The present invention relates to mechanisms for altering the actuation of valves in internal combustion engines; more particularly, to finger follower type rocker arm assemblies capable of changing between high and low or no valve lifts; and most particularly, to an improved oil circuit for a latching mechanism for a two-step finger follower type rocker arm assembly.
Variable valve activation (VVA) mechanisms for internal combustion engines are well known. It is known to be desirable to lower the lift, or even to provide no lift at all, of one or more valves of a multiple-cylinder engine, during periods of light engine load. Such deactivation or cam profile switching can substantially improve fuel efficiency.
Various approaches are known in the prior art for changing the lift of valves in a running engine. One known approach is to provide a latching mechanism in the hydraulic lash adjuster (HLA) pivot end of a rocker arm cam follower, opposite from the valve-actuating end, which locks and unlocks the valve actuator portion from the follower body. The cam follower mechanism is latchable by a hydraulically actuated lock pin whose motion typically is governed in a latching direction by application of pressurized engine oil received from the HLA and in an unlatching direction by a return spring. The lock pin is disposed as a piston in a smooth bore of the follower body and is retained therein by a plug pressed into the end of the bore. The typically cylindrical plug serves to seal the prior art smooth bore, thus forming a hydraulic chamber between itself and an end of the lock pin.
Typically, a two-step roller finger follower (RFF) allows the engine valves to be operated with two different cam profiles, one when the locking pin disengages (unlocks) a high lift follower (low lift mode) and the other when the locking pin is engaged (locked) to the high lift follower (high lift mode). When the HLA oil pressure is low, the return spring moves the locking pin to a retracted position and the locking pin is disengaged from the high lift follower or other valve actuator. When HLA oil pressure is increased, the hydraulic force of the oil pressure in the hydraulic chamber overcomes the spring force and the locking pin moves to an extended position engaging with the high-lift follower or other valve actuator. However, this prior art oil circulation arrangement suffers from several shortcomings.
First, the major diameter of the lock pin, for example, the head diameter if the lock pin includes a head and a shank, determines the magnitude of the hydraulic force generated by the oil pressure in the hydraulic chamber, which may be determined as oil pressure times pin head area, and the total volume of fluid displaced as the pin moves. The lock pin response depends, therefore, on the capacity of the system to flow the required volume of oil. Problems may occur when moving the lock pin from an extended position to a retracted position with cold oil due to the high viscosity of the oil and, thus, a lower flow rate through the HLA, at lower temperatures and at relatively low pressure. A smaller major diameter of the lock pin would reduce the required flow capacity but reduction in size is constrained by the contact stress between the lock pin and the saddle of the high lift follower.
Second, the tolerances on the diameter of the lock pin head and surrounding bore must be controlled to manage the leakage through the clearance. Consequently, manufacturing costs are increased. The axial length of the lock pin head also influences the leakage. While a longer axial length of the lock pin head would reduce the leakage, a shorter length is beneficial for packaging both the return spring and the rocker arm in the cylinder head. However, tight clearances between the lock pin head and the bore combined with shorter pin head length increases the risk of jamming the lock pin head in the bore.
Third, the return spring cavity is typically vented such that clearance between the diameter of the pin shank (minor diameter) and bore is not required to seal against oil leakage. However, tolerances on the minor diameter size cannot be increased, as the diameter must be controlled for reasons of mechanical lash.
Fourth, a passage connecting the bore with the HLA intersects the bore of some prior art two-step finger follower type rocker arm assemblies at an angle, which may limit the hole size of the passage. The lock pin head and retention plug can in some applications partially block the passage and, thus, the flow area. Further, the angled hole may be difficult to drill and may be prone to burrs in critical areas.
What is needed in the art is an improved latching mechanism for a two-step finger follower type rocker arm assembly that more effectively moves the lock pin from a retracted position to an extended position, that eliminates the currently required tight clearances between lock pin head and bore, and that concurs with return spring and rocker arm packaging requirements.
It is a principal object of the present invention to provide a two-step rocker arm assembly having faster response characteristics under cold oil conditions among members of a manufactured population of two-step finger follower type rocker arm assemblies.
It is a further object of the invention to provide an improved oil circuit for a latching mechanism for a two-step finger follower type rocker arm assembly.
It is a still further object of the present invention to provide a lock pin mechanism that reduces the net volume of oil displaced by the lock pin and reduces the required return spring force for equivalent switch pressure.
Briefly described, a rocker arm body includes a first and a second oil passage connecting a lock pin bore with a variable oil pressure source, such as a socket for a hydraulic lash adjuster (HLA). The first oil passage leads from the HLA socket to a hydraulic chamber formed by a lock pin head and a retention plug sealing the lock pin bore. The second oil passage leads from the HLA socket to a return spring cavity. Adding the second oil passage will not increase the number of machining operations compared to the known prior art, since at the same time a venting hole typically machined into the follower body can be eliminated. Therefore, oil flow between the HLA and the return spring cavity and between the HLA and the hydraulic chamber above the lock pin head is enabled. Furthermore, leakage between the lock pin head and the wall of the lock pin head is advantageous rather than detrimental as in prior art. Due to the oil circuit in accordance with the invention, the minor diameter of the lock pin, which may be the diameter of the lock pin shaft, now determines the hydraulic force generated by oil pressure and the net volume of oil displaced by the lock pin motion. Compared to prior art lock pin mechanisms, up to 40% less oil needs to be displaced as the pin translates between the extended and retracted positions. Consequently, the viscous flow losses are reduced and switching response is improved, especially with cold oil. Since a smaller hydraulic force is needed to move the lock pin compared to prior art oil circulation, the required return spring force for equivalent switch pressure is reduced compared to prior art, which enables a superior return spring design, such as having a lower rate and/or shorter length.
Furthermore, with the introduction of the second oil passage that allows oil to flow to both sides of the lock pin head in accordance with the invention, oil leakage past the lock pin head is no longer a concern and, therefore, related tolerances can be relaxed compared to known prior art latching assemblies and the axial length of the pin head can be reduced. The shorter axial length of the lock pin head combined with larger allowable clearances between the lock pin head and the wall of the lock pin bore alleviates the lock pin head jamming concern of the prior art and reduces the chance of the lock pin head partially blocking the original passage where the passage breaks into the lock pin bore.
In an alternate embodiment in accordance with the invention, the first oil passage connecting the HLA socket with the hydraulic chamber above the lock pin head is completely eliminated and the lock pin is modified to permit oil flow between the return spring cavity and hydraulic chamber as well as from the HLA socket to and from the return spring cavity. Eliminating the original oil passage connecting the HLA socket with the hydraulic chamber is advantageous, since the typically angled flow passage is difficult to manufacture and may have potential for burrs in critical regions of the passage.
These and other features and advantages of the invention will be more fully understood and appreciated from the following description of certain exemplary embodiments of the invention taken together with the accompanying drawings, in which:
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.
The advantages and benefits afforded to a two-step roller finger follower (RFF) in accordance with the invention may be better appreciated by first considering a prior art two-step roller finger follower. Such a two-step RFF is suitable for use in a variable valve activation system of an internal combustion engine.
Referring to
Rocker arm 18 includes a body 26 having a first end 28 and a second end 30. A socket 32 is included at first end 28 for pivotably mounting RFF 10 on a hydraulic lash adjuster (HLA) (not shown). A pad 34 is included at second end 30 for actuating a valve stem (not shown). A latching mechanism 40 disposed in body 26 of rocker arm 18 at the first end 28 thereof selectively latches high-lift follower 12 in position to actuate the valve stem in response to the high-lift cam lobe base circle and eccentric, or selectively unlatches high-lift follower 12 to follow the high-lift cam lobe base circle and eccentric in lost motion.
Latching mechanism 40 includes a stepped bore 42, preferably cylindrical, in body 26. Latching mechanism further includes a piston 44 defining a locking pin having a head portion 46 and a shank portion 48. Head portion 46 of piston 44 may be biased outwards in bore 42 by a return spring 50 (retracted position) or shank portion 48 may be extend toward high-lift follower 12. Bore 42 is closed by a plug 52 forming a hydraulic chamber 54 that is in communication via passage 56 with socket 32.
Pressurized oil is supplied to chamber 54 in known fashion from the hydraulic lash adjuster (not shown) upon command from an engine control module to cause piston 44 to become hydraulically biased toward high-lift follower 12. The diameter of head portion 46 of piston 44 determines the magnitude of the hydraulic force (oil pressure times area of head portion 46) available to overcome the spring force and also the total volume of fluid displaced as the piston 44 moves. When such biasing occurs and overcomes the counter-bias of return spring 50, shank portion 48 extends from bore 42 towards high-lift follower 12 and locks high-lift follower 12 in position. Oil leaking past head portion 46 of piston 44 is vented from return spring cavity 58 via a vent hole (not shown) included in body 26 in the cutaway section.
When the engine control module determines, in known fashion from various engine operating parameters, that high-lift follower 12 should be unlocked, a reduced oil pressure is supplied to chamber 54, allowing return spring 50 to bias piston 44 away from high-lift follower 12, and high-lift follower 12 is again free to pivotally slide in central opening 16. As long as the lower oil pressure is supplied to chamber 54, latching mechanism 40 remains disengaged from high-lift follower 12. To minimize oil leakage from hydraulic chamber 54 into a return spring cavity 58 past head portion 46 of piston 44, the clearance between the diameter of head portion 46 and bore 42 must be carefully controlled with appropriate tolerances. Leakage through the clearance between head portion 46 and bore 42 may be controlled by designing head portion 46 of piston 44 to have a longer axial length 45, but axial length 45 is constrained by the packaging requirements for return spring 50. Too tight clearances between head portion 46 and bore 42 combined with too short axial head length may cause jamming of head portion 46 in bore 42. All of these relationships are known in the RFF prior art and need not to be further elaborated here.
Referring now to
Latching mechanism 140 includes a stepped bore 142, preferably cylindrical, and integrated in body 126 proximate to the first end 128. Bore 142 is closed by a plug 152 that is secured by a retaining clip 153. Latching mechanism 140 further includes a locking pin 144 that includes a pin head 146 having a first diameter 147 and a pin shank 148 having a smaller second diameter 149. Locking pin 144 is operated as a piston and is axially positioned within bore 142 such that pin head 146 faces plug 152. Plug 152 seals bore 142 and functions as a stop for locking pin 144. Pin shank 148 receives a return spring 150. Locking pin 144 is biased outwards in bore 142 by return spring 150.
A first hydraulic chamber 154 that is in communication via a first passage 156 with socket 132 is formed between plug 152 and pin head 146, and therefore above pin head 146. A second hydraulic chamber 158 that is in communication via a second passage 160 with socket 132 is formed by the cavity housing return spring 150 and is positioned below pin head 146. Body 126 further includes a spray hole 162 that is in communication with first hydraulic chamber 154.
In operation, upon command from an engine control module pressurized oil is supplied via first passage 156 to first hydraulic chamber 154 in known fashion from the hydraulic lash adjuster (HLA, not shown) inserted in socket 132 and simultaneously discharged from second hydraulic chamber 158 via second passage 160. It may further be possible to supply pressurized oil to second hydraulic chamber 158 via second passage 160 and to simultaneously discharge oil from first hydraulic chamber 154 via first passage 156. In any case, first hydraulic chamber 154 and second hydraulic chamber 158 contain oil at substantially the same pressure. The oil pressure in first hydraulic chamber 154 and second hydraulic chamber 158 causes locking pin 144 to become hydraulically biased towards cam-actuated slider member 112 due to the force produced by the pressure acting on the cross-sectional area of the pin shank diameter 149. When such biasing occurs and overcomes the counter-bias force of return spring 150, the end of pin shank 148 positioned opposite from pin head 146 is urged axially into a locking engagement with slider member 112. Locking pin 144 is now in extended position (shown in
By designing two-step finger follower rocker arm assembly 100 to include a first passage 156 and a second passage 160, oil is present in first hydraulic chamber 154 positioned above pin head 146 and in second hydraulic chamber 158 positioned below pin head 146 concurrently. The minor diameter of locking pin 144, which is the second diameter 149 of pin shank 148, determines the hydraulic force generated by the oil pressure available to move locking pin 144 to the extended position and the net volume of oil displaced by the motion of locking pin 144. The hydraulic force can be calculated from oil pressure times cross-sectional area of pin shank 148. With the benefit of the counter-force exerted on locking pin 144 by second passage 160, if the area of pin shank 148 is about 40% smaller than the area of pin head 146, the net force exerted by is the oil pressure against return spring 150 would be 40% less than the force that would be exerted against the return spring without the benefit of second passage 160. Thus, a smaller return spring 150 would be needed than return spring 50. Further, if the area of pin shank 148 is about 40% smaller than the area of pin head 146, then 40% less oil is displaced for the same pin travel compared to prior art rocker arm 18 that includes only passage 56, as shown in
When the engine control module determines, in known fashion from various engine operating parameters, that a retracted position of locking spring 144 and, thus, disengagement from slider member 112, is desired, a reduced oil pressure is supplied to first hydraulic chamber 154 and to second hydraulic chamber 158, allowing return spring 150 to again bias lock pin 144 away from slider member 112. While moving towards plug 152, locking pin 144 pushes oil contained in first hydraulic chamber 154 out through first passage 156 back to the HLA inserted in socket 132. Simultaneously, oil enters second hydraulic chamber 158 through second passage 160. As long as the reduced oil pressure is maintained in first chamber 154 and second chamber 158, latching mechanism 140 remains in retracted position (shown in
Since two-step finger follower rocker arm assembly 100, as illustrated in
Referring now to
As shown in
In operation, upon command from an engine control module, pressurized oil is supplied via passage 260 to second hydraulic chamber 158 in known fashion from the hydraulic lash adjuster (HLA, not shown) inserted in socket 132. The pressurized oil entering second hydraulic chamber 158 also flows into first hydraulic chamber 154 through cross hole 204 and axial hole 202 integrated in locking pin 244. The pressurized oil entering first hydraulic chamber 154 and second hydraulic chamber 158 causes locking pin 244 to become hydraulically biased towards cam-actuated slider member 112. When such biasing occurs and overcomes the counter-bias of return spring 150, the end of pin shank 148 positioned opposite from pin head 246 is urged axially into a locking engagement with slider member 112. Locking pin 244 is now in extended position (shown in
When the engine control module determines, in known fashion from various engine operating parameters, that a retracted position of locking spring 244 and, thus, disengagement from slider member 112, is desired, a reduced oil pressure is supplied to second hydraulic chamber 158 and to first chamber 154 through cross hole 204 and axial hole 202, allowing return spring 150 to again bias lock pin 244 away from slider member 112. While moving locking pin 244 towards plug 152, oil contained in first hydraulic chamber 154 flows out of first hydraulic chamber 154 through axial hole 202 and cross hole 204 into second hydraulic chamber 158 and from second hydraulic chamber 158 through passage 260 to socket 132. The oil flowing out of passage 260 into socket 132 is received by a hydraulic lash adjuster (HLA, not shown). As long as the reduced oil pressure is maintained in second chamber 158, latching mechanism 240 remains in retracted position (shown in
The hydraulic force generated by the oil pressure and the net volume of oil displacement by motion of locking pin 244 are the same as described above in connection with
While the invention has been described as relating to a two-step rocker arm assembly, it is understood that it can relate to a deactivating rocker arm assembly whereby, instead of a lower valve lift, no valve lift is applied. Furthermore, while the invention has been described in connection with a two-step RFF 10, it may be applicable for other cylinder activation operations.
While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.