This invention relates to the displacement of a valve of an internal combustion engine with the aid of a cam and a cam follower.

5 Usually the cam follower is externally in the form of a circular cylinder and it slides in a slideway which is of circular internal cross-section and can turn about its axis within the slideway. However, United States Patent Specification No. 3,730,150 (in the name of S.J. 10. Codner) discloses a cam follower of hexagonal shape, as seen when looking in the direction in which it slides, which slides in a slideway of hexagonal cross-section. Although it is not stated in the specification, it is reasonable to suppose that all six sides of each hexagon 15 are of equal length.

According to the present invention there is provided apparatus including a valve of an internal combustion engine,, a cam mounted on a camshaft and a cam follower including a body which is mounted in a guideway 20 and serves to displace the valve as the cam is turned, said body having two opposite side surfaces which are planar and parallel to one another and slide on two opposite parallel and planar surfaces of the guideway, characterised in that externally the body is elongate as

25 seen when looking in the direction in which it slides.

Such a construction is superior to the usual construction and the construction disclosed in the U.S. specification as regards coping with the various stresses occurring in operation of the engine. For equal

30 dimensions, engine speeds, etc, use of the invention, as compared with the prior proposals, reduces the distortion of the cam follower caused by the fact that the area of contact between the cam and the follower moves to an fro across the axis of .said body. Consequently, if the

35 present invention is employed the follower body can be made lighter than the follower body of the prior art if made of the same material.

Preferably, the above-mentioned, body is externally substantially rectangular, i.e. not square but oblong, as seen when looking in the direction in which it slides, but the corners may be rounded. It is also possible to have the external shape of the body, as seen when looking

; in the direction in which it slides, approximately the same as the shape of a conventional athletics track, which has two opposite and parallel straight sides and two approximately semi-circular ends. Preferably the length of the body, as seen when looking in the direction in which it slides, is at least one and a half times, better still at least twice, its width.

Although the body could itself directly contact the cam, preferably the body does not contact the cam but is formed with a trough of part-circular cross-section in which a member in the form of a segment of a circular cylinder is mounted so that its curved surface faces the interior surface of the trough and a planar side surface on the member faces the cam surface. The apparatus is especially, but not exclusively, useful in an engine in which the cam surface of the cam has an outline, in a section plane containing the axis of the camshaft, which is not parallel to that axis and there are means for displacing the camshaft lengthwise in order to change the effect which rotation of the camshaft has on the movement of the valve.

Examples in accordance with the invention are described below with reference to the accompanying drawings, in which:- FIGURE 1 shows a valve of an internal combustion engine and, in section, means for displacing it,

FIGURE 2 shows a cross-section of what is shown in Figure 1, taken as indicated by the arrows X in Figure 1, FIGURE 3 shows a plan view of a cam follower, FIGURE 4 shows a modification of what is shown in Figure 1,

FIGURE 5 shows a control system of the

modification according to Figure 4,

FIGURE 6 shows a view from below of a cam follower body,

FIGURES 7 and 8 show sectional side views of the cam follower body, taken as indicated by the arrows Z and V, respectively, in Figure 6, and show also side views of a member which forms another part of the cam follower,

FIGURE 9 diagrammatically shows a cam follower in a slideway, FIGURE 10 shows a perspective view of a shim, and FIGURE 11 shows a perspective view of a member formed to receive the shim.

Figures 1 to 3 show a casting 10 in which are mounted an overhead camshaft 4, which carries a cam 5 and more such cams, and a piston 7 separated from the camshaft by a thrust race 8. Piston 7 is located for axial movement within a cylinder 9; an inlet channel 18 communicates with that end of cylinder 9 which is remote from camshaft 4. As Figure 1 shows, the cam surface 5a of the cam 5 has an outline, in a section plane containing the axis 4a of the camshaft 4, which is straight but not parallel to that axis 4a. The cam surface 5a makes contact with a member 16 which is exactly half of a circular cylinder, which is seated in a trough 6C in the cam follower body 6 which is called a tappet. The body 6 has no laterally projecting parts which would increase its width as seen in Figure 1; this is in contrast to what is shown in

European Patent Specification No. 0108238 (in the name of Fiat Auto S.p.A) but it could have upwardly projecting parts, as shown in Figure 11. The curved surface of the member 16 faces and slides on the interior surface of the trough, which has a shape corresponding to the surface of a segment of a circular cylinder. The member 16 could be in the form of a segment, less than half, of a circular cylinder. It is free to rotate about an axis 16a (see

Figure 3). At least the flat surface of the member 16. is

formed of a temperature resistant material of extreme, hardness - e.g. jthe ceramic material Syalon. Cam follower body 6 acts against the upper end of stem la of a valve 1 through adjusting shims 17. The valve stem is held within casting 10 by retainers 2 and is urged upwardly by compression springs 3.

Overhead camshaft 4 is driven by a drive gear 14 which is located in casting 10, between thrust washers 15, and is connected to camshaft 4 via a spline 13. A compression spring 11 acts against camshaft 4 via a thrust: race 12, tending to urge the camshaft lengthwise, or axially, to the left as seen in Figure 1.

The sectional view of Figure 1 is taken in a plane containing the axis 4a of camshaft 4 and the axis of movement of valve 1. The axis of rotation 16a of member 16 is perpendicular to the plane of Figure 1.

In use, camshaft 4 is able to undergo axial displacement to a limited extent. During operation of the engine, oil under pressure is supplied via inlet 18 to cylinder 9 and causes axial displacement of camshaft 4 to the right against the action of spring 11. At any instant, the extent to which the cam pushes the body 6 downwardly depends upon the angle through which the cam has turned from a particular position and the extent and direction of the lengthwise movement of the camshaft from a particular position. Hence by selecting a predetermined cam shape, the valve opening point, the duration of lift and extent of lift of the valve can all be controlled by the extent of axial displacement of camshaft 4.

The oil under pressure supplied to inlet 18 may be provided by a pump which is fed from the main engine oil supply by a bleed-off valve which is actuated by a solenoid or by an electronic means which in turn is controlled by a microprocessor. In this way, it is possible to adjust the opening and closing of the valves to engine speed and load requirements, the microprocessor

--S- being programmed to give the desired operation of -the valves.

In the example according to Figure 4, where like reference numerals designate like parts as in Figures 1 to 3, the spring 11 and thrust bearing 12 are no longer required and the piston 7 is replaced by a shorter piston 71 having a piston rod 27 fixedly secured thereto (or integral therewith as illustrated) in a non rotatable manner. Seals 20, 26 are provided for the piston 71 and the rod 27 respectively. A chamber 29 is formed between the seals 20 and 26, and there is a further inlet 19 leading into the chamber 29. A disc 22 is threaded into the cylinder head casting 10 and seals the chamber 29 with an 'o' ring seal 21. The rod 27 is threaded at an end 30 and is screwed into a bearing carrier 23. The rod 27 is locked against rotation by pin 28. The bearing carrier 23 is mounted on the camshaft 4 via a crossed cylindrical roller bearing 81. This enables the camshaft to be moved axially in both directions, while being free to rotate without causing rotation of the piston rod. Circlips 24 and 25 and shoulders in the carrier 23 and on the camshaft 4 retain the bearing 81.

Referring now to Figure 5, a high pressure pump 32 serves to introduce oil from a sump 33 into a cylinder 91, corresponding to cylinder 9 in Figure 1, and into the chamber 29 via a relief valve 34 and pressure gauge 40. Oil can flow from the cylinder 91 and chamber 29 via respective solenoid or "quick-acting" electro- hydraulic servo valves 35, 36 to the sump 33.

The modified mechanism enables the camshaft to be moved axially with a faster response time than the arrangement described above with reference to Figures 1 to 3, enabling it to follow more closely changes in engine speed and load during use. This provides an even more effective electronically controlled engine management system for optimising engine output,

1 emmissions and fuel economy. Control of the axial movement of the camshaft in the modified mechanism is effected by a microprocessor 37, which controls the servovalves 35, 36 to open one or the other to permit oil 5 to flow from cylinder 91 or chamber 29 according to the direction and magnitude of camshaft movement required.. A linear transducer 38 senses the axial movement of the camshaft to provide a feedback signal to the microprocessor 37 to control that movement. The camshaft

10 is driven by a wheel 90 and its speed is sensed by a sensor, consisting of an inductive pick-off 91 near a disc 92 which is formed with radial slots and mounted on the camshaft, and a signal representing the speed of the camshaft is supplied to the microprocessor 37.

15 The cam follower body 6 shown in Figures 1 to 5 is hollow and could be as shown in Figures 6 to 8, i.e. an inverted thin-walled cup with a closed top 97 formed with the trough 6C, an endless rectangular side wall 93 and a central web 94, formed with holes 96 to make it lighter,

20 extending between the longer sides of the rectangle and the lower end of which engages the valve stem, directly or through a shim. If desired, the central web 94 could be made shorter or omitted entirely so that the valve stem la then projects into the cup-shaped body.

25. _. The longer sides of the body 6,- as seen in Figures 3 and 6, are more than one and a half times as long, and in fact more than twice as long, as the shorter sides. The length direction of the elongate outline of- the body 6, which outline is seen when looking in the direction in

30 which the body 6 slides, is perpendicular to a plane which contains the axis of the camshaft 4 and the centre-line of the body 6, along which centre-line the body 6 slides; this plane is the plane of the^ paper in Figures 1 and 4. The length direction- of the

35 above-mentioned outline is also parallel to a tangent to the surface 5a of the cam where that surface touches the member 16. The area of contact between the cam surface

5a and the member 16 runs, along- *hβ- member 16 and parallel to the length of the above-mentioned contour of the body 6, i.e. horizontally, considering Figures 3 and 6, when the camshaft is not moving lengthwise Preferably, the slideway for the body 6 has the two opposite and parallel planar guide surfaces 10B and IOC, and, as shown in Figure 9, two more opposite and planar guide surfaces 10D and 10E and recesses 10F at the corners of the rectangular shape formed by these guide surfaces.

Instead of providing the shim 17, chosen from a set of such shims of differing thicknesses so as to provide the correct valve clearance, a shim 95 as shown in Figure 10 may be placed on the member 16 to achieve the same purpose. The shim consists of an oblong flat plate portion 95A, and the shims of a set of such shims, from which the illustrated shim is selected, will have differing thicknesses of this plate portion, and two flanges 95B depending from its longer sides. The member 16 then has upward projections 16K at its ends to retain the selected shim. At least the upper surfaces of the plate portions 95 of the shims are hard, to resist wear.

The member 16 could be made taller, considering

Figures 1, 2, 4, 7 and 8, than is shown in those Figures. Above a part which has the shape of half of a circular cylinder, there could be an upwardly-extending part. This could be simply a parallelepiped, so that the width of its upper surface is the same as the diameter of the half-cylinder. Alternatively, it could be a part the width of which is greater at the top than the diameter of the half-cylinder, for example a portion in the form of a parallelepiped having a width equal to the diameter of the half-cylinder , above that another portion in the form of a parallelepiped having a width greater than the diameter of the half-cylinder and, between the two parallelepipeds, a transition portion with curved or straight sides, the whole member being symmetrical about

a "Central plane containing the axis- of the ^" half-cylinder, and about a central plane perpendicular to the axis of the .half-cylinder, both these planes being vertical, considering Figures 7 and 8. In all cases the cam could directly engage the upper surface of the member 16, which is then made hard, or a shim could be fitted over the upper surface of the member 16, for example a shim as shown in Figure 10.

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