GB2247061A - Variable event valve timing - Google Patents

Abstract

In an internal combustion engine in which variable event valve timing is achieved by the engine having a camshaft, cams mounted on the camshaft and capable of rotating relative to the camshaft through a limited angle to each side of a central position and springs for applying a return force to the respective cams to urge the cams into their central position, the magnitude of the spring force acting on a cam varying as a predetermined function of the angular displacement of the cam from its central position, the cams 14,16 are formed integrally with collars 18, each collar 18 having an arcuate recess 20 in its axial surface engaged by an arcuate projection 22 on a ring 24 slidable on, but fast in rotation with, the camshaft 10, the ring 24 being urged by a helical spring 26 located about the camshaft in the direction of the collar 18. <IMAGE>

Description

VARIABLE EVENT VALVE TINING The present invention relates to an internal combustion engine in which variable event valve timing is achieved by the engine having a camshaft, cams mounted on the camshaft and capable of rotating relative to the camshaft through a limited angle to each side of a central position and springs for applying a return force to the respective cams to urge the cams into their central position, the magnitude of the spring force acting on a cam varying as a function of the angular displacement of the cam from its central position.

Such an internal combustion engine is known from German Offenlegungsschrift 26 47 332. In this publication, the valve cams are formed as lobed rings which can rotate backwards and forwards on the camshaft to advance and retard the valve timing. The camshaft has a radial recess containing a sprung plunger with a cam shaped end to work with the cam ring inner surface. The plunger is lifted in proportion to engine speed variations. A pressure chamber formed in the camshaft recess has a lubricating oil pressure inlet and outlet to control the plunger lift. The outlet height is varied by an engine speed dependent peg. At low engine speed the plunger face is forced downwards by the force from the rocker arm action on the cam ring lobe and the cam action is retarded.

The above proposal uses hydraulic fluid in the plunger to limit the maximum plunger displacement. The fluid acts as a hydraulic damper and this has a desirable effect in that a relatively weak spring suffices to act on the plunger.

However, the attendant disadvantages are that there will be pumping losses tending to reduce valve train efficiency and naturally the provision of several sealed plungers, a pump and a control system for the speed dependent pegs all add significantly to manufacturing cost.

The present Invention therefore seeks to provide an internal combustion engine of type described above which can be constructed less expensively and can operate with less energy loss.

According to the present invention, there is provided an internal combustion engine having a camshaft, cams mounted on the camshaft and capable of rotating relative to the camshaft through a limited angle to each side of a central position and springs for applying a return force to the respective cams to urge the cams into their central position, the magnitude of the spring force acting on a cam varying as a function of the angular displacement of the cam from its central position, characterised in that the cams are formed integrally with collars, each collar having an arcuate recess in its axial surface engaged by an arcuate projection on a ring slidable on, but fast in rotation with, the camshaft, the ring being urged by a helical spring located about the camshaft in the direction of the collar.

By virtue of this configuration, it is possible to use a stronger spring than in the case of DE-OS-26 47 332 and it is possible in this way to absorb the energy of the valve springs before they come to a stop without the need for hydraulic damping. The energy of the valve springs is stored in the compressed springs surrounding the camshaft and is returned to the camshaft after the valve has closed so that the efficiency of the valve train is not reduced.

If one were to rely exclusively on the springs to limit the angular displacement of the cams, there would be variations caused by differences in spring rate and frictional resistance which would make it difficult to predict the calibration of the valve event.

In order to enable the valve timing to be set accurately, it is preferred to provide abutment surfaces for mecnonically limiting the maximum angular displacement of each cam relative to the camshaft before the entire energy imparted to the cam by the valve spring is absorbed by the axial spring, the abutment surfaces for all the engine cylinders being movable in unison as a function of an engine operating parameter, such as speed.

If desired, the cams may be lobed on their internal surfaces and may be engaged by spring biased plungers as in the case of DE-OS-26 47 332 in order to supplement the force of the axial springs or to form part of the means defining the abutment surfaces required for mechanically limiting the maximum angular displacement of the individual cams relative to the camshaft.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side view of variable valve timing mechanism constructed in accordance with a first embodiment of the invention, Figure 2 is a section on the line A-A in Figure 1, Figure 3 is a section on the line B-B in Figure 1, Figure 4 is a longitudinal section along the the axis of the camshaft in Figure 1, Figure 5 is a longitudinal section similar to that of Figure 4 through an alternative embodiment of the invention.

In Figure 1, a camshaft 10 rotatable in the usual manner by the engine crankshaft has rotatably mounted on it a cam sleeve 12 with two cams 14 and 16 for operating respective inlet valves of the same cylinder of a multi valve engine. The invention is also capable of being used for engines with a single inlet valve per cylinder.

The cam sleeve 12 has an integral collar 18 formed with a shaped recess 20 in its axial end surface. The recess 20 is engaged by a shaped projection 22 on a ring 24 which is free to slide along the length of the camshaft 10 but is fast in rotation with it. The ring 24 is urged to the left as viewed by an axial spring 26 which surrounds the camshaft 10 and is trapped between the ring 24 and a stop consituted by a circlip 28 and an annular retainer disc 30.

During engine operation, as the camshaft rotates, the cam sleeve will encounter a resistance from the valve springs tending to rotate the cam sleeve 12 relative to the camshaft 10. In so doing, the projection 22 slides along one side of the recess 20 because the ring 24 cannot rotate on the camshaft 10, which results in the spring 26 being compressed. Therefore, the cams 14 and 16 will be arrested while the camshaft rotates thereby retarding the opening of the valves.

On the closing flank of the cams 14 and 16, the effect of the valve springs will be to tend to cause the cam sleeve to rotate faster than the camshaft 10 and the compression of the spring 26 will also act in the direction to centre the cam sleeve on the camshaft 10. Therefore the closing of the valve will be advanced and the event duration will have been reduced by the rocking of the cam sleeve 12 about the camshaft 10.

If desired, the centring force exerted by the spring 26 can be supplemented by a spring movable radially. In Figure 3, it is seen that the cam sleeve 12 may have a lobed inner surface 32 engages by a plunger 34 under the action of a radial spring 36. However, the provision of such a supplementary spring is not deemed necessary in most circumstances and it would in any event be preferable to redesign the engine to permit a second collar, similar to the collar 18, to be formed on the other end of the cam sleeve 12.

The total angular displacement of the cam sleeve 12, if limited only by the spring 26 would result in unpredicatable variation of the event timing and duration with engine speed and engine temperature. It is desirable therefore to include mechanical abutments to limit the extent to which the cam sleeve 12 can rock about the camshaft 10 and this can be achieved in a variety of ways.

The embodiment of Figures 1 to 4 is an example of one way of limiting the rocking movement of the cam sleeve 12. As shown in Figures 2 and 4, the cam sleeve is provided on its inner surface with a recess 40 in which engages a tapered plumger 42. The plunger 42 has a weak return spring 44 and is not intended to absorb energy from the valve spring in the same way as the plunger 34 shown in Figure 3. As the cam sleeve 12 rocks about the camshaft 10, the plunger 42 is depressed into the camshaft and if movement of the plunger 42 is limited, then the cam sleeve 12 will also be limited in the extent to which it can rock about the camshaft 10.

To limit the extent of the rocking motion and therefore the extent of the collapse of the duration of the valve event, an axially slidable ramp 50, better shown in Figure 4 is located beneath the plunger 42. The plunger can only move radially until it abuts an end stop formed by a follower 52 which slides along the ramp 50. The same control shaft is used to move all the ramps 50 associated with the different engine cylinders and in this way accurate determination of the valve timing and valve event duration can be achieved.

The bottoming of the plunger 42, in order to be 'ive, should take place before all the energy of the cams 14, 16 is absorbed by compression of the axial spring 26 but in order to avoid wear and noise, the cams 14 and 16 should be moving as slowly as possible relative to the camshaft 10 at the time that they are brought to a stop.

Figure 5 shows an alternative embodiment in which the cams 14 and 16 are arrested by limiting the axial displacement of the ring 24. In this case, the modified ring 24' has near its end a tapering surface 60. As the ring 24' slides to the right as viewed, it depressed a plunger 62 which is generally analogous to the previously described plunger 42. This plunger cooperates with a follower 64 and a ramp 70in the same manner as described previously to limit the extent to which the ring can move axially.

As so far described, the cam advance and retard is always effected against the constant return force of the spring 26 but if desired, by axially adjusting the position of the abutment surface formed by the disc 30, it is possible to modify the extent of advance and retard at any given engine speed.

The axial adjustment of the shaft on which the various ramps 50 or 70 are mounted may be effected by any desired control mechanism responsive to speed and/or load. If the adjustment need only be speed dependent, then a mechanical governor with centrifugal fly weights would suffice to achieve the necessary positioning of the ramps.

Claims (8)

1. An internal combustion engine having a camshaft, cams mounted on the camshaft and capable of rotating relative to the camshaft through a limited angle to each side of a central position and springs for applying a return force to the respective cams to urge the cams into their central position, the magnitude of the spring force acting on a cam varying as a predetermined function of the angular displacement of the cam from its central position, characterised in that the cams are formed integrally with collars, each collar having an arcuate recess in its axial surface engaged by an arcuate projection on a ring slidable on, but fast in rotation with, the camshaft, the ring being urged by a helical spring located about the camshaft in the direction of the collar.

2. An internal combustion engine as claimed in claim 1, wherein abutment surfaces are provided for mechanically limiting the maximum angular displacement of the individual cams relative to the camshaft, which abutment surfaces are movable in unison as a function of an engine operating parameter.

3. An internal combustion engine as claimed in claim 1 or 2, wherein the cams are lobed on their internal surface and are engaged by spring biased plungers in order to supplement the force of the axial spring.

4. An internal combustion engine as claimed in any preceding claim, wherein the abutment surfaces for mechanically limiting the maximum angular displacement of the individual cams relative to the camshaft are formed on a shaft located concentrically within camshaft and movable axially along the camshaft as a function of the engine operating parameter.

5. An internal combustion engine as claimed in claim 4 when appended to claim 3, wherein the abutment surfaces are constituted by ramps engaged by the radial plunger to limit the radial displacement of the plunger and hence the angular displacement of cams on the camshaft.

6. An internal combustion engine as claimed in claim 4 as appended to claim 2, wherein the abutment surfaces serve to limit the displacement of the rings slidable along the camshaft.

7. An internal combustion engine as claimed in claim 4, wherein the abutment surfaces project radially from the axially displaceable shaft and cooperate with the sides of profiled recesses formed on the inner surface of the cams to limit the angular displacement of the cams.

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.