GB2245047A

GB2245047A - Phase change mechanism

Info

Publication number: GB2245047A
Application number: GB9013109A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1990-06-12
Filing date: 1990-06-12
Publication date: 1991-12-18
Anticipated expiration: 2010-06-12
Also published as: GB2245047B; GB9013109D0

Abstract

A phase change 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 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 is mounted and a crank element 30 mounted for rotation about an axis 32 which is fixed relative to the drive shaft and the camshaft and supporting a bearing 28 which is arranged eccentrically with respect to the fixed axis 32. A connecting rod 26 journalled about the bearing 28 is coupled at its other end to the carrier 22 so that the position of the carrier 22 varies with the angular position of the crank element. A pair of oppositely acting one-way clutches 34a, 34b act between the crank element 30 and a stationary surface 78 surrounding the crank element 30 to lock the crank element in any desired angular position, and means are provided for selectively releasing the one-way clutches 34a, 34b to permit movement of the crank element 30 in either direction under the action of torque reversals transmitted to the crank element 30 from the camshaft by the connecting rod 26. <IMAGE>

Description

PHASE CHANGE MECHANISM.

The present invention relates to a phase change 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.

An actuator is in 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. Each movable idler pulley is 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 i;;rea- ses 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.

The present invention seeks to provide a phase change mechanism in which a force derived from torque reversals acting on the camshaft is relied upon to effect continuous adjustment of the position of the carrier within a predetermined range.

According to the present invention, there is provided a phase change 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 crank element mounted for rotation about an axis which is fixed relative to the drive shaft and the camshaft and supporting a bearing which is arranged eccentrically with respect to the fixed axis, a connecting rod journalled about the bearing and coupled at its other end to the carrier whereby the position of the carrier varies with the angular position of the crank element, a pair of oppositely acting one-way clutches acting between the crank element and a stationary surface surrounding the crank element to lock the crank element in any desired angular position, and means for selectively releasing the one-way clutches to permit movement oi ç crank element in either direction under the action of torque reversals transmitted to the crank element from the camshaft by the connecting rod.

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 in either direction of the carrier and with it the cranking element is achieved by resisting the vibration forces only in the opposite direction but allowing the crank element to move in sympathy with the forces applied by the connecting rod in the desired 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 crank element oscillate about its fixed axis but if resistance is offered to rotation in only one direction owing to the action of the engaged one-way (overrunning) clutch, then the element will rotate by increments until either the released clutch is re-engaged or the crank element reaches a dead centre position.

Each one-way clutch may comprise a roller, or set of rollers, retained by means of a cage between a cylindrical race and a ramped surface, the rollers being urged by means of springs towards the ramps. In this case, one or other of the one-way clutches may be selectively released by moving the cage in a direction to displace the roller or rollers off the associated ramps.

It is possible to use two oppositely facing one-way clutches with their cages connected to one another or to form an assembly in which oppositely facing ramps are formed on the same surface.

It is preferred that the ramps be formed on the crank element and that the cylindrical race be defined by a stationary cylindrical surface surrounding the crank element.

It will be appreciated that the cage is only capable of limited movement relative to the ramped surface. Therefore if the ramped surface is formed on the crank element, then it will merely follow the position into which the cage is placed. At certain times in the torque reversal cycle, the cage can be moved while encountering little resistance and can be positioned within a wide range. When moved, one of the one-way clutches will be released to cause the crank element to follow the movement of the cage. When the crank element catches up with the cage, the clutches again engage and prevent further movement of the crank element.

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

Advantageously, a spring may be provided for applying a bias constantly to the carrier so that movement in both directions may occur at the same rate.

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 change mechanism in accordance with the invention, Figure 2 is a partial section taken on the line II-II in Figure 1, Figure 3 is a partially cut-away section taken on the line III-III in Figure 2.

Figure 1 shown -. ambodiment of the invention 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 lOa 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 cranking element 30 is mounted about the axis of a pin 32 which is fixed relative to the engine block. Two oppositely facing one-way clutches 34a and 34b (see Figures 2 and 3) ensure that the element 30 is normally locked in position and enable it to rotate in one direction at a time about the axis of the pin 32 upon release of the appropriate clutch 34a or 34b.

A journal bearing 28 mounted on the cranking element 30 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 means of a pair of levers 50 and 54. The lever 50 can pivot about a stationary pin 52 and it other end is connected by a pivot pin 58 to the second lever 54. A point along the lever 50 is connected by a pin 24 to the end of the connecting rod 26 and the other end of the lever 54 is pivotably mounted on the carrier 22 by means of a pin 56.

Each of the one-way clutches 34a, 34b comprises a roller 70 disposed within a cut-out in a cage 72. The roller is jammed by a spring 74 between a ramp 76 on the element 30 and a cylindrical surface 78 on an outer race 80. The ramps face in opposite senses so that, when engaged, the clutch designated 34a in Figure 2 prevents anti-clockwise movement of the crank element 30 relative to the stationary race 80 while the clutch 34b prevents clockwise movement.

An extension arm 82 is provided on the cage 72 to permit it to be positioned by a control system (not shown). In the illustrated position, the cage 72 has been rotated anticlockwise from a rest position to dislodge the roller 70 off its ramp surface 76. The clutch 34a is therefore inoperative whereas the clutch 34b is functional. Thus as is illustrated, the crank element 30 is free to rotate anti-clockwise. Rotation of the cage in the opposite direction would of course have released the crank element 30 for clockwise rotation.

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 26.

The connecting rod 26 acting on the crank element 30 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.

Within the range of the extension arm 82 designated by tne arrow 90, the crank element 30 is always subjected to torque reversal and will move if one or other of the clutches 34a, 34b is released. When the cage is stationary in any position in this range, the clutches are in fact both engaged and the crank element 30 is locked, thereby setting the desired angle of phase shift.

To move the crank element 30 to a new position, the cage 72 is rotated in the desired direction of phase change. The effect of movement is to release one of the clutches in the manner earlier described whereupon the torque acting through the connecting rod 26 on the crank element 28, 30 will rotate it in the direction to follow the cage 72. On reaching the cage, the clutch re-engages automatically and the crank element is again locked and remains in this position until the cage is moved. In other words, the arrangement acts as a position follower.

This position follower configuration makes for a particularly simple design of control system in that little power is needed to position the cage at will and within the adjustment range the set phase angle is determined exclusively by the angular position of the cage.

It is not essential for the clutches to be configured such that the crank element follows the movements of the cage.

If the ramps are formed on the stationary surface, then movement of the cage to one side of a fixed position will result in clockwise rotation of the crank element and movement to the opposite side of the same fixed position will result in anti-clockwise movement of the crank element. In either direction, movement will continue until the crank element reaches a dead centre position in which the torque reversals will not apply a torque for rotating the crank element. In such a configuration, a control system relying on position feedback may be used to ensure that the crank element moves to the desired position.

The clutches 34a ana 4D have been illustrated as acting on a common outer race but they can be formed from commercially available releasable one-way clutches by connecting two such clutches back to back and linking their cages to one another.

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 in one direction than in the other. A spring (not shown) can be used to apply a constant balance to the carrier 22 or the lever 50 to counteract this effect and ensure equal rates of change of phase position in both directions of adjustment.

A further advantage of the phase change 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

1. A phase change 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 crank element mounted for rotation about an axis which is fixed relative to the drive shaft and the camshaft and supporting a bearing which is arranged eccentrically with respect to the fixed axis, a connecting rod journalled about the bearing and coupled at its other end to the carrier whereby the position of the carrier varies with the angular position of the crank element, a pair of oppositely acting one-way clutches acting between the crank element and a stationary surface surrounding the crank element to lock the crank element in any desired angular position, and means for selectively releasing the one-way clutches to permit movement of the crank element in either direction under the action of torque reversals transmitted to the crank element from the camshaft by the connecting rod.

2. A phase change mechanism as claimed in claim 1, wherein each one-way clutch may comprise a roller, or set of rollers, retained by means of a cage between a cylindrical race and a ramped surface, the rollers being urged by means of springs towards the ramps.

3. A phase change mechanism as claimed in claim 2, wherein either one of the one-way clutches may be selectively released by moving the cage in a direction to displace the roller or rollers off the associated ramps.

4. A phase change mechanism as claimed in claim or 3, wherein the ramps are formed on the crank element and the cylindrical race is defined by a stationary cylindrical surface surrounding the crank element.

5. A phase change mechanism as claimed in any preceding claim, wherein the connecting rod is indirectly coupled to the carrier by way of a lever linkage providing a mechanical advantage.

6. A phase change mechanism as claimed in any preceding claim, wherein a spring is provided for applying a bias to the carrier so that movement of the crank element in both directions may occur at the same rate.

7. A phase change mechanism constructed1 arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.