Abstract:
A planetary gear train of an automatic transmission for a vehicle may include: an input shaft receiving torque of an engine; an output shaft outputting changed torque; a first planetary gear set including a first rotation element, a second rotation element, and a third rotation element; a second planetary gear set including a fourth rotation element, a fifth rotation element, and a sixth rotation element; a third planetary gear set including a seventh rotation element, an eighth rotation element, and a ninth rotation element; and a fourth planetary gear set including a tenth rotation element, an eleventh rotation element, and a twelfth rotation element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority to and the benefit of Korean Patent Application No. 10-2015-0078003 filed on Jun. 2, 2015, the entire contents of which is incorporated herein for all purposes by this reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    Field of the 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 of a vehicle improves power delivery performance and reduces fuel consumption by achieving nine forward speed stages and widening gear ratio span using a minimum number of constituent elements and securing linearity of step ratios. 
         [0004]    Description of Related Art 
         [0005]    In recent years, a rise in oil price causes dashing into unlimited competition for enhancing fuel efficiency. 
         [0006]    As a result, researches into reduction of a weight and the enhancement of the fuel efficiency through down sizing are conducted in the case of an engine and researches for simultaneously securing operability and fuel efficiency competitiveness through multiple speed stages are conducted in the case of an automatic transmission. 
         [0007]    However, in the automatic transmission, as the number of speed stages increase, the number of internal components increase, and as a result, mountability, cost, weight, transmission efficiency, and the like may still deteriorate. 
         [0008]    Accordingly, development of a planetary gear train which may bring about maximum efficiency with a small number of components may be important in order to increase a fuel efficiency enhancement effect through the multistages. 
         [0009]    In this aspect, in recent years, 8-speed automatic transmissions tend to be implemented and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted. 
         [0010]    However, since a conventional 8-speed automatic transmission has gear ratio span of 6.5-7.5, improvement of fuel economy may not be great. 
         [0011]    In addition, if 8-speed automatic transmission has gear ratio span larger than 9.0, it is hard to secure linearity of step ratios. Therefore, driving efficiency of an engine and drivability of a vehicle may be deteriorated, and thus, development of high efficiency automatic transmissions which achieve at least nine forward speed stages is necessary. 
         [0012]    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 
       [0013]    Various aspects of the present invention are directed to providing a planetary gear train of an automatic transmission for a vehicle having advantages of improving power delivery performance and fuel economy by achieving nine forward speed stages and one reverse speed stage and widening gear ratio span and of securing linearity of step ratios. 
         [0014]    A planetary gear train of an automatic transmission for a vehicle according to an exemplary embodiment of the present invention may include: an input shaft receiving torque of an engine; an output shaft outputting changed torque; a first planetary gear set including first, second, and third rotation elements; a second planetary gear set including fourth, fifth, and sixth rotation elements; a third planetary gear set including seventh, eighth, and ninth rotation elements; and a fourth planetary gear set including tenth, eleventh, and twelfth rotation elements, wherein the input shaft is directly connected to the fourth rotation element and is selectively connected to the eleventh rotation element, the output shaft is directly connected to the eighth rotation element and the twelfth rotation element, the third rotation element is directly connected to the tenth rotation element, the ninth rotation element is directly connected to the eleventh rotation element, the second rotation element is selectively connected to the sixth rotation element, the first rotation element is directly connected to a transmission housing, the third rotation element is selectively connected to the fifth rotation element, the second rotation element is selectively connected to the sixth rotation element, the fifth rotation element is selectively connected to the transmission housing, the seventh rotation element is selectively connected to the transmission housing, and at least two rotation elements among the fourth, fifth, and sixth rotation elements of the second planetary gear set are selectively connected to each other. 
         [0015]    The first, second, and third rotation elements of the first planetary gear set may be a first sun gear, a first planet carrier, and a first ring gear, the fourth, fifth, and sixth rotation elements of the second planetary gear set may be a second sun gear, a second planet carrier, and a second ring gear, the seventh, eighth, and ninth rotation elements of the third planetary gear set may be a third sun gear, a third planet carrier, and a third ring gear, and the tenth, eleventh, and twelfth rotation elements of the fourth planetary gear set may be a fourth sun gear, a fourth planet carrier, and a fourth ring gear. 
         [0016]    Each of the first, second, third, and fourth planetary gear sets may be a single pinion planetary gear set. 
         [0017]    The second planetary gear set may be selectively locked-up by selective connection of the fourth rotation element and the fifth rotation element. 
         [0018]    The planetary gear train may further include: a first clutch selectively connecting the eleventh rotation element to the input shaft; a second clutch selectively connecting the fourth rotation element to the fifth rotation element; a third clutch selectively connecting the third rotation element to the fifth rotation element; a fourth clutch selectively connecting the second rotation element to the sixth rotation element; a first brake selectively connecting the fifth rotation element to the transmission housing; and a second brake selectively connecting the seventh rotation element to the transmission housing. 
         [0019]    A planetary gear train of an automatic transmission for a vehicle according to another exemplary embodiment of the present invention may include: an input shaft receiving torque of an engine; an output shaft outputting changed torque; a first planetary gear set including first, second, and third rotation elements; a second planetary gear set including fourth, fifth, and sixth rotation elements; a third planetary gear set including seventh, eighth, and ninth rotation elements; a fourth planetary gear set including tenth, eleventh, and twelfth rotation elements; a first rotation shaft connected to the first rotation element and directly connected to a transmission housing; a second rotation shaft connected to the second rotation element; a third rotation shaft connecting the third rotation element to the tenth rotation element; a fourth rotation shaft connected to the fourth rotation element and directly connected to the input shaft; a fifth rotation shaft connected to the fifth rotation element, selectively connected respectively to the third rotation shaft and the fourth rotation shaft, and selectively connected to the transmission housing; a sixth rotation shaft connected to the sixth rotation element and selectively connected to the second rotation shaft; a seventh rotation shaft connected to the seventh rotation element and selectively connected to the transmission housing; an eighth rotation shaft connecting the eighth rotation element to the twelfth rotation element and directly connected to the output shaft so as to be operated as an output element; and a ninth rotation shaft connecting the ninth rotation element to the eleventh rotation element and selectively connected to the input shaft. 
         [0020]    The first planetary gear set may be a single pinion planetary gear set and may include 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 a single pinion planetary gear set and may include 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 third planetary gear set may be a single pinion planetary gear set and may include a third sun gear as the seventh rotation element, a third planet carrier as the eighth rotation element, and a third ring gear as the ninth rotation element. The fourth planetary gear set may be a single pinion planetary gear set and may include a fourth sun gear as the tenth rotation element, a fourth planet carrier as the eleventh rotation element, and a fourth ring gear as the twelfth rotation element. 
         [0021]    The planetary gear train may further include: a first clutch selectively connecting the input shaft to the ninth rotation shaft; a second clutch selectively connecting the fourth rotation shaft to the fifth rotation shaft; a third clutch selectively connecting the third rotation shaft to the fifth rotation shaft; a fourth clutch selectively connecting the second rotation shaft to the sixth rotation shaft; a first brake selectively connecting the fifth rotation shaft to the transmission housing; and a second brake selectively connecting the seventh rotation shaft to the transmission housing. 
         [0022]    In one aspect, the first clutch may be disposed at a rear of the fourth planetary gear set, the second clutch and the first brake may be disposed in front of the first planetary gear set, the fourth clutch may be disposed between the first planetary gear set and the second planetary gear set, and the third clutch and the second brake may be disposed between the second planetary gear set and the third planetary gear set. 
         [0023]    In another aspect, the first clutch may be disposed at a rear of the fourth planetary gear set, the first brake may be disposed in front of the first planetary gear set, the fourth clutch may be disposed between the first planetary gear set and the second planetary gear set, and the second and third clutches and the second brake may be disposed between the second planetary gear set and the third planetary gear set. 
         [0024]    The third and fourth clutches and the second brake may be operated at a first forward speed stage, the second and third clutches and the second brake may be operated at a second forward speed stage, the second and fourth clutches and the second brake may be operated at a third forward speed stage, the first and second clutches and the second brake or the first and fourth clutches and the second brake may be operated at a fourth forward speed stage, the first, second, and fourth clutches may be operated at a fifth forward speed stage, the first, second, and third clutches may be operated at a sixth forward speed stage, the first, third, and fourth clutches may be operated at a seventh forward speed stage, the first and third clutches and the first brake may be operated at an eighth forward speed stage, the first and fourth clutches and the first brake may be operated at a ninth forward speed stage, and the fourth clutch and the first and second brakes may be operated at a reverse speed stage. 
         [0025]    An exemplary embodiment of the present invention may achieve nine forward speed stages and one reverse speed stage by combining four planetary gear sets with six control elements. 
         [0026]    In addition, since gear ratio span greater than 9.0 is secured, driving efficiency of the engine may be maximized. 
         [0027]    In addition, since linearity of step ratios is secured, drivability such as acceleration before and after shift, rhythmical engine speed, and so on may be improved. 
         [0028]    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 
         [0029]      FIG. 1  is a schematic diagram of a planetary gear train according to the first exemplary embodiment of the present invention. 
           [0030]      FIG. 2  is a schematic diagram of a planetary gear train according to the second exemplary embodiment of the present invention. 
           [0031]      FIG. 3  is an operation chart of control elements at each speed stage in the planetary gear train according to the first and second exemplary embodiments of the present invention. 
       
    
    
       [0032]    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. 
         [0033]    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 
       [0034]    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. 
         [0035]    Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
         [0036]    However, parts which are not related with the description are omitted for clearly describing the exemplary embodiments of the present invention and like reference numerals refer to like or similar elements throughout the specification. 
         [0037]    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. 
         [0038]      FIG. 1  is a schematic diagram of a planetary gear train according to the first exemplary embodiment of the present invention. 
         [0039]    Referring to  FIG. 1 , a planetary gear train according to the first exemplary embodiment of the present invention includes first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4  disposed on the same axis, and input shaft IS, an output shaft OS, nine rotation shafts TM 1  to TM 9  connected to at least one of rotation elements of the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4 , six control elements C 1  to C 4  and B 1  to B 2 , and a transmission housing H. 
         [0040]    As a result, torque input from the input shaft IS is changed by cooperation of the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4 , and the changed torque is output through the output shaft OS. 
         [0041]    The simple planetary gear sets are disposed in a sequence of the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4  from an engine side. 
         [0042]    The input shaft IS is an input member and power from a crankshaft of an engine is torque-converted through a torque converter to be input into the input shaft IS. 
         [0043]    The output shaft OS is an output member, is disposed in parallel with the input shaft IS, and transmits driving torque to a driving wheel through a differential apparatus. 
         [0044]    The first planetary gear set PG 1  is a single pinion planetary gear set and includes a first sun gear S 1 , a first planet carrier PC 1  rotatably supporting a first pinion P 1  that is externally meshed with the first sun gear Si, and a first ring gear R 1  that is internally meshed with the first pinion P 1  respectively as first, second, and third rotation elements N 1 , N 2 , and N 3 . 
         [0045]    The second planetary gear set PG 2  is a single pinion planetary gear set and includes a second sun gear S 2 , a second planet carrier PC 2  rotatably supporting a second pinion P 2  that is externally meshed with the second sun gear S 2 , and a second ring gear R 2  that is internally meshed with the second pinion P 2  respectively as fourth, fifth, and sixth rotation elements N 4 , N 5 , and N 6 . 
         [0046]    The third planetary gear set PG 3  is a single pinion planetary gear set and includes a third sun gear S 3 , a third planet carrier PC 3  rotatably supporting a third pinion P 3  that is externally meshed with the third sun gear S 3 , and a third ring gear R 3  that is internally meshed with the third pinion P 3  respectively as seventh, eighth, and ninth rotation elements N 7 , N 8 , and N 9 . 
         [0047]    The fourth planetary gear set PG 4  is a single pinion planetary gear set and includes a fourth sun gear S 4 , a fourth planet carrier PC 4  rotatably supporting a fourth pinion P 4  that is externally meshed with the fourth sun gear S 4 , and a fourth ring gear R 4  that is internally meshed with the fourth pinion P 4  respectively as tenth, eleventh, and twelfth rotation elements N 10 , N 11 , and N 12 . 
         [0048]    The third rotation element N 3  is directly connected to the tenth rotation element N 10 , the eighth rotation element N 8  is directly connected to the twelfth rotation element N 12 , and the ninth rotation element N 9  is directly connected to the eleventh rotation element N 11  such that the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4  are operated with nine rotation shafts TM 1  to TM 9 . 
         [0049]    The nine rotation shafts TM 1  to TM 9  will be described in further detail. 
         [0050]    The first rotation shaft TM 1  is connected to the first sun gear S 1  and id directly connected to the transmission housing H. 
         [0051]    The second rotation shaft TM 2  is connected to the first planet carrier PC 1 . 
         [0052]    The third rotation shaft TM 3  connects the third ring gear R 3  to the fourth sun gear S 4 . 
         [0053]    The fourth rotation shaft TM 4  is connected to the second sun gear S 2  and is directly connected to the input shaft IS so as to be continuously operated as an input element. 
         [0054]    The fifth rotation shaft TM 5  is connected to the second planet carrier PC 2 , is selectively connected respectively to the third rotation shaft TM 3  and the fourth rotation shaft TM 4 , and is selectively connected to the transmission housing H. 
         [0055]    The sixth rotation shaft TM 6  is connected to the second ring gear R 2  and is selectively connected to the second rotation shaft TM 2 . 
         [0056]    The seventh rotation shaft TM 7  is connected to the third sun gear S 3  and is selectively connected to the transmission housing H. 
         [0057]    The eighth rotation shaft TM 8  connects the third planet carrier PC 3  to the fourth ring gear R 4  and is directly connected to the output shaft OS so as to be continuously operated as an output element. 
         [0058]    The ninth rotation shaft TM 9  connects the third ring gear R 3  to the fourth planet carrier PC 4  and is selectively connected to the input shaft IS. 
         [0059]    In addition, four clutches C 1 , C 2 , C 3 , and C 4  being control elements are disposed at connection portions between any two rotation shafts among the rotation shafts TM 1  to TM 9  or between the input shaft IS and any one rotation shaft among the rotation shafts TM 1  to TM 9 . 
         [0060]    addition, two brakes B 1  and B 2  being control elements are disposed at connection portions between any one rotation shaft among the rotation shaft TM 1  to TM 9  and the transmission housing H. 
         [0061]    The six control elements C 1  to C 4  and B 1  to B 2  will be described in further detail. 
         [0062]    The first clutch C 1  is disposed between the input shaft IS and the ninth rotation shaft TM 9  and selectively causes the input shaft IS and the ninth rotation shaft TM 9  to be integrally rotate with each other. 
         [0063]    The second clutch C 2  is disposed between the fourth rotation shaft TM 4  and the fifth rotation shaft TM 5  and selectively causes the fourth rotation shaft TM 4  and the fifth rotation shaft TM 5  to integrally rotate with each other. 
         [0064]    The third clutch C 3  is disposed between the third rotation shaft TM 3  and the fifth rotation shaft TM 5  and selectively causes the third rotation shaft TM 3  and the fifth rotation shaft TM 5  to integrally rotate with each other. 
         [0065]    The fourth clutch C 4  is disposed between the second rotation shaft TM 2  and the sixth rotation shaft TM 6  and selectively causes the second rotation shaft TM 2  and the sixth rotation shaft TM 6  to integrally rotate with each other. 
         [0066]    The first brake B 1  is disposed between the fifth rotation shaft TM 5  and the transmission housing H and causes the fifth rotation shaft TM 5  to be operated as a fixed element. 
         [0067]    The second brake B 2  is disposed between the seventh rotation shaft TM 7  and the transmission housing H and causes the seventh rotation shaft TM 7  to be operated as a selective fixed element. 
         [0068]    The control elements including the first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4  and the first and second brakes B 1  and B 2  may be multi-plates friction elements of wet type that are operated by hydraulic pressure. 
         [0069]      FIG. 2  is a schematic diagram of a planetary gear train according to the second exemplary embodiment of the present invention. 
         [0070]    Referring to  FIG. 2 , the second clutch C 2  is disposed in front of the first planetary gear set PG 1  in first exemplary embodiment of the present invention, but the second clutch C 2  is disposed between the second planetary gear train PG 2  and the third planetary gear train PG 3  in the second exemplary embodiment. 
         [0071]    The second clutch C 2  in the second exemplary embodiment, the same as the second clutch C 2  in the first exemplary embodiment, selectively connects the fourth rotation shaft TM 4  to the fifth rotation shaft TM 5  and causes the second planetary gear set PG 2  to become a lock-up state. 
         [0072]      FIG. 3  is an operation chart of control elements at each speed stage in the planetary gear train according to the first and second exemplary embodiments of the present invention. 
         [0073]    As shown in  FIG. 3 , three control elements are operated at each speed stage in the planetary gear train according to the exemplary embodiments of the present invention. 
         [0074]    The third and fourth clutches C 3  and C 4  and the second brake B 2  are operated at a first forward speed stage D 1 . In a state that the third rotation shaft TM 3  is connected to the fifth rotation shaft TM 5  by operation of the third clutch C 3  and the second rotation shaft TM 2  is connected to the sixth rotation shaft TM 6  by operation of the fourth clutch C 4 , torque of the input shaft IS is input to the fourth rotation shaft TM 4 . In addition, the first rotation shaft TM 1  is operated as the fixed element and the seventh rotation shaft TM 7  is operated as the fixed element by operation of the second brake B 2 . Therefore, the torque of the input shaft IS is shifted into the first forward speed stage, and the first forward speed stage is output through the eighth rotation shaft TM 8 . 
         [0075]    The second and third clutches C 2  and C 3  and the second brake B 2  are operated at a second forward speed stage D 2 . In a state that the fourth rotation shaft TM 4  is connected to the fifth rotation shaft TM 5  by operation of the second clutch C 2  and the third rotation shaft TM 3  is connected to the fifth rotation shaft TM 5  by operation of the third clutch C 3 , the torque of the input shaft IS is input to the fourth rotation shaft TM 4 . In addition, the first rotation shaft TM 1  is operated as the fixed element and the seventh rotation shaft TM 7  is operated as the fixed element by operation of the second brake B 2 . Therefore, the torque of the input shaft IS is shifted into the second forward speed stage, and the second forward speed stage is output through the eighth rotation shaft TM 8 . 
         [0076]    The second and fourth clutches C 2  and C 4  and the second brake B 2  are operated at a third forward speed stage D 3 . In a state that the fourth rotation shaft TM 4  is connected to the fifth rotation shaft TM 5  by operation of the second clutch C 2  and the second rotation shaft TM 2  is connected to the sixth rotation shaft TM 6  by operation of the fourth clutch C 4 , the torque of the input shaft IS is input to the fourth rotation shaft TM 4 . In addition, the first rotation shaft TM 1  is operated as the fixed element and the seventh rotation shaft TM 7  is operated as the fixed element by operation of the second brake B 2 . Therefore, the torque of the input shaft IS is shifted into the third forward speed stage, and the third forward speed stage is output through the eighth rotation shaft TM 8 . 
         [0077]    The first and second clutches C 1  and C 2  and the second brake B 2  are operated at a fourth forward speed stage D 4 . In a state that the input shaft IS is connected to the ninth rotation shaft TM 9  by operation of the first clutch C 1  and the fourth rotation shaft TM 4  is connected to the fifth rotation shaft TM 5  by operation of the second clutch C 2 , the torque of the input shaft IS is input to the fourth rotation shaft TM 4 . In addition, the first rotation shaft TM 1  is operated as the fixed element and the seventh rotation shaft TM 7  is operated as the fixed element by operation of the second brake B 2 . Therefore, the torque of the input shaft IS is shifted into the fourth forward speed stage, and the fourth forward speed stage is output through the eighth rotation shaft TM 8 . 
         [0078]    The first and fourth clutches C 1  and C 4  and the second brake B 2  are operated at a fourth forward speed stage D 4 . In a state that the input shaft IS is connected to the ninth rotation shaft TM 9  by operation of the first clutch C 1  and the second rotation shaft TM 2  is connected to the sixth rotation shaft TM 6  by operation of the fourth clutch C 4 , the torque of the input shaft IS is input to the fourth rotation shaft TM 4 . In addition, the first rotation shaft TM 1  is operated as the fixed element and the seventh rotation shaft TM 7  is operated as the fixed element by operation of the second brake B 2 . Therefore, the torque of the input shaft IS is shifted into the fourth forward speed stage, and the fourth forward speed stage is output through the eighth rotation shaft TM 8 . 
         [0079]    The first, second, and fourth clutches C 1 , C 2 , and C 4  are operated at a fifth forward speed stage D 5 . In a state that the input shaft IS is connected to the ninth rotation shaft TM 9  by operation of the first clutch C 1 , the fourth rotation shaft TM 4  is connected to the fifth rotation shaft TM 5  by operation of the second clutch C 2 , and the second rotation shaft TM 2  is connected to the sixth rotation shaft TM 6  by operation of the fourth clutch C 4 , the torque of the input shaft IS is input to the fourth rotation shaft TM 4 . In addition, the first rotation shaft TM 1  is operated as the fixed element. Therefore, the torque of the input shaft IS is shifted into the fifth forward speed stage, and the fifth forward speed stage is output through the eighth rotation shaft TM 8 . 
         [0080]    The first, second, and third clutches C 1 , C 2 , and C 3  are operated at a sixth forward speed stage D 6 . In a state that the input shaft IS is connected to the ninth rotation shaft TM 9  by operation of the first clutch C 1 , the fourth rotation shaft TM 4  is connected to the fifth rotation shaft TM 5  by operation of the second clutch C 2 , and the third rotation shaft TM 3  is connected to the fifth rotation shaft TM 5  by operation of the third clutch C 3 , the torque of the input shaft IS is input to the fourth rotation shaft TM 4 . In addition, the first rotation shaft TM 1  is operated as the fixed element. Therefore, the torque of the input shaft IS is shifted into the sixth forward speed stage, and the sixth forward speed stage is output through the eighth rotation shaft TM 8 . 
         [0081]    The first, third, and fourth clutches C 1 , C 3 , and C 4  are operated at a seventh forward speed stage D 7 . In a state that the input shaft IS is connected to the ninth rotation shaft TM 9  by operation of the first clutch C 1 , the third rotation shaft TM 3  is connected to the fifth rotation shaft TM 5  by operation of the third clutch C 3 , and the second rotation shaft TM 2  is connected to the sixth rotation shaft TM 6  by operation of the fourth clutch C 4 , the torque of the input shaft IS is input to the fourth rotation shaft TM 4 . In addition, the first rotation shaft TM 1  is operated as the fixed element. Therefore, the torque of the input shaft IS is shifted into the seventh forward speed stage, and the seventh forward speed stage is output though the eighth rotation shaft TM 8 . 
         [0082]    The first and third clutches C 1  and C 3  and the first brake B 1  are operated at an eighth forward speed stage D 8 . In a state that the input shaft IS is connected to the ninth rotation shaft TM 9  by operation of the first clutch C 1  and the third rotation shaft TM 3  is connected to the fifth rotation shaft TM 5  by operation of the third clutch C 3 , the torque of the input shaft IS is input to the fourth rotation shaft TM 4 . In addition, the first rotation shaft TM 1  is operated as the fixed element and the fifth rotation shaft TM 5  is operated as the fixed element by operation of the first brake B 1 . Therefore, the torque of the input shaft IS is shifted into the eighth forward speed stage, and the eighth forward speed stage is output through the eighth rotation shaft TM 8 . 
         [0083]    The first and fourth clutches C 1  and C 4  and the first brake B 1  are operated at a ninth forward speed stage D 9 . In a state that the input shaft IS is connected to the ninth rotation shaft TM 9  by operation of the first clutch C 1  and the second rotation shaft TM 2  is connected to the sixth rotation shaft TM 6  by operation of the fourth clutch C 4 , the torque of the input shaft IS is input to the fourth rotation shaft TM 4 . In addition, the first rotation shaft TM 1  is operated as the fixed element and the fifth rotation shaft TM 5  is operated as the fixed element by operation of the first brake B 1 . Therefore, the torque of the input shaft IS is shifted into the ninth forward speed stage, and the ninth forward speed stage is output through the eighth rotation shaft TM 8 . 
         [0084]    The fourth clutch C 4  and the first and second brakes B 1  and B 2  are operated at a reverse speed stage REV. In a state that the second rotation shaft TM 2  is connected to the sixth rotation shaft TM 6  by operation of the fourth clutch C 4 , the torque of the input shaft IS is input to the fourth rotation shaft TM 4 . In addition, the first rotation shaft TM 1  is operated as the fixed element, and the fifth rotation shaft TM 5  and the seventh rotation shaft TM 7  are operated as the fixed elements by operation of the first and second brakes B 1  and B 2 . Therefore, the torque of the input shaft IS is shifted into the reverse speed stage, and the reverse speed stage is output through the eighth rotation shaft TM 8 . 
         [0085]    The planetary gear train according to the exemplary embodiments of the present invention may achieve nine forward speed stages and one reverse speed stage by control of four planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4 , four clutches C 1 , C 2 , C 3 , and C 4 , and two brakes B 1  and B 2 . 
         [0086]    In addition, since linearity of step ratios is secured, drivability such as acceleration before and after shift, rhythmical engine speed, and so on may be improved. 
         [0087]    In addition, since gear ratio span greater than 9.0 is secured, driving efficiency of the engine may be maximized. 
         [0088]    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. 
         [0089]    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.