Abstract:
A system for decelerating a linearly displaceable valve undergoing a closing motion is disclosed. The system includes a housing, a first hydraulic fluid chamber provided in said housing, and a slave piston for displacing the valve in response to the supply of hydraulic fluid to the first hydraulic fluid chamber. Deceleration of the valve may be accomplished by selectively throttling the release of hydraulic fluid in the first chamber to a second chamber. The hydraulic pressure in the second chamber opposes the closing motion of the valve, thereby slowing it gently for a valve seating event. Progressive throttling is used to maintain nearly constant hydraulic pressure in the second chamber during the seating event. The progressive throttling may be accomplished by selection of an appropriate throttling orifice size and shape, as well as an appropriate throttling profile for the orifice.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is related to and claims priority on Provisional Application Ser. No. 60/098,168 filed Aug. 26, 1998, entitled “Valve Seating Control Device With Variable Area Orifice;” and Provisional Application Ser. No. 60/101,411 filed Sep. 22, 1998, entitled “Valve Seating Control Device With Variable Area Orifice.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the operation of poppet valves. In particular, the invention relates to controlling valve seating velocity, and is particularly useful for the seating of internal combustion engine valves. 
     BACKGROUND OF THE INVENTION 
     An 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. FIG. 1 discloses the engine valve actuator of the &#39;141 patent. 
     The problem addressed by this invention is to provide acceptable engine valve seating velocity in a variable valve actuation (VVA) system. Hydraulic lost motion valve actuation systems may be driven with a cam. The hydraulic displacement of an engine valve in such a lost motion system is directly proportional to the displacement provided by the cam during normal operation. In some applications, however, the engine valve must 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 such instances, however, engine valve seating control is required because the rate of closing the valve is governed by the hydraulic 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. Still further, engine valve seating control is required in common rail VVA designs, in which all seating events occur as a result of the release of hydraulic fluid, possibly to an accumulator. 
     Devices designed to gently seat engine valves have been developed in order to address the needs of systems that require valve seating control. For example, the valve catch system  100  shown in FIG. 2 was developed to provide valve seating control. 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 adjust engine valve lash. 
     The plunger  140  serves to selectively limit valve seating speed velocity as the slave piston 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 δ, thus allowing the slave piston  120  to return rapidly until contacting the plunger, within δ of the valve seat. 
     The system  100  operates under the influence of hydraulic fluid provided through a passage  150  in the housing  110 . 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. 2, 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 produce increased pressure, whih may also be detrimental to compliance. Additionally, increased pressure drop may make it difficult to maintain master piston pressure greater than ambient during periods of decreasing cam displacement of high engine speed, which may allow air bubbles to form in the oil. Another difficulty that may be experienced with the valve catch system  100  is increased viscous dissipation, which may increase oil cooling load and parasitic power loss. 
     The valve catch system  200  shown in FIG. 3, which works on valve catch plenum pressure, is considered to have lower parasitic loss than the system shown in FIG.  2 . 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 master piston (not shown) and/or a trigger valve (not shown). 
     The system  200  operates similarly to the system  100  shown in FIG. 2, except that in system  200 , 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 . 
     The embodiments of the present invention distinguish over the valve catch systems  100  and  200  shown in FIGS. 2 and 3. The various embodiments of the present invention include a variable area orifice in the system plunger. The embodiments of the invention have reduced compliance especially during decompression braking, higher master piston pressure during periods of decreasing cam displacement at high engine speed, reduced parasitic power loss and consequently reduced VVA housing cooling load, and reduced slave piston length and weight as compared with the valve catch system shown in FIG.  2 . Furthermore, the embodiments of the innovation have reduced peak valve catch pressure as compared with the valve catch system, shown in FIG.  3 . The variable flow restriction design in the invention is expected to be more robust than the constant flow restriction design with respect to engine valve velocity at the point of valve catch engagement and oil temperature and aeration. The variable flow restriction allows 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. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of the present invention to provide a system for valve seating control that progressively throttles the flow of hydraulic fluid from a hydraulic chamber opposing the valve closing motion. 
     It is yet another object of the present invention to provide a 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 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 system for valve seating control that provides acceptable seating velocity during centered lift events when the valve seats on a high veolicty section of the cam. 
     It is still another object of the present invention to provide a system for valve seating control that has a less deleterious affect on valve opening events. 
     It is still another object of the present invention to provide a 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 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 system for valve seating control with improved hydraulic fluid aeration characteristics. 
     It still another object of the present invention to provide a 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 system for valve seating control of relatively simple and low cost design. 
     SUMMARY OF THE INVENTION 
     In response to the foregoing challenge, Applicants have developed a system for decelerating a linearly displaceable valve undergoing a closing motion. The system may include a housing, a first hydraulic fluid chamber provided in said housing, and an assembly for displacing the valve responsive to the supply of hydraulic fluid to the first hydraulic fluid chamber. The system comprises: a second hydraulic fluid chamber for receiving hydraulic fluid from the first hydraulic fluid chamber during the valve closing motion; and means for throttling the flow of hydraulic fluid between the first and second hydraulic fluid chambers in response to a displacement of the throttling means by the valve displacing means, wherein hydraulic pressure in the second hydraulic fluid chamber opposes the valve closing motion. 
     The present invention is directed to a system for decelerating a linearly displaceable valve undergoing a closing motion. The system includes a housing, a first hydraulic fluid chamber provided in the housing, and assembly for displacing the valve responsive to the supply of hydraulic fluid to the first hydraulic chamber. The system also comprises a second hydraulic fluid chamber, wherein hydraulic pressure in the second hydraulic fluid chamber opposes the valve closing motion as the valve approaches a closed position. The system also comprises an assembly for throttling the flow of hydraulic fluid between the second and first hydraulic fluid chambers in response to a displacement of the throttling assembly by the valve displacing assembly, wherein a throttling flow area is progressively reduced as the valve approaches the closed position. 
     During a seating portion of the valve closing motion, hydraulic pressure in the second hydraulic fluid chamber is approximately constant. 
     The system may further include an assembly for biasing the throttling assembly towards an engagement position with the displacing assembly. 
     In accordance with an embodiment of the present invention, the second hydraulic fluid chamber is provided in a screw body. The assembly for throttling includes a plunger having an internal passage for providing selective hydraulic communication between the first and second hydraulic fluid chambers. The plunger may include a cross-notched face adapted to contact the valve displacing assembly. Alternatively, the plunger may include a nose adapted to contact the valve displacing assembly. The internal passage in the plunger may be partially occluded by the screw body when the plunger is in a home position. Furthermore, the internal passage may include a plurality of holes. An assembly for adjusting the home position of the plunger may also be provided. 
     The internal passage in the plunger may include a vertical passage communicating with the second hydraulic fluid chamber and a cross passage communicating with the first hydraulic fluid passage. 
     Additionally, the second hydraulic fluid chamber may be provided in the valve displacing assembly. 
     In accordance with the present invention, the throttling assembly may include a plunger having a lower end contained within the valve displacing assembly, an upper end extending out of the valve displacing assembly, and an internal passage for providing selective hydraulic fluid communication between the first and second hydraulic fluid chambers. The hydraulic pressure in the second hydraulic fluid chamber during a seating portion of the valve closing motion may be approximately constant. An assembly for biasing the throttling assembly towards an engagement position with the valve displacing assembly may also be provided such that the internal passage in the plunger is partially occluded by the valve displacing assembly when the plunger is in a home position. 
     The present invention is also directed to a system for decelerating a linearly displaceable valve undergoing a closing motion, the system having a housing, a hydraulic fluid chamber provided in the housing, and a assembly for displacing the valve responsive to the supply of hydraulic fluid to the hydraulic fluid chamber. The system further includes a hydraulic circuit for receiving hydraulic fluid from the hydraulic fluid chamber during the valve closing motion, and an assembly for throttling a flow of hydraulic fluid between the hydraulic fluid chamber and the hydraulic circuit in response to a displacement of the throttling assembly by the valve displacing assembly. The hydraulic pressure in the hydraulic fluid chamber opposes the valve closing motion and a throttling flow area is progressively reduced as the valve approaches the closed position. 
     The present invention may further include an assembly for biasing the throttling assembly towards and engagement position with the valve displacing assembly. The throttling assembly may be provided in a screw body. The throttling assembly may further include a plunger having an internal passage for providing selective hydraulic fluid communication between the hydraulic fluid chamber and the hydraulic circuit. The plunger may include a spherical end adapted to mate with a conical depression in the valve displacing assembly, thereby routing flow from the hydraulic fluid chamber to the hydraulic circuit through the throttling assembly. The internal passage in the plunger may be partially occluded by the screw body when the plunger is in a home position. 
     In accordance with the present invention, a loose-fitting plunger may be adapted to mate with the valve displacing assembly, thereby routing flow from the hydraulic fluid chamber to the hydraulic circuit around the plunger through the throttling assembly. The throttling assembly may include a pin attached to the screw body which progressively occludes a fluid passage in the plunger as the valve approaches the closed position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described in connection with the following figures in which like reference numbers refer to like elements and wherein: 
     FIG. 1 is a cross section in elevation of an engine brake timing control device disclosed in U.S. Pat. No. 5,186,141; 
     FIG. 2 is a cross section in elevation of a first valve catch design; 
     FIG. 3 is a cross section in elevation of a second valve catch design; 
     FIG. 4 is a cross section in elevation of a first valve catch system embodiment of the present invention; 
     FIG. 5 is a cross section in elevation of the plunger occluding hole during a first operational position of the valve catch system shown in FIG. 4; 
     FIG. 6 is a cross section in elevation of the plunger occluding hole during a second operational position of the valve catch system shown in FIG. 4; 
     FIG. 7 is a cross section in elevation of second valve catch system embodiment of the present invention; 
     FIG. 8 is a cross section in elevation of third valve catch system embodiment of the present invention; 
     FIG. 9 is a cross section in elevation of fourth valve catch system embodiment of the present invention; 
     FIG. 10 is a cross section in elevation of the plunger occluding pin during a first operational position of the valve catch system shown in FIG. 9; 
     FIG. 11 is a cross section in elevation of the plunger occluding pin during a second operational position of the valve catch system shown in FIG. 9; 
     FIG. 12 is a cross section in elevation of fifth valve catch system embodiment of the present invention; 
     FIG. 13 is a graph of orifice area profile for constant deceleration; and 
     FIG. 14 is a pictorial view of a plunger having longitudinal notches used in an embodiment of the invention. 
    
    
     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. The first embodiment of the invention is shown in FIG. 4, as valve catch system  300 . 
     The system  300  provides valve seating control by developing valve catch plenum pressure with variable flow resistance during valve closing motions. The system  300  is provided with a slave piston  320  disposed in a slave piston chamber  312  provided in an actuator housing  310 . The slave piston  320  is slidable within the housing  310  so that it may open an engine valve (not shown) below it. Hydraulic fluid may flow to and from the slave piston chamber  312  via a feed passage  350 . 
     A screw body  330  extends through the top of the housing  310  and provides an end wall for the slave piston chamber  312 . The screw body  330  may be screwed in and out of the housing  310  to adjust for variation in valve train lash as needed. The screw body  330  may be secured in place by a locking nut  338 . The screw body  330  may include an adjustable screw  332  extending into an interior chamber  336  provided within the screw body. An advantage of using the adjustable screw  332  is that the threads, if not sealed with a substance like Loctite, can provide a leakage path for air which may be trapped in the screw body chamber  336 . 
     With continued reference to FIG. 4, a plunger  340  may be provided with an upper end  342  slidably disposed in the interior chamber  336  and a lower end extending out of the screw body  330  and into the slave piston chamber  312 . The plunger  340  may be biased by a spring  360  towards the slave piston  320 . The spring  360  engages the lower end of the adjustable screw  332 . The plunger  340  may include a vertical passage  344  and a cross passage  346 . The vertical passage and the cross passage collectively provide hydraulic communication between the screw body interior chamber  336  and the slave piston chamber  312 . The bottom of the plunger  340  may include one or more cross notches  348  and a chamfered edge to increase the pressure area acting on slave piston  320  and to reduce the suction force upon separation of the plunger  340  from the slave piston  320  during opening of the engine valve. The plunger  340  preferably has a diameter nearly equal to the slave piston  320  diameter. 
     The cross passage  346  in the plunger  340  is positioned so that it is partially occluded by the screw body  330  at the point at which the slave piston  320  first contacts the plunger as the engine valve is closing. FIG. 5 shows the orientation of the cross passage  346  relative to the screw body  330  at the point of valve catch engagement by the slave piston  320 . 
     The operation of the valve catch system shown in FIG. 4 is now described starting from the condition in which the engine valve is seated and the cam driving the slave piston is on base circle. The spring  360  biases the plunger  340  to rest against the slave piston  320 . As cam lift progresses, hydraulic fluid flows into the slave piston chamber  312  through the feed  350  causing the pressure in the slave piston chamber to increase and force the slave piston downward. As the slave piston  320  opens the engine valve, the plunger  340  may or may not follow it downward. The maximum downward displacement by the plunger  340  is dictated by the engagement of a plunger shoulder  342  with a screw body shoulder  334 . The spring  360  positions the plunger  340  at the maximum downward displacement before the valve seating event. The amount of the maximum downward displacement of the plunger  340  may be adjusted by turning the screw body  330  into or out of the housing  310 . 
     After the valve opening event, the slave piston  320  returns upward under the influence of the cam or the release of pressure from the slave piston chamber  312 . Eventually the bottom of the plunger  340  engages the top of the slave piston (typically at an engine valve lift of less than 1 mm). At this point the engine valve velocity may be approximately 150 in/sec, while the required seating velocity may be less than 15 in/sec. Once the slave piston  320  contacts the plunger  340 , the engine valve (not shown), the slave piston, and the plunger move together. From this point on, the upward motion of the plunger  340  forces the hydraulic fluid in the interior chamber  336  through the vertical passage  344  and the cross passage  346  thereby increasing the pressure in the interior chamber  336  to approximately 6000 psi. As the engine valve approaches its seat, the cross passage  346  is progressively occluded by the screw body  330 , so that the pressure in the interior chamber  336  is maintained approximately constant while the engine valve velocity is reduced. Constant interior chamber  336  pressure results roughly in a constant rate of engine valve deceleration. 
     FIGS. 5 and 6 show the detail of the progressive occlusion of the cross passage  346  during a valve seating event. FIG. 5 shows the detail of the position of the cross passage  346  relative to the screw body  330  at the point that the slave piston  320  engages the plunger  340 . FIG. 6 shows the detail of the position of the cross passage  346  relative to the screw body  330  at the point that the engine valve is fully seated. 
     The system shown in FIG. 4 provides more reliable valve seating over a range of engine operation conditions due to the variable area orifice of the cross passage  346 . The diameter of the lower section of plunger  340  and the orifice are selectively designed to be large enough to keep pressures within the interior chamber  336  low enough that compliance does not cause problems. Compliance is also favorably impacted because the volume of hydraulic fluid that is exposed to high pressure is reduced as compared with existing designs. Furthermore, this embodiment of the present invention also allows for a slave piston of reduced weight and length. While the embodiment shown in FIG. 4 may increase flow resistance through the plunger  340  it does not increase flow resistance in and out of the slave piston chamber  312 . It is noted that embodiments of the present invention should be manufactured with care due to the potential for misalignment and the tight clearances required for the system to operate properly. 
     With reference to a second embodiment of the invention shown in FIG. 7, a variable flow restriction valve catch plunger  440  may be incorporated into the slave piston  420  of the system  400 . With respect to FIG. 7, the system  400  is provided with a slave piston  420  disposed in a slave piston chamber  412  provided in an actuator housing  410 . The slave piston  420  is slidable within the housing  410  so that it may open an engine valve (not shown) below it. Hydraulic fluid may flow to and from the slave piston chamber  412  via a feed passage  450 . 
     A plunger  440  may be provided with a lower end slidably disposed in the interior chamber  436  of the slave piston  420 , and an upper end extending out of the top of the slave piston and into the slave piston chamber  412 . The plunger  440  may be biased upward by a spring  460  towards a lash adjuster  480 . Spring  460  must be stiff enough to overcome inertial effects which tend to make the plunger  440  loose contact with stop  434  during engine valve closing. The distance that the plunger  440  may slide into the slave piston  420  may be adjusted by screwing a plug  432  into and out of the slave piston. 
     The plunger  440  may include a vertical passage  444 , a cross passage  446 , and an optional hole  448 . The vertical passage and the cross passage collectively provide hydraulic communication between the interior chamber  436  and the slave piston chamber  412 . The plunger  440  preferably has a diameter nearly equal to the slave piston  420  diameter. The cross passage  446  in the plunger  440  is positioned so that it is partially occluded by the upper edge of the slave piston  420  at the point at which the plunger  440  first contacts the lash adjuster  480 . The optional fill hole  448  facilitates rapid upward deployment of plunger  440  when the engine valve is open. The lash adjuster  480  extends through the top of the housing  410  and provides a stop for the plunger  440 . The lash adjuster  480  may be screwed in and out of the housing  410  to adjust the point of plunger engagement as needed to compensate for variation in valve train lash. The lash adjuster  480  may be secured in place by a locking nut  482 . 
     The design shown in FIG. 7 should solve a potential problem of there being insufficient clearance of the locking nut  338  in the first embodiment of the invention. The embodiment shown in FIG. 7 should not require a plunger with a cross notched face 
     With respect to a third embodiment of the invention shown in FIG. 8, the system  500  is designed similarly to the system  300  shown in FIG. 4, except that the bottom of the plunger  540  is spherical in order to seal against a conical seat provided in the slave piston  520 . In the system  500 , the hydraulic fluid flow path extends from the slave piston chamber  512  through the passage  524 , peripheral recess  522 , and feed passage  550  to a master piston and/or accumulator (not shown). 
     A screw body  530  extends through the top of the housing  510 . The screw body  530  may be screwed in and out of the housing  510  to adjust for variation in valve train lash. The screw body  530  may include an adjustable screw or plug  532  extending into an interior chamber  536  provided within the screw body. 
     The lower section of the plunger  540  slides with tight clearance in screw body  530 . The upper section provides a stop which limits extension of the plunger  540  into the slave piston chmaber  512 . The plunger  540  may be biased by a spring  560  towards the slave piston  520 . The plunger  540  may include a vertical passage  544  and a cross passage  546 . The vertical passage and the cross passage collectively provide hydraulic communication between the screw body interior chamber  536  and the slave piston chamber  512 . The bottom the plunger  540  may be spherical, as noted above. 
     With continued reference to FIG. 8, during initial valve opening the plunger  540  is pushed up creating a large flow area. Prior to valve seating, the main flow area is cut off by mating of the spherical end of the plunger  540  with the conical seat or depression in the top of the slave piston  520 , forcing the flow through the passages in the plunger. As the plunger moves up, the cross passage  546  is occluded by the screw body  530 . The combined leakage of hydraulic fluid past the conical seat  526  and around the plunger  540  must be small compared to the flow through the occluding cross passage  546 . An advantage of this design compared to the designs in FIGS. 4-7 is that high pressure acts over the entire slave piston area during engine valve seating. This increased pressure area results in lower peak pressure, which favorably impacts compliance. A potential disadvantage is increased parasitic loss and a consequent increased hydraulic cooling requirement. 
     FIG. 9 discloses a system which provides a variable flow area using the central passage  644  in the plunger  640  and a pin  633  attached to the plug  632 . The plunger  640  may have a loose clearance so its orientation will adjust to seal the hole in the top of the slave piston  620 . With reference to FIGS. 9 and 14, the plunger  640  may have one or more longitudinal notches  646  to facilitate additional hydraulic fluid flow between the interior chamber  636  and the slave piston chamber  612 . FIGS. 10 and 11 provide a detailed illustration of the interaction of the pin  633  with the upper end of the plunger  642  during valve seating. As in the system disclosed in FIG. 8, high pressure acts over the entire slave piston area during engine valve seating. 
     A fifth embodiment of the invention is shown in FIG.  12 . The system  700  shown in FIG. 12 includes: a housing  714 ; a screw body  716 ; a cup  730 ; a nose on the bottom of the cup  738 ; a fill hole  736 ; orifice holes  750 ; a spring  742 ; a snap ring  740  and a snap ring groove  718 . 
     With continued reference to FIG. 12, the screw body  716  is threaded into the housing  714  over a slave piston  720  providing adjustment of the axial position of the valve actuator  710  relative to the point of valve seating to compensate for variation in valve train lash. A locking nut (not shown) may be provided to prevent the position of the screw body  716  from changing relative to the housing  714 . The cup  730  fits over the bottom of the screw body  716  with a tight diametrical clearance. The snap ring  740  attaches the cup  730  to the screw body  716  and provides hard stops for the maximum and minimum cup displacement. Alternatively, the snap ring groove  718  may be designed so that contact between the cup  730  and the screw body  716  limits cup displacement. In this case, vertical notches in the top of the cup  730  may facilitate assembly of snap ring  740 . The screw body  716  includes an open chamber  744 , or valve actuator plenum, at its end facing the cup  730 . Spring  742  is located within the plenum  744 . The spring  742  biases the cup  730  toward the slave piston  720 , in an extended position. A nose  738  is provided on the bottom of the cup  730  to reduce the suction force upon separation of the cup  730  from the slave piston  720  during the opening of the engine valve. A hole  736  is provided in the bottom of the cup  730  to fill the valve actuator plenum  744  with fluid, equalizing pressure and allowing the cup  730  to extend rapidly. 
     The invention includes some number of holes  750  in the side of the cup  730  which are partially occluded by the screw body  716  during valve seating. The holes  750  provide increased resistance to the flow of fluid out of the valve actuator plenum  744  as the engine valve approaches its seat. 
     The assembly of the valve actuator shown in FIG. 12 will now be described. The valve actuator is screwed down until the snap ring  740  contacts the top of the groove  718  on the screw body  716  or alternatively, the cup  730  contacts the screw body  716 . At that point, the cup  730  contacts the top of the slave piston  720 , and the slave piston contacts the crosshead or engine valve (not shown), while the engine valves are held closed by the stiff valve springs (not shown). At this point, the sealing edge  731  is either in line or slightly above the bottom of the orifice holes  750 . From this minimum displacement hard stop position, the screw body  716  is backed off a specified amount (typically 0.3 mm), which is chosen to ensure that the valve actuator will never reach its minimum displacement hard stop before the engine valve seats. This procedure, similar to lash adjustment, compensates for manufacturing variations. 
     At the start of an engine valve lift event, the nose  738  on the cup increases the pressure area on the top of the slave piston  720  and reduces the suction effect as the slave piston pulls away from the valve actuator cup  730 . The spring  742  pushes the cup  730  down 1-2 mm as fluid fills through the hole  736  in the bottom of the cup  730 . 
     Prior to engine valve seating, the extended cup  730  contacts the top of the slave piston sealing off the hole  736 . Fluid is forced out through the occluding holes  750 , which builds pressure in the valve actuator plenum  744 , and slows the slave piston and engine valve assembly. The flow area of the occluding holes  750  decreases with decreasing engine valve and cup  730  lift. The valve actuator is designed in order to provide a roughly constant rate of deceleration of the slave piston  720  and engine valve assembly during valve seating. This requires a constant retarding force, a constant valve actuator plenum pressure, and an occluding orifice hole area proportional to engine valve velocity. The required seating velocity is typically ten to twenty times less than the maximum engine valve velocity prior to the slave piston  720  contacting the cup  730 . Factors such as tolerances also affect the optimal occluding orifice configuration. 
     The graph shown in FIG. 13 illustrates the approximate orifice area required for near constant engine valve deceleration for given distances between valve catch engagement and valve catch seating. The number of occluding holes  750 , their diameter, and their location in cup  730  are chosen to have approximately the proper profile of total orifice area vs. engine valve lift for constant deceleration of the engine valve between valve catch engagement and engine valve seating. The diameter and location of multiple occluding holes may be different. 
     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, in the embodiments mentioned above, various changes may be made to the shape and size of the components used. Furthermore, the shape and positioning of the variable area orifice may be changed so long as the desired deceleration profile for the engine valve is maintained. 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.