Power transmission system of a hybrid electric vehicle

Abstract

Power transmission system of a hybrid electric vehicle, comprising: a drive shaft (IS) suitable for receiving a torque of an engine (ENG); a first planetary gear set (PG1) having first, second and third rotating elements, the first rotating element being directly connected to a first motor generator (MG1), and the third rotating element being connected to the input shaft (IS); a second planetary gear set (PG2) having a fourth, a fifth and a sixth rotating element, the fourth rotating element being directly connected to the second rotating element and a second motor generator (MG2), and the fifth rotating element being selectively connected to the first rotating element; an output gear (OG) connected to the sixth rotating element of the second planetary gear set (PG2); a first clutch (CL1) adapted to selectively connect the first rotating element of the first planetary gear set (PG1) to the fifth rotating element of the second planetary gear set (PG2); a second clutch (CL2) adapted to selectively connect two rotating elements of the rotating elements of the first planetary gear set (PG1) to each other; a first brake (BK1) arranged between the fifth rotating element of the second planetary gear set (PG2) and a gear case (H); and a second brake (BK2) arranged between the fourth rotating element of the second planetary gear set (PG2) connected to the second motor generator (MG2) and the transmission case (H), wherein the first planetary gear set (PG1) is a double planetary gear set having as its rotating elements a first sun gear (S1), a first planetary gear carrier (PC1) and a first ring gear (R1), wherein the first sun gear (S1) is selectively connected to the first ring gear (R1) via the second clutch (CL2) and is selectively connected to the fifth rotating element of the second planetary gear set (PG2), the first planetary gear carrier (PC1) is connected to the second motor generator (MG2), is optionally connected to the transmission case (H) via the second brake (BK2), and is directly connected to the fourth rotating element of the second planetary gear set (PG2), and the first ring gear (R1) is directly connected to the drive shaft (IS), wherein the second planetary gear set (PG2) is a double planetary gear set having as its rotating elements a second sun gear (S2), a second planetary gear carrier (PC2) and a second ring gear (R2), wherein the second sun gear (S2) is directly connected to the first planetary gear carrier (PC1) of the first planetary gear set (PG1), is connected to the second motor generator (MG2), and is connected to the transmission housing (H) via the second brake (BK2), the second planetary gear carrier (PC2) is optionally connected to the first sun gear (S1) of the first planetary gear set (PG1) via the first clutch (CL1) and is optionally connected to the transmission housing (H) via the first brake (BK1), and the second ring gear (R2) is connected to the output gear (OG).

Description

[0001] The invention relates to a power transmission system of a hybrid electric vehicle, and more particularly to a power transmission system of a hybrid electric vehicle that realizes at least two torque split modes and at least four fixed gear modes.

[0002] In general, a hybrid electric vehicle is a vehicle that is powered by at least two different types of power sources.

[0003] Such a hybrid electric vehicle typically includes an internal combustion engine and a motor generator. The hybrid electric vehicle uses a motor generator with relatively better low-speed torque characteristics as the main power source at low speeds, and uses an internal combustion engine with relatively better high-speed torque characteristics as the main power source at high speeds.

[0004] Since the hybrid electric vehicle stops the operation of a fossil fuel-using internal combustion engine and uses a motor generator in a low-speed range, fuel consumption can be improved and exhaust gas can be reduced.

[0005] The power transmission system of a hybrid electric vehicle is divided into a single-mode type and a multi-mode type.

[0006] In the single-mode type, no torque output device such as clutches and brakes is required for shift control, but fuel consumption is high due to deterioration of efficiency in a high-speed range, and an additional torque multiplying device must be used in a large vehicle.

[0007] Since the multi-mode type has high efficiency and is able to multiply the torque independently, the multi-mode type can be used in a large-capacity car.

[0008] Therefore, the multi-mode type is used instead of the single-mode type as the power transmission system of a hybrid electric vehicle and is also constantly being developed.

[0009] The multi-mode type power transmission system includes a plurality of planetary gear sets, a plurality of motor generators operated as a motor and / or a generator, a plurality of torque output devices controlling rotating elements of the planetary gear sets, and a battery used as a drive source of the motor generators.

[0010] The multi-mode type power transmission system has various operating mechanisms depending on connections of the planetary gear sets, motor generators and torque output device.

[0011] In addition, the multi-mode power transmission system has various characteristics, such as durability, power output efficiency, and size, which depend on the interconnections of the planetary gear sets, motor generators, and torque output devices. Therefore, the interconnection structure designs of the power transmission system of a hybrid electric vehicle are also constantly evolving.

[0012] DE 10 2005 038 287 A1 and KR 10 2011 0 062 140 A each describe a power transmission system of a hybrid electric vehicle, comprising a drive shaft suitable for receiving a torque of an engine, a first planetary gear set comprising a first, a second and a third rotating element, wherein the first rotating element is directly connected to a first motor generator, and the third rotating element is connected to the drive shaft, a second planetary gear set comprising a fourth, a fifth and a sixth rotating element, wherein the fourth rotating element is directly connected to the second rotating element and a second motor generator, and the fifth rotating element is selectively connected to the first rotating element, an output gear connected to the sixth rotating element of the second planetary gear set, a first clutch suitableto selectively connect the first rotating element of the first planetary gear set with the fifth rotating element of the second planetary gear set, a second clutch adapted to selectively connect two rotating elements of the rotating elements of the first planetary gear set to each other, a first brake disposed between the fifth rotating element of the second planetary gear set and a transmission housing, and a second brake disposed between the fourth rotating element of the second planetary gear set, which is connected to the second motor generator, and the transmission housing.

[0013] The object of the invention is to provide a power transmission system of a hybrid electric vehicle which realizes an electric vehicle (EV) mode, at least two torque split modes comprising a drive split mode and a compound split mode, and at least four fixed gear modes by means of at least one torque output device.

[0014] According to the invention, the object is achieved by a power transmission system of a hybrid electric vehicle according to the features of claim 1 or 2. Advantageous further developments are described in the subclaims.

[0015] According to one aspect of the invention, a power transmission system of a hybrid electric vehicle comprises: a drive shaft adapted to receive torque from an engine, a first planetary gear set comprising first, second, and third rotating elements, the first rotating element being directly connected to a first motor generator, and the third rotating element being connected to the drive shaft, a second planetary gear set comprising fourth, fifth, and sixth rotating elements, the fourth rotating element being directly connected to the second rotating element and a second motor generator, and the fifth rotating element being selectively connected to the first rotating element, an output gear connected to the sixth rotating element of the second planetary gear set, a first clutch adapted toto selectively connect the first rotating element of the first planetary gear set with the fifth rotating element of the second planetary gear set, a second clutch adapted to selectively connect two rotating elements of the rotating elements of the first planetary gear set to each other, a first brake disposed between the fifth rotating element of the second planetary gear set and a transmission housing, and a second brake disposed between the fourth rotating element of the second planetary gear set, which is connected to the second motor generator, and the transmission housing.

[0016] The first planetary gear set is a double planetary gear set having as its rotating elements a first sun gear, a first planetary carrier, and a first ring gear, wherein the first sun gear is selectively connected to the first ring gear via the second clutch and selectively connected to the fifth rotating element of the second planetary gear set, the first planetary carrier is connected to the second motor generator, selectively connected to the transmission case via the second brake, and directly connected to the fourth rotating element of the second planetary gear set, and the first ring gear is directly connected to the input shaft.

[0017] The second planetary gear set is a double planetary gear set having as its rotating elements a second sun gear, a second planetary carrier, and a second ring gear, wherein the second sun gear is directly connected to the first planetary carrier of the first planetary gear set, is connected to the second motor generator, and is connected to the transmission housing via the second brake, the second planetary carrier is selectively connected to the first sun gear of the first planetary gear set via the first clutch and is selectively connected to the transmission housing via the first brake, and the second ring gear is connected to the output gear.

[0018] According to another aspect of the invention, a power transmission system of a hybrid electric vehicle comprises: a first planetary gear set having as its rotating elements a first sun gear, a first planetary gear carrier, and a first ring gear; a second planetary gear set having as its rotating elements a second sun gear, a second planetary gear carrier, and a second ring gear; first and second motor generators operable as a motor or a generator; and a torque output device having first and second clutches and first and second brakes; the first sun gear is selectively connected to the second planetary gear carrier and connected to the first motor generator; the first planetary gear carrier is directly connected to the second sun gear, connected to the second motor generator, and selectively connected to a transmission housing via the second brake;the first ring gear is directly connected to a drive shaft and is optionally connected to the first sun gear via the second clutch, the second planetary gear carrier is optionally connected to the transmission housing via the first brake, and the second ring gear is connected to an output gear.

[0019] The first planetary gear set and the second planetary gear set may be double planetary gear sets.

[0020] The first brake may be operated in an electric vehicle (EV) mode, the first brake may be operated in a drive split mode, the first clutch may be operated in a compound split mode, the second clutch and the first brake may be operated in a fixed first stage mode, the first clutch and the first brake may be operated in a fixed second stage mode, the first clutch and the second clutch may be operated in a fixed third stage mode, or the first clutch and the second brake may be operated in a fixed fourth stage mode.

[0021] The invention is explained in more detail with reference to the drawing. The drawing shows: Fig. 1 is a schematic diagram of a power transmission system according to an exemplary embodiment of the invention; Fig. 2 is an operation table of a torque output device used in a power transmission system according to an exemplary embodiment of the invention in each operation mode; Fig. 3 is a diagram illustrating connections between components of a power transmission system according to an exemplary embodiment of the invention; Fig. 4 is a lever diagram for a power transfer system according to an exemplary embodiment of the invention in an electric vehicle (EV) mode and a drive split mode; Fig. 5 is a lever diagram for a power transmission system according to an exemplary embodiment of the invention in a compound branch mode; Fig. 6 is a lever diagram for a power transmission system according to an exemplary embodiment of the invention in a fixed first stage mode; Fig. 7 is a lever diagram for a power transmission system according to an exemplary embodiment of the invention in a fixed second stage mode; Fig. 8 is a lever diagram for a power transmission system according to an exemplary embodiment of the invention in a fixed third stage mode; and Fig. 9 is a lever diagram for a power transmission system according to an exemplary embodiment of the invention in a fixed fourth stage mode.

[0022] In the figures, identical parts are designated by the same reference numerals.

[0023] With reference to Fig. 1, a power transmission system of a hybrid electric vehicle according to an exemplary embodiment of the invention inputs torque of a motor ENG, which is a drive source, via a drive shaft IS, changes the torque according to a traveling state of a vehicle, and outputs the changed torque via an output gear OG.

[0024] The power transmission system includes first and second planetary gear sets PG1 and PG2, first and second motor generators MG1 and MG2, and a torque output device having first and second clutches CL1 and CL2 and first and second brakes BK1 and BK2.

[0025] One rotating element of the first planetary gear set PG1 is directly connected to one rotating element of the second planetary gear set PG2, and another rotating element of the first planetary gear set PG1 is selectively connected to another rotating element of the second planetary gear set PG2 such that the first planetary gear set PG1 and the second planetary gear set PG2 operate as a compound planetary gear set.

[0026] The first motor generator MG1 and the second motor generator MG2 are independent drive sources and operate as a motor and a generator in an otherwise conventional manner.

[0027] The first motor generator MG1 is operated as a motor that supplies torque to the other rotating element of the first planetary gear set PG1 selectively connected to the other rotating element of the second planetary gear set PG2, or is operated as a generator that generates electric power when rotated by the torque of the other rotating element.

[0028] The second motor generator MG2 is operated as a motor that supplies torque to the one rotating element of the first planetary gear set PG1 that is directly connected to the one rotating element of the second planetary gear set PG2, or is operated as a generator that generates electric power when rotated by the torque of the one rotating element.

[0029] For this purpose, the first motor generator MG1 and the second motor generator MG2 have stators fixed to a gear case H and rotors connected to the other rotating element of the second planetary gear set PG2 and the one rotating element of the first planetary gear set PG1, respectively.

[0030] The first clutch CL1 selectively connects the other rotating element of the first planetary gear set PG1 with the other rotating element of the second planetary gear set PG2, and the second clutch CL2 selectively connects two rotating elements of three rotating elements of the first planetary gear set PG1.

[0031] The first brake BK1 selectively connects the other rotating element of the second planetary gear set PG2 to the gear housing H such that the other rotating element of the second planetary gear set PG2 selectively operates as a fixed element.

[0032] The second brake BK2 is arranged in parallel with the second motor generator MG2 and causes the one rotating element of the first planetary gear set PG1 connected to the second motor generator MG2 to selectively operate as a fixed element.

[0033] The first and second planetary gear sets PG1 and PG2 and the first and second motor generators MG1 and MG2 are arranged on the same axis.

[0034] The torque output device includes the first and second clutches CL1 and CL2 and the first and second brakes BK1 and BK2, which are conventional wet-type multi-disk friction elements operated by hydraulic pressure. It should be noted that other types of clutches and brakes, such as dry-type friction elements, may be used.

[0035] Herein, a clutch is a friction element that connects one rotating element to another rotating element, and a brake is a friction element that connects one rotating element to a non-rotating element.

[0036] The components of a power transmission system according to an exemplary embodiment of the invention are described in detail below.

[0037] The first planetary gear set PG1 is a double planetary gear set and has three rotating elements consisting of a first sun gear S1, a first planetary gear carrier PC1 and a first ring gear R1.

[0038] The first planetary gear carrier PC1 rotatably supports a first planetary gear P1 which meshes with the first sun gear S1 and the first ring gear R1.

[0039] The second planetary gear set PG2 is a planetary gear set with double planetary gears and has three rotating elements consisting of a second sun gear S2, a second planetary gear carrier PC2 and a second ring gear R2.

[0040] The second planetary gear carrier PC2 rotatably supports a second planetary gear P2 which meshes with the second sun gear S2 and the second ring gear R2.

[0041] The first planetary gear carrier PC1 is directly connected to the second sun gear S2, and the first sun gear S1 is optionally connected to the second planetary gear carrier PC2.

[0042] In addition, the first ring gear R1 is directly connected to the input shaft IS, which is an input element, and the second ring gear R2 is directly connected to the output gear OG, which is an output element.

[0043] The first motor generator MG1 is directly connected to the first sun gear S1 of the first planetary gear set PG1 and selectively to the second planetary gear carrier PC2 of the second planetary gear set PG2, such that the first sun gear S1 and the second planetary gear carrier PC2 are driven simultaneously, or only the first sun gear S1 is driven. Furthermore, the first motor generator MG1 can operate as a generator.

[0044] The second motor generator MG2 is directly connected to the first planetary carrier PC1 of the first planetary gear set PG1 and the second sun gear S2 of the second planetary gear set PG2, so that the first planetary carrier PC1 and the second sun gear S2 are driven simultaneously. Furthermore, the second motor generator MG2 can function as a generator.

[0045] The first clutch CL1 is arranged between the first sun gear S1 of the first planetary gear set PG1 and the second planetary gear carrier PC2 of the second planetary gear set PG2 and selectively connects the first sun gear S1 to the second planetary gear carrier PC2.

[0046] The second clutch CL2 is arranged between the first sun gear S1 and the first ring gear R1 of the first planetary gear set PG1 such that the first planetary gear set PG1 can be locked. In this case, the first planetary gear set PG1 rotates as a rotating element.

[0047] The first brake BK1 is arranged between the second planetary gear carrier PC2 of the second planetary gear set PG2 and the gear housing H such that the second planetary gear carrier PC2 selectively operates as a fixed element.

[0048] The second brake BK2 is arranged in parallel with the second motor generator MG2 and selectively operates the first planetary carrier PC1 of the first planetary gear set PG1 and the second sun gear S2 of the second planetary gear set PG2 as a fixed element.

[0049] With reference to the operating table in Fig. 2 describes in detail the torque output device operating in each gear stage.

[0050] In an electric vehicle (EV) mode, the first brake BK1 is operated. In a drive split mode, the first brake BK1 is operated. In a compound split mode, the first clutch CL1 is operated. In a first stage fixed mode, the second clutch CL2 and the first brake BK1 are operated. In a second stage fixed mode, the first clutch CL1 and the first brake BK1 are operated. In a third stage fixed mode, the first clutch CL1 and the second clutch CL2 are operated. In a fourth stage fixed mode, the first clutch CL1 and the second brake BK2 are operated.

[0051] As described above, the power transmission system according to an exemplary embodiment of the invention can realize one EV mode, two torque split modes, and four fixed gear modes.

[0052] With reference to the Fig. 3 and Fig. 4 the first brake BK1 is operated in EV mode.

[0053] The engine ENG remains stopped, the first planetary gear set PG1 does not affect the shifting, and the shifting in EV mode is carried out by three rotating elements of the second planetary gear set PG2.

[0054] That is, in a state where the second motor generator MG2 operates to input its torque to the second sun gear S2, the second planetary carrier PC2 is operated as a fixed member by the operation of the first brake BK1, and a reduced speed is output via the second ring gear R2 which is the output member.

[0055] Furthermore, in a state where the torque of the second sun gear S2 is input to the first planetary gear carrier PC1, the engine ENG is stopped. Therefore, the first ring gear R1 operates as a stationary element, and the first motor generator MG1 connected to the first sun gear S1 rotates inversely.

[0056] At this time, when the engine ENG is started by controlling the first motor generator MG1, the drive split mode is achieved.

[0057] With reference to the Fig. 3 and Fig. 5, since the first clutch CL1 operates in the compound split mode, the first and second planetary gear sets PG1 and PG2 are driven by four rotating elements. In this state, the first motor generator MG1 and the second motor generator MG2 are connected to the engine ENG or the driven gear OG via the rotating elements of the planetary gear sets PG1 and PG2.

[0058] Therefore, the engine ENG, the first motor generator MG1 and the second motor generator MG2 can generate a drive torque, one of them supplying a torque to the other of them so as to generate electric power, and all the gear ratios required for driving the vehicle can be achieved by appropriately changing their rotational speeds.

[0059] With reference to the Fig. 3 and Fig. 6, the second clutch CL2 and the first brake BK1 are operated in the fixed first stage mode.

[0060] Since the first planetary gear set PG1 is in the direct coupling state due to the operation of the second clutch CL2, three rotating elements of the second planetary gear set PG2 are involved in the shifting.

[0061] In this state, the torque of the engine ENG is input to the second sun gear S2 of the second planetary gear set PG2 via the first planetary carrier PC1 of the first planetary gear set PG1, and the second planetary carrier PC2 is operated as a fixed element by the operation of the first brake BK1. Therefore, a reduced speed is output at a predetermined gear ratio, and the first-stage fixed mode is achieved.

[0062] With reference to the Fig. 3 and Fig. 7, in the fixed second stage mode, the first clutch CL1 and the first brake BK1 are operated.

[0063] By operating the first clutch CL1, the first and second planetary gear sets PG1 and PG2 are operated by means of four rotating elements.

[0064] In this state, the torque of the engine ENG is input to the second sun gear S2 of the second planetary gear set PG2 via the first planetary gear set PG1, and the first sun gear S1 and the second planetary carrier PC2 are operated as fixed elements by the operation of the first brake BK1. Therefore, a reduced speed is output at a predetermined gear ratio, and the second-speed fixed mode is achieved.

[0065] With reference to the Fig. 3 and Fig. 8, in the third-stage fixed mode, the first brake BK1, which was operated in the second-stage fixed mode, is released and the second clutch CL2 is operated.

[0066] Since the first planetary gear set PG1 is in the direct coupling state due to the operation of the second clutch CL2, three rotating elements of the second planetary gear set PG2 are involved in the shifting.

[0067] In this state, the torque of the engine ENG is input to the second sun gear S2 and the second planetary gear carrier PC2 of the second planetary gear set PG2 via the first planetary gear set PG1.

[0068] Therefore, the second planetary gear set PG2 is in the direct coupling state, and a speed equal to a speed of the engine ENG is output, and the third-stage fixed mode is achieved.

[0069] With reference to the Fig. 3 and Fig. 9, in the fourth-stage fixed mode, the second clutch CL2, which was operated in the third-stage fixed mode, is released and the second brake BK2 is operated.

[0070] By the operation of the first clutch CL1, the first sun gear S1 is connected to the second planetary gear carrier PC2, and the first planetary gear set PG1 and the second planetary gear set PG2 are operated by four rotating elements.

[0071] In this state, the torque of the engine ENG is input to the first ring gear R1, and the first planetary carrier PC1 and the second sun gear S2 are driven as stationary elements by the operation of the second brake BK2. Therefore, an increased speed is output at a predetermined gear ratio. That is, an overdrive mode with a fixed gear ratio is achieved.

[0072] According to the invention, an EV mode, two torque split modes consisting of a drive split mode and a compound split mode, and four fixed shift stage modes are achieved.

[0073] Accordingly, according to the invention, a multi-mode type is achieved in which fixed gear ratios and variable gear ratios are combined. Therefore, high efficiency at all gear ratios of a vehicle and improved fuel economy can be achieved.

Claims

[1] Power transmission system of a hybrid electric vehicle, comprising: a drive shaft (IS) suitable for receiving a torque of an engine (ENG); a first planetary gear set (PG1) having first, second and third rotating elements, the first rotating element being directly connected to a first motor generator (MG1), and the third rotating element being connected to the drive shaft (IS); a second planetary gear set (PG2) having a fourth, a fifth and a sixth rotating element, the fourth rotating element being directly connected to the second rotating element and a second motor generator (MG2), and the fifth rotating element being selectively connected to the first rotating element; an output gear (OG) connected to the sixth rotating element of the second planetary gear set (PG2); a first clutch (CL1) adapted to selectively connect the first rotating element of the first planetary gear set (PG1) to the fifth rotating element of the second planetary gear set (PG2); a second clutch (CL2) adapted to selectively connect two rotating elements of the rotating elements of the first planetary gear set (PG1) to each other; a first brake (BK1) arranged between the fifth rotary element of the second planetary gear set (PG2) and a transmission case (H); and a second brake (BK2) arranged between the fourth rotating element of the second planetary gear set (PG2) connected to the second motor generator (MG2) and the transmission case (H), wherein the first planetary gear set (PG1) is a double planetary gear set having as its rotating elements a first sun gear (S1), a first planetary gear carrier (PC1) and a first ring gear (R1), wherein the first sun gear (S1) is selectively connected to the first ring gear (R1) via the second clutch (CL2) and is selectively connected to the fifth rotary element of the second planetary gear set (PG2), the first planetary gear carrier (PC1) is connected to the second motor generator (MG2), is optionally connected to the transmission housing (H) via the second brake (BK2), and is directly connected to the fourth rotating element of the second planetary gear set (PG2), and the first ring gear (R1) is directly connected to the drive shaft (IS), wherein the second planetary gear set (PG2) is a double planetary gear set having as its rotating elements a second sun gear (S2), a second planet carrier (PC2) and a second ring gear (R2), wherein the second sun gear (S2) is directly connected to the first planetary gear carrier (PC1) of the first planetary gear set (PG1), is connected to the second motor generator (MG2), and is connected to the transmission housing (H) via the second brake (BK2), the second planetary gear carrier (PC2) is optionally connected to the first sun gear (S1) of the first planetary gear set (PG1) via the first clutch (CL1) and is optionally connected to the transmission housing (H) via the first brake (BK1), and the second ring gear (R2) is connected to the output gear (OG). [2] A power transmission system of a hybrid electric vehicle, comprising: a first planetary gear set (PG1) having as its rotating elements a first sun gear (S1), a first planetary gear carrier (PC1) and a first ring gear (R1); a second planetary gear set (PG2) having as its rotating elements a second sun gear (S2), a second planetary carrier (PC2) and a second ring gear (R2); a first and a second motor generator (MG1, MG2) which can be operated as a motor or a generator; and a torque output device having a first and a second clutch (CL1, CL2) and a first and a second brake (BK1, BK2), wherein the first sun gear (S1) is selectively connected to the second planetary gear carrier (PC2) and is connected to the first motor generator (MG1), the first planetary gear carrier (PC1) is directly connected to the second sun gear (S2), is connected to the second motor generator (MG2), and is optionally connected to a gearbox housing (H) via the second brake (BK2), the first ring gear (R1) is directly connected to a drive shaft (IS) and is optionally connected to the first sun gear (S1) via the second clutch (CL2), the second planet carrier (PC2) is optionally connected to the gearbox housing (H) via the first brake (BK1), and the second ring gear (R2) is connected to an output gear (OG). [3] A power transmission system according to claim 2, wherein the first planetary gear set (PG1) and the second planetary gear set (PG2) are double pinion planetary gear sets. [4] Power transmission system according to claim 2, wherein the first brake (BK1) is operated in an electric vehicle (EV) mode, the first brake (BK1) is operated in a drive split mode, the first clutch (CL1) is operated in a compound split mode, the second clutch (CL2) and the first brake (BK1) are operated in a fixed first stage mode, the first clutch (CL1) and the first brake (BK1) are operated in a fixed second stage mode, the first clutch (CL1) and the second clutch (CL2) are operated in a fixed third stage mode, or the first clutch (CL1) and the second brake (BK2) are operated in a fixed fourth stage mode.

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