WO2012042408A1

WO2012042408A1 - Cam summation engine valve system

Info

Publication number: WO2012042408A1
Application number: PCT/IB2011/053790
Authority: WO
Inventors: Ian Methley
Original assignee: Mechadyne PLC
Current assignee: Mechadyne PLC
Priority date: 2010-09-30
Filing date: 2011-08-30
Publication date: 2012-04-05
Anticipated expiration: 2013-03-30
Also published as: GB201016451D0; GB2484123B; GB2484123A

Abstract

An engine valve system is described which comprises two cams (16,18) mounted coaxially, a summation rocker (40) coupled to followers of both cams and movable in proportion to the instantaneous sum of the lifts of the respective cams and a valve actuating rocker (42) coupled to the summation rocker and operative to open an engine valve in dependence upon the movement of the summation rocker. The first cam (16) is driven by the engine crankshaft and the second cam (18) is coupled to the first cam by way of an actuator (30) that enables the relative phase of the two cams 16 and 18 to be controlled. In the invention, the actuator (30) driving the second cam (18) is acted upon by an auxiliary load (32, 34, 36) which is such that the reaction torque experienced by the actuator during an engine cycle undergoes periodic reversals of direction. The actuator (30) includes mechanisms for selectively preventing relative rotation of the two cams in opposite directions, so that reaction torques acting on the actuator can be utilised to vary the relative phase of the two cams in both directions.

Description

CAM SUMMATION ENGINE VALVE SYSTEM

Field of the invention The present invention relates to an engine valve system comprising two cams mounted coaxially, a summation rocker coupled to followers of both cams and movable in proportion to the instantaneous sum of the lifts of the respective cams and a valve actuating rocker coupled to the summation rocker and operative to open an engine valve in dependence upon the movement of the summation rocker, wherein a first cam is driven by the engine crankshaft and the second cam is coupled to the first cam by way of an actuator that enables the relative phase of the two cams to be controlled.

Background of the invention

Conventional camshafts experience a reaction torque that reverses direction during each engine cycle. This is because individual cams encounter a retarding torque when attempting to open a valve but encounter an accelerating torque when resisting the closing of an open valve.

Figure 1 of the accompanying drawings shows a typical camshaft drive torque characteristics at different engine speeds for a four cylinder engine in which the torque fluctuates either side of zero. There is a relatively small mean drive torque required due to friction of the camshaft bearing journals and cam followers. It can also be seen from Figure 1 that while the torque characteristic changes considerably with speed, the mean drive torque required varies by only a small amount.

Camshaft phasing systems are well known that are able to change the angle of the camshaft with respect to the engine crankshaft, and these would typically be specified to overcome the mean drive torque of the camshaft throughout the operating range of the engine. It follows that a

camshaft phaser may not be sufficiently powerful to overcome the peak drive torque under all conditions, but this does not prevent the camshaft from being rotated to the desired position over a number of cycles. In some cases the phasing system will be fitted with a torque spring acting in the opposite direction to the mean camshaft drive torque that gives the system a more neutral phasing torque

characteristic or biases the phasing system towards one extreme of its operating range.

The present invention is concerned with a cam summation engine valve system as described in EP 1417399 which enables the valve event duration and lift to be altered by phasing two cam profiles relative to each other. Cam summation systems combine two different cams in order to produce the valve lift characteristic. In such systems, one cam may control only the valve opening events whilst the other cam controls the valve closing events. Though the total torque required to drive both the opening cam and the closing cam is similar to that of a conventional camshaft as shown in Figure 1, the torque required to drive all of the opening cams or all of the closing cams in isolation is quite different .

Typical torque characteristics for the opening cam lobes and the closing cam lobes of a cam summation valve system are shown in Figures 2A and 2B, respectively, of the accompanying drawings. In order to change the lift and duration of the valve events, the relative phasing of the opening and closing cam lobes needs to be controlled. This has been achieved in the prior art by connecting the opening and closing cam lobes to one another via a rotary actuator that is similar in operating principle to a phasing system used to change the timing of a fixed camshaft relative to the engine crankshaft, but generally has a larger angular operating range. Unlike a conventional phasing system however, the mean drive torque that the rotary actuator needs to overcome can be quite significant and, as can be seen from Figures 2A and 2B, will always act to reduce the system to its minimum lift setting. Hence if the actuator is used to drive the closing cam lobes it will be biased in an advancing direction, and if the actuator is used to drive the opening cam lobes, it will be biased in a retarding direction. A typical family of valve lift characteristics that could be produced by a cam summation system of this type having a fixed valve opening timing and a variable closing timing is shown in Figure 3. The drive torque characteristic of the closing cams would tend to force the system towards its minimum lift setting by advancing the timing of the valve closing. A family of valve lift curves of this type might be used for so-called "valve head throttling" wherein the airflow into the cylinder of an internal combustion engine is controlled by the lift of the intake valves, rather than by a throttle plate in the intake system.

Reducing the valve lift and opening duration would therefore be used to reduce the output of the engine, whilst

increasing lift and opening duration would increase the output of the engine.

The problem with the natural bias of the system towards its lowest lift setting is that in the event of a control system failure the valve lift would return to its minimum setting, thus restricting the engine output to the point where the vehicle would become undrivable.

Although most engines using valve head throttling would still retain a throttle plate in the intake system, the throttle plate would only be able to control the engine output if the valve lift defaults to a high lift setting in the event of a control system failure. Therefore if the system can be designed to have a default position with a high valve lift, it becomes possible to overcome a failure of the valve lift control system by operating the engine in a conventional manner using the throttle plate. This will allow the vehicle to continue to operate, although there would be some degradation in performance and fuel economy resulting from the inability to control the valve lift.

The asymmetric torque characteristic applied to the rotary actuator thus causes the following problems: - · The actuator must be specified to overcome the high mean drive torque in all operating conditions.

• The response rate of the actuator is different in the advancing and retarding directions.

• The default characteristic of the system to return to its minimum lift setting may be unacceptable for engine start-up or 'limp home' operating modes.

• A torque spring of sufficient strength to overcome the mean torque cannot easily be integrated with the actuator, particularly in view of the large angular range of the actuator.

Object of the invention

The invention seeks therefore to provide a design of camshaft and rotary actuator for a cam summation system that is able to move away from its minimum lift setting against the natural bias of the cam summation system under all operating conditions. Summary of the invention

According to the present invention, there is provided an engine valve system comprising two cams mounted

coaxially, a summation rocker coupled to followers of both cams and movable in proportion to the instantaneous sum of the lifts of the respective cams and a valve actuating rocker coupled to the summation rocker and operative to open an engine valve in dependence upon the movement of the summation rocker, wherein a first cam is driven by the engine crankshaft and the second cam is coupled to the first cam by way of an actuator that enables the relative phase of the two cams to be controlled, wherein the actuator driving the second cam is acted upon by an auxiliary load which is such that the reaction torque experienced by the actuator during an engine cycle undergoes periodic reversals of direction, and wherein the actuator includes two locking mechanisms acting to prevent relative rotation of the two cams in opposite directions, the locking mechanisms being selectable to enable the reaction torque to vary the

relative phase of the two cams in both directions. The auxiliary load may comprise a spring applying a steady torque in the opposite direction to the mean reaction torque of the second cam.

The auxiliary load may additionally or alternatively comprise a fuel pump or a vacuum pump. If the reaction torque of the auxiliary load fluctuates cyclically, its phase should be synchronised with the cam lobe reaction torque to ensure that the resultant torque undergoes direction reversals.

The auxiliary load may either be operated via an additional cam lobe that is driven by the actuator or it may be directly connected for rotation with the actuator. The locking may conveniently comprise selectable back- to-back one-way clutches but it is alternatively possible to employ an actuator constructed as a vane-type phaser having selectable one-way valves controlling the communication between different working chambers of the phaser. 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, as earlier described, shows typical torque characteristics of a conventional camshaft at different engine speeds,

Figure 2A and 2B, as earlier described, show typical torque characteristics for the opening cams and the closing cams of a cam summation valve system,

Figure 3, as earlier described, shows how the valve event may be varied from a low lift to a high lift by altering the relative phase of the two cams of a cam summation valve system,

Figure 4 shows a perspective view of a cam summation valve system of the invention having an actuator for changing the timing of the closing cam lobe with respect to the pair of opening cam lobes and various auxiliary loads driven by the actuator so that the reaction torques that it experiences undergo reversals of direction during an engine operating cycle,

Figure 5 shows an end view of the cam summation system in Figure 4 in which the cam summation rockers can be seen more clearly.

Figure 6 is a section through the cam summation valve system of Figure 4 showing how a vacuum pump is driven by the inner shaft of single cam phaser camshaft to place an auxiliary load on the actuator,

Figure 7 shows the torque characteristics of the cam summation drive system of the invention at different engine speeds,

Figure 8 shows only the high speed torque

characteristic of Figure 7 with the areas of retarding torque shaded, and Figure 9 is a section through an actuator that uses two back-to-back one-way clutches to change the relative phase of the cams using the reaction torque reversals. Detailed description of the preferred embodiment

Figures 4 to 6 show a cam summation valve system that uses a concentric camshaft. As seen in Figure 6, the

camshaft comprises an outer tube 12 and an inner shaft. Two opening cams 16 are fast in rotation with the outer tube 12. A closing cam 18 is freely rotatable about the outer surface of the outer tube 12 and is connected by a pin 20 for rotation with the inner shaft 14. The pin 20 passes through a circumferentially elongated slot in the outer tube 12. The outer tube 12 is driven by the engine crankshaft and a phaser may be incorporated in the drive train to alter the timing of the valve event relative to the rotation of the engine crankshaft. A summation rocker 40 (see Figure 5) has a pair of cam followers at one end in contact with the opening cams 16 and a single cam follower at the other end in contact with closing cam 18. Two actuating rockers 42 are pivotably connected to the summation rocker 40, one end of each actuating rocker 42 being in contact with the tip of a respective poppet valve 44 and the other with a lash

adjuster 46.

As so far described, the cam summation valve system is known. By rotating the inner shaft 14 relative to the outer tube 12, the relative phase of the opening and closing cams is changed to enable the valve event to be varied in the manner shown in Figure 3. Rotation of the inner shaft 14 relative to the outer tube 12 to change the relative phase of the cams 16 and 18 is carried out by an actuator 30, an embodiment of which will be described in greater detail below by reference to Figure 9. As explained above, if the only reaction torque acting on the actuator 30 were the closing cams of the different engine cylinders, that torque would fluctuate in magnitude but would be uni-directional . In the event of failure of the control system setting the relative phase of the cams 16 and 18, the actuator 30 would migrate steadily in the direction of the reaction torque and if this is the position of minimum lift then the engine would become undrivable. To avoid this problem, various additional loads are provided in the illustrated embodiment of the invention to cause the reaction torques to undergo reversals in direction during each engine cycle. A first such load comprises a spring 32 that opposes the uni-direction reaction torques of the closing cams. In this case, the spring torque has a substantially constant value that opposes the mean torque of the closing cams.

Instead of the reaction torque varying between zero and a positive peak value, the superimposition by the spring of a constant negative torque will result in the reaction torque varying between a negative value and a smaller positive peak value . Of itself, the spring 32 may not be sufficient to provide the required reverse torque. For this reason, in the illustrated embodiment, the inner shaft 14 is further connected to a cam 38 that operates a fuel pump 36 the latter serving as an auxiliary that superimposes a

fluctuating negative reaction torque upon that of the closing cam 18. The fuel pump cam 38 is driven via a connecting pin to the inner drive shaft 14 in an identical manner to each of the closing cams. If correctly phased to coincide with the periods of zero reaction torque from the cam 16, the total reaction torque experienced by the shaft 14 will undergo reversals in direction. The central bolt 31 that connects the actuator 30 to the inner drive shaft 14, is also used to engage with an Oldham coupling 50 by which a vacuum pump 34 is driven. It is already known to drive auxiliary loads presented by ancillary equipment such as vacuum, oil and fuel pumps from the camshaft. Conventionally these loads would be driven by the outer tube of the camshaft, i.e. the element of the actuator driven by the engine crankshaft. By

contrast, in the present invention, such auxiliary systems are driven by the actuator 30 used to adjust the relative phase of the opening and closing cams.

The resultant drive torque characteristic of the inner shaft, as shown in Figure 7, is similar to that for the closing cam lobes shown in the lower graph of Figure 2, but there will be some offset due to the retarding effect of the bias spring 32 and/or and auxiliary loads 34 and 36 driven by the inner shaft.

It should be noted that in the example of Figure 7 the high mean torque caused by the series of uni-directional torque spikes from the closing cam lobes is not totally cancelled out by the retarding torque of the bias spring and/or pump. It should also be noted that in this example the resultant drive shaft torque is illustrated as being offset by a constant torque from the closing cam torques - as might be the case with a bias spring. Figure 8 uses the example of the high speed torque curve from Figure 7 to illustrate the areas of retarding torque that have been introduced by superimposing a

retarding torque onto the advancing torque characteristic of the closing cams. It will be noted that there is still a substantial mean torque in the advancing direction, but that the overall time duration of the advancing and retarding torques is now quite similar. Despite the presence of both advancing and retarding torque spikes in the resultant drive shaft torque, the high mean torque in the advancing direction still makes the use of a conventional hydraulically operated actuator difficult. The system will still default to its minimum lift setting, and the phaser may still be unable to move the system to its maximum lift setting in conditions of low oil pressure.

In order to overcome these problems, it is proposed that the rotary actuator 30 should be of a type that moves under the influence of camshaft torque reversals, rather than using oil pressure to drive the camshaft to the

required position. This will ensure that the actuator is always able to move towards the required fail-safe position regardless of what oil pressure is available.

Various designs of camshaft phasing systems that move under the action of camshaft torque reversals are known. One example is shown in US 7,444,968 which is incorporated herein by reference. This design uses one-way valves to control the direction of oil flow within a vane-type phaser.

Figure 9 shows an alternative design of the actuator 30 which uses back to back one-way clutches as taught by EP 1,216,344 to change the relative phase in both directions under the action of camshaft torque reversals. The inner element 62 of the actuator 30 is connected to the inner shaft 14 of the SCP camshaft and the outer element 64 is connected to its outer tube 12. The two elements 62 and 64 can rotate relative to one another but are prevented from doing so by two hydraulically releasable one-way clutches.

Three radial recesses 63 are provided around the circumference of the inner element 62. The central region of each recess is flat and supports two rolling elements 66 and 68 that are forced apart by a spring 70 to jam between the flat base of the recess and the cylindrical inner surface of the outer element 64. Relative rotation in either direction is thereby prevented in that such rotation will attempt to force one or other of the rolling elements 66, 68 into a gap of progressively decreasing width that is narrower than the diameter of the rolling element.

By applying hydraulic pressure to the working chambers designated 72, the rolling elements 68 can be moved against the action of the springs 70. This will allow the inner element 62 to rotate clockwise, as viewed, relative to the outer element 64. Likewise, if the working chambers 74 are pressurised, the inner element 62 will be able to rotate counter clockwise within the outer element 64. The advantage of using a camshaft torque driven

actuator is that it is not dependent upon oil pressure to overcome the mean drive torque, but it uses selective locking to allow free rotation of the actuator under the action of the applied drive torque. Hence it is only

important for the drive torque characteristic to have defined areas of advancing and retarding torque (as

illustrated in Figure 8) in order to allow the actuator to operate . In the event of a control system failure, it is only necessary to ensure that the actuator locking system

defaults to the position where the phaser will be able to move freely in one direction in order to move the system to one of its extreme settings. Thus it presents no particular problem for the system to default to its maximum lift setting under such circumstances.

Claims

1. An engine valve system comprising two cams mounted coaxially, a summation rocker coupled to followers of both cams and movable in proportion to the instantaneous sum of the lifts of the respective cams and a valve actuating rocker coupled to the summation rocker and operative to open an engine valve in dependence upon the movement of the summation rocker, wherein a first cam is driven by the engine crankshaft and the second cam is coupled to the first cam by way of an actuator that enables the relative phase of the two cams to be controlled, characterised in that the actuator driving the second cam is acted upon by an

auxiliary load which is such that the reaction torque experienced by the actuator during an engine cycle undergoes periodic reversals of direction, and in that the actuator includes two locking mechanisms acting to prevent relative rotation of the two cams in opposite directions, the locking mechanisms being selectable to enable the reaction torque to vary the relative phase of the two cams in both directions.

2. An engine valve system as claimed in claim 1, wherein the auxiliary load comprises a spring.

3. An engine valve system as claimed in claim 1 or 2, wherein the auxiliary load comprises a fuel pump.

4. An engine valve system as claimed in any

preceding claim, wherein the auxiliary load comprises a vacuum or oil pump.

5. An engine valve system as claimed in claim 3 or 4, wherein the reaction torque of the auxiliary load fluctuates cyclically and is synchronised with the cam lobe reaction torque to ensure that the resultant torque undergoes

direction reversals.

6. An engine valve system as claimed in any of claims 3 to 5, wherein the auxiliary load is operated via an additional cam lobe that is driven by the actuator.

7. An engine valve system as claimed in any of claims 3 to 5, wherein the auxiliary load system is directly connected for rotation with the actuator.

8. An engine valve system as claimed in any preceding claim, wherein the movement of the actuator is controlled by selectable back-to-back one-way clutches.

9. An engine valve system as claimed in any of claims 1 to 7, wherein the actuator is constructed as a vane-type phaser having selectable one-way valves controlling the communication between different working chambers of the phaser .