Abstract:
A primary shift portion has a first predetermined speed ratio at a first shift-speed, a second predetermined speed ratio at a second shift-speed, a speed ratio of “1” at third and fifth shift-speeds, and a third predetermined speed ratio at fourth and sixth shift-speeds. A secondary shift portion has a predetermined reduction speed ratio at first, second, third, and fourth shift-speeds, and a speed ratio of “1” at fifth and sixth shift-speeds. The method realizes a 6-speed automatic transmission being capable of sequentially up- and down-shifting and a plurality of skip shifts with an existing 5-speed automatic transmission.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a shift control method for an automatic transmission. More particularly, the present invention relates to a 6-speed shift control method for an automatic transmission to improve power performance and fuel economy by obtaining 6-speeds from an existing 5-speed automatic transmission.  
         BACKGROUND OF THE INVENTION  
         [0002]    Presently, 4-speed and 5-speed automatic transmissions are commonly used in vehicles. Such automatic transmissions are developed differently by vehicle makers depending on the type of vehicle.  
           [0003]    However, current shift control methods have a number of drawbacks. For example, it is difficult to improve power performance and fuel economy in prior shift control methods. Furthermore, customer requirements for more speeds have not yet been satisfied.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention provides a shift control method that provides six forward speeds with an existing 5-speed automatic transmission. Control of shifting is easier, power performance and fuel economy with optimal gear ratios are improved, shift quality is improved such that shift shock is minimized, and the weight of an automatic transmission is decreased.  
           [0005]    Consequently, the present invention provides a shift control method of an automatic transmission, the automatic transmission includes a primary shift portion including two single pinion planetary gear sets, three clutches, two brakes, and a first one-way clutch. The automatic transmission also includes a secondary shift portion including a single pinion planetary gear set, a fourth clutch, a third brake, and a second one-way clutch.  
           [0006]    In use, the automatic transmission is controlled at a first shift-speed, such that the primary shift portion has a first predetermined speed ratio and the secondary shift portion has a predetermined reduction ratio. The automatic transmission is then controlled at a second shift-speed such that the primary shift portion has a second predetermined speed ratio and the secondary shift portion has said predetermined reduction ratio. The automatic transmission is also controlled at a third shift-speed such that the primary shift portion has a speed ratio of “1” and the secondary shift portion has said predetermined reduction ratio. The automatic transmission is controlled at a fourth shift-speed such that the primary shift portion has a third predetermined speed ratio and the secondary shift portion has said predetermined reduction ratio. Additionally, the automatic transmission is controlled at a fifth shift-speed such that the primary shift portion has a speed ratio of “1” and the secondary shift portion has a speed ratio of “1”. Finally, the automatic transmission is controlled at a sixth shift-speed such that the primary shift portion has said third predetermined speed ratio and the secondary shift portion has speed ratio of “1”. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a schematic diagram of an automatic transmission to which a preferred embodiment of the present invention is applied;  
         [0008]    [0008]FIG. 2 is a lever diagram showing the process of shifting according to a preferred embodiment of the present invention; and  
         [0009]    [0009]FIG. 3 is an operational chart showing friction elements engaged at each shift-speed according to a preferred embodiment of the present invention. 
     
    
       [0010]    Preferred embodiments of the present invention will be described in detail below while referring to the attached figures. Like reference numerals refer to corresponding parts throughout the several views of the drawings.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0011]    [0011]FIG. 1 is a schematic diagram of a 5-speed automatic transmission to which the present invention is applied. Reference numerals of parts in subsequent figures will refer to the same of FIG. 1, which have been explained above.  
         [0012]    The 5-speed automatic transmission is composed of a primary shift portion M including two single pinion planetary gear sets  4  and  6 , and a secondary shift portion S including a single pinion planetary gear set. The primary shift portion M receives power from a turbine of a torque converter through an input shaft  2 . The primary shift portion subsequently transmits torque to first and second single pinion planetary gear sets  4  and  6 . Power is then transmitted to the secondary shift portion S through a transfer drive gear  10  connected with a planetary carrier  8  of the first single pinion planetary gear set  4 .  
         [0013]    A first planetary carrier  8  and a second ring gear  20  of the primary shift portion M are fixedly connected as an output element of the primary shift portion. A first ring gear  14  and a second planetary carrier  18  are fixedly connected to each other. A first sun gear  12 , the second planetary carrier  18  and a second sun gear  16  are variably connected to the input shaft  2  by interposition of a first clutch C 1 , a second clutch C 2 , and a third clutch C 3 , respectively. Therefore, the primary shift portion has three input elements.  
         [0014]    A connecting-member  22  which connects the first ring gear  14  and the second planetary carrier  18  is variably fixed to a housing  24  through a first brake B 1  and a first one-way clutch F 1 . The second sun gear  16  is variably fixed to the housing  24  through a second brake B 2 . Therefore, the primary shift portion has two fixed elements.  
         [0015]    A third ring gear  32  of the secondary shift portion S is an input element of the secondary shift portion. A third planetary carrier  30  is an output element of the secondary shift portion. A third sun gear  28  and the third planetary carrier  30  are connected by interposition of a fourth clutch C 4 . The third sun gear  28  is connected to the housing  24  through a third brake B 3  and a second one-way clutch F 2 .  
         [0016]    Accordingly, the above automatic transmission can develop five forward speeds and one reverse speed by engaging the first clutch C 1  and the third brake B 3  at a first speed; the first clutch C 1 , the second brake B 2 , and the third brake B 3  at a second speed; the first clutch C 1 , the second clutch C 2 , and the third brake B 3  at a third speed; the second clutch C 2  and the second brake B 2  at a fourth speed; the second clutch C 2 , the fourth clutch C 4 , and the second brake B 2  at a fifth speed; and the third clutch C 3 , the first brake B 1 , and the third brake B 3  at a reverse speed.  
         [0017]    [0017]FIGS. 2 and 3 are an alignment chart and a friction element engagement table, respectively, to describe the process of shifting according to the present invention. Referring to FIG. 2, in the primary shift portion, a first node N 1  of a lever is set as the first sun gear  12 , a second node N 2  as the first planetary carrier  8  and the second ring gear  20 , a third node N 3  as the first ring gear  14  and the second planetary carrier  18 , and a fourth node N 4  as the second sun gear  16 . The first planetary carrier  8  and the second ring gear are fixedly connected, and the first ring gear  14  and the second planetary carrier  18  are fixedly connected.  
         [0018]    In a secondary shift portion, a fifth node N 5  of the lever is set as the third ring gear  32 , a sixth node N 6  as the third planetary carrier  30 , and a seventh node N 7  as the third sun gear  28 . The above node-setting method is well known by those skilled in the art.  
         [0019]    First, the first brake B 1  and the third brake B 3  are engaged in N and P ranges. When a driver changes a gear select lever into drive (D range) to drive a vehicle, a transmission control unit (TCU) engages the first clutch C 1  and the first brake B 1 . When the first clutch C 1  is engaged, power from the vehicle&#39;s engine is inputted through the first sun gear  12 , and the first ring gear  14  and the second planetary carrier  18  operate as a fixed element by engaging the first brake B 1 . Therefore, a first speed in the primary shift portion is outputted through the second node N 2 , while the first node N 1  operates as an input element and the third node N 3  operates as a fixed element.  
         [0020]    The output of the primary shift portion is inputted into the third ring gear  32  of the secondary shift portion through a transfer drive gear, and the third sun gear  28  operates as a fixed element by operation of the second one-way clutch F 2 . Therefore, a first speed in the secondary shift portion is outputted through the sixth node N 6  to be finally reduced, while the fifth node N 5  operates as an input element and the seventh node N 7  operates as a fixed element.  
         [0021]    When a vehicle speed is increased in the first speed state, the TCU disengages the first brake B 1  and engages second brake B 2 . Then, in the primary shift portion M, the fixed element is converted from the third node N 3  to the fourth node N 4 , and the secondary shift portion has no change. Therefore, a second speed having a lower speed ratio than a first speed ratio is outputted.  
         [0022]    When a vehicle speed is increased in the second speed state, the TCU disengages the second brake B 2  to be disengaged, and engages the second clutch C 2  and the third brake B 3 . Then, because power from the engine is transmitted through the first clutch C 1  and the second clutch C 2 , the output speed ratio of the primary shift portion becomes 1. Also, because the third brake B 3  functions as the second one-way clutch F 2 , the secondary shift portion has the same reduction ratio as the first and second speeds, and a third speed is finally outputted.  
         [0023]    When a vehicle speed is increased in the third speed state, the TCU disengages the first clutch C 1  and engages the second brake B 2 . Then, in the primary shift portion, because the third node N 3  operates as an input element and the fourth node N 4  operates as a fixed element, overdrive occurs. Also, because the secondary shift portion has no change, a fourth speed is finally outputted.  
         [0024]    When a vehicle speed is increased in the fourth speed state, the TCU disengages both the second brake B 2  and the third brake B 3 , and engages the first clutch C 1  and the fourth clutch C 4 . Then, the output speed ratio of the primary shift portion becomes 1 as in the third speed, the speed ratio of the secondary shift portion becomes 1 by engaging the fourth clutch C 4 . Therefore, a fifth speed is outputted wherein the overall speed ratio is 1.  
         [0025]    When a vehicle speed is increased in the fifth speed state, the TCU disengages the first clutch C 1  and engages the second brake B 2 . Then, overdrive occurs in the primary shift portion as in the fourth speed, the secondary shift portion outputs a speed ratio of “1” as in the fifth speed. Therefore, a sixth speed having the lowest speed ratio is outputted.  
         [0026]    When a driver changes a select lever into R range to back-up the vehicle, the TCU engages the third clutch C 3 , the first brake B 1 , and the third brake B 3 . Then, in the primary shift portion, the fourth node N 4  operates as an input element and the third node N 3  operates as a fixed element, and in the secondary shift portion, the fifth node N 5  operates as an input element and the seventh node N 7  operates as a fixed element. Additionally, a reverse output occurs in the primary shift portion, it is reduced in the secondary shift portion, and reverse shifting is finally completed.  
         [0027]    It is described in FIG. 2 that the revolution direction of the primary shift portion is the same as the revolution direction of the secondary shift portion, but the directions are actually opposite each other in the case that the transfer drive gear  10  is directly engaged with the transfer driven gear  36 . Both directions are the same in the case in which an idle gear or a chain is located between the two gears.  
         [0028]    The above shift relation may be shown in the following table.  
                                                 TABLE 1                                   Shift                   Range   Primary Shift Portion   Secondary Shift Portion                                    D   1   Reduction   Reduction       (Forward)   2   Reduction   Reduction           3   Speed Ratio of “1”   Reduction           4   Overdrive   Reduction           5   Speed Ratio of “1”   Speed Ratio of “1”               (same as 3rd)           6   Overdrive   Speed Ratio of “1”               (same as 4th)       R       Reduction   Reduction       (Reverse)                  
 
         [0029]    The automatic transmission to which the present invention is applied is capable of skip shifting 3→1, 4→2, 5→3, 5→2, 6→4, and 6→3. This shifting can be achieved by directly controlling the engagement of friction elements of a target shift range in a current shift range during skip shifting.  
         [0030]    Furthermore, 6→3 skip-shift control may be accomplished by passing through a fourth speed. Skip shifting directly from a sixth speed to a third speed is not easy to control because three elements have to be simultaneously disengaged or engaged. Therefore, by passing through a fourth speed shifting process, that is, by disengaging one element C 4 , then by successively disengaging one element B 2  and engaging one element C 1 , shift quality may be improved. Fundamentally the same as in the above, skip-shift control may be also accomplished by passing through a third speed during 5→2 shifting.  
         [0031]    In FIG. 3, “O” in the brake and one-way clutch columns indicates that the elements can be alternatively engaged or disengaged. This is because the first one-way clutch F 1  can function as the first brake B 1  when it is operated, and the second one-way clutch F 2  can function as the third brake B 3  when it is operated.  
         [0032]    The skip-shifting processes are as follows.  
         [0033]    To achieve 3→1 shifting, the TCU disengages the second clutch C 2  and the third brake B 3  (at the third speed wherein the first clutch C 1 , the second clutch C 2 , and the third brake B 3  are engaged). At this time, control of the first brake B 1  is unnecessary because of the operation of the first one-way clutch F 1 .  
         [0034]    To achieve 4→2 shifting, the TCU disengages the second clutch C 2  and engages the first clutch C 1  (at the fourth speed wherein the second clutch C 2 , the second brake B 2 , and the third brake B 3  are engaged).  
         [0035]    To achieve 5→3 shifting, the TCU disengages the fourth clutch C 4  (at the fifth speed wherein the first clutch C 1 , the second clutch C 2 , and the fourth clutch C 4  are engaged). At this time, control of the third brake B 3  is unnecessary because of the operation of the second one-way clutch F 2 .  
         [0036]    To achieve 5→2 shifting, the TCU disengages the second clutch C 2  and the fourth clutch C 4  and engages the second brake B 2  (at the fifth speed wherein the first clutch C 1 , the second clutch C 2 , and the fourth clutch C 4  are engaged). At this time, control of the third brake B 3  is unnecessary because of the operation of the second one-way clutch F 2 .  
         [0037]    To achieve 5→2 shifting, as in the other method, after shifting into a third speed by disengaging the fourth clutch C 4 , the TCU disengages the second clutch C 2  and engages the second brake B 2 . Control of the third brake B 3  is unnecessary because of the operation of the second one-way clutch F 2 .  
         [0038]    To achieve 6→4 shifting, the TCU disengages the fourth clutch C 4  (at the sixth speed wherein the second clutch C 2 , the fourth clutch C 4 , and the second brake B 2  are engaged). At this time, control of the third brake B 3  is unnecessary because of the operation of the second one-way clutch F 2 .  
         [0039]    To achieve 6→3 shifting, the TCU disengages both the fourth clutch C 4  and the second brake B 2  and engages the first clutch C 1  (at the sixth speed wherein the second clutch C 2 , the fourth clutch C 4 , and the second brake B 2  are engaged). At this time, control of the third brake B 3  is unnecessary because of the operation of the second one-way clutch F 2 .  
         [0040]    To achieve 6→3 shifting, as in the other method, after shifting into a fourth speed by disengaging the fourth clutch C 4 , the TCU disengages the second brake B 2  and engages the first clutch C 1 . At this time, control of the third brake B 3  is unnecessary because of the operation of the second one-way clutch F 2 .  
         [0041]    As described above, according to the present invention, because it is not difficult to simultaneously control two shafts by properly using the one-way clutches disposed in the primary and secondary shift portions, it is easy to control up and down shifting and skip shifting. Additionally, because the sixth speed is obtained without changing the structure of a 5-speed automatic transmission, it is easy to install the automatic transmission in a vehicle. Also, the weight of the automatic transmission is less than that in which additional parts are installed.  
         [0042]    Furthermore, because an automatic transmission can have appropriate gear ratios and ratios between shift ranges, optimal power performance and fuel economy characteristics can be achieved. The invention may improve shift quality by mainly using brake control rather than clutch control. Also, it may improve the durability of an automatic transmission by controlling so as to revolve the whole of the secondary shift portion at fifth and sixth speeds.