Abstract:
An eight-speed automatic transmission having a compound planetary gear set, a second planetary gear set, and a third planetary gear set each driven by three clutches and fixed to four brakes is provided.

Description:
FIELD OF THE INVENTION 
     The present invention relates to automatic transmissions, and more particularly to an eight-speed automatic transmission for use in a motor vehicle. 
     BACKGROUND OF THE INVENTION 
     A typical automatic transmission in a motor vehicle has two or three planetary gear sets, one of which receives a torque input from an engine, another one of which is coupled to a drive shaft for providing a torque output. During operation, a set of frictional units couple the torque input from the engine to one or more members of the planetary gear sets. Simultaneously, another set of frictional units holds stationary one or more members of the planetary gear sets. These frictional units provide different ratios of input-to-output torque to the vehicle. It is desirable to provide an automatic transmission for a vehicle that includes a wider range and a larger number of transmission ratios. 
     Automatic transmissions are typically controlled by a hydraulic control system. These hydraulic control systems are used to engage and disengage the frictional units of the transmission according to the ratio of torque needed. A typical hydraulic control system is disclosed in U.S. Pat. No. 6,159,124 to Redinger et al., herein incorporated by reference. The typical hydraulic control system is composed of various valves that direct and regulate hydraulic pressure to the frictional units via various fluid passages. 
     SUMMARY OF THE INVENTION 
     The eight-speed automatic transmission according to the principles of the present invention has a compound planetary gear set, a second planetary gear set, and a third planetary gear set. The compound planetary gear set has a small sun gear, a large sun gear, a first carrier, and a first ring gear. The second planetary gear set has a second sun gear drivingly engaged with the first carrier, a second carrier drivingly engaged with the large sun gear, and a second ring gear. The third planetary gear set has a third sun gear, a third carrier drivingly engaged with the second ring gear and drivingly engaged with an output shaft, and a third ring gear drivingly engaged with the second carrier. 
     The eight-speed automatic transmission further includes a first clutch selectively receiving an input from an engine and drivingly engaged with the third sun gear, a second clutch selectively receiving an input from the engine and drivingly engaged with the large sun gear and the second carrier, and a third clutch selectively receiving an input from the engine and drivingly engaged with the small sun gear. A first brake is engaged with the third ring gear and the second carrier for selectively fixing the third ring gear and the second carrier from rotation. A second brake is engaged with the first ring gear for selectively fixing the first ring gear from rotation. A third brake is engaged with the first carrier for selectively fixing the first carrier from rotation. A fourth brake is engaged with the small sun gear for selectively fixing the small sun gear from rotation. Selectively engaging one of the clutches and one of the brakes provides an input-to-output torque ratio corresponding to one of eight forward gear speeds and at least one reverse speed. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a schematic diagram of an eight-speed automatic transmission according to the principles of the present invention; 
     FIG. 2 is a table showing the combination of clutches and brakes to be applied to achieve specific torque ratios according to the principles of the present invention; and 
     FIG. 3 is a detailed view of the eight-speed automatic transmission according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring to FIGS. 1 and 3, an eight-speed automatic transmission for use in a motor vehicle is generally indicated by reference numeral  10 . The eight-speed automatic transmission  10  is linked to an engine, not shown, through an engine output shaft  2 . Rotational output from the engine output shaft  2  is received by the eight-speed automatic transmission  10  through a torque converter assembly  4 . The torque converter assembly  4  then transfers the rotational output through a gear set  6 , as will be described in greater detail below, to a transmission output shaft  8  and then on to the drivetrain of the motor vehicle. 
     The gear set  6  of the eight-speed automatic transmission  10  comprises a compound planetary gear set  12 , a second planetary gear set  14 , and a third planetary gear set  16 . In the preferred embodiment, the compound planetary gear set  12  is a Ravigneaux Planetary Gear Set as is well known in the art. However, various other gear set types may be employed. The compound planetary gear set  12  includes a small sun gear  18  engaged with a plurality of first pinions  20  (one of which is shown), and a large sun gear  22  engaged with a plurality of second pinions  24  (one of which is shown). The first and second pinions  20 ,  24  are rotatably supported on a carrier  26  and the plurality of first pinions  20  are engaged with a ring gear  28 . The large sun gear is provided with a splined connection to a first intermediate shaft  29 . 
     The second planetary gear set  14  includes a second sun gear  30  engaged with a plurality of pinions  32  (one of which is shown). The plurality of pinions  32  are rotatably supported on a second carrier  34  and engaged with a second ring gear  36 . The second sun gear  30  is drivingly engaged with the carrier  26  of the compound gear set  12 . The second carrier  34  is drivingly connected to the first intermediate shaft  29 . 
     The third planetary gear set  16  includes a third sun gear  38  engaged with a plurality of pinions  40 . The plurality of pinions  40  are rotatably supported on a third carrier  42  and engaged with a third ring gear  44 . The third carrier  42  is drivingly engaged with the second ring gear  36  of the second planetary gear set  14 . The third ring gear  44  is drivingly engaged with the second carrier  34  of the second planetary gear set  14 . The third carrier  42  rotates to produce a torque output to the transmission output shaft  8 . The third sun gear  38  is provided with a splined connection to a second intermediate shaft  45  concentrically disposed with said first intermediate shaft  29 . 
     The eight-speed automatic transmission  10  further includes a first clutch  46 , a second clutch  48 , and a third clutch  50 . Clutches  46 ,  48 , and  50  are each selectively engagable to receive the torque input from the torque converter assembly  4  via a transmission input shaft  51 . The first clutch  46  is drivingly connected to the third sun gear  38  of the third planetary gear set  16  via shaft  45 . The second clutch  48  is drivingly connected to the large sun gear  22  of the compound planetary gear set  12  as well as to the second carrier  34  of the second planetary gear set  14  via shaft  29 . The third clutch  50  is drivingly connected to the small sun gear  18  of the compound planetary gear set  12  via a third intermediate shaft  53 . 
     The eight-speed automatic transmission  10  further includes a first brake  52 , a second brake  54 , a third brake  56 , and a fourth brake  58 . The first brake  52  is drivingly connected to the third ring gear  44  of the third planetary gear set  16 , the second carrier  34  of the second planetary gear set  14 , as well as the first intermediate shaft  29 . The second brake  54  is drivingly connected to the ring gear  28  of the compound planetary gear set  12 . The third brake  56  is drivingly connected to the carrier  26  of the compound planetary gear set  12 , as well as the second sun gear  30  of the second planetary gear set  14 . The fourth brake  58  is drivingly connected to the small sun gear  18  of the compound planetary gear set  12  via the third intermediate shaft  53 . Each of the brakes  52 ,  54 ,  56 ,  58  is selectively fixable such that the brakes  52 ,  54 ,  56 ,  58  prevent rotation of corresponding attached planetary gear set components. 
     During operation of the eight-speed automatic transmission  10 , the torque input from the input shaft  51  is transferred through one of the clutches  46 ,  48 ,  50  to the planetary gear sets  12 ,  14 ,  16  and on to the third carrier  42  providing the torque output to output shaft  8 . The third carrier  42  is connected to the transmission output shaft  8  which transmits the rotational output of the engine to the drivetrain of the motor vehicle. To achieve specific torque input-to-output ratios, one or more of the clutches  46 ,  48 ,  50  and brakes  52 ,  54 ,  56 ,  58  are engaged to receive torque input from the engine and/or to prevent rotation of attached gears and/or carriers. There is shown in FIG. 2 a table illustrating the combination of clutches and brakes engagable to achieve specific torque input-to-output ratios. Each clutch and brake combination corresponds to one of eight forward gear speeds, two reverse speeds, and two neutral speeds, each of which correspond to a torque input-to-output ratio. 
     The above description of the invention is merely exemplary in nature and, thus, variations that do not depart from the general scheme of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.