Patent Publication Number: US-2021178887-A1

Title: Power transmission apparatus of hybrid electric vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Korean Patent Application No. 10-2019-0164615 filed on Dec. 11, 2019, the entire contents of which is incorporated herein for all purposes by this reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a power transmission apparatus of a hybrid electric vehicle. 
     Description of Related Art 
     An environment-friendly technology of a vehicle is a core technology which controls survival of a future vehicle industry, and advanced vehicle makers have focused their energy on the development of an environment-friendly vehicle to achieve environmental and fuel efficiency regulations. 
     Therefore, vehicle makers have developed an electric vehicle (EV), a hybrid electric vehicle (HEV), a fuel cell electric vehicle (FCEV), and the like, as future vehicle technologies. 
     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 conventional fossil fuel and a motor-generator driven by electrical 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 may be implemented according to the combination of the engine and the motor. Furthermore, the hybrid electric vehicle can provide a significant improvement of fuel efficiency through an idle stop function of stopping the engine when the vehicle stops, and also through a regenerative braking, where a motor-generator is driven as a generator to generate electricity by a kinetic energy of the vehicle under a braking situation, such generated electricity is stored in a battery, and the stored electricity is reused in driving the vehicle. 
     A transmission for hybrid electric vehicle performs shifting operation based on torques of the engine and the motor-generator. Such a transmission may realize multi-speed, e.g., six speeds, of various modes by additionally employing an engine clutch to be variably connected to the engine to a traditional multi-speed, e.g., six-speed, automatic transmission. 
     Such a transmission for a hybrid electric vehicle may typically include three planetary gear sets, six operational elements, and at least one one-way clutch OWC, similarly to a conventional six-speed automatic transmission, as well as the additional engine clutch. By such a scheme, the transmission for a hybrid electric vehicle may not be understood to be best optimized for a hybrid electric vehicle, and may be improved to provide better efficiency, better performance, and better fuel consumption, or less production cost. 
     The information included in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and may 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 apparatus of a hybrid electric vehicle having advantages of, while simplifying the structure of a transmission, realizing various shifting modes, such as an engine mode and a parallel hybrid mode respectively having multiple speeds, an electronically-controlled continuously variable shifting mode (eCVT mode), and an electric vehicle mode (EV mode), reducing a production cost, and realizing fuel consumption characteristic and power performance above an equivalent transmission. 
     An exemplary power transmission apparatus is for a hybrid electric vehicle having power sources of an engine and a motor-generator, and includes, an input shaft receiving a torque of the engine, a first planetary gear set having first, second, and third rotation elements and mounted on the input shaft, a second planetary gear set having fourth, fifth, and sixth rotation elements and mounted on the input shaft, a first shaft fixedly connected to the first rotation element and the motor-generator and selectively connectable to the input shaft, a second shaft fixedly connected to the second rotation element and the sixth rotation element and selectively connectable to the input shaft, a third shaft fixedly connected to the third rotation element and selectively connectable to the transmission housing, a fourth shaft fixedly connected to the fourth rotation element and selectively connectable to the transmission housing, a fifth shaft fixedly connecting the fifth rotation element and an output gear and selectively connectable to the third shaft, and a plurality of engagement elements including at least one clutch and at least one brake. 
     The first planetary gear set may be formed as a single pinion planetary gear set having a first sun gear as the first rotation element, a first planet carrier as the second rotation element, and a first ring gear as the third rotation element. The second planetary gear set may be formed as a single pinion planetary gear set having a second sun gear as the fourth rotation element, a second planet carrier as the fifth rotation element, and a second ring gear as the sixth rotation element. 
     The plurality of engagement elements may include a first clutch mounted between the first shaft and the input shaft, a second clutch mounted between the third shaft and the fifth shaft, a third clutch mounted between the second shaft and the input shaft, a first brake mounted between the fourth shaft and the transmission housing, and a second brake mounted between the third shaft and the transmission housing. 
     The first brake may be mounted between the second planetary gear set and the motor-generator. The second brake may be mounted radially external to the first planetary gear set. 
     The first brake and the second brake may be mounted between the second planetary gear set and the motor-generator. 
     The first planetary gear set may be formed as a double pinion planetary gear set having a first sun gear as the first rotation element, a first ring gear as the second rotation element, and a first planet carrier as the third rotation element. The second planetary gear set may be formed as a single pinion planetary gear set having a second sun gear as the fourth rotation element, a second planet carrier as the fifth rotation element, and a second ring gear as the sixth rotation element. 
     The plurality of engagement elements may include a first clutch mounted between the first shaft and the input shaft, a second clutch mounted between the third shaft and the fifth shaft, a third clutch mounted between the second shaft and the input shaft, a first brake mounted between the fourth shaft and the transmission housing, and a second brake mounted between the third shaft and the transmission housing. 
     Furthermore, a one-way clutch may be mounted between the input shaft and the engine output shaft. 
     According to a power transmission apparatus of a hybrid electric vehicle according to an exemplary embodiment of the present invention, while simplifying the structure of a transmission by employing only two planetary gear sets, various shifting modes, such as an engine mode and a parallel hybrid mode respectively having four shifting stages, an electronically-controlled continuously variable shifting mode (eCVT mode), and an electric vehicle mode (EV mode) having four shifting stages, may be realized, reducing a production cost, and realizing fuel consumption characteristic and power performance above an equivalent transmission. 
     Furthermore, the number of employed planetary gear sets may be decreased in comparison to a conventional six-speed transmission, and therefore, an overall length may be decreased, improving installability into a vehicle. 
     Furthermore, a one-way clutch is mounted between the input shaft and the engine output shaft, and thereby, an engine drag may be prevented in the EV mode and in regenerative braking. 
     Furthermore, effects which may be obtained or expected from exemplary embodiments of the present invention are directly or suggestively described in the following detailed description. That is, various effects expected from exemplary embodiments of the present invention will be described in the following detailed description. 
     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 schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of the present invention. 
         FIG. 2  is an operation chart of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments. 
         FIG. 3  is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of the present invention. 
         FIG. 4  is an operation chart of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments. 
         FIG. 5  is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of the present invention. 
         FIG. 6  is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of the present invention. 
         FIG. 7  is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of the present invention. 
         FIG. 8  is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of the present invention. 
     
    
    
     It may 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 present invention. The specific design features of the present invention as included herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particularly intended application and use environment. 
     In the figures, reference numbers refer to the same or equivalent portions 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 present invention(s) will be described in conjunction with exemplary embodiments of the present invention, it will be understood that the present description is not intended to limit the present invention(s) to those exemplary embodiments. On the other hand, the present invention(s) is/are intended to cover not only the exemplary embodiments of the present invention, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the present invention as defined by the appended claims. 
     Exemplary embodiments of the present application will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the present invention are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. 
     The drawings and description are to be regarded as illustrative in nature and not restrictive, and like reference numerals designate like 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 schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of the present invention. 
     Referring to  FIG. 1 , a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments receives torques from power sources of an engine ENG and a motor-generator MG and includes an input shaft IS, first and second planetary gear sets PG 1  and PG 2 , five shafts TM 1  to TM 5  respectively connecting rotation elements of the first planetary gear set and the second planetary gear set PG 1  and PG 2 , and engagement elements of three clutches C 1  to C 3  and two brakes B 1  and B 2 . 
     As a result, the torques input from the engine ENG and the motor-generator MG is shifted by a cooperative operation of the first planetary gear set and the second planetary gear set PG 1  and PG 2 , and a shifted torque is output through an output gear OG. 
     The engine ENG is a primary power source, and may be implemented as one of various types such as a gasoline engine or a diesel engine. 
     The motor-generator MG is fixedly connected to the planetary gear set portion PG and is used as an auxiliary power source. 
     The motor-generator MG may act as a motor and also as a generator, and includes a stator ST and a rotor RT, where the stator ST is fixed to a transmission housing H, and the rotor RT is internally mounted within the stator ST and rotatable relative to the stator ST. 
     The input shaft IS is an input member, and may receive the torque of the engine ENG through an engine output shaft. 
     The output gear OG is an output element, and may deliver a shifted torque to a driveshaft through a differential apparatus. 
     The first planetary gear set PG 1  is a single pinion planetary gear set, and may include a first rotation element N 1  of a first sun gear S 1 , a second rotation element N 2  of a first planet carrier PC 1  rotatably supporting a plurality of first pinion gears P 1  externally gear-meshed with the first sun gear S 1 , and a third rotation element N 3  of a first ring gear R 1  internally gear-meshed with the plurality of first pinion gears P 1 . 
     The second planetary gear set PG 2  is a single pinion planetary gear set, and includes a fourth rotation element N 4  of a second sun gear S 2 , a fifth rotation element N 5  of a second planet carrier PC 2  rotatably supporting a plurality of second pinion gears P 2  externally gear-meshed with the second sun gear S 2 , and a sixth rotation element N 6  of a second ring gear R 2  internally gear-meshed with the plurality of second pinion gears P 2 . 
     That is, a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments is formed by a combination of first and second planetary gear sets PG 1  and PG 2 , and includes five shafts TM 1 , TM 2 , TM 3 , TM 4 , and TM 5 . 
     The first to sixth rotation elements N 1  to N 6  are fixedly connected to a corresponding one of the shafts TM 1  to TM 5 , and the first planetary gear set and the second planetary gear set PG 1  and PG 2  are operated by a plurality of engagement elements including at least one clutch and at least one brake. 
     The five shafts TM 1  to TM 5  are hereinafter described in detail. 
     The first shaft TM 1  is fixedly connected to the first rotation element N 1  (first sun gear S 1 ) and the motor-generator MG, and while acting as an input element of the motor-generator MG, is selectively connectable to the input shaft IS, selectively acting as an input element of the engine ENG 
     The second shaft TM 2  fixedly connects the second rotation element N 2  (first planet carrier PC 1 ) and the sixth rotation element N 6  (second ring gear R 2 ), and is selectively connectable to the input shaft IS, selectively acting as an input element. 
     The third shaft TM 3  is fixedly connected to the third rotation element N 3  (first ring gear R 1 ), and selectively connectable to the transmission housing H, selectively acting as a fixed element. 
     The fourth shaft TM 4  is fixedly connected to the fourth rotation element N 4  (second sun gear S 2 ), and selectively connectable to the transmission housing H, selectively acting as a fixed element. 
     The fifth shaft TM 5  is fixedly connected to the fifth rotation element N 5  (second planet carrier PC 2 ), selectively connectable to the third shaft TM 3 , and fixedly connected to the output gear OG, always acting as an output element. 
     In an exemplary embodiment of the present invention, when two or more members are described to be “fixedly connected”, where each of the members may be any of a shaft, an input shaft, an output shaft, a rotation member, and a transmission housing, it means that the fixedly connected members always rotate at a same speed. 
     When two or more members are described to be “selectively connectable” by an engagement element, it means that the selectively connectable members rotate separately when the engagement element is not engaged, and rotates at a same speed when the engagement element is engaged. 
     It may be understood that in the case that a member is “selectively connectable” to a transmission housing by an engagement element, the member may be stationary when the engagement element is engaged. 
     Engagement elements of first, second, and third clutches C 1 , C 2 , and C 3  are mounted between a corresponding pair of the five shafts TM 1  to TM 5  and the input shaft, to form selective connections 
     Engagement elements of first and second brakes B 1  and B 2  are mounted between the transmission housing H and a corresponding shaft of the five shafts TM 1  to TM 5 , to form selective connections 
     The five engagement elements of the three clutches C 1 , C 2 , and C 3  and the two brakes B 1  and B 2  are mounted as follows. 
     The first clutch C 1  is mounted between the first shaft TM 1  and the input shaft IS, and selectively connects the first shaft TM 1  and the input shaft IS, controlling power delivery therebetween. 
     The second clutch C 2  is mounted between the third shaft TM 3  and the fifth shaft TM 5 , and selectively connects the second shaft TM 2  and the fifth shaft TM 5 , controlling power delivery therebetween. 
     The third clutch C 3  is mounted between the second shaft TM 2  and the input shaft IS, and selectively connects the first shaft TM 1  and the input shaft IS, controlling power delivery therebetween. 
     The first brake B 1  is mounted between the fourth shaft TM 4  and the transmission housing H, and selectively connects the fourth shaft TM 4  to the transmission housing H. 
     The second brake B 2  is mounted between the third shaft TM 3  and the transmission housing H, and selectively connects the third shaft TM 3  to the transmission housing H. 
     The first brake B 1  is mounted between the second planetary gear set PG 2  and the motor-generator MG, and the second brake B 2  is mounted radially external to the first planetary gear set PG 1 . 
     The engagement elements of the first, second, and third clutches C 1 , C 2 , and C 3  and the first brake and the second brake B 1  and B 2  may be realized as multi-plate hydraulic pressure friction devices that are frictionally engaged by hydraulic pressure, however, it may not be understood to be limited thereto, since various other configuration that are electrically controllable may be available. 
       FIG. 2  is an operation chart of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of  FIG. 1 . 
     Referring to  FIG. 2 , a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments realizes an engine mode (hybrid mode) and an EV mode respectively having four fixed shifting stages, and also realizes an eCVT mode facilitating electronically-controlled continuously variable shifting. 
     In the engine mode (hybrid mode), the engine ENG is operated. In the instant state, the torque of the engine ENG is input to the first planetary gear set PG 1  by operation of at least one of the first and third clutches C 1  and C 3 , and the second clutch C 2  and the first brake and the second brake B 1  and B 2  are controlled to realize four fixed shifting stages. 
     In the eCVT mode, the engine ENG is operated at a fixed rotation speed, and the third clutch C 3  is engaged to transmit the torque of the engine ENG to the second shaft TM 2 . Simultaneously, the motor-generator MG connected to the first shaft TM 1  is operated to deliver a motor torque to the first rotation element N 1 , and the rotation speed of the motor-generator MG is varied to vary gear ratio of the transmission to achieve gear ratios for a high gear (gear ratio below 1.0). 
     In the EV mode, the first clutch C 1  is released to disconnect the first planetary gear set PG 1  from the engine ENG and simultaneously, the torque of the motor-generator MG is transmitted to the first rotation element N 1  through the first shaft TM 1 . Similarly to the engine mode, the second and third clutches C 2  and C 3  and the first brake and the second brake B 1  and B 2  are controlled to realize four EV fixed shifting stages. 
     Here, the engine mode (hybrid mode) and the eCVT mode may be combined to realize a shifting mode of at least five speeds. 
     That is, while maintaining the sift-stages of first forward speed to fourth forward speed in the engine mode, an overdrive OD may be realized by the eCVT mode performing electronically-controlled continuously variable shifting to realize a gear ratio appropriate for a high gear. 
     Hereinafter, an operation of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments is described in detail in connection with respective modes. 
     [Engine Mode First Forward Speed (Hybrid Mode First Forward Speed)] 
     In an engine mode first forward speed, the engine ENG is operated, and the first clutch C 1  and the first brake and the second brake B 1  and B 2  are simultaneously operated. 
     Accordingly, the fourth shaft TM 4  acts as a fixed element by the operation of first brake B 1 , and the third shaft TM 3  acts as a fixed element by the operation of second brake B 2 . In the instant state, by the operation of the first clutch C 1 , the torque of the engine ENG is input to the first rotation element N 1  through the first shaft TM 1 . 
     As a result, a reduced speed output is delivered to the fifth rotation element N 5  of the first planetary gear set and the second planetary gear set PG 1  and PG 2 , and the torque of the engine mode first forward speed is output through the output gear OG connected to the fifth rotation element N 5  through the fifth shaft TM 5 . 
     Here, when the motor-generator MG is driven, the torque of the motor-generator MG is added to the first shaft TM 1 , and therefore, a hybrid mode first forward speed may be realized. 
     [Engine Mode Second Forward Speed (Hybrid Mode Second Forward Speed)] 
     In an engine mode second forward speed, the engine ENG is operated, and the first clutch and second clutch C 1  and C 2  and the first brake B 1  are simultaneously operated. 
     Accordingly, the fourth shaft TM 4  acts as a fixed element by the operation of first brake B 1 . In the instant state, the third shaft TM 3  and the fifth shaft TM 5  are interconnected by the operation of the second clutch C 2 , and by the operation of the first clutch C 1 , the torque of the engine ENG is input to the first rotation element N 1  through the first shaft TM 1 . 
     As a result, a reduced speed output is delivered to the fifth rotation element N 5  of the first planetary gear set and the second planetary gear set PG 1  and PG 2 , and the torque of the engine mode second forward speed is output through the output gear OG connected to the fifth rotation element N 5  through the fifth shaft TM 5 . 
     Here, when the motor-generator MG is driven, the torque of the motor-generator MG is added to the first shaft TM 1 , and therefore, a hybrid mode second forward speed may be realized. 
     [Engine Mode Third Forward Speed (Hybrid Mode Third Forward Speed)] 
     In an engine mode third forward speed, the engine ENG is operated, and the first and the third clutch C 1  and C 3 , first brake B 1  are simultaneously operated. 
     Accordingly, the fourth shaft TM 4  acts as a fixed element by the operation of first brake B 1 . In the instant state, by the operation of the first and third clutches C 1  and C 3 , the torque of the engine ENG is input to the first rotation element N 1  through the first shaft TM 1 , and is input to the second and sixth rotation elements N 2  and N 6  through the second shaft TM 2 . 
     As a result, a reduced speed output is delivered to the fifth rotation element N 5  of the first planetary gear set and the second planetary gear set PG 1  and PG 2 , and the torque of the engine mode third forward speed is output through the output gear OG connected to the fifth rotation element N 5  through the fifth shaft TM 5 . 
     Here, when the motor-generator MG is driven, the torque of the motor-generator MG is added to the first shaft TM 1 , and therefore, a hybrid mode third forward speed may be realized. 
     [Engine Mode Fourth Forward Speed (Hybrid Mode Fourth Forward Speed)] 
     In an engine mode fourth forward speed, the engine ENG is operated, and the first and the second and third clutches C 1 , C 2 , and C 3  are simultaneously operated. 
     Accordingly, the third shaft TM 3  and the fifth shaft TM 5  are interconnected by the operation of the second clutch C 2 , and the torque of the engine ENG is simultaneously input to the first shaft TM 1  and the second shaft TM 2  by the operation of the first and third clutches C 1  and C 3 . Therefore, the first planetary gear set and the second planetary gear set PG 1  and PG 2  integrally rotates. 
     As a result, in the first planetary gear set and the second planetary gear set PG 1  and PG 2 , an input torque is directly output (at a ratio of 1:1) through the output gear OG connected to the fifth rotation element N 5  through the fifth shaft TM 5 , outputting a torque of engine mode fourth forward speed. 
     Here, when the motor-generator MG is driven, the torque of the motor-generator MG is added to the first shaft TM 1 , and therefore, a hybrid mode fourth forward speed may be realized. 
     In such an engine mode, while a vehicle is stopped with the engine ENG running, the first clutch C 1  may be operated to transmit the torque of the engine ENG to the motor-generator MG to generate electricity to recharge a battery. 
     [eCVT Mode] 
     In the eCVT mode, the engine ENG is operated at a fixed rotation speed, and the second and third clutches C 2  and C 3  are operated. 
     Accordingly, the third shaft TM 3  and the fifth shaft TM 5  are interconnected by the operation of the second clutch C 2 , the third clutch C 3  is operated to transmit the torque of the engine ENG to the second shaft TM 2 , and simultaneously, the torque of the motor-generator MG connected to the first shaft TM 1  is transmitted to the first rotation element N 1 . 
     In such a state, an eCVT mode having gear ratios appropriate for a high gear may be realized by varying the rotation speed of the motor-generator MG. 
     [EV Mode First Speed] 
     In an EV mode first speed, the first clutch C 1  is released to disconnect the engine ENG In the instant state, the first brake and the second brake B 1  and B 2  are simultaneous operated, and the motor-generator MG is operated. 
     Accordingly, the fourth shaft TM 4  and the third shaft TM 3  act as fixed elements by the operation of the first brake and the second brake B 1  and B 2 . In the instant state, the torque of the motor-generator MG is input to the first rotation element N 1  through the first shaft TM 1 . 
     As a result, a reduced speed output is delivered to the fifth rotation element N 5  of the first planetary gear set and the second planetary gear set PG 1  and PG 2 , and the torque of the EV mode first speed is output through the output gear OG connected to the fifth rotation element N 5  through the fifth shaft TM 5 . 
     [EV Mode Second Speed] 
     In an EV mode second speed, the first clutch C 1  is released to disconnect the engine ENG. In the instant state, the second clutch C 2  and the first brake B 1  are simultaneous operated and the motor-generator MG is operated. 
     Accordingly, the third shaft TM 3  and the fifth shaft TM 5  are interconnected by the operation of the second clutch C 2 , and the fourth shaft TM 4  acts as a fixed element by the operation of first brake B 1 . In the instant state, the torque of the motor-generator MG is input to the first rotation element N 1  through the first shaft TM 1 . 
     As a result, a reduced speed output is delivered to the fifth rotation element N 5  of the first planetary gear set and the second planetary gear set PG 1  and PG 2 , and the torque of the EV mode second speed is output through the output gear OG connected to the fifth rotation element N 5  through the fifth shaft TM 5 . 
     [EV Mode Third Speed] 
     In an EV mode third speed, the first clutch C 1 , the third clutch C 3 , and the first brake B 1  are simultaneous operated and the motor-generator MG is operated. 
     Accordingly, the first shaft TM 1  and the second shaft TM 2  are interconnected by the operation of the first clutch C 1  and the third clutch C 3 , and the fourth shaft TM 4  acts as a fixed element by the operation of first brake B 1 . In the instant state, the torque of the motor-generator MG is input to the first rotation element N 1  through the first shaft TM 1  and to the second rotation element N 2  through the input shaft IS and the second shaft TM 2 . 
     As a result, the first planetary gear set PG 1  integrally rotates and a reduced speed is output at the fifth rotation element N 5  of the second planetary gear set PG 2 . Accordingly, the torque of the EV mode third speed is output through the output gear OG connected to the fifth rotation element N 5  through the fifth shaft TM 5 . 
     [EV Mode Fourth Speed] 
     In an EV mode fourth speed, the first, second, and third clutches C 1 , C 2 , and C 3  are simultaneous operated, and the motor-generator MG is operated. 
     Accordingly, the first planetary gear set PG 1  integrally rotates by the operation of the first and third clutches C 1  and C 3 , and the second planetary gear set PG 2  integrally rotates by the operation of the second clutch C 2 , since the torque of the motor-generator MG is simultaneously input to the fifth and sixth rotation elements N 5  and N 6 . Therefore, the first planetary gear set and the second planetary gear set PG 1  and PG 2  integrally rotates. 
     As a result, in the first planetary gear set and the second planetary gear set PG 1  and PG 2 , an input torque is directly output (at a ratio of 1:1) through the output gear OG connected to the fifth rotation element N 5  through the fifth shaft TM 5 , outputting a torque of EV mode fourth speed. 
       FIG. 3  is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments.  FIG. 4  is an operation chart of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments. 
     Referring to  FIG. 3 , a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments in  FIG. 3  differ from the various exemplary embodiments in  FIG. 1  in that a one-way clutch OWC is applied between the input shaft IS and the engine output shaft EOS. 
     Since the one-way clutch OWC is mounted between the input shaft IS and the engine output shaft EOS, an engine drag may be prevented in the EV mode and in regenerative braking. 
     As a result, a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments merely differ from the various exemplary embodiments in the feature of employing the one-way clutch, and other arrangement remains the same, which is not further described in detail. 
     Furthermore, referring to  FIG. 4 , shifting modes may be realized in the same way as in the various exemplary embodiments in  FIG. 1  except for only the operation of the one-way clutch OWC, and therefore shifting operation is not further described in detail. 
       FIG. 5  is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of the present invention. 
     Referring to  FIG. 5 , a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments differ from the various exemplary embodiments in  FIG. 1  in the locations of the first brake and the second brake B 1  and B 2 . 
     That is, in the various exemplary embodiments of the present invention in  FIG. 1 , the first brake B 1  is located between the second planetary gear set PG 2  and the motor-generator MG and the second brake B 2  is located radially external to the first planetary gear set PG 1 . However, in the various exemplary embodiments of the present invention of  FIG. 5 , the first brake and the second brake B 1  and B 2  are located between the second planetary gear set PG 2  and the motor-generator MG. 
     As a result, a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of  FIG. 5  merely differ from the various exemplary embodiments of  FIG. 1  in the locations of the first brake and the second brake B 1  and B 2 , and other arrangement remains the same, which is not further described in detail. 
     Furthermore, shifting modes may be realized in the same way as in the various exemplary embodiments of the present invention in  FIG. 1 , and therefore shifting operation is not further described in detail. 
       FIG. 6  is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of the present invention. 
     Referring to  FIG. 6 , a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments differ from the various exemplary embodiments in  FIG. 5  in that a one-way clutch OWC is applied between the input shaft IS and the engine output shaft EOS. 
     Since the one-way clutch OWC is mounted between the input shaft IS and the engine output shaft EOS, an engine drag may be prevented in the EV mode and in regenerative braking. 
     As a result, a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of  FIG. 6  merely differ from the various exemplary embodiments in  FIG. 5  in the feature of employing the one-way clutch, and other arrangement remains the same, which is not further described in detail. 
     Furthermore, shifting modes may be realized in the same way as in the various exemplary embodiments of  FIG. 3  except for only the operation of the one-way clutch OWC, while others remain the same, and therefore shifting operation is not further described in detail. 
       FIG. 7  is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of the present invention. 
     Referring to  FIG. 7 , a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments differs from the various exemplary embodiments in  FIG. 5  in an arrangement of the first planetary gear set PG 1 . 
     That is, in the various exemplary embodiments of the present invention in  FIG. 5 , the first planetary gear set PG 1  is formed as a single pinion planetary gear set. However, in the various exemplary embodiments of the present invention in  FIG. 7 , the first planetary gear set PG 1  is formed as a double pinion planetary gear set. 
     That is, the first planetary gear set PG 1  of various exemplary embodiments in  FIG. 7  is a double pinion planetary gear set, and includes a first rotation element N 1  of a first sun gear S 1 , a second rotation element N 2  of a first ring gear R 1  internally gear-meshed with a plurality of first pinion gears P 1  that are externally gear-meshed with the first sun gear S 1 , and a third rotation element N 3  of a first planet carrier PC 1  rotatably supporting the plurality of first pinion gears P 1 . 
     As a result, a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments in  FIG. 7  merely differ from the various exemplary embodiments in  FIG. 5  in employing a different type of the first planetary gear set PG 1 , and other arrangement remains the same, which is not further described in detail. 
     Furthermore, shifting modes may be realized in the same way as in the various exemplary embodiments of the present invention in  FIG. 1 , and therefore shifting operation is not further described in detail. 
       FIG. 8  is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments of the present invention. 
     Referring to  FIG. 8 , a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments differs from the various exemplary embodiments in  FIG. 7  in that a one-way clutch OWC is applied between the input shaft IS and the engine output shaft EOS. 
     Since the one-way clutch OWC is mounted between the input shaft IS and the engine output shaft EOS, an engine drag may be prevented in the EV mode and in regenerative braking. 
     As a result, a power transmission apparatus of a hybrid electric vehicle according to various exemplary embodiments in  FIG. 8  merely differ from the various exemplary embodiments in  FIG. 7  in the feature of employing the one-way clutch, and other arrangement remains the same, which is not further described in detail. 
     Furthermore, shifting modes may be realized in the same way as in the various exemplary embodiments in  FIG. 3  except for only the operation of the one-way clutch OWC, and therefore shifting operation is not further described in detail. 
     As described above, a power transmission apparatus of a hybrid electric vehicle according to exemplary embodiments employs only two planetary gear sets PG 1  and PG 2 , simplifying the structure of a transmission. Furthermore, an engine mode and a parallel hybrid mode respectively having six speeds and an electronically-controlled continuously variable shifting mode (eCVT mode) may be combined to realize various shifting modes having more than four speeds, reducing a production cost, and realizing fuel consumption characteristic and power performance above an equivalent transmission. 
     Furthermore, the number of employed planetary gear sets may be decreased in comparison to a conventional six-speed transmission, and therefore, an overall length may be decreased, improving installability. 
     Furthermore, a one-way clutch OWC is applied between the input shaft IS and the engine output shaft EOS, and thereby, an engine drag may be prevented in the EV mode and in regenerative braking. 
     For convenience in explanation and accurate definition in the appended claims, the terms “upper”, “lower”, “inner”, “outer”, “up”, “down”, “upwards”, “downwards”, “front”, “rear”, “back”, “inside”, “outside”, “inwardly”, “outwardly”, “internal”, “external”, “inner”, “outer”, “forwards”, and “backwards” are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures. It will be further understood that the term “connect” or its derivatives refer both to direct and indirect connection. 
     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 present 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 present invention and their practical application, to 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 present invention be defined by the Claims appended hereto and their equivalents.