Abstract:
A planetary gear train of an automatic transmission may include: a first shaft receiving engine torque; a second parallel shaft; a first planetary gear set on the first shaft including a first element operated as a fixed element or an output element, a second element directly connected to the first shaft, and a third element; a compound planetary gear set formed by combining a second planetary gear set and a third planetary gear set including a fourth element engaging the first shaft and the third element through gears, a fifth element connected to an output gear, a sixth element engaging the first element through a gear, and a seventh element engaging the third element through a gear; three transfer gears forming the gears; and frictional elements selectively interconnecting the elements or selectively connecting the element to a transmission housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority of Korean Patent Application Number 10-2012-0143089 filed Dec. 10, 2012, the entire contents of which application is incorporated herein for all purposes by this reference. 
       BACKGROUND OF INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a planetary gear train of an automatic transmission for a vehicle that can improve mountability by reducing a length thereof and reduce fuel consumption by improving power delivery performance. 
         [0004]    2. Description of Related Art 
         [0005]    Typically, a planetary gear train is realized by combining a plurality of planetary gear sets and friction members. It is well known that when a planetary gear train realizes a greater number of shift speeds, speed ratios of the planetary gear train can be more optimally designed, and therefore a vehicle can have economical fuel mileage and better performance. For that reason, the planetary gear train that is able to realize more shift speeds is under continuous investigation. 
         [0006]    Though achieving the same number of speeds, the planetary gear train has a different operating mechanism according to a connection between rotation elements (i.e., sun gear, planet carrier, and ring gear). In addition, the planetary gear train has different features such a durability, power delivery efficiency, and size depend on the layout thereof. Therefore, designs for a combining structure of a gear train are also under continuous investigation. 
         [0007]    In addition, the planetary gear train realizes a plurality of shift-speeds. However, another friction member must be operated after one friction member is released so as to shift to a neighboring shift-speed from a view of shift control. In addition, a step ratio between the neighboring shift-speeds should be controlled to be suitable according to the planetary gear train. 
         [0008]    The information disclosed in this Background 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 
       [0009]    Various aspects of the present invention provide for a planetary gear train of an automatic transmission for a vehicle having advantages of improving mountability by shortening a length thereof and reducing fuel consumption by improving power delivery performance as a consequence of achieving eight forward speeds and one reverse speed having excellent operating condition of frictional elements by combining three planetary gear sets separately disposed on a first shaft and a second shaft, three externally-meshing gears, and five frictional elements. 
         [0010]    A planetary gear train of an automatic transmission for a vehicle according to various aspects of the present invention may include: a first shaft receiving torque of an engine; a second shaft disposed in parallel with the first shaft; a first planetary gear set disposed on the first shaft, and including a first rotation element selectively operated as a fixed element or an output element, a second rotation element directly connected to the first shaft so as to be operated as an input element, and a third rotation element operated as a selective output element; a compound planetary gear set formed by combining a second planetary gear set and a third planetary gear set, and including a fourth rotation element selectively connected to the first shaft and the third rotation element through externally-meshed gears, a fifth rotation element connected to an output gear so as to be always operated as an output element, a sixth rotation element selectively connected to the first rotation element through an externally-meshed gear, and a seventh rotation element selectively connected to the third rotation element through an externally-meshed gear; three transfer gears forming the externally-meshed gears; and frictional elements selectively interconnecting the rotation elements or selectively connecting the rotation element to a transmission housing. 
         [0011]    The first planetary gear set may be a single pinion planetary gear set including a first sun gear, a first planet carrier, and a first ring gear as rotation elements thereof, the second planetary gear set may be a double pinion planetary gear set including a second sun gear, a second planet carrier, and a second ring gear as rotation elements thereof, and the third planetary gear set may be a single pinion planetary gear set including a third sun gear, a third planet carrier, and a third ring gear as rotation elements thereof. 
         [0012]    The first rotation element may have the first sun gear, the second rotation element may have the first planet carrier, the third rotation element may have the first ring gear, the fourth rotation element may have the second sun gear, the fifth rotation element may have the third ring gear, the sixth rotation element may have the second ring gear and the third planet carrier, and the seventh rotation element may have the second planet carrier and the third sun gear. 
         [0013]    The three transfer gears may include: a first transfer gear disposed between the first shaft and the fourth rotation element; a second transfer gear disposed between the third rotation element and the fourth rotation element; and a third transfer gear disposed between the first rotation element and the sixth rotation element. 
         [0014]    The frictional elements may include: a first clutch disposed between the fourth rotation element and the first transfer gear; a second clutch disposed between the second transfer gear and the fourth rotation element; a third clutch disposed between the second transfer gear and the seventh rotation element; a fourth clutch disposed between the first rotation element and the third transfer gear; and a first brake disposed between the first rotation element and the transmission housing. 
         [0015]    The first brake and the first and fourth clutches may be operated at a first forward speed, the first brake and the second and fourth clutches may be operated at a second forward speed, the first, second, and fourth clutches may be operated at a third forward speed, the second, third, and fourth clutches may be operated at a fourth forward speed, the first, third, and fourth clutches may be operated at a fifth forward speed, the first, second, and third clutches may be operated at a sixth forward speed, the first brake and the first and third clutches may be operated at a seventh forward speed, the first brake and the second and third clutches may be operated at an eighth forward speed, and the first brake and the third and fourth clutches may be operated at a reverse speed. 
         [0016]    A planetary gear train of an automatic transmission for a vehicle according to various aspects of the present invention may include: a first shaft receiving torque of an engine; a second shaft disposed in parallel with the first shaft; a first planetary gear set disposed on the first shaft, and including a first rotation element selectively operated as a fixed element, a second rotation element directly connected to the first shaft so as to be operated as an input element, and a third rotation element; a compound planetary gear set formed by combining a second planetary gear set and a third planetary gear set, and including a fourth rotation element selectively connected to the first shaft and the third rotation element, a fifth rotation element connected to an output gear so as to be always operated as an output element, a sixth rotation element selectively connected to the first rotation element, and a seventh rotation element selectively connected to the third rotation element; a first transfer gear disposed between the first shaft and the fourth rotation element; a second transfer gear disposed between the third rotation element and the fourth rotation element; a third transfer gear disposed between the first rotation element and the sixth rotation element; a first clutch disposed between the fourth rotation element and the first transfer gear; a second clutch disposed between the second transfer gear and the fourth rotation element; a third clutch disposed between the second transfer gear and the seventh rotation element; a fourth clutch disposed between the first rotation element and the third transfer gear; and a first brake disposed between the first rotation element and the transmission housing. 
         [0017]    The first planetary gear set may be a single pinion planetary gear set and may include a first sun gear being the first rotation element, a first planet carrier being the second rotation element, and a first ring gear being the third rotation element. The compound planetary gear set may be formed by combining the second planetary gear set being a double pinion planetary gear set and the third planetary gear set being a single pinion planetary gear set, and may include a second sun gear being the fourth rotation element, a third ring gear being the fifth rotation element, a second ring gear and a third planet carrier being the sixth rotation element, and a second planet carrier and a third sun gear being the seventh rotation element. 
         [0018]    The first brake and the first and fourth clutches may be operated at a first forward speed, the first brake and the second and fourth clutches may be operated at a second forward speed, the first, second, and fourth clutches may be operated at a third forward speed, the second, third, and fourth clutches may be operated at a fourth forward speed, the first, third, and fourth clutches may be operated at a fifth forward speed, the first, second, and third clutches may be operated at a sixth forward speed, the first brake and the first and third clutches may be operated at a seventh forward speed, the first brake and the second and third clutches may be operated at an eighth forward speed, and the first brake and the third and fourth clutches may be operated at a reverse speed. 
         [0019]    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 
         [0020]      FIG. 1  is a schematic diagram of an exemplary planetary gear train according the present invention. 
           [0021]      FIG. 2  is an operational chart of friction members at each shift-speed applied to an exemplary planetary gear train according to the present invention. 
           [0022]      FIG. 3A  is a lever diagram of an exemplary planetary gear train at the first forward speed according to the present invention. 
           [0023]      FIG. 3B  is a lever diagram of an exemplary planetary gear train at the second forward speed according to the present invention. 
           [0024]      FIG. 3C  is a lever diagram of an exemplary planetary gear train at the third forward speed according to the present invention. 
           [0025]      FIG. 3D  is a lever diagram of an exemplary planetary gear train at the fourth forward speed according to the present invention. 
           [0026]      FIG. 3E  is a lever diagram of an exemplary planetary gear train at the fifth forward speed according to the present invention. 
           [0027]      FIG. 3F  is a lever diagram of an exemplary planetary gear train at the sixth forward speed according to the present invention. 
           [0028]      FIG. 3G  is a lever diagram of an exemplary planetary gear train at the seventh forward speed according to the present invention. 
           [0029]      FIG. 3H  is a lever diagram of an exemplary planetary gear train at the eighth forward speed according to the present invention. 
           [0030]      FIG. 3I  is a lever diagram of an exemplary planetary gear train at a reverse speed according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    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 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. 
         [0032]    Description of components that are not necessary for explaining the illustrated embodiments will be omitted, and the same constituent elements are denoted by the same reference numerals in this specification. 
         [0033]    In the detailed description, ordinal numbers are used for distinguishing constituent elements having the same terms, and have no specific meanings. 
         [0034]      FIG. 1  is a schematic diagram of a planetary gear train according to various embodiments of the present invention. 
         [0035]    Referring to  FIG. 1 , a planetary gear train according to various embodiments of the present invention includes first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3 , five frictional elements B 1 , C 1 , C 2 , C 3 , and C 4 , and three transfer gears TF 1 , TF 2 , and TF 3 . 
         [0036]    The first planetary gear set PG 1  is disposed on a first shaft IS 1 , and the second and third planetary gear sets PG 2  and PG 3  are disposed on a second shaft IS 2  disposed apart from and in parallel with the first shaft IS  1 . 
         [0037]    The first shaft IS 1  is an input member, supports the first planetary gear set PG 1 , and transmits torque from an engine to the first planetary gear set PG 1 . 
         [0038]    The second shaft IS 2  supports the second and third planetary gear sets PG 2  and PG 3  and transmits torque selectively transmitted from the first shaft IS  1  and the first planetary gear set PG 1  to the second planetary gear set PG 2 . 
         [0039]    Therefore, the torque input from the first shaft IS 1  is changed into eight forward speeds and one reverse speed by cooperation of the first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3 , and is then output through an output gear OG. 
         [0040]    The first planetary gear set PG 1  is a single pinion planetary gear set, and has a first sun gear S 1 , a first ring gear R 1 , and a first planet carrier PC 1  rotatably supporting a first pinion P 1  engaged with the first sun gear S 1  and the first ring gear R 1  as rotation elements thereof. 
         [0041]    The second planetary gear set PG 2  is a double pinion planetary gear set, and has a second sun gear S 2 , a second ring gear R 2 , and a second planet carrier PC 2  supporting a second pinion P 2  engaged with the second sun gear S 2  and the second ring gear R 2  are rotation elements thereof. 
         [0042]    The third planetary gear set PG 3  is a single pinion planetary gear set, and has a third sun gear S 3 , a third ring gear R 3 , and a third planet carrier PC 3  rotatably supporting a third pinion P 3  engaged with the third sun gear S 3  and the third ring gear R 3  as rotation elements thereof. 
         [0043]    The first planetary gear set PG 1  is operated as a simple planetary gear train, and the second and third planetary gear sets PG 2  and PG 3  are operated as a compound planetary gear set CPG. 
         [0044]    Therefore, the first planetary gear set PG 1  includes three rotation elements N 1 , N 2 , and N 3 . 
         [0045]    The first rotation element N 1  includes the first sun gear S 1 , is selectively connected to a transmission housing H so as to be operated as a selective fixed element, and is operated as a selective output element. 
         [0046]    The second rotation element N 2  includes the first planet carrier PC 1 , and is directly connected to the first shaft IS  1  so as to be operated as an input element. 
         [0047]    The third rotation element N 3  includes the first ring gear R 1  and is operated as a selective output element. 
         [0048]    In addition, the second and third planetary gear sets PG 2  and PG 3  includes four rotation elements N 4 , N 5 , N 6 , and N 7  by directly connecting the second ring gear R 2  to the third planet carrier PC 3  and directly connecting the second planet carrier PC 2  to the third sun gear S 3 . 
         [0049]    The fourth rotation element N 4  includes the second sun gear S 2  and is selectively connected to the first shaft IS  1  and the third rotation element N 3  through externally-meshed gears. 
         [0050]    The fifth rotation element N 5  includes the third ring gear R 3  and is directly connected to the output gear OG so as to be always operated as a final output element. 
         [0051]    The sixth rotation element N 6  includes the second ring gear R 2  and the third planet carrier PC 3 , and is selectively connected to the first rotation element N 1  through an externally-meshed gear. 
         [0052]    The seventh rotation element N 7  includes the second planet carrier PC 2  and the third sun gear S 3 , and is selectively connected to the third rotation element N 3  through an externally-meshed gear. 
         [0053]    In addition, the rotation elements are combined by the first, second, and third transfer gears TF 1 , TF 2 , and TF 3  and frictional elements consisting of the first brake B 1  and the first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4 . 
         [0054]    The first, second, and third transfer gears TF 1 , TF 2 , and TF 3  respectively have first, second, and third transfer drive gears TF 1   a , TF 2   a , and TF 3   a  and first, second, and third transfer driven gears TF 1   b , TF 2   b , and TF 3   b  externally meshed with each other. 
         [0055]    The first transfer gear TF 1  includes the first transfer drive gear TF 1   a  connected to the first shaft IS  1  and the first transfer driven gear TF 1   b  connected to the fourth rotation element N 4 , and connects the first shaft IS  1  with the fourth rotation element N 4 . 
         [0056]    The second transfer gear TF 2  includes the second transfer drive gear TF 2   a  connected to the third rotation element N 3  and the second transfer driven gear TF 2   b  selectively connected to the fourth rotation element N 4  and the seventh rotation element N 7 , and selectively connects the third rotation element N 3  to the fourth rotation element N 4  and the seventh rotation element N 7 . 
         [0057]    The third transfer gear TF 3  includes the third transfer drive gear TF 3   a  selectively connected to the first rotation element N 1  and the third transfer driven gear TF 3   b  connected to the sixth rotation element N 6 , and connects the first rotation element N 1  to the sixth rotation element N 6 . 
         [0058]    Therefore, the rotation elements connected to each other by the first, second, and third transfer gears TF 1 , TF 2 , and TF 3  are rotated in opposite direction to each other. Gear ratios of the first, second, and third transfer gears TF 1 , TF 2 , and TF 3  are set according to speed ratios demanded at shift-speeds. 
         [0059]    Arrangements of the frictional elements B 1 , C 1 , C 2 , C 3 , and C 4  will be described in detail. 
         [0060]    The first brake B 1  is disposed between the first rotation element N 1  and the transmission housing H. 
         [0061]    The first clutch C 1  is disposed between the first transfer gear TF 1  and the fourth rotation element N 4 . 
         [0062]    The second clutch C 2  is disposed between the second transfer gear TF 2  and the fourth rotation element N 4 . 
         [0063]    The third clutch C 3  is disposed between the second transfer gear TF 2  and the seventh rotation element N 7 . 
         [0064]    The fourth clutch C 4  is disposed between the first rotation element N 1  and the third transfer gear TF 3 . 
         [0065]    The frictional elements consisting of the first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4  and the first brake B 1  are conventional multi-plate friction elements of wet type that are operated by hydraulic pressure. 
         [0066]    It is illustrated in the drawing but is not limited that the first shaft IS  1  and the third rotation element N 3  are connected to the fourth rotation element N 4  through the second shaft IS 2 . That is, the first shaft IS  1  and the third rotation element N 3  may be connected to the fourth rotation element N 4  through an additional rotating member disposed at an external circumferential portion of the second shaft IS 2  without rotational interference therebetween. 
         [0067]      FIG. 2  is an operational chart of friction members at each shift-speed applied to a planetary gear train according to various embodiments of the present invention. 
         [0068]    As shown in  FIG. 2 , three frictional elements are operated at each shift-speed in the planetary gear train according to various embodiments of the present invention. 
         [0069]    The first brake B 1  and the first and fourth clutches C 1  and C 4  are operated at a first forward speed 1ST. 
         [0070]    The first brake B 1  and the second and fourth clutches C 2  and C 4  are operated at a second forward speed 2ND. 
         [0071]    The first, second, and fourth clutches C 1 , C 2 , and C 4  are operated at a third forward speed 3RD. 
         [0072]    The second, third, and fourth clutches C 2 , C 3 , and C 4  are operated at a fourth forward speed 4TH. 
         [0073]    The first, third, and fourth clutches C 1 , C 3 , and C 4  are operated at a fifth forward speed 5TH. 
         [0074]    The first, second, and third clutches C 1 , C 2 , and C 3  are operated at a sixth forward speed 6TH. 
         [0075]    The first brake B 1  and the first and third clutches C 1  and C 3  are operated at a seventh forward speed 7TH. 
         [0076]    The first brake B 1  and the second and third clutches C 2  and C 3  are operated at an eighth forward speed 8TH. 
         [0077]    The first brake B 1  and the third and fourth clutches C 3  and C 4  are operated at a reverse speed REV. 
         [0078]      FIG. 3A  to  FIG. 3I  are lever diagrams of the planetary gear train at each shift-speed according to various embodiments of the present invention, and illustrate shift processes of the planetary gear train according to various embodiments of the present invention by lever analysis method. 
         [0079]    Referring to  FIG. 3A  to  FIG. 3I , three vertical lines of the first planetary gear set PG 1  are set as the first rotation element N 1 , the second rotation element N 2 , and the third rotation element N 3 , and four vertical lines of the compound planetary gear set CPG are set as the fourth rotation element N 4 , the fifth rotation element N 5 , the sixth rotation element N 6 , and the seventh rotation element N 7  from the left to the right. 
         [0080]    In addition, a middle horizontal line represents a rotation speed of “0”, an upper horizontal line represents a rotation speed of “1.0”, and a lower horizontal line represents a rotation speed of “−1.0”. 
         [0081]    “−” means that rotational elements is rotated in an opposite direction of a rotational direction of the engine. It is because the rotation elements are externally meshed with each other through the first, second, and third transfer gears TF 1 , TF 2 , and TF 3  without an idling gear. 
         [0082]    In addition, the rotation speed of “1.0” represents the same rotational speed as the first shaft IS  1  which is an input shaft. Distances between the vertical lines of the first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  are set according to each gear ratio (teeth number of a sun gear/teeth number of a ring gear). 
         [0083]    Hereinafter, referring to  FIG. 2  and  FIG. 3A  to  FIG. 3I , the shift processes of the planetary gear train according to various embodiments of the present invention will be described in detail. 
         [0084]    First Forward Speed 
         [0085]    Referring to  FIG. 2 , the first brake B 1  and the first and fourth clutches C 1  and C 4  are operated at the first forward speed 1ST. 
         [0086]    As shown in  FIG. 3A , the rotation speed of the first shaft IS  1  is input to the second rotation element N 2  and is changed according to the gear ratio of the first transfer gear TF 1  and is then input to the fourth rotation element N 4  as an inverse rotation speed by operation of the first clutch C 1 . 
         [0087]    At this state, the first rotation element N 1  and the sixth rotation element N 6  are operated as fixed elements by operation of the first brake B 1  and the fourth clutch C 4 . Therefore, the rotation elements of the compound planetary gear set CPG form a first shift line SP  1  and D 1  is output through the fifth rotation element N 5  that is the output element. 
         [0088]    Second Forward Speed 
         [0089]    The first clutch C 1  that was operated at the first forward speed 1ST is released and the second clutch C 2  is operated at the second forward speed 2ND. 
         [0090]    As shown in  FIG. 3B , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2 , and the rotation speed of the third rotation element N 3  is changed according to the gear ratio of the second transfer gear TF 2  and is then input to the fourth rotation element N 7  as an inverse rotation speed by operation of the second clutch C 2 . 
         [0091]    At this state, the first rotation element N 1  and the sixth rotation element N 6  are operated as the fixed elements by operation of the first brake B 1  and the fourth clutch C 4 . Therefore, the rotation elements of the compound planetary gear set CPG form a second shift line SP 2  and D 2  is output through the fifth rotation element N 5  that is the output element. 
         [0092]    Third Forward Speed 
         [0093]    The first brake B 1  that was operated at the second forward speed 2ND is released and the first clutch C 1  is operated at the third forward speed 3RD. 
         [0094]    As shown in  FIG. 3C , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2  and is changed according to the gear ratio of the first transfer gear TF 1  and is then input to the fourth rotation element N 4  as the inverse rotation speed by operation of the first clutch C 1 . 
         [0095]    In addition, the first rotation element N 1  is connected to the sixth rotation element N 6  through the third transfer gear TF 3  by operation of the fourth clutch C 4 , and the third rotation element N 3  is connected to the fourth rotation element N 4  through the second transfer gear TF 2  by operation of the second clutch C 2 . 
         [0096]    Therefore, the rotation elements of the compound planetary gear set CPG form a third shift line SP 3  and D 3  is output through the fifth rotation element N 5  that is the output element. 
         [0097]    Fourth Forward Speed 
         [0098]    The first clutch C 1  that was operated at the third forward speed 3RD is released and the third clutch C 3  is operated at the fourth forward speed 4TH. 
         [0099]    As shown in  FIG. 3D , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2 , and the first rotation element N 1  is connected to the sixth rotation element N 6  through the third transfer gear TF 3  by operation of the fourth clutch C 4 . In addition, the third rotation element N 3  is connected to the fourth rotation element N 4  through the second transfer gear TF 2  by operation of the second clutch C 2  and is connected to the seventh rotation element N 7  through the second transfer gear TF 2  by operation of the third clutch C 3 . 
         [0100]    Therefore, the rotation elements of the compound planetary gear set CPG form a fourth shift line SP 4  and D 4  is output through the fifth rotation element N 5  that is the output element. 
         [0101]    Fifth Forward Speed 
         [0102]    The second clutch C 2  that was operated at the fourth forward speed 4TH is released and the first clutch C 1  is operated at the fifth forward speed 5TH. 
         [0103]    As shown in  FIG. 3E , the rotation speed of the first shaft IS  1  is input to the second rotation element N 2  and is changed according to the gear ratio of the first transfer gear TF 1  and is then input to the fourth rotation element N 4  as the inverse rotation speed by operation of the first clutch C 1 . 
         [0104]    In addition, the first rotation element N 1  is connected to the sixth rotation element N 6  through the third transfer gear TF 3  by operation of the fourth clutch C 4 , and the third rotation element N 3  is connected to the seventh rotation element N 7  through the second transfer gear TF 2  by operation of the third clutch C 3 . 
         [0105]    Therefore, the rotation elements of the compound planetary gear set CPG form a fifth shift line SP 5  and D 5  is output through the fifth rotation element N 5  that is the output element. 
         [0106]    Sixth Forward Speed 
         [0107]    The fourth clutch C 4  that was operated at the fifth forward speed 5TH is released and the second clutch C 2  is operated at the sixth forward speed 6TH. 
         [0108]    As shown in  FIG. 3F , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2  and is changed according to the gear ratio of the first transfer gear TF 1  and is then input to the fourth rotation element N 4  as the inverse rotation speed by operation of the first clutch C 1 . 
         [0109]    At this state, the third rotation element N 3  is connected to the fourth rotation element N 4  through the second transfer gear TF 2  by operation of the second clutch C 2  and is connected to the seventh rotation element N 7  through the second transfer gear TF 2  by operation of the third clutch C 3 . 
         [0110]    Therefore, the rotation elements of the compound planetary gear set CPG form a sixth shift line SP 6  and D 6  is output through the fifth rotation element N 5  that is the output element. 
         [0111]    Seventh Forward Speed 
         [0112]    The second clutch C 2  that was operated at the sixth forward speed 6TH is released and the first brake B 1  is operated at the seventh forward speed 7TH. 
         [0113]    As shown in  FIG. 3G , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2  and is changed according to the gear ratio of the first transfer gear TF 1  and is then input to the fourth rotation element N 4  as the inverse rotation speed by operation of the first clutch C 1 . 
         [0114]    In addition, the first rotation element N 1  is operated as the fixed element by operation of the first brake B 1 , and the third rotation element N 3  is connected to the seventh rotation element N 7  through the second transfer gear TF 2  by operation of the third clutch C 3 . 
         [0115]    Therefore, the rotation elements of the compound planetary gear set CPG form a seventh shift line SP 7  and D 7  is output through the fifth rotation element N 5  that is the output element. 
         [0116]    Eighth Forward Speed 
         [0117]    The first clutch C 1  that was operated at the seventh forward speed 7TH is released and the second clutch C 2  is operated at the eighth forward speed 8TH. 
         [0118]    As shown in  FIG. 3H , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2  and the first rotation element N 1  is operated as the fixed element by operation of the first brake B 1 . 
         [0119]    In addition, the third rotation element N 3  is connected to the fourth rotation element N 4  through the second transfer gear TF 2  by operation of the second clutch C 2  and is connected to the seventh rotation element N 7  through the second transfer gear TF 2  by operation of the third clutch C 3 . 
         [0120]    Therefore, the rotation elements of the compound planetary gear set CPG form an eighth shift line SP 8  and D 8  is output through the fifth rotation element N 5  that is the output element. 
         [0121]    Reverse Speed 
         [0122]    As shown in  FIG. 2 , the first brake B 1  and the third and fourth clutches C 3  and C 4  are operated at the reverse speed REV. 
         [0123]    As shown in  FIG. 3I , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2 , the first rotation element N 1  and the sixth rotation element N 6  are operated as the fixed elements by operation of the first brake B 1  and the fourth clutch C 4 . 
         [0124]    In addition, the third rotation element N 3  is connected to the seventh rotation element N 7  through the second transfer gear TF 2  by operation of the third clutch C 3 . 
         [0125]    Therefore, the rotation elements of the compound planetary gear set CPG form a reverse shift line RS and REV is output through the fifth rotation element N 5  that is the output element. 
         [0126]    Since three planetary gear sets are separately disposed on the first shaft and the second shaft disposed apart from and in parallel with each other in the planetary gear train according to various embodiments of the present invention, a length thereof may be reduced and mountability may be improved. 
         [0127]    In addition, optimum gear ratios may be set due to ease of changing gear ratios by using three external-meshing gears as well as the planetary gear sets. Since gear ratios can be changed according to target performance, starting performance may be improved. Therefore, a start-up clutch instead of a torque converter may be used. 
         [0128]    Since three frictional elements are operated at each shift-speed, non-operated frictional element may be minimized and drag torque may be reduced. In addition, fuel consumption may be reduced by increasing power delivery efficiency. 
         [0129]    In addition, since torque load of each frictional element can be reduced, compact design is possible. 
         [0130]    For convenience in explanation and accurate definition in the appended claims, the terms upper or lower, and etc. are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures. 
         [0131]    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 in order 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.