EP1646770A1

EP1646770A1 - Valve gear

Info

Publication number: EP1646770A1
Application number: EP04743111A
Authority: EP
Inventors: Christopher Paulet Melmoth Walters
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-06-30
Filing date: 2004-06-28
Publication date: 2006-04-19
Also published as: WO2005003524A1

Abstract

Valve gear for an internal combustion engine comprises a camshaft (8) carrying a cam (10) which has a profile which varies in the direction of the camshaft axis (12), the camshaft (8) being axially displaceable to vary the action of the cam (10) on the valve (14). A pair of fingers (30, 32) are interposed between a cam (10) and an associated valve stem (20). The upper finger (30) has a recess (48) along which moves a roller (62) carried by the lower finger (32). The profile of the surface of the recess (48) is such that rotation of the cam (10) is converted into a desired acceleration profile of the valve stem (20), without requiring a re-entrant profile of the cam (10). In one embodiment, the position of the pivot axis of the lower finger (32) can be adjusted during running of the engine to vary the lift characteristic of the valve.

Description

VALVE GEAR

This invention relates to valve gear for operating a valve in an internal combustion engine.

In many overhead camshaft engines, valves are operated by cams on a camshaft by buckets which slide in guides under the action of the cams. GB 2190140 discloses valve gear of this kind in which the valve lift, duration and timing can be adjusted while the engine is running. Developments of this concept are disclosed in GB 2341659 and GB 2359608. The adjustment of valve operation is achieved by varying the profile of each cam along its axis, so that part of the cam surface, as viewed from the side, is inclined to the axis about which the cam rotates. The camshaft is axially displaceable to move the cam across the bucket so as to vary the effect of the cam on valve operation. To accommodate the varying inclination of the cam surface as it passes over the bucket, the bucket has a half- roller having a flat surface which contacts the cam and a cylindrical surface which engages the bucket, so that the half-roller can rock relatively to the bucket as the cam rotates .

Many modern overhead camshaft engines, particularly those used in high performance applications, do not employ buckets as cam followers but instead use a so- called finger follower. A finger follower is a lever which is pivotable about a pivot axis parallel to that of the camshaft . The finger follower is acted upon by the cam at a position between its ends. The end of the finger follower away from the pivot axis acts on a valve stem to operate the valve. Finger followers provide a mechanical advantage between the cam and the valve so enabling a greater valve lift area for a given cam angle duration due to the higher velocities that are possible with this system. Finger followers also allow greater freedom in cylinder head design, since the camshafts do not need to be positioned directly above the valve.

Finger followers usually have a roller which is acted upon by the cam to pivot the finger follower. It is difficult to incorporate such a roller into a system as disclosed in GB 2190140. Furthermore, finger follower arrangements sometimes require a re-entrant cam profile if required valve velocities, accelerations and lift are to be achieved, and such profiles cannot cooperate satisfactorily with a flat follower surface as is disclosed in GB 2190140.

According to the present invention there is provided valve gear for operating a valve in an internal combustion engine, the valve gear comprising a camshaft carrying a cam which has a profile which varies in the direction of the camshaft axis, the camshaft being axially displaceable to vary the action of the cam on the valve, characterised in that the cam acts on a first pivotable finger, the first finger having an actuating surface which contacts a second pivotable finger, the second pivotable finger being in driving engagement with the valve

In a typical multicylinder engine, the camshaft will have separate cams for each cylinder. Furthermore, there may be separate camshafts for the inlet and exhaust valves of the cylinders.

In one practical embodiment in accordance with the present invention, the pivotable fingers are each pivotable about a respective axis which is parallel to the axis of the camshaft. The pivot axis of the first finger may lie in a plane extending transversely of the direction of valve movement, which plane lies between the axis of the camshaft and the contact point between the base circle of the cam and the first finger. The pivot axis of the second finger may lie in a plane extending transversely of the direction of valve movement and be positioned so that the point of contact between the second finger and the valve moves across that plane as the valve opens and closes.

The valve gear may include a mechanism for varying the valve lift, duration and timing, for example in accordance with GB 2190140, GB 2341659 or GB 2359608. Thus, the cam may have a profile which varies in the direction of the camshaft axis, the camshaft being axially displaceable to vary the action of the cam on the first finger. The first finger may thus have a cam engaging element which is pivotable with respect to the finger about an axis which lies in a plane perpendicular to the camshaft axis. The cam engaging element preferably has a flat cam contact surface which is engaged by the cam. The cam engaging element may be in the form of a half-roller.

The actuating surface of the first finger is preferably in the form of a profiled track along which travels a contact element carried by the second finger. The track is preferably at least partly concave and, in a preferred embodiment, comprises a first convex, then concave arcuate portion at a radially inner end of the track with respect to the pivot axis of the first finger. A straight region of the track extends generally radially outwardly from the first arcuate portion and, in the embodiment, terminates at a second concave arcuate portion at the radially outer end of the track. The straight portion of the track may be inclined to the cam contact surface so that the distance between the track and the cam contact surface increases in the direction away from the pivot axis of the first finger.

In an engine having two inlet valves per cylinder, the second finger may have two valve engaging portions for engaging the pair of valves . The contact element on the second finger may comprise a cylindrical roller which extends parallel to the pivot axis of the second finger between the valve engaging portions.

A similar structure may be employed for the exhaust valves if the engine has two exhaust valves per cylinder

In one embodiment, the pivot axis of the first finger is fixed with respect to the cylinder head which supports the valve gear.

In an alternative embodiment, the pivot axis of the first finger may be movable with respect to the cylinder head. For example, the pivot axis of the first finger may be defined by contact between the first finger and a movable support element. Thus, the first finger may have a recess with which the movable support element cooperates to define the pivot axis. The movable support element may be moved in dependence on the position of the second finger. For example, the movable support element may comprise a support cam which is pivotable about a fixed axis by means of an operating lever which moves with the second finger about the pivot axis of the second finger.

In another embodiment in accordance with the present invention, the second pivotable finger is pivotably supported by a support element which is movable relatively to a cylinder head of the internal combustion engine.

With valve gear in accordance with this embodiment of the present invention, adjustment of the support element changes the geometric relationship between the valve and the cam which drives it. Consequently, this relationship can be adjusted for each valve (or pair of valves operated by the same cam) , enabling the operation of the valves to be optimised for any operating condition of the engine, for example at idling.

The support element is preferably movable in a direction which lies in a plane perpendicular to the axis of the camshaft, which direction may be parallel to the direction of valve movement. However, in some embodiments, it may be desirable for the support element to be movable in a direction inclined to the direction of valve movement. In a preferred embodiment, the movement of the support element is achieved under hydraulic control. For example, each support element may be associated with a valve, such as a solenoid valve, which admits oil under pressure to an operating chamber to displace the valve in one direction, or to allow the oil to drain to enable the support element to move in the opposite direction.

Movement of the support element may be controlled in response to air flow through the valve (or pair of valves) actuated by the respective second pivotable finger. In a multi cylinder engine, the support elements for the second pivotable fingers of the respective valves may be adjusted independently in order to achieve uniform air flows through each of the valves. Thus, even if manufacturing tolerances result in dimensional differences, or effects such as differential thermal expansion occur, the operation of all of the valves can be equalised. It is also possible to disable each cylinder individually under map control. For example some cylinders could be disabled to achieve a balanced (in terms of firing order) effective temporary reduction in engine capacity, and therefore in fuel consumption and emissions, during idling in traffic. Alternatively, or in addition, cylinder disablement may be achieved using a special cam profile as disclosed in my earlier International patent application published as WO 01/88345. To achieve this control, an air flow sensor may be provided in the air flow path through the valve (or pair of valves) of each cylinder.

The support element may cooperate with the respective second pivotable finger in any suitable manner, but in a preferred embodiment the support element has a hemispherical surface which engages a complementary surface in the respective second pivotable finger. Such a connection between the support element and the second pivotable finger enables the second pivotable finger to move relatively to the support element about axes extending in any direction. An advantage of this arrangement is that, in an engine having two inlet valves and/or two exhaust valves per cylinder, the second pivotable finger may tilt relatively to the support element about an axis extending perpendicular to a plane containing the directions of movement of the valves. This enables the second pivotable finger to adopt a position in which it acts on both inlet valves and both exhaust valves equally, avoiding different operation of the two valves as a result of, for example, different valve clearances.

Another aspect of the present invention provides an internal combustion engine including a camshaft carrying a plurality of cams, the camshaft being axially displaceable relatively to a cylinder head of the engine by means of a motor having a rotary output shaft, the camshaft being connected to the motor by a coupling element which is rotatable but axially fixed with respect to the camshaft, the coupling element being axially displaceable but rotationally fixed to a housing of the motor, and being connected to the output shaft of the motor by a connection by which rotation of the output shaft of the motor causes axial displacement of the connection element.

Preferably, the motor is disposed coaxially with respect to the camshaft and is accommodated, at least partially, within a drive element for driving the camshaft in rotation. Hydraulic means, for example, in the form of an annular piston surrounding the motor, may be provided for rotating the camshaft relative to the drive means .

Another aspect of the present invention provides a second pivotable finger for use in valve gear as described above, the second pivotable finger having features, separately or in combination, as described more specifically below.

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: -

Figure 1 shows valve gear of an internal combustion engine in a first position;

Figure 2 shows the valve gear of Figure 1 in a second position;

Figure 3 is a view in the direction of the arrow III in Figure 1 ; Figure 4 is a view in the direction of the arrow IV in Figure 1 ;

Figures 5a to 5d are views of a component of the valve gear;

Figures 6a to 6c are views of another component of the valve gear;

Figure 7 is a perspective view of the component shown in Figures 5a to 5d;

Figure 8 is a perspective view of the component shown in Figures 6a to βc ;

Figure 9 corresponds to Figure 1 but shows an alternative embodiment;

Figures 10 to 12 correspond respectively to Figures 1, 3 and 4 but show a further alternative embodiment;

Figure 13 shows a forward part of valve gear of another alternative embodiment of an internal combustion engine;

Figure 14 is an enlarged partial view of the valve gear of Figure 13 in a different position;

Figure 15 is a sectional view generally on the_. line XV- XV in Figure 13 ;

Figures 16 and 17 are views of a component of the valve gear of the engine of Figures 13 to 15; Figures 18 and 19 are views of another component of the valve gear;

Figure 20 is a diagrammatic view showing sensor positions in the engine of Figure 13;

Figure 21 is a diagrammatic view showing control circuitry in the engine of Figure 13 ; and

Figure 22 is a valve lift diagram for the engine of Figure 13.

Figures 1 to 4 show a cylinder block 2 having a cylinder 4, and a cylinder head 5 which, in the embodiment illustrated, has a separate cam carrier 6. A camshaft 8 is supported by the cam carrier 6, and has a series of cams 10 distributed along it, only one of which is shown in Figures 1 to 3. The camshaft is rotatable about a camshaft axis 12.

The admission of fuel-air mixture into the cylinder 4 is controlled by two inlet valves 14 (see Figures 3 and 4) and the exhaust of combustion products is controlled by two exhaust valves 16. Each inlet valve comprises a valve head 18 and a valve stem 20. The valve stems move in valve guides 22 and are biased to the closed position (upwardly as shown in Figure 1) by valve springs 24 in a conventional manner.

As shown in Figure 3, the cam 10 varies in profile along the axis 12. Although the base circle 26 is parallel to the axis 12, parts of the cam which extend beyond the base circle, as exemplified by the line 28 in Figure 3, vary in distance from the axis 12 in the axial direction. The cam 10, or the entire camshaft 8, is movable axially so that the part of the cam 10 that is effective to operate the valves 14 changes to alter the valve lift, duration and timing. The mechanism for displacing the camshaft 8 may be as described in GB 2190140, GB 2341659 or GB 2359608.

The action of the cam 10 is transferred to the valves 14 through a first pivotable finger 30 and a second pivotable finger 32. The first finger 30 is pivotable about an axis 34 which is fixed to the cam carrier 6. The axis 34 is defined by a pivot pin 36. The first finger 30 is cranked, having a first portion 38 which, in the valve-closed condition shown in Figure 1, extends obliquely from the pivot pin 36 towards the valve stem 20, and a second portion 40 which extends substantially transversely of the lengthwise direction of the valve stem 20.

The first finger 30 is shown in greater detail in Figures 5a to 5d and Figure 7. It comprises a bore 42 for receiving the pin 36. The portion 40 includes a part cylindrical recess 44 which extends in the lengthwise direction of the portion 40 and receives a cam engaging element 46 (Figures 1, 2 and 5b) which is in the form of a half roller having a part cylindrical surface cooperating with the recess 44 and a flat surface which engages the cam 10. The cam engaging element 46 is thus able to rock in the recess 44 so that the flat surface can align with the engaging surface of the cam 10 which, as will be appreciated from Figure 3, varies in inclination relatively to the axis 12 as the cam rotates . The surface of the portion 40 of the first finger 30 includes, on the side opposite the recess 44, a recess 48 which acts as a guide track as will be described below.

The second finger 32 is mounted for pivotal movement about an axis 50 defined by pivot pins 52 mounted in the cam carrier 6. The second finger is shown in more detail in Figures 6a to 6c and Figure 8. The second finger 32 extends generally radially of the axis 50 from aligned bores 54 for receiving the pins 52. At the end away from the bores 54, the second finger 32 terminates at a downwardly directed foot 56. The foot 56 is notched at 90 to receive the ends of the valve stems to enhance lateral location of the front of the second finger 32 against the valve stems 20. At the transition between the radially extending part of the second finger 32 and the foot 56, there is a second bore 58, parallel to the bores 54 and half way along this bore 58 there is a notch 60 which is sufficiently wide to receive the portion 40 of the first finger 30.

In the assembled condition, the second bore 58 receives a track engaging contact element in the form of a roller 62. As shown in Figures 1 and 2, the roller 62 engages the track 48 in the first finger 30. Also, the foot 56 engages the end of the valve stem 20. The pivot axes 34 and 50 and that of the roller 62 are parallel to the camshaft axis. '

In the position shown in Figure 1, the base circle 26 of the cam 10 contacts the half roller 46. The first finger 30 is consequently in its uppermost position about the pivot axis 34. Since the position of the second finger 32^" is determined by abutment of the pin 62 with the track 48, the second finger 32 is also in its uppermost position about the axis 50 under the influence of the valve spring 24. The valve 14 is thus closed.

Rotation of the camshaft causes the nose of the cam 10 to come into contact with the half roller 46, so pivoting it in a clockwise direction about the axis 34. This in turn causes the second finger 32 to rotate about the pivot axis 50, also in a clockwise direction, to cause the valve to begin to open. Eventually, as shown in Figure 2, the valve 14 reaches its fully open position, whereafter further rotation of the camshaft 8 will cause the valve 14 to close again under the action of the spring 24.

The profile of the track 48 governs the relationship between the motion of the first finger 30 and the second finger 32. It will be appreciated that the track can be any shape commensurate with the valve motion required and acceptable track/roller contact stress. In a preferred embodiment as shown in Figure 5b, the track 48 is largely concave, and comprises a convex arcuate transmission 63 into a first arcuate portion 64 at the radially inner end of the track 48 with respect to the bore 42, a straight portion 66 which extends radially outwardly from the arcuate portion 64, and a second arcuate portion 68 at the radially outer end of the track 48. The straight portion 66 is inclined to the lengthwise direction of the portion 40 and thus, as viewed in the plane of Figure 1, to the flat surface of the half roller 46. The effect of this profile of the track 48 is that, from the position shown in Figure 1, the initial pivoting of the first finger 30 produces a lower rate of pivoting of the second finger 32 as the roller 62 moves along the first arcuate portion 64. Subsequently, when the roller 62 reaches the straight portion 66, the motion of the second finger 32 accelerates with respect to the first finger 30, under the influence of the inclination of the straight portion 66 and because the point of contact between the roller 62 and the track 48 moves further from the pivot axis 34 of the first finger 30. Further acceleration of the second finger 32, and consequently of the valve 14, is achieved as the roller 62 travels over the second arcuate portion 68. Consequently, rapid opening and/or closing of the valve 14 to its full extent can be achieved without requiring a re-entrant profile on the cam 10.

A similar effect is achieved on further rotation of the camshaft 8 as the valve returns to the closed position.

Figure 9 shows an arrangement similar to that of Figures 1 to 8, except that the second finger is supported by a hydraulic adjuster 70 instead of the pivot pin 52. On start-up of the engine, the adjuster 70 receives oil under pressure to take up any clearance between the foot 56 of the second finger 32 and the valve stem 20 when the valve is fully closed.

The adjuster 70 has a pin 72 which is moved upwardly under hydraulic pressure. The pin 72 has a hemispherical tip which is received in a correspondingly-shaped recess in the second finger 32. More than one adjuster 70 may be provided along the pivot axis 50 in order to provide stability to the second finger 32.

A further embodiment of the valve gear is shown in Figures 10 to 12. In these Figures, the parts corresponding to those of the preceding embodiments are designated by the same reference numbers .

In the embodiment of Figures 10 to 12, the pivot axis 34 for the first finger 30 is defined by a pivot 74 with sideways locating end plates integral with body 30 on a lever 76 which is mounted in the cam carrier on a pin 78 for rotation about an axis 80. The lever 76 also has a lug 82 which is retained between the arms of a yoke 84 secured to the second finger 32.

A clearance recess 86 is provided in the first finger 30 to accommodate the lever 76 as the finger 30 moves. Unlike the cranked first finger 30 of the embodiments of Figures 1 to 9, the first finger 30 of the embodiment of Figures 10 to 12 is generally straight although it is profiled at its lower surface (ie the surface closest to the valve stem 20) to provide the track 48. In this embodiment, the track 48 comprises a convex transmission 63 into a concave radially inner arcuate portion 64 and a straight portion 66, but the straight portion 66 merges into a convex transition 88 extending to the free end of the first finger 30. It will be appreciated that the straight portion 66 , both in this embodiment and the embodiments of Figures 1 to 9, may not be perfectly straight, but may instead be slightly convex or concave. In operation of the embodiment of Figures 10 to 11, pivoting of the second finger 32 causes rotation of the lever 76 by means of the yoke 84 engaging the lug 82. This changes the position of the pivot axis 34 in the cam carrier and so adjusts the position of the first finger 30, and consequently the track 48, for any particular position of the cam 10.

Thus, starting from the position shown in Figure 10 which corresponds to the position shown in Figure 1, clockwise displacement of the first finger 30 causes corresponding clockwise movement of the second finger 32. This causes anti-clockwise rotation of the lever 76, so moving the pivot axis 34 downwards and to the left as seen in Figure 10. Since the first finger 30 can be regarded as pivoting about a fulcrum constituted by the contact point between the cam 10 and the half roller 46, a vertical component of movement of the track 48 is superimposed on the overall clockwise movement of the first finger 30, so effectively changing the speed of rotation of the second finger 32. Thus, the lever 76 provides a further facility for adjusting the speed and timing of valve travel.

Figures 13 to 19 show an alternative embodiment. Features shown in Figures 1 to 12 are designated in Figures 13 to 19 by the same reference numbers. In Figures 13 to 15, only the valve stems 20 are shown, and the exhaust valves are not shown.

As in the previous embodiments, . the cam 10, or the entire camshaft 8, is movable axially so that the part of the cam 10 that is effective to operate the valves changes to alter the valve lift, duration and timing. The mechanism for displacing the camshaft 8 will be described below with reference to Figures 13 and 14, but alternatively it may be as described in GB 2190140, GB 2341659 or GB 2359608.

As before, the first finger 30 is pivotable about an axis 34 which is fixed to the cam carrier 6. In the present embodiment, however, the axis 34 is defined not by a pin 36, but by cylindrical bosses 37 which project to each side from the finger 30, as shown in Figures 16 and 17. These bosses 37 serve to support the first finger 30 against forces applied to it by the cam 10 in the direction of the camshaft axis 12, and so avoid the transmission of these forces to the second finger 32 and the valve stems 20. Bosses of alternative shapes to those shown in Figures 16 and 17 may be used, or the first finger 30 may have a generally triangular shape, widening towards the pivot axis 34.

The second finger 32 is mounted for pivotal movement about axes passing through a point 150 defined by a support element 152 mounted in the cam carrier 6. The support element 152 has a hemispherical end 154 centred on the point 150, and the finger 32 has a complementary hemispherical recess 157 which receives the end 154 of the support element 152. The second finger is shown in more detail in Figures 18 and 19. The second finger 32 extends generally radially of an axis passing through the point 150 and parallel to the camshaft axis 12. At the end away from the support element 152, the second finger 32 terminates at a downwardly directed foot 156. At the transition between the radially extending part of the second finger 32 and the foot 156, there is a bore 158, approximately parallel to the camshaft axis 12. Half way along this bore 158 there is a notch 160 which is sufficiently wide to receive the portion 40 of the first finger 30. The first finger 30 thus supports the second finger 32 against lateral forces, i.e. forces parallel to the camshaft axis 12. As in the embodiment shown in Figures 6a, 6b and 6c, the foot 156 has two portions 90 for engaging the respective valve stems 20. As shown in Figure 19, the foot 156 has a continuous straight surface for engagement at its ends with the tips of the valve stems 20. However, in an alternative embodiment, the foot 156 includes valve engaging portions in the form of recesses 159 (Figure 15) for receiving the valve stems 20 to assist stable location of the finger 32.

The shape of the second finger 32 as shown in Figures 18 and 19 results in a higher stiffness/mass ratio at the valve compared with that of the second finger 32 shown in Figures 6 and 8, and contributes to an overall system having high stiffness compared with known variable valve lift systems. A system as described herein may therefore be suitable for high-speed engines (i.e. engines operating at speeds in excess of 7000 RPM for road use) .

In the assembled condition, the bore 158 receives a track engaging contact element in the form of a roller 162. As shown in Figures 13 and 15, the roller 162 engages the track 48 in the first finger 30. Also, the foot 156 engages the ends of the valve stems 20.

Operation of the mechanism is generally as described above with reference to Figures 1 to 3. However, it will be appreciated that, because the second finger 32 of the embodiment of Figures 13 to 19 is supported by the support element 152 at the hemispherical end 154, the finger 32 is able to pivot not only about an axis parallel to the camshaft axis 12, but also about other axes passing through the point 150. In particular, the finger 32 can pivot about an axis passing through the point 150 and perpendicular to a plane containing the lengthwise axes of the valve stems 20. Consequently, the finger 32 can pivot about the point 150 to equalise itself on the two valve stems 20, so taking up any difference in valve clearance between the valves. To assist in this lateral rocking motion of the second finger 32, the roller 162 engaging the track 48 in the first finger 30 may be slightly barrelled, i.e. it may have a convex surface as viewed perpendicular to its axis. Furthermore, the side faces of the notch 160 in the second rocker 32 may diverge from each other in the upwards direction (as seen in Figure 18) with a convex surface to accommodate the rocking motion of the finger 32 relative to the first finger 30.

The recesses 159, as shown in Figure 15, could be replaced by elongated holes (elongated in the direction towards and away from the point 150) , into which convex radiussed plugs are inserted for engagement with the valve stems 20. Alternatively, as shown in Figures 13 and 15, the recesses could be defined by vertical tags 164 that project downwardly on each side of each valve stem. In either form of the recesses 159, the bottom surface of each recess is preferably convex as viewed in two perpendicular planes, again to ensure satisfactory seating of the second finger 32 on the tips of the valve stems 20. In the valve gear of Figures 1 to 4, the axis about which the second finger 32 pivots during opening and closing of the valves is substantially fixed (although it may undergo minor adjustment in the manner of a hydraulic tappet to take up valve clearances during operation, as described with reference to Figure 9) . A consequence of this is that any variations in valve clearances between the cylinders of the engine cannot be adjusted for. Furthermore, other variations in operation between the cylinders of the engine may occur, for example as a result of differential thermal expansion between the cylinder head 5 and the camshaft 8. Such thermal effects can result in the active part of each cam 10 varying between the cylinders of the engine. Such effects may be particularly marked at idle or tick over of the engine. In the embodiment shown in Figures 13 to 15, these effects can be eliminated by adjustment of the support element 152 in the direction shown by an arrow 163. For example, as indicated in dashed outline in Figure 15, raising the support element 152, assuming a constant position for the valve stem 20, causes the roller 162 to move upwards and to the right away from the track 48. In practice, of course, the second finger 32 will be biased upwardly by the valve spring 24, acting through the valve stem 20, to bring the roller 162 into contact with the track 48. However, it will be appreciated that, by adjustment of the support element 152, it is possible to vary the degree of valve opening for any particular rotational position of the cam 10. By suitable control of the support elements 152 for the valves of each cylinder of an engine, it is thus possible to ensure that, for each rotational camshaft position, the degree of valve opening (or more precisely, the air mass flow through the valve) can be equalised across all of the cylinders of the engine.

The camshaft 8 is driven in rotation from the crankshaft of the engine by means of a belt 166 which drives a drive element which, in this embodiment, is a pulley 168. The drive element may alternatively be a sprocket or gear wheel. The pulley 168 is supported by two plain bearings, on opposite sides of the belt 166, to avoid structure borne noise as compared with roller bearings. This arrangement provides a stiff and light independent support structure for the pulley 168 (or other drive element) . The camshaft 8 is movable axially to vary the maximum lift and opening duration of the valves of the engine, and the rotational position of the camshaft 8 relative to the pulley 168 can also be varied in order to change the valve timings. These adjustments are achieved by a camshaft control mechanism 170 situated towards the front of the engine. This mechanism is shown in Figure 13 and, on an enlarged scale, in Figure 14.

The camshaft 8 is rotationally supported on the cam carrier 6 by bearing caps 172. The forward end of the camshaft 8 (i.e. the right-hand end as seen in Figure 13) is hollow and accommodates a sleeve 174 on which the camshaft is rotatable. Thrust bearings 176 are provided to support axial thrust between the camshaft 8 and the sleeve 174. A retaining ring 178 is screwed into the camshaft 8 to retain the sleeve 174. A split support housing 180, 182 is secured to the cam carrier 6 and supports the pulley 168 for rotation. An annular cylinder 184, screwed onto the pulley 168, is accommodated in the housing part 182, and receives an annular piston 186. The piston 186 is biased into the cylinder 184 (i.e. to the right as seen in Figure 13) by a spring 188 which is retained in the pulley 168 by a lip 169. The housing part 182 is secured to the engine (cylinder head or block) by any suitable means (not shown) and may form part of an all-enveloping cover for the front of the engine .

A brush or stepper motor 190 comprises a casing 192, a casing nose 194 and an output shaft 196. The casing 192 is secured to the housing part 182, and extends within the annular cylinder 184. The casing nose 194 is provided with axial splines 198 which cooperate with axial splines 100 formed within the sleeve 174. The sleeve 174 also has an internal screwthread 104 which cooperates with an external screwthread 106 on the output shaft 196 of the motor 190 to provide a non- overhauling slow thread enabling accurate and backlash- free positioning of the camshaft 8 on the output shaft 196.

The annular piston 186 has on its inner surface an axial spline 108 which cooperates with an external axial spline 110 on the camshaft 8. On its external surface, the piston 186 has a helical spline 112, which cooperates with an internal helical spline 114 on the pulley 168.

It will be appreciated that the casing 192 of the motor 190 is fixed with respect to the cam carrier 6, and consequently to the cylinder head 5. The sleeve 174 is thus rotationally fixed with respect to the cylinder head 5 by virtue of the splines 198, 100. However, the sleeve 174, taking with it the camshaft 8, can be displaced axially with respect to the cylinder head 5 upon rotation of the output shaft 196, by virtue of the screwthreads 104, 106. Consequently, by controlled rotation of the motor 190, valve lift and duration can be adjusted by axial displacement of the camshaft 8.

Oil can be admitted or discharged from the interior of the annular cylinder 184 by means of passageways 116, controlled by suitable valves. If oil is admitted to the cylinder 184, the piston 186 is displaced to the left, as seen in Figures 13 and 14, against the action of the spring 188. By virtue of the helical splines 112, 114, this displacement of the piston 186 causes it to rotate with respect to the pulley 168, this rotation being transmitted to the camshaft 8 by virtue of the axial splines 108, 110. If the supply of oil to the cylinder 184 is terminated and a drain path opened, the piston 186 is returned to the right, as seen in Figures 13 and 14, by the spring 188. Consequently, by controlling the supply and return of oil to and from the cylinder 184, the relative rotational positions of the pulley 168 and the camshaft 8 can be adjusted continuously, so as to vary the valve timing of the engine. It will be appreciated that adjustment of the valve timing by the cylinder and piston unit 184, 186 and adjustment of valve lift and duration by the motor 190, can be effected independently of each other.

By situating the motor 190 and the cylinder and piston unit 184, 186 axially of the camshaft 8 and extending within the pulley 168, and by using the interior of the annular cylinder 184 to accommodate the motor 190, the packaging of the control mechanism can be made very compact, taking up little more space, if any, than the drive arrangement of a camshaft having no facility for adjustment. It will be appreciated that the forward end of the camshaft is enlarged so that the portion carrying the splines 110 can move over the motor casing 192 when the camshaft is fully advanced by the action of the screw threads 104, 106. The use of the motor 190, acting on the camshaft through the sleeve 174, enables highly accurate positioning of the camshaft 8.

Figures 20 and 21 show, in schematic form, control circuitry for use in the engine of Figures 13 to 15. Figure 20 shows an electronic control unit (ECU) 118 which receives inputs from the various sensors, as follows :

• a pulley sensor 120, responsive to rotation of the pulley 168

• a relative camshaft rotation sensor 122 responsive to rotation of the camshaft 8 relative to the pulley 168 as caused by displacement of the piston 186.

• a camshaft displacement sensor 124 responsive to axial displacement of the camshaft 8 by operation of the motor 190

• a crankshaft sensor 126 responsive to crankshaft rotation

• other sensors 128.

On the basis of outputs from the sensors 120 to 128, the ECU 118 is able to determine the current operating status of the various components of the valve gear and to provide output signals for controlling the motor 190 and valves controlling the supply or discharge of oil from and to the cylinder 184 in order to adjust valve lift, duration and valve timing.

Figure 21 shows, schematically, an engine 2 having four cylinders, each provided with an air intake duct 130. Within the cylinder head 5, each intake duct 130 is branched into two passages controlled by the respective valves represented by the valve stems 20 in Figure 1. An air flow sensor 132 is disposed in each intake duct 130. The output signal from each sensor 132 is fed to the ECU 118. Figure 21 shows the second pivotable finger 32 for each cylinder as well as the support element 152 for each second finger 32. An oil gallery 136 extends through the cylinder head 5 for selective connection, via a solenoid valve (not shown) to a bore 138 (Figure 15) within which each support element 152 is accommodated.

The ECU 118 also receives control inputs from other sensors 140 as well as engine map data 142 governing operation of the engine under various conditions.

In operation, for example at tick over of the engine, the map data 142 specifies an idling speed for the engine, as sensed by the crankshaft sensor 126. In achieving the idling speed, the ECU 118 adjusts the valve lift, duration and timing by appropriate control of the piston 186 and the motor 190. However, at the idling speed, the air flow rate through the intake ducts 130 as sensed by the sensors 132 may vary between cylinders. This variation may have undesirable consequences on fuel efficiency, emissions and engine operating stability. To avoid these effects, the sensors 132 provide signals to the ECU 118, and the solenoids controlling the flow of oil from the gallery 136 to the individual chambers 138 containing the support elements 152 are controlled in order to raise or lower the respective support elements 152 so as to equalise the air flows through the intake ducts 130. As a result, all of the cylinders operate under the same conditions, regardless of differences in manufacturing tolerances and/or thermal or other differences.

Furthermore, valve gear as described above enables continuous adjustment of valve operation to compensate for changes occurring during engine warm-up . For example, when the engine is cold, valve clearances will be intentionally increased, and consequently there is little danger of any individual clearances closing completely and causing valves to remain permanently open. However, it is preferable for the camshaft 8 to be dimensioned so that it positions the cams 10 accurately when the engine is at its normal operating temperature. This means that the cams 10 will be displaced relatively from the valve stems 20 when the engine is cold. Consequently, when the engine is cold, the critical adjustment to be achieved by displacing the support elements 152 is to compensate for the incorrect positions of the cams 10. However, as the engine warms up, this adjustment becomes less critical, and instead it becomes important to compensate for variations in valve clearance. The ability of the support elements 152 to vary continuously in position means that active valve clearance adjustment is possible by active correction to the fixed map valve clearance at the particular measured airflow, ensuring optimum operation of the engine at all temperatures. The above features will allow benefits to be achieved by lowering emissions and fuel consumption and improving engine smoothness

Figure 22 is a lift diagram for a valve controlled by valve gear in accordance with the present invention. At full power, represented by a line A, the maximum valve lift is approximately 10.5 mm. Line B is a minimum lift line, and represents valve movement at tick over, assuming a fixed support element 152 (i.e. the support element 152 is in the same position when the engine is at tick over (line B) as when the engine is at maximum power (line A) .

By adjusting the position of the support element 152, the cylinder in question can be caused to operate within the shaded area in Figure 22, and consequently both the maximum lift and the duration of valve opening can be reduced within a continuous range. Thus, by controlling valve lift within the shaded region by suitable control of the support element 152, the operating conditions of all of the cylinders of the engine can be equalised. Furthermore, because the second finger 32 can tilt about an axis perpendicular to the plane containing the longitudinal axes of the valve stems 20, the operation of the valves of each pair can also be equalised.

Valve gear as described above has significant cost benefits over known variable valve lift systems. Since adjustment of the support elements can compensate for manufacturing variations, component tolerances can be relatively relaxed, leading to savings in component manufacturing costs. Furthermore, several features of the valve gear, for example the use of the motor 190 to displace the camshaft 10, lead to a reduction in part count, resulting in reduced assembly and component stocking and management costs.

The present invention thus enables use of the same conical (swashed or un-swashed) cam and rectangular half-roller design as with the variable lift duration and timing bucket system of, for example, GB2190140 in a finger follower actuation system that employs two fingers. A variable track in the bottom of the top rocker is acted upon by a roller in the top of the bottom rocker, the effect of which is to modify the motion between the valve and cam such that the cam profile always remains convex so that it effectively becomes a bucket actuation system at the cam with the cam acting on a flat surface.

This allows the more advantageous higher velocity (and lower friction) re-entrant cam/finger follower system type lift to be obtained at the valve but with a flat follower system at the cam and the retention of all the valve lift, duration and timing variability that was achievable with the bucket systems disclosed in GB2190140, GB 2341659 and GB 2359608.

In the embodiments described above, the lower, second rocker 32 is radiussed and is shaped in plan view to allow two valves of a 4-valve combustion chamber to be opened and closed simultaneously when acted upon by the vertical movement of the upper, first rocker 30 which communicates its motion to the roller 62 and lower rocker 32 via the upper rocker track 48.

The lower rocker 32 can accept lateral loads from the upper rocker half-roller 46 communicated through the forward lower vertical side faces of the upper rocker 30 to the slot 60 in the lower rocker top surface and is itself wide-based for efficient distribution of these loads into the cam carrier/cylinder head structure 5,6.

The lower rocker 32 has suitably high vertical stiffness deriving from the vertical material at the front of the rocker and above the valves and the structure is further stiffened by being made as a 1- piece component which is "tied-together" in the middle by the horizontal surface 92 formed between the two rocker halves which also provides an excellent bearing carrier for the roller 62 in its central slot 60.

The lower rocker 32 has only to accept the fairly low lateral loads communicated from the half-roller 46 to the upper rocker 30 and then the lower rocker 32.

The lower rocker of the embodiment of Figures 1 to 4 has also only to accept relatively low loads in its structure between the wide based double shear pivot pins 52 and the roller 62 because the roller 62 is disposed near the valve stem 20 and this structure can therefore be made lighter than the two conventional finger followers which it replaces - each conventional follower has to accept all the loading at each valve.

Owing to the effect of the supporting pivot and the actuation of the two valves being effected by the single cam lobe 10 and half-roller 46 acting on the single upper rocker 30, the effective mass of the upper rocker 30 at each valve is low and the lower rocker mass can be reduced compared with two conventional rockers. Therefore the total mass at the valve of this system is approximately equivalent to a conventional roller rocker finger system at each valve.

As the valves lift the disparate radial arcs of movement at the roller 62 resulting from the pivoting of the two rockers about their respective axes are such that a large differential motion occurs between the track 48 and the roller 62 which allows the design of the track/roller to be disposed in such a way that the desired valve motion can be achieved by a combination of asymmetric cam design and track shape, which ensures that the complex conical camshaft profile always remains convex so that it operates with 100% contact across the half-roller face.

The contact stress between the roller 62 and the track 48 can be kept within reasonable bounds and this design lends itself to good roller support, bearing action and lubrication.

By suitable design of the components, valve lift velocities can match those of roller/finger follower systems (which tend to be higher than those of bucket systems) whilst retaining the half-roller concept that uses flat follower tangential variable valve lift, duration and timing. With this system lift at the cam normally associated with a bucket system can be translated into a valve lift that can normally only be obtained with a re-entrant finger follower cam/convex finger pad system.

A system in accordance with the present invention can be designed to occupy the same packaging space as a conventional roller finger follower system.

Also, the ability to employ only a single cam lobe and half-roller upper follower 30 and a single 1-piece lower follower 32 and a cam profile having no reentrant profile to operate two valves provides a relatively inexpensive mechanism. A system in accordance with the present invention has approximately the same friction (or slightly higher) as a roller finger follower system as only a single cam lobe/bucket is required for two valves. Possibly slightly higher friction resulting from the wider cam lobe with its slightly wider half-roller type bucket giving slightly greater sliding friction than a roller, is approximately offset by the need for only a single cam/follower for two valves) . The overall friction is likely to be lower than occurs in a bucket system.

Claims

1. Valve gear for operating a valve (14) in an internal combustion engine, the valve gear comprising a camshaft (8) carrying a cam (10) which has a profile which varies in the direction of the camshaft axis (12) , the camshaft (8) being axially displaceable to vary the action of the cam (10) on the valve (14) , characterised in that the cam (10) acts on a first pivotable finger (30) , the first finger (30) having an actuating surface (48) which contacts a second pivotable finger (32) , the second pivotable finger (32) being in driving engagement with the valve (14) .

2. Valve gear as claimed in claim 1, characterised in that the pivotable fingers (30, 32) are each pivotable about a respective axis (34, 50) which is parallel to the axis (12) of the camshaft (8) .

3. Valve gear as claimed in claim 2, characterised in that the pivot axis (34) of the first finger (30) lies in a plane extending transversely of the direction of valve movement, which plane lies between the axis (12) of the camshaft (8) and the contact point between the base circle (26) of the cam (10) and the first finger (30) .

4. Valve gear as claimed in claim 2 or 3 , characterised in that the pivot axis (50) of the second finger (32) lies in a plane extending transversely of the direction of valve movement and positioned so that the point of contact between the second finger (32) and the valve (14) moves across that plane as the valve (14) opens and closes.

5. Valve gear as claimed in any one of the preceding claims, characterised in that the first finger (30) has a cam-engaging element (46) which is pivotable with respect to the first finger (30) about an axis which lies in a plane perpendicular to the camshaft axis (12) .

6. Valve gear as claimed in claim 5, characterised in that the cam-engaging element (46) has a flat cam contact surface which is engaged by the cam (10) .

7. Valve gear as claimed in claim 6, characterised in that the cam-engaging element (46) is in the form of a half-roller.

8. Valve gear as claimed in any one of the preceding claims, characterised in that the actuating surface (48) of the first finger (30) is in the form of a profiled track along which travels a contact element (62) carried by the second finger (32) .

9. Valve gear as claimed in claim 8, characterised in that the track (48) is at least partly concave.

10. Valve gear as claimed in claim 9, characterised in that the track (48) comprises a first convex then concave arcuate portion (64) at a radially inner end of the track (48) with respect to the pivot axis (34) of the first finger (30) , a straight portion (66) of the track extending generally radially outwardly from the first arcuate portion (64) and terminating at a second concave arcuate portion (68) at the radially outer end of the track (48) .

11. Valve gear as claimed in claim 10, characterised in that the straight portion (66) of the track (48) is inclined to the cam contact surface so that the distance between the track (48) and the cam contact surface increases in the direction away from the pivot axis (34) of the first finger (30).

12. Valve gear as claimed in any one of the preceding claims, in an engine having two inlet valves (14) and/or two exhaust valves (16) per cylinder, characterised in that the second finger (32) has two valve engaging portions (90, 116) for engaging the two valves (14, 16) .

13. Valve gear as claimed in claim 12, characterised in that the contact element (62) on the second finger (32) comprises a cylindrical roller which extends parallel to the pivot axis (50) of the second finger (32) between the valve engaging portions (90, 116) .

14. Valve gear as claimed in any one of the preceding claims, characterised in that the pivot axis (34) of the first finger (30) is fixed with respect to a cylinder head (5) which supports the valve gear.

15. Valve gear as claimed in any one of claims 1 to 13, characterised in that the pivot axis (34) of the first finger (30) is movable with respect to a cylinder head (5) which supports the valve gear.

16. Valve gear as claimed in claim 15, characterised in that the pivot axis (34) of the first finger (30) is defined by contact between the first finger (30) and a movable support element (74) .

17. Valve gear as claimed in claim 16, characterised in that the first finger (30) has a recess (86) with which the movable support element (74) cooperates to define the pivot axis (30) .

18. Valve gear as claimed in claim 17, characterised in that the movable support element (74) is movable in dependence on the position of the second finger (32).

19. Valve gear as claimed in claim 18, characterised in that the movable support element (74) comprises a support cam which is pivotable about a fixed axis (80) by means of an operating lever (82) which moves with the second finger (32) about the pivot axis (50) of the second finger (32) .

20. Valve gear as claimed in any one of the preceding claims, characterised in that the second pivotable finger is pivotably supported by a support element (152) which is movable relatively to a cylinder head (5) of the internal combustion engine.

21. Valve gear as claimed in claim 20, characterised in that the support element (152) is movable in a direction which lies in a plane perpendicular to the axis of the camshaft (12) .

22. Valve gear as claimed in claim 20 or 21, characterised in that the support element (152) is movable in a direction parallel to the direction of valve movement .

23. Valve gear as claimed in claim 20 or 21, characterised in that the support element (152) is movable in a direction inclined to the direction of valve movement .

24. Valve gear as claimed in any one of the preceding claims, characterised in that the support element (152) is movable under hydraulic control.

25. Valve gear as claimed in claim 24, characterised in that the supply of hydraulic fluid to move the support element (152) is controlled by means of a valve .

26. Valve gear as claimed in claim 24 or 25, characterised in that the movement of the support element (152) is controlled in response to air flow through the valve (14) .

27. Valve gear as claimed in claim 26, in a multicylinder engine, characterised in that the movement of the support element (152) for a valve (14) of each cylinder is controlled in response to variations in airflow between the valves (14) of the respective cylinders.

28. Valve gear as claimed in claim 27, characterised in that an airflow sensor (132) is provided in the air flow path through the valve (14) of each cylinder.

9. Valve gear as claimed in any one of claims 20 to 28, characterised in that the support element (152) is provided with a hemispherical contact surface (154) engaging the second pivotable finger (32) .

30. Valve gear as claimed in any one of claims 20 to 29, in an engine having two inlet valves (14) and/or two exhaust valves (16) per cylinder, characterised in that the second finger (32) has two valve engaging portions (90, 159) for engaging the two valves (14) .

31. Valve gear as claimed in claim 30 when appendant to claim 29, in which the support element (152) permits tilting of the second finger (32) about an axis extending perpendicular to a plane containing the directions of movement of the valves (14) .

32. An internal combustion engine provided with valve gear in accordance with any one of the preceding claims .