GB2245647A - Variable i.c. engine valve timing - Google Patents

Abstract

A cam follower 32 has a cylindrically concave surface 34 facing towards and concentric with the axis of a cam 30 and a profiled element 36 has a matching cylindrically convex surface 38 transversely slidable along the concave surface of the cam follower, whereby the point of contact of the cam with the profiled element may be moved around the surface of the cam without affecting the radial distance of the cam follower from the axis of the cam when the profiled element is on the base circle of the cam. The extent of transverse movement of the element 36 by the cam 30 is determined by stops 42, 46 having relatively movable parts (42a, 42b, Fig. 2) spring-biased to maintain contact with a roller (44) carried by element or located by ball joints (160, Fig. 4) at the ends of the element. The stops 42, 45 may be positioned dependent upon engine speed and load. <IMAGE>

Description

VARIABLE VALVE TIMING The present invention relates to valve timing in an internal combustion engine.

It is well known that the performance of an internal combustion engine is affected by the duration and the timing of the intake and exhaust events. The relative phase and the event duration both have an optimum setting which varies with the operating conditions of the engine.

The duration of a valve opening event in a conventional engine is determined exclusively by the shape of the cam operating the valve. The selection of the cam profile is inevitably a compromise as a large event is desirable for power at high speed and a short event is preferable for improved economy and power at low speed.

In order to enable the event duration to be varied during operation, it has already been proposed to incorporate two valve trains including cams of different profile and to switch between valve trains in dependence upon the operating conditions. Such a system is clearly expensive in view of the duplication of components and also increases the frictional losses in the valve operating mechanism.

It has also been proposed to make the cams of variable profile. One way of achieving this aim is to form a three dimensional cam having a profile which varies along the axis of the cam, the cam being moved axially as a function of the engine operating conditions. The disadvantage of this proposal resides in the difficulty of manufacturing the three dimensional cams and the fact that a pivoting follower must be provided to achieve line contact with the cam under all conditions.

A still further proposal has been to torm the cams of relatively movable parts so that the cam profile may be varied by phase shifting of the cam components relative to one another. Such a construction is once again difficult and costly to manufacture and places limitations on the minimum and maximum durations on the valve event.

A still further known proposal for achieving variable event duration is to use a cam of fixed profile but to rotate the cam with non-uniform angular velocity. Such systems have high friction under part throttle which detracts from the advantages to be gained from variable event timing.

According to the present invention, there is provided an internal combustion engine comprising a poppet valve, a cam acting on the valve by way of a cam follower to open and close the valve and a profiled element movable transversely relative to the cam follower and interposed between the cam follower and the cam, the valve position at any instant being dependent upon both the angular position of the cam and the lateral position of the profiled element relative to the cam and the cam follower.

Preferably, the cam follower has a cylindrically concave surface facing towards and concentric with the axis of the cam and the profiled element has a matching cylindrically convex surface slidable along the concave surface of the cam follower, whereby the point of contact of the cam with the profiled element may be moved around the surface of the cam without affecting the radial distance of the cam follower from the axis of the cam when the profiled element is on the base circle of the cam.

The profiled element thus provides a means of altering the phasing of the valve timing inasmuch as the position of the element determines the angular position of the cam at which the valve will open and close.

In the preferred embodiment of the invention, X profiled element is not static but movable during the valve opening event in synchronism with the movement of the cam thereby also permitting the event duration to be modified. It will be appreciated that if the profiled element is positioned to retard the valve opening at the start of the event and to advance the valve opening at the end of the event, then the event duration will have been reduced.

A variety of mechanisms may be employed to effect the synchronised transverse or lateral movement of the profiled element. Preferably, each profiled element is positioned between a pair of pivotable guides mounted on or driven by separately controlled shafts. In a multicylinder engine, all the guides may be mounted on the common shafts to ensure that balance is maintained between engine cylinders.

In order to avoid any loss of lift height, it is preferred to position the profiled element centrally of the cam when the valve is fully opened.

Because of the shape of the profiled element and the cam, it will tend to move laterally in one direction while the valve is being opened and in the opposite direction while the engine is being closed. The guides are not therefore required to move the profiled element but only to limit its displacement under the action of the cam. The guides are thus effectively variable end stops and the force requires for moving the profiled elements is derived from the cam itself.

In view of this method of operation, the profiled elements may impact against the guides during operation and for this reason it is preferred to design the guides to afford a degree of cushioning to prevent wear. Thus it is advantageous for the guide to be formed in two parts, one being the variable stop and the other being biased to follow the lateral movement of vrle profiled element, damping means acting between the two parts of the guide.

The damping means may be a layer of oil or a spring acting between the two parts.

At the start of a valve event, the profiled element must rest against one of the guides and if not already in that position would, as earlier described be urged into that position by the cam. To avoid possible noise from such an abrupt impact, it would be possible to ensure that the profiled element is constantly spring biased towards the opening guide and this may be ensured by including springs in both guides and arranging for one of the springs to be stiffer than the other.

When referring in the present specification to a cam follower, it should be made clear that this term is intended to include any device, for example a rocker or a bucket, which is interposed between the cam and the valve stem to transfer movement of the cam surface to the valve.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of part of the cylinder head of an internal combustion engine showing a valve, a cam and a cam follower in the form of a rocker.

Figures 2a, 2b and 2c are partial schematic views showing the profiled element in different lateral positions for the embodiment of Figure 2, and Figure 3 is a view similar to that of Figure 1 showing an engine with a bucket cam follower, Figure 4 is a view similar to that of Figure 2a showing an alternative design of the profiled element.

The engine to which the invention may be apps may generally be conventional and will not be described in detail. The invention is concerned with the valve train and only the valve train for one of the poppet valves will be described in detail. It is envisaged that the modified valve train will be used for the inlet valves but it can also or alternatively be used for the exhaust valves In Figure 1, a poppet valve 10 seals against valve seat 12 in a cylinder head 14 and its stem 16 is guided by a valve guide 18. The part of the stem 16 projecting from the valve guide 18 is fitted with a valve spring 20 held in place by a spring retainer 22 attached by collets (not shown) to the valve stem 16.

The overhead cam 30 acts on the valve 10 by way of a rocker 32 which is modified to enable its geometry to be varied during the course of an operating cycle. The side of the rocker 32 facing the cam 30 has a cylindrical recess 34 centred on the rotational axis of the cam 30. A profiled element 36, having a matching convex surface 38, sits within the recess and its opposite surface 40 contacts the cam 30. Guides and stops 42 and 46 mounted on the cylinder head 14 straddle a roller 44 projecting from profiled element 36 to determine the limits of lateral movement of the profiled element 36 during operation.

Under high speed operating conditions, the guides and stops 42 and 46 are set in the position shown in Figure 1 and cannot move. The roller 44 can only move up and down between the stops 42 and 46 and the profiled element remains at all times in its central position in the recess 34. The opening and closing times of the valves under these conditions are dictated solely by the cam profile.

Because of the shape of the surface 40, the overall thickness of the rocker while in contact with the base circle of the cam does not depend on the lateral position of the profiled element. Thus it is possible to use a hydraulic tappet 50, as illustrated schematically to take up play in the valve train.

Turning now to Figures 2a, 2b and 2c, it will be seen that each guide and stop is formed to two elements. One element 42a, 46a is a pivoted spring biased guide which permanently remains in contact with the roller 44 and the other element is a stop which sets the maximum lateral displacement of the profiled element 36 and is adjusted in dependence upon the engine speed and load. A mechanism (not shown) is connected to all the stops so that all cylinders may be adjusted at one and the same time.

Assuming that the stops 42b and 46b are set as shown in Figures 2a to 2c to permit lateral movement of the profiled element 36 then the profiled element will move as the cam rotates clockwise taking up the position shown in Figure 2a at the commencement of valve opening and the position shown in Figure 2c at the end of the event. With the profiled element in the position shown in Figure 2a, the opening of the valve is delayed. The profiled element is moved into this position either by the nett force of springs 60 and 62 acting on it or by the action of the cam itself. Once the cam has passed the dead centre position, it will urge the profile element 36 in the opposite direction towards the position illustrated in Figure 2c passing temporarily through the position illustrated in Figure 2b. With the profiled element in this position, the valve will close earlier.

Thus the invention allows the duration of the event to be shortened at one or both ends at will by appropriate setting of the position of the stops 42b and 46b. The fact that the elements 42 are in two parts, one stationary and the other movable with the profiled element 36 permits wear and noise to be cut down by allowing the movement of the profiled element 36 to be damped.

The embodiment of Figure j differs from that of Figure 1 in that the rocker 32 has been replaced by a bucket 132 and the guides and stops 142, 146 instead of acting on a roller projecting from the profiled element act directly on the ends of the element 136. The operation of this embodiment is otherwise identical with that previously described.

Figure 4 shows schematically that the ends of he profiled element 132' may be fitted with shoes 160 which are connected by a ball joint to the profiled element 132' and which can slide in grooves machined in the guide elements 162, 164. This provides for better control over the movement of the profiled element and improves durability.

The embodiment of Figure 4 is preferred in that the mechanism for adjusting the position of the stops can be implemented readily without the need for a complicated linkage to avoid interfering with the valve mechanism.

However, in the embodiment of Figure 1 it is still possible by the use of gears or levers to effect simultaneous adjustment of all the stops by the use of a common adjusting mechanism.

Claims (8)

1. An internal combustion engine comprising a poppet valve, a cam acting on the valve by way of a cam follower to open and close the valve and a profiled element movable transversely relative to the cam follower and interposed between the cam follower and the cam, the valve position at any instant being dependent upon both the angular position of the cam and the lateral position of the profiled element relative to the cam and the cam follower.

2. An engine as claimed in claim 1, wherein the cam follower has a cylindrically concave surface facing towards and concentric with the axis of the cam and the profiled element has a matching cylindrically convex surface slidable along the concave surface of the cam follower, whereby the point of contact of the cam with the profiled element may be moved around the surface of the cam without affecting the radial distance of the cam follower from the axis of the cam when the profiled element is on the base circle of the cam.

3. An engine as claimed in claim 2, wherein the profiled element is movable during the valve opening event in synchronism with the movement of the cam thereby permitting the event duration to be modified.

4. An engine as claimed in claim 3, wherein each profiled element is positioned between a pair of pivotable guides mounted on or driven by separately controlled shafts.

5. An engine as claimed in claim 4, having a plurality of cylinder and wherein all the guides are controlled by common shafts to ensure that balance is maintained between engine cylinders.

6. An engine as claimed in claim 3, 4 or 5, wherein in order to avoid any loss of lift height, the profiled element is positioned centrally of the cam when the valve is fully opened.

7. An engine as claimed in claim 4, 5 or 6, wherein each guide is formed in two parts, one being a variable stop and the other being biased to follow the lateral movement of the profiled element, damping means being provided to act between the two parts of the guide.

8. An internal combustion engine constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.