Camshaft adjuster and method for operating a camshaft adjuster

Abstract

Method for operating a camshaft adjuster (1) which comprises an adjusting gear (2) with an input shaft (7), an output shaft (6) provided for rotationally fixed connection to a camshaft, and an adjusting shaft (8), wherein the adjusting shaft (8) is driven by an actuator (3), the actuator (3) drives the adjusting shaft (8) by overcoming a torque (MB) which is dependent on its angular position (φ), characterized in that the torque (MB) transmitted from the actuator (3) to the adjusting shaft (8) fluctuates periodically, wherein one cycle of the torque fluctuations extends over less than half a revolution of the adjusting shaft (8).

Description

Field of the invention

[0001] The invention relates to a camshaft adjuster intended for use in an internal combustion engine and to a method for operating a camshaft adjuster. Background of the invention

[0002] DE 102 48 355 A1 discloses an electrically driven camshaft adjuster with an adjusting gear, which can be designed as a double eccentric gear or a double planetary gear. The adjusting gear features low friction and a high reduction ratio of, for example, 1:250.

[0003] DE 10 2004 038 695 A1 discloses another camshaft adjuster that uses an internal eccentric gear or a planetary gear as the adjusting gear. A planetary gear is also part of a camshaft adjuster known from DE 100 54 797 A1, although in this case, the adjustment can be performed hydraulically or electrically. DE 10 2011 004 077 A1 discloses a wave gear suitable for a camshaft adjuster. In general, wave gears for camshaft adjusters can be designed either as pot gears or as parallel-shaft gears. A wave gear is a three-shaft gear.

[0004] Further camshaft adjusters and operating methods for these are known from DE 10 2010 006 392 B3, WO 01 / 88 344 A1, DE 10 2005 037 158 A1 and DE 10 2010 039 426 A1. Object of the invention

[0005] The invention is based on the object of further developing an electrically driven camshaft adjuster compared to the aforementioned prior art, particularly with regard to energy aspects. Description of the invention

[0006] This object is achieved according to the invention by a method for operating a camshaft adjuster according to claim 1. Furthermore, the object is achieved by a camshaft adjuster suitable for carrying out this method according to claim 5. Embodiments and advantages of the invention explained below in connection with the camshaft adjuster also apply mutatis mutandis to the operating method and vice versa.

[0007] The camshaft adjuster comprises an adjusting gear with an input shaft, an output shaft, and an adjusting shaft. The input shaft is driven by a crankshaft of an internal combustion engine via a traction mechanism, and the output shaft is rotationally fixedly connected to the camshaft of the internal combustion engine. The adjusting shaft is driven by an actuator, which is preferably designed as an electric motor. However, a hydraulic actuator can also be provided instead of an electric actuator. The adjusting gear is preferably a three-shaft gear; embodiments as a four-shaft gear are also feasible.

[0008] According to the invention, the torque applied by the actuator to rotate the adjustment shaft depends on the angular position of the adjustment shaft. The drive torque applied by the actuator fluctuates periodically, with one cycle of drive torque fluctuations extending over less than half a revolution of the adjustment shaft. The fluctuations of the torque acting in the adjustment shaft can, for example, describe a sinusoidal or sawtooth-shaped curve. Other periodically fluctuating torque curves dependent on the angular position of the adjustment shaft are also possible, which can be described—at least approximately—by a polynomial or a trigonometric function.

[0009] Regardless of the exact shape of the curve described by the torque curve, the torque acting between the actuator and the adjusting shaft during one full revolution of the adjusting shaft of the load-free adjusting gear preferably passes through at least two minima and maxima, for example at least four or ten minima and maxima.

[0010] The fluctuations in the torque acting on the adjustment shaft when it is rotated correspond to defined preferred positions of the output shaft in relation to the input shaft. From a preferred position, the output shaft can only be adjusted with a torque of the adjustment shaft that increases in both directions of rotation. If the camshaft adjuster is in a preferred position, this means that the camshaft adjuster is set to a particularly energy-efficient position compared to other output shaft positions. The advantage of the angular dependence of the torque required to adjust the camshaft adjuster, which is to be transmitted from the actuator to the adjustment shaft, is that the camshaft adjuster can be held in a preferred position with relatively little energy expenditure.

[0011] The angle-dependent fluctuations in the torque applied to the adjustment shaft far exceed any torque fluctuations in conventional engine-gearbox arrangements. The difference between the maximum and minimum torque transmitted by the actuator to the adjustment shaft preferably corresponds to at least 20% of the average torque acting in the adjustment shaft. Even a change in the sign of the torque in the adjustment shaft during an adjustment in a constant direction is possible. This means that the camshaft adjuster is automatically pulled into its preferred position. As soon as the camshaft adjuster is in a preferred position, the actuator can no longer be energized.

[0012] The adjusting gear of the camshaft adjuster is designed, for example, as a strain wave gear. Whether designed as a pot gear or as a parallel-shaft gear, a strain wave gear comprises an elastic, toothed component. In addition to this component, in a preferred embodiment, other components of the adjusting gear are designed to be at least slightly elastically flexible. This can, in particular, be a bearing ring of a rolling bearing in the adjusting gear. The advantages of an elastic bearing ring are particularly evident when the corresponding rolling bearing has an even number of rolling elements.

[0013] Since load maxima occur at diametrically opposite points on a rolling bearing in a strain wave gear, the even number of rolling elements means that at the corresponding points on the bearing rings there are always either two rolling elements or two gaps. The rolling bearing, which is part of a wave generator in the variable speed drive designed as a strain wave gear, is preferably preloaded in such a way that a deflection of the bearing rings occurs, which depends largely on whether the areas of maximum force introduction, offset by 180° from one another, are located in circumferential sections of the rolling bearing where the bearing rings are supported by rolling elements or are more easily flexible due to a gap between adjacent rolling elements. A lesser degree of flexibility corresponds to easier rotation of the adjusting shaft. In extreme cases, minima of the torque curve are designed as detent positions.

[0014] In the simplest case, significant torque fluctuations during camshaft adjuster operation are achieved by designing a bearing ring, particularly the outer ring, of a rolling bearing acting as a component of a wave generator with such thin walls that it is elastically flexible, thus creating a detent effect. Alternatively, an inner ring or a shaft of the rolling bearing contacting the rolling elements can be designed with a wavy periphery. It is also possible for at least one bearing ring of the rolling bearing to have a wall thickness that varies around its periphery. A targeted reduction in the radial stiffness of the rolling bearing can also be achieved by drilling holes below the rolling element raceway.

[0015] A detent effect of a rolling bearing in a variable speed drive is also possible by using rolling elements with a non-circular cross-section. For example, non-circular rollers or needles can have a slightly elliptical or polygonal cross-section. Likewise, different rolling elements with slightly different diameters can be used within the rolling bearing. For example, two smaller rolling elements and one larger rolling element can alternate along the circumferential direction of the rolling bearing.

[0016] The adjustment gear of the camshaft adjuster has a high reduction ratio, which ensures a much finer detent of the output shaft, relative to the angular position of the input shaft, even with only a coarse detent of the adjustment shaft. Preferably, there are at least 30 detent positions for the output shaft. The output shaft can thus be held in numerous positions, namely preferred positions, between its mechanical end stops, whereby at most a low torque needs to be applied by the actuator to hold the output shaft, i.e. to fix the camshaft in relation to the crankshaft. While the actuator is at least largely unloaded in the preferred positions, the output shaft is held largely or entirely by resistances within the adjustment gear.

[0017] Such independent locking of a transmission output element is, in principle, also present in any self-locking transmission. However, the adjustment mechanism of the camshaft adjuster according to the invention differs fundamentally in that the automatic locking of the transmission output shaft only occurs at specific angular positions. The average torque required to adjust the camshaft adjuster, however, is significantly lower than with a self-locking transmission. Accordingly, the efficiency of the adjustment mechanism according to the invention can exceed 50%, which is not the case with a self-locking transmission.

[0018] The variable-speed gear used in the camshaft adjuster is also referred to as a quasi-self-locking gear or gear with ratcheted self-locking. It combines the advantages of a self-locking gear—namely, the automatic locking of a gear output element—with the essential advantage of a non-self-locking gear—namely, the significantly higher efficiency compared to a self-locking gear.

[0019] Regardless of the design of the adjustment gear, the torque required to rotate the adjustment shaft is preferably significantly lower than with a conventional, electrically operated camshaft adjuster, at least in a narrowly limited angular range corresponding to a preferred position, even if the latter—as is usual—has a non-self-locking gear. In contrast, in an angular range between two specific or nearly specific preferred positions, a torque may be required that is greater than the torque required to operate a conventional camshaft adjuster and is of a magnitude typical for a self-locking gear, or even higher.Due to the averaging of the torque during the adjustment process and due to the fact that the camshaft adjuster according to the invention is operated in one of the numerous preferred positions during most of its operating time, the energy requirement of the camshaft adjuster is ultimately significantly reduced compared to the prior art.

[0020] Several embodiments of the invention are explained in more detail below with reference to a drawing. In the drawings: Short description of the drawing Fig. 1 a camshaft adjuster with a flat wave gear in a schematic sectional view, Fig. 2 a camshaft adjuster with a strain gear in pot design in a representation analogous Fig. 1, Fig. 3 a rolling bearing intended for a wave gear with angle-dependent elastic compliance in different angular positions, Fig. 4 a detail from Fig. 3, Fig. 5 shows a further embodiment of a rolling bearing for a wave gear of a camshaft adjuster, Fig. 6 a torque curve in an adjusting shaft when operating a camshaft adjuster. Detailed description of the drawing

[0021] Parts that correspond to one another or have the same function are identified by the same reference numerals in all figures.

[0022] The Fig. 1 and Fig. 2 each show, in a highly simplified manner, an embodiment of an electrically operated camshaft adjuster 1 suitable for use in an internal combustion engine, in particular in a gasoline engine, with regard to the basic function of which reference is made to the prior art cited at the outset.

[0023] The camshaft adjuster 1 comprises an adjusting gear 2 and an actuator 3, namely an electric motor, wherein in the exemplary embodiments a clutch 4 is connected between the actuator 3 and the adjusting gear 2. The adjusting gear 2 is in the exemplary embodiment according to Fig. 1 as well as in the embodiment according to Fig. 2 is designed as a wave gear. In both cases, a sprocket 5 serves as the drive element of the camshaft adjuster 1, while an output shaft 6 of the adjusting gear 2 is fixedly connected to a camshaft (not shown). Instead of the sprocket 5, a belt pulley could also be provided in the case of a belt-driven camshaft. Connected to the sprocket 5 is a drive ring gear 7, which represents the input shaft 7 of the adjusting gear 2. The actuator 3 can drive an adjusting shaft 8 as the third shaft of the adjusting gear 2 via the clutch 4. As long as the adjusting shaft 8 rotates at the speed of the input shaft 7, the output shaft 6 also rotates at this speed.

[0024] The angular relationship between the camshaft and the crankshaft of the internal combustion engine thus remains unchanged in this operating state. The camshaft is adjusted by the actuator 3 driving the adjusting shaft 8 at a speed that differs from the speed of the sprocket 5. In each of the embodiments according to the Fig. 1 and Fig. 2, the adjusting gear 2 is a high-geared gear, so that a change in the angular relationship between the adjusting shaft 8 and the input shaft 7 by a certain amount leads to a change in the angular relationship between the input shaft 7 and the chain wheel 5 on the one hand and the output shaft 6 on the other hand by a much smaller amount.

[0025] The adjusting shaft 8 is in the two in the Fig. 1 and Fig. 2, each of the embodiments outlined is provided for actuating a wave generator 9. The wave generator 9 comprises a rolling bearing 10, which (in the Fig. 1 and Fig. 2 not visible) is elliptically shaped. Here, an inner ring 11 of the rolling bearing 10 is elliptically shaped, while a relatively thin-walled outer ring 12 adapts to the shape of the inner ring 11. Balls roll between the inner ring 11 and the outer ring 12 as rolling elements 13.

[0026] In the design of the variable speed drive 2 according to Fig. 1, a spur gear 14 is located directly on the outer ring 12, which is also deformable and has the same width measured in the axial direction as the outer ring 12. An external toothing of the spur gear 14 meshes with an internal toothing of the drive ring gear 7, which takes up just under half the width of the spur gear 14. The toothing of the spur gear 14 on the one hand and the drive ring gear 7 on the other hand engage only at two points offset by 180° from each other, in the arrangement according to Fig. 1 in the upper and lower areas of the variable speed gear 2. In all other angular ranges, the spur gear 14 is lifted off the drive ring gear 7 due to the elliptical shape of the rolling bearing 10.

[0027] Similarly, the spur gear 14 interacts with an output ring gear 15, which is arranged axially adjacent to the drive ring gear 7 at a short distance and is also internally toothed. Due to a different number of teeth on the drive ring gear 7 on the one hand and the output ring gear 15 on the other, the output ring gear 15 is slightly rotated relative to the drive ring gear 7 during a full rotation of the inner ring 11. For example, the number of teeth on the drive ring gear 7 differs by two from the number of teeth on the output ring gear 15. The output ring gear 15 is rigidly connected to the output shaft 6.

[0028] The embodiment according to Fig. 2 corresponds in terms of basic kinematics to the embodiment according to Fig. 1, whereby in the arrangement according to Fig. 2, instead of the spur gear 14 and the output ring gear 15, a single, pot-shaped output gear 16 is provided, which is connected to the output shaft 6. The output gear 16 has external teeth that mesh with the internal teeth of the drive ring gear 7, the number of teeth of which differs slightly, for example, by two, from the number of teeth of the internal teeth of the drive ring gear 7. At least in the area of ​​the teeth, the output gear 16 is sufficiently elastic to be deformed by the wave generator 9.

[0029] The Fig. 3 rolling bearings 10 shown in different states are arranged both in the arrangement according to Fig. 1 as well as in the arrangement according to Fig. 2 can be used as a component of the wave generator 9. A main load direction HR, in which a force acts on the rolling bearing 10, is in Fig. 3 is symbolized by a radially directed arrow. The direction of rotation of the inner ring 11 driven by the actuator 3 is indicated by an arrow in the circumferential direction. Fig. 3 left state of the rolling bearing 10, a force flow from the drive ring gear 7 to the inner ring 11 occurs in a straight line through a rolling element 13. A deformation of the thin-walled outer ring 12, which is pressed directly onto a rolling element 13 by the toothing of the drive ring gear 7, is practically impossible in this arrangement.

[0030] In the Fig. In the arrangement shown on the right in Figure 3, the inner ring 12 is rotated so that the force acting in the main load direction HR is directed centrally between two adjacent rolling elements 13. Due to the thin-walled design of the outer ring 12, this is in the corresponding area, as shown in Fig. 4, indicated by a dashed line. The load zone of the outer ring 12, in which the deformation is intended, is designated BZ. The compression of the outer ring 12 in the load zone BZ, provided it is located between two rolling elements 13, creates a desired locking effect of the adjusting gear 2. Since the output shaft 6 of the camshaft adjuster rotates many times slower than the adjusting shaft 8 and the adjusting shaft 8 can already assume a multitude of locking positions, namely a number corresponding to the number of rolling elements 13, the output shaft 6 can be locked extremely precisely.

[0031] In Fig. 5 shows a variant of the rolling bearing 10 of the variable speed gear 2, which can also be used in each of the arrangements according to the Fig. 1 and Fig. 2. A locking effect of the rolling bearing 10 is created by a non-circular design of the inner ring 11. The inner ring 11 has on its circumference a number of flattened portions 17 corresponding to the number of rolling elements 13, at which the surface of the inner ring 11, starting from its cylindrical basic shape, is recessed by a depth t. The depth t is between 0.2% and 20% of the rolling element diameter designated dk. Fig. 5 also shows a cage 18 intended to guide the rolling elements 13. The rolling elements 13 are, as in the variant according to the Fig. 3 and Fig. 4 balls. Alternatively, needles or cylindrical rollers can also be provided as rolling elements of the rolling bearing 10.

[0032] In Fig. 6 shows the curve of the torque acting in the adjusting shaft 8 when the camshaft adjuster 1 is actuated. The illustration according to Fig. 6 applies to both the variant of the rolling bearing 10 according to the Fig. 3 and Fig. 4 as well as for the variant according to Fig. 5. The diagram shows the dependence of the torque acting in the adjusting shaft 8, designated as the actuating torque ME, on its angular position, whereby the angle designated φ in the diagram is Fig. 6 covers several cycles of torque fluctuations. As can be seen from Fig. As can be seen from Figure 6, the actuating torque MB is approximately sinusoidal; an average actuating torque is designated MB_av, a minimum actuating torque MB_min, and a maximum actuating torque MB_max. The minima of the Fig. 6 corresponds to the curve in Fig. 4 shown state of the rolling bearing 10. The output shaft 6, like the input shaft 7, is in a preferred position in which it can be held by the actuator 3 with minimal energy expenditure.

[0033] In general, the relationship between the torque MB acting in the input shaft 7 and an output torque acting in the output shaft 6, designated MA, can be described as follows: MB(φ)=MA / (i_BA×eta(φ)), where i_BA is the transmission ratio of the variable speed gear 2 and eta is a transmission factor which depends on the angle φ. The angle-dependent fluctuation of the transmission factor eta is reflected in the Fig. 5. For a transmission whose transmission characteristics are independent of the angular position of the input and output shafts, a non-angle-dependent efficiency would have to be substituted for eta (φ). For the variable speed drive 2 of the camshaft adjuster 1, the average value of the transmission factor eta, averaged over all angles φ, is greater than 0.5. Therefore, the variable speed drive 2 cannot be classified as a self-locking transmission.

[0034] If the curve plotted as a function of the angle φ of the adjusting shaft 8, which describes an approximately harmonic oscillation and indicates the actuating torque MB required to rotate the adjusting shaft 8, deviates from Fig. 6 into the negative range, this means that the adjusting shaft 8 is automatically pulled into the corresponding preferred positions, whereby the average actuating torque MB_av is also positive in this case. In this embodiment, energization of the actuator 3 is only required to adjust the output shaft 6 connected to the camshaft from a first detent position to another detent position. The number of detent or preferred positions results from the number of revolutions required to adjust the output shaft 6 from one end stop to the second end stop, multiplied by the number of rolling elements 13 of the rolling bearing 10.

[0035] For example, if the transmission ratio i_BA is 90 and the angle of rotation between the end stops of the output shaft 6 is exactly 60°, i.e., one-sixth of a full revolution, the adjusting shaft 8 must be rotated by 90 / 6 = 15 revolutions to move from one end stop to the second end stop. If the rolling bearing 10 of the wave generator 9, as shown in Fig. 3, eighteen rolling elements 13, whereby between each two adjacent rolling elements 13 there is a preferred position of the adjusting shaft 8, so during the 15 revolutions there are a total of 15 x 18 = 270 preferred positions, which are evenly distributed over the aforementioned 60° wide adjustment range of the output shaft 6. Reference number list BZ stress zone dk rolling element diameter eta transfer factor HR main load direction i_BA gear ratio MA output torque MB operating torque, torque MB_av average actuation torque MB_min minimum actuation torque MB_max maximum actuation torque t depth φ angle 1 camshaft adjuster 2 variable speed gears 3 Actuator 4 Clutch 5 sprocket 6 Output shaft 7 Input shaft, drive ring gear 8 Adjustment shaft 9 wave generator 10 rolling bearings 11 Inner ring 12 Outer ring 13 rolling elements 14 Spur gear 15 Output ring gear 16 Output gear 17 Flattening 18 cage

Claims

[1] Method for operating a camshaft adjuster (1), which comprises an adjusting gear (2) with an input shaft (7), an output shaft (6) provided for rotationally fixed connection to a camshaft, and an adjusting shaft (8), wherein the adjusting shaft (8) is driven by an actuator (3), the actuator (3) drives the adjusting shaft (8) by overcoming a torque (MB) dependent on its angular position (φ), characterized by that the torque (MB) transmitted from the actuator (3) to the adjusting shaft (8) fluctuates periodically, one cycle of the torque fluctuations extending over less than half a revolution of the adjusting shaft (8). [2] Method according to claim 1, characterized by that a torque (MB) with changing sign acts between the actuator (3) and the adjusting shaft (8) during an adjustment in the same direction. [3] Method according to claim 1 or 2, characterized bythat during one full revolution of the adjusting shaft (8) of the load-free adjusting gear (2), the torque (MB) acting between the actuator (3) and the adjusting shaft (8) passes through at least two minima (MB_min) and maxima (MB_max). [4] Method according to one of claims 1 to 3, characterized by that the difference between the maximum torque acting between the actuator (3) and the adjusting shaft (8) (MB_max) and the minimum torque acting between the actuator (3) and the adjusting shaft (8) (MB_min) corresponds to at least 20% of the average torque acting between the actuator (3) and the adjusting shaft (8) (MB_av). [5] Camshaft adjuster (1), comprising - an adjusting gear (2) with an input shaft (7), an output shaft (6) provided for rotationally fixed connection to a camshaft, and an adjusting shaft (8), wherein a plurality of preferred positions of the output shaft (6) exist, from which the output shaft (6) can be adjusted with a torque (MB) increasing in both directions of rotation, - an actuator (3) provided for driving the adjusting shaft (8), which is designed to transmit a torque (MB) that varies depending on the angle to the adjusting shaft (8). [6] Camshaft adjuster (1) according to claim 5, characterized by that the adjusting gear (2) is designed as a wave gear. [7] Camshaft adjuster (1) according to claim 6, characterized by that the adjusting gear (2) comprises a rolling bearing (10) with an even number of rolling elements (13). [8] Camshaft adjuster (1) according to claim 6 or 7, characterized bythat at least one of the components bearing inner ring (11) and bearing outer ring (12) of the adjusting gear (2) has a geometric design that varies over the respective circumference. [9] Camshaft adjuster (1) according to one of claims 5 to 8, characterized by that the preferred positions of the output shaft (6) are designed as locking positions in which the output shaft (6) remains without torque application by the actuator (3).

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