# Patent

## Title
Planetary gear train of automatic transmission for vehicle

## Abstract

A planetary gear train of an automatic transmission for a vehicle may include an input shaft to receive torque of an engine, an output shaft to output changed torque of the engine, a first planetary gear set including first, second, and third rotation elements, a second planetary gear set including fourth, fifth, and sixth rotation elements, a third planetary gear set including seventh, eighth, and ninth rotation elements, and a fourth planetary gear set including tenth, eleventh, and twelfth rotation elements.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
The present application claims priority to Korean Patent Application No. 10-2015-0170987, filed Dec. 2, 2015, the entire contents of which is incorporated herein for all purposes by this reference.
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a planetary gear train of an automatic transmission for a vehicle that improves power delivery performance and fuel economy by achieving at least nine forward speed stages and widening speed ratio (or gear ratio) with a minimum number of constituent elements being used, and secures linearity of step ratios.
Description of Related Art
Recent increases in oil prices are triggering hard competition among auto-makers in enhancing fuel consumption of a vehicle.
In this sense, research is being conducted on engines in terms of reducing weight and improving fuel efficiency by down-sizing, and research is also being conducted to ensure both drivability and competitiveness for maximizing fuel efficiency by implementing an automatic transmission with multiple speed stages.
However, in the case of the automatic transmission, the number of internal components increases as the number of gear shift stages is increased, which may deteriorate installability, production cost, weight and/or power flow efficiency.
Therefore, in order to maximally enhance fuel consumption of an automatic transmission having more shift-stages, it is important that better efficiency is achieved using a smaller number of parts.
In this aspect, in recent years, 8-speed automatic transmissions tend to be implemented and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
However, since a conventional 8-speed automatic transmission has gear ratio span of 6.5-7.5, improvement of power performance and fuel economy may not be great.
In addition, if 8-speed automatic transmission has gear ratio span larger than 9.0, it is hard to secure linearity of step ratios. Therefore, driving efficiency of an engine and drivability of a vehicle may be deteriorated, and thus, development of high efficiency automatic transmissions which achieve at least nine forward speed stages is necessary.
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 of ordinary skill in the art.
BRIEF SUMMARY
Various aspects of the present invention are directed to providing a planetary gear train of an automatic transmission for a vehicle having advantages of improving power delivery performance and fuel economy by achieving nine forward speed stages and one reverse speed stage, and widening gear ratio span and of securing linearity of step ratios.
According to various aspects of the present invention, a planetary gear train of an automatic transmission for a vehicle may include an input shaft to receive torque of an engine, an output shaft to output changed torque of the engine, a first planetary gear set including first, second, and third rotation elements, a second planetary gear set including fourth, fifth, and sixth rotation elements, a third planetary gear set including seventh, eighth, and ninth rotation elements, and a fourth planetary gear set including tenth, eleventh, and twelfth rotation elements, in which the input shaft may be directly connected to the first rotation element, the output shaft may be directly connected to the twelfth rotation element, the second rotation element may be directly connected to the sixth rotation element and the ninth rotation element, the fifth rotation element may be directly connected to the eleventh rotation element, the eighth rotation element may be directly connected to the tenth rotation element, and the second rotation element may be selectively connected to a transmission housing.
The third rotation element may be selectively connected to the transmission housing, the fourth rotation element may be selectively connected to the transmission housing, the input shaft may be selectively connected to the eleventh rotation element, the input shaft may be selectively connected to the seventh rotation element, and the third rotation element may be selectively connected to the seventh rotation element.
The first, second, and third rotation elements may be a first sun gear, a first ring gear, and a first planet carrier, the fourth, fifth, and sixth rotation elements may be a second sun gear, a second planet carrier, and a second ring gear, the seventh, eighth, and ninth rotation elements may be a third sun gear, a third planet carrier, and a third ring gear, and the tenth, eleventh, and twelfth rotation elements may be a fourth sun gear, a fourth planet carrier, and a fourth ring gear.
The planetary gear train may further include a first clutch selectively connecting the input shaft to the eleventh rotation element, a second clutch selectively connecting the input shaft to the seventh rotation element, a third clutch selectively connecting the third rotation element to the seventh rotation element, a first brake selectively connecting the second rotation element to the transmission housing, a second brake selectively connecting the third rotation element to the transmission housing, and a third brake selectively connecting the fourth rotation element to the transmission housing.
According to various aspects of the present invention, a planetary gear train of an automatic transmission for a vehicle may include an input shaft receiving torque of an engine, an output shaft outputting changed torque of the engine, a first planetary gear set including first, second, and third rotation elements, a second planetary gear set including fourth, fifth, and sixth rotation elements, a third planetary gear set including seventh, eighth, and ninth rotation elements, a fourth planetary gear set including tenth, eleventh, and twelfth rotation elements, a first rotation shaft directly connected to the first rotation element and directly connected to the input shaft, a second rotation shaft directly connecting the second rotation element to the sixth rotation element and the ninth rotation element, and selectively connected to a transmission housing, a third rotation shaft directly connected to the third rotation element and selectively connected to the transmission housing, a fourth rotation shaft directly connected to the fourth rotation element and selectively connected to the transmission housing, a fifth rotation shaft directly connecting the fifth rotation element to the eleventh rotation element and selectively connected to the input shaft, a sixth rotation shaft directly connected to the seventh rotation element and selectively connected to the input shaft or the third rotation shaft, a seventh rotation shaft directly connecting the eighth rotation element to the tenth rotation element, and an eighth rotation shaft directly connected to the twelfth rotation element and directly connected to the output shaft.
The first planetary gear set may be a double pinion planetary gear set, the first rotation element may be a first sun gear, the second rotation element may be a first ring gear, and the third rotation element may be a first planet carrier, the second planetary gear set may be a single pinion planetary gear set, the fourth rotation element may be a second sun gear, the fifth rotation element may be a second planet carrier, and the sixth rotation element may be a second ring gear, the third planetary gear set may be a single pinion planetary gear set, the seventh rotation element may be a third sun gear, the eighth rotation element may be a third planet carrier, and the ninth rotation element may be a third ring gear, and the fourth planetary gear set may be a single pinion planetary gear set, the tenth rotation element may be a fourth sun gear, the eleventh rotation element may be a fourth planet carrier, and the twelfth rotation element may be a fourth ring gear.
The planetary gear train may further include a first clutch selectively connecting the input shaft to the fifth rotation shaft, a second clutch selectively connecting the input shaft to the sixth rotation shaft, a third clutch selectively connecting the third rotation shaft to the sixth rotation shaft, a first brake selectively connecting the second rotation shaft to the transmission housing, a second brake selectively connecting the third rotation shaft to the transmission housing, and a third brake selectively connecting the fourth rotation shaft to the transmission housing.
A first forward speed stage may be achieved by operation of the third clutch and the first and third brakes, a second forward speed stage may be achieved by operation of the third clutch and the second and third brakes, a third forward speed stage may be achieved by operation of the second and third clutches and the third brake, a fourth forward speed stage may be achieved by operation of the first and third clutches and the third brake, a fifth forward speed stage may be achieved by operation of the first and second clutches and the third brake, a sixth forward speed stage may be achieved by operation of the first, second, and third clutches, a seventh forward speed stage may be achieved by operation of the first and second clutches and the second brake, an eighth forward speed stage may be achieved by operation of the first and third clutches and the second brake, a ninth forward speed stage may be achieved by operation of the first and third clutches and the first brake, and a reverse speed stage may be achieved by operation of the second clutch and the first and third brakes.
According to various aspects of the present invention, a planetary gear train of an automatic transmission for a vehicle may include an input shaft receiving torque of an engine, an output shaft outputting changed torque of the engine, a first planetary gear set which may be a double pinion planetary gear set including first, second, and third rotation elements, a second planetary gear set which may be a single pinion planetary gear set including fourth, fifth, and sixth rotation elements, a third planetary gear set which may be a single pinion planetary gear set including seventh, eighth, and ninth rotation elements, a fourth planetary gear set which may be a single pinion planetary gear set including tenth, eleventh, and twelfth rotation elements, a first rotation shaft directly connected to the first rotation element and directly connected to the input shaft, a second rotation shaft directly connecting the second rotation element to the sixth rotation element and the ninth rotation element, and selectively connected to a transmission housing, a third rotation shaft directly connected to the third rotation element and selectively connected to the transmission housing, a fourth rotation shaft directly connected to the fourth rotation element and selectively connected to the transmission housing, a fifth rotation shaft directly connecting the fifth rotation element to the eleventh rotation element and selectively connected to the input shaft, a sixth rotation shaft directly connected to the seventh rotation element and selectively connected to the input shaft or the third rotation shaft, a seventh rotation shaft directly connecting the eighth rotation element to the tenth rotation element, an eighth rotation shaft directly connected to the twelfth rotation element and directly connected to the output shaft, a first clutch selectively connecting the input shaft to the fifth rotation shaft, a second clutch selectively connecting the input shaft to the sixth rotation shaft, a third clutch selectively connecting the third rotation shaft to the sixth rotation shaft, a first brake selectively connecting the second rotation shaft to the transmission housing, a second brake selectively connecting the third rotation shaft to the transmission housing, and a third brake selectively connecting the fourth rotation shaft to the transmission housing.
Various embodiment of the present invention achieve at least nine forward speed stages and one reverse speed stage by combining four planetary gear sets, which are simple planetary gear sets, with six control elements.
In addition, since gear ratio span greater than 9.0 is secured, driving efficiency of the engine may be maximized.
Furthermore, since linearity of step ratios can be secured due to multiple speed stages, drivability such as acceleration before and after shift, rhythmical engine speed, and on the like may be improved.
It is understood that the term “vehicle” or “vehicular” or other similar terms as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g., fuel derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example, both gasoline-powered and electric-powered vehicles.
The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a planetary gear train according to various embodiments of the present invention.
FIG. 2 is an operation chart of control elements at each speed stage in the planetary gear train according to various embodiments of the present invention.



It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.
DETAILED DESCRIPTION
Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.
FIG. 1 is a schematic diagram of a planetary gear train according to various embodiments of the present invention.
Referring to FIG. 1, a planetary gear train according to various embodiments of the present invention includes first, second, third, and fourth planetary gear sets PG1, PG2, PG3, and PG4 disposed on the same axis, an input shaft IS, an output shaft OS, eight shafts TM1 to TM8 connected to at least one of rotation elements of the first, second, third, and fourth planetary gear sets PG1, PG2, PG3, and PG4, three clutches C1 to C3 and three brakes B1 to B3 that are control elements, and a transmission housing H.
Torque input from the input shaft IS is changed by cooperation of the first, second, third, and fourth planetary gear sets PG1, PG2, PG3, and PG4, and the changed torque is output through the output shaft OS.
The planetary gear sets are disposed in a sequence of the first, second, third, and fourth planetary gear sets PG1, PG2, PG3, and PG4 from an engine.
The input shaft IS is an input member and the torque from a crankshaft of an engine, after being torque-converted through a torque converter, is input into the input shaft IS.
The output shaft OS is an output member, is disposed in parallel with the input shaft IS, and transmits driving torque to a driving wheel through a differential apparatus.
The first planetary gear set PG1 is a double pinion planetary gear set and includes a first sun gear S1, a first ring gear R1 internally engaged with a first pinion P1 externally engaged with the first sun gear S1, and a first planet carrier PC1 rotatably supporting the first pinion P1 respectively as first, second, and third rotation elements N1, N2, and N3.
The second planetary gear set PG2 is a single pinion planetary gear set and includes a second sun gear S2, a second planet carrier PC2 rotatably supporting a second pinion P2 that is externally meshed with the second sun gear S2, and a second ring gear R2 that is internally meshed with the second pinion P2 respectively as fourth, fifth, and sixth rotation elements N4, N5, and N6.
The third planetary gear set PG3 is a single pinion planetary gear set and includes a third sun gear S3, a third planet carrier PC3 rotatably supporting a third pinion P3 that is externally meshed with the third sun gear S3, and a third ring gear R3 that is internally meshed with the third pinion P3 respectively as seventh, eighth, and ninth rotation elements N7, N8, and N9.
The fourth planetary gear set PG4 is a single pinion planetary gear set and includes a fourth sun gear S4, a fourth planet carrier PC4 rotatably supporting a fourth pinion P4 that is externally meshed with the fourth sun gear S4, and a fourth ring gear R4 that is internally meshed with the fourth pinion P4 respectively as tenth, eleventh, and twelfth rotation elements N10, N11, and N12.
The second rotation element N2 is directly connected with the sixth rotation element N6 and the ninth rotation element TM9, the fifth rotation element N5 is directly connected with the eleventh rotation element N11, and the eighth rotation element N8 is directly connected with the tenth rotation element N10 such that the first, second, third, and fourth planetary gear sets PG1, PG2, PG3, and PG4 includes eight rotation shafts TM1 to TM8.
The eight shafts TM1 to TM8 will be described in further detail.
The first rotation shaft TM1 is directly connected to the first rotation element N1 (first sun gear S1) and is directly connected to the input shaft IS so as to be operated as an input element continuously.
The second rotation shaft TM2 directly connects the second rotation element N2 (first ring gear R1) with the sixth rotation element N6 (second ring gear R2) and the ninth rotation element N9 (third ring gear R3), and is selectively connected to the transmission housing H so as to be operated as a selectively fixed element.
The third rotation shaft TM3 is directly connected to the third rotation element N3 (first planet carrier PC1) and is selectively connected to the transmission housing H so as to be operated as a selectively fixed element.
The fourth rotation shaft TM4 is directly connected to the fourth rotation element N4 (second sun gear S2) and is selectively connected to the transmission housing H so as to be operated as a selectively fixed element.
The fifth rotation shaft TM5 directly connects the fifth rotation element N5 (second planet carrier PC2) with the eleventh rotation element N11 (fourth planet carrier PC4) and is selectively connected to the input shaft IS so as to be operated as a selective input element.
The sixth rotation shaft TM6 is directly connected to the seventh rotation element N7 (third sun gear S3), is selectively connected to the input shaft IS so as to be operated as a selective input element, and is selectively connected to the third rotation shaft TM3.
The seventh rotation shaft TM7 directly connects the eighth rotation element N8 (third planet carrier PC3) with the tenth rotation element N10 (fourth sun gear S4).
The eighth rotation shaft TM8 is directly connected to the twelfth rotation element N12 (fourth ring gear R4) and is directly connected to the output shaft OS so as to be operated as an output element continuously.
In addition, three clutches C1, C2, and C3 that are control elements are disposed at positions at which two rotation shafts among the rotation shafts TM1 to TM8 are selectively connected to each other.
In addition, three brakes B1, B2, and B3 that are control elements are disposed at positions at which any one rotation shaft among the rotation shafts TM1 to TM8 is selectively connected to the transmission housing H.
Arrangements of the six control elements C1 to C3 and B1 to B3 are described in detail.
The first clutch C1 is disposed between the input shaft IS and the fifth rotation shaft TM5 and selectively connects the input shaft IS with the fifth rotation shaft TM5.
The second clutch C2 is disposed between the input shaft IS and the sixth rotation shaft TM6 and selectively connects the input shaft IS with the sixth rotation shaft TM6.
The third clutch C3 is disposed between the third rotation shaft TM3 and the sixth rotation shaft TM6 and selectively connects the third rotation shaft TM3 with the sixth rotation shaft TM6.
The first brake B1 is disposed between the second rotation shaft TM2 and the transmission housing H and causes the second rotation shaft TM2 to be operated as the selectively fixed element.
The second brake B2 is disposed between the third rotation shaft TM3 and the transmission housing H and causes the third rotation shaft TM3 to be operated as the selectively fixed element.
The third brake B3 is disposed between the fourth rotation shaft TM4 and the transmission housing H and causes the fourth rotation shaft TM4 to be operated as the selectively fixed element.
The control elements including the first, second, and third clutches C1, C2, and C3 and the first, second, and third brakes B1, B2, and B3 may be multi-plates friction elements of wet type that are operated by hydraulic pressure.
FIG. 2 is an operation chart of control elements at each speed stage in the planetary gear train according to various embodiments of the present invention.
As shown in FIG. 2, three control elements are operated at each speed stage in the planetary gear train according to various embodiments of the present invention.
The third clutch C3 and the first and third brakes B1 and B3 are simultaneously operated at a first forward speed stage D1. In a state that the third rotation shaft TM3 is connected to the sixth rotation shaft TM6 by operation of the third clutch C3, the torque of the input shaft IS is input to the first rotation shaft TM1. In addition, the second and fourth rotation shafts TM2 and TM4 are operated as the fixed elements by operation of the first and third brakes B1 and B3. Therefore, the torque of the input shaft IS is shifted into the first forward speed stage, and the first forward speed stage is output to the output shaft OS connected to the eighth rotation shaft TM8.
The third clutch C3 and the second and third brakes B2 and B3 are simultaneously operated at a second forward speed stage D2. In a state that the third rotation shaft TM3 is connected to the sixth rotation shaft TM6 by operation of the third clutch C3, the torque of the input shaft IS is input to the first rotation shaft TM1. In addition, the third and fourth rotation shafts TM3 and TM4 are operated as the fixed elements by operation of the second and third brakes B2 and B3. Therefore, the torque of the input shaft IS is shifted into the second forward speed stage, and the second forward speed stage is output to the output shaft OS connected to the eighth rotation shaft TM8.
The second and third clutches C2 and C3 and the third brake B3 are simultaneously operated at a third forward speed stage D3. In a state that the input shaft IS is connected to the sixth rotation shaft TM6 by operation of the second clutch C2 and the third rotation shaft TM3 is connected to the sixth rotation shaft TM6 by operation of the third clutch C3, the torque of the input shaft IS is input to the first rotation shaft TM1 and the sixth rotation shaft TM6. In addition, the fourth rotation shaft TM4 is operated as the fixed element by operation of the third brake B3. Therefore, the torque of the input shaft IS is shifted into the third forward speed stage, and the third forward speed stage is output to the output shaft OS connected to the eighth rotation shaft TM8.
The first and third clutches C1 and C3 and the third brake B3 are simultaneously operated at a fourth forward speed stage D4. In a state that the input shaft IS is connected to the fifth rotation shaft TM5 by operation of the first clutch C1 and the third rotation shaft TM3 is connected to the sixth rotation shaft TM6 by operation of the third clutch C3, the torque of the input shaft IS is input to the first rotation shaft TM1 and the fifth rotation shaft TM5. In addition, the fourth rotation shaft TM4 is operated as the fixed element by operation of the third brake B3. Therefore, the torque of the input shaft IS is shifted into the fourth forward speed stage, and the fourth forward speed stage is output to the output shaft OS connected to the eighth rotation shaft TM8.
The first and second clutches C1 and C2 and the third brake B3 are simultaneously operated at a fifth forward speed stage D5. In a state that the input shaft IS is connected to the fifth rotation shaft TM5 by operation of the first clutch C1 and is connected to the sixth rotation shaft TM6 by operation of the second clutch C2, the torque of the input shaft IS is input to the first, fifth, and sixth rotation shafts TM1, TM5, and TM6. In addition, the fourth rotation shaft TM4 is operated as the fixed element by operation of the third brake B3. Therefore, the torque of the input shaft IS is shifted into the fifth forward speed stage, and the fifth forward speed stage is output to the output shaft OS connected to the eighth rotation shaft TM8.
The first, second, and third clutches C1, C2, and C3 are simultaneously operated at a sixth forward speed stage D6. In a state that the input shaft IS is connected to the fifth rotation shaft TM5 by operation of the first clutch C1 and is connected to the sixth rotation shaft TM6 by operation of the second clutch C2 and the third rotation shaft TM3 is connected to the sixth rotation shaft TM6 by operation of the third clutch C3, the torque of the input shaft IS is input to the first, fifth, and sixth rotation shafts TM1, TM5, and TM6. Therefore, all of the first, second, third, and fourth planetary gear sets PG1, PG2, PG3, and PG4 become lock-up state. Therefore, the torque of the input shaft IS is shifted into the sixth forward speed stage, and the sixth forward speed stage is output to the output shaft OS connected to the eighth rotation shaft TM8. The same rotational speed as the input shaft IS is output at the sixth forward speed stage.
The first and second clutches C1 and C2 and the second brake B2 are simultaneously operated at a seventh forward speed stage D7. In a state that the input shaft IS is connected to the fifth rotation shaft TM5 by operation of the first clutch C1 and is connected to the sixth rotation shaft TM6 by operation of the second clutch C2, the torque of the input shaft IS is input to the first, fifth, and sixth rotation shafts TM1, TM5, and TM6. In addition, the third rotation shaft TM3 is operated as the fixed element by operation of the second brake B2. Therefore, the torque of the input shaft IS is shifted into the seventh forward speed stage, and the seventh forward speed stage is output to the output shaft OS connected to the eighth rotation shaft TM8.
The first and third clutches C1 and C3 and the second brake B2 are simultaneously operated at an eighth forward speed stage D8. In a state that the input shaft IS is connected to the fifth rotation shaft TM5 by operation of the first clutch C1 and the third rotation shaft TM3 is connected to the sixth rotation shaft TM6 by operation of the third clutch C3, the torque of the input shaft IS is input to the first rotation shaft TM1 and the fifth rotation shaft TM5. In addition, the third rotation shaft TM3 is operated as the fixed element by operation of the second brake B2. Therefore, the torque of the input shaft IS is shifted to the eighth forward speed stage, and the eighth forward speed stage is output to the output shaft OS connected to the eighth rotation shaft TM8.
The first and third clutches C1 and C3 and the first brake B1 are simultaneously operated at a ninth forward speed stage D9. In a state that the input shaft IS is connected to the fifth rotation shaft TM5 by operation of the first clutch C1 and the third rotation shaft TM3 is connected to the sixth rotation shaft TM6 by operation of the third clutch C3, the torque of the input shaft IS is input to the first rotation shaft TM1 and the fifth rotation shaft TM5. In addition, the second rotation shaft TM2 is operated as the fixed element by operation of the first brake B1. Therefore, the torque of the input shaft IS is shifted into the ninth forward speed stage, and the ninth forward speed stage is output to the output shaft OS connected to the eighth rotation shaft TM8.
The second clutch C2 and the first and third brakes B1 and B3 are simultaneously operated at a reverse speed stage REV. In a state that the input shaft IS is connected to the sixth rotation shaft TM6, the torque of the input shaft IS is input to the first rotation shaft TM1 and the sixth rotation shaft TM6. In addition, the second and fourth rotation shafts TM2 and TM4 are operated as the fixed elements by operation of the first and third brakes B1 and B3. Therefore, the torque of the input shaft IS is shifted into the reverse speed stage, and the reverse speed stage is output to the output shaft OS connected to the eighth rotation shaft TM8.
The planetary gear train according to various embodiments of the present invention may achieve at least nine forward speed stages and one reverse speed stage by combining four planetary gear sets PG1, PG2, PG3, and PG4 with the three clutches C1, C2, and C3 and the three brakes B1, B2, and B3.
In addition, since gear ratio span greater than 9.3 is secured, driving efficiency of the engine may be maximized.
In addition, step ratios between neighboring speed stages except between the seventh forward speed stage and the eighth forward speed stage are of magnitude 1.2 or more in the planetary gear train according to various embodiments of the present invention. In addition, since linearity of step ratios can be secured due to multiple speed stages, drivability such as acceleration before and after shift, rhythmical engine speed, and the like may be improved.
The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.

## Claims

1. A planetary gear train of an automatic transmission for a vehicle comprising:
an input shaft to receive torque of an engine;
an output shaft to output changed torque of the engine;
a first planetary gear set including a first rotation element, a second rotation element, and a third rotation element;
a second planetary gear set including a fourth rotation element, a fifth rotation element, and a sixth rotation element;
a third planetary gear set including a seventh rotation element, an eighth rotation element, and a ninth rotation element; and
a fourth planetary gear set including a tenth rotation element, an eleventh rotation element, and a twelfth rotation element,
wherein the input shaft is directly connected to the first rotation element,
the output shaft is directly connected to the twelfth rotation element,
the second rotation element is directly connected to the sixth rotation element and the ninth rotation element,
the fifth rotation element is directly connected to the eleventh rotation element,
the eighth rotation element is directly connected to the tenth rotation element, and
the second rotation element is selectively connected to a transmission housing.

2. The planetary gear train of claim 1, wherein the third rotation element is selectively connected to the transmission housing,
the fourth rotation element is selectively connected to the transmission housing,
the input shaft is selectively connected to the eleventh rotation element,
the input shaft is selectively connected to the seventh rotation element, and
the third rotation element is selectively connected to the seventh rotation element.

3. The planetary gear train of claim 1, wherein the first rotation element, the second rotation element, and the third rotation element comprise a first sun gear, a first ring gear, and a first planet carrier,
the fourth rotation element, the fifth rotation element, and the sixth rotation element comprise a second sun gear, a second planet carrier, and a second ring gear,
the seventh rotation element, the eighth rotation element, and the ninth rotation element comprise a third sun gear, a third planet carrier, and a third ring gear, and
the tenth rotation element, the eleventh rotation element, and the twelfth rotation element comprise a fourth sun gear, a fourth planet carrier, and a fourth ring gear.

4. The planetary gear train of claim 2, further comprising:
a first clutch selectively connecting the input shaft to the eleventh rotation element;
a second clutch selectively connecting the input shaft to the seventh rotation element;
a third clutch selectively connecting the third rotation element to the seventh rotation element;
a first brake selectively connecting the second rotation element to the transmission housing;
a second brake selectively connecting the third rotation element to the transmission housing; and
a third brake selectively connecting the fourth rotation element to the transmission housing.

5. A planetary gear train of an automatic transmission for a vehicle comprising:
an input shaft configured for receiving torque of an engine;
an output shaft configured for outputting changed torque of the engine;
a first planetary gear set including a first rotation element, a second rotation element, and a third rotation element;
a second planetary gear set including a fourth rotation element, a fifth rotation element, and a sixth rotation element;
a third planetary gear set including a seventh rotation element, an eighth rotation element, and a ninth rotation element;
a fourth planetary gear set including a tenth rotation element, an eleventh rotation element, and a twelfth rotation element;
a first rotation shaft directly connected to the first rotation element and directly connected to the input shaft;
a second rotation shaft directly connecting the second rotation element to the sixth rotation element and the ninth rotation element, and selectively connected to a transmission housing;
a third rotation shaft directly connected to the third rotation element and selectively connected to the transmission housing;
a fourth rotation shaft directly connected to the fourth rotation element and selectively connected to the transmission housing;
a fifth rotation shaft directly connecting the fifth rotation element to the eleventh rotation element and selectively connected to the input shaft;
a sixth rotation shaft directly connected to the seventh rotation element and selectively connected to the input shaft or the third rotation shaft;
a seventh rotation shaft directly connecting the eighth rotation element to the tenth rotation element; and
an eighth rotation shaft directly connected to the twelfth rotation element and directly connected to the output shaft.

6. The planetary gear train of claim 5, wherein the first planetary gear set is a double pinion planetary gear set, the first rotation element is a first sun gear, the second rotation element is a first ring gear, and the third rotation element is a first planet carrier,
the second planetary gear set is a single pinion planetary gear set, the fourth rotation element is a second sun gear, the fifth rotation element is a second planet carrier, and the sixth rotation element is a second ring gear,
the third planetary gear set is a single pinion planetary gear set, the seventh rotation element is a third sun gear, the eighth rotation element is a third planet carrier, and the ninth rotation element is a third ring gear, and
the fourth planetary gear set is a single pinion planetary gear set, the tenth rotation element is a fourth sun gear, the eleventh rotation element is a fourth planet carrier, and the twelfth rotation element is a fourth ring gear.

7. The planetary gear train of claim 5, further comprising:
a first clutch selectively connecting the input shaft to the fifth rotation shaft;
a second clutch selectively connecting the input shaft to the sixth rotation shaft;
a third clutch selectively connecting the third rotation shaft to the sixth rotation shaft;
a first brake selectively connecting the second rotation shaft to the transmission housing;
a second brake selectively connecting the third rotation shaft to the transmission housing; and
a third brake selectively connecting the fourth rotation shaft to the transmission housing.

8. The planetary gear train of claim 7, wherein a first forward speed stage is achieved by operation of the third clutch and the first and third brakes,
a second forward speed stage is achieved by operation of the third clutch and the second and third brakes,
a third forward speed stage is achieved by operation of the second and third clutches and the third brake,
a fourth forward speed stage is achieved by operation of the first and third clutches and the third brake,
a fifth forward speed stage is achieved by operation of the first clutch and the second clutch and the third brake,
a sixth forward speed stage is achieved by operation of the first, second, and third clutches,
a seventh forward speed stage is achieved by operation of the first clutch and the second clutch and the second brake,
an eighth forward speed stage is achieved by operation of the first and third clutches and the second brake,
a ninth forward speed stage is achieved by operation of the first and third clutches and the first brake, and
a reverse speed stage is achieved by operation of the second clutch and the first and third brakes.

9. A planetary gear train of an automatic transmission for a vehicle comprising:
an input shaft configured for receiving torque of an engine;
an output shaft configured for outputting changed torque of the engine;
a first planetary gear set comprising a double pinion planetary gear set including a first rotation element, a second rotation element, and a third rotation element;
a second planetary gear set comprising a single pinion planetary gear set including a fourth rotation element, a fifth rotation element, and a sixth rotation element;
a third planetary gear set comprising a single pinion planetary gear set including a seventh rotation element, an eighth rotation element, and a ninth rotation element;
a fourth planetary gear set comprising a single pinion planetary gear set including a tenth rotation element, an eleventh rotation element, and a twelfth rotation element;
a first rotation shaft directly connected to the first rotation element and directly connected to the input shaft;
a second rotation shaft directly connecting the second rotation element to the sixth rotation element and the ninth rotation element, and selectively connected to a transmission housing;
a third rotation shaft directly connected to the third rotation element and selectively connected to the transmission housing;
a fourth rotation shaft directly connected to the fourth rotation element and selectively connected to the transmission housing;
a fifth rotation shaft directly connecting the fifth rotation element to the eleventh rotation element and selectively connected to the input shaft;
a sixth rotation shaft directly connected to the seventh rotation element and selectively connected to the input shaft or the third rotation shaft;
a seventh rotation shaft directly connecting the eighth rotation element to the tenth rotation element;
an eighth rotation shaft directly connected to the twelfth rotation element and directly connected to the output shaft;
a first clutch selectively connecting the input shaft to the fifth rotation shaft;
a second clutch selectively connecting the input shaft to the sixth rotation shaft;
a third clutch selectively connecting the third rotation shaft to the sixth rotation shaft;
a first brake selectively connecting the second rotation shaft to the transmission housing;
a second brake selectively connecting the third rotation shaft to the transmission housing; and
a third brake selectively connecting the fourth rotation shaft to the transmission housing.

10. The planetary gear train of claim 9, wherein the first rotation element, the second rotation element, and the third rotation element are a first sun gear, a first ring gear, and a first planet carrier,
the fourth rotation element, the fifth rotation element, and the sixth rotation element are a second sun gear, a second planet carrier, and a second ring gear,
the seventh rotation element, the eighth rotation element, and the ninth rotation element are a third sun gear, a third planet carrier, and a third ring gear, and
the tenth rotation element, the eleventh rotation element, and the twelfth rotation element are a fourth sun gear, a fourth planet carrier, and a fourth ring gear.

11. The planetary gear train of claim 9, wherein a first forward speed stage is achieved by operation of the third clutch and the first and third brakes,
a second forward speed stage is achieved by operation of the third clutch and the second and third brakes,
a third forward speed stage is achieved by operation of the second and third clutches and the third brake,
a fourth forward speed stage is achieved by operation of the first and third clutches and the third brake,
a fifth forward speed stage is achieved by operation of the first clutch and the second clutch and the third brake,
a sixth forward speed stage is achieved by operation of the first, second, and third clutches,
a seventh forward speed stage is achieved by operation of the first clutch and the second clutch and the second brake,
an eighth forward speed stage is achieved by operation of the first and third clutches and the second brake,
a ninth forward speed stage is achieved by operation of the first and third clutches and the first brake, and
a reverse speed stage is achieved by operation of the second clutch and the first and third brakes.