Patent Publication Number: US-9841082-B2

Title: Planetary gear train of automatic transmission for a vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Korean Patent Application No. 10-2015-0078001 filed Jun. 2, 2015, 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 an automatic transmission for a vehicle. More particularly, the present invention relates to a planetary gear train for an automatic transmission of a vehicle that improves power delivery performance and fuel efficiency as a consequence of achieving ten forward speed stages using a minimum number of constituent elements, enlarging a span of gear ratios, and almost linearly increasing or decreasing step ratios between transmission steps. 
     Description of Related Art 
     In recent years, increasing oil prices have forced auto manufacturers into extreme competition in order to enhance fuel efficiency. 
     As a result, research is being conducted on engines in terms of reducing weight and improving fuel efficiency by down-sizing, and research is also being conducted to ensure both drivability and competitiveness for maximizing fuel efficiency by implementing an automatic transmission with multiple speed stages. 
     However, in the automatic transmission, as a number of transmission speed stages increases, the number of internal components increases, and as a result, the automatic transmission may be difficult to mount, the manufacturing cost and weight may be increased, and power transmission efficiency may be deteriorated. 
     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 multiple speed stages. 
     In this aspect, in recent years, 8-speed automated transmissions have tended to be achieved and the research and development of a planetary gear train capable of implementing more transmission steps has also been actively conducted. 
     Since a span of gear ratios of the recent 8-speed automatic transmission is merely 6.5 to 7.5, the 8-speed automatic transmission has no great effect of improving fuel efficiency. 
     In addition, since step ratios between transmission step may not be increased or decreased linearly in a case in which a span of shift ratios of the 8-speed automatic transmission is greater than or equal to 9.0, driving efficiency of an engine and drivability of a vehicle may be deteriorated. Accordingly, there is a need for development of a high efficient automatic transmission with 9 or more forward speed stages. 
     The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art. 
     BRIEF SUMMARY 
     Various aspects of the present invention are directed to providing a planetary gear train of an automatic transmission for a vehicle that improves power delivery performance and fuel efficiency by achieving ten forward speed stages and two reverse speed stages using a minimum number of constituent elements, by enlarging a span of gear ratios, and by almost linearly increasing or decreasing step ratios between transmission steps. 
     Various aspects of the present invention are directed to providing 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 of the engine, 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, and seven friction elements disposed to selectively connect the rotation elements with each other and selectively connect the rotation elements with a transmission housing, in which the input shaft may be continuously connected to the first rotation element, the output shaft may be continuously connected to the eleventh rotation element, the second rotation element may be connected to the first rotation element, the third rotation element may be continuously connected to the fifth rotation element, the sixth rotation element may be continuously connected to the seventh rotation element, the sixth rotation element may be continuously connected to the tenth rotation element, the ninth rotation element may be continuously connected to the eleventh rotation element, and three friction elements among a seven friction elements are operated at each speed stage. 
     The second rotation element may be selectively connected to the transmission housing, the third rotation element may be selectively connected to the transmission housing, the fourth rotation element may be selectively connected to the output shaft, the sixth rotation element may be selectively connected to the second rotation element, the eighth rotation element may be selectively connected to the input shaft, and the twelfth rotation element may be selectively connected to the transmission housing. 
     The first, second, and third rotation elements of the first planetary gear set are a sun gear, a planet carrier, and a ring gear, 
     the fourth, fifth, and sixth rotation elements of the second planetary gear set may be a sun gear, a planet carrier, and a ring gear, the seventh, eighth, and ninth rotation elements of the third planetary gear set may be a sun gear, a planet carrier, and a ring gear, and the tenth, eleventh, and twelfth rotation elements of the fourth planetary gear set may be a sun gear, a planet carrier, and a ring gear. 
     According to various aspects of the present invention, 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 of the engine, 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, seven friction elements disposed to selectively connect the rotation elements with each other and selectively connect the rotation elements with a transmission housing, a first rotation shaft including the first rotation element and directly connected to the input shaft, a second rotation shaft including the second rotation element and selectively connected to the first rotation shaft and the transmission housing, a third rotation shaft including the third and fifth rotation elements, a fourth rotation shaft including the fourth rotation element and selectively connected to the transmission housing, a fifth rotation shaft including the sixth, seventh, and tenth rotation elements and selectively connected to the second rotation shaft, a sixth rotation shaft including the eighth rotation element and selectively connected to the input shaft, a seventh rotation shaft including the ninth and eleventh rotation shafts and directly connected to the output shaft, and an eighth rotation shaft including the twelfth rotation element and selectively connected to the transmission housing. 
     The first planetary gear set may be a single-pinion planetary gear set, in which the first rotation element may be a first sun gear, the second rotation element may be a first planet carrier, and the third rotation element may be a first ring gear, the second planetary gear set may be a single-pinion planetary gear set, in which the fourth rotation element may be a second sun gear, the fifth rotation element may be a second planet carrier, and the sixth rotation element may be a second ring gear, the third planetary gear set may be a single-pinion planetary gear set, in which the seventh rotation element may be a third sun gear, the eighth rotation element may be a third planet carrier, and the ninth rotation element may be a third ring gear, and the fourth planetary gear set may be a single-pinion planetary gear set, the tenth rotation element may be a fourth sun gear, the eleventh rotation element may be a fourth planet carrier, and the twelfth rotation element may be a fourth ring gear. 
     The seven friction elements may include a first clutch selectively connecting the fourth and seventh rotation shafts, a second clutch selectively connecting the first and second rotation shafts, a third clutch selectively connecting the input shaft and the sixth rotation shaft, a fourth clutch selectively connecting the second and fifth rotation shafts, a first brake selectively connects the first rotation shaft and the transmission housing, a second brake selectively connects the fourth rotation shaft and the transmission housing, and a third brake selectively connects the eighth rotation shaft and the transmission housing. 
     Shift speed stages achieved by selectively operating the seven friction elements may include a first forward speed stage achieved by simultaneous operation of the fourth clutch and the second and third brakes, a second forward speed stage achieved by simultaneous operation of the second and fourth clutches and the third brake, a third forward speed stage achieved by simultaneous operation of the second clutch and the second and third brakes, a fourth forward speed stage achieved by simultaneous operation of the third clutch and the second and third brakes, a fifth forward speed stage achieved by simultaneous operation of the second and third clutches and the second brake, a sixth forward speed stage achieved by simultaneous operation of the second, third and fourth clutches, a seventh forward speed stage achieved by simultaneous operation of the third and fourth clutches and the second brake, an eighth forward speed stage achieved by simultaneous operation of the third and fourth clutches and the first brake, a ninth forward speed stage achieved by simultaneous operation of the third clutch and the first and second brakes, a tenth forward speed stage achieved by simultaneous operation of the first and third clutches and the first brake, a first reverse speed stage achieved by simultaneous operation of the first, second and third brakes, and a second reverse speed stage achieved by simultaneous operation of the first clutch and the first and third brakes. 
     The second rotation element may be selectively connected to the first rotation element, 
     According to various aspects of the present invention, 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, second and third rotation elements, a second planetary gear set including a fourth, fifth and sixth rotation elements, a third planetary gear set including a seventh, eighth and ninth rotation elements, a fourth planetary gear set including a tenth, eleventh and twelfth rotation elements, a first rotation shaft including the first rotation element and directly connected to the input shaft, a second rotation shaft including the second rotation element and selectively connected to the first rotation shaft and the transmission housing, a third rotation shaft including the third and fifth rotation elements, a fourth rotation shaft including the fourth rotation element and selectively connected to the transmission housing, a fifth rotation shaft including the sixth, seventh, and tenth rotation elements and selectively connected to the second rotation shaft, a sixth rotation shaft including the eighth rotation element and selectively connected to the input shaft, a seventh rotation shaft including the ninth and eleventh rotation shafts and directly connected to the output shaft, an eighth rotation shaft including the twelfth rotation element and selectively connected to the transmission housing, a first clutch selectively connecting the fourth and seventh rotation shafts, a second clutch selectively connecting the first and second rotation shafts, a third clutch selectively connecting the input shaft and the sixth rotation shaft, a fourth clutch selectively connecting the second and fifth rotation shafts, a first brake selectively connecting the first rotation shaft and the transmission housing, a second brake selectively connecting the fourth rotation shaft and the transmission housing, and a third brake selectively connecting the eighth rotation shaft and the transmission housing. 
     Various embodiments of the present invention may achieve ten forward speed stages and two reverse speed stages by combining four planetary gear sets that are simple planetary gear sets with seven friction elements. 
     In addition, engine driving efficiency may be maximized by achieving a span of shift ratios to be greater than or equal to 9.0. 
     In addition, drivability such as acceleration before and after the shift and rhythm of engine speed may be improved by almost linearly increasing or decreasing step ratios between transmission steps. 
     It is understood that the term “vehicle” or “vehicular” or other similar terms as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g., fuel derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example, both gasoline-powered and electric-powered vehicles. 
     The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram of an exemplary planetary gear train according to the present invention. 
         FIG. 2  is an operation table for each of transmission steps of respective friction elements applied to the exemplary planetary gear train according to the present invention. 
     
    
    
     It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
       FIG. 1  is a configuration diagram of a planetary gear train according to various embodiments of the present invention. 
     Referring to  FIG. 1 , the planetary gear train according to various embodiments of the present invention includes a first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4  disposed on the same axis, and an input shaft IS, an output shaft OS, eight rotation shafts TM 1  to TM 8  directly connecting to each other respective rotation elements of the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4 , seven friction elements C 1  to C 4  and B 1  to B 3 , and a transmission housing H. 
     As a result, torque input from the input shaft IS is transmitted by an inter-complementation operation of the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4  to be output through the output shaft OS. 
     The respective 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  form an engine side. 
     The input shaft IS is an input member and rotational power from a crankshaft of an engine is torque-converted through a torque converter to be input into the input shaft IS. 
     The output shaft OS is an output member, is disposed on the same axis with the input shaft IS, and transfers transmitted driving torque to a driving shaft through a differential apparatus. 
     The first planetary gear set PG 1  as a single-pinion planetary gear set includes a first sun gear S 1  which is a first rotation element N 1 , a first planetary carrier PC 1  which is a second rotation element N 2  that supports a first pinion P 1  which outer-engages with the first sun gear S 1  which is the first rotation element N 1 , and a first ring gear R 1  which is a third rotation element N 3  which inner-engages with the first pinion P 1  as rotation elements. 
     The second planetary gear set PG 2  as a single-pinion planetary gear set includes a second sun gear S 2  which is a fourth rotation element N 4 , a second planet carrier PC 2  which is a fifth rotation element N 5  that supports a second pinion P 2  which outer-engages with the second sun gear S 2  which is the fourth rotation element N 4 , and a second ring gear R 2  which is a sixth rotation element N 6  which inner-engages with the second pinion P 2  as rotation elements. 
     The third planetary gear set PG 3  as a single-pinion planetary gear set includes a third sun gear S 3  which is a seventh rotation element N 7 , a third planet carrier PC 3  which is an eighth rotation element N 8  that supports a third pinion P 3  which outer-engages with the third sun gear S 3  which is the seventh rotation element N 7 , and a third ring gear R 3  which is a ninth rotation element N 9  which inner-engages with the third pinion P 3  as the rotation elements. 
     The fourth planetary gear set PG 4  as a single-pinion planetary gear set includes a fourth sun gear S 4  which is a tenth rotation element N 10 , a fourth planet carrier PC 4  which is an eleventh rotation element N 11  that supports a fourth pinion P 4  which outer-engages with the fourth sun gear S 4  which is the tenth rotation element N 10 , and a fourth ring gear R 4  which is a twelfth rotation element N 12  which inner-engages with the fourth pinion P 4  as the rotation elements. 
     In the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4 , the third rotation element N 3  is directly connected to the fifth rotation element N 5 , the sixth rotation element N 6  is directly connected to the seventh and tenth rotation elements N 7  and N 10 , and the ninth rotation element N 9  is directly connected to the eleventh rotation element N 11  so as to be operated with a total of eight rotation shafts TM 1  to TM 8 . 
     Configurations of the eight rotation shafts TM 1  to TM 8  will be described below. 
     The first rotation shaft TM 1  includes the first rotation element N 1  (the first sun gear S 1 ) and is directly connected to the input shaft IS so as to be continuously operated as an input element. 
     The second rotation shaft TM 2  includes the second rotation element N 2  (the first planet carrier PC 1 ) and is selectively connected to the first rotation shaft TM 1  and the transmission housing H. 
     The third rotation shaft TM 3  includes the third rotation element N 3  (the first ring gear R 1 ) and the fifth rotation element N 5  (the second planet carrier PC 2 ). 
     The fourth rotation shaft TM 4  includes the fourth rotation element N 4  (the second sun gear S 2 ) and is selectively connected to the transmission housing H. 
     The fifth rotation shaft TM 5  includes the sixth rotation element N 6  (the second ring gear R 2 ) and the seventh rotation element N 7  (the third sun gear S 3 ) and tenth rotation element N 10  (the fourth sun gear S 4 ), and is selectively connected to the second rotation shaft TM 2 . 
     The sixth rotation shaft TM 6  includes the eighth rotation element N 8  (the third planet carrier PC 3 ) and is selectively connected to the input shaft IS. 
     The seventh rotation shaft TM 7  includes ninth rotation element N 9  (the third ring gear R 3 ) and the eleventh rotation element N 11  (the fourth planet carrier PC 4 ) and is selectively connected to the fourth rotation shaft TM 4 , and is directly connected to the output shaft OS. 
     The eighth rotation shaft TM 8  includes the twelfth rotation element N 12  (the fourth ring gear R 4 ) and is selectively connected to the transmission housing H. 
     In addition, among the rotation shafts TM 1  to TM 8 , four clutches C 1 , C 2 , C 3 , and C 4  which are friction elements are disposed at connection portions where the rotation shafts are connected to each other. 
     In addition, among the rotation shafts TM 1  to TM 8 , three brakes B 1 , B 2  and B 3  which are friction elements are disposed at connection portions between any one rotation shaft and the transmission housing H. 
     The seven friction elements C 1  to C 4  and B 1  to B 3  will be described in further detail. 
     The first clutch C 1  is disposed between fourth rotation shaft TM 4  and the seventh rotation shaft TM 7  and selectively connects the fourth rotation shaft TM 4  and the seventh rotation shaft TM 7 . 
     The second clutch C 2  is disposed between first rotation shaft TM 1  and the second rotation shaft TM 2  and selectively connects the first rotation shaft TM 1  and the second rotation shaft TM 2 . 
     The third clutch C 3  is disposed between input shaft IS and the sixth rotation shaft TM 6  and selectively connects the input shaft IS and the sixth rotation shaft TM 6 . 
     The fourth clutch C 4  is disposed between second rotation shaft TM 2  and the fifth rotation shaft TM 5  and selectively connects the second rotation shaft TM 2  and the fifth rotation shaft TM 5 . 
     The first brake B 1  is disposed between second rotation shaft TM 2  and the transmission housing H and causes the second rotation shaft TM 2  to be operated as a selective fixed element. 
     The second brake B 2  is disposed between fourth rotation shaft TM 4  and the transmission housing H and causes the fourth rotation shaft TM 4  to be operated as a selective fixed element. 
     The third brake B 3  is disposed between eighth rotation shaft TM 8  and the transmission housing H and causes the eighth rotation shaft TM 8  to be operated as a selective fixed element. 
     The friction elements including the first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4  and the first, second, and third brakes B 1 , B 2 , and B 3  may be multi-plate friction elements of a wet type that are operated by hydraulic pressure. 
       FIG. 2  is an operation chart of friction elements at each speed stage in the planetary gear train according to various embodiments of the present invention. 
     As shown in  FIG. 2 , three friction elements are operated at each speed stage in the planetary gear train according to various embodiments of the present invention. Shifting processes in the various embodiments of the present invention will be described in further detail. 
     The fourth clutch C 4  and the second and third brakes B 2  and B 3  are simultaneously operated at a first forward speed stage D 1 . In a state that the second rotation shaft TM 2  is connected to the fifth rotation shaft TM 5  by operation of the fourth clutch C 4  and the input is made into the first rotation shaft TM 1 , and the fourth and eighth rotation shaft TM 4  and TM 8  are operated as the fixed elements by operation of the second and third brakes B 2  and B 3 . Therefore, the first forward speed stage D 1  is achieved. 
     The second and fourth clutches C 2  and C 4  and the third brake B 3  are simultaneously operated at a second forward speed stage D 2 . In a state that the second rotation shaft TM 2  is connected to the first and fifth rotation shafts TM 1  and TM 5  by operation of the second and fourth clutches C 2  and C 4  and the input is made into the first rotation shaft TM 1 , and the eighth rotation shaft TM 8  is operated as the fixed element by operation of the third brake B 3 . Therefore, second forward speed stage D 2  is achieved. 
     The second clutch C 2  and the second and third brakes B 2  and B 3  are simultaneously operated at a third forward speed stage D 3 . In a state that the first rotation shaft TM 1  is connected to the second rotation shaft TM 2  by operation of the second clutch C 2  and the input is made into the first rotation shaft TM 1 , and the fourth and eighth rotation shafts TM 4  and TM 8  are operated as the fixed element by operation of the second and third brakes B 2  and B 3 . Therefore, the third forward speed stage D 3  is achieved. 
     The third clutch C 3  and the second and third brakes B 2  and B 3  are simultaneously operated at a fourth forward speed stage D 4 . In a state that the input shaft IS is connected to the sixth rotation shaft TM 6  by operation of the third clutch C 3  and the input is made into the first and sixth rotation shafts TM 1  and TM 6 , and the fourth and eighth rotation shafts TM 4  and TM 8  are operated as the fixed elements by operation of the second and third brakes B 2  and B 3 . Therefore, the fourth forward speed stage D 4  is achieved. 
     The second and third clutches C 2  and C 3  and the second brake B 2  are simultaneously operated at a fifth forward speed stage D 5 . In a state that the first rotation shaft TM 1  is connected to the second rotation shaft TM 2  by operation of the second clutch C 2  and the input shaft IS is connected to the sixth rotation shaft TM 6  by operation of the third clutch C 3  and the input is made into the first and sixth rotation shafts TM 1  and TM 6 , and the fourth rotation shaft TM 4  is operated as the fixed element by operation of the second brake B 2 . Therefore, the fifth forward speed stage D 5  is achieved. 
     The second, third, and fourth clutches C 2 , C 3 , and C 4  are simultaneously operated at a sixth forward speed stage D 6 . In a state that the second rotation shaft TM 2  is connected to the first and fifth rotation shafts TM 1  and TM 5  by operation of the second and fourth clutches C 2  and C 4 , and the input shaft IS is connected to the sixth rotation shaft TM 6  by operation of the third clutch C 3 . Therefore, the sixth forward speed stage D 6  for just outputting the input is achieved as all of the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4  are integrally rotate at the same speed. 
     The third and fourth clutches C 3  and C 4  and the second brake B 2  are simultaneously operated at a seventh forward speed stage D 7 . In a state that the input shaft IS is connected to the sixth rotation shaft TM 6  by operation of the third clutch C 3  and the second rotation shaft TM 2  is connected to the fifth rotation shaft TM 5  by operation of the fourth clutch C 4  and the input is made into the first and sixth rotation shafts TM 1  and TM 6 , and the fourth rotation shaft TM 4  is operated as the fixed element by operation of the second brake B 2 . Therefore, the seventh forward speed stage D 7  is achieved. 
     The third and fourth clutches C 3  and C 4  and the first brake B 1  are simultaneously operated at an eighth forward speed stage D 8 . In a state that the input shaft IS is connected to the sixth rotation shaft TM 6  by operation of the third clutch C 3  and the second rotation shaft TM 2  is connected to the fifth rotation shaft TM 5  by operation of the fourth clutch C 4  and the input is made into the first and sixth rotation shafts TM 1  and TM 6 , and the second rotation shaft TM 2  is operated as the fixed element by operation of the first brake B 1 . Therefore, the eighth forward speed stage D 8  is achieved. 
     The third clutch C 3  and the first and second brake B 1  and B 2  are simultaneously operated at a ninth forward speed stage D 9 . In a state that the input shaft IS is connected to the sixth rotation shaft TM 6  by operation of the third clutch C 3  and the input is made into the first and sixth rotation shafts TM 1  and TM 6 , and the second and fourth rotation shafts TM 2  and TM 4  are operated as the fixed elements by operation of the first and second brake B 1  and B 2 . Therefore, the ninth forward speed stage D 9  is achieved. 
     The first and third clutches C 1  and C 3  and the first brake B 1  are simultaneously operated at a tenth forward speed stage D 10 . In a state that the fourth rotation shaft TM 4  is connected to the seventh rotation shaft TM 7  by operation of the first clutch C 1  and the input shaft IS is connected to the sixth rotation shaft TM 6  by operation of the third clutch C 3  and the input is made into the first and sixth rotation shafts TM 1  and TM 6 , and the second rotation shaft TM 2  is operated as the fixed element by operation of the first brake B 1 . Therefore, the tenth forward speed stage D 10  is achieved. 
     The first, second and third brakes B 1 , B 2 , and B 3  are simultaneously operated at a first reverse speed stage REV 1 . In a state that the input is made into the first rotation shaft TM 1  and the second, fourth and eighth rotation shafts TM 2 , TM 4 , and TM 8  are operated as the fixed elements by operation of the first, second and third brakes B 1 , B 2 , and B 3 . Therefore, the first reverse speed stage REV 1  is achieved. 
     The first and third brakes B 1  and B 3  and the first clutch C 1  are simultaneously operated at a second reverse speed stage REV 2 . In a state that the fourth rotation shaft TM 4  is connected to the seventh rotation shaft TM 7  by operation of the first clutch C 1  and the input is made into the first rotation shaft TM 1 , and the second and eighth rotation shafts TM 2  and TM 8  are operated as the fixed elements by operation of the first and second brakes B 1  and B 2 . Therefore, the second reverse speed stage REV 2  is achieved. 
     The planetary gear train according to various embodiments of the present invention may achieve ten forward speed stages and two reverse speed stages by control of four planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4  with four clutches C 1 , C 2 , C 3 , and C 4 , and three brakes B 1 , B 2  and B 3 . 
     In addition, engine driving efficiency may be maximized by achieving a span of shift ratios to be greater than or equal to 9.0. 
     In addition, drivability such as acceleration before and after the shift and rhythm of engine speed may be improved by almost linearly increasing or decreasing step ratios between transmission steps. 
     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.