Field of the Invention

The present invention relates to a valve train assembly. Background of the Invention

Valve train assemblies for controlling the operation of valves in internal combustion engines are well known. One type of typical valve train assembly comprises an inlet rocker arm mounted for pivoting in response to the rotation of an intake cam to operate an intake valve during the intake part of an engine cycle and an exhaust rocker arm mounted for pivoting in response to the rotation of an exhaust cam to operate an exhaust valve during the exhaust part of an engine cycle. Each cam is mounted on a common rotatable cam-shaft and has a base circle and a lift portion which, as the cam rotates, engages (either directly or indirectly via an intermediate linkage) a rocker arm causing the rocker arm to pivot and move a valve stem against the force of a valve spring to open the valve. As the cam continues to rotate, a peak of the lift portion passes first into engagement with the rocker (at the point of maximum valve lift) and then out of engagement with the rocker allowing the valve spring to return the valve and the rocker arm to a position in which the valve closes. When the base circle of the cam engages the rocker, the valve is closed. The camshaft is driven by a timing belt (chain or gears) and this and the relative positioning and geometry of the intake cam and exhaust cam lift portions ensure that the intake and exhaust valves are operated at the correct points of an engine cycle.

In a basic engine operation, an intake valve and a corresponding exhaust valve are op- erated once per engine cycle. In more complicated engine operations, it is necessary to operate one or other of the intake and exhaust valves more than once per engine cycle. For example, in some known systems, if the engine makes use of internal exhaust gas recirculation (EGR) technology a smaller additional opening of the exhaust valve is performed during the intake part of the cycle (or a smaller additional opening of the intake valve is performed dur- ing the exhaust part of the cycle) to cause some exhaust gas to be in the combustion cylinder during the combustion part of the cycle. As is well known, EGR reduces NOx emissions. In one known system an exhaust cam (or intake cam) is provided with, in addition to its main lift portion, a smaller lift portion for providing an additional valve lift during the intake cycle (or exhaust cycle). It is desirable to provide a valve train assembly that has as an alternative ar- rangement for providing an additional valve lift.

Summary of the Invention

According to the invention there is provided a valve train assembly comprising: a first rocker arm for pivoting in response to a first rotating cam to open a first engine valve during an engine cycle; a second rocker arm for pivoting in response to a second rotating cam to open a second engine valve during the engine cycle; and a first member for moving in response to the first rotating cam to cause the second rocker arm to pivot in order to open the second valve an additional time during the engine cycle.

According to the invention there is also provided a method of operating a valve train assembly, the method comprising: pivoting a first rocker arm in response to a first rotating cam to open a first engine valve during an engine cycle; pivoting a second rocker arm in response to a second rotating cam to open a second engine valve during the engine cycle; and moving a first member in response to the first rotating cam to cause the second rocker arm to pivot in order to open the second valve an additional time during the engine cycle.

Advantageously, in embodiments of the invention, because the additional opening of the second valve is in response to the first rotating cam (i.e. the cam that controls the first valve) there is no need for the second cam (i.e. the cam that controls the second valve) to be provid- ed with an smaller lift profile, in addition to its main lift profile, to cause the additional opening of the second valve. The arrangement of cams in embodiments of the present invention is therefore simpler than in known arrangements in which additional valve openings are provided during an engine cycle. Brief Description of the Drawings Arrangements embodying the invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a schematic perspective view of a valve train assembly embodying the invention; Figure 2a is a schematic side view of the valve train assembly as configured in an EGR active mode with the exhaust valve closed;

Figure 2b is a schematic side view of the valve train assembly as configured in the EGR active mode with the exhaust valve open;

Figure 3a is a schematic side view of the valve train assembly as configured in an EGR non - active mode with the exhaust valve closed and a lever in an upper position;

Figure 3b is a schematic side view of the valve train assembly as configured in the EGR active mode with the exhaust valve closed and a lever in a lower position;

Figure 4 is a graph of valve lift as a function of cam shaft rotation.

Figures 5 a to 5 c are perspective schematic views of an alternative valve train assembly.

Description

Figures 1 to 3 illustrate a valve train assembly 1 comprising an exhaust rocker arm 2 and an intake rocker arm 3, arranged side by side, each pivotally mounted on a common fixed axle 4. A first end of the exhaust rocker arm 2 has a first roller 5 for engaging an exhaust cam 6 and a corresponding first end of the intake rocker arm 3 has a second roller 7 for engaging an intake cam 8. The exhaust cam 6 and the intake cam 8 are mounted on a cam shaft 9. A second end of the exhaust rocker arm 2 has a first hydraulic lash adjuster 10 comprising a spigot 10a at its base for acting on the head of an exhaust valve 11 of an engine cylinder (not shown). Likewise, a corresponding end of the intake rocker arm 3 comprises a second hydraulic lash adjuster (not visible in the Figures) for acting on the head of an intake valve 12 of the engine cylinder (not shown). The hydraulic lash adjustors are standard components for accommodating any slack in the valve train assembly (such as between a cam and a roller) and so will not be described in any more detail.

The exhaust cam 6 has a base circle portion 6a and a lobe or lift portion 6b and the intake cam 8 also has a base circle portion 8a and a lift portion 8b which is angularly offset rela- tive to the lift portion 6b of the exhaust cam 6. During an engine cycle, as the cam shaft 9 rotates, the intake cam's lift portion 8b comes into engagement with the second roller 7 causing the intake rocker arm 3 to pivot about the axle 4 and 'lift' the intake valve 12 (i.e. move it downward in Figure 1) against the force of an intake valve spring (not shown) to open the intake valve 12. As the cam shaft 9 continues to rotate, the peak of the lift portion 8b passes first into engagement with the second roller 7 (at the point of maximum valve lift) and then passes out of engagement with the second roller 7 allowing the valve spring (not shown) to return the intake valve 12 and the intake rocker arm 3 to respective positions in which the intake valve 12 closes. When the base circle 8a of the intake cam 8 engages the second roller 7, the intake valve 12 is closed.

Thus, in Figure 4, the curve labelled X represents the lift that occurs when the intake cam's 8 lift portion 8b engages the second roller 7. The peak of the curve X represents the maximum lift of the intake valve 12 during the intake part of an engine cycle.

As the cam shaft 9 continues to rotate through an engine cycle, the exhaust cam's lift portion 6b comes into engagement with the first roller 5, causing the exhaust rocker arm 2 to pivot about the axle 4 and 'lift' the exhaust valve 11 (i.e. move it downward in Figure 1) against the force of an exhaust valve spring (not shown) to open the exhaust valve 11. The peak of the lift portion 6b passes first into engagement with the first roller 5 (at the point of maximum valve lift) and then passes out of engagement with the first roller 5 allowing a valve spring (not shown) to return the exhaust valve 11 and the exhaust rocker arm 3 to respective positions in which the exhaust valve 11 is closed. Thus, in Figure 4, the curve labelled Y represents the lift that occurs when the exhaust cam's lift portion 6b engages the first roller 5. The peak of the curve Y represents the maximum lift of the exhaust valve 11 during the exhaust part of an engine cycle. As can be seen from the overlap of the curves Y and X in Figure 5, the intake valve 12 begins to open just before the exhaust valve 11 becomes fully closed.

The valve train assembly 1 further comprises an assembly 20 operable, in response to the rotation of the intake cam 8, to cause an additional opening of the exhaust valve 11, which coincides with the opening of the intake valve 12, providing internal EGR to the engine (not shown).

The assembly 20 comprises a rotatable eccentric shaft 21, a linkage 22, and a lever 23. The eccentric shaft 21 comprises first 21a and second 21b cylindrical shaft members, of equal diameter, arranged co-axially and spaced apart in a line. The eccentric shaft further comprises a third 21c cylindrical shaft member solidly fixed between the first 21a and second 21b members. The third member 21c has a smaller diameter than the first 21a and second members 21b and its axis is offset from the common axis of the first 21a and second 21b members.

The linkage 22 comprises first 22a, second 22b and third 22c blade like link members. The first link member 22a is pivotally mounted, toward one of its ends, on the third shaft member 21c and is pivotally mounted, toward the other of its ends, on an axle 24. The second link member 22b is pivotally mounted, toward one of its ends, on the axle 24 and is pivotally mounted, towards the other of its ends, on the lever 23 and the third link member 22c is pivotally mounted, toward one of its ends, on the axle 24 and is pivotally mounted, towards the other of its ends, to the intake rocker arm 3.

The lever 23 comprises: a tubular member 23a rotatably mounted on a fixed axle 25; a con- nector portion 23b that extends substantially perpendicularly from one end of the tubular member 23a and to which the link member 22b is pivotally connected; and a cam portion 23c that extends substantially perpendicularly from the other end of the tubular member 23a and which has an end face defining a raised cam profile 23 d which faces a roller (or cam follower) 26 supported on top of the exhaust rocker arm 2 towards its end where the hydraulic lash ad- juster 10 is located.

An actuator 27 is operable to rotate the eccentric shaft 21 (as indicated by the curved double headed arrow in Figure 2a) between a fully advanced position as shown in Figures 2a and 2b, in which the assembly 1 provides an additional exhaust valve lift of maximum height and du- ration, and a fully retracted position, as shown in Figures 3a and 3b, in which the assembly 1 provides no additional exhaust valve lift, or to any position between these two extremes, in which position the assembly 1 provides an additional exhaust valve lift, the height and dura- tion of which depend upon how close the eccentric shaft 21 is to the fully advanced position (the closer it is the higher and of longer duration is the additional lift). The actuator 27 may for example, be hydraulic, electric or a pneumatic cylinder. As illustrated in Figure 2a, when the eccentric shaft 21 is in the fully advanced position (i.e. when the off centre third member 21c is furthest to the right as viewed in the page) and when and the intake cam's base circle portion 8a engages the second roller 7 (i.e. the intake valve 13 is closed), the first 22a, second 22b and third 22c link members are arranged in ' shape (the second 22b and third 22c link members are aligned vertically in the plane of the page and the first 22a link member is horizontal in the plane of the page), and the lever 23 is supported in a substantially horizontal orientation on the axle 25 so that there is a relatively small distance between the raised cam portion 23 d and the roller 26.

As the cam shaft 9 rotates, the intake cam's lift portion 8b comes into engagement with the second roller 7 causing the intake rocker arm 3 to pivot about the axle 4 thereby 'lifting' the intake valve 12 as described above. The pivoting of the intake rocker arm 3 pushes the first 22a, the second 22b and third 22c link members upwards so that the third 22c link members exerts a force on the lever 23 causing the lever 23 to pivot about the axle 25 as the intake rocker arm 3 pivots about the axle 4. The lever 23 pivots through an initial angle without coming into contact with and exerting a force upon the roller 26 but, at this point in the rotation of the lever 23, the raised cam profile 23 d comes into contact with the roller 26 exerting a force on it and hence causing the exhaust rocker arm 3 to pivot about the axle 4 and 'lift' the exhaust valve 11 to provide an additional exhaust valve EGR event. As illustrated in Figure 2b, when the peak of intake cam's lift portion 8b engages the second roller 7, the first 22a, second 22b and third 22c link members are arranged in a distorted 'T' shape and the right hand side of the lever 23 (as viewed in the page) is at the bottom point of its pivoting path or stroke. It is at this point that the additional lift of the exhaust valve 11 is at its maximum. As the cam shaft 9 continues to rotate, the peak of the intake cam's lift por- tion 8b moves out of engagement with the second roller 7 and the intake rocker arm 3 starts pivoting back to its closed position. The linkage 22 is pulled downwards pivoting the lever 23 back towards its starting position so that the raised cam profile 23 d no longer engages the roller 26 and the exhaust valve shuts 11 under the action of its return spring (not shown).

Thus, in Figure 4, the curve labelled 'Wl ' represents the additional exhaust valve 11 lift that occurs when the intake cam's lift portion 8b engages the second roller 7. The peak of the curve ' Wl ' represents the maximum additional lift of the exhaust valve 11 and it can be seen that this is substantially coincident in time with the maximum lift of the intake valve 12 during the intake part of the engine cycle (although it is not essential that the high points of the lifts occur at the same time). The height of this lift is also indicated by the arrows and lines at the right hand side of Figure 2b.

As illustrated in Figure 3 a, when the eccentric shaft 21 is in the fully retarded position (i.e. when the off centre third member 21c is furthest to the right as viewed in the page) and the intake cam's base circle portion 8a engages the second roller 7, the first 22a, second 22b and third 22c link members are arranged in a Ύ' shape, and the lever 23 is supported in a position on the axle 25 that is rotated anti - clockwise relative to its corresponding position in Figure 2a. As can be seen by comparing Figures 2a and 3 a the angular distance between the raised cam profile 23d and the roller 26 is greater in the Figure 3a position than it is in the Figure 2a position. In this position, again, as the cam shaft 9 rotates, the intake cam's raised portion 8b comes into engagement with the second roller 7 causing the intake rocker arm 3 to pivot about the axle 4 thereby 'lifting' the intake valve 12 and pushing the first 22a, the second 22b and third 22c link upwards causing the lever 23 to pivot about the axle 25 as described above. However, the assembly 20 is arranged so that the angular distance between the raised cam profile 23d and the roller 26 is so great that as the lever 23 pivots clockwise it never contacts the roller 26 even when, as illustrated in Figure 3b, the lever 23 (as viewed in the page) is at the bottom point of its pivoting path or stroke. Accordingly, no force is exerted on the exhaust valve 11 and hence there is no additional exhaust valve EGR event. Advantageously, when engine conditions (e.g. load) are such that EGR is required, placing the eccentric shaft in the advanced position activates an EGR mode, and when engine conditions are such that EGR is not required (e.g. at loads where EGR would unacceptably reduce engine performance), placing the eccentric shaft into the retarded position deactivates the EGR mode.

Preferably, the actuator 27 is operable to rotate the eccentric shaft 21 into any position between the fully advanced position and the fully retracted position to provide an EGR mode in which the height and duration of the lift depends upon the selected position of the eccentric shaft 21. The closer the selected position is to the fully advanced position the closer the starting position of the raised cam portion 23 is to the roller 26 and consequently the longer the two are in contact during the stroke of the lever 23 and hence the higher the additional lift and the longer its duration. Accordingly, at any given time, an engine control system (not shown) can control the actuator 27 so that the amount of internal EGR is matched to the current engine conditions. In Figure 4, the curves labelled W2 and W3 represent the additional exhaust valve 11 lifts when the eccentric shaft is at two respective positions between the fully advanced and fully retracted positions, the position for lift W2 being closer to the fully advanced position than is the position for the lift W3.

Figures 5 a to 5 c show an alternative arrangement in which like reference numerals refer to like features in the above described embodiment. In this arrangement, the lever 23 is directly rotatably mounted on an eccentric shaft 21 and the connector portion 23b, which is integrally connected to the lever 23, engages a roller 30a supported by a support arm 30. The support arm 30 is itself pivotally connected at an end of the intake rocker arm 3. The eccentric shaft 21 comprises a first cylindrical shaft member 21a which is integrally connected between a pair of second cylindrical shaft members 21c. The second cylindrical shaft members 21c have an equal diameter smaller than that of the first cylindrical shaft member 21a and are ar- ranged co-axially. The common longitudinal axis of the second cylindrical shaft members 21c is offset from that of the first 21a cylindrical shaft member. The eccentric shaft 21 is rotatably mounted by way of the second cylindrical shaft members 21c in a support block 31 and is rotatable about the longitudinal axis of the second cylindrical shaft members 21c. The support block 31 also supports the axle 4 on which the exhaust rocker arm 2 and the intake rocker arm 3 are pivotally mounted. The exhaust cam 6 engages with the exhaust rocker arm 2 by means of a first push rod 40 and the intake cam 8 engages with the intake rocker arm 3 by means of a second push rod 42. An actuator (not shown) is provided for rotating the eccentric shaft 21 between the fully retracted position shown in Figures 5a to 5c and the fully advanced position (not illustrated).

In the fully advanced position, as the cam shaft 9 rotates, the intake cam's lobe 8b comes into engagement with the second push rod 42 pushing it upwards to cause the intake rocker arm 3 to pivot about the axle 4 thereby 'lifting' the intake valve 12. The pivoting intake rocker arm 3 pushes the lever 23 via the connector portion 23b causing the lever 23 to pivot about the eccentric shaft 21 as the intake rocker arm 3 pivots about the axle 4 so that the raised cam profile 23 d is brought into contact with the roller 26 exerting a force on it and hence causing the exhaust rocker arm 2 to pivot about the axle 4 and 'lift' the exhaust valve 11 to provide the additional exhaust valve EGR event described above. However, again as with the embodiment described above, when in the fully retracted position, the relative positioning of the lever 23 and the exhaust rocker arm 2 is such that as the intake rocker 3 and consequently the lever 23 rotate, the raised cam profile 23 d fails to contact the roller 26 and hence there is no additional valve event. With this arrangement, the eccentric shaft 21 can be positioned at any intermediate position between the fully advanced and fully retracted positions so as to vary the additional lift profile. In an alternative arrangement, not illustrated, the eccentric shaft 21 can be positioned only in the fully advanced position (EGR ON) or in the fully retracted position (EGR OFF) but not in between the two. Accordingly, only a single additional lift profile is possible, for example, that of Wl in Figure 4.

In the described embodiments the assembly 1 is configured to provide an additional opening of the exhaust valve in response to the rotating intake cam, it may of course instead be easily configured in a corresponding fashion so as to provide an additional opening of the intake valve in response to the rotating exhaust cam.

Although in the described embodiments the intake rocker controls a single intake valve and the exhaust rocker controls a single exhaust valve, the intake rocker may be configured to simultaneously operate two intake valves and the exhaust rocker may be configured to simultaneously operate two exhaust valves. To achieve this, as is well known, the lash adjuster of each rocker may act upon a bridge portion from which its respective valves are suspended. Many other variations within the scope of the appended claims will be apparent to the skilled person.