Field of the invention

The present invention relates to a variable valve timing mechanism for an internal combustion engine.

Background of the invention

It is known to incorporate a lost motion coupling within the valve train of an internal combustion engine. The effect of such a lost motion coupling is that when the cam is being rotated in the direction to open the valve all the free play in the drive train is taken up and the valves always open at preset crank angles. However, while a valve is closing, because of the presence of free play or lost motion, the reaction force of the valve spring can accelerate the cam or the entire camshaft to make it lead the crank shaft and result in the valve closing at an earlier crank angle. The extent to which the valve closing is advanced in this manner will vary with the length of time that the reaction force acts to accelerate the cam or camshaft and consequently the degree of advance will automatically reduce with increasing engine speed. The free play is taken up again when the cam meets resistance in opening the valve in following engine cycle.

Such a drive mechanism with built in free play is not practical because of the high impact shocks that cause severe noise and wear problems. Spring biased lost motion couplings have therefore been proposed to moderate the acceleration of the cams and to take up the free play gently while the valve is on the base circle of the cam.

To this end, a coil spring has been used as the coupling member that jams against a pad when it is unwound and takes up the lost motion by winding up. Though this construction

reduced the noise problem significantly, impact shock was still excessive causing the spring the fracture.

A system using hydraulic damping has also been proposed in the prior art but such a system is complex and costly to implement.

Object of the invention

The present invention seeks to provide a variable valve timing mechanism which introduces lost motion into the valve train but which mitigates noise nd wear problems.

Summary of the invention

According to the present invention, there is provided a drive mechanism for transmitting driving torque from the crankshaft to a camshaft of an internal combustion engine and incorporating means for permitting a limited degree of rotation of one of the shafts relative to the other, the drive mechanism comprising concentric drive and driven members connectible respectively to the crankshaft and the camshaft, and a lever pivotably supported on one of the members and having a force transmitting surface that is biassed by a spring to remain in contact at all times with a reaction surface on the other member, the position of the point of contact between the two surfaces being dependent upon the relative angular position of the drive and driven members and being operative to move progressively to vary the mechanical advantage of the lever as the shafts rotate relative to one another under the action of the torque reversals on the camshaft.

In the present invention, the spring does not act directly between the drive and driven members but does so through a variable ratio lever. Under direct drive conditions (when

all the lost motion is taken up) , the point of contact between the drive surface of the lever and the reaction surface on the other member is nearly coincident with the pivot point of the lever, resulting in the effective lever arms having a very large ratio and increasing the drive torque transmitted. On the other hand, when the torque on the camshaft is reversed by a valve spring acting on a cam, the point of contact moves away from the pivot point of the lever, first reducing the transmitted torque and then allowing the reverse torque to accelerate the camshaft and make it lead the crankshaft within the permissible limited lost motion. When the reverse torque ceases, through the valve returning to the base circle of its drive cam, then the spring biassed lever will pivot to take up the lost motion gradually and progressively while remaining in contact at all times with the reaction surface. In this way, noise and wear are reduced and impact shocks are avoided.

For better balance and to reduce the forces transmitted at each contact point, it is preferred to use two levers, symmetrically positioned about the axis of rotation of the drive mechanism.

Advantageously, the surface of the lever and the reaction surface with which it makes contact are profiled in such a manner that the surface of the lever rolls without slipping on the reaction surface. This avoids wear due to friction and reduces the lubrication requirements of the mechanism.

The strength of the valve springs and the friction on the valve train determine the applied torque to effect the phase change and the rate of change of the phase during such torque reversal will also depend upon the moment of inertia of the drive mechanism. The amplitude of the phase change, i.e. the extent to which the duration of the valve event is reduced, further depends on the time that the torque acts,

in other words upon the engine speed. Of the above parameters, the valve springs and the moment of inertia are constant and the extent of variation will vary only and automatically with engine speed, assuming no change in friction. It is possible to apply a variable drag between the drive and driven members, for example by the use of a magnetic clutch, to reduce the phase change occurring at any given engine speed.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a transverse section through a drive mechanism, taken along the section line I-I in Figure 2, and

Figure 2 is an axial section through the drive mechanism, taken along the section line II-II in Figure 1.

Description of the preferred embodiment

A drive mechanism comprises a driven member 10 connected to the camshaft 12 of an engine, and a drive member 14 driven through a belt 16 by the engine crankshaft. The drive member 14 is journalled about the camshaft 12 and can rotate relative to it. The coupling between the drive and driven members 14 and 10 is effected by means of pins 18 in the driven member 10 about which there are pivoted two levers 20 that are both biassed to rotate in a clockwise direction as viewed in Figure 1 by helical springs 22. The pins 18 and levers 20 are located within a deep recess in the drive member 14 bounded by reaction surfaces 24 against which the levers 20 are urged by the springs 22.

An electromagnetic clutch 30 is included to apply a controllable drag between the drive and driven members 14 and 10. A disk 32 is formed in close proximity to the drive member 14 and fixed for rotation with the camshaft 12 and an electromagnet 34 powered by a suitable control circuit 36 can apply a variable magnetic field to a thin film of magnetic fluid 38 disposed between the disk 32 and the drive member 14.

When torque is being applied to drive the camshaft in the normal direction, the mechanism adopts the relative position shown in solid lines in Figure 1 in which the point of contact between the lever 20 and the reaction surface 24 is immediately adjacent the pins 18. This is effectively a direct drive position in which the springs 22 takes no part in the torque transmission path.

When a cam on the camshaft 12 rotates beyond its peak position and the spring of the valve that was opened by the cam acts against the closing profile of the cam, the camshaft is subjected to a torque reversal acting to accelerate the camshaft 12 to make it to rotate faster than the drive member 14.

The pins 18 now move away from the reaction surfaces 24. The positions of the reaction surfaces 24 relative to the levers 20 during this torque reversal are shown by dashed lines in Figure 1 from which it would be seen that the phase of the camshaft has been allowed to advance to cause an earlier closing of the valve. At the same time the points of contact between the levers 20 and the two reaction surfaces 24 move progressively away from the pins 18 towards the other ends of the levers 20. The compression springs 22 act to maintain the levers 20 in permanent contact with the reaction surfaces 24. The relative movement between the drive and driven members 14, 10 can cause the springs 22 to

be compressed further or less during torque reversal. If the springs 22 are compressed further, then the extent of phase advance is reduced. Conversely, if the geometry is such that the springs 22 expand during the torque reversal, then the springs 22 will act to enhance the torque reversal and increase the extent of phase advance. Clearly it is also possible to design the contacting surfaces in such a manner that the springs 22 do not change length appreciably and act merely to maintain contact between the levers 20 and the reaction surfaces 24.

When the cam in due course rotates to a position in which the valve is aligned with the base circle of the cam then the reverse torque would cease and the forward torque will act to take up the lost motion and return the drive and driven members to the relative position shown in Figure 1 in solid lines. In the present invention the lost motion is taken up by the gradual and progressive movement of the point of contact between the levers 20 and the reaction surfaces 24. This movement takes place without noise, wear or shock.

The purpose of the clutch 30 is to apply a variable drag between the drive and driven members 14 and 10 to moderate the extent of phase change at any given engine speed. The illustrated embodiment incorporates a position sensor 40 that provides a position feedback signal to the control circuit 36 so that the phase change can be accurately closed loop controlled.

Though the illustrated embodiment has a curved lever 20 resting against a flat reaction surface 24, both surface may ^¬ be curved or the reaction surfaces alone may be curved. By suitable selection of the geometry, it is possible to achieve rolling contact rather than sliding contact at all times, thereby reducing wear caused by friction and alleviating the difficulty of lubricating the mechanism.