Abstract:
A multiple speed power transmission is disclosed having an input, first and second input shafts, first and second clutches releasably coupling the input to the first and second input shafts, respectively, and a set of selectable power paths connecting the first and second input shafts to the output, the set of selectable power paths having at least one power path that includes both the first and second input shafts.

Description:
FIELD 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. 
   BACKGROUND OF THE INVENTION 
   Automatic transmissions for transmitting power between an input and an output, either over a continuously variable range of speed ratios or in discrete step changes among speed ratios, have associated with them several sources of parasitic losses, which adversely affect fuel economy. These losses are associated with a torque converter, open hydraulic friction clutches and brakes, hydraulic pump, and gear meshes. 
   To improve fuel economy in a motor vehicle having an automatic transmission, an automated shift manual (ASM) transmission can be used to eliminate or substantially reduce all of these parasitic losses except gear mesh losses. An ASM transmission generally performs gear ratio changes by first interrupting torque transmitted from the engine to the transmission input, preparing the transmission components associated with the next speed ratio, and then restoring torque at the input. A primary functional feature of ASM transmissions is the need to interrupt power transmitted from the engine to the transmission input shaft before or during each gear ratio change. 
   Dual clutch layshaft transmissions are essentially two ASM transmissions, one providing odd numbered gears and one providing even numbered gears. Shifts between odd numbered gears 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 next even numbered gear. Dual clutch transmissions have parasitic losses only slightly higher than ASM transmissions. 
   When a motor vehicle is accelerated from rest, the mechanical power generated by the engine exceeds the power utilized by the vehicle. The transmission must dissipate the difference, generally as heat. Open torque converters are very efficient at converting the excess mechanical power into heat in the working fluid. Friction clutches, as used in ASM and dual clutch transmissions, are limited in the rate at which they can dissipate the excess power. The amount of energy that must be dissipated is determined by the torque level, the speed difference across the clutch, and the duration of the event. 
   The most effective way to limit the power that must be dissipated by the clutch is to provide additional torque multiplication in the gearbox. This has two benefits. First, it reduces the torque which the clutch must transmit. Second, it reduces the duration of the event because the gearbox input will become equal to the engine speed at a lower vehicle speed. The need for similar top gear ratios, which is dictated by cruising fuel economy, is unchanged, so the resulting gearbox must have substantially more total span. The difference between adjacent gear ratios is limited by the ability to make comfortable shifts. As a result, it is also necessary to increase the number of discrete gear ratios. 
   Traditionally, one reverse ratio has been considered sufficient, since speed is relatively low and fuel efficiency in reverse is not a significant concern. However, if the gear multiplication is high enough to satisfy clutch thermal considerations, it may be excessive for normal reverse driving, even at those relatively low speeds. Therefore, it is beneficial to provide a reverse ratio similar to the traditional reverse ratio in addition to one that has much more multiplication. 
   One known way to increase the gear multiplication is to increase ratio of the tooth counts for individual gear pairs. This would require increasing the distance between shafts due to limitations on how small the gears can be relative to the shaft diameter. Adding an additional forward and reverse ratio would ordinarily require at least four additional gears and an additional synchronizer sleeve. The resulting transmission would be much larger and likely would not fit into the package space available. 
   SUMMARY OF THE INVENTION 
   To meet the needs of the industry and address the shortcomings of prior transmissions, the present invention provides a multiple speed power transmission having an input, first and second input shafts, first and second clutches releasably coupling the input to the first and second input shafts, respectively, and a set of selectable power paths connecting the first and second input shafts to the output, said set of selectable power paths including at least one power path which includes both the first and second input shafts. 
   A transmission according to this invention may be configured similarly to a dual clutch transmission with modest span. However, a selectable torque path between the two input shafts is added such that, when this path is activated, the input shaft associated with even gears rotates slower than the input shaft associated with odd gears by a pre-determined ratio. This torque path requires a new synchronizer, but may re-use gearing that was already present. Depending on the layout of an original gearbox, it is often possible to combine this new synchronizer with an existing synchronizer to form a three position sleeve (connecting a shaft to either of two gears or neither). 
   First gear is engaged by activating the new synchronizer in combination with the second gear synchronizer and the odd gear clutch. If the existing reverse ratio is driven by the even gear input shaft, then an extra low reverse ratio is also created. This low reverse is engaged by activating the new synchronizer in combination with the reverse synchronizer and the odd gear clutch. In fact, there is an additional ratio created below every even numbered ratio in the original transmission. However, only the ratios below first and reverse provide utility. 
   In a similar manner, new ratios are created above each odd numbered ratio. These ratios are engaged by activating the new synchronizer in combination with the corresponding odd gear synchronizer and the even gear clutch. Of these, only the ratio higher than the highest odd numbered ratio provides utility. For example, there would be a ratio available higher than the fifth gear ratio. The step size from fifth to this new ratio is the same as the step size between first and second, which is too large to utilize as sixth gear. However, if a traditional sixth gear is present, this new ratio is usable as a seventh gear. This new gear ratio utilizes the same clutch as sixth gear ratio, so the final upshift must be accomplished with a torque interruption like an ASM. 
   In total, a five forward speed single reverse gearbox with modest span can be transformed into a gearbox with seven forward speeds, two reverse speeds, and very large span. Similarly, a four forward speed single reverse gearbox with modest span can be transformed into a gearbox with five forward speeds, two reverse speeds, and very large span. 
   Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram showing of a five forward speed front wheel drive transaxle according to the present invention; 
       FIG. 2  is a chart containing a preferred number of teeth for each of the gears of the transaxle of  FIG. 1 ; 
       FIG. 3  is a chart containing the speed ratios between the input and output and steps between the speed ratios for each of the forward and reverse speeds of the transaxle of  FIG. 1 , the gears having the number of teeth shown in  FIG. 2 ; 
       FIG. 4  is a schematic diagram showing of a seven forward speed, two reverse speed front wheel drive transaxle according to the present invention; 
       FIG. 5  is a chart containing a preferred number of teeth for each of the gears of the transmission of  FIG. 4 ; 
       FIG. 6  is a chart containing the speed ratios between the input and output and steps between the speed ratios for each of the forward and reverse speeds of the transmission of  FIG. 4 , the gears having the number of teeth shown in  FIG. 5 ; 
       FIG. 7  is a schematic diagram showing of a seven forward speed, two reverse speed rear wheel drive transmission according to the present invention; 
       FIG. 8  is a chart containing a preferred number of teeth for each of the gears of the transmission of  FIG. 7 ; and 
       FIG. 9  is a chart containing the speed ratios between the input and output and steps between the speed ratios for each of the forward and reverse speeds of the transmission of  FIG. 7 , the gears having the number of teeth shown in  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a transaxle according to the present invention includes an input  10  for driveably connecting a power source, such as an internal combustion engine or electric motor, to the transmission, and an output  36  for driving a load, such as the driven wheels of a motor vehicle, through a powertrain that may include a drive shaft, differential mechanism, and axle shafts. A first friction clutch  20 , consisting of a clutch housing and a clutch disc  22 , alternately connects and disconnects a first input shaft  14  as clutch  20  is engaged and disengaged, respectively. A second friction clutch  16 , consisting of a clutch housing and a clutch disc  18 , connects and disconnects a second input shaft  12  as clutch  16  is engaged and disengaged, respectively. 
   A first layshaft  26  supports a first output pinion  30 , which is secured to layshaft  26  in continuous meshing engagement with an output ring gear  34 , secured to output  36 . A second layshaft  24  supports a second output pinion  32 , which is secured to the layshaft  24  in continuous meshing engagement with output ring gear  34 . 
   The first input shaft  14  supports two pinions  50  and  52  which are secured to shaft  14 . The second input shaft  12  supports one pinion  48  which is secured to shaft  12  and two pinions  44  and  46  which may rotate about shaft  12 . Gear  42  is supported on layshaft  26  for rotation relative to layshaft  26 , and in continuous meshing engagement with pinion  52 . Auxiliary shaft  28  is a hollow shaft supported on layshaft  26  for rotation relative to layshaft  26 . The auxiliary shaft  28  supports gears  38  and  40  which are secured to shaft  28  and in continuous meshing engagement with pinions  46  and  50  respectively. Gear  54  is secured to layshaft  24  and in continuous meshing engagement with pinion  44 . Gears  56  and  58  are supported on layshaft  24  for rotation relative to layshaft  24  and in continuous meshing engagement with pinion  48  and gear  42 , respectively. 
   Couplers  60 ,  62 , and  64  are preferably synchronizers of the type used in automotive manual transmissions to connect a gear or pinion to a shaft, after synchronizing the speed of the shaft and that of the pinion or gear. Each coupler may also disconnect the shaft and the associated pinion or gear. Alternatively, each coupler may be a dog clutch having teeth that are engaged with dog teeth on a gear or pinion. This invention may use couplers in any combination of synchronizers and dog clutches. Each coupler is composed of a hub secured to the shaft and a sleeve which is supported on the hub for sliding movement leftward or rightward into engagement with dog teeth on the adjacent gear or pinion. In the case where a coupler is a synchronizer, it is provided with a conical surface, which engages mutually with a corresponding conical surface located on the gear or pinion. When the synchronizer is engaging either of its adjacent gears, these conical surfaces are forced together into frictional contact, and that frictional engagement synchronizes the speed of the gear to that of the shaft before the dog teeth engage. Other types of sychronizers or couplers, now know or later invented, may also be used. 
   Coupler  60  connects second input shaft  12  to pinion  44 , pinion  46 , or disconnects it from both. Coupler  62  connects layshaft  26  to gear  40 , gear  42 , or disconnects it from both. Coupler  64  connects layshaft  24  to gear  56 , gear  58 , or disconnects it from both. 
   Engaging coupler  60  to pinion  46  activates a power path between the first and second input shaft comprising pinion  50 , gear  40 , auxiliary shaft  28 , gear  38 , pinion  46 , and coupler  60 . 
   To accelerate the vehicle using the first forward speed, the transmission is configured with coupler  60  engaging pinion  46  and coupler  62  engaging gear  42 . Then, clutch  16  is engaged. The power path for this speed comprises input  10 , clutch  16 , input shaft  12 , coupler  60 , pinion  46 , gear  38 , auxiliary shaft  28 , gear  40 , pinion  50 , input shaft  14 , pinion  52 , gear  42 , coupler  62 , layshaft  26 , output pinion  30 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  12  through clutch  16 . Shaft  12  is driveably connected to pinion  46  through coupler  60 . Pinion  46  drives gear  38 , auxiliary shaft  28 , gear  40 , pinion  50 , shaft  14 , pinion  52 , and gear  42 . Gear  42  is driveably connected to layshaft  26  through coupler  62 . Pinion  30  is secured to layshaft  26  and drives ring gear  34  and output  36 . 
   To shift from the first forward speed to the second forward speed, clutch  20  is progressively engaged while clutch  16  is progressively released. Following the shift, coupler  60  may be moved to the neutral position, but in any event must be moved to the neutral position before the next odd-to-even upshift. The power path for this speed comprises input  10 , clutch  20 , input shaft  14 , pinion  52 , gear  42 , coupler  62 , layshaft  26 , output pinion  30 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  14  and pinion  52  through clutch  20 . Pinion  52  drives gear  42 , which is driveably connected to shaft  26  through coupler  62 . Pinion  30  is secured to shaft  26  and drives ring gear  34  and output  36 . 
   To shift from the second forward speed to the third forward speed, the transmission is configured by displacing coupler  64  to engage gear  56 , then clutch  16  is progressively engaged while clutch  20  is progressively released. Following the shift, coupler  62  may be moved to the neutral position. The power path for this speed comprises input  10 , clutch  16 , input shaft  12 , pinion  48 , gear  56 , coupler  64 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  12  and pinion  48  through clutch  16 . Pinion  48  drives gear  56 , which is driveably connected to shaft  24  through coupler  64 . Pinion  32  is secured to shaft  24  and drives ring gear  34  and output  36 . 
   To shift from the third forward speed to the fourth forward speed, the transmission is configured by displacing coupler  62  to engage gear  40 , then clutch  20  is progressively engaged while clutch  16  is progressively released. Following the shift, coupler  64  may be moved to the neutral position. The power path for this speed comprises input  10 , clutch  20 , input shaft  14 , pinion  50 , gear  40 , coupler  62 , layshaft  26 , output pinion  30 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  14  and pinion  50  through clutch  20 . Pinion  50  drives gear  40 , which is driveably connected to shaft  26  through coupler  62 . Pinion  30  is secured to shaft  26  and drives ring gear  34  and output  36 . 
   To shift from the fourth forward speed to the fifth forward speed, the transmission is configured by displacing coupler  60  to engage pinion  44 , then clutch  16  is progressively engaged while clutch  20  is progressively released. Following the shift, coupler  62  may be moved to the neutral position. The power path for this speed comprises input  10 , clutch  16 , input shaft  12 , coupler  60 , pinion  44 , gear  54 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  12  through clutch  16 . Shaft  12  is driveably connected to pinion  44  through coupler  60 . Pinion  44  drives gear  54 , shaft  24 , pinion  30 , ring gear  34 , and output  36 . 
   Downshifts are accomplished by reversing the steps of the corresponding upshift. 
   To accelerate the vehicle in reverse, the transmission is configured with coupler  60  engaging pinion  46  and coupler  64  engaging gear  58 . Then, clutch  16  is engaged. The power path for this speed comprises input  10 , clutch  16 , input shaft  12 , coupler  60 , pinion  46 , gear  38 , auxiliary shaft  28 , gear  40 , pinion  50 , input shaft  14 , pinion  52 , gear  42 , gear  58 , coupler  64 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  12  through clutch  16 . Shaft  12  is driveably connected to pinion  46  through coupler  60 . Pinion  46  drives gear  38 , auxiliary shaft  28 , gear  40 , pinion  50 , shaft  14 , pinion  52 , gear  42 , and gear  58 . Gear  58  is driveably connected to layshaft  24  through coupler  64 . Pinion  32  is secured to layshaft  24  and drives ring gear  34  and output  36 . 
   A shift may be accomplished in reverse by progressively engaging clutch  20  while progressively releasing clutch  16 . The power path for this speed comprises input  10 , clutch  20 , input shaft  14 , pinion  52 , gear  42 , gear  58 , coupler  64 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . Following the shift, input  10  is driveably connected to shaft  14  and pinion  52  through clutch  20 . Pinion  52  drives gear  42  and gear  58 , which is driveably connected to shaft  24  through coupler  64 . Pinion  32  is secured to shaft  24  and drives ring gear  34  and output  36 . 
   A chart containing a preferred number of teeth for each of the gears of the transaxle of  FIG. 1  is shown in  FIG. 2 , while  FIG. 3  is a chart containing the speed ratios between the input and output and steps between the speed ratios for each of the forward and reverse speeds of the transaxle of  FIG. 1 . 
   Referring now to  FIG. 4 , a transaxle according to the present invention includes an input  10  for driveably connecting a power source, such as an internal combustion engine or electric motor, to the transmission, and an output  36  for driving a load, such as the driven wheels of a motor vehicle, through a powertrain that may include a drive shaft, differential mechanism, and axle shafts. A first friction clutch  16 , consisting of a clutch housing and a clutch disc  18 , alternately connects and disconnects a first input shaft  12  as clutch  16  is engaged and disengaged, respectively. A second friction clutch  20 , consisting of a clutch housing and a clutch disc  22 , connects and disconnects a second input shaft  14  as clutch  20  is engaged and disengaged, respectively. 
   A first layshaft  26  supports a first output pinion  30 , which is secured to layshaft  26  in continuous meshing engagement with an output ring gear  34 , secured to output  36 . A second layshaft  24  supports a second output pinion  32 , which is secured to the layshaft in continuous meshing engagement with output ring gear  34 . 
   The second input shaft  14  supports two pinions  82  and  84  which are secured to shaft  14 . The first input shaft  12  supports three pinions  76 ,  78 , and  80  which are secured to shaft  12 . Gears  86 ,  88 ,  90 , and  92  are supported on layshaft  24  for rotation relative to layshaft  24  and in continuous meshing engagement with pinions  76 ,  78 ,  82 , and  84  respectively. Gear  70  is supported on layshaft  26  for rotation relative to layshaft  26 , and in continuous meshing engagement with gear  86 . Auxiliary shaft  28  is a hollow shaft supported on layshaft  26  for rotation relative to layshaft  26 . Auxiliary shaft  28  supports gear  72  which is secured to shaft  28  and in continuous meshing engagement with pinion  80 . Gear  74  is supported on shaft  28  for rotation relative to shaft  28  and in continuous meshing engagement with pinion  84 . 
   Coupler  94  connects layshaft  26  to gear  70 , gear  72 , or disconnects it from both. Coupler  96  connects or disconnects auxiliary shaft  28  to gear  74 . Coupler  98  connects layshaft  24  to gear  86 , gear  88 , or disconnects it from both. Coupler  100  connects layshaft  24  to gear  90 , gear  92 , or disconnects it from both. 
   Engaging coupler  96  to gear  74  activates a power path between the first and second input shaft comprising pinion  84 , gear  74 , coupler  96 , auxiliary shaft  28 , gear  72 , and pinion  80 . 
   To accelerate the vehicle using the first forward speed, the transmission is configured with coupler  96  engaging gear  74  and coupler  98  engaging gear  86 . Then, clutch  20  is engaged. The power path for this speed comprises input  10 , clutch  20 , input shaft  14 , pinion  84 , gear  74 , coupler  96 , auxiliary shaft  28 , gear  72 , pinion  80 , input shaft  12 , pinion  76 , gear  86 , coupler  98 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  14  and pinion  84  through clutch  20 . Pinion  84  drives gear  74 , which is driveably connected to auxiliary shaft  28  through coupler  96 . Auxiliary shaft  28  drives gear  72 , pinion  80 , shaft  12 , pinion  76 , and gear  86 . Gear  86  is driveably connected to layshaft  24  through coupler  98 . Pinion  32  is secured to layshaft  24  and drives ring gear  34  and output  36 . 
   To shift from the first forward speed to the second forward speed, clutch  16  is progressively engaged while clutch  20  is progressively released. Following the shift, coupler  96  may be moved to the neutral position. The power path for this speed comprises input  10 , clutch  16 , input shaft  12 , pinion  76 , gear  86 , coupler  98 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  12  and pinion  76  through clutch  16 . Pinion  76  drives gear  86 , which is driveably connected to shaft  24  through coupler  98 . Pinion  32  is secured to shaft  24  and drives ring gear  34  and output  36 . 
   To shift from the second forward speed to the third forward speed, the transmission is configured by displacing coupler  100  to engage gear  92 , then clutch  20  is progressively engaged while clutch  16  is progressively released. Following the shift, coupler  98  may be moved to the neutral position. The power path for this speed comprises input  10 , clutch  20 , input shaft  14 , pinion  84 , gear  92 , coupler  100 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  14  and pinion  84  through clutch  20 . Pinion  84  drives gear  92 , which is driveably connected to shaft  24  through coupler  100 . Pinion  32  is secured to shaft  24  and drives ring gear  34  and output  36 . 
   To shift from the third forward speed to the fourth forward speed, the transmission is configured by displacing coupler  94  to engage gear  72 , then clutch  16  is progressively engaged while clutch  20  is progressively released. Following the shift, coupler  100  may be moved to the neutral position. The power path for this speed comprises input  10 , clutch  16 , input shaft  12 , pinion  80 , gear  72 , coupler  94 , layshaft  26 , output pinion  30 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  12  and pinion  80  through clutch  16 . Pinion  80  drives gear  72 , which is driveably connected to shaft  26  through coupler  94 . Pinion  30  is secured to shaft  26  and drives ring gear  34  and output  36 . 
   To shift from the fourth forward speed to the fifth forward speed, the transmission is configured by displacing coupler  100  to engage gear  90 , then clutch  20  is progressively engaged while clutch  16  is progressively released. Following the shift, coupler  94  may be moved to the neutral position. The power path for this speed comprises input  10 , clutch  20 , input shaft  14 , pinion  82 , gear  90 , coupler  100 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  14  and pinion  82  through clutch  20 . Pinion  82  drives gear  90 , which is driveably connected to shaft  24  through coupler  100 . Pinion  32  is secured to shaft  24  and drives ring gear  34  and output  36 . 
   To shift from the fifth forward speed to the sixth forward speed, the transmission is configured by displacing coupler  98  to engage gear  88 , then clutch  16  is progressively engaged while clutch  20  is progressively released. Following the shift, coupler  100  may be moved to the neutral position. The power path for this speed comprises input  10 , clutch  16 , input shaft  12 , pinion  78 , gear  88 , coupler  98 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  12  and pinion  78  through clutch  16 . Pinion  78  drives gear  88 , which is driveably connected to shaft  24  through coupler  98 . Pinion  32  is secured to shaft  24  and drives ring gear  34  and output  36 . 
   An upshift from the sixth forward speed to the seventh forward speed, unlike all other single step shifts, requires a torque break, i.e., the torsional connection between the input  10  and output  36  is briefly interrupted by disengaging clutch  16  while the state of the couplers are changed. This is mitigated because the 6-7 upshift is never made at high throttle; instead, it usually occurs as a result of the driver reducing power demand when reaching cruising speed. While both clutches are disengaged, coupler  98  is moved to the neutral position, coupler  96  is displaced to engage gear  74 , and coupler  100  is displaced to engage gear  90 . Then, clutch  16  is re-engaged. The power path for this speed comprises input  10 , clutch  16 , input shaft  12 , pinion  80 , gear  72 , auxiliary shaft  28 , coupler  96 , gear  74 , pinion  84 , input shaft  14 , pinion  82 , gear  90 , coupler  100 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . When clutch  16  is re-engaged, input  10  is driveably connected to shaft  12  and pinion  80  through clutch  16 . Pinion  80  drives gear  72  and auxiliary shaft  28 , which is driveably connected to gear  74  through coupler  96 . Gear  74  drives pinion  84 , shaft  14 , pinion  82 , and gear  90 , which is driveably connected to shaft  24  through coupler  100 . Pinion  32  is secured to shaft  24  and drives ring gear  34  and output  36 . 
   Downshifts are accomplished by reversing the steps of the corresponding upshift. 
   To accelerate the vehicle in reverse, the transmission is configured with coupler  96  engaging gear  74  and coupler  94  engaging gear  70 . Then, clutch  20  is engaged. The power path for this speed comprises input  10 , clutch  20 , input shaft  14 , pinion  84 , gear  74 , coupler  96 , auxiliary shaft  28 , gear  72 , pinion  80 , input shaft  12 , pinion  76 , gear  86 , gear  70 , coupler  94 , layshaft  26 , output pinion  30 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  14  and pinion  84  through clutch  20 . Pinion  84  drives gear  74 , which is driveably connected to auxiliary shaft  28  through coupler  96 . Auxiliary shaft  28  drives gear  72 , pinion  80 , shaft  12 , pinion  76 , gear  86 , and gear  70 . Gear  70  is driveably connected to layshaft  26  through coupler  94 . Pinion  30  is secured to layshaft  26  and drives ring gear  34  and output  36 . 
   A shift may be accomplished in reverse by progressively engaging clutch  16  while progressively releasing clutch  20 . The power path for this speed comprises input  10 , clutch  16 , input shaft  12 , pinion  76 , gear  86 , gear  70 , coupler  94 , layshaft  26 , output pinion  30 , output gear  34 , and output  36 . Following the shift, input  10  is driveably connected to shaft  12  and pinion  76  through clutch  16 . Pinion  76  drives gear  86  and gear  70 , which is driveably connected to shaft  26  through coupler  94 . Pinion  30  is secured to shaft  26  and drives ring gear  34  and output  36 . 
   A chart containing a preferred number of teeth for each of the gears of the transaxle of  FIG. 4  is shown in  FIG. 5 , while  FIG. 6  is a chart containing the speed ratios between the input and output and steps between the speed ratios for each of the forward and reverse speeds of the transaxle of  FIG. 4 , the gears having the number of teeth shown in  FIG. 5 . 
   Referring to  FIG. 7 , a rear wheel drive transmission according to the present invention includes an input  10  for driveably connecting a power source, such as an internal combustion engine or electric motor, to the transmission, and an output  36 , which is coaxial with input  10 , for driving a load, such as the driven wheels of a motor vehicle, through a powertrain that may include a drive shaft, differential mechanism, and axle shafts. A first friction clutch  20 , consisting of a clutch housing and a clutch disc  22 , alternately connects and disconnects a first input shaft  14  as clutch  20  is engaged and disengaged, respectively. A second friction clutch  16 , consisting of a clutch housing and a clutch disc  18 , connects and disconnects a second input shaft  12  as clutch  16  is engaged and disengaged, respectively. 
   A first layshaft  26  supports a first output pinion  30 , which is secured to layshaft  26  in continuous meshing engagement with an output gear  34 , secured to output  36 . A second layshaft  24  supports a second output pinion  32 , which is secured to the layshaft in continuous meshing engagement with output gear  34 . 
   The first input shaft  14  supports three pinions  112 ,  114 , and  116  which are secured to shaft  14 . The second input shaft  12  supports two pinions  108  and  110  which may rotate about shaft  12 . Gear  106  is supported on layshaft  26  for rotation relative to layshaft  26 , and in continuous meshing engagement with pinion  116 . Auxiliary shaft  28  is a hollow shaft supported on layshaft  26  for rotation relative to layshaft  26 . The auxiliary shaft  28  supports gears  102  and  104  which are secured to shaft  28  and in continuous meshing engagement with pinions  110  and  112  respectively. Gear  118  is secured to layshaft  24  and in continuous meshing engagement with pinion  108 . Gears  120  and  122  are supported on layshaft  24  for rotation relative to layshaft  24  and in continuous meshing engagement with pinion  114  and gear  106 , respectively. 
   Coupler  124  connects layshaft  26  to gear  104 , gear  106 , or disconnects it from both. Coupler  126  connects input shaft  12  to output gear  34 , pinion  108 , or disconnects it from both. Coupler  128  connects or disconnects shaft  12  to pinion  110 . Coupler  130  connects layshaft  24  to gear  120 , gear  122 , or disconnects it from both. 
   Engaging coupler  128  to pinion  110  activates a power path between the first and second input shaft comprising pinion  112 , gear  104 , auxiliary shaft  28 , gear  102 , pinion  110 , and coupler  128 . 
   To accelerate the vehicle using the first forward speed, the transmission is configured with coupler  128  engaging pinion  110  and coupler  124  engaging gear  106 . Then, clutch  16  is engaged. The power path for this speed comprises input  10 , clutch  16 , input shaft  12 , coupler  128 , pinion  110 , gear  102 , gear  104 , pinion  112 , input shaft  14 , pinion  116 , gear  106 , coupler  124 , layshaft  26 , output pinion  30 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  12  through clutch  16 . Shaft  12  is driveably connected to pinion  110  through coupler  128 . Pinion  110  drives gear  102 , auxiliary shaft  28 , gear  104 , pinion  112 , shaft  14 , pinion  116 , and gear  106 . Gear  106  is driveably connected to layshaft  26  through coupler  124 . Pinion  30  is secured to layshaft  26  and drives output gear  34  and output  36 . 
   To shift from the first forward speed to the second forward speed, clutch  20  is progressively engaged while clutch  16  is progressively released. Following the shift, coupler  128  may be moved to the neutral position. The power path for this speed comprises input  10 , clutch  20 , input shaft  14 , pinion  116 , gear  106 , coupler  124 , layshaft  26 , output pinion  30 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  14  and pinion  116  through clutch  20 . Pinion  116  drives gear  106 , which is driveably connected to shaft  26  through coupler  124 . Pinion  30  is secured to shaft  26  and drives output gear  34  and output  36 . 
   To shift from the second forward speed to the third forward speed, the transmission is configured by displacing coupler  126  to engage gear  108 , then clutch  16  is progressively engaged while clutch  20  is progressively released. Following the shift, coupler  124  may be moved to the neutral position. The power path for this speed comprises input  10 , clutch  16 , input shaft  12 , coupler  126 , pinion  108 , gear  118 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  12  through clutch  16 . Shaft  12  is driveably connected to pinion  108  through coupler  126 . Pinion  108  drives gear  118 , shaft  24 , pinion  32 , output gear  34 , and output  36 . 
   To shift from the third forward speed to the fourth forward speed, the transmission is configured by displacing coupler  130  to engage gear  120 , then clutch  20  is progressively engaged while clutch  16  is progressively released. Following the shift, coupler  126  may be moved to the neutral position. The power path for this speed comprises input  10 , clutch  20 , input shaft  14 , pinion  114 , gear  120 , coupler  130 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  14  and pinion  114  through clutch  20 . Pinion  114  drives gear  120 , which is driveably connected to shaft  24  through coupler  130 . Pinion  32  is secured to shaft  24  and drives output gear  34  and output  36 . 
   To shift from the fourth forward speed to the fifth forward speed, the transmission is configured by displacing coupler  126  to engage output gear  34 , then clutch  16  is progressively engaged while clutch  20  is progressively released. Following the shift, coupler  130  may be moved to the neutral position. The power path for this speed comprises input  10 , clutch  16 , input shaft  12 , coupler  126 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  12  through clutch  16 . Shaft  12  is driveably connected to output gear  34  and output  36  through coupler  126 . The fifth forward speed is direct drive. 
   To shift from the fifth forward speed to the sixth forward speed, the transmission is configured by displacing coupler  124  to engage gear  104 , then clutch  20  is progressively engaged while clutch  16  is progressively released. Following the shift, coupler  126  may be moved to the neutral position. The power path for this speed comprises input  10 , clutch  20 , input shaft  14 , pinion  112 , gear  104 , coupler  124 , layshaft  26 , output pinion  30 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  14  and pinion  112  through clutch  20 . Pinion  112  drives gear  104 , which is driveably connected to shaft  26  through coupler  124 . Pinion  30  is secured to shaft  26  and drives output gear  34  and output  36 . 
   An upshift from the sixth forward speed to the seventh forward speed, unlike all other single step shifts, requires a torque break, i.e., the torsional connection between the input  10  and output  36  is briefly interrupted by disengaging clutch  20  while the state of the couplers are changed. This is mitigated because the  6 - 7  upshift is never made at high throttle; instead, it usually occurs as a result of the driver reducing power demand when reaching cruising speed. While both clutches are disengaged, coupler  124  is moved to the neutral position, coupler  128  is displaced to engage pinion  110 , and coupler  126  is displaced to engage output gear  34 . Then clutch  20  is re-engaged. The power path for this speed comprises input  10 , clutch  20 , input shaft  14 , pinion  112 , gear  104 , auxiliary shaft  28 , gear  102 , pinion  110 , coupler  128 , input shaft  12 , coupler  126 , output gear  34 , and output  36 . When clutch  20  is re-engaged, input  10  is driveably connected to shaft  14  and pinion  112  through clutch  20 . Pinion  112  drives gear  104 , auxiliary shaft  28 , gear  102 , and pinion  110 . Pinion  110  is driveably connected to shaft  12  through coupler  128 . Shaft  12  is driveably connected to output gear  34  and output  36  through coupler  126 . 
   Downshifts are accomplished by reversing the steps of the corresponding upshift. 
   To accelerate the vehicle in reverse, the transmission is configured with coupler  128  engaging pinion  110  and coupler  130  engaging gear  122 . Then, clutch  16  is engaged. The power path for this speed comprises input  10 , clutch  16 , input shaft  12 , coupler  128 , pinion  110 , gear  102 , gear  104 , pinion  112 , input shaft  14 , pinion  116 , gear  106 , gear  122 , coupler  130 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . Input  10  is driveably connected to shaft  12  through clutch  16 . Shaft  12  is driveably connected to pinion  110  through coupler  128 . Pinion  110  drives gear  102 , auxiliary shaft  28 , gear  104 , pinion  112 , shaft  14 , pinion  116 , gear  106 , and gear  122 . Gear  122  is driveably connected to layshaft  24  through coupler  130 . Pinion  32  is secured to layshaft  24  and drives output gear  34  and output  36 . 
   A shift may be accomplished in reverse by progressively engaging clutch  20  while progressively releasing clutch  16 . The power path for this speed comprises input  10 , clutch  20 , input shaft  14 , pinion  116 , gear  106 , gear  122 , coupler  130 , layshaft  24 , output pinion  32 , output gear  34 , and output  36 . Following the shift, input  10  is driveably connected to shaft  14  and pinion  116  through clutch  20 . Pinion  116  drives gear  106  and gear  122 , which is driveably connected to shaft  24  through coupler  130 . Pinion  32  is secured to shaft  24  and drives ring gear  34  and output  36 . 
   For purposes of this description of the invention and the claims, use of the words “driveably connected”, “driving engagement”, or derivatives of such is meant to include either a direct drive relationship between interacting components, or an indirect drive relationship between identified components that have other components therebetween, including but not limited to gears, pinions, shafts, couplers and clutches. As seen in  FIGS. 1 ,  4  and  7 , the layshafts  24 ,  26 , and auxiliary shaft  28 , are non-coaxial with the input shafts  12 ,  14 . 
   In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.