being amplified from the sun gear  18   s  to the ring gear  18   r  via the planetary gear on the planetary carrier  18   p . Thereafter, the processing flow shifts to step  51 , in which the vehicle is driven by both motors A 12  and B 13 . 
     FIG. 3 is an explanatory diagram relating to start-up of the engine. In step  51 , an engine drive mode is selected, followed by shifting to step  52 . 
     In step  52 , when the engine  11  drive mode is confirmed in step  52 , the flow shifts to step  52 , in which a start command for the engine  11  is issued. 
     The mode identifying operation in step  52  may be omitted, but it can be utilized as an indication to the vehicle driver. 
     In step  53 , the state of the engine  11  is checked. If the engine  11  is in operation and the engine-side transfer element  20   e  is not neutral, the flow returns to step  51 , in which the mode selecting operation is performed again. If the engine  11  is off, the flow shifts to step  54 , in which a start command for the engine  11  is issued. 
     In step  55 , whether the engine-side transfer element  20   e  is neutral or not is judged by the state-of-connection detecting device  25 , and if the answer is negative, the flow shifts to step  56 , in which the engine-side transfer element  20   e  is made neutral. 
     When it is detected by the state-of-connection detecting device  25  that the engine-side transfer element  20   e  is neutral, the flow shifts to step  57 , in which the state of the clamping device  24  is checked by the clamping state detecting device  27 . If the clamping device  24  is released, the flow shifts to step  58 , while if the clamping device  24  clamps the low speed-side transfer element  20   l , the flow shifts to step  61 . 
     The reason why the state of the clamping device  24  is checked is that a dog clutch is assumed as the power transfer switching means. In the dog clutch, with the power transfer switching means connected, it is necessary that the components coupled with each other be equal in the number of rotation. Where the clamping device  24  clamps the low speed-side transfer element  20   l , the engine-side transfer element  20   e  disposed on the engine shaft can be connected to the low speed-side transfer element  20   l . But if the clamping device  24  is released, that is, if the planetary carrier  18   p  in the planetary gear  18  is running idle, it is necessary to perform a control so as to make zero the rotational speed of the planetary carrier  18   p.    
     In steps  58  and  59 , the number of rotation of the motor B 13  is controlled to make zero the rotational speed of the planetary carrier  18   p . This operation can be checked by the number-of-rotation detecting device  26 . 
     In step  60 , the low speed-side transfer element  20   l  on the planetary gear side and the engine-side transfer element  20   e  are coupled together. When the clamping device  24  is found to be in a clamping state and the flow has shifted to step  61 , the low speed-side transfer element  20   l  is coupled to the engine-side transfer element  20   e  immediately and the flow shifts to step  62 , in which the clamped state of the low speed-side transfer element  20   l  by the clamping device  24  is released. 
     In step  63 , the motor B 13  functions as a starter to effect cranking. With the torque of the motor B 13  assisted, the torque is used for cranking the engine at the time of shift to the engine start mode, so that the amount of torque assisting the vehicular torque decreases. In this case, the motor A 12  generates the torque corresponding to the deficiency to suppress the variation in the vehicular driving torque. In step  64  the engine starts operating. 
     The system being considered can be realized even in the case where the dog clutch  20  is a pressure mating clutch. Although in case of a pressure mating clutch it is not necessary to synchronize the rotational speeds of power transfer switching components, a large force is required for controlling the power transfer switching components which are coupled together with a large force at the time of coupling. In the dog clutch, a very small force for coupling suffices if the components to be coupled have the same speed. 
     FIG. 4 is a diagram explaining the rotational speed of the planetary gear at the time of start-up of the engine. In the following description the vehicle is assumed to have been at a rotational speed of  101 . 
     At the beginning, the vehicle runs with a motor alone. With the engine-side transfer element  20   e  neutral and the clamping device  24  released, the planetary carrier  18   p  is at a rotational speed of  102 , while the sun gear  18   s  with the motor B 13  connected thereto is at a rotational speed of  103  and is stopped. 
     Upon issuance of an engine start command, the motor B 13  rotates in a direction opposite to the vehicle and the sun gear  18   s  reaches a rotational speed of  105 . At this time, the planetary carrier  18   p  reaches a rotational speed of  104  and stops. 
     Since the engine is off, the engine-side transfer element  20   e  and the low speed-side transfer element  20   l  are coupled together simultaneously with the time when the planetary carrier  18   p  reaches the rotational speed  104 . 
     Subsequently, the motor B 13  rotates in the same direction as the vehicle and the sun gear  18   s  reaches a rotational speed of  106 , resulting in that the engine  11  comes into a cranking state at a rotational speed of  107  and thus can be started. 
     Since this description explains the rotation in the planetary gear, the rotational direction of the engine is not always coincident with the actual operating direction of the vehicle, including a final gear and a differential gear. 
     FIG. 5 illustrates an electric speed change mode. In this mode, the driving force of the engine is divided into a generated power and a direct driving force by the differential mechanism, and the generated power is fed as a driving force for the motor A 12  which is connected to the vehicle driving shaft  15 . By controlling the balance between the direct driving force and the generated power it becomes possible to effect a stepless speed change while allowing an operating point of the engine to remain fixed. 
     In step  71  the electric speed change mode is selected, followed by shifting to step  72 . 
     When the electric speed change mode is confirmed in step  72 , a start command for the engine  11  is issued in step  73  to start the engine  11 . 
     Subsequently, in step  74  the state of operation of the engine  11  is checked, and if the engine is off, the processing flow shifts again to step  73 . 
     If the engine  11  is starting, the flow shifts to step  75 , in which the speed of the motor B 13  is controlled to determine the vehicle speed. A vehicular driving torque is determined by controlling the torque of the motor A 12  in step  76 . At this time, the motor B 13  is in a power generating state, which generated power is used as a driving force for the motor A 12 . 
     In this way a stepless speed change is realized. Upon issuance of a request for the vehicle driving force, the flow shifts to step  77 , in which the output of the engine is controlled. 
     At the maximum vehicle speed in the electric speed change mode, the generated power of the motor B 13  is made zero, the sun gear  18   s  in the planetary gear  18  is fixed electrically by the motor B 13 , and the driving force of the engine  11  is transmitted to the vehicle via the planetary carrier  18   p , ring gear  18   r , and low speed vehicle-side gear  19 . 
     FIG. 6 is a diagram explanatory of an overdrive (OD) mode. 
     In the OD mode, the driving force of the engine is transmitted to the vehicle at a speed change ratio smaller than the minimum speed change ratio capable of being realized in the electric speed change mode. 
     The OD mode is selected in step  81 , followed by shifting to step  82 . When the OD mode is confirmed in step  82 , the flow shifts to step  83 , in which the state of the engine  11  is judged. 
     If the engine  11  stalls, the flow shifts to step  84 , in which there issues a start command for the engine  11 , causing the engine to start. 
     In step  85 , the rotational speed of the engine-side transfer element  20   e  and that of the high speed-side transfer element  20   h  are detected. If there is any difference between both rotational speeds, the flow shifts to step  86 , in which the motors A 12 , B 13  and the engine  11  are controlled their rotations to synchronize the rotational speed of the engine-side transfer element  20   e  with that of the high speed-side transfer element  20   h.    
     Upon coincidence in the rotational speed between both transfer elements  20   e  and  20   h , the flow shifts to step  88 , in which both transfer elements are coupled together. 
     A deficiency in the vehicle driving torque which occurs during speed change is made up for by assisting of the motor torque. 
     As a result of coupling of the engine-side transfer element  20   e  and the high speed-side transfer element  20   h  the low speed-side transfer element  20   l  becomes free, the sun gear  18   s  in the planetary gear  18  stops due to torque balance in the planetary gear, and the rotation of the motor B 13  stops. 
     Next, the flow shifts to step  89 , in which there is made judgment as to whether it is necessary to assist torque by a motor. If the answer is affirmative, the flow shifts to step  91 , in which it is judged whether the requested vehicle driving torque exceeds the allowable torque of the motor A 12 . If the requested vehicle driving torque is within the allowable torque range of te motor A 12 , the flow shifts to step  90 , in which torque is assisted with the motor A 12  alone. 
     If the requested vehicle driving torque exceeds the allowable torque range of the motor A 12 , the flow shifts to step  92 , in which the speed of the motor B 13  is controlled to make zero the rotational speed of the engine-side transfer element  20   e.    
     Thereafter, the flow shifts to step  93 , in which the engine-side transfer element  20   e  is fixed by the clamping device  24 . As a result, the torque of the motor B 13  is transmitted to the vehicle by the low speed vehicle-side gear  19  via the sun gear  18   s  and ring gear  18   r . In step  95 , the engine  11  is controlled. 
     FIG. 7 shows an example of a time chart in which the operations of components used in this embodiment are illustrated in a time-series arrangement. A vehicle driving torque command τv* is constant and the vehicle stops at time  0 . 
     For start-up of the vehicle it is necessary to use a driving force larger than both static friction and vehicular inertia, so there is selected a 2-motor drive mode, Since the planetary carrier  18   p  in the planetary gear  18  is fixed by the clamping device  24 , there arises a clamping device detecting flag. 
     The motors A 12  and B 13  are both in a state of power running. The torque τb of the motor B 13  is positive in the power generating direction. The power transfer switching means is assumed to be a dog clutch. 
     There is adopted a 1-motor drive mode in which the vehicle travels with the motor A 12  alone as the required driving torque becomes smaller after the vehicle begins to travel. In shifting from 2-motor drive mode to 1-motor drive mode, the supply of an electric current to the motor B 13  is stopped and the clamping device  24  is disengaged. In the 1-motor drive mode, the motor B 13  connected to the planetary gear  18  turns off due to torque balance. 
     When starting the engine, the speed of the motor B 13  is controlled to make zero the speed of the planetary carrier  18   p . Subsequently, the engine-side transfer element  20   e  and the low speed-side transfer element  20   l  are coupled together and a shift is made to the electric speed change mode. At this time there arises a low speed side of a state-of-connection judging flag. 
     In the electric speed change mode, a part of the engine output is generated by the motor B 13 , and the motor A 12  is driven using the power generated by the motor B 13 . In the same mode, the vehicle speed increases in inverse proportion to the number of rotation of the motor B 13  and a maximum speed is reached when the rotational speed of the motor B 13  is zero. 
     As the vehicle speed further increases, a shift is made from the electric speed change mode to the OD mode by operation of the power transfer switching means. The engine-side transfer element  20   e  is separated from the low speed-side transfer element  20   l  but is connected to the high speed-side transfer element  20   h.    
     In this case, it is necessary that the number of rotation of the engine-side transfer element  20   e  and that of the high speed-side transfer element  20   h  be coincident with each other. Simultaneously with the separation of the engine-side transfer element  20   e  the throttle valve of the engine  11  is turned back, and when the number of rotational speed of the engine  11  decreases to a predetermined number, the engine-side transfer element  20   e  is connected to the high speed-side transfer element  20   h . At this time there arises a high speed side of the state-of-connection judging flag. 
     The engine torque is not transmitted to the vehicle until connection of both transfer elements  20   e  and  20   h  with each other. Therefore, the torque generated by the motor A 12  is increased to prevent the occurrence of a shock caused by a deficiency of torque in speed change. 
     In the OD mode, the torque generated by the engine becomes smaller as the vehicle speed increases. Therefore, in the event of engine torque deficiency in a high-speed vehicular running, the torque is assisted by a motor. 
     If a required driving torque is within an allowable torque range of the motor A 12 , electric power is fed to the motor A 12  to effect torque assist. 
     On the other hand, if the required driving torque exceeds the allowable torque range of the motor A 12 , the planetary carrier  18   p  is fixed by the clamping device  24  and the torque of the motor B 13  is transmitted to the vehicle driving shaft. Since the planetary carrier  18   p  in the planetary gear  18  is fixed by the clamping device  24 , there arises the clamping device detecting flag. 
     If the speed of the motor B 13  is controlled to make the speed of the planetary carrier  18   p , zero for coupling prior to fixing the planetary carrier  18   p  by the clamping device  24 , it becomes possible to effect a shock-free mode shift. 
     Other embodiments of hybrid vehicles carrying the transmission according to the present invention will be described below. 
     FIG. 8 shows another embodiment of a vehicle carrying the transmission according to the present invention. As shown in the same figure, a connecting device  201  is disposed between a high speed gear  216  and a differential mechanism  218 , with the motor B 13  being connected to the differential mechanism  218 . The output shaft  21  of the engine  11  is connected or is made neutral to the high speed gear  216  or to the differential mechanism  218  by the connecting mechanism  201 , whereby the driving force of the engine is distributed. 
     FIG. 9 shows a further embodiment of a vehicle carrying the transmission according to the present invention. As shown in the same figure, a connecting device  201  is disposed between a high speed gear  216  and a differential mechanism  218 , the motor B 13  is connected to one gear in the differential mechanism  218 , and the motor A 12  is connected to one gear in the differential mechanism  218  via a gear  131 . The motor A 12  can be made compact because the connection thereof to the differential mechanism  218  is made through the gear  131 . 
     FIG. 10 shows a still further embodiment of a vehicle carrying the transmission according to the present invention. As shown in the same figure, a connecting device  201  is disposed between a high speed gear  216  and a differential mechanism  218 , the motor B 13  is connected to the differential mechanism  218 , and the motor A 12  is connected to the high speed gear  216  via a gear  141 . The motor A 12  can be made compact because it is connected to the high speed gear  216  through the gear  141 . 
     FIG. 11 shows a still further embodiment of a vehicle carrying the transmission according to the present invention. As shown in the same figure, a connecting device  154 , a medium speed gear  152 , and a connecting device  151  are disposed between a high speed gear  216  and a differential mechanism  218 , the medium speed gear  152  being in mesh with a gear  153  which is mounted on the vehicle driving shaft  15 . 
     The connecting device  154  has a function of connecting or neutralizing the engine output shaft  21  with respect to the high speed gear  216 , and the connecting device  151  has a function of connecting or neutralizing the engine output shaft  21  with respect to the high speed gear  216  or the differential mechanism  218 . 
     By setting the speed change ratio between the medium speed gear  152  and the gear  153  at a value lower than the speed change ratio between the high speed gears  216  and  217 , the number of mechanical speed change modes increases by one stage and thus the region of the electric speed change mode can be made narrow without impairing the power performance, with the result that it is possible to reduce the capacity of the motor A 12  and that of the motor B 13 . 
     FIG. 12 shows a still further embodiment of a vehicle carrying the transmission according to the present invention. As shown in the same figure, a differential mechanism  218  has a low speed gear  161  and a low/medium speed gear  162 . The low speed gear  161  is in mesh with a gear  219  which is mounted on the vehicle driving shaft  15 , while the low/medium speed gear  162  is in mesh with a gear  163  also mounted on the shaft  15 . 
     A connecting device  164  is disposed between the low speed gear  161  and the low/medium speed gear  162 . The connecting device  164  has a function of connecting or neutralizing an output of the differential mechanism  218  with respect to the low speed gear  161  or the low/medium speed gear  162 . 
     Consequently, it becomes possible for the region of the electric speed change mode to be enlarged from the speed change ratio between the low speed gear  161  and the gear  219  up to the speed change ratio between the low/medium speed gear  162  and the gear  163 . 
     FIG. 13 shows a still further embodiment of a vehicle carrying the transmission according to the present invention. As shown in the same figure, the engine output shaft  21  has a clutch  171 , so that even in the event of failure of the connecting device  201 , motor A 12  and motor B 13 , the vehicle can be stopped, by releasing the clutch  171 , without stopping the engine  11 . 
     FIG. 14 shows a still further embodiment of a vehicle carrying the transmission according to the present invention. As shown in the same figure, the motor A 12  is connected to the vehicle driving shaft  15  via a clutch  181 . By releasing the clutch  181  it becomes possible to make a follow-up loss associated with the motor A 12  nearly zero in the mode wherein the output of the internal combustion engine is transmitted to the vehicle driving shaft via a high speed gear  216  and a gear  217 . 
     FIG. 15 shows a still further embodiment of a vehicle carrying the transmission according to the present invention. As shown in the same figure, the axis of a gear  217  and that of a gear  219  are offset from each other according to sizes of a differential mechanism  218  and a high speed gear  216 . Therefore, the gears  217  and  216  are connected together using a universal joint  191 . 
     FIG. 16 shows a still further embodiment of the present invention, in which one motor is used as a motor generator  313 . According to this embodiment, when an engine-side transfer element  20   e  is connected to a high speed-side transfer element  20   h , it becomes possible to avoid a follow-up loss associated with the motor generator  313 . Regeneration of the motor generator  313  can be effected by locking a low speed-side transfer element  201 . 
     FIG. 17 shows a still further embodiment of the present invention. As shown in the same figure, the hybrid vehicle of this embodiment has a path for transmitting the driving force of the engine  11  directly to a vehicle driving shaft  15  and a path for transmitting the driving force of the engine  11  to the vehicle driving shaft  15  through a differential mechanism  218  with a motor B 13  connected to one shaft thereof. With this constitution, on a high speed side it is possible to use the driving force of the engine with less loss, while on a low speed side, stepless speed change using the differential mechanism can be realized, thereby allowing the engine to always run in a highly efficient region. 
     Although the present invention has been described above by way of embodiments of vehicles carrying the transmission according to the invention, it goes without saying that the present invention is also applicable to other transportation means such as ships and trains. 
     According to the above embodiments it is possible to diminish a follow-up loss of the power generating motor; besides, since the torque can be assisted by the power generating motor, a hybrid vehicle superior in its accelerating performance can be provided at a low fuel consumption.