Patent Publication Number: US-10316709-B2

Title: Electromechanical valve lash adjuster

Description:
PRIORITY 
     The present application claim priority from U.S. Provisional Application No. 62/221,275 filed Sep. 21, 2015 and from U.S. Provisional Application No. 62/313,440 filed Mar. 25, 2016. 
    
    
     FIELD 
     The present disclosure relates to valvetrains and methods of operating them. 
     BACKGROUND 
     In most internal combustion engines, the valves that control cylinder ports for intake and exhaust are actuated using cams mounted on a cam shaft. Rocker arm assemblies are configured to convert the rotational motion of the cams into linear motion through which the valves open and close. The cams may be shaped in view of the timing with which it is desired to have the valves open and close. 
     The rocker arm assemblies form force transmission pathways between the cams and the valves. Valve lash is a gap or clearance that occurs within one of those pathways over the course of cam shaft rotation. There may be an optimal or preferred amount of lash. Too little lash may result in valve leakage or damage to moving parts. Too much lash may result in improper timing, noise, or excessive wear. 
     A variety of factors may affect lash. Among those factors are manufacturing tolerances, thermal expansion, and wear. In view of those factors, most engines include means for adjusting valve lash. In some engines, the lash adjustment means is designed for manual lash adjustment to be performed after assembly and again later during maintenance. Other engines use hydraulic lash adjusters that adjust lash automatically and dynamically while the engines are operating. 
     SUMMARY 
     The present teachings relate to an internal combustion engine that includes a cylinder head, a poppet valve having a seat within the cylinder head, a cam shaft on which is mounted an eccentrically shaped cam, and a rocker arm assembly comprising a cam follower. The cam follower may be positioned to engage and follow the cam as the cam shaft rotates. The rocker arm assembly may form a force transmission pathway through which force from the cam is transmitted to actuate the poppet valve. 
     According to some aspects of the present teachings, the rocker arm assembly includes an electromechanical lash adjuster operable to control lash in the force transmission pathway. In some of these teachings, the rocker arm assembly includes a rocker arm and the electromechanical lash adjuster provides a fulcrum for the rocker arm. The electromechanical lash adjuster includes a variable length structure that determines a spacing between the fulcrum and the cylinder head. The lash adjuster has an electromechanical actuator operable to vary the length of the structure and thereby control lash. In some of these teachings, the length of the structure is continuously variable over a range of adjustment. In some of these teachings, the variable length structure is the entire lash adjuster. An electromechanical lash adjuster as described herein may provide a lower compliance as compared to a hydraulic lash adjuster. The improved stiffness in the valvetrain may improve valve timing. The design may be very compact. 
     In some of these teachings, the actuator is housed within an outer body of the lash adjuster. The outer body of the lash may be cylindrical or nearly cylindrical. In some of these teachings, the actuator is an electromagnetic motor. Housing an electromagnetic motor within the outer body of the lash adjuster may protect the actuator from metal particles suspended in oil in the surrounding environment. 
     According to some aspects of the present teachings, the electromechanical lash adjuster includes two parts that interface through one or more surfaces that are angled such that rotation of one of the parts about an axis while the other part is prevented from rotating causes a linear displacement between the two parts along the axis. The electromechanical actuator may be configured to drive the rotation and the electromechanical lash adjuster may be operative as a linear actuator that varies the spacing between the fulcrum and the cylinder head in relation to the rotation. In some of these teachings, the interface between the two parts is formed through an angled end surface of one or the other part. The two parts may interface end-to-end. In some others of these teachings, the two parts interface through helical threads on one or both parts. In some of these teachings, the electromechanical actuator comprises an electromagnetic motor. The motor may have a spindle configured to drive the rotation. In some of these teachings, the spindle is parallel to, but offset from, the axis about which the part rotates. The motor may be housed within an outer body of the lash adjuster. The motor may drive a pinion gear that meshes with a larger gear that is fixed to the rotating part. These structural features lend themselves to forming a low cost, low compliance, compact, electro-mechanical lash adjuster that has a high load bearing capacity while employing a small actuator. 
     In some aspects of the present teachings, the electromechanical lash adjuster includes first, second, and third parts and the electromechanical actuator is configured to rotate the second part about an axis and relative to the first and third parts. The second part interfaces with the first part through one or more angled surfaces and with the third part through one or more other angled surfaces. The angles of these surfaces are such that rotation of the second part about the axis while the first and third part are prevented from rotating causes a linear displacement between each pair of parts along the axis. In some of these teachings, the second part has two sets of threads, one having opposite threading (left versus right-hand) from the other. One set of threads may form the interface with the first part and the other the interface with the third part. The second part may include an internally and externally threaded. An electromechanical actuator may be configured to drive rotation of the second part relative to the first and third parts. This relative rotation may cause the third part to extend or retract relative to the first part. This structure may facilitate load bearing by the lash adjuster and may provide leverage for the actuator. Also, lash adjustment may be carried out without relative rotation between the ends of the lash adjuster. In some of these teachings, lash adjustment is carried out without rotation of an end of the lash adjuster on which the rocker arm pivots. In some of these teachings, the two sets of threads on the second part have differing pitches. Varying the pitches of the threads provides a means to control the amount of length adjustment that occurs per unit actuator movement. 
     In some of these teachings, the actuator comprises an electric motor that is positioned above a rocker arm for which the electromechanical lash adjuster provides a fulcrum. In some of these teachings a part of the lash adjuster, which may be a part coupled to the electric motor, passes through an opening in the rocker arm. In some of these teachings, the electric motor is held in a fixed position relative to the cylinder head. 
     A gear set may be provided between an electric motor and a threaded part driven by the motor. In some of these teachings, a gear ratio between the electric motor and a part it drives is ten to one or greater. In some of these teachings, the gear ratio is about 25 to one or greater. In some of these teachings, the gear set includes a planetary gear set. The planetary structure may allow the gears to be very compact. A high gear ratio allows the use of a smaller motor and may stiffen the lash adjuster. 
     According to some aspects of the present teachings, the electromechanical actuator is a linear actuator extensible between a first end and a second end thereof. As the term is used here, a linear actuator is a device that is operative to linearly extend a contact surface while applying a force in the direction of extension. Rotation that accompanies the linear extension is not inconsistent with this definition, although in some of these teachings the contact surface extends without rotation. In some of these teachings, the contact surface is a surface on which a rocker arm pivots. 
     According to some aspects of the present teachings, the electromechanical actuator includes a piezoelectric drive element. In some of the teachings, the actuator is an amplified piezo actuator. In some of these teachings, the actuator is a piezoelectric stepper motor. In some of these teachings, the actuator is a SQUIGGLE® motor such as the motor described in U.S. Pat. No. 7,309,943, which is incorporated herein by reference. A piezoelectric actuator may operate without creating magnetic fields that could attract metal particles suspended in oil. Attraction of such particles could interfere with the operation of a lash adjuster. 
     In some of these teachings, the actuator includes a piezoelectric stepper motor that requires at least 100 cycles to travel through the range of adjustment provided by the lash adjuster. In some of these teachings, the stepper motor requires at least 1000 cycles to travel through the range of adjustment. The range of adjustment may be on the order of 3 mm. Requiring a large number of steps to cover the range of motion provides precision and allows the use of smaller piezoelectric elements. 
     According to some aspects of the present teachings, the electromechanical actuator joins two parts with threaded engagement and is operative through a vibratory mechanism. In a vibratory mechanism, one of the parts is induced to vibrate in two modes. The vibrations may be induced by two or more piezoelectric elements. The vibrations may be at or near a resonant frequency of the actuator. The two modes of vibration may be 90 degrees out of phase. The vibrations may be effective to cause an area of contact between the engaged threads to rotate about an axis, creating torque between the engaged parts and inducing relative rotation. The phase relationship of the two modes of vibration may be changed to alter the direction of relative rotation. 
     In some of these teachings, the lash adjuster includes two parts selectively joined by an actuator. The two parts may be movable relative to one another to provide the variable length structure through which lash is controlled. In some of these teachings, the two parts are telescopically engaged. The actuator may include an electromechanical locking element operative to selectively restrain telescoping of the two parts. The actuator may release engagement to adjust lash and may engage the two parts to maintain the length of the variable length structure. 
     In some of these teachings, the electromechanical lash adjuster is operable over a range of extension through which it resists compression along its length primarily through friction. In some of these teachings, the electromechanical lash adjuster is structured whereby the friction force that resist compression increases as load on the electromechanical lash adjuster increases. In some of these teachings, struts connecting two telescoping parts in a lash adjuster are angled relative to the direction of telescoping, whereby a portion of a compressive force on the lash adjuster is translated into a radial force that increases friction between the two telescoping parts. 
     According to some aspects of the present teachings, the electromechanical lash adjuster is operable according to a clamp-extend-clamp-retract mechanism. An actuator operable according to these teachings may include two electromechanical locking elements spaced apart and joined by a structure that is variable in length. The locking elements may be piezoelectric devices. The connecting structure that is variable in length may also be a piezoelectric device. The actuator may be operative to vary the length of a fulcrum or other part provided by the lash adjuster by keeping the first locking element engaged while disengaging the second locking element, extending or contracting the connecting element to create extension or contraction between the two locking elements, engaging the second locking element, disengaging the first locking element, reversing the extension or contraction of the connecting element, then reengaging the first locking element. This process may allow the actuator to travel along the length of one of the two parts and vary the length of a fulcrum with locomotion similar to that employed by an inchworm. 
     According to some aspects of the present teachings, valve timing is adjusted, over a significant range by varying lash. This variation may increase or decrease an amount of overlap between intake and exhaust valve opening and control an amount of internal exhaust gas recirculation. The cam may be shaped to accommodate this mode of valve timing variation. In some of these teachings, the cam shapes allow the amount of overlap to be varied over a substantial range without significantly changing the opening velocities of the valves. In some of these teachings, the amount of overlap may be varied over the range without significantly changing the closing velocities of the valves. In some of these teachings, the amount of overlap may be varied over the range without significantly changing the rate of acceleration of the valves as they begin to open. In some of these teachings, the amount of overlap may be varied over the range without significantly changing the rate of deceleration of the valves as they approach closing. In some of these teachings, the amount of overlap is varied in relation to the engine&#39;s operating regime. The engine operating regime may relate to one or more of torque, speed, temperature, and/or other factors. In some of these teachings, the amount of overlap is varied without input from an engine control unit (ECU). In some of these teachings, the amount of overlap is varied based on engine speed and or temperature. 
     According to some aspects of the present teachings, the rocker arm assembly comprises a first rocker arm for which the electromechanical lash adjuster provides a fulcrum and an auxiliary rocker arm pivotally linked to the first rocker arm at a joint proximate the fulcrum. The auxiliary rocker arm may be configured to reduce stress on the electromechanical lash adjuster in a direction orthogonal to that in which the lash adjuster is extensible. In some of these teachings, the first rocker arm extends from the joint in the direction of the cam follower and the auxiliary rocker arm extends from the joint in the opposite direction. In some of these teachings, the auxiliary rocker arm has an end distal from the joint and the distal end is pivotally mounted at a position that is substantially fixed relative to the cylinder head. In some of these teachings, the auxiliary rocker arm is pivotally mounted to a cam carrier. 
     In some of these teaching, the electromechanical actuator is in a load bearing position within the rocker arm assembly. In some of these teaching, the electromechanical actuator is in a load bearing position under the fulcrum provided by the lash adjuster. Placing the electromechanical actuator in a load-bearing position may facilitate the use of that actuator to provide feedback for control or diagnostic purposes. An electromechanical actuator in a load bearing position may also be operative as a generator. In some of these teachings, an actuator in a load-bearing position is operated to provide vibration control. 
     According to some aspects of the present teachings, the electromechanical lash adjuster includes a controller. In some of these teachings, the controller is independent from the ECU. In some of these teachings, the controller is operative without crank angle data. In some of these teachings, the controller implements a control algorithm based on measurements that relate to the fraction of time that a cam is applying a force to the rocker arm assembly. In some of these teachings, the data used by the algorithm is provided by detecting when a load greater than a threshold value is applied to the fulcrum. In some of these teachings the load is detected by sensing force or pressure. In some of these teachings, the force is sensed by the actuator. In some of these teachings, the force is detected through a resulting displacement of the poppet valve. The controller may compare the two inputs and adjust the lash accordingly. In some of these teachings, the comparison involves determining a fraction of the cam cycle over which the cam is applying the force to the rocker arm assembly. In some of these teachings, the comparison involves determining a ratio between the length of the cam cycle over which the cam is applying the force and the length of the cam cycle over which the cam is not. The lash may be adjusted to cause the result of one of these determinations to approach a target value or to keep it within a target range. 
     According to some aspects of the present teachings, the lash is not adjusted with the cam follower contacting a base circle portion of the cam. Operation of the actuator to adjust lash may cease before the cam follower has come in contact with the cam. In some of these teachings, the cam does not include a base circle structure. The absence of a base circle structure allows the cam to be smaller and lighter and means the cam follower does not contact the cam throughout much of the cam cycle, which reduces friction and may improve fuel economy. Automatic and dynamic lash adjustment without requiring the cam follower to contact the cam at a base circle position may be accomplished by one of the methods described herein. 
     According to some aspects of the present teachings, the actuator includes a servomotor. A servomotor is a motor that may be operative to actuate to a particular position in response to a command to move to that position. In some of these teachings, the motor action is disabled during a period when the cam may be applying substantial force to the rocker arm assembly. A servomotor may lend itself to making rapid adjustments of the lash toward a desired setting. 
     According to some aspects of the present teachings, the actuator includes a stepper motor. A stepper motor may be operative to move one or a whole number of unit distances (steps) in response to commands. A stepper motor may provide a high degree of positional stability and may simplify control. A stepper motor may also have a low sensitivity to variations in its power supply. According to some aspects of the present teachings, the lash adjuster is operative to maintain its position under load without power being supplied to the actuator. 
     According to some aspects of the present teachings, a component of the rocker arm assembly further comprises a component that is operative to sense a force in proportion to a force applied by the cam to the rocker arm assembly. In some of these teachings, the actuator consumes power to maintain the lash and the power consumption is monitored to sense the load. In some of these teachings, the rocker arm assembly comprises a load cell that is distinct from the actuator. In some of these teachings, the actuator comprises a piezoelectric element in a position to detect load on the lash adjuster. The load sense may be used to control the lash as described elsewhere herein. 
     According to some aspects of the present teachings, the electromechanical lash adjuster includes a sensor or is operative as a sensor. In some of these teachings, the sensor is operative to sense a displacement of the valve or a component of the rocker arm assembly. The sensor may be used to control the lash as described elsewhere herein. In some of these teachings, the displacement sensor is a Hall effect sensor, although other types of displacement sensors may be used instead. 
     In some of these teachings, a sensing functionality used to control lash is also used to detect wear. For example, wear of bearings or valve seats in the rocker arm assembly may be detected by the electromechanical lash adjuster. This diagnostic information may be reported to an engine control unit. In some of these teaching, the sensing functionality may be used to detect vibrations. 
     According to some aspects of the present teachings, the electromechanical lash adjuster is operable to dampen vibrations in the rocker arm assembly. In some of these teachings, the electromechanical actuator is operated to induce cyclic movement of the lash adjuster with a timing selected to dampen vibrations in the rocker arm assembly. In some of these teachings, a current to a piezoelectric actuator is varied according to a periodic function that has the effect of dampening vibrations. 
     Another aspect of the present teachings is a method of operating an internal combustion engine. According to the method, two points in the cam cycle are detected. A first point relates to when the cam begins applying a force to the rocker arm assembly or inducing a displacement in the rocker arm assembly. A second point relates to when the cam ceases applying the force or inducing the displacement. The elapsed times between these points and successive occurrences of these points may be compared and the lash is adjusted on the basis of the comparison. In some of these teachings, the comparison involves the ratio between the length of the period over which the force or displacement is being applied to the rocker arm assembly and the length of the period over which it is not. In some of these teachings, the comparison involves the fraction of the cam cycle over which the force or displacement is being applied to the rocker arm assembly. Either of these parameters may be determined without knowledge of the crank angle. Accordingly, these methods lend themselves to a lash adjuster that is operative without data from an ECU. 
     A lash adjuster according to the present teachings may require little power for actuation. According to some aspects of the present teachings, the actuator is powered with energy produced by a generator that is mounted to the electromechanical lash adjuster. In some of these teachings, a controller for the actuator is also powered by the generator. A lash adjuster-mounted generator may be operative to convert mechanical energy into electricity. Providing a generator as part of the lash adjuster may reduce or eliminate the need to run wires to the lash adjuster. In some of these teachings, the generator is configured to be driven by force from the cam shaft transmitted through the rocker arm assembly. In some of these teachings, the generator is configured to be driven by vibrations of the electromagnetic lash adjuster. In some of these teachings, the generator is an electromagnetic generator. In some of these teachings, the generator is a piezoelectric generator. In some of these teachings, the generator includes a piezoelectric element that is also a part of the actuator. 
     The primary purpose of this summary has been to present certain of the inventors&#39; concepts in a simplified form to facilitate understanding of the more detailed description that follows. This summary is not a comprehensive description of every one of the inventors&#39; concepts or every combination of the inventors&#39; concepts that can be considered “invention”. Other concepts of the inventors will be conveyed to one of ordinary skill in the art by the following detailed description together with the drawings. The specifics disclosed herein may be generalized, narrowed, and combined in various ways with the ultimate statement of what the inventors claim as their invention being reserved for the claims that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like are used in the following detailed description to describe spatial relationships as illustrated in the drawings. Those relationships are independent from the orientation of any illustrated device in actual use. 
         FIG. 1A  is a partial cutaway side view of an internal combustion engine according to some aspects of the present teachings. 
         FIG. 1B  is a perspective view of an electromechanical lash adjuster according to some aspects of the present teachings in a retracted configuration. 
         FIG. 1C  is a perspective view of the electromechanical lash adjuster of  FIG. 1B  in an extended configuration. 
         FIG. 2A  is a cross-sectional view of an electromechanical lash adjuster according to some aspects of the present teachings in a retracted configuration. 
         FIG. 2B  is a perspective view of the electromechanical lash adjuster of  FIG. 2A . 
         FIG. 2C  is a cross-sectional view of the electromechanical lash adjuster of  FIG. 2A  in an extended configuration. 
         FIG. 2D  is a perspective view of the electromechanical lash adjuster of  FIG. 2C . 
         FIG. 3  is a partial cutaway side view of an internal combustion engine according to some other aspects of the present teachings. 
         FIG. 4A  is a partial cutaway side view of an internal combustion engine according to some other aspects of the present teachings. 
         FIG. 4B  is a cross-sectional view of an electromechanical lash adjuster according to some aspects of the present teachings in a retracted configuration. 
         FIG. 4C  is a perspective view of the electromechanical lash adjuster of  FIG. 4B . 
         FIG. 4D  is a cross-sectional view of the electromechanical lash adjuster of  FIG. 4B  in an extended configuration. 
         FIG. 4E  is a perspective view of the electromechanical lash adjuster of  FIG. 4D . 
         FIG. 4F  is a perspective view of an electromechanical actuator that is in accordance with some aspects of the present teachings and is used in the electromechanical lash adjuster of  FIGS. 4B-4E . 
         FIG. 5  is a flow chart of a method used in some aspects of the present teachings 
         FIG. 6A  is a perspective view of an electromechanical actuator that may be used in accordance with some aspects of the present teachings. 
         FIG. 6B  is an exploded view of the actuator of  FIG. 6A . 
         FIGS. 6C-6G  are a series of drawings illustrating the operation of the actuator of  FIG. 6A . 
         FIG. 7  is a flow chart of a method according to some aspects of the present teachings. 
         FIG. 8A  is a perspective view of an electromechanical lash adjuster according to some aspects of the present teachings in a retracted configuration. 
         FIG. 8B  is a perspective view of the electromechanical lash adjuster of  FIG. 8A  in an extended configuration. 
         FIG. 8C  is a cross-sectional view of the electromechanical lash adjuster of  FIG. 8A  in a retracted configuration. 
         FIG. 8D  is the same view as  FIG. 8C  but with the electromechanical lash adjuster in an extended configuration. 
     
    
    
     DETAILED DESCRIPTION 
     In the drawings, some reference characters consist of a number followed by a letter. In this description and the claims that follow, a reference character consisting of that same number without a letter is equivalent to a listing of all reference characters used in the drawings and consisting of that same number followed by a letter. For example, “engine  100 ” is the same as “engine  100 A,  100 B,  100 C,  100 D”. Engine  100  is therefore a generic reference that includes the specific instances engine  100 A, engine  1006 , etcetera. Where options are provided for one instance subject to a generic reference, those options are to be given consideration in connection with all instances subject to that generic reference. 
       FIG. 1  provides a partial cutaway side view of an internal combustion engine  100 A according to some aspects of the present teachings. The view includes a portion of a cylinder head  101 , a poppet valve  102  having a seat  103  within cylinder head  101 , an eccentrically shaped cam  104 A mounted on a cam shaft  105 , and a rocker arm assembly  109 A. Rocker arm assembly  109 A includes a rocker arm  106 A, an electromechanical lash adjuster  111 A, and a cam follower  108 . Cam follower  108  is mounted to rocker arm  106 A and is positioned to engage and follow cam  104 A as cam shaft  105  rotates. Cam follower  108  is a roller follower, although another type of cam follower such as a slider could be used instead. 
     Rocker arm assembly  109 A forms a force transmission pathway through which force from cam  104 A may be transmitted to actuate poppet valve  102 . Lash  107  occurs in this force transmission pathway. Lash  107  is illustrated as occurring between cam  104 A and cam follower  108 , but may occur elsewhere in the force transmission pathway such as between rocker arm  106 A and poppet valve  102 . 
     Electromechanical lash adjuster  111 A is extensible between a first end  131 A and a second end  133 A thereof. First end  131 A provides a fulcrum on which rocker arm  106 A pivots. Electromechanical lash adjuster  111 A includes an electromechanical actuator  115 A operable to vary the length of lash adjuster  111 A, which the distance between first end  131 A and second end  133 A. Adjusting the length of electromechanical lash adjuster  111 A varies the height of first end  131 A above cylinder head  101  and thereby controls lash  107 . Electromechanical actuator  115 A is operable to continuously vary the length of electromechanical actuator  115 A while engine  100 A is operating, although lash adjustment may be prevented when cam  104 A is loading rocker arm assembly  109 A. 
     Electromechanical lash adjuster  111 A includes an upper part  110 A and a lower part  112 A. Lower part  112 A is nearly cylindrical and provides an outer body for lash adjuster  111 A. Electromechanical actuator  115 A is housed within that outer body. In conjunction with upper part  110 A, lower part  112 A protects electromechanical actuator  115 A from metal particles in oil that may be dispersed throughout the environment surrounding lash adjuster  111 A. The metal particles might otherwise be attracted by magnetic components of electromechanical actuator  115 A and interfere with its operation. 
       FIG. 1B  provides a perspective view of electromechanical lash adjuster  111 A in a retracted configuration while  FIG. 1C  provide the same view after actuation to a more extended configuration. Upper part  110 A and lower part  112 A interface through helical threads  114 . Threads  114  are pitched, and therefore angled, such that rotating part  110 A about its axis  150  while part  112 A is prevented from rotating about axis  150  results in relative rotation between these parts, causes a linear displacement between upper part  110 A lower part  112 A, extends or contracts lash adjuster  111 A depending on the direction of relative rotation, and raises or lowers the height of fulcrum  131 A over cylinder head  101  thereby adjusting lash  107 . 
     Upper part  110 A may be, in part, an externally threaded shaft while lower part may be, in part, an internally threaded tube. Electromechanical lash adjuster  111 A is continuously variable in length by relative rotation between upper part  110 A and lower part  112 A. Electromechanical actuator  115 A includes an electromagnetic motor  116  that is coaxial with upper part  110 B and lower part  112 B. Operation of electromagnetic motor  116  may be controlled through a controller (not shown). The controller may be an engine control unit (ECU) or a separate controller associated with lash adjuster  111 A 
       FIGS. 2A-2D  show a different electromechanical lash adjuster  111 B that may be used in engine  100  in place of electromechanical lash adjuster  111 A. Lash adjuster  111 B includes an upper part  110 B, a lower part  112 B, and an intermediate part  131 B. Intermediate part  131 B has internal threads  124  formed on an inner surface  126  and external threads  123  formed on its outer surface. Internal threads  124  and external threads  123  having opposite orientations, one set being left-hand threads and the other being right-hand threads. Intermediate part  131 B may be considered a tube. Internal threads  124  may engage external threads  122  of upper part  110 B. External threads  123  may engage internal threads  125  of lower part  112 B. These threads provided angled surfaces through which these parts interface. Relative rotation between upper part  110 B and lower part  112 B may be prevented by an anti-rotation guide  135 B, which is mounted to lower part  112 B and travels within a slot  132 B in upper part  110 B. Motor  116  may be housed within, and fixed to prevent rotation with respect to, lower part  112 B. A shaft  121  of motor  116  may be coaxial with threads  122 ,  123 ,  124 , and  125  and have a non-circular cross-section, e.g. D-shaped, that mates with an opening  120  in intermediate part  131 B allowing motor  116  to drive rotation intermediate part  131 B. 
       FIGS. 2A and 2B  provide cross-sectional and perspective views of lash adjuster  111 B in a retracted configuration.  FIGS. 2C and 2D  provide corresponding views with lash adjuster  111 B in a relatively more extended configuration. Motor  116  is operative to actuate lash adjuster  111 B between these configurations by rotating shaft  121 . The rotation of intermediate part  131 B by motor  116  results in linear displacement between intermediate part  131 B and each of parts  110 B and  112 B. Moreover, the rotation causes a linear displacement between parts  110 B and  112 B, which varies the length of lash adjuster  111 B, which is characterized by a distance between its first end  131 B and its second end  133 B. 
     Internal threads  124  and external threads  123  may have differing pitches. The ratio between rotations of shaft  121  and units of extension of lash adjuster  111 B may be controlled by varying the pitch of threads  122  and  124  and/or the pitch of threads  123  and  125 . For example, internal threads  124  may have a pitch of about 0.2 mm and external threads  123  may have a pitch of about 0.3 mm. 
       FIG. 3  illustrates an engine  100 C having an electromechanical lash adjuster  111 C Lash adjuster  111 C includes a shaft  112 C and a ball  110 C engaged by threads  114 . Rocker arm  106 C pivots on a rounded upper surface of ball  110 C, which provides a fulcrum  131 C for rocker arm  106 C. The upper surface may be cylindrical or have another suitable shape such that engagement between ball  110 C and rocker arm  106 C may prevent rotation of ball  110 C. Motor  116  may be mounted above rocker arm  106 C in a position that is fixed with respect to cylinder head  101 . 
     If ball  110 C is prevented from rotating relative to rocker arm  106 C, rotation of shaft  112 C by motor  116  may cause ball  110 C to travel along shaft  112 C, raising or lowering the fulcrum  131 C for rocker arm  106 C and thereby adjusting lash. Shaft  112 C may pass through an opening  122  in rocker arm  106 C that allows motor  116  to be mounted above rocker arm  106 C. Motor  116  may be mounted to a cam carrier (not shown) or any part that is held in a fixed position relative to cylinder head  101 . Shaft  112 C may rest atop a load cell  113 , which may provide information useful for diagnostics or control. 
       FIG. 4A  provides a partial cross-section of an engine  100 D having a rocker arm assembly  109 D. Rocker arm assembly  109 D includes a rocker arm  106 D and an electromechanical lash adjuster  111 D. Lash adjuster  111 D provides a fulcrum for rocker arm  106 D. Lash adjuster  111 D is operative as a linear actuator to vary the spacing between that fulcrum and cylinder head  101 . Lash adjuster  111 D includes an upper part  141  and a lower part  143 , which are telescopically engaged, whereby upper part  141  can slide relative to lower part  143  making the length of lash adjuster  111 D continuously variable. Upper part  141  and lower part  143  are joined by an electromechanical actuator  115 D, which is a piezoelectric stepper motor operable through a clamp-extend, clamp-retract mechanism. Upper part  141  provides an outer body for lash adjuster  111 D and houses electromechanical actuator  115 D. 
     Rocker arm assembly  109 D further includes a pair of auxiliary rocker arms  117  flanking rocker arm  106 D and pivotally connected at one end to rocker arm  106 D through axle  118 , which provides a joint proximate the fulcrum. The distal ends of auxiliary rocker arms  117  may be pivotally mounted on an axle  119 . Axle  119  may be mounted to a cam carrier (not shown) or other position fixed relative to cylinder head  101 . Auxiliary rocker arms  117  may be positioned to mitigate off axis forces that might otherwise act against lash adjuster  111 D as cam  104 D actuates valve  102 . In this example, off axis forces are force orthogonal to the direction in which lash adjuster  111 D extends to adjust lash. 
       FIG. 4B-4E  provide additional views of electromechanical lash adjuster  111 D.  FIGS. 4B and 4C  show lash adjuster  111 D in a contracted configuration whereas  FIGS. 4D and 4E  show it in an extended configuration.  FIG. 4F  provides a perspective view of actuator  115 D. As shown by these figures, actuator  115 D includes a first end portion  145 A and a second end portion  145 B joined by a variable length central portion  148 . The length of central portion  148  may be controlled through a piezoelectric element  149 . 
     Each of the end portions  145  includes a resilient element  144 , a mandrel element  146 , and a piezoelectric element  153 . Resilient element  144  may be made of metal and may include struts  152  that are configured such that biasing resilient element  144  against mandrel element  146  causes struts  152  to bear against the bore of lower part  143 , increasing friction between those parts and effectively locking the position of end portion  145  within the bore of lower part  143 . The biasing force may be provided by either a piezoelectric element  153  or by a mechanical force that tends to compress lash adjuster  111 D. In the absence of a sufficient biasing force, resiliency causes struts  152  to pull away from firm contact with the bore of lower part  143 , which may release end portion  145  from locking engagement and allowing it to slide within the bore of lower part  143 . 
       FIG. 5  provides a flow chart of a method  200  through which engine  100 D may be operated. Method  200  begins with step  201 , which verifies that first end portion  145 A is in a locking configuration and that cam  104 D is on base circle or otherwise in a position where it is not significantly loading lash adjuster  111 D. Method  200  proceeds with act  202 , releasing second end portion  145 B from its locking configuration. This may involve changing a voltage applied to a piezoelectric element  153 . Next, act  203  extends middle portion  148 . This operation may involve changing a voltage applied to piezoelectric element  149 . Next, act  204  transitions second end portion  145 B into a locking configuration. Next, act  205  releases first end portion  145 A from its locking configuration. Act  206  is the reverse of act  203  and causes middle portion  148  to return to its contracted configuration. Act  207  returns first end portion  145 A to its locking configuration. These steps may be repeated to extend electromechanical lash adjuster  111 D in a series of increments. The order of these steps may be changed to contract lash adjuster  111 D. Adjustment may be suspended while cam  104 D is loading lash adjuster  111 D. When cam  104 D is applying a load to lash adjuster  111 D, that load may drive both first end portion  145 A and second end portion  145 B into their locking configurations. 
     One or more of the piezoelectric elements of lash adjuster  111 D may undergo periodic loading in conjunction with normal operation of rocker arm assembly  109 D. This loading and unloading produces voltage differentials across these piezoelectric elements. The produced voltages may be detected for diagnostic or control purposes. In addition, these voltages may be tapped, whereby these piezoelectric elements are operative as generators. The electricity may be temporarily stored and subsequently used to operate lash adjuster  111 D or power a controller for it. 
       FIGS. 6A-B  illustrate an electromechanical actuator  115 E that may be used in place of electromechanical actuator  115 A in engine  100 A or in place of electromechanical actuator  115 D in engine  100 D.  FIG. 6A  provides a perspective view of actuator  115 E and  FIG. 6B  provides an exploded view. Actuator  115 E includes a housing  155 . A nut  167  may be secured within an orifice  159  at one end of housing  155 . Nut  167  has internal threads  169  that engage external threads  158  on shaft  157 . A guide bushing  179  having a small clearance around shaft  157  may be secured at the opposite end of housing  155 . At that opposite end, housing  155  may have flanges  161  through which housing  155  may be braced to a lower part  143  such as the one shown in  FIG. 4A  or otherwise held stationary relative to cylinder head  101 . A spherical ball tip  163  or other end piece on threaded shaft  157  may provide a fulcrum for a rocker arm  106  or may be positioned to act against an upper part  141  such as the one shown in  FIG. 4A  that provide a fulcrum for the rocker arm  106 . 
     Four piezoelectric plates  171  are bonded to outside surfaces  173  of housing  155 . Plates  171  are positioned and operative to excite motion of housing  155  in the two orthogonal planes  175  and  177 . The number and structure of piezoelectric elements  171  may be varied provided the elements  171  are operative to excite motion of housing  155  in planes  175  and  177 . Piezoelectric plates  171  are operated through electrodes (not shown). Piezoelectric plates  171  may be driven with a frequency suitable to induce vibration of housing  155  and nut  167  at a resonant frequency in the ultrasonic range. 
     As shown in  FIG. 6C , exciting vibration of housing  155  and nut  167  in planes  175  and  177  with the vibrations 90-degrees out of phase is operative to induce torque between nut  167  and shaft  157  and cause nut  167  to travel along shaft  157 . There is a small clearance between the threads  169  of nut  167  and the threads  158  of shaft  157 . The size of this clearance is exaggerated in the images of  FIG. 6C . The series of images in  FIG. 6C  shows how the bending of plates  171  causes an area of contact between threads  169  and threads  158  to rotate about shaft  157 . This causes nut  167  to orbit shaft  157  and, with friction, generates the torque. Shaft  157  may be driven either upward or downward depending on the phase relationship between the orthogonal modes of vibration. Operation of actuator  115 E may be enhanced by isolating actuator  115 E from oil in the environment surrounding lash adjuster  111 . That isolation may be accomplished by enclosing actuator  115 E within a telescopically engaged upper part  141  and a lower part  143  like actuator  115 D as shown in  FIG. 4A . 
       FIG. 7  provides a flow chart of a method  220  for controlling valve timing in an engine  100  that uses an electromechanical lash adjuster  109 . Method  220  may be used to set the opening time for a valve  102  that controls either an intake or an exhaust port. By applying the method  220  to a pair of valves  102  controlling intake and exhaust ports of a single cylinder, the amount of overlap between the opening periods for those valves may be set to a pre-determined value. 
     Method  220  involves detecting the beginnings and endings of load events on a rocker arm assembly  109 . The presence or absence of such a load event can be determined based on whether the load on a lash adjuster  109  exceeds a critical value. The load may be detected by a load cell  113  such as shown in  FIG. 3  or by a suitably positioned piezoelectric element such as piezoelectric element  145 B shown in  FIG. 4A . Alternatively, the presence of a load exceeding the critical valve can be inferred from a displacement of poppet valve  102 , which may be detected by any suitable sensor. 
     Method  220  begins with acts  221  and  223 , detecting the beginnings of two consecutive load events, and act  225 , detecting the end of a load event. Act  227  determines the period between load events. In this example, the determination is based on the interval between the starts of the preceding two load events. Alternative methods for calculating this period include determining the interval between the ends of two consecutive load events and more complicated methods that use additional load data to make a more accurate determination. Act  229  determines the duration of the last load event. Act  231  is operating the electromechanical lash adjuster  109  to drive a ratio between the load event duration and the load event period toward a target value. Method  220  may then return to act  223  and repeat. 
     One possible variation on method  220  is to use the time between load events in place of the load event period. The length of time between load events may be determined as the interval between the start of a load event and the end of the preceding load event. A ratio of the length of the interval between load events and the load event period is another alternative metric that may be used without changing the effect of method  220 . 
       FIG. 8A-8D  illustrate an electromechanical lash adjuster  111 F according to some aspects of the present teachings. Lash adjuster  111 F may be used in place of lash adjuster  111 A in engine  100 A or in place of lash adjuster  111 D in engine  100 D. Referring to  FIGS. 8C and 8B , lash adjuster  111 F includes two parts, lower part  307  and upper part  311 , that are positioned end-to-end within an outer body  301  in a configuration that permits their relative rotation about axis  150 , which is through the center of lash adjuster  111 F. Lower part  307  and upper part  311  interface through abutting end surfaces  319  and  315 , which are angled such that relative rotation between these parts on axis  150  causes a linear displacement between them along that axis. This capability for linear displacement makes lash adjuster  111 F extensible and continuously variable in length between a first end  133 F and a second end  131 F thereof. End  133 F is adapted to fit within a bore in cylinder head  101  and end  131 F is adapted to provide a fulcrum for a rocker arm  106 . 
     Lower part  307  has radial symmetry with two repeating units. Each unit provides a surface  315  that faces upper part  311 , has a generally flat profile, and angles upward at a slope of 8-10° with respect to axis  150  through most of its 180° arc length. At its uppermost extent, surface  315  has a short flat region  316  out of which there is a protrusion  317  that may have a square cross-section. Protrusion  317  is shaped to ride within a channel  309  formed in upper part  311 . Channel  309  has an arc length that is somewhat less than 180°. Protrusion  317  is adapted to ride freely with channel  309  under relative rotation between upper part  311  and lower part  307  until protrusion  317  encounters an end surface  310  of channel  309 . Protrusion  317  cooperates with channel  309  to provide rotation-limiting stops. 
     Upper part  311  also has, for the most part, radial symmetry with two repeating units. Each unit provides a surface  321  that faces lower part  307 , has a generally flat profile except for channel  309 , and angles with respect to axis  150  with the same slope as surface  315  through most of surface  321 &#39;s 180° arc length. 
     The radial symmetry of upper part  311  is broken by a slot  132 F formed in upper part  311 . A pin  133 F fits through a bore in outer body  301  and rides within slot  132 F to prevent upper part  311  from rotating relative to outer body  301 . Motor  116  is secured to outer body  301  so that upper part  311  does not rotate relative to motor  116 . 
     A pinion gear  303 , which is an annular gear having inward facing teeth, is formed into lower part  307 , whereby it is approximately the largest gear that can be fit within outer body  301 . Motor  116  is positioned off axis  150  within outer body  301  so that motor  116  can directly drive a small gear  305  that meshes with pinion gear  303 . Using a small number of simple parts all fitting within outer body  301 , this arrangement provides a high gear ratio between motor  116  and lower part  307  the rotation of which is driven by motor  116 . 
     Lash adjuster  111 F has stiffness under load. Lash adjuster  111 F resists compression under load through friction. As the load of rocker arm  109  on lash adjuster  111 F increases, the friction force between surfaces  315  and  319  remains larger than the torque that load introduces between parts  307  and  311  due to the angled interface between those surfaces. A slope of 10 degrees is approximately the greatest these surfaces can have without providing one or both of surfaces  315  and  319  with a high friction material such as one of the high friction material used in transmissions. 
     In some aspects of the present teachings, in order to maintain a desired range of motion for lash adjuster  111 F and to maintain its stiffness under load without requiring high friction materials, lash adjuster  111 F does not have radial symmetry. In this alternative configuration, upper part  311  has a surface  321  that interfaces with part  307  and is continuously sloping with respect to axis  150  through a radial arc in the range from 225 to 360 degrees. In some of these teachings, the slope of that surface is in the range from 4 to 7 degrees. 
     The components and features of the present disclosure have been shown and/or described in terms of certain embodiments and examples. While a particular component or feature, or a broad or narrow formulation of that component or feature, may have been described in relation to only one embodiment or one example, all components and features in either their broad or narrow formulations may be combined with other components or features to the extent such combinations would be recognized as logical by one of ordinary skill in the art.