VALVE ACTUATORS

Abstract

An actuator for operating a poppet valve (30) of an internal combustion engine. The actuator comprises a rotary portion (4) and a body portion (2), and the rotary portion defines a cam surface (50, 60). A cam follower (10) is engaged with the cam surface, and a linkage is coupled to the cam follower at one end and coupled to a valve stem (12) at its other end. The cam surface (50, 60) is shaped such that exertion of a closing force on the valve stem, by a piston of the engine following failure of the actuator for example, causes the cam follower (10) to exert a turning force on the rotary portion (4).

Description

FIELD OF THE INVENTION

The present invention relates to a valve actuator. More particularly, it relates to a rotary actuator having a rotary portion which is rotatable relative to a body portion and includes a cam driven linkage. Such an actuator may be used to operate a poppet valve of an internal combustion engine for example.

BACKGROUND TO THE INVENTION

WO 2004/097184 describes a rotary electromagnetic actuator which may be used to open and close a valve of an internal combustion engine. The valve is operable independently of motion of the crankshaft of the engine.

SUMMARY OF THE INVENTION

The present invention provides an actuator for operating a valve, the actuator comprising:

• • a rotary portion and a body portion, wherein the rotary portion is rotatable relative to the body portion about an axis of rotation by actuation of the actuator, and the rotary portion defines a cam surface;
  • a cam follower engaged with the cam surface as the cam surface rotates; and
  • a linkage which is coupled to the cam follower at one end and to be coupled to a valve stem at its other end,
  • wherein rotation of the rotary portion from a first rotational position to a second rotational position causes displacement of the cam follower, which in turn causes the other end of the linkage to move from a first position to a second position, and
  • wherein the cam surface is shaped such that exertion of a force on the other end of the linkage which urges the other end from its second position towards its first position causes the cam follower to exert a turning force on the rotary portion so as to rotate it towards its first rotational position.

In the event of failure or malfunctioning of part of the actuator or associated control systems, there is a risk that the head of a valve could come into contact with a piston of an associated cylinder, causing damage to the valve and/or other parts of the engine. In particular, when the valve is fully open, movement of the valve stem may mechanically blocked by the actuator mechanism, thereby increasing the likelihood of substantial damage to one or more components in the event of piston-to-valve contact. According to the invention, the linkage and cam arrangement may be configured such that, at least over the range of valve travel where piston-to-valve contact could occur, movement of the valve head towards the valve seat is not blocked by the actuator. Therefore, if a pushing force is exerted on the valve head, for example by a piston during a clash scenario, the valve head is able to move, thereby avoiding any damage. In the claimed configuration, the geometry of the cam surface and the cam follower and/or the linkage may be selected such that, at least over the range of valve travel where piston-to-valve contact could occur, a force urging the valve stem towards the valve closed position causes the cam follower to exert a torque on the rotary portion of the actuator via the linkage, rotating the cam surface as it would if driven during valve closing by the actuator, and thereby allowing the linkage and the valve stem to move towards the valve closed position.

The cam follower may be mounted on or coupled to one end of the linkage. The other end of the linkage may be adapted to be coupled or connected to a valve stem. The linkage may be operable to convert displacement of the cam follower by the cam surface of the rotary portion into linear motion of the other end of the linkage.

Preferably the cam surface is shaped such that when the rotary portion is in its second rotational position, a reference line which lies in a plane perpendicular to the axis of rotation of the rotary portion and is normal to the cam surface at a point of contact by the cam follower is spaced from the axis of rotation of the rotary portion. This spacing between the line of action of the cam follower and the axis of rotation of the rotary portion provides a lever arm, such that the undesired force arising from piston-to-valve contact results in a torque which rotates the rotary portion towards its first rotational position.

In a preferred embodiment, the cam surface is shaped such that when the rotary portion is in its second rotational position, or in a rotational position in a range extending from the second rotational position to an intermediate rotational position between the first and second rotational positions, the reference line is spaced from the axis of rotation of the rotary portion. Accordingly, over a portion of the rotation of the rotary portion up to and including the position corresponding to the valve fully open position, the geometry of the linkage and cam arrangement is such that a force on the other end of the linkage which urges the other end of the linkage from its second position towards its first position causes the cam follower to exert a turning force on the rotary portion so as to rotate it towards the first rotational position. This may allow a valve coupled to the other end of the linkage to move towards its closed position when urged to do so, for example as a result of contact between a piston and the valve head.

The cam lift of the cam surface (that is, the difference between (a) the distance of the cam surface at a given point from the axis of rotation of the rotary portion and (b) the distance of the “base circle” (or minimum radius) of the cam surface from the axis of rotation of the rotary portion) may increase continuously over a portion up to and including the point corresponding to the valve open position (which portion forms a valve opening part of the cam surface). An increasing cam lift provides the desired offset between the line of action of a force exerted on the cam surface by the cam follower, and the rotational axis of the rotary portion.

Preferably, the first position of the other end of the linkage corresponds to a valve closed position and the second position corresponds to a valve open position. In particular, the second position may correspond to a valve fully open position.

The actuator may be an electromagnetic actuator, in which the rotary portion comprises a rotor and the body portion comprises a stator. Alternatively, the actuator may be a hydraulic or pneumatic actuator.

The present invention further provides an internal combustion engine including at least one cylinder having at least one inlet or exhaust valve, a piston and an actuator as described herein, wherein the at least one valve is operable independently of rotation of the engine crankshaft, and the other end of the actuator linkage is coupled to the at least one valve to enable the actuator to actuate the at least one valve.

The engine may be a petrol or diesel engine for example. The present disclosure may be particularly beneficial when applied to a diesel engine, as this type of engine tends to operate with a reduced clearance distance between the valves and the piston.

In preferred embodiments, the valve includes a valve stem which is arranged to reciprocate along a valve axis, the piston is arranged to reciprocate along a piston axis, and the valve axis is substantially parallel to the piston axis. In such a configuration, in the event of piston to valve contact, the piston is likely to exert a force on the valve stem which is substantially parallel to the valve axis, thereby reducing a risk of the valve stem being bent as a result of forces exerted on it by the piston.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example and with reference to the accompanying schematic drawings, wherein:

FIG. 1 is a diagram showing an actuator in combination with a valve, with the rotor of the actuator having a symmetrical cam surface;

FIG. 2 is a diagram showing an actuator and valve according to an embodiment of the invention;

FIG. 3 shows the actuator and valve configuration of FIG. 2 after upwards displacement of the valve head; and

FIG. 4 shows a side view of a cam surface profile according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electromagnetic actuator and valve configuration outside the scope of the present invention. The actuator comprises a stator 2 with a rotor 4 mounted therein. The rotor is rotatable about an axis of rotation 6. The rotor defines a cam surface 8 which rotates with it. A cam follower 10 is maintained in contact with the cam surface as the cam surface rotates.

The cam follower may be urged into contact with the cam surface by a biasing arrangement such as a spring for example. Alternatively, in a desmodromic configuration, a second cam and cam follower mechanism may be employed to control the closing motion of the valve stem.

The cam follower 10 is coupled to a valve stem 12 by a mechanical linkage 14. The linkage comprises a cam follower arm 16 and a rocker arm 18 which are rigidly connected together and rotatable about a rocker pivot 20. Cam follower 10 is mounted on (or integrally formed with) a distal end of the cam follower arm 16.

The rocker arm 18 is pivotably coupled to one end 21 of a rigid link arm 22 by a first pivot joint 24. The other end 23 of the link arm 22 is pivotably coupled to the valve stem 12 via a second pivot joint 26.

Valve 30 comprises the valve stem 12, a valve head 32 which is rigidly connected to the valve stem, and a valve seat 34 (supported by the cylinder head of the engine). A valve guide (not shown) permits linear reciprocal movement in direction D by the valve stem along the valve axis to open and close the valve by bringing the valve head 32 into or out of engagement with the valve seat 34.

In operation of the actuator and valve configuration shown in FIG. 1, rotation of the rotor under the control of an actuator control system causes displacement of the cam follower 10 as it follows the cam surface 8. In the position shown in the drawing, the cam follower is in engagement with the point on the cam surface which is the greatest distance from the axis of rotation 6. In this position, the valve is in its fully open position, with the valve head 32 displaced a maximum distance away from the seat 34.

When the rotor is rotated in either direction away from the position shown in FIG. 1, the distance of the point of contact (on the cam surface by the cam follower) away from the axis of rotation reduces. As the cam follower follows the surface, the linkage causes the valve head to move towards its seat. When the cam follower reaches a point on the cam surface at a minimum distance from its axis of rotation (on the “base circle” 40 of the cam surface), the valve head is brought into engagement with the valve seat.

In a configuration of the form shown in FIG. 1, opening and closing of the valve may be achieved by either oscillating the rotor between two positions, or by continuously rotating the rotor in the same rotational direction.

In FIG. 1, it can be seen that when the valve is in its fully open position as depicted in the drawing, a reference line “l” which lies in a plane perpendicular to the axis of rotation of the rotor and is normal to the cam surface at the point of contact of the cam follower with the cam surface passes through the axis of rotation 6. As a result, if the valve head 32 were to be urged towards its closed position as a result of a force acting thereon in the direction F, upwards movement of the valve stem is blocked by the linkage 14 and rotor 4 as a force along line 1 does not generate a turning force on the rotor. If, in a failure condition, a piston was to come into contact with the valve head, the valve head would be blocked from movement with the piston, leading to substantial damage.

FIGS. 2 and 3 show an actuator and valve combination similar to that of FIG. 1. The same reference numerals are used for the same or corresponding features. The configuration of FIGS. 2 and 3 differs from that of FIG. 1 in that the rotor has a modified cam surface 50. A key difference between the shape of cam surface 50 of FIG. 2 and the shape of cam surface 8 shown in FIG. 1 is present at the point on the surface which is engaged by the cam follower 10 when the valve is in its fully open position (which is the case in FIGS. 1 and 2).

With the valve fully open as shown in FIG. 2, it can be seen that a reference line “L” which lies in a plane perpendicular to the axis of rotation 6 of the rotor 4 and is normal to the cam surface 50 at the point of contact by the cam follower 10 is spaced from the axis of rotation of the rotor by a distance “x”. Accordingly, if a force F acts on the valve head to move it towards its valve seat, the linkage 14 causes the cam follower 10 to exert a force on the cam surface along reference line L. As a result of the offset x between the line of action of this valve closing force and the axis of rotation of the rotor, the cam follower exerts a torque on the rotor. This causes the rotor to rotate in an anti-clockwise direction in the view shown in FIGS. 2 and 3, allowing the valve head to move towards its seat. The disposition of the valve and actuator following such a movement is shown in FIG. 3. Thus, if under a failure condition, a force F is exerted on the valve head by a piston, the valve is displaceable towards its seat by the piston, avoiding damage to the valve or actuator.

The geometry and dimensions of the cam surface, cam follower and linkage are selected such that a force on the valve of the magnitude generated during a piston-to-valve contact event (at least for the cam and linkage positions in which this contact could take place) will exert a sufficient torque on the rotary portion of the actuator to overcome readily the maximum torque that the actuator is able to produce. It will be appreciated that this torque is proportional to the distance “x” (as denoted on FIG. 2) and so, over the range of cam follower and cam surface positions in which piston-to-valve contact could occur, this distance should be sufficiently large to generate the necessary torque.

FIG. 4 is an enlarged view of a cam surface profile 60 embodying the invention. It has similar features to the cam surface 50 shown in FIGS. 2 and 3 and will now be described in more detail.

In FIG. 4, the cam surface profile 60 is divided into four consecutive sections 62, 64, 66 and 68, which subtend angles a to d, respectively, at the rotational axis 6. The cam surface is at its minimum distance from the axis 6 along sections 64 and 66, which correspond to the base circle of the cam surface. Section 62 defines a cam surface which gradually and continuously increases its distance from the axis 6 from the base circle of section 64 around to a point 70. Continuing along the cam surface in the same rotational direction, starting from point 70, the distance of the cam surface from the axis 6 continues to increase further, along section 68, before rapidly decreasing to smoothly blend into the base circle at the start of section 66.

Dashed circles 72 and 74 identify possible positions for the cam follower at the start of a valve opening event. They are located at each end of base circle section 66. In order to open the valve, the rotor rotates clockwise when viewed in the direction of FIG. 4, causing the cam follower to move from section 66, onto section 64. Section 64 may correspond to or include a “quietening ramp” on the cam profile. As the rotor continues to rotate, the cam follower then moves onto section 62 of the cam profile which moves the cam follower away from the axis of rotation 6. The linkage 14 translates this movement of the cam follower into an opening action on the valve. The extent to which the valve is opened is controllable by controlling the amount of rotation of the rotor and therefore how far the cam follower travels along the cam surface 62.

In the embodiment of FIG. 4, point 70 at one end of cam section 62 corresponds to the valve fully open position. It can be seen that in this example, reference line L (which lies in a plane perpendicular to the axis of rotation 6 and normal to the cam surface at this point) is spaced from the axis of rotation in accordance with the invention.

In this embodiment, the cam surface continues to increase its distance from the axis of rotation beyond point 70 along a portion 76 at one end of section 68 of the cam surface (that is, the cam lift increases). In normal use, the cam follower would not travel onto this surface. However, it ensures that even if the follower does travel slightly beyond point 70 due to manufacturing tolerances for example, a turning force will be exerted on the rotor if there is piston to valve contact.

As reference line L is offset from the axis of rotation 6, the force has a lever arm relative to the axis causing it to exert a torque on the rotor. This torque overrides the electromagnetic forces acting between the rotor and its stator, and the inertia of the rotor, allowing the valve to close without damage to the valve, piston or any part of the mechanical, electrical or control systems.

Such a cam surface configuration would be intended for use in an oscillating mode to open and close a valve, and not in a full rotation mode, as described with relation to the configuration of FIG. 1. The cam follower would not normally be able to move substantially beyond point 70 onto cam surface portion 76 as the valve head would already be in its nominally fully open position.

The embodiments of FIGS. 2 and 3 illustrate arrangements including an electromagnetic actuator to operate a valve. It will be appreciated that other types of actuator may be used in accordance with the present disclosure. For example, rotary hydraulic or pneumatic actuators may be employed. In hydraulic or pneumatic implementations, it may be appropriate to include a pressure limiting valve to allow fluid to be released when the actuator is caused to rotate by piston-to-valve contact.

The actuator to valve linkages shown in FIGS. 2 and 3 are merely schematic depictions of examples of a suitable linkage for converting movement of the cam follower into linear displacement of a valve stem attached to the other end of the linkage. A range of other suitable linkage configurations will be readily apparent to the skilled person.

Claims

1. An actuator for operating a valve, the actuator comprising: a rotary portion and a body portion, wherein the rotary portion is rotatable relative to the body portion about an axis of rotation by actuation of the actuator, and the rotary portion defines a cam surface;a cam follower engaged with the cam surface as the cam surface rotates; anda linkage which is coupled to the cam follower at one end and to be coupled to a valve stern at its other end,wherein rotation of the rotary portion from a first rotational position to a second rotational position causes displacement of the cam follower, which in turn causes the other end of the linkage to move from a first position to a second position, andwherein the cam surface is shaped such that exertion of a force on the other end of the linkage which urges the other end from its second position towards its first position causes the cam follower to exert a turning force on the rotary portion so as to rotate it towards its first rotational position.

2. The actuator of claim 1, wherein the cam surface is shaped such that when the rotary portion is in its second rotational position, a reference line which lies in a plane perpendicular to the axis of rotation of the rotary portion and is normal to the cam surface at a point of contact by the cam follower is spaced from the axis of rotation of the rotary portion.

3. The actuator of claim 2, wherein the cam surface is shaped such that when the rotary portion is in its second rotational position, or in a rotational position in a range from the second rotational position to an intermediate rotational position between the first and second rotational positions, the reference line is spaced from the axis of rotation of the rotary portion.

4. The actuator of claim 1, wherein the first position corresponds to a valve closed position and the second position corresponds to a valve open position.

5. The actuator of claim 4, wherein the second position corresponds to a valve fully open position.

6. The actuator of claim 1, wherein the actuator is an electromagnetic actuator, the rotary portion comprises a rotor and the body portion comprises a stator.

7. The actuator of claim 1, wherein the actuator is a hydraulic actuator.

8. The actuator of claim 1, wherein the actuator is a pneumatic actuator.

9. An internal combustion engine including at least one cylinder having at least one inlet or exhaust valve, a piston and the actuator of claim 1, wherein the at least one inlet or exhaust valve is operable independently of rotation of a crankshaft of the engine, and the other end of the actuator linkage is coupled to the at least one inlet or exhaust valve to enable the actuator to actuate the at least one inlet or exhaust valve.

10. The engine of claim 9, wherein the engine is a diesel engine.

11. The engine of claim 9, wherein the valve includes a valve stem which is arranged to reciprocate along a valve axis, the piston is arranged to reciprocate along a piston axis, and the valve axis is substantially parallel to the piston axis.