Abstract:
Shortened length, light weight, enhanced durability, and reduced power loss may be achieved when a powertrain of an automatic transmission is formed by combining one single pinion simple planetary gear set and two double pinion simple planetary gear sets. Two simple planetary gear sets are combined to form a compound planetary gear set having four operational elements by fixedly interconnecting two pairs of operating members therein such that the powertrain forms seven operational elements including one permanent input element, two selective input elements, one permanent fixed element, two selective fixed elements, one intermediate output element, and one permanent output element.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an automatic transmission, and more particularly, to a powertrain of an automatic transmission.  
       BACKGROUND OF THE INVENTION  
       [0002]     A multi-stage gearshift mechanism of an automatic transmission includes a plurality of planetary gear sets. A powertrain having such a plurality of planetary gear sets varies the torque in multi-stages and outputs it to an output shaft when receiving a converted engine torque from a torque converter.  
         [0003]     The more speeds the powertrain of an automatic transmission has, the better the power performance and fuel consumption. Therefore, it is desirable for powertrains to have as many speeds as possible.  
         [0004]     Even for the same number of speeds, durability, power transmission efficiency, and size/weight of a transmission are substantially dependent on how planetary gear sets are arranged. Therefore, research for more structural strength, less power loss, and more compact packaging are continuously being conducted.  
         [0005]     Usually, development of a powertrain using planetary gear sets does not devise a wholly new type of planetary gear set. To the contrary, it invokes how single/double pinion planetary gear sets are combined, and how clutches, brakes, and one-way clutches are disposed to the combination of planetary gear sets such that required shift speeds and speed ratios are realized with minimal power loss.  
         [0006]     For a manual transmission, too many speeds cause a driver the inconvenience of excessive manual shifting. However, for an automatic transmission, a transmission control unit automatically executes shifting by controlling the operation of the power train, and therefore, more speeds usually implies more merits.  
         [0007]     Accordingly, research of four-speed and five-speed powertrains has been undertaken, and recently, a powertrain of an automatic transmission enabling six forward speeds and one reverse speed has been developed. An example of such research may be found in the powertrain of an automatic transmission disclosed in U.S. Pat. No. 6,071,208. However, such a conventional powertrain has drawbacks such as large volume and weight due to the use of as many as six frictional elements.  
         [0008]     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore, it may contain information that does not form the prior art that is already known in this country to a person or ordinary skill in the art.  
       SUMMARY OF THE INVENTION  
       [0009]     Embodiments of the present invention provide a six-speed powertrain of an automatic transmission having advantages of shortened length and light weight by combining three planetary gear sets and five frictional elements.  
         [0010]     According to an exemplary embodiment, the number of operating members rotating at high speed is reduced and slip speeds of frictional elements are reduced such that durability may be enhanced. Furthermore, durability is further enhanced and power loss is reduced by simplifying the power delivery path.  
         [0011]     An exemplary six-speed powertrain of an automatic transmission according to an embodiment of the present invention is formed by combining a first planetary gear set of a simple planetary gear set with second and third planetary gear sets of double pinion simple planetary gear sets. The powertrain forms seven operational elements because it includes a compound planetary gear set formed as a combination of two planetary gear sets of the first, second, and third planetary gear sets, and two pairs of operating members of the two planetary gear sets being fixedly interconnected such that the compound planetary gear set forms four operational elements.  
         [0012]     The seven operational elements include a first operational element always acting as a fixed element, a second operational element always rotating at a reduced speed, a third operational element always acting as an input element, a fourth operational element selectively acting as a fixed element or an input element receiving an output of the second operational element, a fifth operational element selectively acting as an input element or a fixed element, a sixth operational element always acting as an output element, and a seventh operational element selectively acting as an input element.  
         [0013]     The compound planetary gear set may be formed as a combination of the first planetary gear set of a simple planetary gear set and the second planetary gear set of a double pinion simple planetary gear set.  
         [0014]     The first, second, and third planetary gear sets may be sequentially arranged from an end of the powertrain.  
         [0015]     The compound planetary gear set may be formed as a combination of the first and second planetary gear sets by fixedly interconnecting a first planet carrier of the first planetary gear set and a second planet carrier of the second planetary gear set, and by fixedly interconnecting a first ring gear of the first planetary gear set and a second ring gear of the second planetary gear set.  
         [0016]     The first planetary gear set may include operating members of a first sun gear, a first ring gear, and a first planet carrier, the second planetary gear set may include operating members of a second sun gear, a second ring gear, and a second planet carrier, and the third planetary gear set may include operating members of a third sun gear, a third ring gear, and a third planet carrier. In this case, the first operational element may be formed by the third planet carrier, the second operational element may be formed by the third ring gear, the third operational element may be formed by the third sun gear, the fourth operational element may be formed by the second sun gear, the fifth operational element may be formed by the first and second ring gears wherein the first and second ring gears are fixedly interconnected, the sixth operational element may be formed by the first and second planet carriers wherein the first and second planet carriers are fixedly interconnected, and the seventh operational element may be formed by the first sun gear.  
         [0017]     The first operational element may be fixedly connected to a transmission housing, the third operational element may be directly connected to an input shaft, the seventh operational element may be variably connected to the input shaft via a first clutch, the fifth operational element may be variably connected to the input shaft via a second clutch and also variably connected to the transmission housing via a first brake, and the fourth operational element may be variably connected to the second operational element via a third clutch and also variably connected to the transmission housing via a second brake.  
         [0018]     The first and second clutches and the first brake may be disposed rearward in the transmission, and the third clutch and the second brake may be disposed forward in the transmission. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a schematic diagram of a powertrain of an automatic transmission according to an exemplary embodiment of the present invention.  
         [0020]      FIG. 2  is an operational chart of a powertrain of an automatic transmission according to an exemplary embodiment of the present invention.  
         [0021]      FIG. 3  is a shift diagram for first and second forward speeds of a powertrain of an automatic transmission according to an exemplary embodiment of the present invention.  
         [0022]      FIG. 4  is a shift diagram for third and fourth forward speeds of a powertrain of an automatic transmission according to an exemplary embodiment of the present invention.  
         [0023]      FIG. 5  is a shift diagram for fifth and sixth forward speeds and a reverse speed of a powertrain of an automatic transmission according to an exemplary embodiment of the present invention.  
         [0024]      FIG. 6A  shows exemplary gear ratios of a powertrain of an automatic transmission according to an exemplary embodiment of the present invention.  
         [0025]      FIG. 6B  shows exemplary gear ratios of a conventional powertrain of an automatic transmission.  
         [0026]      FIG. 7A  shows rotation speeds of respective operating members in a powertrain of an automatic transmission according to an exemplary embodiment of the present invention.  
         [0027]      FIG. 7B  shows rotation speeds of respective operating members in a conventional powertrain of an automatic transmission.  
         [0028]      FIG. 8A  shows slip speeds of non-operated frictional elements in a powertrain of an automatic transmission according to an exemplary embodiment of the present invention.  
         [0029]      FIG. 8B  shows slip speeds of non-operated frictional elements in a conventional powertrain of an automatic transmission.  
         [0030]      FIG. 9A  shows power delivery paths of a powertrain of an automatic transmission according to an exemplary embodiment of the present invention.  
         [0031]      FIG. 9B  shows power delivery paths of a conventional powertrain of an automatic transmission.  
         [0032]      FIG. 10  is a schematic diagram of a conventional powertrain of an automatic transmission.  
         [0033]      FIG. 11  is an operational chart of a powertrain of the automatic transmission shown in  FIG. 10 . 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0034]     Embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings.  
         [0035]     As shown in  FIG. 1 , a powertrain of an automatic transmission according to an exemplary embodiment of the present invention includes first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  arranged on an input shaft  100  connected to an engine output side via a torque converter.  
         [0036]     The first planetary gear set PG 1  is formed as a single pinion planetary gear set having operating members of a first sun gear S 1 , a first ring gear R 1 , and a first planet carrier PC 1  rotatably supporting a pinion gear P 1  engaged with the first sun gear S 1  and the first ring gear R 1 .  
         [0037]     The second planetary gear set PG 2  is formed as a double pinion planetary gear set having operating members of a second sun gear S 2 , a second ring gear R 2 , and a second planet carrier PC 2  rotatably supporting two pinion gears P 2   a  and P 2   b  engaged with the second sun gear S 2  and the second ring gear R 2 .  
         [0038]     The third planetary gear set PG 3  is formed as a double pinion planetary gear set having operating members of a third sun gear S 3 , a third ring gear R 3 , and a third planet carrier PC 3  rotatably supporting two pinion gears P 3   a  and P 3   b  engaged with the third sun gear S 3  and the third ring gear R 3 .  
         [0039]     The first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  are arranged in the order of the third, second, and first planetary gear sets PG 3 , PG 2 , and PG 1  from the engine. Among the operating members, the first and second planet carriers PC 1  and PC 2  and the first and second ring gears R 1  and R 2  are fixedly interconnected, respectively. According to such fixed interconnections of the first and second planet carriers PC 1  and PC 2  and the first and second ring gears R 1  and R 2 , the first and second planetary gear sets PG 1  and PG 2  form one compound planetary gear set.  
         [0040]     The third sun gear S 3  is fixedly connected to the input shaft  100  so as to always act as an input element. The first sun gear S 1  and the first ring gear R 1  are variably connected to the input shaft  100  via first and second clutches C 1  and C 2  so as to variably act as input elements.  
         [0041]     In addition, a connecting member  102  interconnecting the first and second ring gears R 1  and R 2  is variably connected to a transmission housing  104  via a first brake B 1 . The third planet carrier PC 3  is fixedly connected to the housing  104  so as to always act as a fixed element. The second sun gear S 2  and the third ring gear R 3  are variably connected via a third clutch C 3 .  
         [0042]     The second sun gear S 2  is variably connected to the housing  104  via a second brake B 2 . The second planet carrier PC 2  is provided with an output gear  106  so as to always act as an output element.  
         [0043]     According to such an arrangement, the first and second clutches C 1  and C 2  and the first brake B 1  are disposed rearward in the transmission, and the third clutch C 3  and the second brake B 2  are disposed forward in the transmission.  
         [0044]     Such a powertrain may be operated according to an operational chart shown in  FIG. 2  to realize six forward speeds and one reverse speed. That is, the first clutch C 1  and the first brake B 1  are operated for the first forward speed, the first clutch C 1  and the second brake B 2  are operated for the second forward speed, the first clutch C 1  and the third clutch C 3  are operated for the third forward speed, the first and second clutches C 1  and C 2  are operated for the fourth forward speed, the second and third clutches C 2  and C 3  are operated for the fifth forward speed, the second clutch C 2  and the second brake B 2  are operated for the sixth forward speed, and the third clutch C 3  and the first brake B 1  are operated for the reverse speed.  
         [0045]     According to the powertrain of an exemplary embodiment of the present invention, one single pinion planetary gear set and two double pinion planetary gear sets are combined by fixedly connecting the first and second planet carriers PC 1  and PC 2  and the first and second ring gears R 1  and R 2  such that the powertrain may form seven operational elements as shown in  FIG. 3 .  
         [0046]     Therefore, a first node N 1  (hereinafter called a first operational element) is formed by the third planet carrier PC 3 . A second node N 2  (hereinafter called a second operational element) is formed by the third ring gear R 3 . A third node N 3  (hereinafter called a third operational element) is formed by the first sun gear S 3 . A fourth node N 4  (hereinafter called a fourth operational element) is formed by the second sun gear S 2 . A fifth node N 5  (hereinafter called a fifth operational element) is formed by the first and second ring gears R 1  and R 2 . A sixth node N 6  (hereinafter called a sixth operational element) is formed by the first and second planet carriers PC 1  and PC 2 . A seventh node N 7  (hereinafter called a seventh operational element) is formed by the first sun gear S 1 .  
         [0047]      FIG. 3  to  FIG. 5  show shift diagrams of a powertrain of an exemplary embodiment of the present invention in the case that gear ratios of respective planetary gear sets are predetermined as shown in  FIG. 6A . Hereinafter, a shifting operation of the exemplary powertrain according to an embodiment of the present invention will be described in detail.  
         [0048]     Firstly for the first forward speed, the first clutch C 1  and the first brake B 1  are operated.  
         [0049]     Then, the seventh node N 7  (i.e., the first sun gear S 1 ) receives an input of an engine speed, and the fifth node N 5  acts as a fixed element due to the operation of the first brake B 1 .  
         [0050]     Therefore, a speed line of the first forward speed is formed as a speed line L connecting the seventh node N 7 , rotating at the engine speed, and the fifth node N 5 , remaining stationary. Therefore, the output element of the sixth node N 6  rotates at a speed D 1 , and the first forward speed is realized.  
         [0051]     In this case, the third planetary gear set PG 3  does not take part in forming the first forward speed. That is, although the third sun gear S 3  receives the input of the engine speed and the third planet carrier PC 3  acts as the fixed element, the third ring gear R 3  freely rotates since the third clutch C 3  is disengaged.  
         [0052]     For the second forward speed, the first brake B 1  is released and the second brake B 2  is operated from the first forward speed.  
         [0053]     Then, the fixed element is changed to the second sun gear S 2  (i.e., the fourth node N 4 ), while the first sun gear S 1  receives the input of the engine speed as in the first forward speed. Therefore, a speed line of the second forward speed is formed as a speed line L 2  connecting the seventh node N 7 , rotating at the engine speed, and the fourth node N 4 , remaining stationary. Therefore, the output element of the sixth node N 6  rotates at a speed D 2 , and the second forward speed is realized.  
         [0054]     In this case, the third planetary gear set PG 3  does not take part in forming the second forward speed, the same as in the first forward speed. That is, although the third sun gear S 3  receives the input of the engine speed and the third planet carrier PC 3  acts as the fixed element, the third ring gear R 3  freely rotates since the third clutch C 3  is disengaged.  
         [0055]     For the third forward speed, the second brake B 2  is released and the third clutch C 3  is operated from the second forward speed.  
         [0056]     Then, the third ring gear R 3  and the second sun gear S 2  are interconnected, while the first sun gear S 1  remains receiving the input of the engine speed. In this case, the seventh node N 7  receives an input of the engine speed, and the fourth node N 4  receives a reduced speed that is reduced from the engine speed by the third planetary gear set PG 3 . Therefore, a speed line of the third forward speed is formed as a speed line L 3  shown in  FIG. 4 . Therefore, the output element of the sixth node N 6  rotates at a speed D 3 , and the third forward speed is realized.  
         [0057]     For the fourth forward speed, the third clutch C 3  is released and the second clutch C 2  is operated from the third forward speed.  
         [0058]     Then, the first sun gear S 1  and the first ring gear R 1  receive the input of the engine speed at the same time. Therefore, a speed line of the fourth forward speed is formed as a speed line L 4 , and the first and second planetary gear sets integrally rotate. Therefore, the output element of the sixth node N 6  rotates at a speed D 4  (i.e., at the same speed of the input engine speed), and the fourth forward speed is realized.  
         [0059]     In this case, the third planetary gear set PG 3  does not take part in forming the fourth forward speed, the same as in the first and second forward speeds. That is, although the third sun gear S 3  receives the input of the engine speed and the third planet carrier PC 3  acts as the fixed element, the third ring gear R 3  freely rotates since the third clutch C 3  is disengaged.  
         [0060]     For the fifth forward speed, the first clutch C 1  is released and the third clutch C 3  is operated from the fourth forward speed.  
         [0061]     Then, the third ring gear R 3  and the second sun gear S 2  are interconnected, while the first ring gear R 1  remains receiving the input of the engine speed. In this case, the fifth node N 5  receives an input of the engine speed, and the fourth node N 4  receives a reduced speed that is reduced from the engine speed by the third planetary gear set PG 3 . Therefore, a speed line of the fifth forward speed is formed as a speed line L 5  shown in  FIG. 5 . Therefore, the output element of the sixth node N 6  rotates at a speed D 5  faster than the input engine speed, and the fifth forward speed is realized.  
         [0062]     For the sixth forward speed, the third clutch C 3  is released and the second brake B 2  is operated from the fifth forward speed.  
         [0063]     Then, the second sun gear S 2  acts as the fixed element, while the first ring gear R 1  remains receiving the input of the engine speed. Therefore, a speed line of the sixth forward speed is formed as a speed line L 6  connecting the fifth node N 5 , rotating at the engine speed, and the fourth node N 4 , remaining stationary. Therefore, the output element of the sixth node N 6  rotates at a speed D 6  faster than the input engine speed, and the sixth forward speed is realized.  
         [0064]     In this case, the third planetary gear set PG 3  does not take part in forming the sixth forward speed, the same as in the first, second, and fourth forward speeds. That is, although the third sun gear S 3  receives the input of the engine speed and the third planet carrier PC 3  acts as the fixed element, the third ring gear R 3  freely rotates since the third clutch C 3  is disengaged.  
         [0065]     For the reverse speed, the third clutch C 3  and the first brake B 1  are operated.  
         [0066]     Then, the third ring gear R 3  and the second sun gear S 2  are interconnected, while the fifth node N 5  of the first and second ring gears R 1  and R 2  acts as the fixed element. Therefore, the fourth node N 4  receives the reduced speed from the planetary gear set PG 3 . Therefore, a speed line of the reverse speed is formed as a speed line Lr connecting the fourth node N 4 , rotating at the reduced speed, and the fifth node N 5 , remaining stationary. Therefore, the output element of the sixth node N 6  rotates at a speed R reversal to the input engine speed, and the reverse speed is realized.  
         [0067]      FIGS. 6A and 6B ,  FIGS. 7A and 7B ,  FIGS. 8A and 8B , and  FIGS. 9A and 9B  compare operating states of a powertrain of an exemplary embodiment of the present invention and a conventional powertrain.  
         [0068]      FIGS. 6A and 6B  respectively show exemplary gear ratios of a powertrain of an exemplary embodiment of the present invention and a conventional power train. The gear ratios of a powertrain of an exemplary embodiment of the present invention have been prepared to show equivalent values to those of a conventional powertrain, for better comparison thereof.  
         [0069]      FIGS. 7A and 7B  respectively show rotation speeds of respective operating members in a powertrain of an automatic transmission according to an exemplary embodiment of the present invention and in a conventional powertrain.  FIGS. 8A and 8B  respectively show slip speeds of non-operated frictional elements in a powertrain of an automatic transmission according to an exemplary embodiment of the present invention and in a conventional powertrain.  FIGS. 9A and 9B  respectively show power delivery paths of a powertrain of an automatic transmission according to an exemplary embodiment of the present invention and a conventional powertrain of an automatic transmission.  
         [0070]     The numbers shown in  FIGS. 6A  to  8 B may be obviously calculated by a person of ordinary skill in the art, based on the structural features and operational charts of the powertrain of the exemplary embodiment and the conventional powertrain.  
         [0071]     According to the powertrain of the exemplary embodiment of the present invention, no operational element rotates faster than the input shaft at the third speed that is frequently engaged for acceleration (refer to  FIG. 7A ), and therefore, slip speeds of friction elements not operated at the third speed are less than the rotation speed of the input shaft (refer to  FIG. 8A ).  
         [0072]     When the numerals shown in  FIG. 8A  are compared with those in  FIG. 8B , it is apparent that the powertrain of the present embodiment shows less slip speeds of friction elements overall at the forward speeds than the conventional powertrain.  
         [0073]     It is well known that more planetary gear sets implies more loss of power during power transmission. When the numerals shown in  FIG. 9A  are compared with those in  FIG. 9B , it is apparent that the powertrain of the present embodiment has less planetary gear sets involved in the power transmission at many of the shift-speeds and accordingly shows better power efficiency  
         [0074]     According to an exemplary embodiment of the present invention, six forward speeds and one reverse speed are achieved with three planetary gear sets and a minimized number of friction elements such that an automatic transmission becomes light and compact.  
         [0075]     Durability is increased due to reduction of rotation speeds of operational elements at a shift-speed frequently engaged for acceleration. A further increase of durability and reduction of power loss is also achieved by reduction of slip speeds of friction elements. A shortened route of power transmission also contributes to an increase of durability and reduction of power loss.  
         [0076]     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.