GB2244783A - Phase changing mechanism - Google Patents

Abstract

A phase changing mechanism for use in an internal combustion engine for varying the phasing of a camshaft being driven through a continuous belt 16 by a drive shaft, in which engine the belt 16 passes over two movable idler pulleys 18, 20 which enable the geometry of the belt 16 to be varied to alter the lengths of the two runs of the belt extending between toothed drive pulleys 14, 10a on the drive shaft and the camshaft. The mechanism comprises a movable carrier 22 on which at least one of the idler pulleys 18, 20 is mounted, a freewheeling element 30 mounted for rotation in only one direction about an axis 32 which is fixed relative to the drive shaft and the camshaft, a bearing 28 mounted on the freewheeling element 30 and arranged eccentrically with respect to the fixed axis 32, and a connecting rod 26 journalled about the bearing 28 and coupled at its other end to the carrier 22. <IMAGE>

Description

PHASE CHANGING MECHANISM The present invention relates to a phase changing mechanism for use in internal combustion engines.

The optimum timing of the opening of the inlet valves and the exhaust valves relative to one another and relative to the position of the crankshaft varies with engine speed and load. Any system with a fixed valve timing must necessarily provide a compromise between low speed refinement and high speed performance and it is preferable to be able to alter the phasing of the inlet and exhaust valve during engine operation.

A system has already been proposed for varying the phasing of a shaft being driven through a continuous belt by a drive shaft, in which system the belt passes over two movable idler pulleys which enable the geometry of the belt to be varied to alter the lengths of the two runs of the belt extending between toothed drive pulleys of the two shafts. In this context, it should be made clear that the term belt is intended to include a chain and that the term pulleys includes sprockets.

If the length of the run of the belt in tension is varied by one of the idler pulleys, the phasing of the two shafts is altered. The other idler pulley is required to take up the slack in the other run of the belt as the overall length of the belt is of course constant. The second idler pulley is in effect a belt tensioner and may be a conventional spring biased pulley but in view of the extent of movement necessary it is in practice preferred to move both the pulleys in synchronism in order to maintain constant belt tension.

Ideally, the phasing of the inlet and exhaust valves should be varied as a continuous function of engine speed and load. In practice, however, a simplification in the control system is achieved if the phasing is switchable between two fixed timing positions. The ability to select between two fixed calibrations still provides a significant improvement in engine performance.

An actuator is in any event needed to move at least one of the idler pulleys in a direction transverse to the run of the belt extending between the two toothed drive pulleys.

The movable idler pulley or pulleys are mounted on a normally stationary carrier and it is necessary for the actuator to displace the carrier in dependence upon engine speed and load.

Conventionally, a hydraulic motor or an electric motor has been used as an actuator to move the carrier. This increases the cost and complexity of the variable valve phasing mechanism. Furthermore, the motor acts as a power drain since it must generate considerable force in order to move the camshaft against the resistance offered by friction and the valve springs.

With a view to mitigating the foregoing disadvantages, the present invention provides a phase changing mechanism for use in an internal combustion engine for varying the phasing of a camshaft being driven through a continuous belt by a drive shaft, in which engine the belt passes over two movable idler pulleys which enable the geometry of the belt to be varied to alter the lengths of the two runs of the belt extending between toothed drive pulleys on the drive shaft and the camshaft, the mechanism comprising a movable carrier on which at least one of the idler pulleys is mounted, a freewheeling element mounted for rotation in only one direction about an axis which is fixed relative to the drive shaft and the camshaft, a bearing mounted on the freewheeling element and arranged eccentrically with respect to the fixed axis, and a connecting rod journalled about the bearing and coupled at its other end to the carrier.

The invention relies on the fact that the torque reversals on the camshaft will tend to vibrate the carrier. Any conventional actuator must be designed to resist these vibrations and apply enough force to rotate the camshaft.

In the present invention, however, the required movement of the carrier is achieved by resisting the vibration forces in one direction but not in the other, this being achieved by virtue of the fact that the freewheeling member is only allowed to rotate in one direction. Assuming that the eccentric bearing is not in a dead centre position, that is to say a position in which the centre of the eccentric bearing and the fixed axis are not lined up in the direction of the force acting along the connecting rod, the vibrations tend to make the freewheeling element oscillate about its fixed axis but if resistance is offered to rotation in only one direction, for example by the use of a one-way (over-running) clutch or ratchet, then the element will rotate by increments until it reaches the next dead centre position.Thus the freewheeling elements has only two stable positions, each corresponding to one of the dead centre positions of the eccentric bearing and a particular position of the carrier corresponding to a given phase. If the freewheeling element is mechanically displaced from a dead centre position, it will be rotated by the vibrations of the carrier until it reaches its other stable position and it will remain there until such time as it is again mechanically displaced. This therefore provides a very simple actuating mechanism requiring no prime mover and no complicated control mechanism. At the dead centre position, the force required to move the freewheeling element out of equilibrium is small and therefore a simple solenoid can be employed as a trigger to initiate a change in phase.

The connecting rod can be directly connected to the carrier or by way of a lever linkage providing a mechanical advantage.

If desired a plurality of connecting arms and freewheeling elements may be connected in series using a lever linkage in order to provide more than two stable positions. The total number of stable positions in such a case will be 2n, where n is the number of elements connecting in series.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a phase changing mechanism in accordance with a first embodiment of the invention, Figure 2 is a partial section taken on the line II-II in Figure 1, Figure 3 is a view similar to that of Figure 1 of an embodiment in which the connecting rod is indirectly connected to the idler pulley carrier, and Figure 4 is a view similar to that of Figures 1 and 2 of an embodiment in which two elements are connecting in series to provide a total of four possible stable positions of the carrier.

Figure 1 shows an embodiment in which the phase of the exhaust camshaft relative to the crankshaft is fixed but the phase of the intake camshaft is variable. To achieve this, the exhaust camshaft pulley 10 is driven from the crankshaft by a timing belt 12 of fixed geometry. The pulley on the crankshaft (not shown) is of half the circumference of the pulley 10 on the camshaft so that the camshaft rotates at half the crankshaft speed.

The exhaust camshaft also carries a drive pulley 10a which serves to transmit drive to the intake camshaft. The latter carries a pulley 14 of the same size as the pulley 10a and driven by a belt 16 which passes around the two pulleys l0a and 14 and also around two idler pulleys 18, 20. The idler pulleys 18, 20 are mounted on a carrier plate 22 which is pivotable about the axis of the intake camshaft and as it pivots it alters the geometry of the belt 16 to vary the phase of the intake camshaft relative to the exhaust camshaft and therefore relative to the crankshaft. As so far described the mechanism for changing the phase of the intake camshaft is known 'and the invention is concerned primarily with the means for positioning the carrier 22.

A freewheeling element 30 is mounted about the axis of a pin 32 which is fixed relative to the engine block. A oneway clutch 34 ensures that the element 30 can only rotate in one direction about the axis of the pin 32. A journal bearing 28 mounted on the freewheeling element 28 is arranged eccentrically with respect to the axis of the pin 32, and a connecting rod 26 is journalled about the bearing 28 at one end and is coupled at its other end to the carrier 22 by a pivot pin 24. Two diametrically opposed recesses 36 are formed in the periphery of the element 30 which are engageable by the push rod 38 of a solenoid actuator 40.

By virtue of the one-way clutch 34 (see Figure 2) the element 30 can only rotate in the direction of the arrow 42 in Figure 1. The force required to move the carrier 22 is derived from the vibrations of the carrier 22 itself caused by the torque reversals on the camshafts. These vibrations result in a force acting along the connecting rod.

Because the connecting rod is acting effectively on a crank it will apply a torque dependent upon the angle of the crank. In the two dead centre positions, when the centres of the pin 32, the eccentric bearing 28 and the pin 24 are aligned, the torque is zero. At right angles to these dead centre positions, the torque is a maximum. The torque will reverse in directions because of the vibrations but the presence of the one-way clutch will result in the element 30 being driven in the direction of the arrow 42 until it reaches a dead centre, zero torque, position. Each dead centre stable position corresponds to a position of the carrier 22 and therefore a given value of phase shift.

To change phase, it is only necessary to displace the element 30 out of its dead centre position. As this is a no torque position, the effort required for this is minimal and is provided by the solenoid 40 which acts by way of its output rod 38 on the recess 36 in the circumference of the element 30. Each actuation of the solenoid 40 will cause one half turn of the element 30 to change the phase from one stable value to the other. The element 30 is thus a bistable two position phase adjustment device requiring no external source of power to drive it other than the solenoid 40, which is not called upon to provide the torque normally required to rotate the camshaft against the forces of the valve springs and friction.

The embodiment of Figure 3 differs from that of Figure 1 only in that a lever linkage is interposed between the connecting rod 26 and the carrier 22. A first lever arm 50 is pivoted at one end on a fixed pin 52 and a point along its length to the end of the connecting rod 26. A second lever arm 54 is pivoted at 56 to the carrier 22 and at its other end by a pin 58 to the first lever arm 50. The effect of the lever arms 50 and 54 is that the amount movement of the connecting rod 26 differs from the amount of movement of the carrier 22 and the mechanical advantage can be adjusted by appropriate selection of the lengths of the lever arms.

The embodiment of Figure 4 differs from that of Figure 3 in that the two bistable adjustment mechanisms incorporating freewheeling element 30a and 30b are connected in series using a modified lever linkage. Instead of the pin 52 at the end of the first lever arm 50a being fixed, it is mounted on the free end of a second lever arm 50b of which the other end is pivoted on a fixed pin 52b. In this case there are four possible stable positions and one can envisage more bistable adjustment mechanisms being connected in series in this manner to provide finer control over the phase position.

One can incorporate the phase changing mechanism of the invention in any control system and if closed loop control is required then a sensor may be provided to indicate the current value of the phase position.

If a graph is drawn of the variation of force acting on the carrier with time, it will not be found to be symmetrical about the zero force position because friction tends to create a nett force in one direction. This can result in the element 30 moving faster towards one of the dead centre positions than towards the other but a spring can be used to apply a constant balance to the carrier 22 to counteract this effect and ensure equal rates of change of phase position in both directions of adjustment.

A further advantage of the phase changing mechanism of the invention is that it is less prone to wear. This is because all movements are achieved by allowing the camshaft to move in the direction in which it is tending to move and not by allying an external torque opposed by the various forces acting on the camshaft. Lubrication does not therefore present a serious problem and a sealed lubrication system would suffice to meet the needs of the phase change mechanism.

Claims (8)

1. A phase changing mechanism for use in an internal combustion engine for varying the phasing of a camshaft being driven through a continuous belt by a drive shaft, in which engine the belt passes over two movable idler pulleys which enable the geometry of the belt to be varied to alter the lengths of the two ruhs of the belt extending between toothed drive pulleys on the drive shaft and the camshaft, the mechanism comprising a movable carrier on which at least one of the idler pulleys is mounted, a freewheeling element mounted for rotation in only one direction about an axis which is fixed relative to the drive shaft and the camshaft, a bearing mounted on the freewheeling element and arranged eccentrically with respect to the fixed axis, and a connecting rod journalled about the bearing and coupled at its other end to the carrier.

2. A phase change mechanism as claimed in claim 1, wherein a one-way clutch is provided to permit rotation of the freewheeling element in only direction.

3. A phase change mechanism as claimed in claim 1 or 2, further comprising a solenoid for initiating movement of the freewheeling element out of a stable position.

4. A phase change mechanism as claimed in any preceding claim, wherein the connecting rod is coupled to the carrier by means of a lever linkage.

5. A phase change mechanism as claimed in claim 4, wherein a plurality of connecting arms and freewheeling elements are connected in series using a plurality of lever linkages in order to provide more than two stable positions.

6. A phase change mechanism as claimed in any preceding claim, wherein a spring is provided for applying a constant bias to the carrier.

7. A phase change mechanism as claimed in any preceding claim, wherein a sealed lubrication system is provided for lubricating all the moving parts of the phase change mechanism.

8. A phase change mechanism constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.