Patent Application
20170284313
The present invention relates to a switchable rocker arm for a valve train of an internal combustion (IC) engine, and more particularly, to the coupling assembly of a switchable rocker arm that provides two discrete valve lift modes.
More stringent fuel economy regulations in the transportation industry have prompted the need for improved efficiency of the IC engine. Lightweighting, friction reduction, thermal management, variable valve timing and a diverse array of variable valve lift technologies are all part of the technology toolbox for IC engine designers.
Variable valve lift (VVL) systems typically employ a technology in a valve train of an IC engine that allows different engine valve lifts to occur. The valve train consists of the components that are required to actuate an engine valve, including a camshaft (also termed “cam”), the valve, and all components that lie in between. VVL systems are typically divided into two categories: continuous variable and discrete variable. Continuous variable valve lift systems are capable of varying a valve lift from a design lift minimum to a design lift maximum to achieve any of several lift heights. Discrete variable valve lift systems are capable of switching between two or three distinct valve lifts. Components that enable these different valve lift modes are often called switchable valve train components. Typical two-step discrete valve lift systems switch between a full valve lift mode and a partial valve lift mode, often termed cam profile switching, or between a full valve lift mode and a no valve lift mode that facilitates deactivation of the valve. Valve deactivation can be applied in different ways. In the case of a four-valve-per-cylinder configuration (two intake+two exhaust), one of two intake valves can be deactivated. Deactivating only one of the two intake valves can provide for an increased swirl condition that enhances combustion of the air-fuel mixture. In another scenario, all of the intake and exhaust valves are deactivated for a selected cylinder which facilitates cylinder deactivation. On most engines, cylinder deactivation is applied to a fixed set of cylinders, when lightly loaded at steady-state speeds, to achieve the fuel economy of a smaller displacement engine. A lightly loaded engine running with a reduced amount of active cylinders requires a higher intake manifold pressure, and, thus, greater throttle plate opening, than an engine running with all of its cylinders in the active state. Given the lower intake restriction, throttling losses are reduced in the cylinder deactivation mode and the engine runs with greater efficiency. For those engines that deactivate half of the cylinders, it is typical in the engine industry to deactivate every other cylinder in the firing order to ensure smoothness of engine operation while in this mode. Deactivation also includes shutting off the fuel to the dormant cylinders. Reactivation of dormant cylinders occurs when the driver demands more power for acceleration. The smooth transition between normal and partial engine operation is achieved by controlling ignition timing, cam timing and throttle position, as managed by the engine control unit (ECU). Examples of switchable valve train components that serve as cylinder deactivation facilitators include roller lifters, pivot elements, rocker arms and camshafts; each of these components is able to switch from a full valve lift mode to a no valve lift mode. The switching of lifts occurs on the base circle or non-lift portion of the camshaft; therefore the time to switch from one mode to another is limited by the time that the camshaft is rotating through its base circle portion; more time for switching is available at lower engine speeds and less time is available at higher engine speeds. Maximum switching engine speeds are defined by whether there is enough time available on the base circle portion to fully actuate a coupling assembly to achieve the desired lift mode.
The precision of control of the deactivated cylinders varies within the engine industry. For optimum performance of the system, selective cylinder control rather than simultaneous multiple cylinder control is recommended. With selective cylinder control, the timing of the valve deactivation event with respect to the combustion cycle is maintained for each individual cylinder; for example, in a selective cylinder control system, an exhaust charge is normally trapped in the cylinder, which serves as an air spring and aids oil control during the deactivated mode. This is typically accomplished by deactivating the exhaust valve(s) first, followed by deactivation of the intake valve(s) of a given cylinder. With simultaneous multiple cylinder control, the timing of the valve deactivation event with respect to the combustion cycle is not controlled to the extent of the selective cylinder control resulting in intermittent exhaust charge trapping.
In today's IC engines, many of the switchable valve train components that enable valve deactivation for cylinder deactivation contain a coupling or locking assembly that is actuated by an electro-hydraulic system. The electro-hydraulic system typically contains at least one solenoid valve within an array of oil galleries that manages engine oil pressure to either lock or unlock the coupling assembly within the switchable valve train component to enable a valve lift switching event. These types of electro-hydraulic systems require time within the combustion cycle to actuate the switchable valve train component.
In most IC engine applications, switchable valve train components for cylinder deactivation in an electro-hydraulic system are classified as “pressureless-locked”, which equates to:
a). In a no or low oil pressure condition, the spring-biased coupling assembly will be in a locked position, facilitating the function of a standard valve train component that translates rotary camshaft motion to linear valve motion; and,
b). In a condition in which engine oil pressure is delivered to the coupling assembly that exceeds the force of the coupling assembly bias spring, the coupling assembly will be displaced a given stroke to an unlocked position, facilitating valve deactivation where the rotary camshaft motion is not translated to the valve.
“Pressureless-unlocked” electro-hydraulic systems can be found in some cam profile switching systems that switch between a full valve lift and a partial valve lift, which equates to:
a). In a no or low oil pressure condition, the spring-biased coupling assembly will be in an unlocked position, facilitating a partial valve lift event; and,
b). In a condition in which engine oil pressure is delivered to the coupling assembly that exceeds the force of the coupling assembly bias spring, the coupling assembly will be displaced a given stroke to a locked position, facilitating a full valve lift event.
With the successful implementation of cylinder deactivation systems on millions of production engines, engine manufacturers are now looking to expand the operating range. Examples include switching at higher engine speeds along with switching at colder oil temperatures. In addition, a new type of cylinder deactivation is in development that expands the deactivated mode operating range, increases the number of deactivating cylinders, and increases the frequency of switching in and out of a deactivated mode. In this new type of cylinder deactivation, all cylinders, as opposed to a group of cylinders, are continuously switched on and off depending on the demanded engine output. By controlling the engine output over a larger operating range in this way instead of by conventional throttling, pumping losses are reduced even further compared to traditional cylinder deactivation systems and, thus, a higher engine efficiency is achieved.
Vital to the durability and performance of a switchable valve train component is the robustness of the coupling assembly. Two important design attributes of the coupling assembly include: 1). the ability to switch from a locked to an unlocked position very quickly, and 2). a high resistance to wear. However, many times these attributes are in opposition. For example, the locking/unlocking stroke of the coupling assembly to engage/disengage an adjacent component has a direct impact on switching times; a shorter stroke for a given cross-sectional area of a coupling assembly will likely yield a faster switching time. Yet, a shorter stroke typically dictates a smaller contact area with the engaged or disengaged component, meaning that a given load is applied over a smaller area leading to higher contact pressures and subsequent wear. For this reason, various coupling assembly forms, materials, coatings and heat treatments are often employed in an effort to maximize wear resistance in order to minimize the actuation stroke and resultant contact area.
Given the aforementioned design challenges and more stringent switching time demands for switchable valve train components, a coupling assembly for improved wear and actuation times is required. Therefore, a primary objective of this invention is to locate the coupling assembly at a position within a switchable rocker arm that reduces the load and resultant wear on the coupling assembly.
A switchable rocker arm for valve deactivation that pivots about a rocker shaft is provided for a valve train of an internal combustion engine. The switchable rocker arm includes a valve side lever assembly, a cam side lever assembly, and a hydraulically actuated coupling assembly. The hydraulically actuated coupling assembly is located at a position within the switchable rocker arm that minimizes loads and resultant wear on the coupling assembly. The hydraulically actuated coupling assembly facilitates two valve lift modes: a full valve lift mode and a no valve lift mode. The full valve lift mode is achieved when the valve side lever assembly is coupled or locked to the cam side lever assembly; thereby, when a camshaft rotationally actuates the cam side lever assembly, both assemblies pivot together in unison about the rocker shaft, allowing rotary motion of the camshaft to be translated to linear motion of an engine valve. The no valve lift mode results when the valve side lever assembly is uncoupled or unlocked from the cam side lever assembly; thereby, when the camshaft rotationally actuates the cam side lever assembly, only the cam side lever assembly rotates about the rocker shaft while the valve side lever assembly remains stationary, preventing translation of the rotary motion of the camshaft to the engine valve.
The valve side lever assembly includes a first housing with two axially offset arms at one end defining a cavity, and a valve interface and shuttle pin bore that houses a hydraulically actuated shuttle pin at an opposite end. The valve interface can be in the form of a hydraulic lash adjuster, as provided in a first embodiment, or an adjusting screw assembly, as provided in a second embodiment. Each of the two arms has a rocker shaft bore to interface with and pivot about the rocker shaft.
The cam side lever assembly includes a second housing with a cam interface at one end, a locking pin bore at an opposite end, and a rocker shaft bore between the two ends to pivot about the rocker shaft. The cam side lever assembly resides in the cavity formed by the two offset arms of the first housing of the valve side lever assembly in such a way that the two rocker shaft bores of the first housing are axially aligned with the rocker shaft bore of the second housing. A limited rotational position of the cam side lever assembly with respect to the valve side lever assembly is provided by two inwardly protruding stops located on each of the two axially offset arms of the first housing of the valve side lever assembly. The cam interface can be in the form of a roller follower assembly or a sliding pad. The locking pin bore houses a locking pin, in contact with a bias spring or resilient element that is displaced by the adjacent hydraulically actuated shuttle pin. Optionally, a sleeve can be arranged within the locking pin bore to house the locking pin.
The locking pin moves in a longitudinal direction within the locking pin bore (or sleeve) to achieve a first locked position and a second unlocked position. The first locked position results when the locking pin bore of the second housing is axially aligned with the shuttle pin bore of the first housing, enabling engagement of the locking pin with both the locking pin bore and the shuttle pin bore. In this position, the bias spring or resilient element in contact with one end of the locking pin is compressed and provides a pre-load to the locking pin; additionally, the position of the locking pin is defined by a first distance from an outer end of the locking pin to a blind or closed end of the locking pin bore, and the position of the shuttle pin is defined by a second distance from an outer end of the shuttle pin to a blind or closed end of the shuttle pin bore. The first locked position fulfills a full valve lift or activated valve mode during which rotational cam lift is translated to linear valve lift.
The second unlocked position results when hydraulic pressure, typically engine oil pressure, is applied to the adjacent shuttle pin engaged with the locking pin. The force created by the hydraulic pressure acting on the shuttle pin overcomes the pre-load of the compressed bias spring acting on the adjacent locking pin, causing the locking pin to move longitudinally to the second unlocked position at which the locking pin is disengaged with the shuttle pin bore. In this unlocked position, the bias spring contacting the locking pin is compressed further than in the first locked position; additionally, compared to the first unlocked position, the locking pin is closer to the closed end of the locking pin bore, defined by a third distance, and the shuttle pin is further away from the closed end of the shuttle pin bore, defined by a fourth distance. While in the second unlocked mode, the cam side lever assembly is rotationally displaced about the rocker shaft by the camshaft separately from the valve side lever assembly, which remains stationary, fulfilling a no valve lift or deactivated valve mode. A lost motion spring or resilient element is arranged between the cam side and valve side arm assemblies to provide a force during the second unlocked mode that can, a). control the motion of the cam side lever assembly such that separation with the camshaft does not occur at a maximum deactivation engine speed, and, b). act upon the valve side lever assembly to prevent pump-up of the optional hydraulic lash adjuster.
Multiple variations of an oil gallery or fluid passage network within the first housing of the valve side lever assembly are possible to transport oil from the rocker shaft to accommodate the previously described functions and component options. For a fluid passage network of the first embodiment of the switchable rocker arm that contains a hydraulic lash adjuster to serve as the valve interface, the shuttle pin can receive hydraulic fluid from at least one fluid passage within either or both axially offset arms that first feeds the hydraulic lash adjuster, followed by the shuttle pin, in series. In a variation of this fluid passage, separate fluid passages for the shuttle pin and hydraulic lash adjuster can exist within either or both arms for the hydraulic lash adjuster and shuttle pin. A fluid passage network for the second embodiment of the switchable rocker arm that contains an adjusting screw assembly to serve as the valve interface requires only a fluid passage or passages within either or both of the axially offset arms to feed the shuttle pin. The adjusting screw assembly typically does not need an oil feed, however, if one is needed, either of the previously described fluid passage networks could be applied.
A location of the hydraulically actuated coupling assembly within the switchable rocker arm is specified at the valve end of the switchable rocker arm in order to facilitate a maximum distance between a locking interface and the switchable rocker arm pivot point. While in the first locked position that facilitates the full valve lift mode, the switchable rocker arm is subjected to a load throughout the valve lift event causing the locking pin to be loaded in shear due to its partial position within the locking pin bore and the shuttle pin bore. A magnitude of this shear load is proportional to a distance from the rocker arm pivot point to where the locking pin is loaded in shear. Therefore, placement of the coupling assembly at a location that is furthest away from the rocker arm pivot point will yield lower shear loads and subsequently lower wear of the coupling assembly.
The foregoing Summary as well as the following Detailed Description will be best understood when read in conjunction with the appended drawings. In the drawings:
Certain terminology is used in the following description for convenience only and is not limiting. The words “inner,” “outer,” “inwardly,” and “outwardly” refer to directions towards and away from the parts referenced in the drawings. A reference to a list of items that are cited as “at least one of a, b, or c” (where a, b, and c represent the items being listed) means any single one of the items a, b, c or combinations thereof. The terminology includes the words specifically noted above, derivatives thereof, and words of similar import.
Referring to
Referring to
Referring specifically to
The switchable rocker arm 10 captured in
Referring now to
Referring again to
M
11
=F
v
×d
v
To counteract this moment created on the switchable rocker arm 10 by the engine valve, a counter-moment is present about the central axis 11 created by a reactive force F34 of the locking pin 34, hereafter termed “reactive shear force”, multiplied by a magnitude of vector d34. Assuming that a sum of moments about the pivot axis 11 is zero, the reactive shear force F34 can be expressed as shown below:
One can observe that the magnitude of the reactive shear force F34 of the locking pin 34 is inversely proportional to the magnitude of vector d34. Therefore, as the magnitude of vector d34 increases, the reactive shear force F34 applied to the locking pin 34 decreases. Furthermore, with reference to
Quantifying the difference in reactive shear force between the two locking pin locations, a distance of 14 millimeters is assumed for d34A that corresponds with the alternative locking pin 34A shown in broken lines, and a distance of 28 millimeters is assumed for d34 that corresponds with the locking pin 34 shown in solid lines. Using the equation for reactive shear force reduction, a reduction of 50% is achieved by locating the locking pin 34 at the fourth end of the second housing 32 versus the less distant location of the alternative locking pin 34A, providing significantly reduced locking pin stress and resulting wear.
Referring to
Having thus described various embodiments of the present switchable rocker arm in detail, it is to be appreciated and will be apparent to those skilled in the art that many physical changes, only a few of which are exemplified in the detailed description above, could be made in the apparatus without altering the inventive concepts and principles embodied therein. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore to be embraced therein.
