Abstract:
A power transmission system of a hybrid electric vehicle includes: an input shaft; an output shaft; first, second and third planetary gear sets; a first rotational shaft, a second rotational shaft, a third rotational shaft, a fourth rotational shaft; a fifth rotational shaft; a sixth rotational shaft; and three friction elements mutually selectively connecting rotational shafts to each other or selectively connecting the respective rotational shafts and the transmission housing such that the power transmission system of the hybrid electric vehicle can suppress conversion to an ENG mode by providing sufficient power performance at a time of wide open throttle (WOT) launching and maximally use power of an engine at a time of conversion to a first HEV mode and a third HEV mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to and the benefit of Korean Patent Application No. 10-2014-0105268 filed on Aug. 13, 2014, the entire contents of which is incorporated herein for all purposes by this reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power transmission system of a hybrid electric vehicle, and more particularly, to a power transmission system of a hybrid electric vehicle that can suppress conversion to an ENG mode by providing sufficient power performance at the time of wide open throttle (WOT) launching and maximally use power of an engine at the time of conversion to a first HEV mode and a third HEV mode. 
     2. Description of Related Art 
     Eco-friendly technology of a vehicle is a core technology influencing survival of a future vehicle industry and advanced vehicle makers devote all their might to developing eco-friendly vehicles for meeting environmental and fuel efficiency regulations. 
     As a result, the respective vehicle makers have developed an electric vehicle (EV), a hybrid electric vehicle (HEV), a fuel cell electric vehicle (FCEV), and the like as a future vehicle technology. 
     Since the future vehicle has various technological restrictions such as a weight and cost, the vehicle makers have paid attention to the hybrid electric vehicle as an alternative of a realistic problem for meeting exhaust gas regulations and improving fuel efficiency performance and have entered into keen competition for commercializing the hybrid electric vehicle. 
     The hybrid electric vehicle is a vehicle using two or more power sources. Two or more power sources may be combined by various schemes and a gasoline engine or a diesel engine using the existing fossil fuel and a motor/generator driven by electric energy are mixed and used as the power sources. 
     In the hybrid electric vehicle, an EV mode in which the hybrid electric vehicle is driven by only the motor, an HEV mode using both the engine and the motor, and an ENG mode using only the engine can be implemented according to the combination of the engine and the motor. 
     Further, the hybrid electric vehicle can acquire a significant fuel efficiency enhancement effect as compared with the existing vehicle through idle stop of stopping the engine when the vehicle stops, fuel saving by regenerative braking that drives a generator by using kinetic energy of the vehicle instead of braking by the existing friction when the vehicle is braked, and stores in a battery electric energy generated at the time of driving the generator and reuses the stored electric energy in driving the vehicle, and the like. 
     A power transmission system of the hybrid electric vehicle is classified into a single-mode scheme and a multiple-mode scheme. 
     The single-mode scheme has an advantage that torque transmission mechanisms such as a clutch and a brake for transmission control are not required, but disadvantages that when the vehicle travels at a high speed, efficiency deteriorates, and as a result, the fuel efficiency is low and an additional torque increasing device is required to apply the single-mode scheme to large-sized vehicles. 
     The multiple mode scheme has advantages that when the vehicle is driven at the high speed, the efficiency is high and torque can be designed to increase, and as a result, the multiple-mode scheme can be applied to large and medium-sized vehicles. 
     As a result, in recent years, the multiple-mode scheme has primarily been adopted rather than the single-mode scheme and the resulting research has actively progressed. 
     The multiple-mode scheme power transmission system is configured to include a plurality of planetary gear sets, a plurality of motors/generators used as the motor and the generator, a plurality of torque transmission mechanisms (friction elements) capable of controlling rotating elements of the planetary gear sets, a battery used as power sources of the motors/generators, and the like. 
     The multiple-mode scheme power transmission system has different operating mechanisms according to connection configurations of the planetary gear sets, the motors/generators, and the torque transmission mechanisms. 
     In addition, since the multiple-mode scheme power transmission system has characteristics that durability, power transmission efficiency, a size, and the like vary according to the connection configurations, research and development for implementing a power transmission system which is stronger, has no power loss and is compact has been continued, in a power transmission system field of a hybrid electric vehicle. 
     The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art. 
     BRIEF SUMMARY 
     Various aspects of the present invention are directed to providing a power transmission system of a hybrid electric vehicle that can suppress conversion to an ENG mode by providing sufficient power performance at the time of wide open throttle (WOT) launching and maximally use power of an engine at the time of conversion to a first HEV mode and a third HEV mode. 
     The present invention may have also been made in an effort to provide a power transmission system of a hybrid electric vehicle that decreases an electric load by increasing a weight of a mechanical power transmission path to allow large engine power to be used, substitutes an ENG mode at the time of launching to decrease the number of mode conversion times, and minimizes a change in RPM of all rotating elements at the time of mode conversion. 
     The present invention may have also been made in an effort to provide a power transmission system of a hybrid electric vehicle that can provide an ENG mode in which the vehicle can be driven without the electric load of a motor/generator for improving fuel efficiency at the time of high-speed driving. 
     An aspect of the present invention provides a power transmission system of a hybrid electric vehicle including an input shaft into which power of an engine is input, an output shaft outputting transmitted rotational power through an output gear, a first planetary gear set having three rotation elements constituted by a first sun gear, a first planetary carrier, and a first ring gear and disposed on the input shaft, a second planetary gear set having three rotation elements constituted by a second sun gear, a second planetary carrier, and a second ring gear disposed on a coaxial line of the first planetary gear set, a third planetary gear set having three rotation elements constituted by a third sun gear, a third planetary carrier, and a third ring gear and disposed on a coaxial line of the second planetary gear set, a first rotational shaft directly connected with a first motor/generator while directly connecting one rotation element among the rotation elements of the first planetary gear set and one rotation element among the rotation elements of the second planetary gear set, a second rotational shaft directly connected with the output shaft while directly connecting one rotation element of the first planetary gear set excepted from the rotation elements connected to the first rotational shaft and one rotation element of among the rotation elements of the third planetary gear set, a third rotational shaft directly connecting one rotation element of the first planetary gear set excepted from the rotation elements connected to the first or second rotational shaft and the input shaft, a fourth rotational shaft connected to one rotation element of the second planetary gear set excepted from the rotation elements connected to the first rotational shaft to be selectively connected to a transmission housing, a fifth rotational shaft selectively connected to the transmission housing while directly connecting one rotation element of the second planetary gear set excepted from the rotation elements connected to the first or fourth rotational shaft and one rotation element of the third planetary gear set excepted from the rotation elements connected to the second rotational shaft, a sixth rotational shaft directly connected to one rotation element of the third planetary gear set excepted from the rotation elements connected to the second or fifth rotational shaft to be directly connected to a second motor/generator, and three friction elements selectively connecting the rotational shafts to each other or selectively connecting the respective rotational shafts and the transmission housing. 
     Each of all of the first, second, and third planetary gear sets may be configured by single pinion planetary gear set and may have a first rotational shaft connected with a first motor/generator while directly connecting the first sun gear and the second ring gear, a second rotational shaft directly connecting the first planetary carrier and the third planetary carrier and connected with the output shaft, a third rotational shaft connecting the first ring gear and the input shaft, a fourth rotational shaft connected to the second sun gear, a second rotational shaft directly connecting the second planetary carrier and the third planetary carrier, a sixth rotational shaft connecting the third sun gear to the second motor/generator. 
     Each of the three friction elements may include a first brake selectively connecting the fifth rotational shaft with the transmission housing, a second brake selectively connecting the fourth rotational shaft with the transmission housing, and a first clutch as the direct joining means of the third planetary gear set, which selectively connects the second rotational shaft and the fifth rotational shaft. 
     Each of the three friction elements may include a first brake selectively connecting the fifth rotational shaft with the transmission housing, a second brake selectively connecting the fourth rotational shaft with the transmission housing, and a first clutch as the direct joining means of the third planetary gear set, which selectively connects the fifth rotational shaft with the sixth rotational shaft. 
     Each of the three friction elements may include a first brake selectively connecting the fifth rotational shaft with the transmission housing, a first brake selectively connecting the fourth rotational shaft with the transmission housing, and a first clutch as the direct joining means of the third planetary gear set, which selectively connects the second rotational shaft with the sixth rotational shaft. 
     In the case of the first and second brakes, and the first clutch, the first brake may operate in EV mode 1, the first clutch may operate in EV mode 2, the first brake may operate in HEV mode 1, the first clutch may operate in HEV mode 2, the second brake may operate in HEV mode 3, the first and second brakes may operate in ENG mode 1, and the first clutch and the second brake may operate in ENG mode 2. 
     The first planetary gear set may be configured by a double pinion planetary gear set, and each of the second and third planetary gear sets is configured by a single pinion planetary gear set and may have a first rotational shaft connected with the first motor/generator while directly connecting the first sun gear and the second ring gear, a second rotational shaft directly connecting the first ring gear and the third planetary carrier and connected with the output shaft, a third rotational shaft connecting the first planetary carrier and the input shaft, a fourth rotational shaft connected to the second sun gear, a fifth rotational shaft directly connecting the second planetary carrier and the third ring gear, and a sixth rotational shaft directly connecting the third sun gear and the second motor/generator. 
     Each of the first and third planetary gear sets may be configured by the single pinion planetary gear set, and the second planetary gear set is configured by the double pinion planetary gear set, and may have a first rotational shaft connected with the first motor/generator while directly connecting the first sun gear and the second planetary carrier, a second rotational shaft directly connecting the first planetary carrier and the third planetary carrier and connected with the output shaft, a third rotational shaft connecting the first ring gear and the input shaft, a fourth rotational shaft connected to the second sun gear, a fifth rotational shaft directly connecting the second ring gear and the third ring gear, and a sixth rotational shaft directly connecting the third sun gear and the second motor/generator. 
     Each of the first and second planetary gear sets may be configured by the single pinion planetary gear set, and the third planetary gear set is configured by the double pinion planetary gear set, and may have a first rotational shaft connected with the first motor/generator while directly connecting the first sun gear and the second ring gear, a second rotational shaft directly connecting the first planetary carrier and the third ring gear and connected with the output shaft, a third rotational shaft connecting the first ring gear and the input shaft, a fourth rotational shaft connected to the second sun gear, a fifth rotational shaft directly connecting the second planetary carrier and the third planetary carrier, and a sixth rotational shaft directly connecting the third sun gear and the second motor/generator. 
     According to an aspect of the present invention, in an overall configuration, two EV modes, three HEV modes, and two ENG modes can be implemented in a combination of three planetary gear sets, three friction elements, and two motors/generators. 
     Further, a weight of a mechanical power transmission path can be increased by transmitting larger torque than engine torque to an output shaft and power of a larger engine than the same specification of a first motor/generator can be used. 
     Since the larger torque than the engine torque can be transmitted to the output shaft, high-rotation driving having high engine power at the same vehicle speed is enabled at the time of WOT launching and a larger acceleration force can be achieved. 
     In addition, since the larger acceleration force can be achieved in the HEV mode than the ENG mode, the HEV mode need not be converted into the ENG mode at the time of launching, and as a result, a relatively simple system can be configured and friction elements can be reduced depending on mode reduction, thereby further increasing efficiency. 
     The vehicle can be driven without electric loads of first and second motor/generators by providing the ENG mode at the time of the high-speed driving to improve fuel efficiency. 
     The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram of a power transmission system according to various exemplary embodiments of the present invention. 
         FIG. 2  is an operation table for each of operation modes of friction elements applied to the power transmission system according to the various exemplary embodiments of the present invention. 
         FIG. 3  is a configuration diagram of a power transmission system according to various exemplary embodiments of the present invention. 
         FIG. 4  is a configuration diagram of a power transmission system according to various exemplary embodiments of the present invention. 
         FIG. 5  is a configuration diagram of a power transmission system according to various exemplary embodiments of the present invention. 
         FIG. 6  is a configuration diagram of a power transmission system according to various exemplary embodiments of the present invention. 
         FIG. 7  is a configuration diagram of a power transmission system according to various exemplary embodiments of the present invention. 
     
    
    
     It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
     In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     However, parts which are not related with the description are omitted for clearly describing the exemplary embodiment of the present invention and like reference numerals refer to like or similar elements throughout the specification. 
     In the following description, dividing names of components into first, second, and the like is to divide the names because the names of the components are the same as each other and an order thereof is not particularly limited. 
       FIG. 1  is a configuration diagram of a power transmission system according to a first exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , the power transmission system according to the exemplary embodiment of the present invention is configured in a combination of first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3 , first and second motors/generators MG 1  and MG 2 , and three friction elements BK 1 , BK 2 , and CL 1 . 
     The first planetary gear set PG 1  as a single pinion planetary gear set includes a first sun gear S 1 , a first ring gear R 1 , and a first planetary carrier PC 1  that supports first pinions P 1  which outer-engage with each other between the first sun gear S 1  and the first ring gear R 1 . 
     The second planetary gear set PG 2  as the single pinion planetary gear set includes a second sun gear S 2 , a second ring gear R 2 , and a second planetary carrier PC 2  that supports second pinions P 2  which outer-engage with each other between the second sun gear S 2  and the second ring gear R 2 . 
     The third planetary gear set PG 3  as the single pinion planetary gear set includes a third sun gear S 3 , a third ring gear R 3 , and a third planetary carrier PC 3  that rotatably supports third pinions P 3  which outer-engage with each other between the third sun gear S 3  and the third ring gear R 3 . 
     The first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  are sequentially disposed on a coaxial line from an engine ENG and have sixth rotational shafts TM 1  to TM 6  while respective rotating elements are joined to each other. 
     In more detail, the first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  have the sixth rotational shafts TM 1  to TM 6  while the first sun gear S 1  of the first planetary gear set PG 1  is directly connected with the second ring gear R 2  of the second planetary gear set PG 2 , the second planetary carrier PC 2  of the second planetary gear set PG 2  is directly connected with the third ring gear R 3  of the third planetary gear set PG 3 , and the third planetary carrier PC 3  of the third planetary gear set PG 3  is directly connected with the first planetary carrier PC 1  of the first planetary gear set PG 1 . 
     The first rotational shaft TM 1  directly connects the first sun gear S 1  and the second ring gear R 2  and directly connected with the first motor/generator MG 1 . 
     The second rotational shaft TM 2  directly connects the first planetary carrier PC 1  and the third planetary carrier PC 3 , and directly connected even with the output shaft OS to continuously operate as an output element. 
     The third rotational shaft TM 3  directly connects the first ring gear R 1  and the input shaft IS to continuously operate as an input element. 
     The fourth rotational shaft TM 4  is connected to the second sun gear S 2 , and is selectively connected to the transmission housing H to operate as a fixation element. 
     The fifth rotational shaft TM 5  directly connects the second planetary carrier PC 2  and the third ring gear R 3 , and is selectively connected to the transmission housing H to operate as the fixation element. 
     The sixth rotational shaft TM 6  is directly connected with the third sun gear S 3  and the second motor/generator MG 2 . 
     The first motor/generator MG 1  and the second motor/generator MG 2  as independent power sources have functions as the motor and the generator. 
     The first motor/generator MG 1  operates as a motor that is directly connected with the first rotational shaft TM 1  to supply rotational power or serves as a generator that generates electricity while rotating by rotational force of the first rotational shaft TM 1 . 
     The second motor/generator MG 2  operates as a motor that is directly connected with the sixth rotational shaft TM 6  to supply rotational power or serves as a generator that generates electricity while rotating by rotational force of the sixth rotational shaft TM 6 . 
     The first and second brakes BK 1  and BK 2  among the friction elements are the friction elements that selectively connect a rotation element (rotational shaft) and a fixation element (transmission housing), and the first clutch CL 1  as a friction element that selectively connects the rotation element and the fixation element may include multi-disk type hydraulic friction elements that are friction joined to each other by hydraulic pressure. 
     The first brake BK 1  is disposed to selectively connect the fifth rotational shaft TM 5  and the transmission housing H, and the second brake BK 2  is disposed to selectively connect the fourth rotational shaft TM 4  and the transmission housing H. 
     The first clutch CL 1  connects three rotation elements of the third planetary gear set PG 3  to each other by two rotational shafts of three rotational shafts TM 2 , TM 5 , and TM 6  including three rotation elements of the third planetary gear set PG 3  to allow the third planetary gear set PG 3  to be directly joined, and in  FIG. 1 , it is disclosed that the first clutch CL 1  is disposed between the second rotational shaft TM 2  and the fifth rotational shaft TM 5 . 
     In  FIG. 1 , it is disclosed that the engine ENG is disposed at a front side of the first planetary gear set PG 1 , but the present invention is not limited thereto and the engine ENG may be disposed at a rear side of the third planetary gear set PG 3 . 
       FIG. 2  is an operation table for each of operation modes of friction elements applied to the power transmission system according to the first exemplary embodiment of the present invention. 
     Referring to  FIG. 2 , operation states of the friction elements for each operation mode will be described below. 
     In EV mode 1, the first brake BK 1  operates and in EV mode 2, the first clutch CL 1  operates. 
     In HEV mode 1, the first brake BK 1  operates, in HEV mode 2, the first clutch CL 1  operates, and in HEV mode 3, the second brake BK 2  operates. 
     In ENG mode 1, the first and second brakes BK 1  and BK 2  operate and in ENG mode 2, the second brake BK 2  and the first clutch CL 1  operate. 
     As described above, the power transmission system according to the first exemplary embodiment of the present invention may implement two EV modes, three HEV modes, and two ENG modes. 
     Hereinafter, an operation principle for each mode will be described below. 
     [EV Mode 1] 
     The EV mode is a mode that drives the vehicle with power of the motor/generator by supplying power of a battery to the motor/generator while the engine stops. 
     The EV mode exerts a large influence on fuel efficiency improvement because the engine stops, has an advantage that the vehicle may be driven backward without an additional backward device, operates at the time of starting and low-speed driving after stopping, and requires a deceleration transmission ratio in which a power source rotates more rapidly than an output member for slip-down prevention or rapid acceleration on a slope. 
     Under such a condition, in EV mode 1, the second motor/generator MG 2  is operation-controlled while the fifth rotational shaft TM 5  operates as the fixation element by operating the first brake BK 1 , and as a result, a deceleration output is performed according to a gear ratio of the third planetary gear set PG 3  while an input in the sixth rotational shaft TM 6  is performed. 
     [EV Mode 2] 
     The motor/generator has characteristics that the efficiency depends on a rotational speed and torque and this means that a ratio of the electric energy is converted into mechanical energy of rotation and torque varies even though the same current is supplied. 
     That is, current of the battery used in the EV mode is energy accumulated by combustion of fuel or regenerative braking in the engine and efficiently using the accumulated energy regardless of a generation path is directly linked with fuel efficiency improvement. 
     Due to such a reason, in recent years, there has been a tendency that a transmission having two or more steps is mounted even in an electric vehicle and since it is advantageous that two or more transmission steps are provided in the EV mode of the hybrid electric vehicle, EV mode 2 is provided even in the exemplary embodiment of the present invention. 
     When a transmission process of EV mode 2 is examined by considering such a point, a vehicle speed increases to cancel the operation of the first brake BK 1  at a point in which the efficiency of the second motor/generator MG 2  is low, and the first clutch CL 1  is operation-controlled, during driving in EV mode 1 in EV mode 2. 
     Then, since the first clutch CL 1  which is a direct joining means of the third planetary gear set PG 3  operates, the third planetary gear set PG 3  is in a direct joining state, and as a result, while all of the rotational shafts TM 2 , TM 5 , and TM 6  rotate at the same speed, the input is just output. 
     [HEV Mode 1] 
     In HEV mode 1, the power of the engine is transmitted to the output member through a mechanical path and an electric path, the power is distributed by the planetary gear set, and since the engine and the motor/generator connected to the planetary gear set may arbitrarily control the rotational speed regardless of the vehicle speed, the engine and the motor/generator serve as an electronic continuously variable transmission. 
     Accordingly, an engine speed and torque is fixed to with respect to a given vehicle speed in the existing transmission, while the electronic continuously variable transmission may arbitrarily change the engine speed and the torque, driving efficiency of the engine may be maximized and fuel efficiency improvement may be achieved. 
     By considering such a point, in EV mode 1, the second rotational shaft TM 2  is restrained in connection with the output shaft OS and the residual first and third rotational shafts TM 1  and TM 3  are rotatable in the first planetary gear set PG 1 . 
     Therefore, after the engine ENG starts by using the first motor/generator MG 1 , the speeds of the engine ENG and the first motor/generator MG 1  may be controlled regardless of the vehicle speed. 
     Further, since torque of the first motor/generator MG 1  is applied clockwise regardless of the rotational direction, the sum of the torques of the engine ENG and the first motor/generator MG 1  are transmitted to the output shaft to generate high drive power. 
     In addition, when the first motor/generator MG 1  rotates counterclockwise, the first motor/generator MG 1  serves as the generator and when the second motor/generator MG 2  rotates clockwise (of course, in this case, the rotation speed of the engine ENG is lower than before), the second motor/generator MG 2  serves as the motor. 
     Since the engine ENG and the first motor/generator MG 1  may be continuously variably controlled as necessary in HEV mode 1 (input branch), HEV mode 1 may show very excellent performance in terms of fuel efficiency and power performance. 
     [HEV Mode 2] 
     In an exemplary embodiment of the present invention, the input branch mode may be set as two types, a rotational speed ratio of the engine and the motor/generator to the vehicle speed may be, in overall, set as two types by changing a gear ratio of the second planetary gear set PG 2 , and a rotational speed level for each rotation element deteriorates in overall to assist the fuel efficiency improvement. 
     In EV mode 2, the second rotational shaft TM 2  of the first planetary gear set PG 1  is restrained in connection with the output shaft OS and the residual first and third rotational shafts TM 1  and TM 3  are rotatable. 
     Therefore, when the engine ENG and the first motor/generator MG 1  are controlled, the speeds of the engine ENG and the first motor/generator MG 1  may be continuously variably controlled regardless of the vehicle speed. 
     In addition, when the first motor/generator MG 1  rotates counterclockwise, the first motor/generator MG 1  serves as the generator and when the first motor/generator MG 1  rotates clockwise (of course, in this case, the rotation speed of the engine ENG is lower than before), the first motor/generator MG 1  serves as the motor. 
     As such, since the engine ENG and the first motor/generator MG 1  may be continuously variably controlled as necessary, EV mode 2 may show very excellent performance in terms of fuel efficiency and power performance. 
     [HEV Mode 3] 
     In a hybrid input branch mode, the rotation speed of the motor/generator connected to the output member is restrained to the vehicle speed, and as a result, it is difficult to efficiently operate the motor/generator and decrease a capacity of the motor/generator. 
     In particular, when the vehicle speed is high and the rotation speed of the motor/generator chained to the vehicle speed is thus high, the efficiency of the motor/generator deteriorates, and as a result, optimal fuel efficiency may not be implemented. 
     Under such a condition, when the first planetary gear set PG 1  connected with the engine ENG and two different elements of the third planetary gear set PG 3  connected with an output gear are joined to each other to control the rotational speeds of the engine ENG and two motors/generators MG 1  and MG 2  regardless of the vehicle speed, a function of the continuously variable transmission operates once again to achieve the fuel efficiency improvement. 
     As a result, when the second brake BK 2  operates, the speed and the torque of the second motor/generator MG 2  is restrained by the speed and the torque of the engine ENG through the fifth rotational shaft TM 5 , and since the first planetary gear set PG 1  and the third planetary gear set PG 3  are connected to each other by the second rotational shaft TM 2 , the speeds and the torques are restrained from each other. 
     In addition, the first and second motors/generators MG 1  and MG 2  need to have an electric energy balance to each other, and while all rotation elements of the first and second planetary gear sets PG 1  and PG 2  are correlated to each other in terms of the speed and the torque, the function of the electronic continuously variable transmission is performed. 
     [ENG Mode 1] 
     Core technology for the fuel efficiency improvement of the hybrid electric vehicle may be recovery and reuse of regenerative energy and arbitrary control of an engine operation point. 
     In addition, the control of the engine operation point accompanies two energy conversion processes of a process in which the mechanical energy of the engine is converted into the electric energy in the motor/generator and a process in which the electric energy of the motor/generator is converted into the mechanical energy again in the motor/generator again. 
     During the energy conversion, total energy is not output and the loss is generated in the middle of the conversion and under a predetermined driving condition, the fuel efficiency may be more excellent in the ENG mode in which the vehicle is driven by only the engine than the HEV mode. 
     That is, in ENG mode 1, when the second brake BK 2  and the first brake BK 1  are coupled to each other, the first rotational shaft TM 1  stops, the third planetary carrier PC 3  of the third planetary gear set PG 3  rotates clockwise through the second rotational shaft TM 2  according to the rotational speed of the engine ENG, and the third ring gear R 3  stops, the deceleration transmission is performed. 
     In this case, since the power of the first and second motors/generators MG 1  and MG 2  is not required, the ENG mode in which the vehicle is driven by only the power of the engine is established. 
     [ENG Mode 2] 
     In ENG mode 2, when the first clutch CL 1  and the second brake BK 2  are coupled to each other, all rotation elements TM 2 , TM 5 , and TM 6  of the third planetary gear set PG 3  integrally rotate, the fifth rotational shaft TM 5  that connects the third ring gear R 3  and the second planetary carrier PC 2  rotates at a speed which is the same as an output speed, and the first rotational shaft TM 1  is accelerated to rotate according to the gear ratio of the second planetary gear set PG 2 . 
     Further, the third rotational shaft TM 3  connected with the engine ENG rotates at a lower speed than the second rotational shaft TM 2 , and as a result, an acceleration transmission ratio is formed. 
     In this case, since the power of the first and second motors/generators MG 1  and MG 2  is not required, the ENG mode in which the vehicle is driven by only the power of the engine is established. 
     As described above, according to the first exemplary embodiment of the present invention, in an overall configuration, two EV modes, three HEV modes, and two ENG modes may be implemented in a combination of three planetary gear sets PG 1 , PG 2 , and PG 3 , three friction elements BK 1 , BK 2 , and CL 1 , and two motors/generators MG 1  and MG 2 . 
     Further, in the first exemplary embodiment of the present invention, the power of the engine ENG is input into the first ring gear R 1  of first planetary gear set and the power of the first motor/generator MG 1  is input into the first sun gear S 1 , and as a result, torque which is larger than the torque of the engine ENG is transmitted to the output shaft OS to increase a weight of a mechanical power transmission path and use larger engine power for the same specification of the first motor/generator MG 1 . 
     Further, in the first exemplary embodiment of the present invention, since the larger torque than the engine torque may be transmitted to the output shaft, high-rotation driving having high engine power at the same vehicle speed is enabled at the time of WOT launching and a larger acceleration force may be achieved. 
     In addition, in the first exemplary embodiment of the present invention, since the larger acceleration may be achieved in the HEV mode than the ENG mode, the HEV mode need not be converted into the ENG mode at the time of launching, and as a result, a relatively simpler system may be configured and friction elements may be reduced depending on mode reduction, thereby further increasing efficiency. 
     The vehicle may be driven without the electric load of the first and second motors/generators MG 1  and MG 2  by providing the ENG mode at the time of the high-speed driving to improve fuel efficiency. 
       FIG. 3  is a configuration diagram of a power transmission system according to a second exemplary embodiment of the present invention. 
     Referring to  FIG. 3 , the first clutch CL 1  which is the direct joining means of the third planetary gear set PG 3  is disposed between the second rotational shaft TM 2  and the fifth rotational shaft TM 5  in the first exemplary embodiment, but the first clutch CL 1  which is the direct joining means of the third planetary gear set PG 3  is disposed between the fifth rotational shaft TM 5  and the sixth rotational shaft TM 6  in the second exemplary embodiment. 
     Since the second exemplary embodiment is different from the first exemplary embodiment only in a layout position of the first clutch CL 1  and the second exemplary embodiment is the same as the first exemplary embodiment in an operation effect, a detailed description thereof will be omitted. 
       FIG. 4  is a configuration diagram of a power transmission system according to a third exemplary embodiment of the present invention. 
     Referring to  FIG. 4 , the first clutch CL 1  which is the direct joining means of the third planetary gear set PG 3  is disposed between the second rotational shaft TM 2  and the seventh rotational shaft TM 7  in the first exemplary embodiment, but the first clutch CL 1  which is the direct joining means of the third planetary gear set PG 3  is disposed between the second rotational shaft TM 2  and the sixth rotational shaft TM 6  in the second exemplary embodiment. 
     Since the third exemplary embodiment is different from the first exemplary embodiment only in the layout position of the first clutch CL 1  and the second exemplary embodiment is the same as the first exemplary embodiment in the operation effect, a detailed description thereof will be omitted. 
       FIG. 5  is a configuration diagram of a power transmission system according to a fourth exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , the first planetary gear set PG 1  is configured by a single pinion planetary gear set in the first exemplary embodiment, but the first planetary gear set PG 1  is configured by a double-pinion planetary gear set in the fourth exemplary embodiment. 
     As a result, the first rotational shaft TM 1  related with the first planetary gear set PG 1  is configured by connecting the first sun gear S 1  and the second ring gear R 2 , the second rotational shaft TM 2  is configured by connecting the first ring gear R 1  and the third planetary carrier PC 3 , and the third rotational shaft TM 3  is configured to include the first planetary carrier PC 1 . 
     Since the fourth exemplary embodiment is different from the first exemplary embodiment only in configurations of the second and third rotational shafts TM 2  and TM 3  and the fourth exemplary embodiment is the same as the first exemplary embodiment in the operation effect, a detailed description thereof will be omitted. 
       FIG. 6  is a configuration diagram of a power transmission system according to a fifth exemplary embodiment of the present invention. 
     Referring to  FIG. 6 , the second planetary gear set PG 2  is configured by the single pinion planetary gear set in the first exemplary embodiment, but the second planetary gear set PG 2  is configured by the double-pinion planetary gear set in the fifth exemplary embodiment. 
     As a result, the first rotational shaft TM 1  related with the second planetary gear set PG 2  is configured by connecting the first sun gear S 1  and the second planetary carrier PC 2 , the fourth rotational shaft TM 4  is configured to be connected to the second sun gear S 2 , and the fifth rotational shaft TM 5  is configured by connecting the second ring gear R 2  and the third ring gear R 3 . 
     Since the fifth exemplary embodiment is different from the first exemplary embodiment only in configurations of the first and fifth rotational shafts TM 1  and TM 5  and the fifth exemplary embodiment is the same as the first exemplary embodiment in the operation effect, a detailed description thereof will be omitted. 
       FIG. 7  is a configuration diagram of a power transmission system according to a sixth exemplary embodiment of the present invention. 
     Referring to  FIG. 7 , the third planetary gear set PG 3  is configured by the single pinion planetary gear set in the first exemplary embodiment, but the third planetary gear set PG 3  is configured by the double-pinion planetary gear set in the sixth exemplary embodiment. 
     As a result, the second rotational shaft TM 2  related with the third planetary gear set PG 3  is configured by connecting the first planetary carrier PC 1  and the third ring gear R 3 , the fifth rotational shaft TM 5  is configured by connecting the second planetary carrier PC 2  and the third planetary carrier PC 3 , and the sixth rotational shaft TM 6  is configured to be connected to the third sun gear S 3 . 
     Since the fifth exemplary embodiment is different from the first exemplary embodiment only in configurations of the second and seventh rotational shafts TM 2  and TM 7  and the fifth exemplary embodiment is the same as the first exemplary embodiment in the operation effect, a detailed description thereof will be omitted. 
     For convenience in explanation and accurate definition in the appended claims, the terms “upper”, “lower”, “inner” and “outer” are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures. 
     The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.