Patent Publication Number: US-7905806-B2

Title: Power train for hybrid electronic vehicles and method of controlling the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based on, and claims priority from, Korean Applications Serial Number 10-2006-0050808, filed on Jun. 7, 2006, 10-2006-0050805, filed on Jun. 7, 2006 and 10-2006-0010953, filed on Feb. 6, 2006, the disclosures of which are hereby incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a power train having dual modes for hybrid electric vehicles and to a method of controlling the power train and, more particularly, to a technique in which the method of operating the power train is varied depending on the transmission gear ratio of the vehicle, thus transmitting power more efficiently. 
     BACKGROUND OF THE INVENTION 
     As well known to those skilled in the art, a hybrid power transmission device using two planetary gear sets and two motor generators controls the speed of the motor generators without a separate transmission, and thus is able to serve as a variable transmission that is electrically operated. Furthermore, the hybrid power transmission device can operate in a motor mode, an engine mode, a hybrid mode and a regenerative braking mode by controlling the speed of the motor generators. Due to the hybrid power transmission device, if desired, the engine can be turned on or off, so that the fuel consumption ratio is increased. In addition, when braking, the hybrid power transmission device minimizes the use of a frictional brake and thus increases the efficiency of power recovery when braking. 
     An input split type power train, in which one of two motor generators is directly fixed to an output shaft, is a representative example of conventional power trains using two motor generators for hybrid electric vehicles. 
     The conventional input split type power train having the above-mentioned construction exhibits the highest efficiency at a transmission gear ratio which forms a mechanical point at which the speed of the other motor generator, which is not coupled to the output shaft, becomes zero. On the basis of this mechanical point, as the transmission gear ratio is increased or reduced, the efficiency of the power train is reduced. Such a reduction in efficiency of the power train when the transmission gear ratio is reduced is marked, compared to when the transmission gear ratio is increased. 
     In other words, the conventional power train is problematic in that, as the transmission gear ratio is reduced after passing the mechanical point (as the speed of the vehicle increases), the efficiency of the power train rapidly decreases. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide a power train for hybrid vehicles and a method of controlling the power train which increase the range of the transmission gear ratio in which the efficiency of the power train is superior, so that the power train can maintain a relatively high efficiency even though the transmission gear ratio varies. 
     A power train having dual modes for hybrid electric vehicles according to an exemplary embodiment of the present invention includes an engine, gear sets and clutches. A first planetary gear set is provided with a first carrier coupled to the engine. A second planetary gear set has a second sun gear. A third planetary gear set has a third ring gear. A first clutch couples or decouples a second sun gear of the second planetary gear set to or from the third ring gear of the third planetary gear set. A second clutch converts the third ring gear of the third planetary gear set between a stationary state and a rotatable state. A first motor generator is coupled to either a third sun gear or a third carrier of the third planetary gear set. A second motor generator is coupled to either a first sun gear of the first planetary gear set or the second sun gear of the second planetary gear set. One element of either the first planetary gear set or the second planetary gear set is coupled to a drive wheel. Two elements of the first planetary gear set are coupled to two respective elements of the second planetary gear set. A remaining one of the third sun gear and the third carrier of the third planetary gear set which is not coupled to the first motor generator is coupled to one of the elements of the first planetary gear set and the second planetary gear set. 
     To control the power train, in the case where the first motor generator is coupled to the third carrier of the third planetary gear set, the second motor generator is coupled to the second sun gear of the second planetary gear set, a second carrier of the second planetary gear set is coupled to the drive wheel, the first sun gear of the first planetary gear set is coupled to a second ring gear of the second planetary gear set, a first ring gear of the first planetary gear set is coupled to the second carrier of the second planetary gear set, and the third sun gear of the third planetary gear set is coupled to the second ring gear of the second planetary gear set, whereas, on the basis of a mechanical point at which the speed of the second motor generator is zero, in a portion of a high transmission gear ratio region, the first clutch is engaged and the second clutch is engaged, and, in a low transmission gear ratio region, the first clutch is disengaged and the second clutch is engaged. 
     Furthermore, in the high transmission gear ratio region, relative to the mechanical point at which the speed of the second motor generator is zero, depending on the increase in the transmission gear ratio of the power train, when the efficiency of the power train in a state in which the first clutch is disengaged and the second clutch is engaged is higher than the efficiency of the power train in a state in which the first clutch is engaged and the second clutch is disengaged, the first clutch is disengaged, and the second clutch is engaged. 
     Meanwhile, in the case where the first motor generator is coupled to the third carrier of the third planetary gear set, the second motor generator is coupled to the first sun gear of the first planetary gear set, the second ring gear of the second planetary gear set is coupled to the drive wheel, the first sun gear of the first planetary gear set is coupled to the second sun gear of the second planetary gear set, the first carrier of the first planetary gear set is coupled to the second carrier of the second planetary gear set, and the third sun gear of the third planetary gear set is coupled to the first ring gear of the first planetary gear set, on the basis of a mechanical point at which the speed of the second motor generator is zero, in a high transmission gear ratio region, the first clutch is disengaged and the second clutch is engaged, and, in a low transmission gear ratio region, the first clutch is engaged and the second clutch is disengaged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the nature and objects of the present invention, reference should be made to the following detailed description with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a power train having dual modes for hybrid electric vehicles, according to a first embodiment of the present invention; 
         FIG. 2  is a schematic diagram of the power train of  FIG. 1 , which is operated in a first mode; 
         FIG. 3  is a schematic diagram of the power train of  FIG. 1 , which is operated in a second mode; 
         FIG. 4  is a lever analysis diagram of the power train of  FIG. 1 ; 
         FIGS. 5  (A) and (B) are a lever analysis diagram showing formation of mechanical points in the first mode of the power train according to the first embodiment of the present invention; 
         FIGS. 6  (A) and (B) are a lever analysis diagram showing formation of mechanical points in the second mode of the power train according to the first embodiment of the present invention; 
         FIG. 7  is a graph showing the efficiency of the power train as a function of a transmission gear ratio according to a method of controlling the power train of the first embodiment; 
         FIG. 8  is a schematic diagram of a power train having dual modes for hybrid electric vehicles, according to a second embodiment of the present invention; 
         FIG. 9  is a schematic diagram of the dual power train of  FIG. 8  which is operated by a first mode; 
         FIG. 10  is a schematic diagram of the power train of  FIG. 8  which is operated by a second mode; 
         FIG. 11  is a lever analysis diagram of the power train of  FIG. 8 ; 
         FIGS. 12  (A) and (B) are a lever analysis diagram showing formation of mechanical points in the first mode of the power train according to the second embodiment of the present invention; 
         FIGS. 13  (A) and (B) are a lever analysis diagram showing formation of mechanical points in the second mode of the power train according to the second embodiment of the present invention; 
         FIG. 14  is a graph showing the efficiency of the power train as a function of transmission gear ratio according to a method of controlling the power train of the second embodiment; and 
         FIG. 15  is a schematic diagram of a power train having dual modes for hybrid electric vehicles, according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the attached drawings. 
     Referring to  FIG. 1 , a power train according to the first embodiment of the present invention includes a first planetary gear set  9 , a second planetary gear set  21 , a third planetary gear set  31 , a first clutch  33  and a second clutch  35 . The first planetary gear set  9  includes a first carrier  3  coupled to an engine  1 , a first sun gear  5 , and a first ring gear  7 . The second planetary gear set  21  includes a second ring gear  11  coupled to the first sun gear  5 , a second carrier  15  coupled both to the first ring gear  7  and to a drive wheel  13 , and a second sun gear  19  coupled to a second motor generator  17 . The third planetary gear set  31  includes a third sun gear  23  coupled to the second ring gear  11 , a third carrier  27  coupled to a first motor generator  25 , and a third ring gear  29 . The first clutch  33  couples or decouples the second sun gear  19  of the second planetary gear set  21  to or from the third ring gear  29  of the third planetary gear set  31 . The second clutch  35  converts the third ring gear  29  of the third planetary gear set  31  between a stationary state and a rotatable state. 
     That is, the first motor generator  25  is coupled to the third carrier  27  of the third planetary gear set  31 . The second motor generator  17  is coupled to the second sun gear  19  of the second planetary gear set  21 . The second carrier  15  of the second planetary gear set  21  is coupled to the drive wheel  13 . The first sun gear  5  of the first planetary gear set  9  is coupled to the second ring gear  11  of the second planetary gear set  21 . The first ring gear  7  of the first planetary gear set  9  is coupled to the second carrier  15  of the second planetary gear set  21 . The third sun gear  23  of the third planetary gear set  31  is coupled to the second ring gear  11  of the second planetary gear set  21 . 
     In this embodiment, the second clutch  35  is provided between a power train case  37  and the third ring gear  29  of the third planetary gear set  31  to convert the third ring gear  29  between a stationary state and a rotatable state relative to the power train case  37 . The second clutch  35  may be provided between the third ring gear  29  and a separate vehicle body part other than the power train case  37 . Each of the first planetary gear set  9 , the second planetary gear set  21  and the third planetary gear set  31  comprises a single pinion planetary gear. 
     The power train having the above-mentioned construction is operated in a first mode or in a second mode, each of which is a compound split mode, depending on the states of the first and second clutches  33  and  35 . 
     Hereinafter, the case of  FIG. 2 , in which the first clutch  33  is in a disengaged state while the second clutch  35  is in an engaged state, will be called the first mode, and the case of  FIG. 3 , in which the first clutch  33  is in an engaged state while the second clutch  35  is in a disengaged state, will be called the second mode. 
       FIG. 4  is a lever analysis diagram of the power train of the first embodiment.  FIG. 5  is a lever analysis diagram of some states of the power train being operated in the first mode. In  FIG. 5 , the upper view illustrates a transmission gear ratio that forms a mechanical point M 1 - 1  at which the speed of the first motor generator  25  is zero, and the lower view illustrates a transmission gear ratio that forms a mechanical point M 1 - 2  at which the speed of the second motor generator  17  is zero. 
     In the case where the power train of the present invention is in the first mode, the two mechanical points M 1 - 1  and M 1 - 2  shown in  FIG. 5  are attained while the transmission gear ratio is varied. Supposing that no battery is provided between the first motor generator  25  and the second motor generator  17 , that electricity that is generated at one side is completely consumed by the other side, so that the sum of the generated and wasted amounts is zero, and that energy loss for maintaining the speed of the motor generator at zero is negligible, the efficiency of the power train becomes 1, which is the maximum value, at the two mechanical points. This is confirmed in the graph of  FIG. 7 , which illustrates the efficiency of the power train depending on variation in the transmission gear ratio. 
     Referring to  FIG. 7 , it can be seen that the line that shows the efficiency in the first mode attains the maximum efficiency value of “1” twice. Furthermore, it can be seen that the positions at which the efficiency of the power train is maximum are the two mechanical points M 1 - 1  and M 1 - 2  of the power train in the first mode. In addition, as will be explained later herein, it can be seen that the efficiency in the first mode is lower than in the second mode in a portion of the region in which the transmission gear ratio is increased from the position designated as a mode conversion point, and the efficiency in the first mode is higher than in the second mode in the region in which the transmission gear ratio is reduced from the mode conversion point. 
     For reference, in  FIGS. 4 through 6 , the character O denotes output, I denotes input (engine), MG 1  denotes the first motor generator, and MG 2  denotes the second motor generator. 
     I is spaced apart from O by a distance of 1. 
     When a distance from O to MG 1  is α and a distance from O to MG 2  is β, in the case of the first mode, α=−3.5 and β=−5. In the case of the second mode, α=−2.857 and β=−5. It is understood that both the first mode and the second mode are compound split modes because α and β are not zero. The distances from O to 1, MG 1  and MG 2  correspond to gear ratios in the real power train. 
     The graph of  FIG. 7  is defined by the following Equation. 1: 
     
       
         
           
             
               
                 
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     Here, eff denotes the efficiency of the power train. 
     α and β denote the above-mentioned values depending on the mode. 
     γ denotes the transmission gear ratio. 
     η a  and η b  respectively denote efficiencies when the first motor generator  25  and the second motor generator  17  are charged and discharged (consumed). Here, 0.949 and 1/0.951 are used as values of η a  and η b . That is, when charged, it has a value less than 1 (in this case, 0.949). When discharged, it has a value greater than 1 (in this case, 1/0.951). 
       FIG. 6  is lever analysis diagrams of some states of the power train being operated in the second mode. In  FIG. 6 , the upper view shows a transmission gear ratio that forms a mechanical point M 2 - 1  at which the speed of the first motor generator  25  is zero, and the lower view shows a transmission gear ratio that forms a mechanical point M 2 - 2  at which the speed of the second motor generator  17  is zero. 
     As such, in the second mode, the two mechanical points M 2 - 1  and M 2 - 2  shown in  FIG. 6  are attained while the transmission gear ratio is varied. Supposing that no battery is provided between the first motor generator  25  and the second motor generator  17 , that electricity that is generated at one side is completely consumed by the other side so that the sum of the generated and wasted amounts is zero, and that energy loss for maintaining the speed of the motor generator at zero is negligible, the efficiency of the power train becomes 1, which is the maximum value, at the two mechanical points. This is confirmed in the graph of  FIG. 7 , which illustrates the efficiency of the power train depending on variation in the transmission gear ratio. 
     Referring to  FIG. 7 , it can be seen that the line that shows the efficiency of the power train in the second mode attains the maximum efficiency value of “1” twice. Furthermore, it can be seen that the positions at which the efficiency of the power train is maximum are the mechanical points M 2 - 1  and M 2 - 2  of the power train in the second mode, and, at the position designated as the mode conversion point, the efficiency of the power train in the first mode is maximum and, simultaneously, the efficiency thereof in the second mode is also maximum, that is, the mechanical points M 1 - 2  and M 2 - 2  are the same. Furthermore, it is understood that, because the mechanical point M 2 - 1  is located to the right of the mode conversion point, the efficiency of the power train in the second mode is higher than in the first mode in the area around the mechanical point. 
     The mode conversion point corresponds to a transmission gear ratio at which the speed of the second motor generator  17  in the first mode becomes zero, and also corresponds to a transmission gear ratio at which the speed of the second motor generator  17  in the second mode becomes zero. 
     Therefore, in the present invention, on the basis of the mode conversion point, which is the mechanical point at which the speed of the second motor generator  17  is zero, in the region where the transmission gear ratio is lower, the first clutch  33  is disengaged and the second clutch  35  is engaged such that the power train is operated in the first mode. In a portion of the region where the transmission gear ratio is higher, the first clutch  33  is engaged and the second clutch  35  is disengaged such that the power train is operated in the second mode. Thus, the range of the transmission gear ratio in which the efficiency of the power train is superior is increased, so that the power train can maintain a relatively high efficiency even though the transmission gear ratio is varied. 
     As shown in  FIG. 7 , the graph of the second mode crosses the graph of the first mode at point X in the region where the transmission gear ratio is increased from the mode conversion point. As the transmission gear ratio is increased from the point X, the efficiency of the first mode, in which the first clutch  33  is in the disengaged state while the second clutch  35  is in the engaged state, becomes higher than the efficiency of the second mode, in which the first clutch  33  is in the engaged state while the second clutch  35  is in the disengaged state. Therefore, to operate the power train more efficiently, in the region where the transmission gear ratio is increased from the point X, the first clutch  33  is disengaged and the second clutch  35  is engaged such that the power train is operated in the first mode again. 
     Meanwhile, in the first mode and the second mode, the first motor generator  25  and the second motor generator  17  alternately conduct charging and discharging depending on the transmission gear ratio. The power generated by the first or second motor generator  25  or  17  is added to the power of the engine  1  and transmitted to the drive wheel  13 . 
     Referring to  FIG. 8 , a power train according to a second embodiment of the present invention includes a first planetary gear set  9 , a second planetary gear set  21 , a third planetary gear set  31 , a first clutch  33  and a second clutch  35 . The first planetary gear set  9  includes a first carrier  3  coupled to an engine  1 , a first sun gear  5  coupled to a second motor generator  17 , and a first ring gear  7 . The second planetary gear set  21  includes a second sun gear  19  coupled to the first sun gear  5 , a second carrier  15  coupled to the first carrier  3 , and a second ring gear  11  coupled to a drive wheel  13 . The third planetary gear set  31  includes a third sun gear  23  coupled to the first ring gear  7 , a third carrier  27  coupled to a first motor generator  25 , and a third ring gear  29 . The first clutch  33  couples or decouples the second sun gear  19  of the second planetary gear set  21  to or from the third ring gear  29  of the third planetary gear set  31 . The second clutch  35  converts the third ring gear  29  of the third planetary gear set  31  between a stationary state and a rotatable state. 
     That is, the first motor generator  25  is coupled to the third carrier  27  of the third planetary gear set  31 . The second motor generator  17  is coupled to the first sun gear  5  of the first planetary gear set  9 . The second ring gear  11  of the second planetary gear set  21  is coupled to the drive wheel  13 . The first sun gear  5  of the first planetary gear set  9  is coupled to the second sun gear  19  of the second planetary gear set  21 . The first carrier  3  of the first planetary gear set  9  is coupled to the second carrier  15  of the second planetary gear set  21 . The third sun gear  23  of the third planetary gear set  31  is coupled to the first ring gear  7  of the first planetary gear set  9 . 
     In this embodiment, the second clutch  35  is provided between a power train case  37  and the third ring gear  29  of the third planetary gear set  31  to convert the third ring gear  29  between a stationary state and a rotatable state relative to the power train case  37 . The second clutch  35  may be provided between the third ring gear  29  and a separate vehicle body part other than the power train case  37 . Each of the first planetary gear set  9 , the second planetary gear set  21  and the third planetary gear set  31  comprises a single pinion planetary gear. 
     The power train having the above-mentioned construction is operated in a first mode or in a second mode, each of which is a compound split mode, depending on the states of the first and second clutches  33  and  35 . 
     Hereinafter, the case of  FIG. 9 , in which the first clutch  33  is in a disengaged state while the second clutch  35  is in a engaged state, will be called the first mode, and the case of  FIG. 10 , in which the first clutch  33  is in a engaged state while the second clutch  35  is in a disengaged state, will be called the second mode. 
       FIG. 11  is a lever analysis diagram of the power train according to the second embodiment.  FIG. 12  is lever analysis diagrams showing some states of the power train while operated by the first mode. In  FIG. 12 , the upper view illustrates a transmission gear ratio that forms a mechanical point M 1 - 1  at which the speed of the first motor generator  25  is zero, and the lower view illustrates a transmission gear ratio that forms a mechanical point M 1 - 2  at which the speed of the second motor generator  17  is zero. 
     In the case where the power train of the present invention is in the first mode, the two mechanical points M 1 - 1  and M 1 - 2  shown in  FIG. 12  are attained while the transmission gear ratio is varied. Supposing that no battery is provided between the first motor generator  25  and the second motor generator  17 , that electricity that is generated at one side is completely consumed by the other side, so that the sum of the generated and wasted amounts is zero, and that energy loss for maintaining the speed of the motor generator at zero is negligible, the efficiency of the power train becomes 1, which is the maximum value, at the two mechanical points. This is confirmed in the graph of  FIG. 14 , which illustrates the efficiency of the power train depending on variation in the transmission gear ratio. 
     Referring to  FIG. 14 , it can be seen that the line that shows the efficiency in the first mode attains the maximum efficiency value of “1” twice. Furthermore, it can be seen that the positions at which the efficiency of the power train is maximum are the two mechanical points M 1 - 1  and M 1 - 2  of the power train in the first mode. In addition, it can be seen that the efficiency in the first mode is lower than in the second mode in the region in which the transmission gear ratio is reduced from the position designated as a mode conversion point, and the efficiency in the first mode is higher than in the second mode in the region in which the transmission gear ratio is increased from the mode conversion point. 
     For reference, in  FIGS. 11 through 13 , the character O denotes output, I denotes input (engine), MG 1  denotes the first motor generator, and MG 2  denotes the second motor generator. I is spaced apart from O by a distance of 1. 
     When a distance from O to MG 1  is α and a distance from O to MG 2  is β, in the case of the first mode, α=−0.667 and β=3.5. In the case of the second mode, α=−2.111 and β=−3.5. It is understood that both the first mode and the second mode are the compound split modes because α and β are not zero. The distances from O to 1, MG 1  and MG 2  correspond to gear ratios in the real power train. The graph of  FIG. 14  is defined by Equation. 1, in the same manner as in the first embodiment. 
     In  FIG. 13 , the upper view illustrates a transmission gear ratio that forms a mechanical point M 2 - 1  at which the speed of the first motor generator  25  is zero, and the lower view illustrates a transmission gear ratio that forms a mechanical point M 2 - 2  at which the speed of the second motor generator  17  is zero. 
     In the case where the power train of the present invention is in the second mode, the two mechanical points M 2 - 1  and M 2 - 2  shown in  FIG. 13  are attained while the transmission gear ratio is varied. Supposing that no battery is provided between the first motor generator  25  and the second motor generator  17 , that electricity that is generated at one side is completely consumed by the other side, so that the sum of the generated and wasted amounts is zero, and that energy loss for maintaining the speed of the motor generator at zero is negligible, the efficiency of the power train becomes 1, which is the maximum value, at the two mechanical points. This is confirmed in the graph of  FIG. 14 , which illustrates the efficiency of the power train depending on variation in the transmission gear ratio. 
     Referring to  FIG. 14 , it can be seen that the line that shows the efficiency in the second mode attains the maximum efficiency value of “1” twice. Furthermore, it can be seen that the positions at which the efficiency of the power train is maximum are the two mechanical points M 2 - 1  and M 2 - 2  of the power train in the second mode, and, at the position designated as the mode conversion point, the efficiency of the power train in the first mode is maximum and, simultaneously, the efficiency thereof in the second mode is also maximum, that is, the mechanical points M 1 - 2  and M 2 - 2  are the same. Furthermore, it is understood that, because the mechanical point M 2 - 1 , at which the efficiency of the power train is maximized, is located to the left of the mode conversion point, the efficiency of the power train in the second mode is higher than in the first mode in the left region relative to the mechanical point. 
     The mode conversion point corresponds to a transmission gear ratio at which the speed of the second motor generator  17  in the first mode becomes zero, and also corresponds to a transmission gear ratio at which the speed of the second motor generator  17  in the second mode becomes zero. 
     Therefore, in the present invention, on the basis of the mode conversion point, which is the mechanical point at which the speed of the second motor generator  17  is zero, in the region where the transmission gear ratio is higher, the first clutch  33  is disengaged and the second clutch  35  is engaged such that the power train is operated in the first mode. In the region where the transmission gear ratio is lower, the first clutch  33  is engaged and the second clutch  35  is disengaged such that the power train is operated in the second mode. Thus, the range of the transmission gear ratio in which the efficiency of the power train is superior is increased, so that the power train can maintain a relatively high efficiency even though the transmission gear ratio is varied. 
     That is, when a relatively high transmission gear ratio is required because the vehicle is moving at a low speed, the first mode is used. When a relatively low transmission gear ratio is required because the vehicle is moving at a high speed, the second mode is used. 
     In the first mode and the second mode, the first motor generator  25  and the second motor generator  17  alternately conduct charging and discharging depending on the transmission gear ratio. The power generated by the first or second motor generator  25  or  17  is added to the power of the engine  1  and transmitted to the drive wheel  13 . 
     Referring to  FIG. 15 , a power train according to a third embodiment of the present invention includes a first planetary gear set  9 , a second planetary gear set  21 , a third planetary gear set  31 , a first clutch  33  and a second clutch  35 . The first planetary gear set  9  includes a first carrier  3  coupled to an engine  1 , a first sun gear  5  coupled to a drive wheel  13 , and a first ring gear  7 . The second planetary gear set  21  includes a second sun gear  19  coupled to a second motor generator  17 , a second carrier  15  coupled to the first carrier  3 , and a second ring gear  11  coupled to the first ring gear  7 . The third planetary gear set  31  includes a third sun gear  23  coupled to a first motor generator  25 , a third carrier  27  coupled to the second ring gear  11 , and a third ring gear  29 . The first clutch  33  couples or decouples the second sun gear  19  of the second planetary gear set  21  to or from the third ring gear  29  of the third planetary gear set  31 . The second clutch  35  converts the third ring gear  29  of the third planetary gear set  31  between a stationary state and a rotatable state. 
     That is, the first motor generator  25  is coupled to the third sun gear  23  of the third planetary gear set  31 . The second motor generator  17  is coupled to the second sun gear  19  of the second planetary gear set  21 . The first sun gear  5  of the first planetary gear set  9  is coupled to the drive wheel  13 . The first carrier  3  of the first planetary gear set  9  is coupled to the second carrier  15  of the second planetary gear set  21 . The first ring gear  7  of the first planetary gear set  9  is coupled to the second ring gear  11  of the second planetary gear set  21 . The third carrier  27  of the third planetary gear set  31  is coupled to the second ring gear  11  of the second planetary gear set  21 . 
     In this embodiment, the second clutch  35  is provided between a power train case  37  and the third ring gear  29  of the third planetary gear set  31  to convert the third ring gear  29  between a stationary state and a rotatable state relative to the power train case  37 , in the same manner as that of the above-mentioned embodiments. The second clutch  35  may be provided between the third ring gear  29  and a separate vehicle body part other than the power train case  37 . Each of the first planetary gear set  9 , the second planetary gear set  21  and the third planetary gear set  31  comprises a single pinion planetary gear. 
     The operation and effect of the third embodiment are similar to those of the first and second embodiments, therefore further explanation is deemed unnecessary. 
     As is apparent from the foregoing, the present invention provides a power train for hybrid vehicles which increases the range of the transmission gear ratio within which the efficiency of the power train is superior, and in which a method of operating the power train is varied depending on the transmission gear ratio, so that the power train can be operated with superior efficiency. 
     Furthermore, because the mode which realizes superior efficiency of the power train is selected and the power train is operated in the selected mode, the maximum mechanical load applied to first and second motor generators is reduced. Therefore, the capacity of the motor generator can be reduced and still exhibit the same efficiency.