Variable valve activation (VVA) mechanisms for internal combustion engines are well known. It is known 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 valve deactivation or valve lift switching can substantially improve fuel efficiency.
A Roller Finger Follower (RFF), as a type of rocker arm, acts between a rotating eccentric camshaft lobe and a pivot point on the engine, such as a Hydraulic Lash Adjuster (HLA), to open and close an engine valve. Switchable RFFs may be a “deactivation” type or a “two-step” type. The term switchable deactivation RFF, as used herein, means the switchable RFF is capable of switching from a valve lift mode to a no lift mode. The term switchable two-step RFF, as used herein, means the switchable RFF is capable of switching from a first valve lift mode to a second and lesser valve lift mode that is greater than no lift. When the term “switchable RFF” is used herein, by itself, it includes both types.
A typical switchable RFF includes an outer arm and an inner arm. The inner arm is movably connected to the outer arm. It can be switched by a locking member, from a coupled mode wherein the inner arm is immobilized relative to the outer arm, to a decoupled state wherein the inner arm can move relative to the outer arm. Typically, the outer arm of the switchable RFF is pivotally supported at a first end by the HLA. A second end of the outer arm operates against an associated engine valve for opening and closing the valve by the rotation of an associated eccentric cam lobe acting on an inner arm contact surface which may be a roller. The inner arm is connected to the outer arm for pivotal movement about the outer arm's second end with the contact surface of the inner arm disposed between the first and second ends of the outer arm. Typically, the locking member includes a locking pin disposed in a bore in the first end of the outer arm, the locking pin being selectively moved to engage the inner arm to thereby couple the inner arm to the outer arm when engaged, and decouple the inner arm from the outer arm when disengaged.
In a switchable two-step RFF, the outer arm typically supports a pair of rollers carried by a shaft. The rollers are positioned to be engaged by associated low-lift eccentric cam lobes that cause the outer arm to pivot about the HLA, thereby actuating an associated engine valve to a low-lift. The inner arm, in turn, is positioned to engage an associated high-lift eccentric cam lobe sandwiched between the aforementioned low-lift lobes. The switchable two-step RFF is then selectively switched between a coupled and a decoupled mode by the locking member. In the coupled mode, with the inner arm locked to the outer arm, the rotational movement of the central high-lift lobe is transferred from the inner arm, through the outer arm to cause pivotal movement of the RFF about the HLA, which, in turn, opens the associated valve to a high-lift. In the decoupled mode, the inner arm is no longer locked to the outer arm and is permitted to move relative to the outer arm against a lost motion spring that biases the inner arm away from the outer arm. In turn, the rollers of the outer arm engage their associated low-lift lobes. The rotational movement of the low-lift lobes is transferred directly through the outer arm, and the associated valve is reciprocated by the outer arm to a low-lift.
A switchable deactivation RFF typically includes an outer arm and an inner arm. The inner arm supports a roller carried by a shaft. The roller is engaged by an eccentric lifting cam lobe for actuating an associated engine valve. Like the switchable two-step RFF, the switchable deactivation RFF is selectively switched between a coupled and a decoupled mode by a movable locking member. In the coupled mode the inner arm of the switchable deactivation RFF is locked to the outer arm and the rotational movement of the associated lifting cam lobe is transferred from the inner arm, through the outer arm to cause pivotal movement of the RFF about the HLA, which, in turn, opens the associated valve to a prescribed lift. In the decoupled mode, the inner arm becomes unlocked from the outer arm and is permitted to pivot relative to the outer arm against a lost motion spring. In the decoupled mode, the rotational movement of the lifting cam lobe is absorbed by the inner arm in lost motion and is not transferred to the outer arm. Thus, the associated valve remains closed when the switchable deactivation RFF is in its decoupled mode.
In a first switchable deactivation RFF design, the inner arm makes contact with an associated cam lobe while the outer arm does not. The lost motion spring biases the inner arm away from the outer arm and, with the outer arm supported by the HLA, serves to load the inner arm against its associated cam lobe in the decoupled mode. In a switchable deactivation RFF having a lost motion spring with an effective force exerted on the HLA that is higher than the opposing force of an associated HLA spring, the opposing forces must be properly managed to prevent reactive pump-down of the HLA induced by the force of the lost motion spring. For this purpose, an expansion travel limiter is incorporated in the switchable deactivation RFF to limit the movement of the inner arm relative to the outer arm. Thus, when the switchable deactivation RFF is in its decoupled mode, and the inner arm of the RFF follows the cam lobe, the lost motion spring will push the outer arm until the expansion travel limiter is engaged. At that point, further movement of the outer arm relative to the inner arm ceases, HLA pump-down is prevented and HLA leak-down recovery is initiated. Moreover, at that point, since the effective preload force of the lost motion spring is greater than the expansion force of the HLA, pump-up of the HLA is prevented. Note also that, when the inner arm of the RFF follows the base circle of the associated cam lobe, the expansion travel limiter further serves to set a clearance gap or mechanical lash between the locking pin and the inner arm to assure proper alignment of the locking pin with the inner arm when the RFF switches between its decoupled and coupled modes and to define the total mechanical lash in the valve train.
In an alternate switchable deactivation RFF design, in order to increase its resistance to HLA pump-up beyond that provided by the installed load of the lost motion spring, null pads may be added on the outer arm of the switchable deactivation HLA for contacting zero-lift, constant radius null lobes disposed on either side of the associated lifting lobe. In this design, when the inner arm contacts the expansion travel limiter, the inner arm of the RFF is prevented from contacting the base circle of its associated cam lobe by the null pads first contacting with the zero-lift null lobes. Since the inner arm is held away from the base circle of the cam by the expansion travel limiter, the force of the lost motion spring cannot pump-down the HLA. By the null pads making contact with the zero-lift null pads, pump-up of the HLA is prevented by the opposing installed load of the associated valve closing spring. The expansion travel limiter establishes the mechanical lash between the locking pin and the inner arm; as well as the clearance (lash) between the inner arm and the base circle of the cam. The pin lash plus the cam lash establishes the total mechanical lash of the valve train.
Various lost motion expansion limiters used in switchable RFFs are known in the art. For example, in U.S. Pat. No. 6,532,920, a switchable two-step RFF is shown wherein the roller shaft of the outer arm contacts a throughbore in the inner arm to limit inner arm travel. As shown in U.S. Pat. Nos. 5,544,626, 5,653,198 and 6,314,928, bumper pads or projections formed at the lower end of the inner arm are used to limit inner arm travel of the switchable RFFs. The disadvantage of these devices in the prior art is that the stop position cannot be precisely controlled resulting in sometimes too small or too large of a mechanical lash between the locking pin and the inner arm or, in the case of a switchable deactivation RFF with null pads, resulting in a clearance between the inner arm and base circle of the associated cam lobe that is too too small, or even non-existing. A mechanical lash that is too small may result in the locking pin being unable to reliably engage the inner arm. A lash that is too large may permit excess pump-down of the HLA thereby delaying the opening point, decreasing the lift and advancing the closing point of the associated valve in the coupled mode which is known to contribute to engine roughness at idle and/or emission problems. A cam clearance that is too small (in the case of a switchable deactivation RFF with null pads), between the inner arm and its base circle, increases total lash when the inner arm is allowed to contact the cam base circle and may similarly affect the opening, closing and lift characteristics of its associated valve.
What is needed in the art is a device that precisely limits the amount of upward pivotable movement of the inner arm relative to the outer arm caused by the force of the lost motion spring.
It is a principal object of the present invention to provide an inner arm stop to precisely position the inner arm relative to the outer arm thereby controlling mechanical lash and HLA pump-down.