Abstract:
An automatic transmission is disclosed providing eight forward ratios and one reverse ratio wherein the first forward ratio is selected by engaging exactly one friction brake using an electro-mechanical actuator. An operating method is disclosed for a vehicle having a transmission with this characteristic. The disclosed operating method permits the engine to be shut off to save fuel while the vehicle is stationary and then to start moving before an engine driven pump creates hydraulic pressure to engage the clutches.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. Pat. No. 8,187,139 issued May 29, 2012, the disclosure of which is incorporated in its entirety by reference herein. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to the field of automatic transmissions for motor vehicles. More particularly, the disclosure pertains to a method of operating a motor vehicle designed to enable stopping the internal combustion engine when the vehicle is stationary. 
     BACKGROUND 
     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 transmission is adapted for this idle engine stop feature by adding an electrically driven pump to provide hydraulic pressure to engage the appropriate friction elements. 
     SUMMARY 
     The disclosed transmission and associated operating method utilize electro-mechanically actuated brakes to engage the launch gear ratio. The electro-mechanically actuated brakes can be operated without hydraulic pressure, permitting the controller to engage a forward launch gear ratio of the transmission while the engine is shut down and therefore not driving a hydraulic pump. In some embodiments, a reverse gear ratio can also be engaged using only electro-mechanically actuated brakes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a transmission. 
         FIG. 2  is a table showing proposed tooth numbers for each of the gears in  FIG. 1 . 
         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 flow chart further illustrating a method of operation of the transmission of  FIG. 1 . 
         FIG. 5  is a flow chart further illustrating a method of operation of the transmission of  FIG. 1 . 
         FIG. 6  is a flow chart further illustrating an alternate method of operation of the transmission of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that can be embodied in various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     A transmission 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 . 
     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 can 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. 
     Operation of the transmission is illustrated in the flow charts of  FIGS. 4 and 5 . When the vehicle is stationary in drive (forward) mode  100 , 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 an engine start condition, typically by releasing the brake pedal, the engine is quickly started  102 . 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 in first gear  104 . 
     To shift to second gear  106 , motor  92  is used to progressively engage brake  64 , maintaining brake  68  in the fully applied state in step  108 . 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 gear  106  to third gear  110 , clutch  60  is progressively engaged while brake  64  is progressively released in step  112 . To shift from third gear  110  to fourth gear  114 , clutch  62  is progressively engaged while clutch  60  is progressively released in step  116 . 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 gear  114  to fifth gear  118 , clutch  60  is progressively engaged while brake  68  is progressively released in step  120 . Fifth gear  118  is a direct drive gear. To shift from fifth gear  118  to sixth gear  122 , brake  64  is progressively engaged while clutch  60  is progressively released in step  124 . To shift from sixth gear  122  to seventh gear  126 , brake  70  is progressively engaged while brake  64  is progressively released in step  128 . Clutch  62  is maintained in the fully applied state through all of these transitions. 
     Eighth gear  130  provides improved fuel economy for high speed driving. Unfortunately, it is not possible to shift directly from seventh gear  126  to eighth gear  130  without interrupting the flow of power through the transmission. There are two ways to get into eighth gear. The first method is to first interrupt the engine torque  132 , then release clutch  62  and brake  70  and apply clutch  60  and brake  64  in step  134 , and then restore engine torque  136 . 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  in step  138 , maintaining brake  64  in 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, certain embodiments can be implemented, with some functional limitations, with hydraulic actuation of brakes  64  and  70 . Specifically, if either brake  64  or brake  70  is 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, embodiments can be implemented without torque converter  80  by driveably connecting gearbox input shaft  16  to transmission input shaft  10 , preferably via a torsional damper. Operation of such an embodiment is illustrated in the flow chart of  FIG. 6 . In drive idle state  100 ′, no clutches or brakes are engaged. When the driver releases the brake pedal, the engine is started  102 ′ and then brake  68  is gradually engaged  140 . All remaining shifts are accomplished in the manner described above and illustrated in  FIGS. 4 and 5 . 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  can 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. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments can be combined to form further embodiments of the invention.