Abstract:
A multiple speed power transmission, comprising: a gearbox input shaft; an output shaft; and a collection of gears, friction clutches, and passive couplers capable of transmitting power from the gearbox input shaft to the output shaft with eight forward speed ratios and one reverse speed ratio wherein the first forward speed ratio is selected by engaging friction brakes using electromechanical actuators.

Description:
BACKGROUND AND SUMMARY 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. 
     When a motor vehicle is stationary for a period of time, such as while waiting at a traffic light, it is desirable to shut off the engine to save fuel. The engine must then be quickly re-started when the driver signals that he is ready to move again, usually by removing his foot from the brake pedal and applying pressure to the accelerator pedal. If the delay in delivering torque to the drive wheels is excessive, the driver will be unsatisfied with the vehicle. In order to minimize the delay, it is important that the transmission be prepared to transmit torque in first gear as soon as the engine is running. Traditionally, an automatic transmissions is adapted for this idle engine stop feature by adding an electrically driven pump to provide hydraulic pressure to engage the appropriate friction elements. 
     This invention avoids the need for an electrically driven pump by employing electro-mechanical actuation for the friction elements used in first gear. Electro-mechanical actuation is operable when the engine is not running. However, electromechanical actuation is limited to friction elements on the periphery of the gearbox, such as brakes, whereas hydraulic actuation can be utilized for friction elements that are located under rotating shells. With the kinematic arrangement of this invention, only brakes need to be applied to engage first gear. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a transmission according to a first embodiment of the present invention having three brakes and two rotating clutches. 
         FIG. 2  is a table showing proposed tooth numbers for each of the gears. 
         FIG. 3  is a table indicating the states of the clutches and resulting speed ratio of the transmission in  FIG. 1  when the gears have the number of teeth indicated in  FIG. 2 . 
         FIG. 4  is a schematic diagrams of an alternative embodiment of the present invention having three brakes and three rotating clutches. 
         FIG. 5  is a table indicating the states of the clutches and resulting speed ratio of the transmission in  FIG. 4  when the gears have the number of teeth indicated in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A transmission according to a first embodiment of the present invention is illustrated in  FIG. 1 . The transmission contains four simple planetary gear set assemblies  20 ,  30 ,  40 , and  50 . Each simple planetary gear set assembly has a sun gear, a ring gear with an internal mesh, a planet carrier, and a set of planet gears supported for rotation on the carrier and meshing with both the sun gear and ring gear. A recommended number of gear teeth for each of these gears is shown in  FIG. 2 . Simple planetary gear sets constrain the carrier to rotate at a speed that is a weighted average of the speeds of the sun gear and the ring gear, with the weighting factors determined by the respective number of gear teeth. Other types of epicyclic gearing assemblies, including double pinion planetary gear sets, stepped pinion planetary gear sets, and co-planar gear loops as describe in U.S. Pat. Nos. 5,030,184 and 6,126,566, offer alternative means of constraining the speed of an element to be the weighted average of the speeds of two other elements. 
     Gearbox input shaft  16  is driven by the vehicle&#39;s engine via torque converter assembly  80 . The second sun gear  32  is fixed to gearbox input shaft  16 . The first carrier  26  is connected to the fourth ring gear  54 . The third sun gear  42  is connected to the fourth sun gear  52 . The first ring gear  24 , second carrier  36 , and third ring gear  44  are mutually connected. A gearbox output shaft  12  drives the vehicle wheels, preferably via a driveshaft, a differential assembly, and rear axle shafts. Gearbox output shaft  12  is fixed to the third carrier  46 . A transmission case  14  provides support for the gear sets, input shaft, and output shaft. 
     Clutches  60  and  62  are preferably hydraulically actuated friction clutches which releasably connect two elements when hydraulic pressure is applied and disconnect those elements when the hydraulic pressure is released. The hydraulic pressure is provided by an engine driven hydraulic pump and the pressurized fluid is distributed to the clutches via channels within gearbox input shaft  16 . Clutch  60  releasably connects gearbox input shaft  16  to the first carrier  26  and fourth ring gear  54 . Clutch  62  releasably connects gearbox input shaft  16  to the first ring gear  24 , the second carrier  36 , and the third ring gear  44 . Clutch  62  forces all three elements of planetary gear set  30  to rotate at the same speed. This effect may alternatively be accomplished by a clutch releasably connecting any two of sun gear  32 , carrier  36 , and ring gear  34  to each other. 
     Brakes  64 ,  68 , and  70  are preferably electro-mechanically actuated friction brakes which hold an element against rotation in response to the rotation of an electric motor and release said element when the electric motor is rotated in the opposite direction. U.S. Pat. No. 6,699,153 describes a number of suitable electro-mechanically actuated brake assemblies. Brake  64  releasably holds the first sun gear  22  against rotation. Brake  64  is applied by rotating motor  92  which moves piston  94  to create pressure on the friction plates. Brake  68  releasably holds the third sun gear  42  and fourth sun gear  52  against rotation. Brake  70  releasably holds the first carrier  26  and fourth ring gear  54  against rotation. Brakes  68  and  70  are applied by motor  96  and piston  98 . Rotating motor  96  in one direction pushes piston  98  against brake  68  and rotating it in the opposite direction pushes piston  98  against brake  70 . An intermediate position releases both brakes. 
     One way brake  66  is a passive coupler which allows the first carrier  26  and fourth ring gear  54  to rotate freely in a positive direction but prevents rotation in the opposite direction. 
     Torque converter assembly  80  comprises an impeller  82 , stator  86 , and turbine  84 . Impeller  82  is driven by transmission input shaft  10 . The stator  86  is connected to the transmission case  14  by one way brake  88 . Torque is transmitted from the impeller to the turbine hydro-dynamically by fluid that circulates among the three elements. When the turbine is substantially slower than the impeller, one way brake  88  holds the stator stationary and it provides a reaction torque to create torque multiplication between the impeller and turbine. The one way brake overruns when the turbine speed is near or greater than the impeller speed. Hydraulically actuated lock-up clutch  90  connects the turbine to the impeller eliminating the hydro-dynamic losses of the torque converter. Careful design of the hydraulic system can reduce leakage of fluid from the torque converter sufficiently to allow idle engine shutdown for periods of several minutes, which exceeds the requirement for realizing the majority of the fuel saving benefits. 
     The transmission ratio is selected by applying two of the clutches and brakes as indicated in  FIG. 3 . In first gear, however, it is only necessary to apply one friction brake because one way brake  66  will engage passively. 
     When the vehicle is stationary in drive (forward) mode, the engine will generally be off, unless the idle stop feature of the control strategy is temporarily disabled for some reason, such as a drained torque converter, low coolant temperature, etc. The transmission is prepared for forward motion in first gear by rotating motor  96  to apply brake  68 . When the driver signals that he is ready to move, the engine is quickly started. Of course, if the idle stop feature has been disabled, the engine will already be running, so this step can be skipped. The engine drives impeller  82 , and hydro-dynamic forces within the torque converter generate torque on turbine  84  and gearbox input shaft  16 . Brake  68  and one way brake  66  provide a reaction torque such that a multiple of the input torque is transferred to output shaft  12 , accelerating the vehicle. 
     To shift to second gear, motor  94  is used to progressively engage brake  64 , maintaining brake  68  in the fully applied state. As the torque capacity of brake  64  increases, one way brake  66  will overrun. 
     The engine driven hydraulic pump begins building up pressure in the valve body shortly after the engine is started. By the time the vehicle is ready to shift to third gear, hydraulic pressure is available. To shift from second to third gear, clutch  60  is progressively engaged while brake  64  is progressively released. To shift from third to fourth gear, clutch  62  is progressively engaged while clutch  60  is progressively released. Brake  68  is maintained in the fully applied state through all of these transitions. It is advantageous to apply lock-up clutch  90  soon after hydraulic pressure is available in order to minimize the energy loss associated with an open torque converter. 
     To shift from fourth to fifth gear, clutch  60  is progressively engaged while brake  68  is progressively released. Fifth gear is a direct drive gear. To shift from fifth to sixth gear, brake  64  is progressively engaged while clutch  60  is progressively released. To shift from sixth to seventh gear, brake  70  is progressively engaged while brake  64  is progressively released. Clutch  62  is maintained in the fully applied state through all of these transitions. 
     Eighth gear provides improved fuel economy for high speed driving. Unfortunately, it is not possible to shift directly from seventh gear to eighth gear without interrupting the flow of power through the transmission. There are two ways to get into eighth gear. The first method is to release clutch  62  and brake  70  then apply clutch  60  and brake  64 . Engine torque must be interrupted during this transition because the transmission is not capable of transmitting torque to the wheels. The second method is to bypass seventh gear and shift from sixth gear into eighth gear by progressively engaging clutch  60  while progressively releasing clutch  62 , maintaining brake  64  is the applied state. 
     Downshifting to a lower gear is accomplished by reversing the steps described above for the corresponding upshift. 
     The transmission is also capable of operation with an idle engine stop strategy in reverse. The transmission is prepared for reverse motion by rotating motors  92  and  96  to apply brakes  64  and  70 , respectively. When the driver signals his intent to move, the engine is quickly started. The engine drives impeller  82 , and hydro-dynamic forces within the torque converter generate torque on turbine  84  and gearbox input shaft  16 . Brakes  64  and  70  provide a reaction torque such that a multiple of the input torque, in the opposite direction of the input torque, is transferred to output shaft  12 , accelerating the vehicle. 
     Although brakes  64 ,  70  and  68  are all preferably electro-mechanically controlled, the invention can be practiced, with some functional limitations, with hydraulic actuation for these clutches. Specifically, if either brake  64  or brake  70  are hydraulically actuated, the idle engine stop feature would not be available in reverse. Furthermore, if brake  64  is hydraulically actuated, then the shift from first gear to second gear could not be initiated until the engine driven hydraulic pump has had time to produce sufficient pressure in the valve body. 
     Also, the invention may be practiced without torque converter  80  by driveably connecting gearbox input shaft  16  to transmission input shaft  10 , preferably via a torsional damper. In this alternative embodiment, a forward vehicle launch is accomplished by the gradual engagement of brake  68 . All shifts are accomplished in the manner described above. A vehicle launch in reverse is accomplished by fully engaging either brake  70  or brake  64  and then gradually engaging the other one. 
     Optionally, one way brake  66  may be omitted and its function accomplished by brake  70 . If one way brake  66  is omitted, however, brakes  68  and  70  would need to be operated together in first gear, and would therefore require independent actuation. 
     A transmission according to a second embodiment of the present invention is illustrated in  FIG. 4 . The transmission contains four simple planetary gear set assemblies  20 ,  30 ,  40 , and  50 . Gearbox input shaft  16  is driven by the vehicle&#39;s engine via torque converter assembly  80 . The second sun gear  32  is fixed to gearbox input shaft  16 . The first carrier  26  is connected to the fourth ring gear  54 . The third sun gear  42  is connected to the fourth sun gear  52 . The second carrier  36  is connected to the third ring gear  44 . A gearbox output shaft  12  drives the vehicle wheels, preferably via a driveshaft, a differential assembly, and rear axle shafts. Gearbox output shaft  12  is fixed to the third carrier  46 . A transmission case  14  provides support for the gear sets, input shaft, and output shaft. 
     Clutches  60 ,  100 , and  102  are preferably hydraulically actuated friction clutches which releasably connect two elements when hydraulic pressure is applied and disconnect those elements when the hydraulic pressure is released. Clutch  60  releasably connects gearbox input shaft  16  to the first carrier  26  and fourth ring gear  54 . Clutch  100  releasably connects the first ring gear  24  to the second ring gear  34  and fourth carrier  56 . Clutch  102  releasably connects the first ring gear  24  to the second carrier  36  and third ring gear  44 . 
     Brakes  68  and  70  are preferably electro-mechanically actuated friction brakes which hold an element against rotation in response to the rotation of an electric motor and release said element when the electric motor is rotated in the opposite direction. Brake  68  releasably holds the third sun gear  42  and fourth sun gear  52  against rotation. Brake  70  releasably holds the first carrier  26  and fourth ring gear  54  against rotation. Brakes  68  and  70  are applied by motor  96  and piston  98 . Rotating motor  96  in one direction pushes piston  98  against brake  68  and rotating it in the opposite direction pushes piston  98  against brake  70 . An intermediate position releases both brakes. 
     Brake  64  releasably holds the first sun gear  22  against rotation. Brake  64  may be either electro-mechanically actuated or hydraulically actuated. 
     One way brake  66  is a passive coupler which allows the first carrier  26  and fourth ring gear  54  to rotate freely in a positive direction but prevents rotation in the opposite direction. 
     The transmission ratio is selected by applying three of the clutches and brakes as indicated in  FIG. 5 . An X in parenthesis indicates that the clutch is ordinarily applied in that gear but does not carry any torque. In first gear, it is only necessary to apply one friction brake because one way brake  66  will engage passively and clutch  102  is not required to establish the power path. 
     When the vehicle is stationary in drive (forward) mode, the transmission is prepared for forward motion in first gear by rotating motor  96  to apply brake  68 . When the driver. signals that he is ready to move, the engine is quickly started. The engine drives impeller  82 , and hydro-dynamic forces within the torque converter generate torque on turbine  84  and gearbox input shaft  16 . Brake  68  and one way brake  66  provide a reaction torque such that a multiple of the input torque is transferred to output shaft  12 , accelerating the vehicle. 
     The engine driven hydraulic pump begins building up pressure in the valve body shortly after the engine is started. By the time the vehicle is ready to shift to second gear, hydraulic pressure is available. Clutch  102  is engaged as soon as hydraulic pressure becomes available. To shift to second gear, brake  64  is progressively engaged, maintaining brake  68  and clutch  102  in the fully applied state. As the torque capacity of brake  64  increases, one way brake  66  will overrun. It is advantageous to apply lock-up clutch  90  soon after hydraulic pressure is available in order to minimize the energy loss associated with an open torque converter. 
     To shift from second to third gear, clutch  60  is progressively engaged while brake  64  is progressively released. To shift from third to fourth gear, clutch  100  is progressively engaged while clutch  60  is progressively released. Brake  68  and clutch  102  are maintained in the fully applied state through all of these transitions. 
     To shift from fourth to fifth gear, clutch  60  is progressively engaged while brake  68  is progressively released. Fifth gear is a direct drive gear. To shift from fifth to sixth gear, brake  64  is progressively engaged while clutch  60  is progressively released. Clutch  100  and clutch  102  are maintained in the fully applied state through these transitions. 
     To shift from sixth to seventh gear, clutch  60  is progressively engaged while clutch  102  is progressively released, maintaining clutch  100  and brake  64  fully engaged. To shift from seventh to eighth gear, clutch  102  is progressively engaged while clutch  100  is progressively released, maintaining clutch  60  and brake  64  fully engaged. All shifts may be performed without interrupting the power flow. 
     An additional gear ratio, which falls between seventh and eighth, is available by applying brake  70 , clutch  100 , and clutch  102 . However, it is not possible to shift into this ratio from either seventh gear or eighth gear without interrupting the flow of power. A shift from sixth gear into this supplemental ratio is accomplished by progressively engaging brake  70  while progressively releasing brake  64 , maintaining clutch  100  and clutch  102  fully engaged. The control strategy may elect to utilize this extra ratio instead of seventh and eighth gear in circumstances where the regular eighth gear is too deep of an overdrive, such as when towing a heavy trailer. 
     Downshifting to a lower gear is accomplished by reversing the steps described above for the corresponding upshift. 
     When the vehicle is stationary in reverse mode, the transmission is prepared for reverse motion by applying clutch  102 , brake  64 , and brake  70 . The engine must be running in order to supply hydraulic pressure to clutch  102 . The engine drives impeller  82 , and hydro-dynamic forces within the torque converter generate torque on turbine  84  and gearbox input shaft  16 . Brakes  64  and  70  provide a reaction torque such that a multiple of the input torque, in the opposite direction of input torque, is transferred to output shaft  12 , accelerating the vehicle. 
     As with the first embodiment, the invention may be practiced according to the second embodiment, with some functional limitations, without the one way brake or the torque converter or with all hydraulic actuation. 
     In accordance with the provisions of the patent statutes, the structure and operation of the preferred embodiment has been described. A number of variations have also been suggested. However, it should be noted that alternate embodiments can be practiced otherwise than as specifically illustrated and described.