OPERATING MECHANISMS FOR VALVES

Abstract: An operating mechanism (10) for a valve (12) comprises a cam (40) mounted for rotation about an axis (42) thereof, a rocker (50) having an abutment surface (70) which is movable to operate said valve, and a follower (80) mounted for pivoting motion. The follower (80) has a point of contact with the outer surface (46) of the cam (40) so that, as the cam rotates, the follower is caused to pivot. The follower (80) also has a point of contact with the rocker (50) so that, as the follower pivots, the rocker is caused to pivot to thereby operate the valve (12). The mechanism also comprises adjustment means operable to move the axis (82) of the follower (80) relative to the axis (42) of the cam and the axis (52) of the rocker (50) to thereby shift the point of contact between the cam and the follower to thereby alter the timing of the operation of the valve relative to the rotation of the cam.

Description: OPERATING MECHANISMS FOR VALVES This invention is concerned with operating mechanisms for valves, eg the valves of internal combustion engines. Conventional internal combustion engines have valves to control the flow of a fuel and air mixture into the cylinders of the engine and further valves to control the flow of exhaust gases out of the cylinders. Most such valves are of the so-called "poppet" type which comprise a valve closure member which can engage a seat around an opening into the cylinder to prevent gas flow, or can be displaced from the seat to open said opening to allow gas flow. A stem projecting from valve closure member has a cap thereon against which a spring acts. The spring urges the valve closure member against its seat, ie into a valve- closed condition. In order to operate the valve, a valve operating mechanism moves the stem (against the force of the spring) to open the valve and subsequently allows the spring to close the valve by moving the stem in the opposite direction. A conventional valve operating mechanism for an internal combustion engine comprises a camshaft which is rotated about a central axis thereof in synchronisation with a crankshaft of the engine. The camshaft has a plurality of cams projecting therefrom (conventionally there is one cam per valve of the engine) . The cams are arranged at various orientations about the central axis of the camshaft so that each valve of a particular cylinder can be operated at the appropriate times and the cylinders of the engine can be operated in the appropriate sequence. Each cam has an outer surface which varies in distance from the central axis of the camshaft, the furthest distance from the axis representing the valve open condition. The operating surface bears (either directly or via a rocker or other intermediate member) on the stem of a valve in order to operate the valve. Thus, the timing of the operation of the valve depends on the variation in the distance of the cam's outer surface from the axis of the cam and, in a conventional operating mechanism, there is no possibility of varying this timing within the operating cycle. It is well known that the particular time in the cycle of operation of an internal combustion engine's cylinder when the valves open and close profoundly affects the efficiency of combustion and hence the fuel consumption and exhaust characteristics of the engine. It is also well known that the optimum times for the valves to open and close depends on the speed of operation of the engine. Hence, in most engines the timing is set at a compromise between the optimum timing for low speed operation and the optimum timing for high speed operation. This results in the engine operating most of the time at a less than optimum timing. There have been many suggestions for operating mechanisms which enable the timing to be altered during operation of the engine so that the timing can be made more appropriate at different operating speeds. Examples are disclosed in GB 2 273 956 A, GB 2 272 022 A and WO 94/08129. However, in most cases, these suggestions involve complicated mechanisms which are expensive, unreliable and occupy an excessive amount of space. Accordingly, it is an object of the present invention to provide an operating mechanism which allows the timing to be altered during the operation of the engine, the mechanism being simple, reliable and compact. The invention provides an operating mechanism for a valve, the mechanism comprising a cam mounted for rotation about an axis thereof, a rocker having an abutment surface which is movable to operate said valve, the rocker being mounted for pivoting motion about an axis thereof which is parallel to the axis of the cam, and a follower mounted for pivoting motion about an axis thereof which is also parallel to the axis of the cam, the cam having an outer surface which varies in distance from its axis, the follower having a point of contact with the outer surface of the cam so that, as the cam rotates, the follower is caused to pivot about its axis, the follower also having a point of contact with the rocker so that, as the follower pivots about its axis, the rocker is caused to pivot about its axis to thereby operate the valve, characterised in that the mechanism also comprises adjustment means operable to move the axis of the follower relative to the axes of the cam and of the rocker to thereby shift the point of contact between the cam and the follower to thereby alter the timing of the operation of the valve relative to the rotation of the cam. In an operating mechanism according to the invention the timing of the operation of the valve can be altered in a simple and reliable manner by a mechanism which does not occupy a large space. In addition to alteration of the timing of the operation of the valve, it is possible to configure an operating mechanism according to the invention, ie arrange the relative positions of the axes, so that increased lift is achieved in the valve. The follower may be mounted on an eccentric and the adjustment means may be operable to turn the eccentric about an axis which is parallel to the axis of the cam. The eccentric may be turned about an axis which is coincident with the axis on which the rocker is mounted, eg the eccentric may be mounted on a shaft on which the rocker is also mounted. The point of contact between the cam and the follower may be provided by a roller mounted on the follower. This enables the roller to roll along the cam surface when the adjustment means is operated. The rocker may be adapted to operate a plurality of valves simultaneously, eg the rocker may operate two or more valves of a cylinder of an internal combustion engine. The cam may be mounted on a camshaft on which a further cam is mounted for rotation about the axis of the cam, and the mechanism may also comprise a further rocker having an abutment surface which is moveable to operate a further valve, a further follower having a point of contact with the outer surface of the further cam and a point of contact with the further rocker, and further adjustment means operable to move the axis of the further follower relative to the axes of the further cam and of the further rocker to alter the timing of the operation of the further valve. This enables a single camshaft to operate both the inlet and outlet valves of a cylinder of an internal combustion engine. The adjustment means and the further adjustment means may be operable independently. The adjustment means may be operable in response to changes in the speed of operation of an engine of which the valve forms part. For example, the adjustment means may have its operation controlled by an engine management system. There now follows a detailed description, to be read with reference to the accompanying drawings, of an operating mechanism which is illustrative of the invention. In the drawings: Figure 1 is a cross-sectional view taken through the illustrative operating mechanism and an associated engine; Figure 2 is a perspective view of a portion of the illustrative operating mechanism; Figure 3 is a diagrammatic view of adjustment means of the illustrative operating mechanism; and Figure 4 is a graphical representation of the operation of the illustrative operating mechanism. The illustrative operating mechanism 10 is for operating a valve 12 (Figure 1) of an internal combustion engine. The engine comprises a block 13 and a head 14 which define a plurality of cylinders 16 in each of which a piston (not shown) reciprocates. The cylinders 16 are arranged in a line extending normally of the plane of Figure 1 so that only one cylinder 16 is shown in that Figure. Each cylinder 16 has two inlet ports 20 defined by the head 14 (only one port 20 is visible in Figure 1) through which a fuel and air mixture can enter the cylinder 16. The head 14 also defines two exhaust ports 22 (one only visible in Figure 1) through which exhaust gases can exit from the cylinder 16. A spark plug 24 projects into each cylinder 16 to cause ignition of the fuel and air mixture in the cylinder. The valve 12 is operable to open and close one of the inlet ports 20 but a further valve (not shown) beside the valve 12 is operable to open and close the other inlet port 20. A further valve 18 is operable to open and close one of the outlet ports 22 but a further valve (not shown) beside the valve 18 is operable to open and close the other outlet port 22. Since the valves 12 and 18 are identical, only the valve 12 is described herein in detail, like parts in the valve 18 being given the same reference numerals. The valve 12 is a conventional "poppet" valve and comprises a closure member 30 which is within the cylinder 16 and is movable between a closed position thereof (shown in Figure 1) , in which the member 30 engages a seat around the exit of the port 20, and an open position in which the member 30 is displaced from said seat and allows gas to enter the cylinder 16 through the port 20. The valve 12 also comprises a stem 32 which is fixed to the member 30 and projects through a guide 34 mounted on the head 14 which communicates with the port 20. Beyond the guide 34, a cap 36 is fixed to the stem 32 and a coil spring 38, in a recess of the head 14, acts on this cap 36 to urge the member 30 into its closed position. In order to move the member 30 into its open position, the stem 32 is pressed towards the cylinder 16 against the force of the spring 38. The illustrative mechanism 10 also comprises a cam 40 mounted for rotation about an axis 42 thereof. The cam 40 is mounted on a camshaft which extends above the head 14 parallel to the line of cylinders 16. The cam 40 has an outer surface 46 which extends longitudinally of the camshaft. The surface 46 varies in distance from the axis 42. The cam 40, as will appear from the description below, is arranged to operate the valve 12 and the valve of the other inlet port 20. A further cam 48 is mounted on the camshaft to operate the valve 18 and the valve of the other outlet port 22. Further cams (not shown) are also mounted on the camshaft to operate the valves of the other cylinders 16. The illustrative mechanism also comprises a rocker 50 which is mounted for pivoting motion about an axis 52 thereof which is parallel to the axis 42 of the cam 40. The axis 50 is a central axis of a shaft 54 which extends parallel to the camshaft. The shaft 54 is mounted on bearings (not shown) to turn about the axis 52. A further shaft 56 is mounted on bearings (not shown) to turn about an axis 58 which is parallel to the axes 42 and 52. A further rocker 60 is mounted on the shaft 56 for pivoting motion about the axis 58. The rockers 50 and 60 are of identical construction so that only the rocker 50 will be described in detail, like parts of the rocker 60 being given the same reference numerals. The rocker 50 has a central portion which defines a bore through which the shaft 54 is received. The rocker 50 is a loose fit on the shaft so that the shaft 54 can turn about the axis 52 without the rocker 50 moving. Also, the rocker 50 can pivot about the axis 52 without the shaft 54 turning. The central portion of the rocker 50 is positioned longitudinally of the shaft 54 at a position which is displaced relative to the cam 40. The rocker 50 also comprises a projection 62 which projects transversely and longitudinally of the shaft 54 so that an end portion of the projection 62 is positioned above and in alignment with the cam 40. The end portion of the projection 62 has a convexly-curved lower surface 64 (figure 2) whose purpose will appear from the description below. The rocker 50 also comprises a further projection 66 which extends generally in the opposite direction to the projection 62. The projection 66 is forked to provide two spaced abutments 68. Each abutment 68 has a lower abutment surface 70. One surface 70 engages the top of the stem 32 of the valve 12 while the other surface 70 engages the top of the stem 32 of the valve of the other inlet port 20. The surfaces 70 are movable to operate said valves. Specifically, the rocker 50 pivots about the axis 52 in one direction (clockwise viewing Figure 1) to cause the surfaces 70 to press down the stems 32 to open the valves, and pivots in the opposite direction to allow the springs 38 to close the valves. The illustrative operating mechanism 10 also comprises a follower 80 mounted for pivoting motion about an axis 82 thereof which is also parallel to the axis 42 of the cam 40. The axis 82 is provided by the central axis of an eccentric 84 mounted on the shaft 54, the axis 82 being off-set from the axis 52. The eccentric 84 is adjacent to the central portion of the rocker 50 and is aligned with the cam 40. The eccentric 84 is pinned to the shaft 54 by a pin 86 so that, when the shaft 54 turns about the axis 52, the eccentric 84 also turns about the axis 52 thereby displacing the axis 82 relative to the axes 52 and 42. A further follower 88 is associated with the rocker 60 and is mounted on a further eccentric 90 on the shaft 56 for pivoting motion about an axis 92 which is parallel to the axes 42 and 58. The followers 80 and 88 are of identical construction so that only the follower 80 will be described in detail, like parts of the follower 88 being given the same reference numerals. The follower 80 has a portion which is rotatably mounted on the outer surface of the eccentric 84, and two parallel arms projecting radially of the shaft 54. Between these arms, the follower 80 supports a roller 94 which provides a point of contact with the outer surface 46 of the cam 40 (or, in the case of the follower 88, with the cam 48) so that, as the cam rotates, the follower 80 is caused to pivot about its axis 82 (or 92 for the follower 88) . The follower 80 also comprises an abutment 98 which bridges the arms of the follower 80 above the roller 94 thereof. The abutment 98 has a concavely-curved upper surface 100 which engages the surface 64 of the rocker 50 and, thereby, provides a point of contact with the rocker 50. As the follower 80 pivots about its axis 82, because of the contact between the surfaces 64 and 100, the rocker 50 is caused to pivot about its axis 52 to thereby operate the valve 12. The illustrative operating mechanism 10 also comprises adjustment means (not shown) operable to move the axis 82 of the follower 80 relative to the axes 42 and 52 of the cam 40 and of the rocker 50 to thereby shift the points of contact between the cam 40 and the follower 80, and the follower 80 and the rocker 50 to thereby alter the timing of the operation of the valve 12 relative to the rotation of the cam 40. The adjustment means is operable to turn the shaft 54 about the axis 52 thereby turning the eccentric 84 also about the axis 52. The turning of the eccentric 84 moves the axis 82 arcuately about the axis 52, thereby bodily moving the follower 80 relative to the cam 40 and the rocker 50. The movement of the follower 80 relative to the cam 40 causes the roller 94 to roll along the surface 46 of the cam 40 so that the point of contact between the surface 46 and the follower 80 moves to a different point on the surface 46. This means that, as the cam 40 rotates about the axis 42, the time when the valve 12 is opened is altered. The adjustment means comprises an arm 102 (figure 3) which projects radially from an end portion of the shaft 54, and a hydraulic piston and cylinder assembly 108 having a piston rod 106 pivotally connected to the arm 102. The assembly 108 is operable to move the arm 102 and thereby to turn the shaft 54 about its axis 52. The assembly 108 can be controlled manually but, preferably, it is controlled by an engine management system in accordance with the speed of the engine. Thus, at low speeds, the assembly 108 may hold the arm in a first position in which the roller 94 is in one position relative to the axis 42 but, at higher speeds, the assembly 108 may move the arm into a second position in which the roller 94 is in a second position relative to the axis 42. Operation of the assembly 108 is effective to move all the rockers 50 mounted on the shaft 54 so that the timing of all the inlet valves 12 of all the cylinders 16 are advanced or retarded simultaneously and by the same amount. A further hydraulic piston and cylinder assembly (not shown) provides further adjustment means operable to turn the shaft 56 relative to the axis 58 thereof, thereby moving the axis 92 relative to the axis 42 of the cam 48. This moves the followers 88 relative to the cam surface, thereby advancing or retarding the timing of the valves 14 of the outlet ports 22 simultaneously and by the same amount. The adjustment means and the further adjustment means are operable independently of one another so that the relative timing of the opening of the inlet ports 20 and the outlet ports 22 can be varied. Figure 4 is a graph showing the valve operating lift diagram of the valve 12. The x-axis is in degrees of rotation of the cam 40 (one complete turn equals 360°) and the y-axis is in millimetres of lift of the closure member 30 of the valve 12 away from its seat. The curve 110 shows the operation of the valve 12 at low engine speeds when the shaft 54 is held in one position. The curve 112 shows the operation of the valve 12 at higher engine speeds. A comparison of the curves 110 and 112 shows that the curve 112 is off-set from the curve (the off-set is 12.7°) so that the valve 12 opens later and closes later. Furthermore, the maximum lift is greater on the curve 112 than on the curve 110 and the area under the curve 112 is greater than that under the curve 110 (giving greater gas flow) .

Claims: CLAIMS An operating mechanism (10) for a valve (12) , the mechanism comprising a cam (40) mounted for rotation about an axis (42) thereof, a rocker (50) having an abutment surface (70) which is movable to operate said valve, the rocker (50) being mounted for pivoting motion about an axis (52) thereof which is parallel to the axis (42) of the cam, and a follower (80) mounted for pivoting motion about an axis (82) thereof which is also parallel to the axis (42) of the cam, the cam (40) having an outer surface (46) which varies in distance from its axis (42) , the follower (80) having a point of contact with the outer surface (46) of the cam so that, as the cam (40) rotates, the follower (80) is caused to pivot about its axis (82) , the follower (80) also having a point of contact with the rocker (50) so that, as the follower (80) pivots about its axis (82) , the rocker (50) is caused to pivot about its axis (52) to thereby operate the valve (12) , characterised in that the mechanism also comprises adjustment means (108) operable to move the axis (82) of the follower (80) relative to the axes (42, 52) of the cam (40) and of the rocker (50) to thereby shift the point of contact between the cam and the follower to thereby alter the timing of the operation of the valve (12) relative to the rotation of the cam (40) . An operating mechanism according to claim 1, characterised in that the follower (80) is mounted on an eccentric (84) and the adjustment means (108) is operable to turn the eccentric (84) about an axis (52) which is parallel to the axis (42) of the cam (40) . An operating mechanism according to claim 2 , characterised in that the eccentric (84) is turned about an axis (52) which is co-incident with the axis on which the rocker (50) is mounted. An operating mechanism according to any one of claims 1 to 3 , characterised in that the point of contact between the cam (40) and the follower (80) is provided by a roller (94) mounted on the follower. An operating mechanism according to any one of claims 1 to 4, characterised in that the rocker (50) is adapted to operate a plurality of valves simultaneously. An operating mechanism according to any one of claims 1 to 4 , characterised in that the cam (40) is mounted on a camshaft on which a further cam (48) is mounted for rotation about the axis (42) of the cam, and the mechanism also comprises a further rocker (60) having an abutment surface which is moveable to operate a further valve (18) , a further follower (88) having a point of contact with the outer surface of the further cam and a point of contact with the further rocker, and further adjustment means operable to move the axis of the further follower relative to the axes of the further cam and of the further rocker to alter the timing of the operation of the further valve. An operating mechanism according to claim 6, characterised in that the adjustment means (108) and the further adjustment means are operable independently. An operating mechanism according to any one of claims 1 to 7, characterised in that the adjustment means (108) is operable in response to changes in the speed of operation of an engine of which the valve forms part.

Inventors: SLEE ROGER HARVEY, SLEE, ROGER, HARVEY
Applicants: SLEE ROGER HARVEY, T & N TECHNOLOGY LTD, SLEE, ROGER, HARVEY, T & N TECHNOLOGY LIMITED
Application year: 1998
Main classification: F01L1
IPC codes: F01L1, F01L13