Abstract:
A system for controlling the seating of an engine valve is disclosed. The system is designed to bring a hydraulically actuated engine valve to a soft landing on its valve seat. The velocity of the engine valve is reduced as it approaches its seat by progressively throttling the escape of hydraulic fluid from a chamber. The chamber is pressurized as a result of the valve approaching its seat. Accordingly, as the valve approaches its seat, the pressure in the chamber increases, causing the force that opposes the closing motion of the engine valve to increase.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is related to and claims priority on U.S. provisional patent application Serial No. 60/154,035 filed Sep. 16, 1999, and U.S. provisional patent application Serial No. 60/154,472 filed Sep. 17, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the control of engine valves, such as intake and exhaust valves. In particular, the invention relates to methods and apparatus for controlling valve seating velocity. 
     BACKGROUND OF THE INVENTION 
     Engine combustion chamber valves, such as intake and exhaust valves, are almost universally of a poppet type. These engine valves are typically spring loaded toward a valve closed position. In many internal combustion engines the engine cylinder intake and exhaust valves may be opened and closed by fixed profile cams in the engine, and more specifically by one or more fixed lobes which may be an integral part of each of the cams. The use of fixed profile cams makes it difficult to adjust the timings and/or amounts of engine valve lift to optimize valve opening times and lift for various engine operating conditions, such as different engine speeds. 
     A variety of systems exist to regulate the timing of engine valve opening by controlling the hydraulic pressure that acts on a slave piston which actuates the engine valve. These systems include “common rail” systems in which a solenoid control valve opens a path from a source of high pressure fluid to the top of the slave piston at precise times. One such common rail system is described in Cosma et al., U.S. Pat. No. 5,619,964, assigned to the assignee of the present application. 
     Another method of adjusting valve timing and lift, given a fixed cam profile, has been to incorporate a “lost motion” device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage means. In a lost motion system, a cam lobe may provide the “maximum” (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve. 
     This variable length system (or lost motion system) may, when expanded fully, transmit all of the cam motion to the valve, and when contracted fully, transmit none or a minimum amount of the cam motion to the valve. An example of such a system and method is provided in Hu, U.S. Pat. Nos. 5,537,976 and 5,680,841, which are assigned to the same assignee as the present application and which are incorporated herein by reference. 
     In the lost motion system of U.S. Pat. No. 5,680,841, an engine cam shaft may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may include a solenoid valve and a check valve in communication with the hydraulic circuit including the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the motion of the master piston, which in turn displaces hydraulic fluid in direct response to the motion of a cam. When the solenoid is opened temporarily, the circuit may partially drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston. 
     Another example of an engine valve actuator is disclosed in U.S. Pat. No. 5,186,141, “engine Brake Timing Control Mechanism,” issued to D. Custer on Feb. 16, 1993 (the “&#39;141 patent”), incorporated by reference herein. The actuator disclosed in the &#39;141 patent does not provide for engine valve seating control, although it could benefit from such control. 
     Engine valves are required to open and close very quickly, therefore the valve spring is typically very stiff. When the valve closes, it may impact the valve seat with such force that it eventually erodes the valve or the valve seat, or even cracks or breaks the valve. In mechanical valve actuation systems that use a valve lifter to follow a cam profile, the cam lobe shape provides built-in valve-closing velocity control. In common rail hydraulically actuated valve assemblies, however, there is no cam to self-dampen the closing velocity of an engine valve. Furthermore, in some lost motion applications the engine valve needs to be closed at an earlier time than that provided by the cam profile. This earlier closing may be carried out by rapidly releasing hydraulic fluid to an accumulator in the lost motion system. In hydraulic lost motion systems, a rapid draining of fluid from the slave piston may allow an engine valve to “free fall” and seat with an unacceptably high velocity. Free fall results when the rate of closing the engine valve is governed by the hydraulic fluid flow to the accumulator instead of by the fixed cam profile. Engine valve seating control may also be required for applications (e.g. centered lift) in which the engine valve seating occurs on a high velocity region of the cam. Electromagnetic valve actuation may also require valve seating control. 
     As a result of the foregoing there is a need to limit valve seating velocities. The need for limited valve seating velocities conflicts with the need for rapid valve opening rates. Some attempts have been made to solve the problem by providing separate fill and drain ports for slave pistons. U.S. Pat. No. 5,577,468 discloses one system for limiting valve seating velocity. Existing methods for controlling engine valve seating velocity may be costly, inaccurate, and cause excessive valve closing variations. Existing systems also fail to accommodate the need for adjustments due to variations in engine valve lash between cylinders. 
     Applicants approached the valve seating challenge with the understanding that valve seating velocity should be less than approximately 15 in/sec (0.38 m/sec). Absent steps to control valve seating velocity, the valves could seat at a velocity that is an order of magnitude greater. Applicants also determined that valve seating control preferably should be designed to function when the closing valve gets within 0.5 to 0.75 mm of the valve seat. The combination of valve thermal growth, valve wear, and tolerance stack-up can exceed 0.75 mm, resulting in the complete absence of seating velocity control or in an exceedingly long seating event if measures are not taken to adjust the lash of the valve seating control to account for such variations. It is also assumed that, preferably, valve seating control should not significantly reduce initial engine valve opening rate, and valve seating control should be capable of operating over a wide range of valve closing velocities and oil viscosities. 
     Valve catch devices used to control valve seating velocity may use hydraulic fluid flow restriction to produce pressure that acts on an area of the slave piston to develop a force to slow the slave piston and reduce seating velocity. The area on which the pressure acts may be very small in such devices which in turn requires that the pressure opposing the valve return spring be high, and the controlling flow rate be low. Low controlling flow rates result in an increased sensitivity to leakage and manufacturing tolerances. In addition, these devices may restrict the hydraulic fluid flow that produces valve opening. 
     A known valve catch (seating) system developed to provide valve seating control is disclosed in co-pending U.S. patent application Ser. No. 09/383,987, filed Aug. 26, 1999, hereby incorporated by reference and which is shown as system  100  in FIG.  1 . The system  100  includes a slave piston  120  disposed within an actuator housing  110 . The slave piston  120  is slidable within the housing  110  so that it may open an engine valve (not shown) below it. A screw body  130  extends through the top of the housing  110  and abuts against the slave piston  120  when the latter is in a resting position (i.e. engine valve closed). A plunger  140  is disposed within the screw body  130  and is biased towards the slave piston  120  by a spring  160 . The screw body  130  may be twisted into and out of the housing  110  to manually adjust engine valve lash. 
     The plunger  140  serves to selectively limit valve seating velocity as the slave piston  120  approaches its home position (engine valve closed), thereby allowing the engine valve to close more gently than it otherwise might. The plunger  140  is mechanically limited from extending beyond the screw body  130  by more than a preset distance, thus allowing the slave piston  120  to return rapidly until contacting the plunger. 
     The system  100  operates under the influence of hydraulic fluid provided through a passage  150  in the housing  110 . Preferably, the hydraulic fluid provided by the passage  150  is high pressure. During the downward (valve opening) displacement of the slave piston  120 , hydraulic fluid flows through the passage  150  in the housing  110  and through the passages in the slave piston so that the slave piston is forced downward against the engine valve. During the upward (valve closing) displacement of the slave piston  120 , the hydraulic fluid flows back through the passages in the slave piston  120  and out of the passage  150  in the housing  110 . As the slave piston  120  approaches its home position, it forms a seal with the plunger  140 . The seal between the plunger  140  and the slave piston  120  results in the building of hydraulic pressure in the space between the slave piston and the end wall of the housing  110  as the slave piston progresses towards its home position. The building hydraulic pressure opposes the upward motion of the slave piston  120 , thereby slowing the slave piston and assisting in seating the engine valve. 
     While the valve catch system  100  shown in FIG. 1, which works on slave piston pressure, has achieved acceptable valve seating velocity over a wide range of engine speeds and oil temperatures, improvements are still needed. For example, the valve catch system  100  tends to hold the engine valve open longer than is desirable for optimum engine breathing at high engine speeds. The system is also prone to reduce valve velocity to nearly zero prior to seating and thereafter accelerate the valve so that it seats at an unacceptable velocity. This type of valve catch system also may require a complicated slave piston design, which increases high-pressure volume, increases the length and flow resistance of the fluid path between the slave piston and the passages leading to the master piston, trigger valve, or plenum, and increases the required slave piston height and weight. Increased high-pressure volume may be detrimental to compliance. Increased flow path length and flow resistance provide increased pressure drop and therefore increased parasitic power and oil cooling load. Additionally, increased pressure drop may make it difficult to maintain master piston pressure greater than ambient during periods of decreasing cam displacement during high engine speed, which may allow air bubbles to form in the oil. 
     A second valve catch system  200  is disclosed in the co-pending 09/383,987 application referenced above, and is shown in FIG.  2 . The valve catch system  200  works on valve catch plenum pressure, and is considered to have lower parasitic loss than the system shown in FIG.  1 . The system  200  includes a slave piston  220  disposed within an actuator housing  210 . The slave piston  220  is slidable within the housing  210  so that it may open an engine valve (not shown) below it. A screw body  230  extends through the top of the housing  210  and abuts against the slave piston  220  when the latter is in a resting position (i.e. engine valve closed). A plunger  240  is disposed within the screw body  230  and biased towards the slave piston  220  by a spring  260 . The screw body  230  may be twisted into and out of the housing  210  to adjust engine valve lash. A fluid passage  250  through the housing  210  leads to a high pressure hydraulic source such as a master piston (not shown) and/or a trigger valve (not shown). 
     The system  200  operates similarly to the system  100  shown in FIG. 1, except that the hydraulic pressure that opposes the upward movement of the slave piston  220  is built inside the screw body  230 . Although performance may be improved using the system  200 , compliance difficulties may still be encountered due to the high pressures required and the increased compliance associated with the smaller area of plunger  240 . 
     In view of the foregoing there is a need for a system for valve seating control that operates well in a high pressure regime requiring fine control of hydraulic fluid flow through the system. There is also a need for a system that does not adversely effect hydraulic fluid flow for valve opening and which is less susceptible to leakage sensitivity. In particular, there is a need for valve seating that is improved by a flow control that becomes more restrictive as the valve approaches the seat. 
     There is also a need for a valve catch that adjusts for lash differences between the engine valve and the valve catch. Although most variable valve actuation (VVA) systems are inherently self lash adjusting, valve seating control is not. Systems that do not need manual adjustment, either initially, or as the system ages, are desirable. Previous valve seating control mechanisms have required a manual lash adjustment or a separate set of lash adjustment hardware. The design of a conventional hydraulic lash adjustor capable of transmitting compression-release braking loads would be challenging due to structural and compliance requirements. 
     Unlike the valve catch systems  100  and  200  shown in FIGS. 1 and 2, the various valve catch embodiments of the present invention include a variable area orifice in the system plunger. Accordingly, the various valve catch embodiments of the invention may have reduced parasitic power loss and consequently reduced VVA housing cooling load, and reduced slave piston length and weight as compared with previous valve catch systems. The valve catch embodiments of the present invention may also experience reduced peak valve catch pressure as compared with the previous valve catch systems. Furthermore, the variable flow restriction design of the valve catch embodiments of the present invention is expected to be more robust than the constant flow restriction design in terms of engine valve velocity control at the point of valve catch engagement, and in terms of oil temperature and aeration control. Variable flow restriction may allow the displacement at the point of valve catch/slave piston engagement to be reduced, so that the valve catch has less undesired effect on the breathing of the engine. 
     The present invention meets the aforementioned needs and provides other benefits as well. The claimed invention provides acceptable engine valve seating velocity in a VVA system, such as a lost motion or common rail system. For a lost motion VVA system, engine valve seating control is provided for early engine valve closing, where the rate of closing is governed by the hydraulic flow from the slave piston to the accumulator as opposed to a cam profile. Engine valve seating control also may be provided for a high velocity region of the cam and/or for common rail VVA designs. 
     The valve seating velocity control provided by this invention also may be applied to camless variable valve actuation designs in which the engine valve is not spring loaded toward a valve-closed position. One example is the electromagnetic concept (Aura, FEV, BMW, Daimler Benz, Siemens) in which there are opposing springs acting in both the valve closed and valve open directions, in order to create an oscillating spring-mass system, and two solenoids, which latch the valve in either the closed or full-open position. In this system, valve seating velocity control could be provided by precisely controlling the current to the solenoids; however, in practice, a separate valve seating control device may be required to assure acceptable valve seating under all conditions. Another example is the electrohydraulic common rail concept (Ford) in which there are no valve springs and two high-speed solenoid valves are used to alternately connect a source of high-pressure hydraulic fluid and drain to either side of a piston connected to the engine valve. In this system, valve seating velocity control could be provided by precisely controlling the timing of the high-speed solenoid valves; however, in practice, a separate valve seating control device may be required to assure acceptable valve seating under all conditions. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of the present invention to provide a method and system for controlling valve seating. 
     It is a further object of the present invention to provide a method and system for controlling valve seating that operates well under high hydraulic pressure. 
     It is another object of the present invention to provide a method and system for seating an engine valve with fine control over valve seating velocity. 
     It is yet a further object of the present invention to provide a method and system for controlling valve seating that is less likely to adversely affect hydraulic fluid flow for valve opening. 
     It is still another object of the present invention to provide a method and system for controlling valve seating that progressively restricts hydraulic fluid flow as the valve approaches its seat. 
     It still yet another object of the present invention to provide a method and system for controlling valve seating velocity that is self lah adjusting for valve thermal growth, valve wear, and tolerance stack-up. 
     It is yet another object of the present invention to provide a method and system for valve seating control that provides a nearly constant deceleration of the valve before seating. 
     It is still yet a further object of the present invention to provide a method and system for valve seating control that provides acceptable seating velocity during early valve closing events. 
     It is still another object of the present invention to provide a method and system for valve seating control that provides acceptable seating velocity during centered lift events when the valve seats on a high velocity section of the cam. 
     It is still another object of the present invention to provide a method and system for valve seating control that reduces the volume of hydraulic fluid in the master-slave piston circuit in order to reduce system compliance. 
     It is a further object of the present invention to provide a method and system for valve seating control with reduced parasitic loss and consequently reduced cooling requirements. 
     It is another object of the present invention to provide a method and system for valve seating control with improved hydraulic fluid aeration characteristics. 
     It is still another object of the present invention to provide a method and system for valve seating control that utilizes a slave piston of reduced length and weight as compared to previous systems. 
     It is still a further object of the present invention to provide a method and system for valve seating control of relatively simple and low cost design. 
     It is still a further object of the present invention to provide a method and system for reliable valve seating. 
     It is another object of the present invention to provide a method and system for controlled seating velocity over a wide range of valve closing velocities. 
     It is still another object of the present invention to provide a method and system for controlled seating velocity over a wide range of oil viscosities. 
     It is still a further object of the present invention to provide a method and system for valve seating control that does not require closely held concentricity of the control elements. 
     It is still a further object of the present invention to provide a method and system for valve seating control that includes a means to dissipate the heat generated during valve seating. 
     Additional objects and advantages of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention. 
     It is a further object of the present invention to provide a method and system for valve seating control that does not require closely held concentricity of the control elements. 
     It is a further object of the present invention to provide a method and system for valve seating control that includes a means to dissipate the heat generated during valve seating. 
     SUMMARY OF THE INVENTION 
     In response to the foregoing challenges, Applicants have developed an engine valve seating system having a piston adapted to be bi-directionally displaced in response to the filling and draining of hydraulic fluid from an hydraulic chamber in communication with the piston, the system comprising means for guiding hydraulic fluid from the chamber during draining; and means for throttling hydraulic fluid flow through the guiding means at a preselected rate in response to a change in position of the guiding means relative to the throttling means during draining. 
     Applicants have additionally developed a system for controlling seating velocity of an internal combustion engine valve comprising a housing having a bore formed therein for receipt of a piston; a piston positioned in and adapted for bi-directional displacement in the bore; an hydraulic chamber defined by an end of the piston; a piston stop extending into the chamber; and a disk having at least a central opening, the disk positioned in the chamber and being adapted to cooperate with the piston stop to control valve seating velocity. 
     Applicants have additionally developed a system for controlling seating velocity of an internal combustion engine valve comprising a housing having a bore formed therein for receipt of a piston, and a recess formed in an end wall of the bore; a recess shoulder formed along the intersection of the recess and the bore; a piston positioned in and adapted for bi-directional displacement in the bore; an hydraulic chamber defined by the bore end wall and the piston; means for providing hydraulic fluid flow to and from the chamber; a disk having at least a central opening, said disk positioned between the piston and the bore end wall; a spring adapted to bias the disk against the recess shoulder when the piston is in a retracted position; and an elongated stop having a fluted end extending from the piston, through the chamber, through the disk, and into the recess, wherein a minimized hydraulic passage is formed between the disk and the elongated stop when the piston is in the retracted position. 
     Applicants have additionally developed a system for controlling seating velocity of an internal combustion engine valve comprising a housing having a bore formed therein for receipt of a piston; a piston positioned in and adapted for bi-directional displacement in the bore, the piston having a recess formed in an upper end thereof; an hydraulic chamber defined by an end wall of the bore and the upper end of the piston; a recess shoulder formed along the intersection of the recess and the chamber; a disk having a central opening, the disk positioned between the piston and the bore end wall; a spring adapted to bias the disk against the recess shoulder when the piston is in a retracted position; and an elongated stop having a fluted end extending from the bore end wall, through the chamber, through the disk, and into the recess, wherein a minimized hydraulic passage is formed between the disk and the elongated stop when the piston is in the retracted position. 
     Applicants have also developed a method of controlling the seating velocity of an engine valve comprising the steps of filling a fluid chamber responsive to an opening motion of the engine valve; expulsing fluid from the fluid chamber responsive to a closing motion of the engine valve; and progressively throttling the expulsion of fluid from the fluid chamber during at least a portion of the engine valve closing motion. 
     Applicants have additionally developed a method of controlling the seating velocity of an engine valve and providing automatic lash take up, the said method comprising the steps of providing leakage filling of a first fluid chamber to automatically take up lash; filling a fluid chamber responsive to an opening motion of the engine valve; expulsing fluid from the fluid chamber responsive to a closing motion of the engine valve; and progressively throttling the expulsion of fluid from the fluid chamber during at least a portion of the engine valve closing motion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various embodiments and elements of the invention are shown in the following figures, in which like reference numerals are intended to refer to like elements. 
     FIG. 1 is a cross-section in elevation of a valve catch design disclosed in a co-pending application assigned to the assignee of the present application. 
     FIG. 2 is a cross-section in elevation of a second valve catch design disclosed in a co-pending application assigned to the assignee of the present application. 
     FIG. 3 is a cross-section in elevation of a first embodiment of the present invention. 
     FIG. 4 is a cross-section in elevation of a second embodiment of the present invention. 
     FIG. 5 is a cross-section in elevation of a third embodiment of the present invention. 
     FIG. 6 is a cross-section in elevation of a fourth embodiment of the present invention. 
     FIG. 7 is a pictorial view of an elongated stop or control pin with two flutes implemented as flat s for u se in the first embodiment of the present invention. 
     FIG. 8 is a pictorial view of an elongated stop or control pin with two flutes implemented as grooves for use in an embodiment of the present invention. 
     FIG. 9 is a pictorial view of one means of limiting the motion of the check disk relative to the control pin for use in the first embodiment of the present invention. 
     FIG. 10 is a pictorial view of a another means of limiting the motion of the check disk relative to the control pin for use in an embodiment of the present invention. 
     FIG. 11 is a cross-section in elevation of a fifth embodiment of the present invention. 
     FIG. 12 is a second cross-section in elevation of the valve catch shown in FIG.  11 . 
     FIGS. 13-16 are graphs that illustrate operational parameters of the fifth embodiment of the present invention. 
     FIG. 17 is a cross-section in elevation of a sixth embodiment of the present invention. 
     FIG. 18 is a cross-section in elevation of a seventh embodiment of the present invention. 
     FIG. 19 is a cross-section in elevation of an eighth embodiment of the present invention. 
     FIG. 20 is a cross-section in elevation of a ninth embodiment of the present invention. 
     FIG. 21 is a cross-section in elevation of an alternative slave piston and check disk arrangement for use in the systems shown in FIGS. 19-20. 
     FIG. 22 is a graph of disk flow area verses slave piston to disk gap for the arrangement shown in FIG.  21 . 
     FIG. 23 is a cross-section in elevation of a tenth embodiment of the present invention. 
     FIG. 24 is a cross-section in elevation of an eleventh embodiment of the present invention. 
     FIG. 25 is a cross-section in elevation of a twelve embodiment of the present invention illustrating and adaption of the slave piston and check disk arrangement of FIG. 21 for use in the system shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to a first embodiment of the present invention, an example of which is illustrated in the accompanying drawings. With reference to FIG. 3, a first embodiment of the self adjusting valve catch (SAVC) may be provided with engine oil in much the same manner as the camshaft journals. The SAVC  100  comprises a housing  700 , a slave piston  300 , an elongated stop  400 , and a disk  500 . 
     The housing  700  includes a housing bore  710  in which the slave piston  300  is slidably disposed. A hydraulic fill/drain port  720  through the housing  700  comprises a means for providing hydraulic fluid to and from the housing bore  710 . A recess  750  is provided in the end wall  712  of the housing  700 . The recess  750  receives the fluted end  420  of the elongated stop  400  when the slave piston  300  is in a retracted position. The recess  750  may open on the chamber  740  defined by the end wall  712 , the housing bore  710 , and the slave piston  300 . 
     The slave piston  300  may be generally cylindrically shaped such that it is capable of forming a sliding seal with the housing bore  710 . The slave piston  300  may include a chamber or recess  350  extending into the slave piston from the upper end there. The lower end  340  of the slave piston  300  may be adapted to contact a contact stem  600 , an engine valve stem, or a valve bridge (not shown). 
     The elongated stop  400  may be cylindrically shaped with a non-fluted end  410 , and a fluted end  420 . An elongated stop shoulder  430  is formed at the intersection of the non-fluted end  410  and the fluted end  420 . The non-fluted end  410  of the elongated stop is disposed within the slave piston recess  350 . A spring  440  is placed in compression between the non-fluted end  410  and the bottom of the slave piston recess  350 . The fluted end  420  extends into the housing recess  750  when the slave piston  300  is in its retracted position. 
     The fluted end  420  includes one or more flutes  422  which provide a means for guiding hydraulic fluid to and from the chamber  740 . The flutes  422  may be of a uniform or non-uniform depth along the length of the fluted end  420 . In a preferred embodiment of the invention, the flutes  422  are tapered to have a progressively decreased depth as the flutes near the non-fluted end  410  (see FIG.  7 ). The taper is shown as linear, but may be non-linear to accomplish the desired seating velocity in alternative embodiments. A cap  425  may be positioned over the fluted end  420 . The  10  cap  425  may be connected to the elongated stop by a crimp/swage, press fit joint, or a pinned joint. 
     The relationship of the cap  425  to the elongated stop is illustrated in FIG.  9 . 
     The disk  500  is provided with a central bore adapted to receive the fluted end  420  of the elongated stop. The central bore in the disk  500  makes it self-aligning on the fluted end  420 , thereby simplifying assembly. The diameter of the central bore is selected relative to that of the fluted end  420  so that a minimum flow area may be formed between the fluted end and the disk  500 . The spring  510  biases the disk  500  towards the end wall  712 . In this embodiment the disk  500  seats at a predetermined valve lift at which point the flow from the volume above the slave piston begins to be throttled. 
     Controlled valve seating is accomplished because as the slave piston  300  approaches its fully retracted position, the relative movement of the tapered end of the flutes  422  past the disk  500  throttles the flow of hydraulic fluid from the chamber  740  to the recess  750  more and more until the flow is reduced to a minimal value and the engine valve seats. 
     Operation of the system  100  starts with the slave piston  300  in its retracted position as shown. In this position there may be a gap between the slave piston and the valve stem, bridge, or contact stem  600 . High or low pressure hydraulic fluid entering port  720  flows through flutes  422  or around disk  500  and fills the chamber  740 . The low pressure hydraulic fluid in the chamber  740  causes the slave piston  300  to move down and eliminates the lash between the slave piston and the contact stem  600 . The elimination of the lash initially creates a gap between the elongated stop  400  and the upper end of the housing recess  750 . After this initial period, the elongated stop  400  slowly moves upward under the action of the spring  440  and eliminates this gap as a result of leakage flow into the recess  350 . Leakage flow into the recess  350  provides constant self-adjusting lash take-up for the system. The elongated stop  400  can move upward until it contacts the upper end of the housing recess  750 , at which point the stiff engine valve springs prevent any further motion. 
     During valve actuation high pressure hydraulic fluid enters the system through the port  720 , unseats and flows past the disk  500 , and moves the slave piston  300  downward. The disk design provides minimal throttling of the hydraulic fluid during filling of the hydraulic chamber between the bore and the slave piston. 
     During valve closing, hydraulic fluid is vented through the port  720 , allowing the slave piston  300  to return to its retracted position. The return or seating velocity of the slave piston  300  may be controlled by the selection of the flow area between the disk  500  and the elongated stop  400 , as well as by the design of the cap  425 . The flow past the disk  500  on closing is initially high due to the cap  425  keeping the disk  500  unseated. Once the cap  425  moves upward enough to let the disk  500  seat, flow is controlled solely by the design of the flutes  422 . The flow area is designed to be relatively large (unrestricting) when the slave piston is in an extended position. As the slave piston  300  approaches its retracted position, the flow area decreases during the last portion (e.g. &gt;&gt;0.75 mm) of valve lift. 
     Auto-lashing of the elongated stop is also provided by means of leakage to the slave piston recess  350 . While the slave piston  300  and the elongated stop  400  move nearly together during valve actuation, the stop actually moves slowly upward relative to the slave piston as hydraulic fluid leaks into the slave piston recess  350 . The shoulder  430  should provide a surface for the hydraulic fluid in the hydraulic chamber  740  to act on the stop  400  to keep the stop moving with the slave piston  300  in the presence of inertial forces. 
     The net upward displacement of the elongated stop  400  relative to the slave piston  300  caused by the leakage of hydraulic fluid into the slave piston recess  350  (typically 0.025 mm) may cause the elongated stop  400  to contact the housing  700  before the slave piston  300  is fully retracted. The relatively small diameter of the elongated stop  400  produces high pressure in the slave piston recess  350 . The upward pressure of the valve spring (not shown) on the elongated stop  400  squeezes the hydraulic fluid in the slave piston recess  350  back out until the slave piston  300  is fully retracted. The process of squeezing the extra hydraulic fluid out of the slave piston recess  350  provides additional valve seating velocity control over the last few hundredths of a millimeter of valve closure. 
     With continued reference to FIG. 3, the fluted end  420  of the elongated stop  400  may include two flutes  422 . It is recognized that the number, length, depth, and taper angle of the flutes  422  may be varied without departing from the scope of the invention. In fact, the flutes  422  may constitute flat portions, or “flats,” on the elongated stop  400 , as shown in FIG.  7 . 
     With reference to FIG. 4, in which like reference numerals refer to like elements, a system  100  similar to that shown in FIG. 3 is disclosed. The system shown in FIG. 4 includes an inverted elongated stop  400  as compared to the system of FIG.  3 . An explanation of the operation of the system shown in FIG. 4 is apparent from the following explanation of the operation of the system  100  shown in FIG.  5 . 
     With reference to FIG. 5, in which like reference numerals refer to like elements in the other figures, a system  100  for valve actuation and valve seating control in accordance with a third embodiment is shown. The system  100  comprises a housing  700 , a slave piston  300 , an elongated stop integrated into a lash adjusting screw  400 , and a disk  500 . The system  100  shown in FIG. 5 may be combined with an external lash take up device (not shown). 
     The housing  700  includes a housing bore  710  in which the slave piston  300  is slidably disposed. A hydraulic fill/drain port  720  through the housing  700  comprises a means for providing hydraulic fluid to and from the housing bore  710 . The housing  700  may also have a threaded opening  730  for receipt of the elongated stop  400 . The threaded opening  730  may extend through the wall of the housing  700  so that it opens on a chamber  740  defined by the end wall  712  and the side wall  714  of the housing bore  710 . 
     The slave piston  300  may be cylindrically shaped such that it is capable of forming a sliding seal with the housing bore side wall  714 . The slave piston  300  may include a peripheral indent  310 , a recess  320  in the upper end of the slave piston, and a feed passage  330  that provides communication between the indent and the recess. The lower end  340  of the slave piston  300  may be adapted to contact an engine valve or contact stem  600 . 
     The elongated stop  400  may be cylindrically shaped with a non-fluted end  410  (threaded as shown in FIG.  5 ), and a fluted end  420 . An elongated stop shoulder  430  is formed at the intersection of the non-fluted end  410  and the fluted end  420 . When threaded as shown, the non-fluted end  410  may be screwed into the housing  700  to a preselected depth. The extension of the fluted end  420  into the housing  700  may be adjusted by backing out or twisting in the elongated stop  400  relative to the housing. The fluted end  420  extends into the slave piston recess  320  when the slave piston  300  is in its retracted position. 
     The fluted end  420  includes one or more flutes  422  which provide a means for guiding hydraulic fluid to and from the chamber  740 . The flutes  422  may be of a uniform or non-uniform depth along the length of the fluted end  420 . In a preferred embodiment of the invention, the flutes  422  are tapered to have a progressively decreased depth as the flutes near the non-fluted end  410 . The taper is shown as linear, but may be non-linear to accomplish the desired seating velocity in alternative embodiments. 
     The disk  500  is provided with a central bore adapted to receive the fluted end  420  of the elongated stop  400 . The diameter of the central bore is selected relative to that of the fluted end  420  so that a minimum flow area may be formed between the fluted end and the disk  500 . The disk  500  may be biased towards the upper end of the slave piston  300  by a spring  510 . 
     With continued reference to FIG. 5, the system  100  may be operated starting from the position shown, in which the slave piston  300  is retracted. In order to displace the slave piston  300  downward for a valve opening event, pressurized hydraulic fluid is provided through the fill/drain port  720  to the housing bore  710 . The hydraulic fluid flows around the peripheral indent  310  and through the feed passage  330  into the recess  320 . As hydraulic pressure builds in the recess  320 , the disk  500  may be displaced slightly upward against the bias of spring  510 , allowing the chamber  740  to also fill with hydraulic fluid. After the recess  320  and the chamber  740  are filled with hydraulic fluid, the addition of still more hydraulic fluid to the recess  320  forces the slave piston  300  downward. As the slave piston  300  moves downward, the disk  500  follows the slave piston under the influence of the spring  510 . The disk  500  does not cut off hydraulic communication between the recess  320  and the chamber  740  because the flutes  422  on the elongated stop permit hydraulic fluid to flow past the disk  500  as it slides down the stop. The downward motion of the slave piston  300  causes the engine valve or contact stem  600  to open against the bias of a valve spring (not shown). 
     In an alternative embodiment of the present invention, the slave piston  300  shown in FIG. 5 may simply follow the engine valve/contact stem  600  as it moves downward in response to a separate valve opening means (not shown). 
     Following the valve opening event, the engine valve must be returned gently to its seat during a valve closing event. In order to close the valve, the hydraulic fluid must be drained from the chamber  740 . The hydraulic fluid may be drained back through the feed passage  330  and out of the fill/drain port  720 . As the hydraulic fluid is drained, the slave piston  300  retracts. The upper end of the retracting slave piston  300  engages the disk  500 , and pushes the disk up along the fluted end  420  of the elongated stop. As the disk  500  travels towards the bore end wall  712 , the hydraulic fluid in the chamber  740  escapes to the recess  320  through the open space between the flutes  422 . This open space decreases to the point of being just the annular clearance at the tapered portion of the flutes  422 . As the open space decreases, the rate of hydraulic fluid flow from the chamber  740  to the recess  320  decreases in like proportion. The progressively decreased drain or flow of hydraulic fluid from the chamber  740  as a result of the disk  500  riding up the fluted end  420  of the elongated stop brings the slave piston  300  ( and thus the engine valve) to a soft landing against the elongated stop  400 . 
     With reference to FIG. 6, in which like reference numerals refer to like elements, a fourth embodiment of a system  100  for valve actuation and valve seating control is shown. In the embodiment shown in FIG. 6, the fluted end  420  of the elongated stop is integrally formed with the body of the slave piston  300 . Instead of providing a recess in the slave piston  300 , a recess  750  is provided in the end wall  712  of the housing  700 . The recess  750  receives the fluted end  420  of the elongated stop  400  when the slave piston  300  is in a retracted position. The spring  510  biases the disk  500  towards the end wall  712 . The fill/drain passage  720  provides and drains hydraulic fluid directly to the recess  750  without passing through a feed passage in the slave piston  300 . Valve seating is accomplished in the same manner in the embodiments of the invention shown in FIGS. 5 and 6. As the slave piston approaches its fully retracted position, the relative movement of the tapered end of the flutes  422  past the disk  500  throttles the flow of hydraulic fluid from the chamber  740  to the recess  750  more and more until the flow is reduced to zero and the valve seats. 
     With reference to FIGS. 7 and 8, in which like reference numerals refer to like elements in the other drawing figures, the operative engagement of the disk  500  and the fluted end  420  of an elongated stop  400  is illustrated. In these embodiments of the invention, the flutes  422  are of non-uniform depth, and have a non-linear taper. The movement of the disk  500  along the longitudinal axis of the elongated stop  400  towards the tapered end of the flutes produces progressive throttling of the hydraulic fluid flow between the disk and the elongated stop. The progression of throttling, which is proportional to the decrease in flow area, is apparent from progressive illustration of flow area (shown shaded) in FIGS. 7 and 8. The sequence illustrates the decrease in flow area between the disk  500  and the elongated stop  400  as the disk moves downward on the elongated stop. This reduction in area may extend down to just the annular clearance between the elongated stop  400  and the disk  500  as shown in FIGS. 7 and 8. Limiting the travel of the disk  500  relative to the elongated stop  400  allows for a more compact design because it eliminates the need to size the maximum flute area  401  in FIGS. 7 and 8 for unrestricted valve closing. This pertains to the embodiments in FIGS. 3-6 and  11 - 12 . Several possible means for limiting the travel of the disk are shown in FIGS. 9 and 10. FIG. 9 shows a cap  425  which is crimped onto the fluted end  420  of the elongated stop. FIG. 10 shows the stop  425  as a feature on the elongated stop  400 . In the embodiment of FIG. 10, the elongated stop assembly could be fabricated from a central pin, which is upset to form the flute and disk retention stop, and a cylindrical sleeve, which is swaged onto the central pin after assembling the disk and disk spring on the central pin. 
     With reference to FIGS. 11 and 12, in which like reference numerals refer to like elements in the other drawing figures, a fifth embodiment of the valve catch portion of the present invention is illustrated. FIG. 11 is a cross-section in elevation of the system  100 , which includes a cut line. FIG. 12 is a cross-section in elevation of system  100  as viewed along the cut line in FIG.  11 . In the fifth embodiment, the system  100  is similar to that described in relation to FIG. 3, with the following differences. 
     The elongated stop  400  shown in FIGS. 11 and 12 is configured differently than in FIGS. 3-6. The elongated stop comprises two separate pieces to facilitate assembly, an upper stop  450  and a lower stop  460 . The upper stop  450  includes a plurality of flutes in fluted section  420 , and two bosses  466 . The bosses  466  limit the upward movement of the disk  500  relative to the elongated stop  400 . The fluted section  420  of the upper stop  450  controls the flow of the hydraulic fluid between the chamber  740  and the recess  750 . The bosses  466  may prevent the disk  500  from coming off the end of the stop  400 . The bosses  466  may also hold the disk  500  off of the seat when piston  300  is at high lift, thereby providing additional flow area from chamber  740  to the recess  750 . 
     FIGS. 13-16 are graphs that illustrate the operational parameters of the embodiment of the invention shown in FIGS. 11 and 12. The data provided in FIGS. 13-16 is not intended to limit the invention in any way. It is understood that the operational parameters of the various embodiments of the invention may vary widely without departing from the scope of the invention. 
     A sixth embodiment of the valve catch portion of the present invention is shown as system  100  in FIG.  17 . The system  100  may include an inner tappet  810 , an outer tappet  820 , an hydraulic fluid line  830  extending from a trigger accumulator  890  to an inner tappet plenum  860 , a check valve  840  in an hydraulic circuit  845  connecting a low pressure reservoir  880  with the valve seating plenum  870 , a partially occludable orifice  850  located at a juncture of the valve seating plenum  870  and the hydraulic circuit  845 , and an inter-tappet plenum  860 . 
     During engine valve opening, both the inner tappet  810  and the outer tappet  820  may move downward following the engine valve. During this time the valve seating plenum  870  may be filled with hydraulic fluid through the check valve  840  and the orifice  850 . The flow through the check valve  840  may be required to prevent cavitation in the valve seating plenum  870  because the orifice  850  is designed to be partially occluded at this point. 
     As the engine valve closes (i.e. element  600  moves upward), the check valve  840  closes and hydraulic fluid is forced through the partially occluded orifice  850  from the valve catch plenum  870  back to the low pressure reservoir  880 . The partially occluded orifice  850 , formed by the upper edge of the outer tappet  820  and the hole in the side wall of the plenum  870 , is designed to progressively restrict the flow of hydraulic fluid from the plenum  870  as the engine valve approaches its seat. The ideal orifice flow area profile would maintain a constant valve catch plenum pressure between the point at which the orifice starts to occlude, at typically, but not limited to, 1 mm engine valve lift, to the point of valve seating. 
     The system  100  shown in FIG. 17 may also be used to provide VVA in an alternative embodiment. In a VVA embodiment, the inner tappet  810  is displaced by a valve train element such as a cam (not shown). The outer tappet  820  follows the engine valve/contact stem  600 . Variable valve timing may be achieved by opening the trigger valve  890 , which permits the flow of oil from the inter-tappet plenum  860 . Automatic lash take up may be provided by a device (not shown) located between the outer tappet  820  and element  600 . 
     With reference to FIG. 18, the design shown in FIG. 17 may be adapted to reside on the push-tube side of a rocker in a cam-in-block engine design. The assembly shown in FIG. 18 is essentially a flipped over version of the assembly shown in FIG.  17 . In FIG. 18, the outer tappet is on the engine valve side of the valve train and the inner tappet is on the cam side of the valve train. The operation of the assembly shown in FIG. 18 is the same as that for the assembly shown in FIG.  17 . 
     FIGS. 19 and 20 show two different embodiments of the invention in which like reference numerals refer to like elements shown in the other figures. In both FIGS. 19 and 20, the valve stem or follower end  600  is shown in its rest position with the engine valve against its seat. 
     FIG. 19 shows a lash adjustment piston  900  located above the elongated stop  400 . The elongated stop  400  is integrally formed with the seating piston  300 . Low pressure oil from the supply duct  720  leaks past the lash piston clearance  902  into the lash chamber  910 , pushing the lash piston  300  against the stop  400  which in turn contacts the valve/follower end  600 . The lash piston  900  may also be biased downward by a lash spring  920 . 
     When the engine valve lifts off its seat due to the action of the follower, oil will flow past the check disk  500  and push the seating piston  300  down against its maximum travel stop  301 . During the time the engine valve is off of its seat, some oil will leak through clearance  902  and cause the lash piston  900  to move down following the stop  400 . This controlled leakage is small enough that it has no effect during the time that the engine valve is open (on the order of milliseconds). When the engine valve returns and approaches its seat, the valve/follower end  600  will contact the seating piston  300  and push it upward. The upward travel of the seating piston  300  is controlled by the oil flow through the clearance between the stop  400 , the inside diameter of the check disk  500 , and the flow through the flow control channels  422 . The varying nature of these flow areas causes the engine valve to approach its seat at a controlled velocity. In between valve events oil will leak in or out of the lash chamber  910  through the clearance  902  at a rate fast enough to adjust for any changes in valve length due to thermal growth (tens of seconds) and component wear (months). 
     FIG. 20 shows an embodiment of the invention similar to that shown in FIG. 19 where the lash piston  900  (shown as a shell) is located below the seating piston  300 . Oil from the supply duct  720  enters through a stationary check disk seat  520 . In all other aspects, the components in FIG. 20 act the same as those in FIG.  19 . 
     FIG. 21, in which like reference numerals refer to like elements, shows an alternative seating piston  300  and check disk  500  arrangement that can be used in the systems  100  shown in FIGS. 19 and 20. The disk  500  includes a central flow opening  502 , an off-set flow opening  504 , and a bore alignment feature  506 . The progressive occlusion of the central opening  502  provides the required throttling for valve seating. FIG. 22 shows the flow area past the check disk  500  as a function of the separation (Δ) of the seating piston  300  and the disk. 
     FIG. 23 shows an alternative seating piston  300  and check disk  500  arrangement to that shown and described in connection with FIG.  24 . FIG. 23 shows a one piece or self-contained version of FIG.  19 . In this version the stroke of the seating piston  300  is not affected by lash adjustment. Automatic lash take up is provided by the flow of leakage oil to the lash chamber  910 . Leakage flow to the chamber  910  causes the entire assembly packaged within the tube  304  to move downward and take up any lash. 
     With continued reference to FIG. 23, valve seating velocity occurs as a result of seating the check disk  500  against an upper seat. The check disk  500  is provided with a constant orifice  502 . Fluid flow may also occur around the outer perimeter of the check disk  500  as a result of aligmnent feature  506 . The flow past the alignment feature  506  is throttled as the disk  500  approaches its upper seat during engine valve closing. 
     FIG. 24 shows a two-piece construction with the lash adjustment piston  900  separate from the valve seating piston assembly  300 . The seating piston assembly  300  includes a disk seat member  302  and a tube  304 . This is primarily a manufacturing concern, but the two separate pistons may permit two different diametrical clearances. The lash adjustment piston  900  may be closely fit within the bore  710  to prevent excess leakage of the high pressures generated during valve seating. The seating piston assembly  300  may have a much larger clearance in the bore  710  to generate sufficient cooling flow around the outside of the seating piston tube  304  when supplied by the low pressure oil source  720 . The internal clearance between the seating control piston lower member  306  and the tube  304  is similar to that of the lash piston since it experiences similar pressures. This form of cooling is self regulating. As the oil becomes hotter its viscosity becomes lower and the leakage flow around the tube  304 , and therefore cooling, is increased. The two piece design will separate during engine valve lift. During engine valve closing, the seating piston assembly  300  moves back toward the lash piston  900  and then the elongated stop  400  will regulate the valve seating velocity. This hardware is shown with a central hole  502  in the check disk  500  that is covered by the end of the elongated stop  400  to regulate the flow area past the disk  500 . In some designs the off-center hole  504  is not required and the disk  500  has only the central hole  502 . A fluted elongated stop passing through the check disk could be used as illustrated in the previous designs. 
     Furthermore, it will be apparent to those skilled in the art that various modifications and variations can be made in the construction, configuration, and/or operation of the present invention without departing from the scope or spirit of the invention. For example, the shape, size, width, depth, and length of the fluted end of the elongated stop, and the flutes themselves, may be varied to achieve a particular hydraulic fluid flow profile suitable for a particular engine valve arrangement. Furthermore, the number of flutes on the elongated stop may also be varied to achieve a particular fluid flow profile. Still further, it is appreciated that the references throughout the specification to a slave piston encompass a piston other than that used in a traditional master-slave system, and in fact include all pistons whether used in lost motion systems or not. Thus, it is intended that the present invention cover the modifications and variations of the invention provided they come within the scope of the appended claims and their equivalents.