GB2323630A - Variable timing valve operating mechanism, eg for i.c. engines - Google Patents

Abstract

The mechanism comprises a cam 12, a rocker 20, which may operate a pair of valves, and a pivotally-mounted lever 50 having points of contact with both the cam 12 and the rocker 20. Adjustment means which comprise a shaft 24 and eccentric 53 are operable to move the axis 52 of the lever 50 relative to the axes 16,22 of the cam 12 and of the rocker 20 so that the point of contact between the cam 12 and the lever 50 is shifted and hence the timing of the valve relative to the rotation of the cam can be varied. The lever 50 comprises two cam followers 60,62 which are arranged to engage the cam 12 at different orientations relative to the axis 16 of the cam. Cam follower 62 is mounted on a carriage 70 which can be pivoted by an actuator 72. At low engine speeds, only follower 60 engages the cam 12 but at higher speeds the actuator 72 also presses follower 62 against the cam to hold the valve open for longer.

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 valveclosed 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 outer 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 breathing and 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 for 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 lever 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 lever having a point of contact with the outer surface of the cam so that, as the cam rotates, the lever is caused to pivot about its axis, the lever also having a point of contact with the rocker so that, as the lever pivots about its axis, the rocker is caused to pivot about its axis to thereby operate the valve, wherein the mechanism also comprises adjustment means operable to move the axis of the lever relative to the axes of the cam and of the rocker to thereby shift the point of contact between the cam and the lever to thereby alter the timing of the operation of the valve relative to the rotation of the cam, the lever also comprising abutment means which provides the point of contact with the cam, and wherein the abutment means comprises a plurality of cam engagers arranged to engage the cam at different orientations relative to the axis of the cam, and positioning means operable to alter the relative positions of the cam engagers on the lever to alter which of the cam engagers engages 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. The mechanism is also versatile since the adjustment means can be operated to advance or retard valve opening while the positioning means can be operated to lengthen or shorten the time during which the valve remains open.

The lever 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 rocker is adapted to operate a plurality of valves simultaneously. This is advantageous in the case of an engine which has two or more inlet and/or outlet valves per cylinder.

The adjustment means may be operable in response to changes in the speed of operation of an engine of which the valve forms part.

Said cam engagers may, conveniently, comprise a first cam engager which is fixed in position relative to the lever, and at least one further cam engager which is movable relative to the lever by operation of the positioning means. Said further cam engager may be mounted on a carriage which is movable relative to the lever by operation of the positioning means. The cam engagers are, preferably, provided by rollers.

The positioning means may be operable in response to changes in the speed of operation of an engine of which the valve forms part.

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 side-elevational view, partly in section, of the illustrative operating mechanism; and Figures 2a to 2d are timing diagrams illustrating times of operation of a valve operated by the illustrative operating mechanism with respect to a crankshaft of an engine.

The illustrative operating mechanism 10 is for operating a conventional poppet-type valve (not shown) of an internal combustion engine. The valve is operable to open and close one of the ports of a cylinder of the engine.

The illustrative mechanism 10 comprises a cam 12 mounted on a camshaft 14 for rotation about an axis 16 thereof. The cam 12 has an outer surface 18 which extends longitudinally of the camshaft 14. The surface 18 varies in distance from the axis 16. The cam 12, as will appear from the description below, is arranged to operate the valve and a further valve.

The illustrative mechanism also comprises a rocker 20 which is mounted for pivoting motion about an axis 22 thereof which is parallel to the axis 16 of the cam 12.

The axis 22 is a central axis of a hollow shaft 24 which extends parallel to the camshaft 14. The hollow shaft 24 is mounted on bearings (not shown) to turn about the axis 22.

The rocker 20 has a central portion which defines a bore through which the shaft 24 is received. The rocker 20 is a loose fit on the shaft so that the shaft 24 can turn about the axis 22 without the rocker 20 moving. Also, the rocker 20 can pivot about the axis 22 without the shaft 24 turning. The central portion of the rocker 20 is positioned longitudinally of the shaft 24 at a position which is displaced relative to the cam 12. The rocker 20 also comprises a projection 26 which projects transversely and longitudinally of the shaft 24 so that an end portion of the projection 26 is positioned above and in alignment with the cam 12. The end portion of the projection 26 has a convexly-curved lower surface 28 whose purpose will appear from the description below. The rocker 20 also comprises a further projection 30 which extends generally in the opposite direction to the projection 26. The projection 30 is forked to provide two spaced abutments 32 (only one visible in the drawing). Each abutment 32 has a lower abutment surface 34. One surface 34 engages the top of the stem of said valve while the other surface 34 engages the top of the stem of the further valve. The surfaces 34 are movable to operate said valves.

Specifically, the rocker 20 pivots about the axis 22 in one direction (clockwise viewing the drawing) to cause the surfaces 34 to press down the stems to open the valves, and pivots in the opposite direction to allow springs (not shown) to close the valves in the conventional manner.

The illustrative operating mechanism 10 also comprises a lever 50 mounted for pivoting motion about an axis 52 thereof which is also parallel to the axis 16 of the cam 12. The axis 52 is provided by the central axis of an eccentric 54 mounted on the shaft 24, the axis 52 being off-set from the axis 22. The eccentric 54 is adjacent to the central portion of the rocker 20 and is aligned with the cam 12. The eccentric 54 is pinned to the shaft 24 by a pin 56 so that, when the shaft 24 turns about the axis 22, the eccentric 54 also turns about the axis 22 thereby displacing the axis 52 relative to the axes 22 and 16.

The lever 50 also comprises abutment means 56 which provides the point of contact with the cam 12. The abutment means 56 comprises a two cam engagers 60 and 62, in the form of rollers, arranged to engage the cam 12 at different orientations relative to the axis 16 of the cam.

Specifically, a pin 64 is fixedly mounted on the lever 50 and extends parallel to the axes 16 and 22. Mounted on this pin 64 is a first of the cam engagers 60 so that it engages the outer surface 18 of the cam 12. Thus, rotation of the cam 12 causes the lever 50 to pivot about the axis 52. The lever 50 also has a concavely-curved abutment surface 66 which forms a point of contact with the surface 28 of the rocker 20. Thus, when the cam 12 causes the lever 50 to pivot about the axis 52, the lever 50 causes the rocker 20 to pivot about the axis 22 to thereby operate the valves.

The second cam engager 62 is mounted for rotation on a pin 68 which is parallel to the pin 64. The pin 68 is mounted on a carriage 70 which is movable relative to the lever 50. Specifically, the carriage 70 is pivotally mounted on the pin 64.

The illustrative operating mechanism 10 also comprises positioning means 72 operable to alter the relative positions of the cam engagers 60 and 62 to alter which of the cam engagers engages the cam 12, thereby altering the timing of the operation of the valves relative to the rotation of the cam 12. The positioning means 72 comprises a hydraulic piston and cylinder assembly comprising a cylinder 74 in which a piston 78 reciprocates. The assembly 74, 78 is mounted on the lever 50 and has a piston rod 80 projecting from the piston 78 and engaging the carriage 70. The cylinder 74 is arranged to be supplied with hydraulic fluid from the interior of the hollow shaft 24 through a passage 82. The passage 82 communicates with a chamber 84 within the piston 78 and this chamber 84 communicates through a non-return valve 96 with a closed end portion of the cylinder 74.

When hydraulic fluid under pressure is supplied to the interior of the shaft 24, the hydraulic fluid passes through the passage 82 into the chamber 84 and then through the non-return valve 96. This moves the piston 78 so that the piston rod 80 pivots the carriage 70 about the pin 64 (anti-clockwise viewing Figure 1). Rotation of the cam 12, during part of the cycle, tries to force the piston 78 into the cylinder 74 but this is resisted by the substantially incompressible hydraulic fluid in the closed end of the cylinder 74. When hydraulic pressure in the interior of the shaft 24 is removed, a spring (not shown) of the positioning means 72 acts to move the carriage 70 so that the piston 78 is moved into the cylinder 74, the hydraulic fluid gradually leaking past the piston 78 and falling into the sump of the engine from which it can be recycled to the interior of the shaft 24. The spring is a coil spring wrapped around the shaft 24 and having its ends abutting the lever 50 and the carriage 70.

The assembly 74, 78 is arranged to be operated in response to changes in the speed of operation of an engine of which the valves form part. Specifically, when the engine is operating at low speed, the spring holds the cam engager 62 out of contact with the cam 12 and the cam engager 60 moves the lever 50 which, in turn, moves the rocker 20 to operate the valves. Upon the engine speed rising, the assembly 74, 78 operates to pivot the carriage 70 about the pin 64 to press the cam engager 62 into engagement with the cam 12, so that both cam engagers 60 and 62 are in engagement with the cam. This has the effect of holding the valves in their open condition for longer in the cycle of operation.

The illustrative operating mechanism 10 also comprises adjustment means (not shown) operable to move the axis 52 of the lever 50 relative to the axes 16 and 22 of the cam 12 and of the rocker 20 to thereby shift the points of contact between the cam 12 and the cam engagers 60 and 62, and the lever 50 and the rocker 20 to thereby alter the timing of the operation of the valves relative to the rotation of the cam 12.

The adjustment means is operable to turn the shaft 24 about the axis 22 thereby turning the eccentric 54 also about the axis 22. The turning of the eccentric 54 moves the axis 52 arcuately about the axis 22, thereby bodily moving the lever 50 relative to the cam 12 and the rocker 20. The movement of the lever 50 relative to the cam 12 causes the cam engager 60 (and the cam engager 62 if it is in engagement with the cam 12) to roll along the surface 18 of the cam 12 so that the point of contact between the surface 18 and the lever 50 moves to a different point on the surface 18. This means that, as the cam 12 rotates about the axis 16, the times when the valves open and close is altered.

The adjustment means comprises an arm which projects radially from an end portion of the shaft 24, and a hydraulic piston and cylinder assembly which is operable to move the arm and thereby to turn the shaft 24 about its axis 22. The assembly 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 may hold the arm in a first position in which the cam engager 60 is in one position relative to the axis 16 but, at higher speeds, the assembly may move the arm into a second position in which the cam engager 60 is in a second position relative to the axis 16.

Figures 2a to 2d illustrate the opening and closing positions of the valves. In each case, the rotation is clockwise with the top-dead-centre piston position at the top of the diagram.

Figure 2a shows the condition in which the cam engager 60 alone engages the cam 12 and the axis 52 of the lever 50 is in a central position thereof. The point "a" represents the opening time of the inlet valve and the point "b" represents its closing time. The points "c" and "d" represent, respectively, the opening and closing times of the outlet valve (which is operated by a similar mechanism to the mechanism 10).

Figure 2b illustrates the effect of operating the adjustment means to shift the axis 52 of the lever 50.

This advances the points a and b to points a1 and b1 or retards them to points a2 and b2 The points c and d can also be advanced or retarded in a similar way.

Figure 2c illustrates the effect of operating the positioniing means 72 to bring the second cam engager 62 into engagement with the cam 12. This moves the point b to b3 without moving the point a.

Figure 2d illustrates the effect of operating the adjustment means and the positioning means 72. This can, for example, move point a to a4 and point b to point b4, a4 and b4 being further apart than a and b.

Claims (10)

1 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 lever 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 lever having a point of contact with the outer surface of the cam so that, as the cam rotates, the lever is caused to pivot about its axis, the lever also having a point of contact with the rocker so that, as the lever pivots about its axis, the rocker is caused to pivot about its axis to thereby operate the valve, wherein the mechanism also comprises adjustment means operable to move the axis of the lever relative to the axes of the cam and of the rocker to thereby shift the point of contact between the cam and the lever to thereby alter the timing of the operation of the valve relative to the rotation of the cam, the lever also comprising abutment means which provides the point of contact with the cam, and wherein the abutment means comprises a plurality of cam engagers arranged to engage the cam at different orientations relative to the axis of the cam, and positioning means operable to alter the relative positions of the cam engagers on the lever to alter which of the cam engagers engages the cam.

2 An operating mechanism according to claim 1, wherein the lever is mounted on an eccentric and the adjustment means is operable to turn the eccentric about an axis which is parallel to the axis of the cam.

3 An operating mechanism according to claim 2, wherein the eccentric is turned about an axis which is co incident with the axis on which the rocker is mounted.

4 An operating mechanism according to any one of claims

1 to 3, wherein the rocker is adapted to operate a plurality of valves simultaneously.

5 An operating mechanism according to any one of claims

1 to 4, wherein the adjustment means is operable in response to changes in the speed of operation of an engine of which the valve forms part.

6 An operating mechanism according to any one of claims

1 to 5, wherein the said cam engagers comprise a first cam engager which is fixed in position relative to the lever, and at least one further cam engager which is movable relative to the lever by operation of the positioning means.

7 An operating mechanism according to claim 6, wherein said further cam engager is mounted on a carriage which is movable relative to the lever by operation of the positioning means.

8 An operating mechanism according to any one of claims

1 to 7, wherein the cam engagers are provided by rollers.

9 An operating mechanism according to any one of claims

6 to 8, wherein the positioning means is operable in response to changes in the speed of operation of an engine of which the valve forms part.

10 An operating mechanism for a valve substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.