Patent Publication Number: US-10774693-B2

Title: Variable length piston assemblies for engine valve actuation systems

Description:
FIELD 
     This disclosure relates generally to systems and methods for actuating valves in internal combustion engines. More particularly, this disclosure relates to variable length piston assemblies that may be used in various applications, including applications relating to lost motion assemblies in engine valve trains and lost motion features that may be integrated into valve rocker arms and rocker arm pivots. The disclosure further relates to variable length piston assemblies that may be used in bleeder brake systems. 
     BACKGROUND 
     Internal combustion engines require valve actuation systems to control the flow of combustible components, typically fuel and air, to one or more combustion chambers during operation. Such systems control the motion and timing of intake and exhaust valves during engine operation. In a positive power mode, intake valves are opened to admit fuel and air into a cylinder for combustion and exhaust valves are subsequently opened to allow combustion products to escape the cylinder. This operation is typically called a “main event” operation of the valves. 
     In addition to positive power main event operation, valve actuation systems may be configured to facilitate “auxiliary events” during engine operation. These may include, but are not limited to, engine braking, exhaust gas recirculation (EGR) and internal exhaust gas recirculation (iEGR). During these auxiliary events, valve timing and motion may be controlled to cause the engine to recirculate exhaust gases to achieve improved emissions, or to cause the engine to absorb energy from the engine load in engine braking operations. 
     Valve movement during main event positive power modes of operation is typically controlled by one or more rotating cams as motion sources. Cam followers, push rods, rocker arms and other elements disposed in a valve train provide for direct transfer of motion from the cam surface to the valves. For auxiliary events, “lost motion” devices may be utilized in the valve train to facilitate auxiliary event valve movement. Lost motion devices refer to a class of technical solutions in which valve motion is modified compared to the motion that would otherwise occur as a result of actuation by a respective cam surface alone. Lost motion devices may include devices whose length, rigidity or compressibility is varied and controlled in order to facilitate the selective occurrence of auxiliary events in addition to, or as an alternative to, main event operation of valves. Lost motion devices may be viewed as a subclass of a larger category of variable length piston assemblies, which may have application beyond those involving lost motion. 
     Prior art lost motion systems typically rely upon hydraulic or pneumatic working fluids (i.e., oil or air) for their operation. As a result, such systems are not readily adaptable to engines that do not utilize such working fluids, or that use such fluids at relatively low pressures that aren&#39;t sufficient to actuate lost motion systems. 
     In addition to lost motion systems, prior art valve actuation systems may require variable length elements that may be actuated to provide other functions, such as braking action provided by bleeder brake components. Such variable length elements may be used to selectively actuate engine valves to cause a bleeder brake operation to occur. 
     With the use of mechanically interacting elements and surfaces, another challenge in the art is to provide actuating assemblies that can withstand the rapid and repeated cycling and stresses that occur in an engine valve train environment and which reduce the potential for excessive peak stresses to develop during operation. 
     It would therefore be advantageous to provide systems and methods that address the aforementioned shortcoming and others in the prior art. 
     SUMMARY 
     Responsive to the foregoing challenges, Applicants provide various embodiments of variable length assemblies and systems, which may be used in applications involving components that may expand or contract in length, such as lost motion assemblies in engine valve actuation systems, or in other applications, such as bleeder brake actuation, where a variable length device is useful in a valve train. 
     According to one aspect, lost motion assemblies and systems are provided which eliminate the need for hydraulic or pneumatic working fluids for operation and which facilitate start-stop decompression and rapid cycling during valve system operation. In one embodiment, the lost motion assembly is integrated into a valve rocker arm pivot. A piston or base of the pivot is retained in stationary position within a bore in a pedestal, which may be mounted on an engine cylinder head and may support valve train components for several cylinders. An actuating plate is mounted for rotational movement relative to the piston. A biasing element such as a spring may be disposed in a bore or recess within the piston to provide a biasing force on the actuating plate. The actuating plate may include an actuation post that enables an actuating assembly, which may include a solenoid and an actuating arm mounted for pivoting movement on the pedestal. The piston and actuating plate are provided with working surfaces that interact when the actuating plate is rotated relative to the piston to configure the lost motion assembly in an “off” state, in which motion may be absorbed, and an “on” state in which the lost motion assembly acts as a solid element that transmits full cam motion. In one embodiment, the working surfaces include lower flat portions, ramped transition portions, and upper flat portions. 
     According to another aspect, the working surfaces may include continuous shallow ramp portions. The ramp angle is shallow enough to prevent counter-rotation of the actuating plate under load from the rocker arm. In this embodiment, flat portions and discontinuities in the working surface may be reduced or eliminated. The shallow ramps of the working surfaces of the actuating plate and piston remain in close contact as the lost motion assembly is actuated, reducing the potential for partial engagement of the contact surfaces and resulting in excessive contact stress. 
     According to a further aspect, lost motion assemblies may be provided in conjunction with valve train linkages that utilize lash adjusters. The lost motion assembly may include an internal spring which exerts a force on the lost motion assembly actuating plate and piston. The force is sufficient to resist compression of the spring, and thus the lost motion assembly, when forces resulting from the attempted expansion of a lash adjuster are exerted on the valve train. The lost motion assembly stroke may be adjusted using a shim situated between a shoulder on the piston and the pedestal surface or a retaining plate surface. This support structure provides a simple way to adjust the stroke of the lost motion assembly. The spring may engage a spring cap that includes a central circular projection on which the actuating plate may rotate. The spring cap also engages a surface within the bore. The actuating plate may be provided with a small clearance such that when the lost motion assembly piston is in a maximum position (i.e., maximum lift from the shim), the spring counters forces exerted in the valve train by the lash adjuster and the actuating plate remains unloaded. This facilitates actuation of the actuating plate with relatively little force. 
     According to a further aspect, retaining assemblies for mounting lost motion assemblies are provided. The retaining assembly may include a bore in a pedestal used for mounting rocker arm pivots, rocker arms and other valve actuation components. The piston and actuating plate assembly may be retained in the bore. An anti-rotation key may be provided on the piston body to prevent rotation of the piston within the pedestal. An annular retaining plate may engage a shoulder on the lost motion assembly piston and may be fastened to the pedestal with threaded fasteners to thereby retain the piston and actuating plate assembly within the bore of the pedestal. 
     According to a further aspect, actuating assemblies for lost motion assemblies are provided. In one embodiment, an actuating post on the actuating plate may extend through a slot formed in the pedestal and may engage an actuating arm that is pivotably mounted on the pedestal. An actuating solenoid includes a plunger that engages and pivots the actuating arm when actuation of the lost motion assembly is desired. This motion rotates the actuating plate relative to the piston and changes the state of the lost motion assembly from an “off” state, where motion may be absorbed, to an “on” state where the lost motion assembly is rigid and does not absorb motion. 
     According to one aspect, a lost motion assembly comprising a variable-length assembly may be integrated into a rocker arm. A lost motion actuating assembly is also integrated into the rocker arm. The lost motion assembly may be integrated into a motion imparting end of the rocker arm, e.g., where it contacts a valve stem. An actuating plate having a working surface may rotate relative to a piston, which may move axially but not rotationally within the rocker arm. An actuator, which, for example, may be a pneumatic, electromagnetic or hydraulic actuator that utilizes oil flowing within the rocker arm shaft and through ports in the rocker arm may move linearly and, through a linkage, may rotate the actuating plate to change the state of the lost motion assembly from an “off” state, where motion may be absorbed, to an “on” state where the lost motion assembly is rigid and does not absorb motion. 
     According to a further aspect, a variable-length assembly may be integrated into a bleeder brake housing and used to actuate one or more valves for a bleeder brake application. A fixed plate is secured within the bleeder brake housing and may have an anti-rotation feature. A piston is mounted for rotational and axial movement within the housing. Both the fixed plate and piston have interacting working surfaces that engage one another to provide axial movement of the piston when the piston rotates relative to the fixed plate. An actuating ring with a pin may engage a pocket on the piston and permit rotational movement of the piston. A spring may be disposed on the pin and within the pocket on the piston to provide a relatively low biasing force on the piston in a direction towards the fixed plate and away from the engine valves during positive power operation. A solenoid actuating assembly may be integrated into the housing and provide axial movement which, through a linkage, results in rotational movement of the piston within the housing. Actuation of the solenoid thus results in a state change to the variable-length assembly from an “off” state, where the assembly has a minimum length, to an “on” state where the assembly has a maximum length. 
     According to a further aspect, a variable-length assembly may include a helical interacting surface in which the helical contact surface has a curved profile in a radial direction. This provides more evenly distributed contact stress across the engaged portion of the surface and avoids high contact stresses near the axis of the piston. With such configurations, maximum or peak contact stress during operation can be reduced. 
     Other aspects and advantages of the disclosure will be apparent to those of ordinary skill from the detailed description that follows and the above aspects should not be viewed as exhaustive or limiting. The foregoing general description and the following detailed description are intended to provide examples of the inventive aspects of this disclosure and should in no way be construed as limiting or restrictive of the scope defined in the appended claims. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
       The above and other attendant advantages and features of the invention will be apparent from the following detailed description together with the accompanying drawings, in which like reference numerals represent like elements throughout. It will be understood that the description and embodiments are intended as illustrative examples according to aspects of the disclosure and are not intended to be limiting to the scope of invention, which is set forth in the claims appended hereto. 
         FIG. 1  is a cross sectional view of components of an internal combustion engine environment suitable for implementing aspects of the disclosure. 
         FIG. 2  illustrates curves correlating valve lift and crank angle for main and auxiliary events that may occur during an engine operation. 
         FIG. 3  is a partial cross section showing an example lost motion assembly integrated into a center pivot for a rocker arm. 
         FIG. 4  is an exploded view of the lost motion assembly of  FIG. 3  as well as mounting and actuation details. 
         FIGS. 5A and 5B  are perspective views of an example actuator piston and actuator plate for a lost motion assembly. 
         FIGS. 6A and 6B  are graphical representations of the working surface profiles of the actuator piston and actuator plate of  FIGS. 5A and 5B , respectively. 
         FIGS. 6C and 6D  are graphical representations of an alternative embodiment of working surface profiles that may be used with the actuator piston and actuator plate of  FIGS. 5A and 5B , respectively. 
         FIG. 7  is a top view of the lost motion assembly of  FIG. 4  showing an actuating linkage and solenoid. 
         8 A and  8 B are perspective views showing operational positions of the example actuator piston and actuator plate of  FIGS. 5A and 5B . 
         FIGS. 9A, 9B and 9C  are elevation views of operational positions of the lost motion assembly of  FIGS. 5A and 5B . 
         FIGS. 10A and 10B  are perspective views of alternative example actuator piston and actuator plate, respectively. 
         FIGS. 11A and 11B  are graphical representations of the working surface profiles of the actuator piston and actuator plate of  FIGS. 10A and 10B , respectively. 
         FIGS. 12A and 12B  are assembled elevation views showing operational positions of the example actuator piston and actuator plate of  FIGS. 10A and 10B . 
         FIG. 13  is a top view showing contact surfaces of the actuator piston of  FIG. 10A . 
         FIGS. 14A, 14B, 14C and 14D  are elevation views showing progressive operation of the example actuator piston and actuator plate of  FIGS. 10A and 10B . 
         FIGS. 15A, 15B and 15C  are cross-sections showing operational positions of an alternative lost motion assembly in a system with lash adjusters. 
         FIGS. 16A and 16B  are top views of an actuating system that incorporates a rotary motion limiter for the motion of an actuator plate. 
         FIG. 17  is a top view of an alternative motion limiter. 
         FIG. 18  is a schematic diagram illustrating a three-part lost motion assembly. 
         FIGS. 19A, 19B and 19C  schematically illustrate operational positions of the lost motion assembly of  FIG. 18 . 
         FIG. 20  is an alternative anti-rotation configuration for an actuator piston. 
         FIG. 21  is an exploded view of a rocker arm having a lost motion assembly and actuating assembly integrated therein. 
         FIG. 22  is an assembled view of the rocker arm of  FIG. 21 . 
         FIGS. 23A and 23B  are top and side perspective views of an actuating assembly for the lost motion assembly of  FIG. 21 , showing an “off” position. 
         FIGS. 23C and 23D  are top and side perspective views of an actuating assembly for the lost motion assembly of  FIG. 21 , showing an “on” position. 
         FIG. 24  is a top view of a motion limiter which may function as a lash setting plate on the actuating assembly of  FIG. 21 . 
         FIG. 25  is an exploded view of a variable-length assembly and actuating assembly integrated into a bleeder brake housing. 
         FIG. 26  is an assembled view of the bleeder brake housing, variable-length assembly and actuating assembly of  FIG. 25 . 
         FIGS. 27A and 27B  are top views showing “off” and “on” positions of the actuating assembly of  FIG. 25 . 
         FIGS. 28A and 28B  are top views illustrating an example anti-rotation configuration for a fixed plate and a fixed plate receptacle in a bleeder brake housing, respectively. 
         FIG. 29  is a cross-section showing a lash setting adjustment feature for a variable-length assembly in a bleeder brake housing. 
         FIG. 30  is a perspective of an alternative example actuating ring for a variable-length assembly. 
         FIG. 31  is a cross-section of an actuating ring mounting configuration for a variable-length assembly. 
         FIG. 32  is a cross-section of a biasing spring configuration and actuating ring mounting configuration for a variable-length assembly. 
         FIG. 33  is a perspective of an example piston that may be used with the mounting configuration of  FIG. 32 . 
         FIGS. 34A, 34B and 34C  are views showing respective actuated positions of an example actuator piston. 
         FIG. 35  is a perspective view of an example helical actuator piston surface that may be used in a variable-length assembly. 
         FIG. 36  is a perspective view of an example helical fixed plate surface that may be used in conjunction with the actuator piston surface of  FIG. 35 . 
         FIG. 37  is a schematic illustration of helical interacting surfaces on a piston and fixed plate. 
         FIG. 38  is a detailed perspective view of a helical piston surface showing contour lines. 
         FIGS. 39A-39E  are cross-sections in radial planes  39 A- 39 A,  39 B- 39 B,  39 C- 39 C,  39 D- 39 D and  39 E- 39 E of  FIG. 35 , respectively, showing an example helical shape of ramped surfaces on a lost motion assembly piston. 
         FIG. 40A  is an illustration of contact stresses in a flat ramped helical surface of a lost motion assembly piston.  FIG. 40B  is an illustration of contact stresses in a curved ramped helical surface of a lost motion assembly piston with the contact area biased toward the larger radii of the helix. 
         FIGS. 41A and 41B  are top views showing a gear actuation assembly for a lost motion piston in “off” and “on” positions, respectively. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an internal combustion engine environment  10 , such as that described in U.S. Pat. No. 7,458,350, which is suitable for background purposes and application of aspects of the disclosure. This may be a V-8 engine having overhead valves  12 , which are actuated by rocker arms  50 , each of which pivots on a center ball pivot  20 . Cam followers  14 , which may be rocker or tappet type followers, may drive one or more pushrods  16 , which in turn transfer motion to rocker arms  50 . Motion from each cam lobe may drive a respective follower  14 . Each follower  14  may be linked to and drive two rocker arms  50  of the same type (intake or exhaust), and transfer motion thereto through respective independent lash adjusters. While this engine architecture is suitable for application of the lost motion assemblies and related features described in this application, those skilled in the art will appreciate that other engine architectures may be equally employed. Moreover, the lost motion features may be applied in the context of compression release, bleeder braking, decompression valve lift events and other aspects of engine operation. 
       FIG. 2  illustrates typical engine operational curves, which correlate valve lift and crank angle for main and auxiliary events that may occur during an engine operation. Such operations are described in U.S. Pat. No. 7,905,208, the teachings of which are incorporated herein by this reference. A normal main event valve motion is represented in curve  60 , the lower of the two exhaust event curves (first lift event). During positive power, lift is provided to the valves for the intake and exhaust cycles. During main event operation, lost motion components in the valve train absorb motion such that motion in the valve train that would otherwise cause valve lift at compression release event  62  and brake gas recirculation event  64  does not result in valve lift. During auxiliary events, which may occur as represented by curve  70 , the lost motion components do not absorb motion in the valve train such that compression release event  72  and brake gas recirculation event  74  result in valve lift at the appropriate time in the engine cycle to effectuate these events. 
       FIG. 3  illustrates a partial cross-section of a variable-length assembly  100  used as a lost motion assembly, which may be integrated into a housing, such as a rocker arm pivot for rocker arm  50 , which may be mounted on a pedestal  90  mounted on the cylinder head. As used herein, a housing may comprise a fixed housing or a dynamic housing. As used herein, a component is “fixed” to the extent that it is essentially (i.e., within design parameters and tolerances) immobile relative to valve actuation motions provided by a valve actuation motion source. In contrast, as used herein, a component is “dynamic” to the extent that it is capable of movement driven at least in part by valve actuation motions provided by a valve actuation motion source. As described in various embodiments described below, the housing, when fixed, may be embodied in components such as an engine rocker arm pivot, bleeder brake housing, valve overhead fixture or an engine support structure, or a bracket or fixture mounted thereon, or, when dynamic, the housing may be embodied in any of a number of valve train components including a rocker arm, valve bridge, pushrod or cam follower. The lost motion assembly  100  may be controlled by an actuating linkage, as will be described, to provide lost motion to the rocker arm pivot, and thus to the valve train, on demand. As will be described, the assembly may be actuated to an “off” position or state (also termed a deactivated or lost motion state) and an “on” state (also termed an activated or full motion transfer state). In the deactivated state (curve  60  of  FIG. 2 ), the assembly may undergo a specific deformation before becoming solid and transmitting main event motion, thus absorbing auxiliary event motion and only transmitting main event motion. In the activated state (curve  70  of  FIG. 2 ), the lost motion assembly is in a rigid state and therefore transmits all camshaft motion to the corresponding engine valve(s). 
       FIG. 4  is an exploded perspective of an example lost motion assembly  100  as well as other components for mounting and actuating the same. Pedestal  90  may include a number of stations for mounting a like number of rocker arm pivots and rocker arms. Only one pivot and rocker arm are shown for simplicity. 
     Referring additionally to  FIGS. 5A and 5B , lost motion assembly  100  may include a piston or base  110 , which may be generally cylindrical in shape and may include one or more anti-rotation elements or keys  112  formed in, or fastened to, a side thereof. A rounded pivot surface  114  is provided on a lower portion of piston  110 . An internal recess or bore  118  receives a spring  140 . A piston working surface  116  is provided on the base  110  to interact with an actuator plate working surface  126  on an actuator plate  120 , which includes an actuation post  122  extending therefrom. These elements and their interaction will be further detailed herein. 
     The lost motion assembly  100  may be assembled and installed in a retaining assembly which may include a pedestal  90  and other mounting components in the following manner. The spring  140  is installed in recess  118  and a spring cap  150  is installed on the top of spring  140 . Actuator plate  120  is then installed on spring cap  150  for rotational movement with a circular extension  152  fitting in a corresponding circular recess  128  in actuating plate  120 . These assembled components are then installed in a bore  94  in pedestal  90 , which is shaped to receive the major outer diameter of base  110  as well as the anti-rotation element  112 . Actuation post  122  may extend through a slot  92  in the pedestal  90 . A retaining ring or plate  230  may engage a shoulder  119  formed on the piston  110  and held in place on the pedestal  90  with fasteners  232  thereby retaining the piston  110  and thus the lost motion assembly  100  in position on the pedestal  90 . 
     Referring additionally to  FIG. 7 , the lost motion assembly  100  may be actuated with a solenoid  200 , which is mounted on the pedestal  90 . An actuating arm  210  may be pivotably mounted on the pedestal  90  with a fastener  212 . Solenoid  200  includes a plunger that may selectively engage and apply a force to a surface  211  on the actuating arm  210 , thereby rotating the actuating arm  210 . A return spring  213  provides a biasing force against the plunger action. An aperture  209  in the actuating arm receives the actuating post  122  such that pivoting movement of the actuating arm  210  results in rotational movement of actuating plate  120  and operation of the lost motion assembly  100 . 
       FIGS. 6A and 6B  are graphical representations (not to scale) of an example piston working surface  116  and actuating plate working surface  126 , respectively. It will be understood that the horizontal axis in these graphs is an angle measured from a reference point around the center of the piston or actuating plate. Complementary upper and lower stepped portions are provided with ramped transitions therebetween. More specifically, piston working surface  116  may include two lower plateaus or flat portions  116 . 1 , two ramped transition portions  116 . 2  and two upper plateaus or flat portions  116 . 3 . Similarly, actuating plate working surface  126  may include two upper plateaus or flat portions  126 . 1 , two ramped transition portions  126 . 2  and two lower plateaus or flat portions  126 . 3 . As shown in  FIGS. 6A and 6B , the piston  110  and actuating plate  120  are positioned relative to one another (i.e., rotationally positioned) in accordance with a motion-absorbing state in which the two lower flat portions  116 . 1  of the piston working surface  116  are aligned with the complementary upper flat portions  126 . 1  of the actuating plate working surface  126 . In this state, further described below, the piston working surface  116  and actuating plate working surface  126  may have a gap between them, as they are normally biased away from each other by spring  140  (i.e., there is lash space provided between the working surfaces  116 ,  126 ) but are otherwise free to come into contact with each other when forces in the valve train are sufficient to overcome the biasing force, thereby allowing the lost motion assembly  100  to “lose” motion. The extent of this lash space may be referred to as the stroke of the lost motion assembly. When the actuating plate  120  is rotated relative to the piston, thereby placing the piston  110  and actuating plate  120  in a motion transferring state, the profile represented in graph of  FIG. 6B  shifts relative to the graph of  FIG. 6A  and the working surfaces  116  and  126  interact such that the clearance between them is reduced. Specifically, the rotation of the actuating plate  120  aligns the upper flat portions  126 . 1  of the actuating plate working surface  126  with the upper flat portions  116 . 3  of the piston working surface  116 . In this state, engagement between the upper flat portions  126 . 1  of the actuating plate working surface  126  and the upper flat portions  116 . 3  of the piston working surface  116  prevent relative axial movement between piston  100  and actuating plate  120 , effectively preventing any motion applied to the piston  100  from being lost. The ramped transitions provide for motion similar to screw threads to assist in smooth operation between the two operating states and facilitate minimal wear. As will be recognized, the ramp angle and other parameters of the example components described may be varied to keep operational contact stresses, even during instances of partial engagement, within acceptable limits. For example, to achieve higher lift, a steeper ramp angle and/or increase in angle of relative rotation between the piston and actuating plate may be utilized. For an actuating plate and piston configuration with two ramped transitions, a relative rotation of about 70 degrees may be suitable. In addition, piston diameter, or the diameter of the working surface may be increased to keep contact stresses within acceptable limits. As will be further detailed below, especially with respect to  FIGS. 34A and 34B  and  FIG. 37 , transitions may be smoothed and blended to eliminate instances of excessive contact stress even when partial contact between the working surfaces occurs. 
     Additionally, while the working surface profiles illustrated in  FIGS. 6A and 6B  illustrate pairs of upper  116 . 3 ,  126 . 3  and lower  116 . 1 ,  126 . 1  flat portions that are spaced 180 degrees from each other, it is appreciated that a greater or lesser number of upper/lower pairs of flat portions may be provided on the piston  110  and actuating plate  120 . For example, both the piston  110  and actuating plate  120  could each comprise four pairs of upper/lower flat surfaces (once again separated by ramp portions) such that each pair spans 90 degrees of the overall profile. Furthermore,  FIGS. 6A and 6B  illustrate a single height difference between upper and lower flat portions. However, it is understood that flat portions of multiple heights could be provided such that different or intermediate levels of lost motion assembly operating length can be provided (i.e., an operating state in which the lost motion assembly has an intermediate “solid” operating length that is less than its maximum “solid” length). Such an example of working surfaces  616  and  626  are shown in  FIGS. 6C and 6D . As will be recognized, with the working surface profiles illustrated, the lost motion assembly may assume a first operating state having a first stroke with the working surfaces aligned as shown, wherein ramped surface  626 . 2  is aligned with ramped surface  616 . 2  and the first stroke is defined as the travel surfaces may move before being in contact. A second operating state results with the actuating plate working surface shifted to the right from the position shown in  FIG. 6C , where the ramped surface  626 . 2  is in alignment with the ramped surface  616 . 4  and a second stroke is defined for the lost motion assembly. 
     Referring additionally to  FIGS. 8A and 8B , in which the pedestal has been omitted, in an “off” or deactivated position shown in  FIG. 8A , the flat portions  126 . 1  ( FIG. 6B ) are aligned with flat portions  116 . 1  ( FIG. 6A ) of the piston working surface such that the lost motion assembly  100  can absorb motion as spring  140  compresses up to the point where flat portions  126 . 1  engage flat portions  116 . 1 . When the lost motion assembly  100  is actuated in the direction of the arrow in  FIG. 8B  to a fully “on” or actuated position, the flat portions  126 . 1  of the actuating plate working surface  126  are fully engaged with the flat portions  116 . 3  of the piston working surface  116  such that the lost motion assembly  100  is rigid and will not absorb motion. 
       FIGS. 9A, 9B and 9C  illustrate operational positions of the example lost motion assembly.  FIG. 9A  corresponds to an “off” position or state of the lost motion assembly where the position of the piston  110  corresponds to a base circle or lowest point of the corresponding cam. In this state, the lost motion assembly  100  has a stroke length “S” where it can absorb motion as the inner spring compresses.  FIG. 9B  shows the lost motion assembly in an “off” state where there is peak lost motion, that is, all of the motion intended to be lost is lost and normal valve lift would begin if further motion is imparted from valve train components that interact with the lost motion assembly.  FIG. 9C  shows the lost motion assembly in an “on” state where the position of the piston corresponds to the cam base circle. As will be understood by those of ordinary skill, rotation of the actuation plate thus results in lash take-up with the piston on base circle. 
       FIGS. 10A, 10B, 11A and 11B  illustrate an alternative embodiment for a lost motion assembly. In this embodiment, the working surfaces  516  and  526  are provided with shallow ramp portions. This configuration may be useful where the friction between the parts is sufficient to prevent the load on the piston  110  from inducing rotation of the actuating plate. That is, the rotary force induced by the contact angle is less than the static friction provided by the tangent force and the coefficient of friction. With this configuration, flat surfaces may be unnecessary. Moreover, the stroke of the lost motion assembly may be controlled by appropriate selection of the ramp angle and the degree of rotation of the actuating plate (i.e., solenoid plunger and/or actuating arm travel).  FIGS. 11A and 11B  depict the working surface geometries in detail (not to scale). The angle of the working surfaces may be on the order of 3 degrees and may be determined based on loading and friction against relative rotation. For example, for steel on steel and a lubricated frictional coefficient of 0.1, a 5.7 degree angle may be at approximate equilibrium. With a shallow 3 degree angle, for example, the frictional forces are approximately 90% more than the rotational force tending to counter-rotate the actuation plate  120 . When load is applied the actuator will remain at the angular position at the time of loading. For stroke control the extent or degree of rotation can be managed. The stroke per turn can also be increased if the angle is maintained the same, however a larger diameter piston may be used to increase the perimeter in the calculation. As will be recognized, such gentle inclined or shallow ramp surfaces do not have abrupt transitions and do not require transition from a ramped slope to a flat portion. As such, the potential for partial engagement of bearing surfaces and increased contact stress may be reduced. As will also be recognized, the ramps maintain flat-on-flat contact as relative rotation occurs.  FIGS. 12A and 12B  illustrate operational positions corresponding to lost motion “off” and lost motion “on,” respectively. As can be seen, no abrupt transitions are encountered as the actuating plate rotates relative to the piston.  FIG. 13  depicts the contact area, which is essentially flat-on-flat contact of two segments of a circular ring. As will be recognized by those of ordinary skill in the art, the diameter and width of the contact areas, as well as the length of contact areas may be varied to ensure that contact stresses do not become excessive during operation. 
       FIGS. 14A, 14B, 14C and 14D  depict operational states of a lost motion assembly with ramped slopes on the piston working surface and actuating plate working surface. These figures correspond to actuator plate rotations of 0, 30, 80, and 100 degrees, respectively. As can be seen, the interaction of the working surfaces can provide variable position/stroke of the actuator. The gentle sloped ramp of the actuator allows the piston to rotate easily when unloaded and locks the piston in place via friction when the loading is applied. Actuation can be varied by using a rotary or linear stepper motor or any type of variable position or variable force actuator methods to provide multiple positions of operation. The force required to move and hold the sloped ramp actuator in position is very low due to the low angle slope design, and bias spring that unloads the mechanism in the base circle position of the cam profile. 
     According to another aspect, lost motion assemblies may be configured to operate in systems equipped with lash adjusters. Lash is excessive play in the valve train—the linkage from the cam to the valve, including, for example, pushrod to rocker or pushrod to cam follower interfaces. Lash can lead to excessive noise, impact loading and other problems. A lost motion assembly is designed to have stroke to absorb unwanted events during “normal” main event motion. Likewise, a system with lash adjusters in the camshaft follower (or elsewhere in the system) is also designed to take up slack in the valvetrain. Thus, it is important that a lost motion device&#39;s stroke does not deplete under the forces introduced into the valve train by lash adjusters. The spring force in the lost motion assembly must be sufficient to maintain the assembly&#39;s stroke when a lash adjuster operates to take up slack in the valvetrain while ensuring that the lost motion assembly is only compressed as a result of camshaft motion. Thus, the lash adjuster will typically take the slack out of the system and the stroke of the lost motion assembly needs to be closely controlled to track the lost motion events in the camshaft. The “brake” or lost motion lash does not need to be set. Rather, only the stroke of the system needs to be set. 
       FIGS. 15A, 15B and 15C  depict operational states of a lost motion assembly  100  that may be used in valve actuation systems that include lash adjusters.  FIG. 15A  shows a lost motion assembly in an “off” state with the piston position corresponding to the cam base circle. The stroke “S” of the piston  110  corresponds to the distance between the working surfaces of the actuating plate  120  and the piston  110 . The stroke in this case is equal to the lost motion lift plus a small clearance to allow rotation of the actuating plate  120 . This clearance may be on the order of +/−0.001 inches and may be controlled with the use of a shim  235  situated between the piston shoulder  119  and the retaining plate  230 . Thus, shim thickness (which can be very accurately determined) may be used to control the clearance between the actuating plate and the piston  110 . Moreover, as will be recognized, the shim may be used to set the stroke “S” of the lost motion assembly and lash in the valve system does not need to be set due to the presence of the lash adjusters in the valve train. The lost motion assembly spring  140  exerts sufficient biasing force that it will not be compressed when the lash adjusters in the valve train operate to take up lash.  FIG. 15B  shows the lost motion assembly in an “off” state with the position of the piston  110  corresponding to peak compression release lift, for example. This is the point where the lost motion assembly goes “solid” and positive power lift is provided. The lift “L” is thus lost during the transition between the piston shoulder  119  lifting from the shim  235  to the point where the piston  110  makes solid contact with the actuating plate  120 .  FIG. 15C  shows the lost motion assembly in an “on” position and the piston position corresponding to cam base circle. It will be understood that the small clearance remaining at this point between the actuation plate  120  and the piston  110  allows relatively easy rotation of the actuation plate  120 . Moreover, the actuation plate at this stage does not sustain significant load, and is essentially unloaded since the lost motion assembly spring  140  exerts forces to counter those in the valve train caused by the lash adjusters and also keeps the spring cap  150  essentially bottomed against the floor (ceiling)  98  of the pedestal bore  94 . Thus, actuation of the actuation plate  120  in this state requires very little force and can be implemented without undue wear on the lost motion assembly components, even with frequent operation. The availability of low force actuation also provides quick system response times when actuation commands are initiated to the solenoid or other actuation devices, which may include progressive actuating devices such as stepper motors, variable position actuators and variable force actuators. 
       FIGS. 16A and 16B  illustrate details of a two-position on/off actuation system. Rotational movement of the actuating plate  120  can be limited by providing a slot  92  in the pedestal which defines the angular on and off positions for the actuating plate post  122 . The slot may be machined into the pedestal (see also  FIG. 1 ).  FIG. 16A  illustrates an off position  210 ′ of the actuating arm as well as an on position  210 ″ of the actuating arm and the corresponding position of actuating post  122  of the actuating plate  120  as moved by solenoid  200 . Slot  92  can be modified to achieve different applications and stroke lengths of the lost motion assembly.  FIG. 17  illustrates a further modification of mounting details in which a timing plate  290  may be adjustably secured to the pedestal  90  with a fastener  292  in a first slot  294  which permits rotational adjustment of the plate  290  and thus adjustment of a second slot  296  which adjusts the travel extent of actuating plate post  122 . This implementation may be used to adjust the lash and angular travel of the actuator piston. 
       FIG. 18  illustrates another embodiment of a lost motion assembly  1100  which utilizes two sets of interacting working surfaces to achieve increased effective travel and stroke for a given angular relative rotation. A center actuating plate  1120  may include two working surfaces  1126 . 1  and  1126 . 2 , each of which interact with a working surface on a respective piston  1110 . 1  and  1110 . 2 . Rotational movement of the center actuating plate may be achieved with a radially extending post (not shown) or other extension that may engage an actuating assembly. The pistons  1110 . 1  and  1110 . 2  are mounted to remain rotationally stationary in a retaining assembly. A biasing element  1140  may extend within the components and bias them in a separated configuration.  FIG. 19A  illustrates an “off” position in which the assembly is in a base circle position with the assembly expanded to its maximum stroke.  FIG. 19B  illustrates an “off” position when the full stroke of the assembly is absorbed.  FIG. 19C  illustrates an “on” position in which the lost motion assembly is in a solid, incompressible state and thereby transmits full cam motion. As will be recognized, this configuration permits increased lost motion assembly stroke for a given degree of rotation of the actuating plate  1120 . 
       FIG. 20  illustrates an alternative mounting and anti-rotation configuration for a lost motion assembly piston, such as that shown in  FIG. 1 . Piston  110  may include an anti-rotation recess  112 ′ which receives a complementarily shaped tab or projection  238  on retaining ring  230 , which may be fastened to the pedestal  90  in a manner similar to the retaining ring  230  shown in  FIG. 1 . As will be recognized by those of ordinary skill in the art, various mounting configurations may be implemented for the components of the lost motion assemblies described herein. 
     According to aspects of the disclosure, variable length piston assemblies functioning as lost motion assemblies may be integrated into valve rocker arms. Referring to  FIGS. 21 and 22 , a rocker arm  2150  may include a roller element  2152  secured within journals  2154  by a pin or axle  2156 . Roller element  2152  engages a cam surface (not shown) to affect movement of valve elements (not shown) as is known. A rocker arm shaft journal  2158  receives a rocker arm shaft (not shown) which supports the rocker arm for pivoting movement on an engine. Rocker arm  2150  may include a lost motion piston assembly receptacle or bore  2180  for receiving the components of a variable-length assembly/lost motion assembly  2100  therein. Such components may include a lost motion actuator piston  2110 , biasing spring  2140 , spring cap  2145  and actuating plate  2120  assembled into the lost motion piston assembly bore  2180  and retained therein by a fastener  2170 . Fastener  2170  may engage a slot  2112  in the lost motion actuator piston  2110  to prevent rotation but permit limited axial movement thereof. Lost motion actuator piston  2110  and actuating plate  2120  are provided with respective working surfaces that interact to provide selective axial movement in a manner described above relative to the other embodiments. An actuating pin  2122  extends from the actuating plate  2120  an engages an actuator linkage  2124 . Actuating plate  2120  may rotate relative to the lost motion actuator piston  2110  under force from the actuator linkage  2124 , which is given motive force by actuating components secured within actuator receptacle  2190 . An actuator plate limiter  2126  may be secured to the rocker arm  2150  and may limit movement of the actuating plate pin  2122  and thus the movement of the actuating plate  2120 . Actuating plate limiter  2126  may also adjust the rotational position of the actuating plate and thereby set the lash for the lost motion assembly  2100  as will be described. 
     The actuating components within actuator receptacle  2190  may include an activation piston  2192 , an activation piston biasing element  2194 , a biasing element end plate  2196  and a retaining element  2198 , which may be a spring retainer or “C” clip that expands within a slot in the bore  2190  to retain the elements therein. Referring additionally to  FIGS. 23A and 23C , activation piston  2192  may be actuated hydraulically via an oil passage  2310  which provides hydraulic fluid to a chamber  2312  defined by the activation piston  2192  and receptacle  2190 . Oil may be provided selectively from the rocker shaft using ports in a known manner. Biasing element  2194  tends to counteract the hydraulic forces on the activation piston  2192  and return the activation piston  2192  to its “off” position (to the left in  FIGS. 23A and 23C ). Activation piston  2192  may be secured to the linkage  2124  with a pin  2125  extending from activation piston  2192  through an elongated opening formed in the upper side wall of the receptacle  2190 . Linkage  2124  pivots about a linkage pivot pin  2127 . In operation, the actuating assembly will be in an “off” state shown in  FIG. 23A  in the absence of sufficient hydraulic fluid and pressure in the chamber  2312 . Activation piston  2192  is retracted (to the left in  FIG. 23A ) and actuating plate pin  2122  is in the position shown. As shown in  FIG. 23B , this “off” state corresponds to a shortened length of the stacked actuating plate  2120  and piston  2110 . When hydraulic fluid flows into chamber  2312 , piston  2192  moves to the position shown in  FIG. 23C , thus moving actuating plate pin  2122  to the position shown and causing the stacked actuating plate  2120  to rotate relative to the piston  2110  and an increase in the effective length of the stacked actuating plate and piston  2110 . 
       FIG. 24  shows further details of the limiter  2126 , which may be rotatably secured to the rocker arm by a central pivot  2408  and may include an adjustment slot  2410  for receiving an adjustment fastener  2412 . An actuating pin receiving slot  2414  receives and limits movement of the actuating plate pin  2122 . Thus, rotational adjustment of the limiter  2126  may be used to limit the travel of the actuating plate pin  2122  and thus the extension undergone by the lost motion assembly  2100 . In addition, lash may be set using the limiter  2126 . The fasteners  2412  and  2408  may be loosened and the rack may be moved to the “on” position manually. Then the limiter  2126  may be rotated to a position in which the actuating plate pin  2122  is engaged by the end of slot  2414 . This effectively reduces the extension of the piston beyond a point where any undue lash would exist. As will be recognized, the stroke of the assembly is fixed by the geometry of the slot  2414 . Rotation of the limiter  2126  may thus adjust the “off” position stop (the rightmost extent of slot  2414 ). Rotation of the limiter  2126  in a counterclockwise direction, for example, will move the “off” position stop counterclockwise and the piston will ride up the ramp and be partially extended when the assembly is in the “off” state. 
     According to aspects of the disclosure, variable length piston assemblies may be used in applications other than lost motion applications. For example, such variable length piston assemblies may be utilized in bleeder brake applications. As will be recognized in the art, bleeder brake components provide for the slight lifting of engine valves at appropriate times during the engine cycle to effect braking action.  FIGS. 25 and 26  illustrate an exploded and assembled view, respectively, of an example bleeder brake housing, which includes an integrated variable length piston assembly and actuator assembly. A bleeder brake housing  2510  may be fastened to an engine, e.g., above an exhaust valve or valve train component, using housing fasteners  2512  that extend through fastening bores  2514 . Bleeder brake housing may include a variable length piston assembly receptacle  2516  for receiving components of the variable length piston assembly  2550 . An actuating assembly mounting extension  2571  may include one or more threaded bores  2572  for securing the actuating assembly thereto. 
     An example variable length piston assembly  2550  may include a piston  2552  with a piston working surface  2554  defined thereon and a fixed plate  2556  with a fixed plate working surface  2558  defined thereon. These respective surfaces interact, in a manner that will be further described, to provide for variation in the length of the piston/fixed plate stack in response to relative rotational movement. An actuating ring  2560  may include an actuating ring pin  2562  extending therefrom. A piston biasing spring  2564  may be disposed on the pin  2562 . An adjustment set screw  2569  may abut a rear surface of the fixed plate  2556  and be secured with nut  2567  within the housing  2510  in a threaded bore (hidden in  FIG. 25 ). A snap ring retainer  2561  may secure the variable length piston assembly components within the receptacle  2516 . Further details regarding these components and their interaction will be provided below. 
     An actuating assembly  2570  may be secured to the bleeder brake housing  2510 . Assembly  2570  may include a solenoid coil  2572  secured to a mounting bracket  2573  with a threaded fastener  2574  and having a solenoid plunger  2575  disposed for selective axial movement within the coil  2572 . A spring element  2576  may bias the solenoid plunger in an extended direction. The end of plunger  2575  may include a yoke that is secured to an aperture  2568  within a linkage  2566  that may be pivoted to cause rotational movement of the piston  2552  as will be explained. 
       FIGS. 27A and 27B  illustrate an example actuation operation.  FIG. 27A  is a view from the underside of housing  2510  showing the piston  2552  in an “off” position. Solenoid plunger  2575  is in an extended position in which piston  2552  is in a first rotational position indicated by the position of pin  2562  at a “10 o&#39;clock” position as viewed in the figure. When solenoid  2572  is activated, plunger  2575  retracts to the position shown in  FIG. 27B , whereby piston  2552  is in a second rotational position indicated by the position of pin  2562  at an “8 o&#39;clock” position as viewed in the figure. This “on” position coincides with the maximum length of the piston and fixed plate stack.  FIGS. 41A and 41B  illustrate an alternative actuation assembly  4100  which utilizes a gear interface  4130  to the piston  4152 , which may be provided with a number of gear teeth  4162 , which may be involute elements that result in a constant rotational velocity and force on the piston  4152  when actuated. An actuation arm  4166  may include a number of gear teeth  4168  on an end thereof which engage the piston gear teeth  4162 . Under action from the solenoid plunger  4175 , actuation arm  4166  may cause rotation of the piston  4152  from the “off” position shown in  FIG. 41A  to the “on” position shown in  FIG. 41B . 
       FIGS. 28A and 28B  illustrate an example anti-rotation mounting configuration for a fixed plate and a piston assembly receptacle in a bleeder brake housing  2510 , respectively. The fixed plate  2556  may be provided with one or more notches or recesses  2810  on an outer circumference. Referring additionally to  FIG. 28B , the bleeder brake housing  2510  may be provided with a like number of projections  2820  which are received in the recesses  2810  when the fixed plate is installed in position in the housing  2510 , thereby preventing rotation of the fixed plate within the housing. 
       FIG. 29  is a cross-section illustrating installed positions of the components of an example variable-length assembly and adjustment of the assembly using the adjustment set screw  2567 . Fixed plate  2556  is disposed in a like-sized bore in the housing  2510  and secured against relative rotational movement as described above, in abutting engagement with the end of adjustment set screw  2569 . As will be recognized, the adjustment screw  2567  can be used to adjust the locked height of the fixed plate  2556 . For example, downward adjustment of the screw  2569  may result in displacement of the fixed plate  2556  and piston  2552  stacked components by a distance “G” as indicated. Thus, the configuration is advantageous in permitting lash and other adjustments to the variable-length assembly. 
       FIG. 30  is a perspective of a piston actuating ring  2560  with the piston actuating ring pin  2562  installed therein.  FIG. 31  illustrates an alternative mounting configuration for the piston actuating ring  2560 . According to this example, the variable-length assembly receptacle  2516  may be provided with a counterbore  3110  which acts as a journal or guide for the actuating ring  2560  to ride within. A snap ring  3112  may be used to secure the actuating ring  2560  within the counterbore  3110 . This configuration is advantageous in preventing binding or cocking of the actuating ring  2560  within the receptacle  2516 , mitigating side loading effects on the piston  2552  and providing smooth operation of the variable length piston assembly overall. 
       FIGS. 32 and 33  show further details as to how components of an example variable length piston assembly may be installed in a bleeder brake housing, including features of a piston biasing element. This configuration provides a low-profile piston biasing feature which may be used in applications, such as those with constrained space beneath a valve cover, where space is limited.  FIG. 33  shows details of an example piston, which may include an annular base  3310  and a central raised portion  3312  having a lateral pocket or recess  3314  defined on a side thereof. A circular recess  3316  is also formed in the base  3310  for receiving the spring  2564  and permitting clearance of the end of pin  2562 .  FIG. 32  is a cross-section showing an arrangement of the piston assembly components. Piston  2552  (shown in an inverted position compared to  FIG. 33 ) extends within a bore in the receptacle  2516 . The actuating ring  2560  is retained in a bore  3210  for rotational movement and actuating ring pin  2562  extends upward into the circular pocket  3316  and is disposed in the lateral pocket  3314 . A snap ring  2561  retains the actuating ring and piston in place. As will be recognized, spring  2564  provides a relatively light upward biasing force on the piston  2552  to prevent contact with valves or valve train components when bleeder braking operation is not being provided. 
       FIGS. 34A, 34B and 34C  show different operational positions of an example variable length piston assembly. In  FIG. 34A , the assembly is in an “off” position with the working surfaces of the fixed plate  2556  and piston  2552  in complete complementary engagement such that the effective length of the assembly is minimal. In  FIG. 34B , the piston is rotated to an intermediate position such that the flat portions of the working surface are in partial engagement and the effective length of the assembly is at its full extent. In  FIG. 34C , the piston is rotated completely, increasing the contact area of the working surfaces and maintaining the effective length at the full extent. As will be recognized, working surface profiles may be provided that convert about 70 degrees of rotation into about 1.5 mm of extension and locking with 50 degrees of rotation generating the full 1.5 mm lift and additional rotation providing increased contact area on the working surfaces. 
     According to an aspect of the disclosure, working surface profiles are provided on a variable length piston assembly which advantageously reduce the peak contact stresses experienced by the interacting piston and fixed plate. More particularly, the ramped portions of the working surfaces may include a surface that is the form of a variable pitch helix may be utilized. Such surfaces will result in a “cone-on-cone” contact shape between the piston surface and fixed plate surface in instances of partial engagement, i.e., when the piston is in a partially activated position when the piston surface engages the fixed plate surface. This contact shape may be configured to ensure that contact stresses do not become excessive.  FIGS. 35 and 36  are perspective views which illustrate details of example helical working surfaces on a piston and fixed plate, respectively. Piston  3510  may include a helical working surface with two ramps  3520 . 1  and  3520 . 2  having the form of a variable pitch symmetrical helix. A tangent slope section may be provided in the center. In an example implementation, the lift and pitch of the ramp portions may extend for about 50 degrees of a radial sweep about a central axis of the surface, with flat areas  3530 . 1  and  3530 . 2  beyond this rotation. More particularly, from a radial sweep of zero to about 24 degrees, the pitch may progress from zero (flat surface) to a pitch of 21 (tangent slope). At the pitch of 21, there may be a dwell of 1 degree. At further radial sweep positions of 25 to 50 degrees the pitch may progress (regress) from a pitch of 21 to a pitch of 0. Similarly, the fixed plate  3610  may have two ramps  3620 . 1  and  3620 . 2  in the form of a variable pitch symmetric helix and extend for about 50 degrees, with flat areas  3630 . 1  and  3630 . 2  beyond this rotation. 
       FIG. 37  schematically illustrates the engagement of the respective ramp portions  3520 . 1  and  3620 . 1  of the piston  3510  and fixed plate  3610 .  FIG. 38  illustrates further detail of a complex helix surface profile with contour lines  3550 . 1 ,  3550 . 2  and  3550 . 3  showing the curvature of the surface at three different radii from the center. As will be recognized, the pitch and curvature varies at different radii. At any given radius, the helix is an angular sweep of a surface about an axis while also translating axially at a prescribed pitch. In the case of a complex curved ramp the pitch varies as the sweep rotates through the angle. Starting with zero pitch peaking at the midway point through the angle and finally ending with zero pitch the shape is as shown above. 
       FIGS. 39A-39E  are cross-sections of an example lost motion assembly piston with helical ramped portions, taken in respective radial planes as indicated in  FIG. 35 . The cross-section in  FIG. 39A  shows the elevation of ramps  3520 . 1  and  3520 . 1  just after transition from a lower flat area (proceeding in a counterclockwise direction when viewed from above). One of the elevated flat areas  3530 . 2  is visible in the background on the right.  FIG. 39B  shows a slightly higher elevation of the ramps  3520 . 1  and  3520 . 2 .  FIGS. 39C and 39D  show still higher elevations of the ramps  3520 . 1  and  3520 . 2 , with  FIG. 39D  showing an elevation of ramps  3520 . 1  and  3520 . 2  that is almost flush with the flat area  3530 . 2 .  FIG. 39E  shows a section taken through the flat areas  3530 . 1  and  3530 . 2 . 
     In order to further reduce contact stresses, the variable pitch helical surfaces of the ramps  3520 . 1  and  3520 . 1  may be provided with a slight curvature (i.e., large radius of curvature) in a radial direction and oriented to provide more contact area at the outer radii of the piston.  FIG. 39D  is annotated to show a slight curvature of radius “R” of the surface  3520 . 1  in the radial plane of the cross-section. The curvature radius may be on the order of 300 to 500 mm. Moreover, as indicated, the curvature may be oriented such that the radius origin “O” may be offset from the rotational axis “A” of the piston. This orientation of the curvature may provide a very slightly higher elevation of the surface near the outer radial edge of the piston and thereby bias contact between the piston and the actuating plate such that slightly greater contact forces occur near the outer radial edge of the piston and are thus spread over a greater width area of line contact, reducing contact stresses. 
       FIGS. 40A and 40B  illustrate the advantage of the radial curved surface geometry in reducing contact stress.  FIG. 40A  shows a contact stress result for a ramp that has a flat cross-sectional profile (i.e., where there is no curvature in the radially extending line  3520 . 1  representing the surface in any of  FIGS. 39A-D ).  FIG. 40  B shows a contact stress result for a ramp that has a slight curvature in the cross-sectional profile of the ramp surface. As can be seen, the radial curvature of the ramp surface may result in a substantial decrease in contact stress since the curvature causes contact forces to be distributed such that the outer areas (larger radii) of the piston supports higher contact forces, which are distributed over a slightly wider area of line contact than would be the case for inner areas (smaller radii) of the piston. 
     As will be recognized, the interaction of the variable pitch symmetric helix sections of the respective working surfaces provides cylinder on cylinder line contact between the piston working surface and the fixed plate working surface in the instance where the respective ramp sections of each engage one another, i.e., during surface engagement when the piston is in an intermediate rotational orientation. If an “on” actuation sequence is initiated, in the instance where engagement of the piston and fixed plate occurs when flat areas are not aligned and rather the respective ramp portions engage when the load is applied, contact will be line contact with sufficient breadth that peak contact stresses may be reduced, thus preventing the development of contact stress in excess of material limits. Moreover, in such an event, and when line contact occurs at a place where the slope is greater than 8 degrees, the pitch may permit the piston to rotate under valve train forces back to the off position. 
     As will be recognized, various manufacturing methods may be used to creating variable length pistons and piston assemblies according to aspects of this disclosure. Such methods may include the steps of cold forming, hot forming, powdered metal forming, casting or conventional machining. Post forming hardening may be employed with one or more of these steps. 
     Although the present implementations have been described with reference to specific example embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.