Abstract:
A method for controlling gear shifts in a multiple speed power transmission for a vehicle that includes a first clutch for transmitting power through a first power path producing a reverse gear, and a second clutch for transmitting power through a second power path producing a forward gear. The method includes selecting a reverse range in which the transmission is to operate, preparing the transmission to transmit power alternately through the first power path and second power path; engaging the first clutch and transmitting power through the first power path in the reverse gear, selecting a forward range in which the transmission is to operate, disengaging the first clutch and engaging the second clutch, and transmitting power through the second power path in the forward gear.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation-in-part of the U.S. patent application Ser. No. 11/083,250, filed Mar. 17, 2005 now U.S. Pat. No. 7,246,536. 

   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 transmissions, are limited in the rate at which they can dissipate the excess power. The amount 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 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 to 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. 
   In a layshaft transmission, gears connected to a drive path moving sleeves on a coupler, such as a synchronizer. In a dual clutch transmission (DCT), one or two gears may be selected at any time, provided each gear is associated with a different input clutch. The clutch associated with the gear being selected or deselected must be disengaged while the coupler sleeve is moved. 
   In a DCT that includes a clutch coupler, operation in the lowest forward gear, first gear, requires that two synchronizers to be engaged: a second gear coupler and the clutch coupler. Similarly, operation in the lowest reverse gear, R1 gear, requires that a R2 coupler and the clutch coupler be engaged. 
   The sequence in which these couplers are engaged greatly influences the magnitude of coupler torque required. For example, if second gear is engaged first, the second gear coupler must accelerate only one clutch disc, and it has a moderate torque ratio to that clutch disc. On the other hand, if the clutch coupler is engaged first, subsequent engagement of the second gear coupler requires accelerating both clutch discs concurrently while overcoming a much larger torque ratio. This is particularly troublesome if the clutches are very cold, which causes them to have high drag. 
   There is a need in the automotive industry for a gear shift control strategy that ensures engagement of the clutch coupler is performed last. 
   Rock cycling the vehicle by moving the gear selector between the R-range and D-range is commonly used to move the wheels from snow, ice or mud. Preferably, the gear shift control strategy would only switch clutches and not move any coupler sleeves to switch from a forward ratio to a reverse ratio during rock cycling operation. 
   SUMMARY OF THE INVENTION 
   To meet the needs of the industry and to address the shortcomings of prior transmission gear shift controls, a method has been developed for controlling gear shifts in a multiple speed power transmission for a vehicle that includes a first clutch for transmitting power through a first power path producing a reverse gear, and a second clutch for transmitting power through a second power path producing a forward gear. The method includes selecting a reverse range in which the transmission is to operate, preparing the transmission to transmit power alternately through the first power path and second power path; engaging the first clutch and transmitting power through the first power path in the reverse gear, selecting a forward range in which the transmission is to operate, disengaging the first clutch and engaging the second clutch, and transmitting power through the second power path in the forward gear. 
   A transmission that may be controlled by the gear shift strategy 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 of them). 
   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 useable 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 a five forward speed front wheel drive transaxle; 
       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 a seven forward speed, two reverse speed front wheel drive transaxle; 
       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 ; and 
       FIG. 7  is a state transition diagram illustrating the gear shift control strategy. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a transaxle 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 engagements 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  15  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. Couplers may be 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 synchronizers 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  60  is the clutch coupler. 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  5   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 , and 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 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. 
   Coupler  96  is the clutch coupler. 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 reengaged. 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 . 
     FIG. 7  illustrates the gear shift control strategy with a state transition diagram. The boxes represent control states. Text in the boxes indicates the clutches that would be used to provide requested torque in that state. The arrows indicate conditions that trigger changes in state. 
   Referring to  FIG. 7 , the steps for controlling gear shifts of a dual clutch transmission, such as the transaxle illustrated in  FIG. 4 , begins at step  102  with the power source transmitting power to the input  10  and the transaxle prepared for operation in Park or Neutral or in R2 gear, wherein coupler  94  connects gear  70  and layshaft  26  or in third gear, wherein coupler  100  connects gear  92  and layshaft  24 ; and with clutches  16 ,  20  disengaged. If the gear selector is in the Park or P-range position, the output  36  is also held against rotation by a parking pawl device (not illustrated). 
   When the gear selector is moved by the vehicle operator from Park or Neutral to the Drive or D-range position, control passes to step  104  where R2 gear is disengaged by moving coupler  94  to its neutral position thereby disconnecting gear  70  from layshaft  26 , and second gear is engaged by coupler  98  connecting gear  86  and layshaft  24 . At step  104  with the transaxle so prepared, clutch  16  is engaged, the control responds to displacement of the accelerator pedal, and the vehicle is launched, i.e., accelerated from a stopped condition, in second gear with third gear preselected in anticipation of a 2-3 upshift. 
   If, however, the accelerator pedal is depressed more than a reference displacement, indicating the operator desires faster vehicle speed, or if the length of the period during which second gear is engaged exceeds a reference length, indicating that clutch  16  may be slipping excessively long, control passes to step  106  where third gear is disengaged by moving coupler  100  to its neutral position thereby disconnecting gear  92  from layshaft  24 , and first gear is engaged by causing coupler  96  to connect gear  74  and layshaft  28 . 
   With the transaxle so prepared, clutches  16  and  20  are used together to produce a blended vehicle launch in first gear and second gear by varying the torque capacity of clutches  16  and  20  during the vehicle launch. Preferably in the earliest portion of the vehicle launch, the torque capacity of clutch  20  is greater than the torque capacity of clutch  16 , and in the latter portion of the vehicle launch the torque capacity of clutch  16  is greater than that of clutch  20 . In this way, the 1-2 vehicle launch begins in first gear and ends in second gear with clutch  16  fully engaged and clutch  20  fully disengaged. The blended forward vehicle launch prevents excessive wear of clutch  16  and responds to the driver&#39;s demand for a speedier launch. 
   If, at step  106  during a blended 1-2 vehicle launch, the operator moves the gear selector to the Reverse or R-range position, the transaxle&#39;s gear shift control transitions quickly to and through step  110 , where neutral is produced by disengaging first gear coupler  96  and second gear coupler  86 . Thereafter, control passes promptly to step  112 , where the transaxle is prepared for operation in reverse drive, specifically in the R2 gear with coupler  94  connecting gear  70  and layshaft  26 . With the transaxle so prepared, clutch  16  is engaged and the vehicle accelerates in response to accelerator pedal position in the R2 gear. Alternatively, a reverse vehicle launch in first and second gears upon bleeding operation of clutches  16  and  20  occurs at step  116 , as described below. 
   If, at the initial step  102  with the source transmitting power to the input  10  and the transaxle prepared for operation in Park, R2 gear and third gear and clutches  16 ,  20  disengaged, the operator moves the gear selector to the Reverse or R-range position, control passes to step  114  where R2 gear remains engaged due to its coupler  94  connecting gear  70  and layshaft  26 , and third gear remains engaged due to its coupler  100  connecting gear  92  and layshaft  24 . With the transaxle so prepared, clutch  16  is engaged, the control responds to displacement of the accelerator pedal, and the vehicle is launched in the R2 gear with third gear preselected in anticipation of a R2-3 shift. 
   If, however, the accelerator pedal is depressed more than a reference displacement, indicating the operator desires faster reverse vehicle speed, or if the length of the period during which R2 gear and clutch  16  are engaged exceeds a reference length, indicating that clutch  16  may be slipping excessively long, control passes to step  116  where R2 gear remains engaged, third gear is disengaged by moving coupler  100  to its neutral position, and R1 gear is engaged by causing coupler  96  to connect gear  74  and layshaft  28 . With the transaxle so prepared at step  116 , clutches  16  and  20  are used together to produce a blended reverse vehicle launch in R1 gear and R2 gear by varying the torque capacity of clutches  16  and  20  during the reverse vehicle launch. Preferably in the earliest portion of the reverse vehicle launch, the torque capacity of clutch  20  is greater than the torque capacity of clutch  16 , and in the latter portion of the reverse vehicle launch the torque capacity of clutch  16  is greater than that of clutch  20 . In this way, the R1-R2 launch begins in R1 gear and ends in R2 gear with clutch  16  fully engaged and clutch  20  fully disengaged. The blended reverse vehicle launch prevents wear of clutch  16  and responds to the driver&#39;s demand for a speedier launch. 
   Movement of the gear selector between the D-range position and the R-range position during a forward or reverse vehicle launch indicates that the vehicle is being rock cycled, which is usually performed to free the wheels from ice, snow, mud, sand, or another material that prevent adequate wheel traction on the drive surface. 
   Whether the reverse launch occurs at step  114  or step  116 , if during the reverse launch, the operator moves the gear selector to the Drive or D-range position, control passes to step  118 , where the R2 gear coupler  94  remains engaged with gear  70 , R1 gear is disengaged, and third gear is engaged by causing coupler  100  to connect gear  92  and layshaft  24 . With the transaxle so prepared at step  118 , clutch  20  is engaged, and the control responds to accelerator pedal position and accelerates the vehicle in third gear. 
   If, with the control operating in step  118 , the gear selector is moved to the R-range within a reference period length, thereby indicating that the rock cycling is not completed, the control passes to step  114 , where the vehicle is accelerated in the R2 gear, as described above with reference to step  114 . 
   If, with the control operating in step  118 , the gear selector remains in the D-position longer than a reference period length, or the accelerator pedal is depressed greater than a reference magnitude, control passes from step  118  to step  104  for a vehicle launch in second gear, as described above with reference to step  104 . 
   If, with the control operating in step  104 , the gear selector is not moved to the R-range within a reference period length, thereby indicating that the rock cycling is completed, the control can pass to step  106 , where clutches  16  and  20  are used together to produce a blended vehicle launch in first gear and second gear by varying the torque capacity of clutches  16  and  20  during the vehicle launch. 
   If, with the control operating in step  104 , the gear selector is moved to the R-range, thereby indicating that the rock cycling is not completed, control passes to step  114 , the transaxle is prepared for operation in R2 gear and third gear, clutch  16  is engaged and the vehicle is accelerated in R2 gear in response to depressing the accelerator pedal. 
   Whether the vehicle launch occurs at step  118 , step  104 , or step  106 , when the vehicle launch is completed, control passes to step  108 , where gear shifts to higher forward gears occur as described above with reference to  FIGS. 4 and 6 . If vehicle speed falls below a reference vehicle speed, control passes from step  108  to step  106  for a blended vehicle launch in first and second gears. 
   In any state other than  108 , if the gear selector is moved to the P-position or N-position, control goes to state  102 . 
   In  FIG. 7 , the reference accelerator pedal displacement, reference vehicle speed and reference delays for completing a gear shift are calibratable parameters. 
   This gear shift control strategy ensures that the clutch coupler is engaged after the second gear coupler or the R2 gear coupler, thereby minimizing torque at the coupler at its engagement. The control also allows the transmission to stay in third gear and R2 gear during rock cycling maneuvers.