BACKGROUND OF THE INVENTION 
   This invention relates to the field of power transmissions for motor vehicles. More particularly, it pertains to an automatic transmission for an automotive vehicle having an auxiliary gear unit and a primary gear unit. The auxiliary gear unit produces a direct drive ratio and can either underdrive or overdrive its output. 
   Various techniques have been used to increase the number of gear ratios produced by a multiple speed ratio automatic transmission that employs a simple planetary gearset arranged in series with a compound planetary gearset. For example, U.S. Pat. Nos. 5,755,637 and 5,43,825 disclose an automatic transmission for a motor vehicle that includes a compound planetary gearset coaxial with an input shaft and operating under control of clutches and brakes to establish first to fourth forward drive ratios. A simple planetary gear set coaxial with an output shaft and provides a fifth forward drive ratio by further increasing the pre-established fourth ratio. A chain and sprocket coupler delivers rotary power from the compound planetary gearset to the simple planetary gearset. 
   There is, however, a need for automatic transmissions to produce an increasing larger number of gear ratios and wider ratio spans. Larger ratio spans improve drive-away performance and fuel economy at the same time. Preferably a six-speed transmission would provide a ratio span of at least 6.0, and a seven-speed transmission would have a ratio span of 8.0 or greater. 
   It is desired that a transmission having six or seven forward ratios should provide also multiple reverse gear ratios to enhance performance in winter driving conditions. Preferably, a transmission would also provide engine braking at least optionally over a range of forward ratios. 
   SUMMARY OF THE INVENTION 
   This invention relates to a transmission kinematic arrangement capable of achieving multiple forward gear ratios and two reverse ratios with two planetary gear units arranged in series and controlled by a minimum number of friction clutches and brakes. 
   The transmission can operate in six or seven forward speed ratios with few structural changes to the components or the assembly. The two available reverse gear ratios provide a preferred range of options that is particularly useful to an operator in winter driving or whenever road conditions are slippery. Auxiliary coast clutches arranged in series with overrunning one-way clutches provide engine braking over a range of suitable forward ratios. 
   The overall ratio span produced by the transmission with gears and pinions having-preferred sizes is 6.0 for the six-speed version and 8.0 for the seven-speed version. Larger ratio spans improve drive-away performance and improve fuel economy compared to a transmission having a narrower span of ratios. 
   A multiple ratio transmission according to this invention having these and other advantages includes a planetary primary gear unit producing forward ratios and a reverse ratio, and a planetary auxiliary gear unit in series with the primary gear unit producing a direct drive ratio and either an underdrive ratio or an overdrive ratio. The transmission produces six or seven forward ratios and two reverse ratios by combining the ratios of the primary and auxiliary gear units. Five friction clutches and brakes control operation of the primary unit, and operation of the auxiliary unit is controlled by a clutch and a brake. 
   Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a transmission according to this invention that includes an auxiliary gear unit for alternately producing a direct drive ratio and an underdrive ratio; 
       FIG. 2  is a chart indicating the status of the clutches and brakes for each forward and reverse ratio and the magnitude of the gear ratios and the ratio steps for the transmission of  FIG. 1 ; 
       FIG. 3  is a chart containing a preferred number of teeth of each gear and pinion of the transmission of  FIG. 1 ; 
       FIG. 4  is a schematic diagram of another transmission according to this invention that includes an auxiliary gear unit for alternately producing a direct drive ratio and an overdrive ratio; 
       FIG. 5  is a chart indicating the status of the clutches and brakes for each forward and reverse ratio and the magnitude of the gear ratios and the ratio steps for the transmission of  FIG. 4 ; 
       FIG. 6  is a chart containing a preferred number of teeth of each gear and pinion of the transmission of  FIG. 4 ; 
       FIG. 7  is a chart indicating the status of the clutches and brakes for each forward and reverse ratio and the magnitude of the gear ratios and the ratio steps for a seven speed version of the transmission of  FIG. 1 ; 
       FIG. 8  is a chart containing a preferred number of teeth of each gear and pinion of the transmission of  FIG. 7 ; 
       FIG. 9  is a chart indicating the status of the clutches and brakes for each forward and reverse ratio and the magnitude of the gear ratios and the ratio steps for a seven speed version of the transmission of  FIG. 4 ; 
       FIG. 10  is a chart containing a preferred number of teeth of each gear and pinion of the transmission of  FIG. 12 . 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings, there is illustrated in  FIG. 1  a transmission  10  having a hydrokinetic torque converter  12  driveably connected to an engine crankshaft  14 . The torque converter includes three bladed wheels: an impeller  16 , turbine  18 , and stator  20 , which together define a toroidal fluid flow circuit within the casing of the torque converter. The stator  20  is supported on an overrunning brake  22 , which anchors the stator to the shaft to prevent rotation of the stator in a direction opposite to the direction of rotation of the impeller and to permit free-wheeling rotation in the opposite direction. Turbine  18  is connected to a transmission input  24 . A bypass or lock-up clutch  26 , located between crankshaft  14  and input  24 , direct mechanically connects the engine shaft and input when the clutch is engaged. When clutch  26  is disengaged, engine shaft  14  and input  24  are hydrodynamically connected through operation of the torque converter  12 . 
   A primary gear unit  30  includes two interconnected planetary gearsets. The first gearset includes a sun gear  34 , ring gear  36 , carrier  38  and planetary pinions  40 , rotatably supported on carrier  38  and in continuous meshing engagement with sun gear  34  and ring gear  36 . The second gearset includes a sun gear  40 , ring gear  42 , carrier  44  and planet pinions  46 , rotatably supported on carrier  44  and in continuous meshing engagement with sun gear  40  and ring gear  42 . 
   A forward clutch  46 , a hydraulically actuated friction clutch, releasably connects input  24  and sun gear  34 . A direct clutch  48 , a hydraulically actuated friction clutch, releasably connects input  24  and carrier  44 . A low/reverse brake  50 , releasably holds ring gear  36  and carrier  44  fixed against rotation on the transmission housing  52 . An overrunning low clutch  54 , arranged in parallel with clutch  50 , provides a one-way drive connection between housing  52  and ring gear  36  and carrier  44  in one rotary direction and permits ring gear  36  and carrier  44  to rotate relative to the housing in the opposite direction. An intermediate brake  56 , a band brake, releasably holds sun gear  40  and drum  58  fixed against rotation on the transmission housing  52 . A reverse clutch  60 , a hydraulically actuated friction clutch, releasably connects input  24  and drum  58 , which is driveably secured to sun gear  40 . Carrier  44  is secured to ring gear  36 . Carrier  38  is secured to ring gear  42 . 
   An auxiliary planetary gear unit  62  alternately produces a direct drive ratio or an underdrive ratio depending on the engaged or disengaged state, respectively, of an auxiliary clutch. Gear unit  62  includes a sun gear  64 , ring gear  66 , carrier  68  and planetary pinions  70 , rotatably supported on carrier  68  and in continuous meshing engagement with sun gear  64  and ring gear  66 . Ring gear  66  is driveably connect to carrier  38 , by a chain drive  72  or a set of meshing gears. 
   An auxiliary direct clutch  76 , a hydraulically actuated friction clutch, releasably connects sun gear  64  and carrier  68 . An auxiliary one-way brake  80 , arranged in parallel with coast brake  78 , provides a one-way drive connection between housing  52  and sun gear  64  in one rotary direction and permits sun gear  64  to rotate relative to the housing in the opposite direction. The auxiliary coast brake  78 , a hydraulically actuated friction brake, releasably connects sun gear  64  and the housing  52 . 
   Carrier  68  is driveably connected to a pinion  82  of a final drive unit, the pinion being in meshing engagement with a gear  84 , which is driveably connected to the central bevel gears  86  of a differential mechanism  88 . The differential mechanism transmits power differentially between carrier  68  and the driven wheels of the vehicle through axle shafts  90 ,  92  and side bevel gears  94 ,  96 , which mesh with bevel gears  86 . 
     FIG. 2  is a chart indicating the engaged and disengaged status of the clutches and brakes corresponding to each forward ratio and reverse ratio produced by the transmission of  FIG. 1 . The primary unit  30  produces four forward ratios and a reverse ratio; the auxiliary unit  62  produces a direct drive ratio and an underdrive ratio; and the transmission produces six forward ratios and two reverse ratios. In  FIG. 2 , “X” indicates engagement of the friction clutches and brakes and transmission of torque by the overrunning clutches and brakes. “O/R” indicates a one-way clutch or brake is overrunning. A blank indicates there is no relative rotation across the clutch or brake, or that no torque is being transmitted by the clutch or brake. 
   The first forward ratio is produced by engaging forward clutch  46 , which causes low one-way clutch  54  to drive and hold ring gear  36  and carrier  44  fixed against rotation, and auxiliary one-way brake  80  to drive and hold sun gear  64  fixed against rotation. This action causes the primary unit  30  to produce its first forward ratio and the auxiliary unit  62  to operate in an underdrive ratio. Sun gear  34  is driveably connected to input  24 , ring gear  36  is fixed against rotation, and carrier  38  is underdriven in relation to the speed of input  24 . In the auxiliary unit, ring gear  66  is driven at the speed of carrier  38 , sun gear  64  is held against rotation, and an additional speed reduction occurs in auxiliary gear unit  62 , whereby carrier  68  underdrives output  81  in relation to the speed of input  24 . 
   An upshift to the second forward ratio results by switching the auxiliary unit from underdrive to direct drive operation by engaging the auxiliary direct clutch  76 , which causes the auxiliary one-way clutch  80  to overrun and driveably connects carrier  68  and sun gear  64  mutually. This action allows the primary unit  30  to continue to produce its first forward ratio and the auxiliary unit  62  to operate in a direct drive ratio. The primary unit  30  underdrives its output, carrier  38 , through the same torque path as in the first forward transmission ratio, and the auxiliary gear unit  62  is locked-up due to the drive connection between carrier  68  and sun gear  64  through the auxiliary direct clutch  76 . Output  81  and carrier  68  are underdriven at the same speed as carrier  38 . 
   The third forward ratio is produced by engaging forward clutch  46  and intermediate band  56  and by operating the auxiliary unit  62  in the underdrive ratio. This action causes the primary unit  30  to produce its second forward ratio. With sun gear  40  held fixed against rotation, thereby causing one-way-clutch  54  to overrun, carrier  38  is underdriven in relation to the speed of input  24  and sun gear  34 , and ring gear  66  is driven at that underdriven speed. The auxiliary gear unit  62  further reduces the speed of carrier  68  and output  81  in relation to the speed of the ring gear  66  by releasing auxiliary direct clutch  76  at the same time. 
   The fourth forward ratio is produced when forward clutch  46  is engaged, direct clutch  48  is engaged, and intermediate band  46  is released, thereby causing one-way clutch  54  to continue over-rolled and to keep the auxiliary unit  62  in the underdrive ratio operation mode. This action causes the primary unit  30  to produce its third forward ratio, a direct drive ratio. Sun gear  34  and ring gear  36  rotate at the speed of input  24 , and carrier  38  and ring gear  66  rotate at the speed of input  24 . The auxiliary unit  62  produces a speed reduction because sun gear  64  is fixed against rotation and the output  81  is underdriven in relation to the speed of input  24 . 
   The fifth forward ratio is produced when the primary unit  30  operates in its third forward ratio and auxiliary unit  62  operates in its direct drive ratio. With the transmission  10  so disposed, primary unit  30  is locked-up because ring gear  36  and sun gear  34  are driveably connected to input  24  through direct clutch  48  and forward clutch  46 , respectively. Similarly, the auxiliary gear unit  62  is locked-up because carrier  68  is driveably connected to sun gear  64  through the auxiliary direct clutch  76 . Therefore, output  81  is driven at the speed of input  24 . 
   The sixth forward ratio is produced by disengaging forward clutch  46 , engaging intermediate band  56  and direct clutch  48 , and by operating auxiliary unit  62  in its direct drive ratio. This action causes the primary unit  30  to produce its fourth forward ratio. With sun gear  40  fixed against rotation and providing a torque reaction, input  24  is driveably connected to carrier  44  due to the engagement of direct clutch  48 , and the first output, ring gear  42  and carrier  38 , is overdriven in relation to the speed of input  24 . The auxiliary gear unit  62  is locked-up due to the engagement of auxiliary direct clutch  76 , and the output  81  is overdriven relative to the speed of input  24   
   The primary unit  30  produces a first reverse ratio by engaging low/brake  50  and reverse clutch  60 . Brake  50  holds carrier  44  and ring gear  36  fixed against rotation; reverse clutch  60  driveably connects input  24  to sun gear  40 . This underdrives the first output, ring gear  42  and carrier  38 , in a reverse direction relative to input  24 . Operating the primary unit  30  in its reverse ratio and operating the auxiliary unit  62  in its underdrive ratio by engaging auxiliary coast clutch  78  produce a first transmission reverse ratio. Operating the primary unit  30  in its reverse ratio and operating the auxiliary unit  62  in its direct drive ratio by disengaging auxiliary coast clutch  78  and engaging auxiliary direct clutch  76  produce a second transmission reverse ratio. 
     FIG. 2  shows the gear ratios and ratio steps produced by the primary and auxiliary gear units of the transmission of  FIG. 1  when the gears and pinions have the number of gear teeth indicated in  FIG. 3 .  FIG. 2  also contains the overall ratios produced by the transmission of  FIG. 1  and the ratio steps for an example of this invention in which the gears and pinions have the number of teeth indicated in the chart of  FIG. 3 . The overall transmission ratio span is 5.94. 
   The transmission of  FIG. 1  can operate as described above upon deleting the low/reverse one-way brake  54  from primary unit  30  and one-way brake  80  from auxiliary unit  62 . The low/reverse brake  50  is engaged instead of producing a drive connection through low one-way brake  54  in the first and second forward ratios. Otherwise, the primary unit  30  operates as discussed above with reference to  FIG. 1 . Similarly, the auxiliary unit  62  produces its direct drive ratio by engaging the low brake  78  instead of producing a drive connection through the one-way brake  80  in the first, third and fourth forward ratios and in the first reverse ratio. Otherwise, the auxiliary unit  62  operates as discussed above with reference to  FIG. 1 . 
     FIG. 4  is a schematic diagram of a transmission identical to that of  FIG. 1  except that it includes an auxiliary gear unit  100 , which is substituted for the auxiliary gear unit  62  of  FIG. 1 . Gear unit  100  alternately produces a direct drive ratio and an overdrive ratio of the speed of input  24  in relation to the speed of its input, which is carrier  106 . Gear unit  100  includes a sun gear  102 , ring gear  104 , carrier  106 , and planetary pinions  108 , rotatably supported on carrier  106  and in continuous meshing engagement with sun gear  102  and ring gear  104 . Carrier  106  is driveably connect to carrier  38  by a chain drive  110  or a set of meshing gears, and the output of gearset  100 , taken at ring gear  104 , is driveably connected to a pinion  82  of the final drive unit. 
   An auxiliary overdrive brake  112 , a hydraulically actuated friction brake, releasably secures sun gear  102  against rotation on the housing  52 . An auxiliary coast clutch  114  releasably connects sun gear  102  and carrier  106 . An auxiliary one-way clutch  116 , arranged in parallel with clutch  114 , provides a one-way drive connection between sun gear  102  and carrier  106 . 
     FIG. 5  is a chart indicating the engaged and disengaged status of the clutches and brakes corresponding to each forward and reverse gear ratio of the transmission of  FIG. 4 . In  FIG. 5 , “X” indicates engagement of the friction clutches and brakes and transmission of torque by the overrunning clutches and brakes. “O/R” indicates a one-way clutch or brake is overrunning. A blank indicates there is no relative rotation across the clutch or brake, or that no torque is being transmitted by the clutch or brake. “( )” indicates a clutch is engaged to provide engine braking during a coast condition, i.e., when torque is transmitted from the driven wheels to the engine crankshaft  14 . 
   The primary unit  30  of the transmission of  FIG. 4  produces four forward ratios and a reverse ratio identically as described above with reference to the primary unit  30  of the transmission of  FIG. 1 . The auxiliary unit  100  of the embodiment of  FIG. 4  produces a direct drive ratio and an overdrive ratio. The transmission of  FIG. 4  produces six forward ratios and two reverse ratios; however, the ratios of the auxiliary unit  100  are combined with the forward ratios produced by the primary unit  30  as shown in  FIG. 5 , a different combination than that of  FIG. 1 . 
     FIG. 5  contains the ratios produced by the primary and auxiliary gear units  30 ,  100  of the transmission of  FIG. 4 , the overall ratios produced by the transmission, and ratio steps when the gears and pinions have the number of gear teeth indicated in  FIG. 6 . The overall ratio span is 5.94. 
   The transmission of  FIG. 4  can operate as described above upon deleting the low one-way brake  54  from primary unit  30  and one-way clutch  116  from auxiliary unit  100 . The low/reverse brake  50  is engaging instead of producing a drive connection through low one-way brake  54 . Otherwise, the primary unit  30  operates as discussed above with reference to  FIG. 4 . Similarly, the auxiliary unit  100  produces its direct drive ratio by engaging the low clutch  114  instead of producing a drive connection through the one-way clutch  116 . Otherwise, the auxiliary unit  100  operates as discussed above with reference to  FIG. 4 . 
     FIG. 7  is a chart indicating the engaged and disengaged status of the clutches and brakes corresponding to each forward ratio and reverse ratio produced by a seven speed version of the transmission of  FIG. 1 . The primary unit  30  produces four forward ratios and a reverse ratio; the auxiliary unit  62  produces a direct drive ratio and an underdrive ratio; and the transmission produces seven forward ratios and two reverse ratios. In  FIG. 7 , “X” indicates engagement of the friction clutches and brakes and transmission of torque by the overrunning clutches and brakes. “O/R” indicates a one-way clutch or brake is overrunning. A blank indicates there is no relative rotation across the clutch or brake, or that no torque is being transmitted by the clutch or brake. “( )” indicates a clutch is engaged to provide engine braking during a coast condition, i.e., when torque is transmitted from the driven wheels to the engine crankshaft  14 . 
   The primary unit  30  operates as described above with reference to  FIG. 1  by producing its first forward ratio in the first and second forward transmission ratios; by producing its second forward ratio in the third and fourth forward transmission ratios; by producing its third forward ratio, a direct drive ratio, in the fifth and sixth forward transmission ratios; and by producing its fourth forward ratio in the seventh forward transmission ratio. 
   The auxiliary unit  62  operates as described above by producing its underdrive ratio in the first, third and fifth forward transmission ratios; and by producing its direct drive ratio in the second, fourth, sixth and seventh forward transmission ratios. In this way, an addition ratio, the fourth transmission ratio, is added to the six transmission ratios described above with reference to  FIG. 1 . 
     FIG. 7  contains the ratios produced by the primary and auxiliary gear units  30 ,  62  and the overall ratios produced by the seven-speed version of the transmission of  FIG. 1  when the gears and pinions have the number of teeth indicated in  FIG. 8 . 
   The overall ratio span of the seven-speed transmission of  FIG. 7  is 8.01. 
   The transmission of  FIG. 7  can operate as described above upon deleting the low/reverse one-way brake  54  from primary unit  30  and one-way brake  80  from auxiliary unit  62 . The low/reverse brake  50  is engaging instead of producing a drive connection through low one-way brake  54 . Otherwise, the primary unit  30  operates as discussed above with reference to  FIG. 1 . Similarly, the auxiliary unit  62  produces its direct drive ratio by engaging the coast low brake  78  instead of producing a drive connection through the one-way brake  80  Otherwise the auxiliary unit  62  operates as discussed above with reference to  FIG. 1 . 
     FIG. 9  is a chart indicating the engaged and disengaged status of the clutches and brakes corresponding to each forward ratio and reverse ratio produced by a seven speed version of the transmission of  FIG. 4 . The primary unit  30  produces four forward ratios and a reverse ratio; the auxiliary unit  100  produces a direct drive ratio and an overdrive ratio; and the transmission produces seven forward ratios and two reverse ratios. In  FIG. 9 , “X” indicates engagement of the friction clutches and brakes and transmission of torque by the overrunning clutches and brakes. “O/R” indicates a one-way clutch or brake is overrunning. A blank indicates there is no relative rotation across the clutch or brake, or the clutch or brake is transmitting that no torque. “( )” indicates a clutch is engaged to provide engine braking during a coast condition, i.e., when torque is transmitted from the driven wheels to the engine crankshaft  14 . 
   The primary unit  30  operates as described above with reference to  FIG. 1  by producing its first forward ratio in the first and second forward transmission; by producing its second forward ratio in the third and fourth forward transmission ratios; by producing its third forward ratio, a direct drive ratio, in the fifth and sixth forward transmission ratios; and by producing its fourth forward ratio in the seventh forward transmission ratio. 
   The auxiliary unit  100  operates as described above with reference to  FIG. 4  by producing its overdrive ratio in the second, fourth, sixth and seventh forward transmission ratios, and by producing its direct drive ratio in the first, third and fifth forward transmission ratios. In this way, an addition ratio, the fourth transmission ratio, is added to the six transmission ratios described above with reference to  FIG. 4 . 
     FIG. 9  contains the ratios produced by the primary and auxiliary gear units  30 ,  100  the overall ratios and ratio steps produced by the seven-speed version of the transmission of  FIG. 4  when the gears and pinions have the number of teeth indicated in  FIG. 10 . The overall ratio span of the seven speed transmission is 8.01. 
   The transmission of  FIG. 9  can operate as described above upon deleting the low one-way brake  54  from primary unit  30  and one-way clutch  116  from auxiliary unit  100 . The low/reverse brake  50  is engaging instead of producing a drive connection through low one-way brake  54 . Otherwise, the primary unit  30  operates as discussed above with reference to  FIG. 4 . Similarly, the auxiliary unit  100  produces its direct drive ratio by engaging the low clutch  114  instead of producing a drive connection through the one-way clutch  116 . Otherwise, the auxiliary unit  100  operates as discussed above with reference to  FIG. 4 . 
   In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.
A multiple ratio transmission having a planetary primary gear unit producing forward ratios and a reverse ratio, and a planetary auxiliary gear unit in series with the primary gear unit producing a direct drive ratio and either an underdrive ratio or an overdrive ratio. The transmission produces six or seven forward ratios and two reverse ratios by combining the ratios of the primary and auxiliary gear units. Five friction clutches and brakes plus one one-way clutch control operation of the primary unit, and operation of the auxiliary unit is controlled by a clutch and a brake plus one one-way clutch. The same six or seven ratios can be achieved with a total of seven friction clutches and brakes in the primary and auxiliary unit by deleting the one-way clutches.