Abstract:
A multiple speed power transmission comprises: an input; an output; first and second input shafts releasably coupled to the input by first and second friction clutches, respectively; a countershaft disposed parallel to the first and second input shafts; an output pinion fixed to the countershaft; an output ring gear fixed to the output and meshing with the output pinion; an intermediate shaft disposed coaxially with the countershaft; a first input pinion fixed to the first input shaft and meshing with a first gear fixed to the intermediate shaft; a second input pinion fixed to the second input shaft; a second gear disposed coaxially with the intermediate shaft; an idler gear meshing with the second input pinion and the second gear; a first coupler for releasably coupling the first countershaft to the intermediate shaft; and a second coupler for releasably coupling the intermediate shaft to the second gear.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to automatic transmissions having a layshaft kinematic arrangement, particularly to automatic transmissions having dual input clutches, but no torque converter. 
   Dual clutch layshaft transmissions are essentially two automated manual transmissions in a common housing, one providing odd numbered gears and the other providing even numbered gears. Shifts between odd and even numbered gears can be accomplished without interrupting power flow. While operating in an odd numbered gear, couplers can be actuated to configure the transmission for the desired even numbered gear. Then, power is transferred to the even numbered gear by engaging the even clutch while disengaging the odd clutch in a coordinated fashion. 
   In a front wheel drive vehicle, the axial space available for the transmission is limited by the width of the engine compartment and the length of the engine. For this reason, dual clutch transmissions typically use at least two countershafts so that components can be placed side by side instead of along the main transmission axis. Some arrangements, such as those described in U.S. Pat. Nos. 7,044,014 and 7,077,025, utilize more than two countershafts to achieve very short lengths. 
   A well known method of reducing the length of a two countershaft transmission is having a single pinion on an input shaft drive gears on both countershafts. This reduces the overall length of the transmission by the face width of a gear. A disadvantage of this method is that it reduces the ability to adjust speed ratios by selecting the size of each gear, because a change in the size of the pinion forces a change in the size of both driven gears. This disadvantage is partially alleviated by the fact that there are two final drive ratios which can be adjusted separately to achieve the desired ratio spacing. However, when more than one input pinion is re-used for two gears, the number of degrees of freedom for adjusting ratios is less than the number of ratios. As a result, a designer is forced to accept some ratios that are larger or smaller than desired. 
   It is desirable to have the speed ratio for reverse be about the same as the speed ratio for first gear, since both are used to move the vehicle from rest. However, in a typical layshaft transmission, it is difficult to obtain a reverse speed ratio this high with a single idler gear. The size of the pinion is limited and the size of the driven gear cannot be as large as the driven gear for first because the teeth must clear the teeth on the pinion. As a result, a stepped pinion is sometimes used to provide an additional opportunity to multiply the torque. However, stepped pinions increase the required axial length. 
   BRIEF SUMMARY OF THE INVENTION 
   The claimed invention is a dual clutch transmission which is intended for applications which have limited axial space available. The transmission has two countershafts, each with a final drive pinion that meshes with a common final drive ring gear. The gears on one of the countershafts (fifth, sixth, and reverse) are arranged as a cluster in a way that creates a direction reversing power path between the two input shafts. Reverse is obtained via the even clutch, the direction reversing power path to the odd input shaft, and the first gear power path. The overall length is reduced by using re-using pinions for multiple ratios and by moving the gearing associated with reverse to a location that does not increase axial length. 
   An additional power path between the input shafts is available using the gearing for fifth gear and sixth gear. This power path, in combination with the second gear power path, creates an alternative first gear which is a small step shorter than second gear. An alternative reverse is also available by using the odd gear clutch, the direction reversing power path to the even gear input shaft, and the second gear power path. 
   One aspect of the present invention is a multiple speed power transmission comprising an input; an output; first and second input shafts releasably coupled to the input by first and second friction clutches, respectively; a countershaft disposed parallel to the first and second input shafts; an output pinion fixed to the countershaft; an output ring gear fixed to the output and meshing with the output pinion; an intermediate shaft disposed coaxially with the countershaft; a first input pinion fixed to the first input shaft and meshing with a first gear fixed to the intermediate shaft; a second input pinion fixed to the second input shaft; a second gear disposed coaxially with the intermediate shaft; an idler gear meshing with the second input pinion and the second gear; a first coupler for releasably coupling the first countershaft to the intermediate shaft; and a second coupler for releasably coupling the intermediate shaft to the second gear. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a dual clutch transmission according to an embodiment of the present invention which produces seven forward and two reverse speed ratios. 
       FIG. 2  is a table showing the proposed tooth numbers for the gears and pinions of the transmission illustrated in  FIG. 1 . 
       FIG. 3  is a table indicating the positions of the sleeves and state of the clutches and resulting speed ratio of the transmission in  FIG. 1  when the gears and pinions have the numbers of teeth indicated in  FIG. 2 . 
       FIG. 4  is a schematic diagram of a dual clutch transmission according to a second embodiment of the present invention which produces seven forward and two reverse speed ratios. 
       FIG. 5  is a table showing the proposed tooth numbers for the gears and pinions of the transmission illustrated in  FIG. 4 . 
       FIG. 6  is a table indicating the positions of the sleeves and state of the clutches and resulting speed ratio of the transmission in  FIG. 4  when the gears and pinions have the numbers of teeth indicated in  FIG. 5 . 
       FIG. 7  is a schematic diagram of a dual clutch transmission according to a third embodiment of the present invention which produces nine forward and two reverse speed ratios. 
       FIG. 8  is a table showing the proposed tooth numbers for the gears and pinions of the transmission illustrated in  FIG. 7 . 
       FIG. 9  is a table indicating the positions of the sleeves and state of the clutches and resulting speed ratio of the transmission in  FIG. 7  when the gears and pinions have the numbers of teeth indicated in  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A transmission according to a first embodiment of the present invention is illustrated in  FIG. 1 . A transmission input  10  is driven by the vehicle&#39;s engine. An odd clutch  22  releasably couples the transmission input to a solid input shaft  12 . An even clutch  24  releasably couples the transmission input to a hollow input shaft  14  which is concentric with the solid input shaft. Countershafts  16  and  18  are arranged parallel to the input shafts. Output pinions  50  and  52  are fixed to the countershafts and mesh with output ring gear  54 . The output ring gear is fixed to the carrier of the differential unit (not shown) which drives both half shafts and the front wheels of the vehicle. 
   Pinions  26  and  30  are fixed to solid input shaft  12 . Pinions  28  and  32  are fixed to hollow input shaft  14 . Gear  34  is supported for rotation on countershaft  18  and in continuous meshing engagement with pinion  26 . Gear  38  is supported for rotation on countershaft  18  and in continuous meshing engagement with pinion  30 . Gear  40  is supported for rotation on countershaft  18  and in continuous meshing engagement with pinion  32 . Gear  36  is supported for rotation on countershaft  18  and in continuous meshing engagement with pinion  28 . Hollow intermediate shaft  20  is supported for rotation on countershaft  16 . Gear  44  is fixed to intermediate shaft  20  and in continuous meshing engagement with pinion  30 . Gear  46  is supported for rotation on intermediate shaft  20  and in continuous meshing engagement with pinion  32 . Gear  42  is supported for rotation on intermediate shaft  20 . Idler gear  48  is in continuous meshing engagement with both pinion  28  and gear  42 . 
   Gears which are supported for rotation on a shaft are selectively connected to and disconnected from the shaft by a coupler. These couplers are preferably synchronizers as used in manual transmissions which first match the speeds of the elements and then engage dog teeth. The couplers are actuated by moving a sleeve. Coupler  56  engages gear  34  with countershaft  18  whenever sleeve  70  is moved to the left. Coupler  60  engages gear  38  with countershaft  18  whenever sleeve  70  is moved to the right. Coupler  62  engages gear  40  with countershaft  18  whenever sleeve  72  is moved to the left. Coupler  58  engages gear  36  with countershaft  18  whenever sleeve  72  is moved to the right. Coupler  64  engages gear  44  and intermediate shaft  20  with countershaft  16  whenever sleeve  74  is moved to the right. Coupler  66  engages gear  46  with intermediate shaft  20  whenever sleeve  76  is moved to the left. Coupler  68  engages gear  42  with intermediate shaft  20  whenever sleeve  76  is moved to the right. Moving a sleeve to an intermediate position disengages both couplers with which it is associated. 
   The transmission is prepared to start the vehicle from stationary in the forward direction by moving sleeve  70  to the left to couple gear  34  to countershaft  18  and moving sleeve  72  to the right to couple gear  36  to countershaft  18 . Other sleeves must be in the neutral position. Then, clutch  22  is gradually engaged. Power flows from the transmission input through clutch  22  to input shaft  12 , pinion  26 , gear  34 , coupler  56 , countershaft  18 , output pinion  52 , and output ring gear  54 . When the gears and pinions have the number of teeth shown in  FIG. 2 , the ratio of transmission input speed to front wheel speed will be 19.098. 
   When sufficient vehicle speed has been achieved, the transmission is shifted into second gear by progressively releasing clutch  22  while progressively engaging clutch  24 . Power flows from the transmission input through clutch  24  to input shaft  14 , pinion  28 , gear  36 , coupler  58 , countershaft  18 , output pinion  52 , and output ring gear  54 . When the gears and pinions have the number of teeth shown in  FIG. 2 , the overall speed ratio will be 10.602 at the completion of the this shift. 
   The transmission is prepared for the shift into third gear by moving sleeve  70  to the right, thus disengaging gear  34  from countershaft  18  and coupling gear  38  to countershaft  18 . This action may be performed at any time after the shift into second is completed. The shift is completed by progressively releasing clutch  24  while progressively engaging clutch  22 . Power flows from the transmission input through clutch  22  to input shaft  12 , pinion  30 , gear  38 , coupler  60 , countershaft  18 , output pinion  52 , and output ring gear  54 . When the gears and pinions have the number of teeth shown in  FIG. 2 , the overall speed ratio will be 6.483 at the completion of the this shift. 
   The transmission is prepared for the shift into fourth gear by moving sleeve  72  to the left, thus disengaging gear  36  from countershaft  18  and coupling gear  40  to countershaft  18 . This action may be performed at any time after the shift into third is completed. The shift is completed by progressively releasing clutch  22  while progressively engaging clutch  24 . Power flows from the transmission input through clutch  24  to input shaft  14 , pinion  32 , gear  40 , coupler  62 , countershaft  18 , output pinion  52 , and output ring gear  54 . When the gears and pinions have the number of teeth shown in  FIG. 2 , the overall speed ratio will be 4.830 at the completion of the this shift. 
   The transmission is prepared for the shift into fifth gear by moving sleeve  70  to an intermediate position, thus disengaging gear  38  from countershaft  18  and moving sleeve  74  to the right, thus coupling gear  44  to countershaft  16 . This action may be performed at any time after the shift into fourth is completed. The shift is completed by progressively releasing clutch  24  while progressively engaging clutch  22 . Power flows from the transmission input through clutch  22  to input shaft  12 , pinion  30 , gear  44 , coupler  64 , countershaft  16 , output pinion  50 , and output ring gear  54 . When the gears and pinions have the number of teeth shown in  FIG. 2 , the overall speed ratio will be 3.583 at the completion of the this shift. 
   The transmission is prepared for the shift into sixth gear by moving sleeve  72  to an intermediate position, thus disengaging gear  40  from countershaft  18  and moving sleeve  76  to the left, thus coupling gear  46  to intermediate shaft  20 . This action may be performed at any time after the shift into fifth is completed. The shift is completed by progressively releasing clutch  22  while progressively engaging clutch  24 . When the gears and pinions have the number of teeth shown in  FIG. 2 , the overall speed ratio will be 2.669 at the completion of the this shift. In sixth gear, power flows from the transmission input, through clutch  24  to input shaft  14 , pinion  32 , gear  46 , coupler  66 , intermediate shaft  20 , coupler  64 , countershaft  16 , output pinion  50 , and output ring gear  54 . Sleeve  74  must remain in the right position while the transmission is operated in sixth gear. 
   An alternative first gear ratio is available which provides a smaller speed ratio and a smaller ratio step to second gear. This ratio would be preferable in situations in which the vehicle is lightly loaded because the smaller ratio step enables a more comfortable shift into second gear and the high speed ratio of the regular first gear would not be necessary. The transmission is prepared to start the vehicle from stationary using this alternate first gear by moving sleeve  76  to the left to couple gear  46  to intermediate shaft  20  and moving sleeve  72  to the right to couple gear  36  to countershaft  18 . Other sleeves must be in the neutral position. Then, clutch  22  is gradually engaged. Power flows from the transmission input through clutch  22  to input shaft  12 , pinion  30 , gear  44 , intermediate shaft  20 , coupler  66 , gear  46 , pinion  32 , input shaft  14 , pinion  28 , gear  36 , coupler  58 , countershaft  18 , output pinion  52 , and output ring gear  54 . When the gears and pinions have the number of teeth shown in  FIG. 2 , the ratio of transmission input speed to front wheel speed will be 14.230. The shift from this alternate first gear ratio to the second forward ratio is accomplished by progressively releasing clutch  22  while progressively engaging clutch  24 . Sleeve  76  must be moved to its intermediate position between the completion of the shift into second and preparing the transmission for a shift into third. Operation in higher gears is as described above. 
   The transmission is prepared to start the vehicle from stationary in the reverse direction by moving sleeve  70  to the left to couple gear  34  to countershaft  18  and moving sleeve  76  to the right to couple gear  42  to intermediate shaft  20 . Other sleeves must be in the neutral position. Then, clutch  24  is gradually engaged. Power flows from the transmission input through clutch  24  to input shaft  14 , pinion  28 , idler gear  48 , gear  42 , coupler  68 , intermediate shaft  20 , gear  44 , pinion  30 , input shaft  12 , pinion  26 , gear  34 , coupler  56 , countershaft  18 , output pinion  52 , and output ring gear  54 . When the gears and pinions have the number of teeth shown in  FIG. 2 , the ratio of transmission input speed to front wheel speed will be 19.433. 
   An alternative reverse gear ratio is available. The transmission is prepared to start the vehicle from stationary using this alternate reverse gear by moving sleeve  76  to the right to couple gear  42  to intermediate shaft  20  and moving sleeve  72  to the right to couple gear  36  to countershaft  18 . Other sleeves must be in the neutral position. Then, clutch  22  is gradually engaged. Power flows from the transmission input through clutch  22  to input shaft  12 , pinion  30 , gear  44 , intermediate shaft  20 , coupler  68 , gear  42 , idler gear  48 , pinion  28 , gear  36 , coupler  58 , countershaft  18 , output pinion  52 , and output ring gear  54 . When the gears and pinions have the number of teeth shown in  FIG. 2 , the ratio of transmission input speed to front wheel speed will be 10.419. 
   In applications that do not require wide ratio span, pinion  26 , gear  34 , and coupler  56  could be eliminated, producing an even shorter embodiment. The alternate first gear ratio would be used in place of regular first gear and the alternate reverse gear ratio would be used in place of regular reverse gear. The number of teeth on gear  42  could be adjusted to obtain a more favorable reverse speed ratio. 
     FIG. 4  illustrates an alternate embodiment of the invention. One difference between this embodiment and the embodiment illustrated in  FIG. 1  is the addition of pinion  78 , which is fixed to input shaft  12  and meshes with gear  38 . This removes the ratio redundancy between third and fifth gears and enables a more favorable set of ratio steps. This change will increase the overall length unless sleeve  70  is narrow enough to fit beside pinion  30  and gear  44 . A second difference is that reverse idler  48  has been eliminated and gear  42  now meshes with gear  36 , such that gear  36  accomplishes the role of a reverse idler gear. This decreases the cost but also decreases the freedom to select the reverse gear ratio by adjusting tooth counts. These two variations could be practiced independently of one another. The operation of the embodiment of  FIG. 4  is similar to the operation of the embodiment of  FIG. 1  which is described above. 
     FIG. 7  illustrates a third embodiment that obtains two additional forward speed ratios. It is derived from the embodiment illustrated in  FIG. 1  by adding pinion  80  fixed to input shaft  12 , gear  82  supported for rotation on countershaft  16 , and coupler  84  which engages gear  82  with countershaft  16  whenever sleeve  74  is moved to the left. These parts could be added to the embodiment of  FIG. 4  with similar results. 
   The transmission of  FIG. 7  operates in a similar manner to the transmission of  FIG. 1  up through sixth gear. The shift from sixth gear to seventh gear is accomplished by releasing clutch  24 , moving sleeve  74  to the left to disengage gear  44  from countershaft  16  and engage gear  82  to countershaft  16 , and then engaging clutch  22 . This shift, unlike the shifts described above, requires the interruption of power to the wheels. Sleeve  76  should be maintained in the left position. A shift from seventh to eighth is accomplished by progressively releasing clutch  22  while progressively engaging clutch  24 . 
   In accordance with the provisions of the patent statutes, three preferred embodiment have been described. However, it should be noted that alternate embodiments can be practiced otherwise than as specifically illustrated and described.