Gear selection strategy for a dual clutch transmission

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.

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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIG. 1, a transaxle includes an input10for driveably connecting a power source, such as an internal combustion engine or electric motor, to the transmission, and an output36for 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 clutch20, consisting of a clutch housing and a clutch disc22, alternately connects and disconnects a first input shaft14as clutch20is engaged and disengaged, respectively. A second friction clutch16, consisting of a clutch housing and a clutch disc18, connects and disconnects a second input shaft12as clutch16is engaged and disengaged, respectively.

A first layshaft26supports a first output pinion30, which is secured to layshaft26in continuous meshing engagement with an output ring gear34, secured to output36. A second layshaft24supports a second output pinion32, which is secured to the layshaft24in continuous meshing engagement with output ring gear34.

The first input shaft14supports two pinions50and52which are secured to shaft14. The second input shaft12supports one pinion48which is secured to shaft12and two pinions44and46which may rotate about shaft12. Gear42is supported on layshaft26for rotation relative to layshaft26, and in continuous meshing engagements with pinion52. Auxiliary shaft28is a hollow shaft supported on layshaft26for rotation relative to layshaft26. The auxiliary shaft28supports gears38and40which are secured to shaft28and in continuous meshing engagement with pinions46and50respectively. Gear54is secured to layshaft24and in continuous meshing engagement with pinion44. Gears56and58are supported on layshaft24for rotation relative to layshaft24and in continuous meshing engagement with pinion48and gear42, respectively. Couplers60,62, and64are 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 may15also 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.

Engaging coupler60to pinion46activates a power path between the first and second input shaft comprising pinion50, gear40, auxiliary shaft28, gear38, pinion546, and coupler60.

To shift from the first forward speed to the second forward speed, clutch20is progressively engaged while clutch16is progressively released. Following the shift, coupler60may 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 input10, clutch20, input shaft14, pinion52, gear42, coupler62, layshaft26, output pinion30, output gear34, and output36. Input10is driveably connected to shaft14and pinion52through clutch20. Pinion52drives gear42, which is driveably connected to shaft26through coupler62. Pinion30is secured to shaft26and drives ring gear34and output36.

To shift from the second forward speed to the third forward speed, the transmission is configured by displacing coupler64to engage gear56, then clutch16is progressively engaged while clutch20is progressively released. Following the shift, coupler62may be moved to the neutral position. The power path for this speed comprises input10, clutch16, input shaft12, pinion48, gear56, coupler64, layshaft24, output pinion32, output gear34, and output36. Input10is driveably connected to shaft12and pinion48through clutch16. Pinion48drives gear56, which is driveably connected to shaft24through coupler64. Pinion32is secured to shaft24and drives ring gear34and output36.

To shift from the third forward speed to the fourth forward speed, the transmission is configured by displacing coupler62to engage gear40, then clutch20is progressively engaged while clutch16is progressively released. Following the shift, coupler64may be moved to the neutral position. The power path for this speed comprises input10, clutch20, input shaft14, pinion50, gear40, coupler62, layshaft26, output pinion30, output gear34, and output36. Input10is driveably connected to shaft14and pinion50through clutch20. Pinion50drives gear40, which is driveably connected to shaft26through coupler62. Pinion30is secured to shaft26and drives ring gear34and output36.

To shift from the fourth forward speed to the fifth forward speed, the transmission is configured by displacing coupler60to engage pinion44, then clutch16is progressively engaged while clutch20is progressively released. Following the shift, coupler62may be moved to the neutral position. The power path for this speed comprises input10, clutch16, input shaft12, coupler60, pinion44, gear54, layshaft24, output pinion32, output gear34, and output36. Input10is driveably connected to shaft12through clutch16. Shaft12is driveably connected to pinion44through coupler60. Pinion44drives gear54, shaft24, pinion30, ring gear34, and output36. Downshifts are accomplished by reversing the steps of the corresponding upshift.

A shift may be accomplished in reverse by progressively engaging clutch20while progressively releasing clutch16. The power path for this speed comprises input10, clutch20, input shaft14, pinion52, gear42, gear58, coupler64, layshaft24, output pinion32, output gear34, and output36. Following the shift, input10is driveably connected to shaft14and pinion52through clutch20. Pinion52drives gear42and gear58, which is driveably connected to shaft24through coupler64. Pinion32is secured to shaft24and drives ring gear34and output36.

A chart containing a preferred number of teeth for each of the gears of the transaxle ofFIG. 1is shown inFIG. 2, whileFIG. 3is 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 ofFIG. 1.

Referring now toFIG. 4, a transaxle includes an input10for driveably connecting a power source, such as an internal combustion engine or electric motor, to the transmission, and an output36for 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 clutch16, consisting of a clutch housing and a clutch disc18, alternately connects and disconnects a first input shaft12as clutch16is engaged and disengaged, respectively. A second friction clutch20, consisting of a clutch housing and a clutch disc22, connects and disconnects a second input shaft14as clutch20is engaged and disengaged, respectively.

A first layshaft26supports a first output pinion30, which is secured to layshaft26in continuous meshing engagement with an output ring gear34, secured to output36. A second layshaft24supports a second output pinion32, which is secured to the layshaft in continuous meshing engagement with output ring gear34. The second input shaft14supports two pinions82and84which are secured to shaft14. The first input shaft12supports three pinions76,78, and80which are secured to shaft12. Gears86,88,90, and92are supported on layshaft24for rotation relative to layshaft24and in continuous meshing engagement with pinions76,78,82, and84respectively. Gear70is supported on layshaft26for rotation relative to layshaft26, and in continuous meshing engagement with gear86. Auxiliary shaft28is a hollow shaft supported on layshaft26for rotation relative to layshaft26. Auxiliary shaft28supports gear72which is secured to shaft28and in continuous meshing engagement with pinion80. Gear74is supported on shaft28for rotation relative to shaft28and in continuous meshing engagement with pinion84.

Coupler96is the clutch coupler. Engaging coupler96to gear74activates a power path between the first and second input shaft comprising pinion84, gear74, coupler96, auxiliary shaft28, gear72, and pinion80.

To shift from the first forward speed to the second forward speed, clutch16is progressively engaged while clutch20is progressively released. Following the shift, coupler96may be moved to the neutral position. The power path for this speed comprises input10, clutch16, input shaft12, pinion76, gear86, coupler98, layshaft.24, output pinion32, output gear34, and output36. Input10is driveably connected to shaft12and pinion76through clutch16. Pinion76drives gear86, which is driveably connected to shaft24through coupler98. Pinion32is secured to shaft24and drives ring gear34and output36. To shift from the second forward speed to the third forward speed, the transmission is configured by displacing coupler100to engage gear92, then clutch20is progressively engaged while clutch16is progressively released. Following the shift, coupler98may be moved to the neutral position. The power path for this speed comprises input10, clutch20, input shaft14, pinion84, gear92, coupler100, layshaft24, output pinion32, output gear34, and output36. Input10is driveably connected to shaft14and pinion84through clutch20. Pinion84drives gear92, which is driveably connected to shaft24through coupler100. Pinion32is secured to shaft24and drives ring gear34and output36.

To shift from the third forward speed to the fourth forward speed, the transmission is configured by displacing coupler94to engage gear72, then clutch16is progressively engaged while clutch20is progressively released. Following the shift, coupler100may be moved to the neutral position. The power path for this speed comprises input10, clutch16, input shaft12, pinion80, gear72, coupler94, layshaft26, output pinion30, output gear34, and output36. Input10is driveably connected to shaft12and pinion80through clutch16. Pinion80drives gear72, which is driveably connected to shaft26through coupler94. Pinion30is secured to shaft26and drives ring gear34and output36.

To shift from the fourth forward speed to the fifth forward speed, the transmission is configured by displacing coupler100to engage gear90, then clutch20is progressively engaged while clutch16is progressively released. Following the shift, coupler94may be moved to the neutral position. The power path for this speed comprises input10, clutch20, input shaft14, pinion82, gear90, coupler100, layshaft24, output pinion32, output gear34, and output36. Input10is driveably connected to shaft14and pinion82through clutch20. Pinion82drives gear90, which is driveably connected to shaft24through coupler100. Pinion32is secured to shaft24and drives ring gear34and output36.

To shift from the fifth forward speed to the sixth forward speed, the transmission is configured by displacing coupler98to engage gear88, then clutch16is progressively engaged while clutch20is progressively released. Following the shift, coupler100may be moved to the neutral position. The power path for this speed comprises input10, clutch16, input shaft12, pinion78, gear88, coupler98, layshaft24, output pinion32, output gear34, and output36. Input10is driveably connected to shaft12and pinion78through clutch16. Pinion78drives gear88, which is driveably connected to shaft24through coupler98. Pinion32is secured to shaft24and drives ring gear34and output36.

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 input10and output36is briefly interrupted by disengaging clutch16while 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, coupler98is moved to the neutral position, coupler96is displaced to engage gear74, and coupler100is displaced to engage gear90. Then, clutch16is reengaged. The power path for this speed comprises input10, clutch16, input shaft12, pinion80, gear72, auxiliary shaft28, coupler96, gear74, pinion84, input shaft14, pinion82, gear90, coupler100, layshaft24, output pinion32, output gear34, and output36. When clutch16is re-engaged, input10is driveably connected to shaft12and pinion80through clutch16. Pinion80drives gear72and auxiliary shaft28, which is driveably connected to gear74through coupler96. Gear74drives pinion84, shaft14, pinion82, and gear90, which is driveably connected to shaft24through coupler100. Pinion32is secured to shaft24and drives ring gear34and output36.

A shift may be accomplished in reverse by progressively engaging clutch16while progressively releasing clutch20. The power path for this speed comprises input10, clutch16, input shaft12, pinion76, gear86, gear70, coupler94, layshaft26, output pinion30, output gear34, and output36. Following the shift, input10is driveably connected to shaft12and pinion76through clutch16. Pinion76drives gear86and gear70, which is driveably connected to shaft26through coupler94. Pinion30is secured to shaft26and drives ring gear34and output36.

A chart containing a preferred number of teeth for each of the gears of the transaxle ofFIG. 4is shown inFIG. 5, whileFIG. 6is 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 ofFIG. 4, the gears having the number of teeth shown inFIG. 5.

FIG. 7illustrates 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 toFIG. 7, the steps for controlling gear shifts of a dual clutch transmission, such as the transaxle illustrated inFIG. 4, begins at step102with the power source transmitting power to the input10and the transaxle prepared for operation in Park or Neutral or in R2 gear, wherein coupler94connects gear70and layshaft26or in third gear, wherein coupler100connects gear92and layshaft24; and with clutches16,20disengaged. If the gear selector is in the Park or P-range position, the output36is 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 step104where R2 gear is disengaged by moving coupler94to its neutral position thereby disconnecting gear70from layshaft26, and second gear is engaged by coupler98connecting gear86and layshaft24. At step104with the transaxle so prepared, clutch16is 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 clutch16may be slipping excessively long, control passes to step106where third gear is disengaged by moving coupler100to its neutral position thereby disconnecting gear92from layshaft24, and first gear is engaged by causing coupler96to connect gear74and layshaft28.

With the transaxle so prepared, clutches16and20are used together to produce a blended vehicle launch in first gear and second gear by varying the torque capacity of clutches16and20during the vehicle launch. Preferably in the earliest portion of the vehicle launch, the torque capacity of clutch20is greater than the torque capacity of clutch16, and in the latter portion of the vehicle launch the torque capacity of clutch16is greater than that of clutch20. In this way, the 1-2 vehicle launch begins in first gear and ends in second gear with clutch16fully engaged and clutch20fully disengaged. The blended forward vehicle launch prevents excessive wear of clutch16and responds to the driver's demand for a speedier launch.

If, at step106during a blended 1-2 vehicle launch, the operator moves the gear selector to the Reverse or R-range position, the transaxle's gear shift control transitions quickly to and through step110, where neutral is produced by disengaging first gear coupler96and second gear coupler86. Thereafter, control passes promptly to step112, where the transaxle is prepared for operation in reverse drive, specifically in the R2 gear with coupler94connecting gear70and layshaft26. With the transaxle so prepared, clutch16is 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 clutches16and20occurs at step116, as described below.

If, at the initial step102with the source transmitting power to the input10and the transaxle prepared for operation in Park, R2 gear and third gear and clutches16,20disengaged, the operator moves the gear selector to the Reverse or R-range position, control passes to step114where R2 gear remains engaged due to its coupler94connecting gear70and layshaft26, and third gear remains engaged due to its coupler100connecting gear92and layshaft24. With the transaxle so prepared, clutch16is 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 clutch16are engaged exceeds a reference length, indicating that clutch16may be slipping excessively long, control passes to step116where R2 gear remains engaged, third gear is disengaged by moving coupler100to its neutral position, and R1 gear is engaged by causing coupler96to connect gear74and layshaft28. With the transaxle so prepared at step116, clutches16and20are used together to produce a blended reverse vehicle launch in R1 gear and R2 gear by varying the torque capacity of clutches16and20during the reverse vehicle launch. Preferably in the earliest portion of the reverse vehicle launch, the torque capacity of clutch20is greater than the torque capacity of clutch16, and in the latter portion of the reverse vehicle launch the torque capacity of clutch16is greater than that of clutch20. In this way, the R1-R2 launch begins in R1 gear and ends in R2 gear with clutch16fully engaged and clutch20fully disengaged. The blended reverse vehicle launch prevents wear of clutch16and responds to the driver'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 step114or step116, if during the reverse launch, the operator moves the gear selector to the Drive or D-range position, control passes to step118, where the R2 gear coupler94remains engaged with gear70, R1 gear is disengaged, and third gear is engaged by causing coupler100to connect gear92and layshaft24. With the transaxle so prepared at step118, clutch20is engaged, and the control responds to accelerator pedal position and accelerates the vehicle in third gear.

If, with the control operating in step118, 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 step114, where the vehicle is accelerated in the R2 gear, as described above with reference to step114.

If, with the control operating in step118, 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 step118to step104for a vehicle launch in second gear, as described above with reference to step104.

If, with the control operating in step104, 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 step106, where clutches16and20are used together to produce a blended vehicle launch in first gear and second gear by varying the torque capacity of clutches16and20during the vehicle launch.

If, with the control operating in step104, the gear selector is moved to the R-range, thereby indicating that the rock cycling is not completed, control passes to step114, the transaxle is prepared for operation in R2 gear and third gear, clutch16is engaged and the vehicle is accelerated in R2 gear in response to depressing the accelerator pedal.

Whether the vehicle launch occurs at step118, step104, or step106, when the vehicle launch is completed, control passes to step108, where gear shifts to higher forward gears occur as described above with reference toFIGS. 4 and 6. If vehicle speed falls below a reference vehicle speed, control passes from step108to step106for a blended vehicle launch in first and second gears.

In any state other than108, if the gear selector is moved to the P-position or N-position, control goes to state102.

InFIG. 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.