Abstract:
A planetary gear set of an automatic transmission for a vehicle may include input and output shafts for receiving and outputting torques, first to fourth planetary gear sets respectively forming first to third, fourth to sixth, seventh to ninth, and tenth to twelfth rotational elements, and six control elements each of which selectively interconnects a corresponding pair among the input and output shafts, the rotational elements, and a transmission housing, where the input and output shafts are fixedly connected to the first and eleventh rotational elements respectively, respective pairs of the first and sixth rotational elements, the second and ninth rotational elements, the third and tenth rotational elements, the fifth and eleventh rotational elements are fixedly interconnected, and the seventh rotational element is selectively connected to the transmission housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Korean Patent Application No. 10-2015-0170993, filed Dec. 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. 
     Description of Related Art 
     The recent increase in oil prices has caused car makers to meet global demands for improving fuel efficiency. 
     In this sense, 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. 
     In order to achieve more shift stages for an automatic transmission, the number of internal components is generally increased as the number of gear shift stages is increased, 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. 
     Therefore, in order to maximally enhance fuel consumption of an automatic transmission having more shift stages, it is important for better efficiency to be derived by a smaller number of parts. 
     In this respect, an eight-speed automatic transmission has been recently introduced, and a planetary gear train for an automatic transmission enabling more shift stages is under investigation. 
     Considering that gear ratio spans of recently developed eight-speed automatic transmissions are typically between 6.5 and 7.5, fuel consumption enhancement is not very large. 
     In the case of a gear ratio span of an eight-speed automatic transmission having a level above 9.0, it is difficult to maintain step ratios between adjacent shift stages to be linear, by which driving efficiency of an engine and drivability of a vehicle are deteriorated. 
     Thus, research studies are underway for developing a high efficiency automatic transmission having nine or more speeds. 
     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 having advantages of, by minimal complexity, realizing at least forward ninth speeds and at least one reverse speed, increasing a gear ratio span so as to improve power delivery performance and fuel consumption, and achieving linearity of shift stage step ratios. 
     According to various aspects of the present invention, a planetary gear set of an automatic transmission for a vehicle may include an input shaft for receiving an engine torque, an output shaft for outputting a shifted torque of the engine, a first planetary gear set including first, second, and third rotational elements, a second planetary gear set including fourth, fifth, and sixth rotational elements, a third planetary gear set including seventh, eighth, and ninth rotational elements, a fourth planetary gear set including tenth, eleventh, and twelfth rotational elements, and six control elements for selectively interconnecting the rotational elements and a transmission housing, in which the input shaft may be continuously connected to the first rotational element, the output shaft may be continuously connected to the eleventh rotational element, the first rotational element may be continuously connected to the sixth rotational element, the second rotational element may be continuously connected to the ninth rotational element, the third rotational element may be continuously connected to the tenth rotational element, the fifth rotational element may be continuously connected to the eleventh rotational element, and the seventh rotational element may be selectively connected to the transmission housing. 
     The eighth rotational element may be selectively connected to the transmission housing, the twelfth rotational element may be selectively connected to the transmission housing, the input shaft may be selectively connected to the eighth rotational element, the third rotational element may be selectively connected to the eighth rotational element, and the fourth rotational element may be selectively connected to the twelfth rotational element. 
     The first, second, and third rotational elements may be respectively a sun gear, a ring gear, and a planet carrier of the first planetary gear set, the fourth, fifth, and sixth rotational elements are respectively a sun gear, a planet carrier, and a ring gear of the second planetary gear set, the seventh, eighth, and ninth rotational elements are respectively a sun gear, a planet carrier, and a ring gear of the third planetary gear set, and the tenth, eleventh, and twelfth rotational elements are respectively a sun gear, a planet carrier, and a ring gear of the fourth planetary gear set. 
     According to various aspects of the present invention, a planetary gear set of an automatic transmission for a vehicle may include an input shaft for receiving an engine torque, an output shaft for outputting a shifted torque, a first planetary gear set including first, second, and third rotational elements, a second planetary gear set including fourth, fifth, and sixth rotational elements, a third planetary gear set including seventh, eighth, and ninth rotational elements, a fourth planetary gear set including tenth, eleventh, and twelfth rotational elements, six control elements for selectively interconnecting the rotational elements and a transmission housing, a first connecting member connected to the first rotational element and the sixth rotational element, and directly connected to the input shaft, a second connecting member connected to the second rotational element and the ninth rotational element, a third connecting member connected to the third rotational element and the tenth rotational element, a fourth connecting member connected to the fourth rotational element, a fifth connecting member connected to the fifth rotational element and the eleventh rotational element, and directly connected to the output shaft, a sixth connecting member connected to the seventh rotational element and selectively connected to the transmission housing, a seventh connecting member connected to the eighth rotational element, selectively connected to the input shaft and the third connecting member, and selectively connected to the transmission housing, and an eighth connecting member connected to the twelfth rotational element, selectively connected to the fourth connecting member, and selectively connected to the transmission housing. 
     The first planetary gear set may be a double pinion planetary gear set, where the first rotational element may be a first sun gear, the second rotational element may be a first ring gear, and the third rotational element may be a first planet carrier, the second planetary gear set may be a single pinion planetary gear set, where the fourth rotational element may be a second sun gear, the fifth rotational element may be a second planet carrier, and the sixth rotational element may be a second ring gear, the third planetary gear set may be a single pinion planetary gear set, where the seventh rotational element may be a third sun gear, the eighth rotational element may be a third planet carrier, and the ninth rotational element may be a third ring gear, and the fourth planetary gear set may be a single pinion planetary gear set, where the tenth rotational element may be a fourth sun gear, the eleventh rotational element may be a fourth planet carrier, and the twelfth rotational element may be a fourth ring gear. 
     The six control elements may include a first clutch selectively connecting the input shaft and the seventh connecting member, a second clutch selectively connecting the third connecting member and the seventh connecting member, a third clutch selectively connecting the fourth connecting member and the eighth connecting member, a first brake selectively connecting the sixth connecting member and the transmission housing, a second brake selectively connecting the seventh connecting member and the transmission housing, and a third brake selectively connecting the eighth connecting member and the transmission housing. 
     Shift stages realized by selective operation of three control elements among the six control elements may include forward first speed formed by simultaneous operation of the second clutch and the second and third brakes, a forward second speed formed by simultaneous operation of the first and second clutches and the third brake, a forward third speed formed by simultaneous operation of the first clutch and the first and third brakes, a forward fourth speed formed by simultaneous operation of the third clutch and the first and third brakes, a forward fifth speed formed by simultaneous operation of the first and third clutches and the first brake, a forward sixth speed formed by simultaneous operation of the first, second, and third clutches, a forward seventh speed formed by simultaneous operation of the second and third clutches and the first brake, a forward eighth speed formed by simultaneous operation of the second and third clutches and the second brake, a forward ninth speed formed by simultaneous operation of the third clutch and the first and second brakes, and a reverse speed formed by simultaneous operation of the first, second, and third brakes. 
     According to various aspects of the present invention, a planetary gear set of an automatic transmission for a vehicle may include an input shaft for receiving an engine torque, an output shaft for outputting a shifted torque, a first planetary gear set as a double pinion planetary gear set including first, second, and third rotational elements, a second planetary gear set as a single pinion planetary gear set including fourth, fifth, and sixth rotational elements, a third planetary gear set as a single pinion planetary gear set including seventh, eighth, and ninth rotational elements, a fourth planetary gear set as a single pinion planetary gear set including tenth, eleventh, and twelfth rotational elements, a first connecting member connected to the first rotational element and the sixth rotational element, and directly connected to the input shaft, a second connecting member connected to the second rotational element and the ninth rotational element, a third connecting member connected to the third rotational element and the tenth rotational element, a fourth connecting member connected to the fourth rotational element, a fifth connecting member connected to the fifth rotational element and the eleventh rotational element, and directly connected to the output shaft, a sixth connecting member connected to the seventh rotational element and selectively connected to the transmission housing, a seventh connecting member connected to the eighth rotational element, selectively connected to the input shaft and the third connecting member, and selectively connected to the transmission housing, an eighth connecting member connected to the twelfth rotational element, selectively connected to the fourth connecting member, and selectively connected to the transmission housing, a first clutch selectively connecting the input shaft and the seventh connecting member, a second clutch selectively connecting the third connecting member and the seventh connecting member, a third clutch selectively connecting the fourth connecting member and the eighth connecting member, a first brake selectively connecting the sixth connecting member and the transmission housing, a second brake selectively connecting the seventh connecting member and the transmission housing, and a third brake selectively connecting the eighth connecting member and the transmission housing. 
     The first planetary gear set may include a first sun gear as the first rotational element, a first ring gear as the second rotational element, and a first planet carrier as the third rotational element, the second planetary gear set may include a second sun gear as the fourth rotational element, a second planet carrier as the fifth rotational element, and a second ring gear as the sixth rotational element, the third planetary gear set may include a third sun gear as the seventh rotational element, a third planet carrier as the eighth rotational element, and a third ring gear as the ninth rotational element, and the fourth planetary gear set may include a fourth sun gear as the tenth rotational element, a fourth planet carrier as the eleventh rotational element, and a fourth ring gear as the twelfth rotational element. 
     According to various aspects of the present invention, a planetary gear set of claim of an automatic transmission for a vehicle may include an input shaft for receiving an engine torque, an output shaft for outputting a shifted torque of the engine, a first planetary gear set as a double pinion planetary gear set including a first sun gear, a first planet carrier, and a first ring gear, a second planetary gear set as a single pinion planetary gear set including a second sun gear, a second planet carrier, and a second ring gear, a third planetary gear set as a single pinion planetary gear set including a third sun gear, a third planet carrier, and a third ring gear, a fourth planetary gear set as a single pinion planetary gear set including a fourth sun gear, a fourth planet carrier, and a fourth ring gear, a first connecting member connected to the first sun gear and the second ring gear, and directly connected to the input shaft, a second connecting member connected to the first ring gear and the third ring gear, a third connecting member connected to the first planet carrier and the fourth sun gear, a fourth connecting member connected to the second sun gear, a fifth connecting member connected to the second planet carrier and the fourth planet carrier, and directly connected to the output shaft, a sixth connecting member connected to the third sun gear and selectively connected to the transmission housing, a seventh connecting member connected to the third planet carrier, selectively connected to the input shaft and the third connecting member, and selectively connected to the transmission housing, an eighth connecting member connected to the fourth ring gear, selectively connected to the fourth connecting member, and selectively connected to the transmission housing, and six control elements selectively connecting the connecting members and the transmission housing. 
     A planetary gear train according to various embodiments of the present invention may realize at least nine forward speeds and at least one reverse speed formed by operating the four planetary gear sets as simple planetary gear sets by controlling six control elements. 
     In addition, a planetary gear train according to various embodiments of the present invention may realize a gear ratio span of more than 9.0, thereby maximizing efficiency of driving an engine. 
     In addition, the linearity of step ratios of shift stages is secured while multi-staging the shift stage with high efficiency, securing linearity of step ratios of shift stages, thereby making it possible to improve drivability such as acceleration before and after a shift, an engine speed rhythmic sense, and the like. 
     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 schematic diagram of an exemplary planetary gear train according to the present invention. 
         FIG. 2  is an operational chart for respective control elements at respective shift stages in 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 schematic diagram of a planetary gear train according to various embodiments of the present invention. Referring to  FIG. 1 , a planetary gear train according to various embodiments of the present invention includes first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4  arranged on a same axis, an input shaft IS, an output shaft OS, eight connecting members TM 1  to TM 8  for interconnecting rotational 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 3  and B 1  to B 3 , and a transmission housing H. Torque input from the input shaft IS is shifted by cooperative operation of the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4 , and then output through the output shaft OS. The simple planetary gear sets are arranged in the order of first, first, second, third and fourth planetary gear sets PG 1 , PG 2 , PG 3  and PG 4 , from an engine side. The input shaft IS is an input member and the torque from a crankshaft of an engine, after being torque-converted through a torque converter, is input into the input shaft IS. The output shaft OS is an output member, and being arranged on a same axis with the input shaft IS, delivers a shifted torque to a drive shaft through a differential apparatus. 
     The first planetary gear set PG 1  is a double pinion planetary gear set, and includes a first sun gear S 1 , a first planet carrier PC 1  that supports a first pinion P 1  externally engaged with the first sun gear S 1 , and a first ring gear R 1  internally engaged with the first pinion P 1 . The first sun gear S 1  acts as a first rotational element N 1 , the first planet carrier PC 1  acts as a second rotational element N 2 , and the first ring gear R 1  acts as a third rotational element N 3 . 
     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  that supports a second pinion P 2  externally engaged with the second sun gear S 2 , and a second ring gear R 2  internally engaged with the second pinion P 2 . The second sun gear S 2  acts as a fourth rotational element N 4 , the second planet carrier PC 2  acts as a fifth rotational element N 5 , and the second ring gear R 2  acts as a sixth rotational element N 6 . 
     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  that supports a third pinion P 3  externally engaged with the third sun gear S 3 , and a third ring gear R 3  internally engaged with the third pinion P 3 . The third sun gear S 3  acts as a seventh rotational element N 7 , the third planet carrier PC 3  acts as an eighth rotational element N 8 , and the third ring gear R 3  acts as a ninth rotational element N 9 . 
     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  that supports a fourth pinion P 4  externally engaged with the fourth sun gear S 4 , and a fourth ring gear R 4  internally engaged with the fourth pinion P 4 . The fourth sun gear S 4  acts as a tenth rotational element N 10 , the fourth planet carrier PC 4  acts as an eleventh rotational element N 11 , and the fourth ring gear R 4  acts as a twelfth rotational element N 12 . 
     In the arrangement of the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4 , the first rotational element N 1  is directly connected with the sixth rotational element N 6 , the second rotational element N 2  is directly connected with the ninth rotational element N 9 , the third rotational element N 3  is directly connected with the tenth rotational element N 10 , and the fifth rotational element N 5  is directly connected with the eleventh rotational element N 11 , by eight connecting members TM 1  to TM 8 . 
     The eight connecting members TM 1  to TM 8  are arranged as follows. 
     The first connecting member TM 1  is connected with the first rotational element N 1  (first sun gear S 1 ) and the sixth rotational element N 6  (second ring gear R 2 ), and directly connected with the input shaft IS, thereby continuously acting as an input element. 
     The second connecting member TM 2  is connected with the second rotational element N 2  (first ring gear R 1 ) and the ninth rotational element N 9  (third ring gear R 3 ). 
     The third connecting member TM 3  is connected with the third rotational element N 3  (first planet carrier PC 1 ) and the tenth rotational element N 10  (fourth sun gear S 4 ). 
     The fourth connecting member TM 4  is connected with fourth rotational element N 4  (second sun gear S 2 ). 
     The fifth connecting member TM 5  is connected with the fifth rotational element N 5  (second planet carrier PC 2 ) and the eleventh rotational element N 11  (fourth planet carrier PC 4 ), and directly connected with the output shaft OS thereby continuously acting as an output element. 
     The sixth connecting member TM 6  is connected with the seventh rotational element N 7  (third sun gear S 3 ), and selectively connected with the transmission housing H, thereby acting as a selective fixed element. 
     The seventh connecting member TM 7  is connected with the eighth rotational element N 8  (third planet carrier PC 3 ), selectively connected with the third connecting member TM 3 , selectively connected with the input shaft IS thereby acting as a selective input element, and selectively connected with the transmission housing H, thereby acting as a selective fixed element. 
     The eighth connecting member TM 8  is connected with the twelfth rotational element N 12  (fourth ring gear R 4 ), selectively connected with the fourth connecting member TM 4 , and selectively connected with the transmission housing H, thereby acting as a selective fixed element. 
     The connecting members TM 1  to TM 8  may be selectively interconnected with one another by control elements of three clutches C 1 , C 2 , and C 3 . 
     The connecting members TM 1  to TM 8  may be selectively connected with the transmission housing H, by control elements of three brakes B 1 , B 2 , and B 3 . 
     The six control elements C 1  to C 3  and B 1  to B 3  are arranged as follows. 
     The first clutch C 1  is arranged between the input shaft IS and the seventh connecting member TM 7 , such that the input shaft IS and the seventh connecting member TM 7  may selectively become integral. 
     The second clutch C 2  is arranged between the third connecting member TM 3  and the seventh connecting member TM 7 , such that the third connecting member TM 3  and the seventh connecting member TM 7  may selectively become integral. 
     The third clutch C 3  is arranged between the fourth connecting member TM 4  and the eighth connecting member TM 8 , such that the fourth connecting member TM 4  and the eighth connecting member TM 8  may selectively become integral. 
     The first brake B 1  is arranged between the sixth connecting member TM 6  and the transmission housing H, such that the sixth connecting member TM 6  may selectively act as a fixed element. 
     The second brake B 2  is arranged between the seventh connecting member TM 7  and the transmission housing H, such that the seventh connecting member TM 7  may selectively act as a fixed element. 
     The third brake B 3  is arranged between the eighth connecting member TM 8  and the transmission housing H, such that the eighth connecting member TM 8  may selectively act as a fixed element. 
     The control elements of the first, second, and third clutches C 1 , C 2 , and C 3  and the first, second, and third brakes B 1 , B 2 , and B 3  may be realized as multi-plate hydraulic pressure friction devices that are frictionally engaged by hydraulic pressure. 
       FIG. 2  is an operational chart for respective control elements at respective shift stages in a planetary gear train according to various embodiments of the present invention. As shown in  FIG. 2 , a planetary gear train according to various embodiments of the present invention performs shifting by operating three control elements at respective shift stages. 
     In the forward first speed shift stage D 1 , the second clutch C 2  and the second and third brakes B 2  and B 3  are simultaneously operated. As a result, the third connecting member TM 3  is interconnected with the seventh connecting member TM 7  by the operation of the second clutch C 2 . In this state, torque of the input shaft IS is input to the first connecting member TM 1 . In addition, the seventh and eighth connecting members TM 7  and TM 8  simultaneously act as fixed elements by the operation of the second and third brakes B 2  and B 3 , thereby realizing the forward first speed by cooperative operation of respective connecting members and outputting a shifted torque through the output shaft OS connected with the fifth connecting member TM 5 . 
     In the forward second speed shift stage D 2 , the first and second clutches C 1  and C 2  and the third brake B 3  are simultaneously operated. As a result, the seventh connecting member TM 7  is interconnected with the input shaft IS by the operation of the first clutch C 1 , and the third connecting member TM 3  is interconnected with the seventh connecting member TM 7  by the operation of the second clutch C 2 . In this state, the torque of the input shaft IS is simultaneously input to the first connecting member TM 1  and the seventh connecting member TM 7 . In addition, the eighth connecting member TM 8  simultaneously acts as a fixed element by the operation of third brake B 3 , thereby realizing the forward second speed by cooperative operation of respective connecting members and outputting a shifted torque through the output shaft OS connected with the fifth connecting member TM 5 . 
     In the forward third speed shift stage D 3 , the first clutch C 1  and the first and third brakes B 1  and B 3  are simultaneously operated. As a result, the seventh connecting member TM 7  is interconnected with the input shaft IS by the operation of the first clutch C 1 . In this state, the torque of the input shaft IS is simultaneously input to the first connecting member TM 1  and the seventh connecting member TM 7 . In addition, the sixth and eighth connecting members TM 6  and TM 8  simultaneously act as fixed elements by the operation of the first and third brakes B 1  and B 3 , thereby realizing the forward third speed by cooperative operation of respective connecting members and outputting a shifted torque through the output shaft OS connected with the fifth connecting member TM 5 . 
     In the forward fourth speed shift stage D 4 , the third clutch C 3  and the first and third brakes B 1  and B 3  are simultaneously operated. As a result, the fourth connecting member TM 4  is interconnected with the eighth connecting member TM 8  by the operation of the third clutch C 3 . In this state, the torque of the input shaft IS is simultaneously input to the first connecting member TM 1  and the seventh connecting member TM 7 . In addition, the sixth and eighth connecting members TM 6  and TM 8  simultaneously act as fixed elements by the operation of the first and third brakes B 1  and B 3 , thereby realizing the forward fourth speed by cooperative operation of respective connecting members and outputting a shifted torque through the output shaft OS connected with the fifth connecting member TM 5 . 
     In the forward fifth speed shift stage D 5 , the first and third clutches C 1  and C 3  and the first brake B 1  are simultaneously operated. As a result, the seventh connecting member TM 7  is interconnected with the input shaft IS by the operation of the first clutch C 1 , and the fourth connecting member TM 4  is interconnected with the eighth connecting member TM 8  by the operation of the third clutch C 3 . In this state, the torque of the input shaft IS is simultaneously input to the first connecting member TM 1  and the seventh connecting member TM 7 . In addition, the sixth connecting member TM 6  simultaneously acts as a fixed element by the operation of first brake B 1 , thereby realizing the forward fifth speed by cooperative operation of respective connecting members and outputting a shifted torque through the output shaft OS connected with the fifth connecting member TM 5 . 
     In the forward sixth speed shift stage D 6 , the first, second, and third clutches C 1 , C 2 , and C 3  are simultaneously operated. As a result, the seventh connecting member TM 7  is interconnected with the input shaft IS by the operation of the first clutch C 1 , the third connecting member TM 3  is interconnected with the seventh connecting member TM 7  by the operation of the second clutch C 2 , and the fourth connecting member TM 4  is interconnected with the eighth connecting member TM 8  by the operation of the third clutch C 3 . In this state, torque is input to the first connecting member TM 1  and the seventh connecting member TM 7 . In addition, the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4  becomes entirely integral by the operation of the second clutch C 2  and the third clutch TM 3 , thereby realizing the forward sixth speed and outputting an inputted torque through the output shaft OS connected with fifth connecting member TM 7 . 
     In the forward seventh speed shift stage D 7 , the second and third clutches C 2  and C 3  and the first brake B 1  are simultaneously operated. As a result, the third connecting member TM 3  is interconnected with the seventh connecting member TM 7  by the operation of the second clutch C 2 , and the fourth connecting member TM 4  is interconnected with the eighth connecting member TM 8  by the operation of the third clutch C 3 . In this state, torque of the input shaft IS is input to the first connecting member TM 1 . In addition, the sixth connecting member TM 6  simultaneously acts as a fixed element by the operation of first brake B 1 , thereby realizing the forward seventh speed by cooperative operation of respective connecting members and outputting a shifted torque through the output shaft OS connected with the fifth connecting member TM 5 . 
     In the forward eighth speed shift stage D 8 , the second and third clutches C 2  and C 3  and the second brake B 2  are simultaneously operated. As a result, the third connecting member TM 3  is interconnected with the seventh connecting member TM 7  by the operation of the second clutch C 2 , and the fourth connecting member TM 4  is interconnected with the eighth connecting member TM 8  by the operation of the third clutch C 3 . In this state, torque of the input shaft IS is input to the first connecting member TM 1 . In addition, the seventh connecting member TM 7  simultaneously acts as a fixed element by the operation of second brake B 2 , thereby realizing the forward eighth speed by cooperative operation of respective connecting members and outputting a shifted torque through the output shaft OS connected with the fifth connecting member TM 5 . 
     In the forward ninth speed shift stage D 9 , the third clutch C 3  and the first and second brakes B 1  and B 2  are simultaneously operated. As a result, the fourth connecting member TM 4  is interconnected with the eighth connecting member TM 8  by the operation of the third clutch C 3 . In this state, torque is input to the first connecting member TM 1 . In addition, the sixth and seventh connecting members TM 6  and TM 7  simultaneously act as fixed elements by the operation of the first and second brakes B 1  and B 2 , thereby realizing the forward eighth speed by cooperative operation of respective connecting members and outputting a shifted torque through the output shaft OS connected with the fifth connecting member TM 5 . 
     In the reverse speed REV, the first, second, and third brakes B 1 , B 2 , and B 3  are simultaneously operated. As a result, torque of the input shaft IS is input to the first connecting member TM 1 , and the sixth, seventh, and eighth connecting members TM 6 , TM 7 , and TM 8  simultaneously act as fixed elements by the operation of the first, second, and third brakes B 1 , B 2 , and B 3 , thereby realizing the reverse speed by cooperative operation of respective connecting members and outputting a shifted torque through the output shaft OS connected with the fifth connecting member TM 5 . 
     As described above, a planetary gear train according to various embodiments of the present invention may realize at least the forward ninth speed and at least one reverse speed formed by operating four planetary gear set PG 1 , PG 2 , PG 3 , and PG 4  by controlling the three clutches C 1 , C 2 , and C 3  and the three brakes B 1 , B 2 , and B 3 . 
     In addition, a planetary gear train according to various embodiments of the present invention may realize a gear ratio span of more than 9.2, thereby maximizing efficiency of driving an engine. 
     In addition, the linearity of step ratios of shift stages is secured while multi-staging the shift stage with high efficiency, thereby making it possible to improve drivability such as acceleration before and after a shift, an engine speed rhythmic sense, and the like. 
     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.