Multiple speed automatic transmission

A multiple speed transmission includes an input and output; a first, second, third and fourth planetary gear sets, each gear set including a sun gear, a ring gear, a carrier, and pinions supported on the carrier and meshing with the sun gear and the ring gear; a first epicyclic gearing assembly including the first gear set, the second gear set, a first clutch, a second clutch, a first brake, and first, second, third and fourth rotating members, said first clutch being operable to couple said first rotating member to the input, said second clutch being operable to couple said second rotating member to the input, and said first brake being operable to hold said fourth rotating member against rotation; and a second epicyclic gearing assembly including the third gear set, the fourth gear set, a third clutch, a second brake, a third brake, and fifth, sixth, seventh and eighth rotating members, the third rotating member being secured to the eighth rotating member, said third clutch being operable to couple said seventh rotating member to the input, said second brake being operable to hold said seventh rotating member against rotation, said third brake being operable to hold said fifth rotating member against rotation, and said sixth rotating member being secured for rotation to the output.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of automatic transmissions for motor vehicles. More particularly, the invention pertains to a kinematic arrangement of gearing, clutches, brakes, and the interconnections among them in a power transmission.

2. Description of the Prior Art

Traditionally, automatic transmissions have a planetary gearbox which provides a finite collection of selectable speed ratios and a torque converter which allows the engine to rotate even when the vehicle is stopped. The gearbox must provide a sufficiently low speed ratio to provide high output torque at low vehicle speed, a sufficiently high top gear ratio to minimize fuel consumption at highway speed, and enough speed ratios in between to enable comfortable shifts. The gearbox must also provide at least one negative speed ratio so that the vehicle can propel itself in reverse.

The primary function of the torque converter is to enable the transition from stationary to moving. A hydrodynamic torque converter transmits power from the engine to the gearbox input shaft whenever the engine speed exceeds the speed of the input shaft. When the gearbox input speed is zero, the torque applied to the gearbox is a multiple of the torque supplied by the engine. This torque ratio decreases as the vehicle accelerates and the speeds become more nearly equal. A substantial amount of the power supplied by the engine is dissipated by the torque converter whenever there is relative speed between the engine and the input shaft. Therefore, modern transmissions include a converter bypass clutch which is engaged at higher vehicle speeds to avoid this energy loss.

It is desirable to replace the torque converter with a launch clutch. This reduces fuel consumption in two ways. First, a torque converter will dissipate energy through slip until the bypass clutch is locked, whereas a launch clutch will be fully engaged at a low vehicle speed. Second, the clutch can be set at zero torque capacity when the vehicle is at rest, whereas a torque converter always places some load on the engine.

Launching a vehicle with a clutch presents several technical challenges. First, a slipping clutch does not multiply the torque as a torque converter does. Second, a clutch is not nearly as effective as a torque converter at dissipating heat. Both of these problems are most effectively addressed by providing lower gear ratios within the gearbox. This reduces the heat associated with a launch by reducing the input torque necessary to achieve a desired output torque and by reducing the time before the clutch is completely engaged. Finally, even when a clutch is disengaged, there is some parasitic drag. This drag tends to be higher for launch clutches than other clutches because the design is optimized for energy dissipation as opposed to drag reduction. If there is a high relative speed across a launch clutch in the higher gears, then this parasitic drag will have a large impact on fuel economy.

Although a lower gear ratio addresses some of the challenges, there are functional compromises associated with an extremely low gear ratio. When the vehicle is lightly loaded, it accelerates quickly. Once the launch is complete, the extreme gear ratio forces the engine to accelerate proportionately faster. The noise that results is considered unpleasant by many drivers and occupants. Also, the first shift occurs at a very low vehicle speed, which many drivers dislike.

Dual clutch transmissions are a class of automatic transmissions that use a blend of traditional automatic transmission components and manual transmission components. Essentially, a dual clutch transmission is a pair of automated manual transmissions; one for even gears and one for odd gears. Shifts between even and odd gears can be performed without torque interruption. Dual clutch transmissions do not typically have torque converters. At least one of the clutches, which can be either a wet or dry clutch, is used as the launch clutch and the advantages mentioned previously are realized. In addition to the functional compromises mentioned previously, these transmissions have other drawbacks relative to a planetary automatic. First, they typically use layshaft gearing, which is less efficient than planetary gearing for most ratios. Second, they are not able to perform two step shifts directly without torque interruption. For example, a shift from fifth to third would need to briefly engage fourth. This makes the vehicle seem less responsive to the driver.

SUMMARY OF THE INVENTION

The transmission of this invention is a planetary gearbox which provides seven forward speed ratios and three reverse speed ratios. The values for those speed ratios and the application pattern for the clutches is optimized for elimination of the torque converter. Two of the clutches are designated as launch clutches. When the remaining clutches are disposed to prepare for a launch, the vehicle can be launched with a very low speed ratio by engaging the first launch clutch or with a more moderate ratio by engaging the second launch clutch. A sophisticated control strategy determines to what extent each clutch is engaged in order to realize the heat dissipation advantages of the lowest gear ratio while avoiding the aforementioned unpleasant characteristics. Furthermore, the launch clutches are either engaged or open with a low relative speed in the higher gears which minimizes the impact on fuel economy.

A multiple speed transmission includes an input and output; first, second, third and fourth planetary gear sets, each gear set including a sun gear, a ring gear, a carrier, and pinions supported on the carrier and meshing with the sun gear and the ring gear; a first epicyclic gearing assembly including the first gear set, the second gear set, a first clutch, a second clutch, a first brake, and first, second, third and fourth rotating members, said first clutch being operable to couple said first rotating member to the input, said second clutch being operable to couple said second rotating member to the input, and said first brake being operable to hold said fourth rotating member against rotation; and a second epicyclic gearing assembly including the third gear set, the fourth gear set, a third clutch, a second brake, a third brake, and fifth, sixth, seventh and eighth rotating members, the third rotating member being secured to the eighth rotating member, said third clutch being operable to couple said seventh rotating member to the input, said second brake being operable to hold said seventh rotating member against rotation, said third brake being operable to hold said fifth rotating member against rotation, and said sixth rotating member being secured for rotation to the output.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, there is illustrated inFIG. 1the kinematic arrangement of an automatic transmission10. The transmission includes an input shaft12which is driven by an engine or electric motor, possibly via a torsion isolator. Although no torque converter is illustrated, a torque converter could be inserted between the engine or electric motor and input shaft12. The transmission also includes an output shaft14which is connected to the vehicle driving wheels via a differential mechanism and an axle.

A planetary gear system includes first, second, third, and fourth gear sets20,30,40, and50. The first gear set20includes a sun gear28, ring gear22, carrier24, and planet pinions26supported on carrier24and meshing with sun gear28and ring gear22. Similarly, the second gear set30includes a sun gear38, ring gear32, carrier34, and planet pinions36supported on carrier34and meshing with sun gear38and ring gear32. The third gear set40includes a sun gear48, ring gear42, carrier44, and planet pinions46supported on carrier44and meshing with sun gear48and ring gear42. The fourth gear set50includes a sun gear58, ring gear52, carrier54, and planet pinions56supported on carrier54and meshing with sun gear58and ring gear52.

Sun gear28is secured to sun gear38for rotation as a unit. Ring gear22and carrier34are mutually driveably connected and are secured by drum80to ring gear52for rotation as a unit. Ring gear32is secured to drum84for rotation as a unit. Ring gear42and carrier54are mutually driveably connected and are secured to drum82for rotation as a unit. Sun gear48is secured to sun gear58for rotation as a unit. Carrier44is secured to the output14.

Input12is alternately driveably connected to and disconnected from sun gear28and sun gear38by launch clutch60(L1). Input12is alternately driveably connected to and disconnected from carrier24by launch clutch62(L2). Input12is alternately driveably connected to and disconnected from ring gear42by clutch68(High). Ring gear32is alternately held against rotation, preferably on the transmission case58, upon engagement of a brake70(OD) and is released for free rotation upon disengagement of brake70. Ring gear42and carrier54are alternately held against rotation, preferably on the transmission case58, upon engagement of a band brake64(Rev) and are released for free rotation upon disengagement of band brake64. Sun gear48and sun gear58are alternately held against rotation, preferably on the transmission case58, upon engagement of a brake66(Fwd) and are released for free rotation upon disengagement of brake66. A one-way brake72(Low) alternately holds ring gear32against rotation on the transmission case58in one rotary direction and releases it to rotate freely in the opposite direction.

Clutches60,62,68and brakes66,70are preferably hydraulically-actuated control devices having sets of interleaved friction discs and spacer plates, the discs being secured to one element of the clutch or brake, the spacer plates secured to another element of the clutch or brake. When hydraulic pressure increases in the cylinder of a servo that actuates a respective friction element, the discs and plates of the respective friction element are forced by displacement of the servo piston into mutual frictional contact, thereby producing a drive connection between the components of the gear units to which the elements of the clutch or brake are secured. When the pressure is vented from the servo cylinder, the clutch or brake is disengaged and the components are free to rotate independently. Preferably band brake64is actuated to engage drum82in response to the magnitude of hydraulic pressure in the cylinder of a servo. A unique feature of this transmission is the fact that band brake64, which provides the torque reaction in reverse, is never involved in any dynamic shift events. Therefore, it is feasible to use a clutch mechanism with very low parasitic drag which would typically be rejected based on poor controllability. This is important because this clutch must have a very high torque capacity and a traditional clutch would contribute excessive drag in high gears and adversely impact fuel economy.

One-way brake72may be actuated by a sprag, roller, rocker or a similar device in response to the rotary direction of drum84relative to housing58. Although the one-way brake72is operative to produce a non-synchronous 1-2 upshift and to reduce open brake viscous drag, brake72can be deleted and its function replaced by brake70.

FIG. 2is a lever diagram having two levers92,94and representing transmission10. On the first lever92, which corresponds to gear sets20and30, node A, a first rotating member A, represents sun gear28and sun gear38; node B, a second rotating member B, represents carrier24; node C, a third rotating member C, represents ring gear22and carrier34; and node D, a fourth rotating member D, represents ring gear32. A torsional reaction is produced by brake70and one-way brake72at rotating member D.

On the second lever94, which corresponds to gear sets40and50, node E, a fifth rotating member E, represents sun gear48and sun gear58; node F, a sixth rotating member F, represents carrier44and output14; node G, a seventh rotating member G, represents ring gear42and carrier54; and node H, an eighth rotating member H, represents ring gear52. A torsional reaction is produced by brake66at node E. A torsional reaction is produced by brake64at node G.

Input12is connected by launch clutch60to rotating member A, by launch clutch62to rotating member B, and by clutch68to rotating member G. Rotating members C and H are continually mutually interconnected.

Operation of the transmission10is described next with reference to the engaged and disengaged state of the friction elements, which states in combination produce each of the gear ratios. Preferably, the states of the clutches and brakes are changed automatically in accordance with execution of a control algorithm by an electronic transmission controller.FIG. 4is a chart indicating the state of engagement and disengagement of the clutches and brakes corresponding to each of the gears. In the chart, symbol “X” indicates an engaged clutch or brake that is engaged to produce the respective gear, “(X)” indicates a clutch or brake that may be engaged, but does not affect operation in the respective gear, and “Alt” indicates a clutch or brake that may be engaged alternately instead of the clutch or brake marked “(X)” for the respective gear. A blank indicates that the corresponding clutch and brake is disengaged or released. In low forward and reverse gears, the “X” for one-way brake72indicates that the brake is producing a drive connection to the housing58and is not overrunning. “CST” in those gears for brake70indicates that brake70must be engaged if it is desired to transmit torque in the negative direction such that the engine provides a braking action while coasting.FIG. 4also shows the speed ratio for each forward and reverse gear when the gear sets have the beta ratios illustrated inFIG. 3. The speed ratio of a transmission is the ratio of the speed of its input to the speed of its output.

The transmission10is prepared for a launch in the forward direction when brake66is engaged and all other friction elements are disengaged. In order to launch the vehicle using the low gear ratio, launch clutch60is gradually engaged. One-way brake72(or brake70) provides a torque reaction at ring gear32and carrier34is underdriven. Carrier34drives ring gear52through drum80. Brake66provides a torque reaction at sun gear58and carrier54is further underdriven. Carrier54drives ring gear42. Brake66provides a torque reaction at sun gear48, driving carrier44and output14at an even lower speed.

A transition from low gear ratio to 1stgear ratio is accomplished by gradually releasing clutch60, gradually engaging clutch62, and maintaining brake66engaged. Typically, this transition would be completed before clutch60becomes fully engaged. As such, the launch is completed in 1stgear, avoiding the unpleasant engine noise and early shift. However, if the vehicle is accelerating slowly, the energy which must be dissipated in the launch clutches may be minimized by completing the launch in low gear. In the later case, the transition to 1stgear would be performed as a regular shift after the vehicle reaches sufficient speed to fully engage clutch62promptly.

A shift from 1stgear to 2ndgear is accomplished by gradually engaging clutch60while maintaining clutch62and brake66engaged. This causes all elements of the first and second gear sets to rotate at the speed of the input. One-way brake72overruns. The third and fourth gear sets provide torque multiplication in the same fashion as for low and 1st.

A shift from 2ndgear to 3rdgear is accomplished by gradually engaging clutch68, gradually releasing clutch62, and maintaining clutch60and brake66engaged. Alternatively, clutch60may be gradually released and clutch62maintained engaged. In 3rdgear, the third gear set provides all of the torque multiplication. Neither clutch60nor62carries any torque in 3rdgear, so both could be released once the shift is complete. However, it is advisable to keep one engaged such that all speeds are determined.

A shift from 3rdgear to 4thgear is accomplished by gradually engaging clutch62, gradually releasing brake66, and maintaining clutch60. This produces a direct drive state in which all planetary elements rotate with the input.

A shift from 4thgear to 5thgear is accomplished by gradually engaging brake70, gradually releasing clutch60, and maintaining clutch62and clutch68engaged.

A shift from 5thgear to 6thgear is accomplished by gradually engaging clutch60, gradually releasing clutch62, and maintaining clutch68and brake70engaged.

The transmission10is prepared for a launch in the reverse direction when brake64is engaged and all other friction elements are disengaged. In order to launch the vehicle using the low reverse gear ratio, launch clutch60is gradually engaged. One-way brake72(or brake70) provides a torque reaction at ring gear32and carrier34is underdriven in the forward direction. Carrier34drives ring gear52through drum80. Brake64provides a torque reaction at carrier54and sun gear58is driven in the reverse direction. Sun gear58drives sun gear48. Brake64provides a torque reaction at ring gear42, driving carrier44and output14at a lower speed in the reverse direction of input.

A transition from low reverse gear ratio to R1 reverse gear ratio is accomplished by gradually releasing clutch60, gradually engaging clutch62, and maintaining brake64engaged. As with a forward launch, this transition could be completed before clutch60becomes fully engaged to avoid an early shift. However, if the vehicle is accelerating slowly, the energy which must be dissipated in the launch clutches may be minimized by completing the launch in low reverse gear and performing a shift to R1 later.

A shift from R1 reverse gear to R2 reverse gear is accomplished by gradually engaging clutch60while maintaining clutch62and brake64engaged. This causes all elements of the first and second gear sets to rotate at the speed of the input. One-way brake72overruns. The third and fourth gear sets provide torque multiplication in the same fashion as for low reverse and R1.

FIG. 5illustrates an alternate kinematic arrangement of an automatic transmission16than the arrangement ofFIG. 1and is represented by the same lever diagram ofFIG. 2. The transmission includes an input shaft12which is driven by an engine or electric motor, possibly via a torsion isolator. Although no torque converter is illustrated, a torque converter could be inserted between the engine or electric motor and input shaft12. The transmission also includes an output shaft14which is connected to the vehicle driving wheels via a differential mechanism and an axle.

A planetary gear system includes first, second, third, and fourth gear sets20,30,40, and50. The first gear set20includes a sun gear28, ring gear22, carrier24, and planet pinions26supported on carrier24and meshing with sun gear28and ring gear22. Similarly, the second gear set30includes a sun gear38, ring gear32, carrier34, and planet pinions36supported on carrier34and meshing with sun gear38and ring gear32. The third gear set40includes a sun gear48, ring gear42, carrier44, and planet pinions46supported on carrier44and meshing with sun gear48and ring gear42. The fourth gear set50includes a sun gear58, ring gear52, carrier54, and planet pinions56supported on carrier54and meshing with sun gear58and ring gear52.

Sun gear28is secured by drum86to ring gear32for rotation as a unit. Carrier34and sun gear48are mutually driveably connected and are secured by drum88to carrier24for rotation as a unit. Carrier44is secured by drum90to ring gear52for rotation as a unit. Ring gear42and carrier54are mutually driveably connected and are secured to the output14.

Input12is alternately driveably connected to and disconnected from sun gear38by launch clutch60(L1). Input12is alternately driveably connected to and disconnected from ring gear22by launch clutch62(L2). Input12is alternately driveably connected to and disconnected from carrier44and ring gear52by clutch68(High). Drum86, sun gear28, and ring gear32are alternately held against rotation, preferably on the transmission case58, upon engagement of a brake70(OD) and is released for free rotation upon disengagement of brake70. Drum90, carrier44, and ring gear22are alternately held against rotation, preferably on the transmission case58, upon engagement of a band brake64(Rev) and are released for free rotation upon disengagement of band brake64. Sun gear58is alternately held against rotation, preferably on the transmission case58, upon engagement of a brake66(Fwd) and are released for free rotation upon disengagement of brake66. A one-way brake72(Low) alternately holds drum86, sun gear28, and ring gear32against rotation on the transmission case58in one rotary direction and releases them to rotate freely in the opposite direction. Although the one-way brake72is operative to produce a non-synchronous 1-2 upshift and to reduce open brake viscous drag, brake72can be deleted and its function replaced by brake70.

The lever diagram ofFIG. 2applies also to transmission16ofFIG. 5. On the first lever92, which corresponds to gear sets20and30, node A, a first rotating member A, represents sun gear38; node B, a second rotating member B, represents ring gear22; node C, a third rotating member C, represents carrier24and carrier34; and node D, a fourth rotating member D, represents sun gear28and ring gear32. A torsional reaction is produced by brake70and one-way brake72at rotating member D.

On the second lever94, which corresponds to gear sets40and50, node E, a fifth rotating member E, represents sun gear58; node F, a sixth rotating member F, represents ring gear42, carrier54, and output14; node G, a seventh rotating member G, represents carrier44and ring gear52; and node H, an eighth rotating member H, represents sun gear48. A torsional reaction is produced by brake66at node E. A torsional reaction is produced by brake64at node G.

Input12is connected by launch clutch60to rotating member A, by launch clutch62to rotating member B, and by clutch68to rotating member G. Rotating members C and H are continually mutually interconnected.

FIG. 7is a chart indicating the state of engagement and disengagement of the clutches and brakes corresponding to each of the gears. In the chart, symbol “X” indicates an engaged clutch or brake that is engaged to produce the respective gear, “(X)” indicates a clutch or brake that may be engaged, but does not affect operation in the respective gear, and “Alt” indicates a clutch or brake that may be engaged alternately instead of the clutch or brake marked “(X)” for the respective gear. A blank indicates that the corresponding clutch and brake is disengaged or released. In low forward and reverse gears, the “X” for one-way brake72indicates that the brake is producing a drive connection to the housing58and is not overrunning. “CST” in those gears for brake70indicates that brake70must be engaged if it is desired to transmit torque in the negative direction such that the engine provides a braking action while coasting.FIG. 7also shows the speed ratio for each forward and reverse gear when the gear sets have the beta ratios shown inFIG. 6. The speed ratio of a transmission is the ratio of the speed of its input to the speed of its output.

Transmission16operates in seven forward gears and three reverse gears when the clutches and brakes have the states of engagement and disengagement shown inFIG. 7, which are identical to those ofFIG. 4. The speed ratios produced in each gear are shown also inFIG. 7, provided A, the ratio of the number of gear teeth of the ring gear to the number of gear teeth of the sun gear for the respective gear sets20,30,40, and50is as set forth inFIG. 6.

A transmission embodiment according to this invention may contain two epicyclic gearing assemblies, each with four members that rotate around a common axis. In each epicyclic gearing assembly, two of the rotating elements have the most extreme speeds at all times, while the speeds of the other two elements are a weighted average of those two speeds. The weighting factors are determined by the configuration of the epicyclic gearing assembly and the ratios of the numbers of gear teeth. These epicyclic gearing assemblies are represented by levers inFIG. 2. The nodes on the endpoints of the levers (A, D, E, and H) represent the elements that have the most extreme speeds, while the interior nodes (B, C, F, and G) represent the nodes whose speeds are a weighted average of the former group.

There are many possible configurations of epicyclic gearing assemblies that will produce any particular desired weighting factors. Two such configurations have been illustrated here for each of the two epicyclic gearing assemblies. It should be noted that other known configurations of epicyclic gearing assemblies, which achieve the same or similar weighting factors can be substituted for those illustrated without departing from the spirit of this invention. Specifically, in some configurations, the planetary gear sets contain two sets of pinion gears instead of one, with an inner set meshing with the sun gear, the outer set meshing with the ring gear, and the two sets meshing with each other. In other configurations, such as a Ravigneaux arrangement, some of the pinion gears are shared between multiple gear sets.