EP2064417A1

EP2064417A1 - Continuous camshaft phase-shifting apparatus

Info

Publication number: EP2064417A1
Application number: EP07842677A
Authority: EP
Inventors: Xiaolan Ai
Original assignee: Timken Co
Current assignee: JTEKT Bearings North America LLC
Priority date: 2006-09-19
Filing date: 2007-09-18
Publication date: 2009-06-03
Also published as: WO2008036650A8; KR20090074161A; US20100064997A1; WO2008036650A1; JP2010504470A

Abstract

A phase shift mechanism (A) for controlling a phase shift angle between a camshaft (12) and a drive (10) comprises a planetary gear train ( 20). The planetary gear train (20) has first and second planet gears (34, 26) that are united to rotate about a common axis at the same angular velocity, and a carrier (32) which includes a first planet bearing (62) and a second planet bearing (64) on which the united first and second planet gears ( 34, 36) respectively rotate and a locking mechanism (24) carried by the carrier (32) for locking the planetary gear train (20) in a locked condition by preventing the planet gears (34, 36) from rotating relative to the carrier (32 ) such that the phase shift angle remains the same and the output shaft 26 rotates with the input shaft (18) at the same angular velocity.

Description

CONTINUOUS CAMSHAFT PHASE-SHIFTING APPARATUS

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U. S. C. § 1 19(e) to United States Provisional Patent Application No. 60/845,692 filed September 19, 2006, the provisional application being incorporated by reference. TECHNICAL FIELD

Camshaft phase shifting mechanisms are used in internal combustion engines to vary valve timing to achieve benefits of improving fuel consumption and improve exhaust gas quality. It is possible, with an adequate camshaft shifter, to vary valve timing for maximum comfort and/or maximum torque and for the highest performance. Currently, most types of camshaft phase shifting mechanisms are hydraulic powered. These camshaft phase shifting mechanisms generally consist of a hydraulic shifter unit, a regulation valve and a control circuit. The shifter unit has to have a low leakage rate and a sufficiently large chamber/piston to ensure adequate stiffness. The regulation valve has to ensure high flow rate during adjustment cycles while providing a precise regulation to fix the set-point angles. Some camshaft phase shifting mechanisms require a separate high-pressure supply. These current mechanisms are complex, expensive and need regular maintenance. Additionally, the performance of these current mechanisms depends mainly on temperature parameters.

It is desirable to have a compact, highly responsive and inexpensive camshaft phase shifting mechanism that is free from complex hydraulic systems while being electronically controlled for simplicity and high precision and free of temperature dependence. BRIEF SUMMARY OF THE DISCLOSURE

This invention relates to a phase shifting mechanism, and more specifically to an electro-mechanical phase shifting mechanism for a camshaft of an internal combustion engine. -9-

Thθ phase shifting mechanism of the present disclosure comprises a positive epicyclic gear train with frictional locking capability. The gear train has three co-axial rotatable branches. The first branch operatively couples to an input shaft (i.e., the crankshaft), the second branch operatively couples to an output shaft and the third branch, a lockable branch, operatively couples to a rotor of an electric machine. The gear train can only be unlocked during phase adjustment by the electric machine that is connected to the third branch, the lockable branch.

In an embodiment, the input shaft couples to the crankshaft, the output shaft couples to the camshaft and the planetary gear train co-axially aligns around the input shaft and the output shaft. An input sun gear of the planetary gear train couples to the input shaft and an output sun gear of the planetary gear train couples to the output shaft. The planetary gear train has first and second planet gears that engage the input and output sun gears, respectively, and are united to rotate about a common axis at the same angular velocity. The carrier includes a planet shaft and a first planet bearing and a second planet bearing on which the united first and second planet gears respectively rotate.

A locking mechanism of the phase shifting mechanism locks the planetary gear train in a locked condition by preventing the planet gears from rotating relative to the carrier to rotate the input sun gear, the output sun gear and the carrier as a unit such that the phase shift angle for the output shaft with respect to the input shaft remains the same and the output shaft rotates with the input shaft at the same angular velocity. During operation, the electric machine applies torque to the locking mechanism in order to unlock the locking mechanism such that the output shaft rotates at a different angular velocity with respect to the input shaft.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS In the accompanying drawings which form part of the specification: Fig. 1 illustrates a schematic view of components of an internal combustion engine illustrating a drive, a camshaft and a phase shifting mechanism constructed in accordance with and embodying the present disclosure;

Fig. 2 illustrates a cross sectional view of an input shaft, an output shaft and the phase shifting mechanism; Fig. 3 illustrates an exploded view of a phase shifting mechanism constructed in accordance with and embodying the present disclosure illustrating a planetary gear train and locking mechanism of the phase shifting mechanism wherein first and second planet gears of the planetary gear train are identical and integrated with one another; Fig. 4 illustrates another exploded view of components of the phase shifting mechanism of Fig. 3;

Fig. 5 illustrates a cross sectional view of the gear train and locking mechanism constructed in accordance with and embodying the present disclosure; Fig. 6 illustrates a perspective view of an input sun gear and an extension of a limiting device of the phase shifting mechanism; and

Fig. 7 illustrates a perspective view of an output sun gear and another embodiment of an extension of a limiting device of the phase shift mechanism. DESCRIPTION OF THE PREFERRED EMBODIMENT

The following detailed description illustrates the disclosure by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the disclosure, describes several embodiments, adaptations, variations, alternatives, and uses of the disclosure, including what is presently believed to be the best mode of carrying out the disclosure.

Referring to the drawings, an electro-mechanic phase shifting mechanism generally shown as A is shown (Figs. 1 and 2). In an embodiment, the phase shifting mechanism A is located between a drive 10 and a camshaft 12 of an internal combustion engine 14. Referring to Figs. 1 -5, the phase shifting mechanism A comprises a sprocket 16, an input shaft 18 -A-

couplθd to the drive 10 (crankshaft) (Fig. 1 ), an epicyclic gear train generally shown as 20 having a locking mechanism generally shown as 22, an electric machine generally shown as 24 and an output shaft 26 coupled to the camshaft 12 (Fig. 1 ). The epicyclic gear train 20 co-axially aligns around the input shaft 18 and the output shaft 26. The epicyclic gear train 20 comprises a first branch in the form of an input sun gear 28, a second branch in the form of an output sun gear 30, a lockable third branch in the form a carrier 32, a first set of planet gears 34 and a second set of planet gears 36. The input sun gear 28 meshes with the first set of planet gears 34, and the output sun gear 30 meshes with the second set of planet gears 36. Each planet gear 34 in the first planet gear set couples to, and thus rotates as a unit with, a corresponding planet gear 36 in the second planet gear set. In an embodiment, the planet gears 34, 36 are substantially identically formed and are integrated as a single gear. Planet gears 34, 36 together form a planetary gear pair to rotate about a common axis at the same angular velocity. The planetary gear pairs are supported by a set of planet shafts 38 (Fig. 1 ), through bearings. The carrier 32 is supported in a housing 40 though bearings 42. As shown in Fig. 2, the input shaft 18 connects to sprocket 16 at one end and to the input sun gear 28 at the other end. The input shaft 18 is supported in the housing 40 though bearing 44. The output shaft 26 connects to the output sun gear 30 at one end and couples to camshaft 12 (Fig. 1 ) at the other end. The electric machine 24 includes a rotor 46 and a stator 48. The rotor

46 fits over the carrier 32 to establish a firm mechanical connection, so that the carrier 32 rotates with the rotor 46 as a unit. As shown, the stator 48 mounts to the housing 40.

To improve supporting stiffness, the input shaft 18 and output shaft 26 may extend beyond the input sun gear 28 and the output sun gear 30 with one piloted on the other through bearing 49 (Fig. 1 ). Input shaft 18 is allowed to rotate with respect to the output shaft 26 when phase shift between the two shafts 18, 26 is desirable. To prevent excessive angular displacement between the two shafts 18, 26 an angular position limiting device generally shown as 50 (Figs. 1 , 3-5) may be employed to provide mechanical stops in both rotating directions.

The limiting device 50 rotatably couples the input sun gear 28 with the output sun gear 30. Referring to Figs. 3 and 4, the limiting device 50, in an embodiment, comprises a slot 52 positioned on a face 54 of the input sun gear 28 and comprises an extension 56 protruding from another face 58 of the output sun gear 30 such that the extension 56 slidably engages with the slot 52. In an embodiment, the extension 56 comprises pins protruding from the output sun gear 30. In other embodiments (Figs. 6 and 7), an extension 60 comprise key members protruding from the face 54 of the input sun gear 28 (Fig. 6) and/or protruding from the face 58 of the output sun gear 30 (Fig. 7). The key members slidably engage the appropriate mating surface positioned on the opposing sun gear. During rotation of the shafts 18, 26, the extensions 56 slidably reciprocate within the opposing slot 52 such that the slots limit travel movement of the extensions 56 to prevent excessive angular displacement between the shafts 18, 26. Returning to Figs. 1 -5, the epicyclic gear train 20 has a basic gear ratio

"SR₀" defined as

CO^ - O)₁

SR ^vo₀ = ^- ω_sι - ω_c (1 )

where ω_Sj = angular velocity of the input sun gear 28 ω_S2 = angular velocity of the output sun gear 30 we = angular velocity of the carrier 32.

The basic gear ratio can be determined by tooth numbers of the gears in the epicyclic gear train 20, as below where

Nsi = the number of teeth for the input sun gear 28 Ns₂ = the number of teeth for the output sun gear 30 Np₁ = the number of teeth for the first planet gear 34 Np₂ = the number of teeth for the second planet gear 36.

The phase shifting angle for the output shaft 26 with respect to the input shaft 18 is determined as

AΘ = {l-SR_ϋ )j{ω_sl - ω_c )dτ . (3)

0

The locking mechanism 22 of the epicyclic gear train 20 is designed to have a configuration and internal geometry that ensure an internal jam or lock when no external torque is applied to the carrier 32. The locking mechanism 22 comprises conical bearings 62, 64, that, under radial load, impose frictional resistant torque on the planetary gear pairs 34, 36 that tend to prevent them from rotating about their support shafts 38. For involute gears, and many other types of gears with conjugant teeth profiles, the radial load is in direct proportion to the amount of torque being transmitted. Thus, the frictional resistant torque is also in proportion to the input and output torque. On the other hand, the input torque from the input sun gear 28 and the output torque from the output sun gear 30 result in a differential torque that tries to rotate the planetary gears 34, 36. If the maximum available frictional torque is greater than the differential applied torque, the epicyclic gear train 20 is frictionally locked. To ensure this condition, the following internal geometry relationship between the planetary gear train 20 and the coefficients of friction between the first planet gear 34 and the first planet bearing 42 and between the second planet gear 36 and the second planet bearing 44 is characterized as

I , ta n or , , ta n a _? ) . . .. _ _π . .. β Λ ^₁ -Λ + A ₂ _²_ > I₁ ^• 1 - S R ₀ (4)

^ C O S yO ₁ C O S p ₂ J where _ N_n

N_sl

A ₌ ^ri

K _ ^r2

^₂ π= the effective bore radius for planet support bearing 62 r₂= the effective bore radius for planet support bearing 64

Rsi= pitch circle radius for the input sun gear 28 Rs₂= pitch circle radius for the output sun gear 30

O₁= pressure angle for the input sun gear 28 and the first planet gear

34

O₂= pressure angle for the output sun gear 30 & the second planet gear

36 βi= half conical angle for planet support bearing 62 β₂= half conical angle for planet support bearing 64 w= maximum available friction coefficient for planet support bearings 62 and 64.

The locking mechanism 22 prevents the planet gears 34, 36 from rotating relative to the carrier 32 to rotate the input sun gear 28, the output sun gear 30 and the carrier 32 as a unit such that the phase shift angle for the output shaft 26 with respect to the input shaft 18 remains the same and the output shaft 26 rotates with the input shaft 18 at the same angular velocity.

The locking mechanism 22 comprises friction torques caused by coefficients of friction between the first planet gear 34 and the first planet bearing 62 and between the second planet gear 36 and the second planet bearing 64.

As shown in Fig, 2, the electric machine 24 couples to the carrier 32.

The electric machine 24 applies a torque to the planetary gear train 20 for unlocking the friction torques enabling the carrier 32 to rotate relative to at least one of the input sun gear 28 and the output sun gear 30 wherein the phase shift angle for the output shaft 26 with respect to the input shaft 18 changes so that the output shaft 26 assumes a different angular velocity with respect to the input shaft 18.

During operation, the electro-mechanic camshaft phase shifting mechanism A has three operation modes. The first operating mode relates to a neutral mode in which the electric machine 24 is switched off (i.e., consuming no electric power or generating any electric power); and thus, exerting no torque on the carrier 32. With no actuation torque exerting on the carrier 32, the epicyclic gear train 20 is frictionally locked or "internally jammed" by the locking mechanism 22. In this locked state, the epicyclic gear train 20 can only be rotated as unit. The output shaft 26 rotates with the input shaft 18 in the same direction at the same angular velocity. Therefore, ω_c = ω_Sh From equation (3), since Δθ = 0, there will be no phase change in this operating mode between the input shaft 18 and the output shaft 26.

The second operating mode relates to a generating mode, in which the electric machine 24 applies a resistant torque to the gear train 20, slowing the rotor 46 down such that ω_c <ω_Si. In doing so, the epicyclic gear train 20 converts mechanical power into electric power, acting as a generator. The resistant torque unlocks the epicyclic gear train 20. Consequently, the output shaft 26 rotates with the input shaft 18 in the same direction but at a faster or slower angular velocity. From equation (3), there will be a continuous phase advancing, if Sf?₀>1 , or retarding, if Sf?o<1 , of the output shaft with respect to the input shaft 18. Accordingly, in this operating mode, there is a continuous phase advancing or retarding of output shaft 26 with respect to the input shaft 18. In the second operating mode, the torque applied by the electric machine 24 to the planetary gear train 20 comprises a resistant torque which unlocks the carrier 32 by overcoming the torque caused by friction between the first planet gear 34 and the first planet bearing 62 and between the second planet gear 36 and the second planet bearing 64. The resistant torque unlocks the carrier 32 to change the phase shift angle such that the output shaft 26 rotates with respect to the input shaft 18 at a different angular velocity. In an embodiment, the output shaft 26 rotates with respect to the input shaft 18 in the same angular direction.

The third operating mode relates to a motoring mode, in which the electric machine 24 applies a driving torque to the gear train 20, speeding the rotor 46 and carrier 32 up such that ω_c >ω_s?. The electric machine 24 draws electric power from a supplier (not shown) and converts it into mechanical power. In doing so, the electric machine 24 acts as a motor. The driving torque unlocks the epicyclic gear train 20. As a result, the output shaft 26 rotates with the input shaft 18 in the same direction but at a slower or faster angular velocity. From equation (3), there will be a continuous phase retarding, if Sf?₀>1 or advancing, if Sf?₀<1 of the output shaft 26 with respect to the input shaft 18. Accordingly, in this operating mode, there is a continuous phase retarding or advancing of output shaft 26 with respect to the input shaft 18.

In the third operating mode, the torque applied by the electric machine 24 to the planetary gear train 20 comprises a driving torque which unlocks the carrier 32 by overcoming the torques caused by friction between the first planet gear 34 and the first planet bearing 62 and between the second planet gear 36 and the second planet bearing 64. The driving torque unlocks the carrier 32 to change the phase shift angle such that the output shaft 26 rotates with respect to the input shaft 18 in the same angular direction and at a different angular velocity. In an embodiment, the output shaft 26 rotates with respect to the input shaft 18 in the same angular direction.

One of the advantages for the phase shifting mechanism A is low manufacturing cost due to the elimination of hydraulic systems. Additionally, since operation occurs under the neutral mode where the epicyclic gear train 20 is internally locked and rotates as a unit, the gears 28, 34, 36 and 30, planet support bearings 62, 64 and pilot bearing 49 experience intermittent usage. Thus, the phase shifting mechanism A uses low cost bearings. For the same reason, the phase shifting mechanism A uses low cost electric machines, such as a switched reluctance motor, to reduce the overall cost. In view of the above, it will be seen that the several objects of the disclosure are achieved and other advantageous results are obtained. As various changes could be made in the above constructions without departing from the scope of the disclosure, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

CLAIMS:

1. In an internal combustion engine including a camshaft and a drive for the camshaft, a phase shift mechanism located between the drive and the camshaft for controlling a phase shift angle between the camshaft and the drive, the phase shift mechanism comprising: an input shaft coupled to the drive; an output shaft coupled to the camshaft; a planetary gear train co-axially aligned around the input shaft and the output shaft, the planetary gear train having an input sun gear coupled to the input shaft, an output sun gear coupled to the output shaft, the planetary gear train further having first and second planet gears that engage the input and output sun gears, respectively, and are united to rotate about a common axis at the same angular velocity, and having a carrier which a first planet bearing and a second planet bearing on which the united first and second planet gears respectively rotate; and a locking mechanism for locking the planetary gear train in a locked condition by preventing the planet gears from rotating relative to the carrier to rotate the input sun gear, the output sun gear and the carrier as a unit such that the phase shift angle for the output shaft with respect to the input shaft remains the same and the output shaft rotates with the input shaft at the same angular velocity.

2. The phase shifting mechanism according to claim 1 wherein the locking mechanism comprises friction torques caused by friction between the first planet gear and the first planet bearing and between the second planet gear and the second planet bearing.

3. The phase shift mechanism according to claim 2 further comprising an electric machine coupled to the carrier, the electric machine configured to apply a torque to the planetary gear train for overcomingg the friction torques enabling the carrier to rotate relative to at least one of the input sun gear and the output sun gear wherein the phase shift angle for the output shaft with respect to the input shaft changes so that the output shaft assumes a different angular velocity with respect to the input shaft.

4. The phase shifting mechanism according to claim 3 wherein the phase shift angle is a function of a gear ratio "SR₀" of the planetary gear train.

5. The phase shifting mechanism according to claim 4 wherein the gear ratio SR₀ is characterized by the equation:

SR₀ = _ ^WS 2

⁽Osi - ⁽O_C where ω_Si = angular velocity of the input sun gear; ω_S2 = angular velocity of the output sun gear and; we = angular velocity of the carrier.

6. The phase shifting mechanism according to claim 5 wherein the change in the phase shift angle is characterized by the equation

Aθ = {l- SR₀ ) j{ω_sl - ω_c )dτ .

7. The phase shifting mechanism according to claim 4 wherein the gear ratio SR₀ is characterized by the equation: where

Nsi = the number of teeth for the input sun gear; Ns₂ = the number of teeth for the output sun gear;

Np₁ = the number of teeth for the first planet gear; and Np₂ = the number of teeth for the second planet gear.

8. The phase shifting mechanism according to claim 7 wherein the change in the phase shift angle is characterized by the equation

Aθ = {l- SR₀ ) j{ω_sl - ω_c )dτ .

9. The phase shifting mechanism according to claim 6 wherein the torque applied by the electric machine to the planetary gear train comprises a resistant torque which unlocks the carrier by overcoming the torque caused by friction between the first planet gear and the first planet bearing and between the second planet gear and the second planet bearing to change the phase shift angle that the output shaft rotates with respect to the input shaft.

10. The phase shifting mechanism according to claim 9 wherein a relationship between geometric parameters of the planetary gear train and the coefficients of friction between the first planet gear and the first planet bearing and between the second planet gear and the second planet bearing is characterized by the equation where

^{1 "} N₅₁

Np₁ = the number of teeth for the first planet gear;

Nsi = the number of teeth for the input sun gear; r_?= the effective bore radius for planet support bearing; r₂= the effective bore radius for planet support bearing;

Rs₁= pitch circle radius for the input sun gear;

Rs₂= pitch circle radius for the output sun gear;

O₁= pressure angle for the input sun gear and the first planet gear; O₂= pressure angle for the output sun gear and the second planet gear; β ₁= half conical angle for planet support bearing; β₂= half conical angle for planet support bearing and; w= maximum available friction coefficient for the first and second planet bearings.

1 1 . The phase shifting mechanism according to claim 6 wherein continuous phase advancing occurs for the phase shift angle if SR₀>1 .

12. The phase shifting mechanism according to claim 6 wherein continuous phase retarding occurs for the phase shift angle if SR₀<1 .

13. The phase shifting mechanism according to claim 6 wherein the torque applied by the electric machine to the planetary gear train comprises a driving torque which unlocks the carrier by overcoming the torques applied by the coefficients of friction between the first planet gear and the first planet bearing and between the second planet gear and the second planet bearing to change the phase shift angle that the output shaft rotates with respect to the input shaft.

14. The phase shifting mechanism according to claim 13 wherein a relationship between geometric parameters of the planetary gear train and the coefficients of friction between the first planet gear and the first planet bearing and between the second planet gear and the second planet bearing is characterized by the equation f . t a n α ^, ta n α , 1 . • u o _n β ^■ U i ΪT + λ ₂ -M > I₁ ^• 1 - S R ₁ y c o s P ₁ C O S p ₂ J where

N_E

¹ N Sl

4 =^

^KS 2 Ν_P1 = the number of teeth for the first planet gear;

Nsi = the number of teeth for the input sun gear; the effective bore radius for planet support bearing; r₂= the effective bore radius for planet support bearing;

Rs₁= pitch circle radius for the input sun gear; Rs₂= pitch circle radius for the output sun gear; O₁= pressure angle for the input sun gear and the first planet gear; O₂= pressure angle for the output sun gear and the second planet gear; β ₁= half conical angle for planet support bearing; β₂= half conical angle for planet support bearing and; 4= maximum available friction coefficient for the first and second planet bearings.

15. The phase shifting mechanism according to claim 13 wherein continuous phase retarding occurs for the phase shift angle if SR₀>1 .

16. The phase shifting mechanism according to claim 14 wherein continuous phase advancing occurs for the phase shift angle if SR₀<1 .

17. The phase shifting mechanisms according to claim 3 further comprising a limiting device that rotatably couples the input sun gear with the output sun gear.

18. The phase shifting mechanism according to claim 17 wherein the limiting device comprises a slot positioned on a face of the input sun gear and comprises an extension protruding from another face of the output sun gear such that the extension slidably engages with the slot.

19. The phase shifting mechanism according to claim 17 wherein the limiting device comprises a slot positioned on a face of the input sun gear and comprises a key member protruding from another face of the output sun gear such that the key member slidably engages with the slot.

20. The phase shifting mechanism according to claim 17 wherein the first and second planet gears are identically formed and are integrated as a single gear.

21 . An internal combustion engine having phase shift control between a camshaft and a drive of the internal combustion engine, the internal combustion engine comprising: an input shaft coupled to the drive; an output shaft coupled to the camshaft; a planetary gear train co-axially aligned around the input shaft and the output shaft, the planetary gear train having an input sun gear coupled to the input shaft, an output sun gear coupled to the output shaft, the planetary gear train further having first and second planet gears that engage the input and output sun gears, respectively, and are integrated as a single gear to rotate about a common axis at the same angular velocity, and having a carrier which includes a first planet bearing and a second planet bearing on which the united first and second planet gears respectively rotate; a locking mechanism comprising friction torques caused by friction between the first planet gear and the first planet bearing and between the second planet gear and the second planet bearing for locking the planetary gear train in a locked condition by preventing the planet gears from rotating relative to the carrier to rotate the input sun gear, the output sun gear and the carrier as a unit such that the phase shift angle for the output shaft with respect to the input shaft remains the same and the output shaft rotates with the input shaft at the same angular velocity; and an electric machine coupled to the carrier, the electric machine configured to apply a torque to the planetary gear train for overcoming the friction torques enabling the carrier to rotate relative to at least one of the input sun gear and the output sun gear wherein the phase shift angle for the output shaft with respect to the input shaft changes.

22. The phase shifting mechanism according to claim 21 wherein the torque applied by the electric machine to the planetary gear train comprises a resistant torque which unlocks the carrier by overcoming the torque applied by the coefficients of friction between the first planet gear and the first planet bearing and between the second planet gear and the second planet bearing to change the phase shift angle such that the output shaft rotates with respect to the input shaft at a different angular velocity.

23. The phase shifting mechanism according to claim 21 wherein the torque applied by the electric machine to the planetary gear train comprises a driving torque which unlocks the carrier by overcoming the torques caused by friction between the first planet gear and the first planet bearing and between the second planet gear and the second planet bearing to change the phase shift angle such that the output shaft rotates with respect to the input shaft in the same angular direction and at a different angular velocity.

24. A method of controlling a phase shift angle between a camshaft and a drive of an internal combustion engine, the method comprising: aligning a planetary gear train aligned around an input shaft coupled to the drive and around an output shaft coupled to the camshaft; locking the planetary gear train by the friction force to rotate an input sun gear, an output sun gear and a carrier of the planetary gear train as a unit, the input sun gear being coupled to the input shaft and the output sun gear being coupled to the output shaft such that the phase shift angle for the output shaft with respect to the input shaft remains the same and the output shaft rotates with the input shaft at the same angular velocity; and applying a torque to the planetary gear train for unlocking the friction force applied to the planetary gear train enabling the carrier to rotate relative to at least one of the input sun gear and the output sun gear wherein the phase shift angle for the output shaft with respect to the input shaft changes so that the output shaft assumes a different angular velocity with respect to the input shaft.

25. The method according to claim 24 wherein applying the torque to the planetary gear train comprises applying a resistant torque which unlocks the carrier by overcoming the torque applied by the friction to change the phase shift angle such that the output shaft rotates with respect to the input shaft at a different angular velocity.

26. The method according to claim 24 wherein applying the torque to the planetary gear train comprises applying a driving torque which unlocks the carrier by overcoming the torque applied by the friction to change the phase shift angle such that the output shaft rotates with respect to the input shaft at a different angular velocity.