Abstract:
A gear train of an automatic transmission for a vehicle includes first and second shifting sections and a plurality of friction members, wherein: the first shifting section includes first and second simple planetary gear sets forming a first input path for directly receiving an input shaft rotation speed and an intermediate output path for outputting a reduced speed of the first shifting section; and the second shifting section includes third and fourth simple planetary gear sets forming a first intermediate input path for variably receiving an input shaft rotation speed, a second intermediate input path for receiving the reduced speed of the first shifting section through the intermediate output path and also for variably receiving an input shaft rotation speed, and a final output path for outputting a modified speed of the second shifting section.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to and the benefit of Korean Patent Application No. 10-2006-0104594 filed in the Korean Intellectual Property Office on Oct. 26, 2006, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   (a) Field of the Invention 
   The present invention relates to a gear train of an automatic transmission for a vehicle. 
   (b) Description of the Related Art 
   A typical shift mechanism of an automatic transmission utilizes a combination of planetary gear sets. A gear train of such an automatic transmission changes rotating speed and torque received from a torque converter of the automatic transmission, and accordingly changes and transmits the changed torque to an output shaft. 
   When a transmission has a large number of shift speeds, speed ratios of the transmission can be more optimally designed and therefore a vehicle can have better fuel mileage and better performance. 
   For a given number of speeds, features of a gear train such as durability, efficiency in power transmission, and size depend a lot on the layout of the planetary gear sets. 
   A manual transmission with a large number of speeds causes the inconvenience of excessively frequent shifting operations by the driver. Therefore, gear trains with large numbers of shift-speeds lend themselves better to automatic transmissions. 
   The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
   SUMMARY OF THE INVENTION 
   The present invention provides a gear train with improved power transmission performance and reduced fuel consumption. 
   An exemplary embodiment of the present invention provides a gear train of an automatic transmission for a vehicle having first and second shifting sections and a plurality of friction members, wherein: the first shifting section includes first and second simple planetary gear sets forming a first input path for directly receiving an input shaft rotation speed and an intermediate output path for outputting a reduced speed of the first shifting section; and the second shifting section includes third and fourth simple planetary gear sets forming a first intermediate input path for variably receiving an input shaft rotation speed, a second intermediate input path for receiving the reduced speed of the first shifting section through the intermediate output path and also for variably receiving an input shaft rotation speed, and a final output path for outputting a modified speed of the second shifting section. 
   Each of the first, second, third, and fourth planetary gear sets may be a single pinion planetary gear set. 
   A sun gear of the first planetary gear set may be directly connected with a sun gear of the second planetary gear set, and a ring gear of the first planetary gear set may be directly connected with a planet carrier of the second planetary gear set, such that the first shifting section includes four operational elements: a first operational element formed by the sun gears of the first and second planetary gear sets; a second operational element formed by a planet carrier of the first planetary gear set; a third operational element formed by the ring gear of the first planetary gear set and the planet carrier of the second planetary gear set; and a fourth operational element formed by a ring gear of the second planetary gear set. 
   The fourth operational element may be directly connected with an input shaft so as to form the first input path. The third operational element may form the intermediate output path. The first and second operational elements may selectively act as a fixed element. 
   A ring gear of the third planetary gear set may be directly connected with a planet carrier of the fourth planetary gear set, and sun gears of the third and fourth planetary gear sets may be variably interconnected, such that the second shifting section includes four operational elements: a fifth operational element formed by a ring gear of the fourth planetary gear set; a sixth operational element formed by the ring gear of the third planetary gear set and the planet carrier of the fourth planetary gear set; a seventh operational element formed by a planet carrier of the third planetary gear set; and an eighth operational element formed by the sun gears of the third and fourth planetary gear sets. 
   The fifth operational element may form the second intermediate input path that is directly connected with the intermediate output path of the first shifting section and variably connected with the input shaft. The sixth operational element may form the first intermediate input path that is variably connected with the input shaft. The seventh operational element may form the final output path. 
   The sixth operational element that forms the first intermediate input path may be variably connected with the transmission housing so as to selectively act as a fixed element. 
   The sun gears of the third and fourth planetary gear sets forming the eighth operational element may be variably connected with the transmission housing such that they may simultaneously act as fixed elements or only one of them may act as a fixed element. 
   The sun gear of the third planetary gear set may be variably connected with the transmission housing. 
   The fifth operational element forming the second intermediate input path may be variably connected with the transmission housing so as to selectively act as a fixed element. 
   An exemplary gear train of an automatic transmission for a vehicle according to another embodiment of the present invention has first and second shifting sections and a plurality of friction members. The first shifting section comprises first and second planetary gear sets wherein a sun gear of the first planetary gear set is directly connected with a sun gear of the second planetary gear set, and a ring gear of the first planetary gear set is directly connected with a planet carrier of the second planetary gear set, such that the first shifting section forms four operational elements. The four operational elements of the first shifting section comprise first and second operational elements selectively acting as a fixed element, a third operational element acting as an intermediate output element, and a fourth operational element that always acts as an input element. The second shifting section comprises third and fourth planetary gear sets wherein a ring gear of the third planetary gear set is directly connected with a planet carrier of the fourth planetary gear set and sun gears of the third and fourth planetary gear sets are variably interconnected, such that the second shifting section forms four operational elements. The four operational elements of the second shifting section comprise a fifth operational element that is directly connected with the third operational element of the first shifting section and variably connected with the input shaft, a sixth operational element that is variably connected with the input shaft and variably connected with the transmission housing, a seventh operational element that always acts as an output element, and an eighth operational element that selectively acts as an idle element or a fixed element. 
   Each of the first, second, third, and fourth planetary gear sets may be a single pinion planetary gear set. 
   The first operational element may be formed by the sun gears of the first and second planetary gear sets. The second operational element may be formed by a planet carrier of the first planetary gear set. The third operational element may be formed by the ring gear of the first planetary gear set and the planet carrier of the second planetary gear set. The fourth operational element may be formed by a ring gear of the second planetary gear set. 
   The fifth operational element may be formed by a ring gear of the fourth planetary gear set. The sixth operational element may be formed by the ring gear of the third planetary gear set and the planet carrier of the fourth planetary gear set. The seventh operational element may be formed by a planet carrier of the third planetary gear set. The eighth operational element may be formed by the sun gears of the third and fourth planetary gear sets. 
   The sun gears of the third and fourth planetary gear sets forming the eighth operational element may be variably connected with the transmission housing such that they may simultaneously act as fixed elements or only one of them may act as a fixed element. 
   The sun gear of the third planetary gear set may be variably connected with the transmission housing. 
   The plurality of friction members may include: a first clutch disposed between the input shaft and the sixth operational element; a second clutch disposed between the input shaft and the fifth operational element; a third clutch disposed between the sun gears of the third and fourth planetary gear sets forming the eighth operational element; a first brake disposed between the second operational element and the transmission housing; a second brake disposed between the transmission housing and the sun gear of the third planetary gear set forming the eighth operational element; a third brake disposed between the first operational element and the transmission housing; and a fourth brake disposed between the sixth operational element and the transmission housing. 
   The first, second, third, and fourth planetary gear sets may be disposed in a sequence of first, second, third, and fourth planetary gear sets. 
   The first and third brakes may be disposed on an engine side of the first planetary gear set. The first and second clutches may be disposed between the second and third planetary gear sets. The third clutch and the second and fourth brakes may be disposed on a side of the fourth planetary gear set opposite the engine. 
   The plurality of friction members may further include a fifth brake disposed between the fifth operational element and the transmission housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a gear train according to a first exemplary embodiment of the present invention. 
       FIG. 2  is an operational chart of frictional members employed in a gear train according to a first exemplary embodiment of the present invention. 
       FIG. 3  is a speed diagram of a gear train according to exemplary embodiments of the present invention. 
       FIG. 4  is a schematic diagram of a gear train according to a second exemplary embodiment of the present invention. 
       FIG. 5  is an operational chart of frictional members employed in a gear train according to a second exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An exemplary embodiment of the present invention will hereinafter be described in detail with reference to the accompanying drawings. 
   Referring to  FIG. 1 , a gear train according to a first exemplary embodiment of the present invention includes first and second shifting sections T 1  and T 2 . The first shifting section T 1  includes first and second planetary gear sets PG 1  and PG 2 , each of which is a single pinion planetary gear set. The second shifting section T 2  includes third and fourth planetary gear sets PG 1  and PG 2 , each of which is a single pinion planetary gear set. A rotation speed input through an input shaft IS is changed by the first and second shifting sections T 1  and T 2 , and then output through an output shaft OS. 
   The input shaft IS may be a turbine shaft of a torque converter that receives engine torque through the torque converter. The output shaft OS outputs a drive torque to vehicle wheels through an output gear and a differential. 
   The first shifting section T 1  selectively receives the engine speed through a single input path IP, and outputs a reduced speed through a single intermediate output path MOP. 
   To this end, a sun gear S 1  of the first planetary gear set PG 1  is directly connected with a sun gear S 2  of the second planetary gear set PG 2 , and a ring gear R 1  of the first planetary gear set PG 1  is directly connected with a planet carrier PC 2  of the second planetary gear set PG 2 . 
   The first shifting section T 1  thus has four operational elements, as shown in  FIG. 3 . The first operational element N 1  is formed by the sun gears S 1  and S 2  of the first and second planetary gear sets PG 1  and PG 2 . The second operational element N 2  is formed by a planet carrier PC 1  of the first planetary gear set PG 1 . The third operational element N 3  is formed by the ring gear R 1  of the first planetary gear set PG 1  and the planet carrier PC 2  of the second planetary gear set PG 2 . The fourth operational element N 4  is formed by a ring gear R 2  of the second planetary gear set PG 2 . 
   Referring back to  FIG. 1 , the first operational element N 1  is connected with a transmission housing H interposing a third brake B 3 , and acts as a fixed element at the second forward speed and the sixth forward speed. The second operational element N 2  is connected with the transmission housing H interposing a first brake B 1 , and acts as a fixed element at the first and seventh forward speeds and the first reverse speed. 
   The third operational element N 3  is connected with one of intermediate input paths of the second shifting section T 2  and forms an intermediate output path MOP. The fourth operational element N 4  is directly connected with the input shaft IS, and forms an input path IP. 
   The second shifting section T 2  receives a rotation speed through first and second intermediate input paths MIP 1  and MIP 2 . The first intermediate input path MIP 1  is variably connected with the input shaft IS. The second intermediate input path MIP 2  is variably connected with the input shaft IS and fixedly connected with the intermediate output path MOP. Thus, the second shifting section T 2  changes rotation speeds received through the first and second intermediate input paths MIP 1  and MIP 2 , and outputs the changed rotation speed through a final output path OP. 
   To this end, a ring gear R 3  of the third planetary gear set PG 3  is directly connect with a planet carrier PC 4  of the fourth planetary gear set PG 4 , and a sun gear S 3  of the third planetary gear set PG 3  is variably connected with a sun gear S 4  of the fourth planetary gear set PG 4  interposing a third clutch C 3 . 
   The second shifting section T 2  thus has four operational elements of fifth, sixth, seventh, and eighth operational elements, as shown in  FIG. 3 . 
   The fifth operational element N 5  is formed by a ring gear R 4  of the fourth planetary gear set PG 4 . The sixth operational element N 6  is formed by the ring gear R 3  of the third planetary gear set PG 3  and the planet carrier PC 4  of the fourth planetary gear set PG 4 . The seventh operational element N 7  is formed by a planet carrier PC 3  of the third planetary gear set PG 3 . The eighth operational element N 8  is formed by the sun gears S 3  and S 4  of the third and fourth planetary gear sets PG 3  and PG 4 . 
   Referring back to  FIG. 1 , the fifth operational element N 5  forms the second intermediate input path MIP 2  by being directly connected with the intermediate output path MOP and being variably connected with the input shaft IS interposing a second clutch C 2 . The fifth operational element N 5  acts as an input element at first, second, third, fourth, and fifth forward speeds and first and second reverse speeds. 
   The sixth operational element N 6  forms the first intermediate input path MIP 1  by being variably connected with the input shaft IS interposing a first clutch C 1 . The sixth operational element N 6  acts as an input element at fourth, fifth, sixth, and seventh forward speeds. The sixth operational element N 6  is variably connected with the transmission housing H interposing a fourth brake B 4 , and it acts as a fixed element at the first and second reverse speeds. 
   The seventh operational element N 7  is connected with the output shaft OS so as to form the final output path OP, and always acts as an output element. 
   Regarding the eighth operational element N 8 , the sun gear S 3  of the third planetary gear set PG 3  is connected with the transmission housing H interposing a second brake B 2 . The eighth operational element N 8  may thus selectively act as a fixed element and an idle element. At the first, second, and third forward speeds, the third clutch C 3  and the second brake B 2  simultaneously operate and thus the eighth operational element N 8  acts as a fixed element. At the fourth forward speed, the eighth operational element N 8  can still act as a fixed element since the second brake B 2  operates although the third clutch C 3  does not. The eighth operational element N 8  acts as an idle element at the fifth, sixth, and seventh forward speeds and the first and second reverse speeds. 
   The first, second, and third clutches C 1 , C 2 , and C 3  and the first, second, third, and fourth brakes B 1 , B 2 , B 3 , and B 4  can each be a multi-plate hydraulic pressure friction device that is frictionally engaged by hydraulic pressure. 
   As shown in  FIG. 2 , three friction members operate in each speed according to a gear train of the first exemplary embodiment of the present invention. 
   In  FIG. 3 , the lower horizontal line indicates 0 (zero) rotation speed, and the upper horizontal line indicates 1.0 rotation speed (i.e., the same rotation speed as the input shaft IS). 
   The four vertical lines of the first shifting section T 1  correspond to, sequentially from left to right, the first operational element N 1 , i.e. the sun gears S 1  and S 2  of the first and second planetary gear sets PG 1  and PG 2 , the second operational element N 2 , i.e. the planet carrier PC 1  of the first planetary gear set PG 1 , the third operational element N 3 , i.e. the ring gear R 1  of the first planetary gear set PG 1  and the planet carrier PC 2  of the second planetary gear set PG 2 , and the fourth operational element N 4 , i.e. the ring gear R 2  of the second planetary gear set PG 2 . The horizontal spacing between the four vertical lines depends on sun gear/ring gear teeth ratios of the first and second planetary gear sets PG 1  and PG 2 . 
   The vertical lines of the second shifting section T 2  correspond to, sequentially from the left to the right, the fifth operational element N 5 , i.e. the ring gear R 4  of the fourth planetary gear set PG 4 , the sixth operational element N 6 , i.e. the ring gear R 3  of the third planetary gear set PG 3  and the planet carrier PC 4  of the fourth planetary gear set PG 4 , the seventh operational element N 7 , i.e. the planet carrier PC 3  of the third planetary gear set PG 3 , and the eighth operational element N 8 , i.e. the sun gears S 3  and S 4  of the third and fourth planetary gear sets PG 3  and PG 4 . The horizontal spacing between the four vertical lines depends on sun gear/ring gear teeth ratios of the third and fourth planetary gear sets PG 3  and PG 4 . 
   &lt;The First Forward Speed&gt; 
   At the first forward speed, the third clutch C 3  and the first and second brakes B 1  and B 2  operate, as shown in  FIG. 2 . 
   Then, in the first shifting section T 1 , the second operational element N 2  acts as a fixed element by the operation of the first brake B 1  while the fourth operational element N 4  receives an input speed. Therefore, a reduced speed is output from the first shifting section T 1  through the third operational element N 3  (refer to  FIG. 3 ). 
   In the second shifting section T 2 , the eighth operational element N 8  acts as a fixed element by the operation of the second brake B 2  and the third clutch C 3  while the reduced speed of the third operational element N 3  is input to the fifth operational element N 5 . Therefore, a first forward speed line SP 1  is formed, and a further reduced speed is output from the seventh operational element N 7 . That is, the final output speed of the gear train is at a height D 1  of the first forward speed line SP 1  at the position of the seventh operational element N 7 , and thus the first forward speed is attained. 
   &lt;The Second Forward Speed&gt; 
   At the second forward speed, the first brake B 1  that operated at the first forward speed is released, and the third brake B 3  is controlled to operate. 
   Then, similarly to the first forward speed, the first operational element N 1  in the first shifting section T 1  acts as a fixed element by the operation of the third brake B 3  while the fourth operational element N 4  receives an input speed. Therefore, a reduced speed is output from the first shifting section T 1  through the third operational element N 3 . 
   In the second shifting section T 2 , the same as in the first forward speed, the eighth operational element N 8  acts as a fixed element by the operation of the second brake B 2  and the third clutch C 3  while the reduced speed of the third operational element N 3  is input to the fifth operational element N 5 . Therefore, a second forward speed line SP 2  is formed, and a further reduced speed is output from the seventh operational element N 7 . That is, the final output speed of the gear train is at a height D 2  of the second forward speed line SP 2  at the position of the seventh operational element N 7 , and thus the second forward speed is attained. 
   &lt;The Third Forward Speed&gt; 
   At the third forward speed, the third brake B 3  that operated at the second forward speed is released, and the second clutch C 2  is controlled to operate. 
   Then, the first shifting section T 1  does not contribute to the speed change and directly output the inputted engine speed, since the first and second planetary gears PG 1  and PG 2  merely integrally rotate. 
   In the second shifting section T 2 , similarly to the second forward speed, the eighth operational element N 8  acts as a fixed element by the operation of the second brake B 2  and the third clutch C 3  while the fifth operational element N 5  receives the engine speed from the first shifting section T 1  and the input shaft IS via the second clutch C 2 . Therefore, a third forward speed line SP 3  is formed, and a reduced speed is output from the seventh operational element N 7 . That is, the final output speed of the gear train is at a height D 3  of the third forward speed line SP 3  at the position of the seventh operational element N 7 , and thus the third forward speed is attained. 
   &lt;The Fourth Forward Speed&gt; 
   At the fourth forward speed, the third clutch C 3  that operated at the third forward speed is released, and the first clutch C 1  is controlled to operate. 
   Then, the same as in the third forward speed, the first shifting section T 1  does not contribute to the speed change and directly output the inputted engine speed, since the first and second planetary gears PG 1  and PG 2  merely integrally rotate by the operation of the second clutch C 2 . 
   In the second shifting section T 2 , the eighth operational element N 8  acts as a fixed element by the operation of the second brake B 2  while the sixth operational element N 6  (more specifically, the ring gear R 3  of the third planetary gear set PG 3 ) receives the engine speed from the input shaft IS via the first clutch C 1 . Therefore, a fourth forward speed line SP 4  is formed, and a reduced speed is output from the seventh operational element N 7 . That is, the final output speed of the gear train is at a height D 4  of the fourth forward speed line SP 4  at the position of the seventh operational element N 7 , and thus the fourth forward speed is attained. 
   &lt;The Fifth Forward Speed&gt; 
   At the fifth forward speed, the second brake B 2  that operated at the fourth forward speed is released, and the third clutch C 3  is controlled to operate. 
   Then, the same as in the third forward speed, the first shifting section T 1  does not contribute to the speed change and directly output the inputted engine speed, since the first and second planetary gears PG 1  and PG 2  merely integrally rotate by the operation of the second clutch C 2 . 
   In addition, in the second shifting section T 2 , the third and fourth planetary gear sets PG 3  and PG 4  integrally rotate due to the simultaneous operation of the first, second, and third clutches C 1 , C 2 , and C 3 . Therefore, a fifth forward speed line SP 5  is formed, and the same speed as the input speed is output from the seventh operational element N 7 . That is, the final output speed of the gear train is at a height D 5  of the fifth forward speed line SP 5  at the position of the seventh operational element N 7 , and thus the fifth forward speed is attained. 
   &lt;The Sixth Forward Speed&gt; 
   At the sixth forward speed, the second clutch C 2  that operated at the fifth forward speed is released, and the third brake B 3  is controlled to operate. 
   Then, the same as in the second forward speed, the first operational element N 1  in the first shifting section T 1  acts as a fixed element by the operation of the third brake B 3  while the fourth operational element N 4  receives an input speed. Therefore, a reduced speed is output from the first shifting section T 1  through the third operational element N 3 . 
   In the second shifting section T 2 , the sixth operational element N 6  receives the engine speed by the operation of the first clutch C 1  while the reduced speed of the third operational element N 3  is input to the fifth operational element N 5 . Therefore, a sixth forward speed line SP 6  is formed, and an increased speed is output from the seventh operational element N 7 . That is, the final output speed of the gear train is at a height D 6  of the sixth forward speed line SP 6  at the position of the seventh operational element N 7 , and thus the sixth forward speed is attained. 
   &lt;The Seventh Forward Speed&gt; 
   At the seventh forward speed, the third brake B 3  that operated at the sixth forward speed is released, and the first brake B 1  is controlled to operate. 
   Then, the same as in the first forward speed, the second operational element N 2  in the first shifting section T 1  acts as a fixed element by the operation of the first brake B 1  while the fourth operational element N 4  receives an input speed. Therefore, a reduced speed is output from the first shifting section T 1  through the third operational element N 3 . 
   In the second shifting section T 2 , the sixth operational element N 6  receives the engine speed by the operation of the first clutch C 1  while the reduced speed of the third operational element N 3  is input to the fifth operational element N 5 . Therefore, a seventh forward speed line SP 7  is formed, and an increased speed is output from the seventh operational element N 7 . That is, the final output speed of the gear train is at a height D 7  of the seventh forward speed line SP 7  at the position of the seventh operational element N 7 , and thus the seventh forward speed is attained. 
   &lt;The First Reverse Speed&gt; 
   At the first reverse speed, the third clutch C 3  and the first and fourth brakes B 1  and B 4  are controlled to operate. 
   Then, in the first shifting section T 1 , the second operational element N 2  acts as a fixed element by the operation of the first brake B 1  while the fourth operational element N 4  receives an input speed. Therefore, a reduced speed is output from the first shifting section T 1  through the third operational element N 3 . 
   In the second shifting section T 2 , the sixth operational element N 6  acts as a fixed element by the operation of the fourth brake B 4  while the reduced speed of the third operational element N 3  is input to the fifth operational element N 5 . Therefore, a first reverse speed line RS 1  is formed, and a negative reduced speed is output from the seventh operational element N 7 . That is, the final output speed of the gear train is at a height SR 1  of the first reverse speed line RS 1  at the position of the seventh operational element N 7 , and thus the first reverse speed is attained. 
   &lt;The Second Reverse Speed&gt; 
   At the second reverse speed, the first brake B 1  that operated at the first reverse speed is released, and the third brake B 3  is controlled to operate. 
   Then, in the first shifting section T 1 , the first operational element N 1  acts as a fixed element by the operation of the third brake B 3  while the fourth operational element N 4  receives an input speed. Therefore, a reduced speed is output from the first shifting section T 1  through the third operational element N 3 . 
   In the second shifting section T 2 , the sixth operational element N 6  acts as a fixed element by the operation of the fourth brake B 4  while the reduced speed of the third operational element N 3  is input to the fifth operational element N 5 . Therefore, a second reverse speed line RS 2  is formed, and a negative reduced speed is output from the seventh operational element N 7 . That is, the final output speed of the gear train is at a height SR 2  of the second reverse speed line RS 2  at the position of the seventh operational element N 7 , and thus the second reverse speed is attained. 
   Referring to  FIG. 4 , a gear train according to a second exemplary embodiment is mostly the same as the gear train according to the first exemplary embodiment. However, an eighth forward speed can be attained in the gear train of the second exemplary embodiment by an a fifth brake B 5  between the transmission housing H and the ring gear R 4  of the fourth planetary gear set PG 4 . 
   Shifting operation for the first to seventh forward speeds and the two reverse speeds according to a gear train of the present exemplary embodiment is the same as in the gear grain of the first exemplary embodiment. 
   As shown in  FIG. 5 , at the eighth forward speed, the first brake B 1  that operated at the seventh forward speed is released, and the fifth brake B 5  is controlled to operate. 
   Then, the first shifting section T 1  does not contribute to the speed change and can merely passively rotate, since none of the first and third brakes B 1  and B 3  and the second clutch C 2  operates. 
   In the second shifting section T 2 , the fifth operational element N 8  acts as a fixed element by the operation of the fifth brake B 5  while the sixth operational element N 6  (more specifically, the ring gear R 3  of the third planetary gear set PG 3 ) receives the engine speed from the input shaft IS via the first clutch C 1 . Therefore, an eighth forward speed line SP 8  (shown as a dotted line in  FIG. 3 ) is formed, and an increased speed is output from the seventh operational element N 7 . That is, the final output speed of the gear train is at a height D 8  of the eighth forward speed line SP 8  at the position of the seventh operational element N 7 , and thus the eighth forward speed is attained. 
   As described above, according to an exemplary embodiment of the present invention, seven or eight forward speeds and two reverse speeds are achieved by using four simple planetary gear sets in combination with three clutches and four or five brakes. According to such a gear train, power transmission performance of an automatic transmission may be improved, and fuel consumption may be reduced. While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.