CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority of Korean Patent Application Number 10-2011-0095035 filed Sep. 21, 2011, the entire contents of which application is incorporated herein for all purposes by this reference. 
     BACKGROUND OF INVENTION 
     1. Field of Invention 
     The present invention relates, in general, to a transmission for a vehicle, and, more particularly, to a structure of a transmission for a vehicle, capable of realizing forward 11-speed and reverse 3-speed gear ratios. 
     2. Description of Related Art 
     Vehicles are equipped with a transmission so as to able to supply power generated from a power source such as an engine to drive wheels at the proper rotational speed and torque converted depending on a driving condition. 
     Such a transmission generally provides a plurality of gear ratios from a maximum gear ratio for meeting maximum hill-climbing capability to a minimum gear ratio for obtaining the highest speed of the vehicle. In theory, the greater the number of gear ratios, the more advantageous it is for securing the optimum driving point of the engine, so that it is possible to contribute to improving the fuel efficiency of the vehicle. 
     For reference, Korean Unexamined Patent Application Publication No. 10-2010-0097706, discloses a structure of a transmission capable of realizing forward 10-speed and reverse 2-speed gear ratios. 
     The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art. 
     SUMMARY OF INVENTION 
     Various aspects of the present invention provide for a transmission for a vehicle, which employs as few parts as possible and provides forward 11-speed and reverse 1-speed gear ratios along with a relatively simple configuration, thereby allowing the state of operation of an engine connected thereto to be the optimal state. Thus, the transmission makes it possible to improve fuel efficiency and to secure smooth driving performance of the vehicle. 
     Various aspects of the present invention provide for a transmission for a vehicle, which includes a first compound planetary gear train in which a first planetary gear train and a second planetary gear train are coupled to each other, a second compound planetary gear train in which a third planetary gear train and a fourth planetary gear train are coupled to each other and at least one rotational element thereof is connected with a rotational element of the first compound planetary gear train, an input shaft which is connected to rotational elements of the first compound planetary gear train by at least two paths and is connected to rotational elements of the second compound planetary gear train by at least two paths, an output shaft which is connected to one rotational element of the second compound planetary gear train, first and second brakes, each of which locks the rotational elements of the first compound planetary gear train, a third brake which locks the rotational element of the second compound planetary gear train, and a first clutch which selectively connects the rotational elements of the first and second planetary gear trains to each other. 
     According to the present invention, the transmission employs as few parts as possible to provide forward 11-speed and reverse 1-speed gear ratios while having a relatively simple configuration, thereby allowing the state of operation of an engine connected thereto to be the optimal state. Thus, the transmission makes it possible to improve fuel efficiency and to secure smooth driving performance of the vehicle. 
     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  shows a structure of an exemplary transmission for a vehicle according to the present invention. 
         FIG. 2  shows an operational chart of operating elements of the transmission of  FIG. 1 . 
         FIG. 3  is a lever diagram explaining the operation of the transmission of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts. 
     While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
     Referring to  FIGS. 1 to 3 , a transmission for a vehicle according to various embodiments of the present invention includes a first compound planetary gear train CG 1  in which a first planetary gear train PG 1  and a second planetary gear train PG 2  are coupled to each other, a second compound planetary gear train CG 2  in which a third planetary gear train PG 3  and a fourth planetary gear train PG 4  are coupled to each other and at least one rotational element thereof is connected with a rotational element of the first compound planetary gear train CG 1 , an input shaft IN which is connected to rotational elements of the first compound planetary gear train CG 1  through at least two paths and is connected to rotational elements of the second compound planetary gear train CG 2  through at least two paths, an output shaft OUT which is connected to one rotational element of the second compound planetary gear train CG 2 , first and second brakes B 1  and B 2 , each of which locks the rotational elements of the first compound planetary gear train CG 1 , a third brake B 3  which locks the rotational element of the second compound planetary gear train CG 2 , and a first clutch CL 1  which selectively connects the rotational elements of the first and second planetary gear trains PG 1  and PG 2  to each other. 
     The input shaft IN is permanently connected to a first rotational element of the second planetary gear train PG 2 . The input shaft IN is selectively connected to a first rotational element of the third planetary gear train PG 3  and a first rotational element of the fourth planetary gear train PG 4  via a second clutch CL 2 . The input shaft IN is selectively connected to a second rotational element of the third planetary gear train PG 3  via a third clutch CL 3 . The input shaft IN is selectively connected to a first rotational element of the first planetary gear train PG 1  via a fourth clutch CL 4 . 
     Further, the output shaft OUT is connected to a second rotational element of the fourth planetary gear train PG 4 . A third rotational element of the first planetary gear train PG 1  is permanently connected to the first rotational element of the third planetary gear train PG 3 . The first clutch CL 1  is installed to selectively connect the third rotational element of the first planetary gear train PG 1  and a second rotational element of the second planetary gear train PG 2 . 
     The first compound planetary gear train CG 1  is configured to permanently connect the second rotational element of the first planetary gear train PG 1  and the third rotational element of the second planetary gear train PG 2  and to selectively connect the third rotational element of the first planetary gear train PG 1  and the second rotational element of the second planetary gear train PG 2  via the first clutch CL 1 . 
     The second compound planetary gear train CG 2  is configured to permanently connect the first rotational element of the third planetary gear train PG 3  and the first rotational element of the fourth planetary gear train PG 4  and to permanently connect the second rotational element of the third planetary gear train PG 3  and the third rotational element of the fourth planetary gear train PG 4 . 
     Herein, the first, second, and third rotational elements of the first planetary gear train PG 1  are sequentially referred to as a first sun gear S 1 , a first carrier C 1 , and a first ring gear R 1 . The first, second, and third rotational elements of the second planetary gear train PG 2  are sequentially referred to as a second sun gear S 2 , a second carrier C 2 , and a second ring gear R 2 . The first, second, and third rotational elements of the third planetary gear train PG 3  are sequentially referred to as a third sun gear S 3 , a third carrier C 3 , and a third ring gear R 3 . The first, second, and third rotational elements of the fourth planetary gear train PG 4  are sequentially referred to as a fourth sun gear S 4 , a fourth carrier C 4 , and a fourth ring gear R 4 . 
     Thus, in the first compound planetary gear train CG 1 , the first carrier C 1  is permanently connected to the second ring gear R 2 , and the first ring gear R 1  is selectively connected to the second carrier C 2  via the first clutch CL 1 . When the selective connection is made by the first clutch CL 1 , the first compound planetary gear train CG 1  is configured as a CR-CR type compound planetary gear train. 
     Further, in the second compound planetary gear train CG 2 , the third sun gear S 3  is permanently connected to a fourth sun gear S 4 , and the third carrier C 3  is permanently connected to the fourth ring gear R 4 . Thereby, the second compound planetary gear train CG 2  is configured as a Simpson type compound planetary gear train. 
     Further, the input shaft IN is permanently connected to the second sun gear S 2 , is selectively connected to a connector between the third sun gear S 3  and the fourth sun gear S 4  via the second clutch CL 2 , is selectively connected to a connector between the third carrier C 3  and the fourth ring gear R 4  via the third clutch CL 3 , and is selectively connected to the first sun gear S 1  via the fourth clutch CL 4 . 
     The output shaft OUT is connected to the fourth carrier C 4 , and the third ring gear R 3  is permanently connected to the first ring gear R 1 . 
     The first brake B 1  is installed so as to be able to selectively lock the connector between the first carrier C 1  and the second ring gear R 2 . The second brake B 2  is installed so as to be able to selectively lock the first sun gear S 1 . The third brake B 3  is installed so as to be able to selectively lock the connector between the third carrier C 3  and the fourth ring gear R 4 . 
     Further, to prevent the connector between the first carrier C 1  and the second ring gear R 2  from rotating in the reverse direction, a one-way clutch F 1  is provided. 
     In the transmission for a vehicle configured as mentioned above according to various aspects of the present invention, the operation of the first to fourth clutches CL 1  to CL 4  and the first to third brakes B 1  to B 3  takes place as shown in the operational chart of  FIG. 2 . Thereby, the configuration of the transmission for a vehicle that was described above realizes forward 1-speed to 11-speed gear ratios and reverse 3-speed gear ratios. This operation is shown in a lever diagram of  FIG. 3   
     As shown on the left side of  FIG. 3 , the first compound planetary gear train CG 1  realizes two forward reduction ratios, one reverse reduction ratio, one step-up ratio, one 1:1 ratio, and one zero ratio. The second compound planetary gear train CG 2  to which power shifted by this gear ratios realizes forward 11-speed and reverse 3-speed gear ratios as shown in the right side of  FIG. 3 . 
     For convenience in explanation and accurate definition in the appended claims, the terms left and right, and etc. are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures. 
     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.
A transmission for a vehicle employs parts as few as possible to provide forward 11-speed and reverse 3-speed gear ratios along with relatively simple configuration, thereby allowing the operational state of an engine connected thereto to achieve an optimal state. Thus, the transmission makes it possible to improve fuel efficiency and to secure smooth driving performance of the vehicle.