Patent Publication Number: US-2020300347-A1

Title: Dual clutch transmission

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Korean Patent Application No. 10-2018-0089897 filed on Aug. 1, 2018, 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 a dual clutch transmission configured for simplifying constituent elements of a transmission to reduce an overall length of the transmission and to improve mountability of the transmission in a vehicle. 
     Description of Related Art 
     An automated manual transmission is automatically shifted by an actuator while driving a vehicle to provide similar driving comfort as an automatic transmission and can contribute to the improvement of fuel efficiency of the vehicle with the power delivery efficiency superior to the automatic transmission. 
     The Dual Clutch Transmission (DCT) can perform the shifting through the handover control for cross-controlling the two clutches during the shifting of the gear, preventing a reduction in the shift feeling caused by the power disconnection of the engine. 
     However, in the case of a conventional DCT structure, since a dual clutch and a plurality of external gears and a plurality of synchronizers are disposed on the same axle (an input shaft and output shaft), so that the overall length of the transmission is increased, there was a more unfavorable problem on the overall length when adding a drive motor. 
     The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and may not be taken as an acknowledgment 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 dual clutch transmission configured for simplifying constituent elements of a transmission to reduce an overall length of the transmission and to improve mountability of the transmission in a vehicle. 
     A dual clutch transmission according to an exemplary embodiment of the present inventions may include a first input shaft and a second input shaft to which a rotational power of an engine is selectively inputted through a first clutch and a second clutch to rotate; an idle shaft and a countershaft disposed in parallel with the first and second input shafts; an output shaft selectively connectable to the countershaft through a fourth clutch; a planetary gear set provided with three rotation elements of which a first rotation element is connected to a gear provided in the idle shaft so that a rotational power is inputted thereto, the rotational power of the engine is selectively inputted to a second rotation element through a third clutch and a third rotation element is connected to the output shaft; one or more first external gear sets, in which two or more external gears among of external gears provided in the first input shaft, the idle shaft and the countershaft are engaged in engagement state, selected by a connecting or disconnecting device; a second external gear set, in which two or more external gears among of an external gear connected to one rotation element of the rotation elements of the planetary gear set and external gears provided in the idle shaft and the countershaft are engaged in engagement state, selected by a connecting or disconnecting device; and a third external gear set, in which the external gears of the second input shaft and the idle shaft are engaged in engagement state, selected by a connecting or disconnecting device. 
     A brake connected between the countershaft and a transmission housing to selectively brake the countershaft, may be further included. 
     A drive motor connected to the first rotation element of the planetary gear set to provide a rotational power, may be further included. 
     A motor input shaft may be disposed in parallel with the first input shaft; the motor input shaft may be provided with a motor input gear; the drive motor may be directly connected to the motor input shaft to provide the rotational power to the motor input gear; and the motor input gear may be engaged with the external gear of the second external gear set directly connected to the first rotation element. 
     A drive motor directly connected to the first clutch and the second clutch to provide a rotation power and an engine clutch disposed between the engine and the drive motor to selectively transmit the rotational power of the engine, may be further included. 
     The planetary gear set may be a pair of single pinion planetary gear set of which the first, second, and third rotation elements are a sun gear, a planet carrier and a ring gear and disposed on the same axle with the first and second input shafts. 
     The planetary gear set may be disposed between the second external gear set and the third external gear set; and the sun gear may be directly connected to any one external gear of the second external gear set, the planet carrier may be directly connected to the third clutch and the ring gear may be directly connected to an external gear provided in the output shaft. 
     The first external gear set may include a 1-1 external gear set in which a 1-1 input gear provided in the first input shaft and a 1-1 counter gear provided in the countershaft are engaged in engagement state but a corresponding external gear mate is selected through a first synchronizer provided in the countershaft, and a first idle gear provided in the idle shaft and the 1-1 counter gear provided in countershaft are engaged in engagement state but a corresponding external gear mate is selected through a second synchronizer provided in the idle shaft; and a 1-2 external gear set in which a 1-2 input gear provided in the first input shaft and a 1-2 counter gear provided in the countershaft are engaged in engagement state but a corresponding external gear mate is selected through the first synchronizer provided in the countershaft. 
     In the second external gear set, a second input gear directly connected to the first rotation element of the planetary gear set and a second counter gear directly connected to the countershaft are engaged in engagement state, the second counter gear and the output shaft are disposed on the same axle, and the countershaft and the output shaft are selectively connectable to each other through a fourth clutch provided therebetween; and a second idle gear provided in the idle shaft and the second counter gear provided in the countershaft are engaged in engagement state and a corresponding external gear mate is selected through a second synchronizer provided in the idle shaft. 
     In the third external gear set, a third input gear provided in the second input shaft and a third idle gear provided in the idle shaft are engaged in engagement state, and a rotational power is transmitted to a corresponding external gear mate through the second clutch. 
     A gear ratio transmitted to an output element of the planetary gear set via the third external gear set and the second external gear set through the second clutch and a gear ratio transmitted to the output element of the planetary gear set through the third clutch may be equal to each other. 
     A dual clutch transmission according to another exemplary embodiment of the present invention may include a first input shaft and a second input shaft to which a rotational power of an engine is selectively inputted through a first clutch and a second clutch to rotate; a countershaft disposed in parallel with the first and second input shafts; an output shaft selectively connectable to the countershaft through a third clutch; a planetary gear set provided with three rotation elements of which a first rotation element is connected to the input shaft so that a rotational power is selectively inputted thereto, a second rotation element is connected to the second input shaft so that a rotational power is selectively inputted thereto and a third rotation element is connected to the output shaft; and an external gear set in which external gears provided in the first input shaft and the countershaft are engaged in engagement state. 
     A brake connected between the countershaft and a transmission housing to selectively brake the countershaft, may be further included. 
     In the external gear set, an input gear directly connected to the first input shaft and a counter gear directly connected to the countershaft are engaged in engagement state but the countershaft and the output shaft are disposed on the same axle, so that the countershaft and the output shaft are selectively connectable to each other through a third clutch disposed therebetween. 
     The present invention through the above-mentioned problem solving means configures a transmission by reducing the number of synchronizers, external gears and shafts compared to a general DCT structure using only external gears and reduces the overall length of the transmission by a drive motor not coaxially disposed with other constituent elements, so that cost and weight of the transmission is reduced, improving mountability of the transmission in 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  is a drawing showing a dual clutch transmission structure of nine forward speeds according to an exemplary embodiment of the present invention; 
         FIG. 2  is a shifting operation chart of the transmission shown in  FIG. 1 ; 
         FIG. 3  is a drawing of various exemplary embodiments structure of a hybrid dual clutch transmission of nine forward speeds according to an exemplary embodiment of the present invention; 
         FIG. 4  is a shifting operation chart of the transmission shown in  FIG. 3 ; 
         FIG. 5  is a drawing of various exemplary embodiments structure of a hybrid dual clutch transmission of nine forward speeds according to an exemplary embodiment of the present invention; 
         FIG. 6  and  FIG. 7  are shifting operation charts of the transmission shown in  FIG. 5 ; 
         FIG. 8  is a drawing of a dual clutch transmission structure of eight forward speeds according to an exemplary embodiment of the present invention; 
         FIG. 9  is a shifting operation chart of the transmission shown in  FIG. 8 ; 
         FIG. 10  is a drawing of a dual clutch transmission structure of seven forward speeds according to an exemplary embodiment of the present invention; 
         FIG. 11  is a shifting operation chart of the transmission shown in  FIG. 10 ; 
         FIG. 12  is a drawing of various exemplary embodiments structure of a dual clutch transmission of six forward speeds according to an exemplary embodiment of the present invention; 
         FIG. 13  is a shifting operation chart of the transmission shown in  FIG. 12 ; 
         FIG. 14  is a drawing of various exemplary embodiments structure of a dual clutch transmission of six forward speeds according to an exemplary embodiment of the present invention; 
         FIG. 15  is a shifting operation chart of the transmission shown in  FIG. 14 ; 
         FIG. 16  is a drawing of a dual clutch transmission structure of five forward speeds according to an exemplary embodiment of the present invention; 
         FIG. 17  is a shifting operation chart of the transmission shown in  FIG. 16 ; 
         FIG. 18  is a drawing of a dual clutch transmission structure of three forward speeds according to an exemplary embodiment of the present invention; 
         FIG. 19  is a shifting operation chart of the transmission shown in  FIG. 18 ; 
         FIG. 20  is a drawing of a dual clutch transmission structure of four forward speeds according to an exemplary embodiment of the present invention; and 
         FIG. 21  is a shifting operation chart of the transmission shown in  FIG. 20 . 
     
    
    
     It may 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 particularly intended application and use environment. 
     In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
     DETAILED DESCRIPTION 
     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 of the present invention, it will be understood that the present description is not intended to limit the invention(s) to those exemplary embodiments. On the other hand, the invention(s) is/are intended to cover not only the exemplary embodiments of the present invention, 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. 
     Exemplary embodiments of the present invention will be described in detail with reference to the attached drawing. 
     A dual clutch transmission of the present invention may be configured to include a first input shaft IP 1  and a second input shaft IP 2 , an idle shaft IDLE and a countershaft CNT, an output shaft OP, a planetary gear set PG and three external gear sets. 
     Explaining the present invention in detail referring to  FIG. 1  to  FIG. 2 , firstly, the first input shaft IP 1  and the second input shaft IP 2  selectively receives a torque of an engine E through a dual clutch. For example, a first clutch Cl, which connects or disconnects the first input shaft IP 1  and the engine E, may be positioned therebetween to be rotatable, and a second clutch C 2 , which connects or disconnects the second input shaft IP 2  and the engine E, may be positioned therebetween to be rotatable. 
     The first input shaft IP 1  and the second input shaft IP 2  may be disposed on a concentric axle, and the first input shaft IP 1  may be formed of a solid shaft and the second input shaft IP 2  may be formed of a hollow shaft so that the first input shaft IP 1  is inserted into the second input shaft IP 2 . 
     Furthermore, the idle shaft IDLE and the countershaft CNT may be disposed side by side on the side of the first input shaft IP 1  and the second input shaft IP 2 . 
     Additionally, the output shaft OP may be selectively connectable to countershaft CNT via the fourth clutch C 4 , and a final reduction gear externally engaged with a differential device OP together with an output gear OPG may be directly connected to the output shaft. 
     At the instant time, the output shaft OP and the countershaft CNT may be disposed in parallel on the same axle, and the output shaft OP may be disposed closer to the engine E than the countershaft CNT. 
     The planetary gear set PG may include three rotation elements of which a first rotation element may be connected to an external gear provided at the idle shaft IDLE so that a rotating power may be inputted, a rotational power of engine E may be selectively inputted to a second rotation element via the third clutch C 3 , and a third rotation element may be connected to the output shaft OP. 
     The planetary gear set PG may be a pair of single pinion planetary gear set of which the first, second, and third rotation elements include a sun gear S, a planet carrier CR and a ring gear R and may be disposed on the same axle together with the first and second input shafts. 
     Additionally, the first external gear set EG 1  may be provided with at least one, and at least two external gears among of the external gears provided at the first input shaft IP 1  and the idle shaft IDLE and the countershaft CNT may be engaged in the engagement state so that the corresponding external gear mate may be selected by a connecting or disconnecting device. 
     For example, the external gear provided at the first input shaft IP 1  and the external gear provided at the countershaft CNT may be engaged, and also, the external gear provided at the idle shaft IDLE and the external gear provided at the countershaft CNT may be engaged. 
     Herein, the connecting or disconnecting device to be described below may be an engagement element in which engagement and disengagement operations are selectively achieved such as a synchronizer, a wet clutch, and the like. 
     Next, in the second external gear set EG 2 , the external gear connected to one among of the rotation elements of the planetary gear set PG and at least two external gears among of the external gears provided at the idle shaft IDLE and the countershaft CNT may be engaged in engagement state, so that the corresponding external gear mate may be selected by the connecting or disconnecting device. 
     For example, one rotation element of the planetary gear set PG and the external gear provided at the countershaft CNT may be engaged, and also, the external gear provided at the idle shaft IDLE and the external gear provided at the countershaft CNT may be engaged. 
     Furthermore, in a third external gear set EG 3 , the external gears of the second input shaft IP 2  and the idle shaft IDLE may be engaged in engagement state, so that the corresponding external gear mate may be selected by the connecting or disconnecting device. 
     Furthermore, the present invention may be configured to include a brake B 1  connected to between the countershaft CNT and a transmission housing H to selectively brake the countershaft CNT. 
     That is, according to the above structure, nine forward speeds and one reverse speed are implemented by five connecting or disconnecting devices including a dual clutch, a pair of planetary gear sets, a synchronizer (two clutches, one brake, two synchronizers) and combination of the external gears, so that the number of a synchronizer, external gear and shaft may be reduced compared to a conventional DCT structure that only utilizes external gears, reducing the cost and weight of the transmission and improving mountability of the transmission in the vehicle. 
     Meanwhile, the present invention may additionally install a drive motor M. 
     As various exemplary embodiments disposing the drive motor M, the drive motor M may be configured to be connected to the first rotation element of the planetary gear set PG to provide a rotational power. 
     To explain referring to  FIG. 3  and  FIG. 4 , a motor input shaft IPM may be disposed side by side on the side of the first input shaft, and the motor input shaft IPM may be directly connected to the motor input gear MIG. Furthermore, the motor input shaft IPM may be directly connected to the drive motor M so that the motor input gear MIG is provided with a rotational power, and the motor input gear MIG may be engaged with the external gear of the second external gear set EG 2  directly connected to the first rotation element. 
     That is, the motor input gear MIG is engaged with the sun gear S of the planetary gear set PG to directly supply a rotational power of the drive motor M to the sun gear S, as shown in  FIG. 4 , so that the shifting stages of the nine forward speeds and one reverse speed may be formed in an engine mode and a parallel mode, and an e-CVT mode and an EV mode may be implemented. 
     Furthermore, it is possible to reduce constituent elements of the transmission and the overall length of the transmission by not disposing the drive motor M on the same axle with another constituent elements, improving mountability of the transmission in the vehicle. 
     Furthermore, as various exemplary embodiments disposing the drive motor M, to explain referring from  FIG. 5  to  FIG. 7 , the drive motor M may be directly connected to the first clutch Cl and the second clutch C 2  to supply a rotational power, and an engine clutch EC may be additionally disposed between the engine E and the drive motor M so that the rotational power of the engine E may be selectively transmitted. 
     That is, as shown in  FIG. 6  and  FIG. 7 , the drive motor M and the engine clutch EC are added to the structure implementing nine forward speeds and one reverse speed, so that a vehicle can travel by implementing the engine mode and the parallel mode as well as EV mode. 
     Meanwhile, referring to  FIG. 1 , the planetary gear set PG of the present invention may be disposed between the second external gear set EG 2  and the third external gear set EG 3 . At the instant time, the sun gear S may be directly connected to any one external gear of the second external gear set EG 2 , the planet carrier CR may be directly connected to the third clutch C 3 , and the ring gear R may be directly connected to the external gear provided at the output shaft OP. 
     For example, the sun gear S may be directly connected to a second input gear IG 2  forming the second external gear set EG 2 , the planet carrier CR may be configured so that a rotational power of the engine E may be inputted via the third clutch C 3 , and the ring gear R may be externally engaged with the output gear OPG directly connected to output shaft OP. 
     Furthermore, the first external gear set EG 1  according to an exemplary embodiment of the present invention may be divided into a 1-1 external gear set EG 1 - 1  and a 1-2 external gear set EG 1 - 2 . 
     First, in the 1-1 external gear set EG 1 - 1 , a 1-1 input gear IG 1 - 1  provided at the first input shaft IP 1  and a 1-1 counter gear CG 1 - 1  provided at the countershaft CNT may be engaged in engagement state, but the shifting stage may be formed by selecting the mate of the 1-1 input gear IG 1 - 1  and the 1-1 counter gear CG 1 - 1  as corresponding external gear mate through a first synchronizer SY 1  provided at the countershaft CNT. 
     Furthermore, in the 1-1 external gear set EG 1 - 1 , a first idle gear IDG 1  provided in the idle shaft IDLE and the 1-1 counter gear CG 1 - 1  provided in the countershaft CNT may be engaged in engagement state, so that the shifting stage may be formed by selecting the mate of the first idle gear IDG 1  and the 1-1 counter gear CG 1 - 1  as corresponding external gear mate through a synchronizer SY 2  provided in the idle shaft IDLE. 
     That is, in the 1-1 external gear set EG 1 - 1 , the 1-1 input gear IG 1 - 1  and the first idle gear IDG 1  may be engaged with the 1-1 counter gear CG 1 - 1 , respectively, to be the structure that three gears are externally engaged. 
     Next, in the 1-2 external gear set EG 1 - 2 , a 1-2 input gear IG 1 - 2  provided in the first input shaft IP 1  and a 1-2 counter gear CG 1 - 2  provided in the countershaft CNT may be engaged in engagement state, so that the shifting stage may be formed by selecting the mate of the 1-2 input gear IG 1 - 2  and the 1-2 counter gear CG 1 - 2  as corresponding external gear mate through the first synchronizer SY 1  provided in the countershaft CNT. 
     That is, in the 1-2 external gear set EG 1 - 2 , the 1-2 input gear IG 1 - 2  may be engaged with the 1-2 counter gear CG 1 - 2  so that two gears are externally engaged. 
     Next, in the second external gear set EG 2  according to an exemplary embodiment of the present invention, the second input gear IG 2  directly connected to the first rotation element of the planetary gear set PG and a second counter gear CG 2  directly connected to the countershaft CNT may be engaged in engagement state, and at the instant time, the second counter gear CG 2  and the output shaft OP may be disposed on the same axle, the countershaft CNT and the output shaft OP are selectively connectable to each other through the fourth clutch C 4  provided between the counter gear and the output shaft OP so that the corresponding shifting stage may be formed. 
     Furthermore, in the second external gear set EG 2 , a second idle gear IDG 2  provided in the idle shaft IDLE and the second counter gear CG 2  provided in countershaft CNT may be engaged in engagement state, so that the second idle gear IDG 2  and the second counter gear CG 2  as the corresponding external gear mate are selected through the second synchronizer SY 2  provided in the idle shaft IDLE to form the shifting stage. 
     That is, in the second external gear set EG 2 , the second input gear IG 2  and the second idle gear IDG 2  may be engaged with the second counter gear CG 2 , respectively, to be the structure that three gears are externally engaged. 
     Next, in the third external gear set EG 3  according to an exemplary embodiment of the present invention, a third input gear IG 3  provided in the second input shaft IP 2  and a third idle gear IDG 3  provided in the idle shaft IDLE may be engaged in engagement state, so that the rotational power of the engine E may be transmitted to the idle shaft IDLE via the third input gear IG 3  and the third idle gear IDG 3  as the corresponding external gear mate through the engagement operation of the second clutch C 2 . 
     That is, in the third external gear set EG 3 , the third input gear IG 3  may be engaged with the third idle gear IDG 3  to be the structure that two gears are externally engaged. 
     Furthermore, the gear ratio transmitted to the output element of the planetary gear set PG via the third external gear set EG 3  and the second external gear set EG 2  through the second clutch C 2  and the gear ratio transmitted to the output element of the planetary gear set PG through the third clutch C 3  may be equal to each other. 
     That is, to explain by referring to  FIG. 1  and  FIG. 2 , in a case of for example the sixth shifting stage, an engine driving torque is transmitted to the third external gear set EG 3  through the second clutch C 2  to rotate the idle shaft IDLE, which is transmitted to the second idle gear IDG 2  through the second synchronizer SY 2  to pass the second counter gear CG 2  and the second input gear IG 2  and then pass the sun gear S and the planet carrier CR of the planetary gear set PG to be outputted through the ring gear R. 
     With this, the engine driving torque is inputted to the planet carrier CR through the third clutch C 3  and then to be outputted through the ring gear R. In the two ways, the gear ratios transmitted to the ring gear R are equal to each other. 
     According to the above mentioned structure, the present invention can form a dual clutch transmission by reducing the number of synchronizers and external gears and shafts compared to a conventional DCT structure using only external gears, reducing cost and weight of the transmission, which allows the vehicle mountability of the transmission to be improved. 
     Meanwhile,  FIG. 8  shows the structure which removes the brake B 1  in the transmission shown in  FIG. 1 , and it can implement the transmission structure of eight forward speeds and one reverse speed as shown in  FIG. 9  by removing only the brake B 1 . 
     Furthermore, through some structure changes that remove some synchro gears or some external gears in the transmission structure shown in  FIG. 1 , it can implement the transmission structure of the seventh, sixth, fifth forward speed, easily changing the transmission structure. 
     For example,  FIG. 10  shows the structure which removes the 1-2 external gear set EG 1 - 2  and the first synchronizer SY 1  in the transmission shown in  FIG. 1 , and it can implement the transmission structure of seven forward speeds and one reverse speed as shown in  FIG. 11 . 
     Additionally,  FIG. 12  shows the structure which removes the 1-2 external gear set EG1-2 and the first synchronizer SY 1  and brake B 1  in the transmission shown in  FIG. 1 , and it can implement the transmission structure of six forward speeds and one reverse speed as shown in  FIG. 13 . 
     Also,  FIG. 14  shows the structure which removes the 1-2 external gear set EG 1 - 2 , the first synchronizer SY 1 , the first idle gear IDG 1  and the second synchronizer SY 2  in the transmission shown in  FIG. 1 , and directly connects the second idle gear IDG 2  with the idle shaft IDLE, and it can implement the transmission structure of six forward speeds as shown in  FIG. 15 . 
     Furthermore,  FIG. 16  shows the structure which removes the 1-2 external gear set EG 1 - 2 , the first synchronizer SY 1 , the first idle gear IDG 1 , the brake B 1  and the second synchronizer SY 2  in the transmission shown in  FIG. 1 , and directly connects the second idle gear IDG 2  with the idle shaft IDLE, and it can implement the transmission structure of five forward speeds as shown in  FIG. 17 . 
     Meanwhile, the present invention can implement a transmission of three forward speeds and four forward speeds, which is configured to include the first input shaft IP 1 , the second input shaft IP 2 , the countershaft CNT, the output shaft OP, the planetary gear set PG and the external gear set EG. 
     To explain referring to  FIG. 18  and  FIG. 19 , firstly, the first input shaft IP 1  and the second input shaft IP 2  may selectively receive a rotational power of the engine E through a dual clutch, for example, the first clutch Cl may be disposed between the first input shaft IP 1  and the engine E to be rotatable, which connects the first input shaft IP 1  with the engine E or disconnects the first input shaft IP 1  from the engine E, and the second clutch C 2  may be disposed between the second input shaft IP 2  and the engine E to be rotatable, which connects the second input shaft IP 2  with the engine E or disconnects the second input shaft IP 2  from the engine E. 
     The first input shaft IP 1  and the second input shaft IP 2  may be disposed on the same axle. That is, the first input shaft IP 1  becomes a solid shaft and the second input shaft IP 2  becomes a hollow shaft so that the first input shaft IP 1  is inserted into the second input shaft IP 2 . 
     Furthermore, the countershaft CNT may be disposed IP 2  side by side on the side of the first input shaft IP 1  and the second input shaft. 
     Furthermore, the output shaft OP may be selectively connectable to the countershaft CNT through the third clutch C 3 , and a final reduction gear externally engaged with a differential device may be directly connected to the output shaft OP together with the output gear OPG. 
     At the instant time, the output shaft OP and the countershaft CNT may be disposed side by side on the same axle, and the output shaft OP may be disposed closer to the engine E than the countershaft CNT. 
     The planetary gear set PG may include three rotation elements of which the first rotation element may be connected to the first input shaft IP 1  so that a rotational power of the engine E is selectively inputted thereto through the first clutch Cl, the second rotation element may be connected to the second input shaft IP 2  so that a rotational power of the engine E is selectively inputted thereto through the second clutch C 2 , and the third rotation element is directly connected to the output shaft OP. 
     Furthermore, in the external gear set EG, the external gear provided in the first input shaft IP 1  and the external gear provided in the countershaft CNT may be engaged in engagement state. 
     For example, the input gear IPG directly connected to the first input shaft IP 1  and the counter gear CG directly connected to the countershaft CNT may be engaged in engagement state, but the countershaft CNT and the output shaft OP may be disposed on the same axle and the countershaft CNT may be selectively connectable to the output shaft OP through the third clutch C 3  provided therebetween, through which the corresponding shifting stage may be formed. 
     That is, the present invention, as like  FIG. 18  and  FIG. 19 , implements three forward speeds through the combination of external gears together with a dual clutch, a pair of planetary gear set, one clutch, so that the number of synchronizers, external gears and shafts may be reduced, reducing cost and weight of the transmission, which allows mountability of the transmission in the vehicle to improve. 
     Furthermore, the present invention may be configured to further include the brake B 1  connected between the countershaft CNT and the transmission housing H to selectively brake the countershaft CNT. 
     That is,  FIG. 20  shows the structure of adding the brake B 1  to the transmission structure shown in  FIG. 18 , and it can Implement the transmission of four forward speeds as like  FIG. 21  by adding only the brake B 1  to  FIG. 18 , so that the constituent elements of the transmission may be reduced to reduce cost and weight of the transmission, improving mountability of the transmission in the vehicle. 
     For convenience in explanation and accurate definition in the appended claims, the terms “upper”, “lower”, “inner”, “outer”, “up”, “down”, “upper”, “lower”, “upwards”, “downwards”, “front”, “rear”, “back”, “inside”, “outside”, “inwardly”, “outwardly”, “internal”, “external”, “inner”, “outer”, “forwards”, and “backwards” 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 to explain certain principles of the invention and their practical application, to 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.