Abstract:
In a Type 2 engine, a valve deactivation hydraulic lash adjuster (DHLA) in accordance with the invention replaces a conventional hydraulic lash adjuster in the train of a gas-exchange valve in a compression-ignited engine. In a Type 3 engine, a similar DHLA is disposed within an articulated rocker arm which is made selectively competent (valve activating) or incompetent (valve deactivating) thereby. A solenoid valve within the assembly diverts hydraulic fluid between support and non-support of a piston slidably disposed in a housing and terminating in a ball head. The valve is force-balanced. The preferred hydraulic fluid is diesel fuel, allowing for smaller diameter passages and cleaner operation than in prior art systems, eliminating the need for an accumulator chamber and accumulator piston as in the prior art. An alternate version of a type 3 engine having a DHLA, in accordance with the invention, is also shown.

Description:
TECHNICAL FIELD 
     The present invention relates to variable valve activation (VVA) and variable valve timing (VVT) mechanism for internal combustion engines; more particularly, to VVA/VVT mechanism for modulating the timing of compression ignited (CI) engines such as diesel engines; and most particularly, to a valve lash adjuster having electro-hydraulic lost-motion capability for varying the lift and/or timing of individual valves in a valve train of a multi-cylinder engine. 
     BACKGROUND OF THE INVENTION 
     Means for varying the timing of valve actuation of internal combustion engines are well known. Such means typically take the form of a camshaft phaser or an element of a valve train, such as rocker arms, roller finger followers, hydraulic valve lifters, or hydraulic lash adjusters, provided with a mechanism for switching between a valve activating mode and a valve deactivating mode. VVA/VVT is especially well known in spark-ignited (SI) engines, in which it is an essential element of various schemes for improving fuel economy. However, camshaft phasers, while readily applied to SI engines, are not as suitable for CI engines since cam phasing introduces the risk of catastrophic valve-to-piston collisions due to the close proximity of the piston crown to the cylinder head at top dead center (TDC) at which point the valves are obliged to be closed. For this reason, an alternative technology, known in the art as “lost motion”, has found increasing favor for VVT/VVA in CI engines, since much of the functionality of existing SI VVT/VVA systems is available without danger of piston/valve collisions. 
     The potential importance of VVT/VVA to CI engine performance is coming to be realized within the engine industry. The firing of an SI engine is readily and accurately controlled by simply controlling the timing of the ignition spark. The firing of a CI engine, and more particularly so the firing of a controlled auto-ignition (CAI) or homogeneous charge compression ignition (HCCI) engine on the other hand, is governed by a plurality of independent or loosely-dependent variables which conspire to cause the fuel/air charge to explode at some resultant combination of temperature, pressure, and mixture. 
     These variables, which include, but may not be limited to, cylinder temperature, cylinder pressure, valve train timing and wear, fuel injection timing and accuracy, homogeneity of the fuel/air charge, and thermal load of the fuel/air charge, can vary from cylinder to cylinder in an individual engine and furthermore can vary for any given cylinder from one firing cycle to the next. Thus, in the prior art the exact point in the compression stroke at which the compressed charge in a cylinder will ignite cannot easily be predicted or controlled to a very high degree of certainty, and in practice the cylinders of a multiple-cylinder HCCI engine may not fire with a degree of uniformity required to meet future performance standards. 
     In a CI engine, the trapped air mass is the “charge volume” in the cylinder upon which compression work is done. Because adiabatic compression of the charge volume is the mechanism by which CI ignition is induced, an important ignition factor is the “Effective Compression Ratio” (ECR) within the cylinder. Thus, direct control of ECR can provide improved control of firing timing both in individual cylinders and among the cylinders in a CI engine. Controlling ECR by increasing the compression ratio can improve cold start characteristics, and by decreasing the compression ratio can improve engine performance. Other engine control strategies that can be attained by strategically controlling the opening, closing and lift of the gas valves in a CI engine, as more fully described in co-pending U.S. patent application Ser. No. 11/027,109, include in-cylinder swirl of intake gases to provide effective mixing of injected fuel and air, and controlled Exhaust Gas Recirculation (EGR) to control combustion initiation and burn rates, while lowering flame temperatures for reduced NO x  emissions. 
     In a Type 2 engine valve train, a roller finger follower (RFF) typically is interposed between an inwardly-opening poppet valve stem tip at one end and a hydraulic lash adjuster (HLA) at the distal end, with a cam lobe providing motivation to the RFF at an intermediate point. For reasons of good dynamic performance at high speed, low friction, and convenient packaging, this mechanism is rapidly becoming the valve train of choice for many new light-duty engines today, both SI and CI. 
     In a Type 3, 4, or 5 valve train, a rocker arm pivots on a rocker shaft, with one end of the rocker arm being motivated by the camshaft either directly or through the medium of a follower and/or pushrod, and the other end actuating the engine valve. For reasons of valve train cost, packaging convenience, or tradition, these systems are frequently used for medium- to heavy-duty engines and may or may not use an HLA. (For simplicity of presentation hereinbelow, Type 3 should be understood to mean all central-pivot rocker arm engines, including Types 4 and 5.) 
     In another version of the Type 3 valve train, the rocker arm pivots on an inverted HLA instead of a rocker shaft. Since the HLA is stationary, this type of valve train offers reduced dynamic mass advantages over other Type 3 valve train. 
     Lost motion means in a valve train element switches the linear motion imparted to the valve train by a rotating cam between either of a valve stem/lifter/pushrod or rocker arm and a lost motion spring/piston/accumulator. In the valve activating mode, the switchable element is mechanically and hydraulically competent to transfer the motion instructions of the cam to the valve; but in the valve deactivating mode, the switchable element collapses by a controlled amount and at the appropriate time in some fashion to “lose” the motion of the cam and belay those instructions to the valve. See, for example, U.S. Pat. No. 6,883,492. 
     Serious drawbacks of such known VVA/VVT systems are that they employ engine lubricating oil as the hydraulic medium, which tends to be dirty, carbon-laden and relatively high viscosity, requiring relatively large passageways to prevent flow failure; they employ a relatively bulky, powerful solenoid control valve which because of its size has a relatively slow speed of response; and they introduce significant additional complexity to the cylinder head that, in so doing, creates problematic packaging and manufacturing issues. 
     It is highly desirable that any apparatus and control system for improved control of ECR be applicable to existing arrangements of Type 2 and Type 3 engine valve trains with a minimum of engine redesign. 
     What is needed in the art is an improved means for controlling engine strategies such as, for example, ECR, EGR and in-cylinder swirl in a CI engine. 
     What is further needed is that such improved means be applicable to, and controllable for, individual cylinders in a multiple cylinder engine. 
     It is a principal object of the present invention to improve control of various engine control strategies in a CI engine. 
     It is a further object of the invention to provide such improved control with minimum redesign requirements for Type 2 and Type 3 engines. 
     It is a still further object of the invention to provide such improved control through novel adaptation of existing fuel injection equipment (FIE) technologies which have been demonstrated to have the speed of response, precision, and durability required for an ECR control system. 
     SUMMARY OF THE INVENTION 
     Briefly described, in a Type 2 engine, a valve deactivation hydraulic lash adjuster (DHLA) in accordance with the invention replaces a conventional hydraulic lash adjuster in the train of a gas-exchange valve in an internal combustion engine, and especially a compression-ignited engine. In a Type 3 engine, a DHLA is disposed within an articulated rocker arm which is made selectively competent (valve activating) or incompetent (valve deactivating) thereby. In the version of the Type 3 engine described above, a DHLA is disposed above the rocker arm at a center pivot point and replaces a conventional hydraulic lash adjuster. 
     In each approach, an electrically controlled solenoid valve within the DHLA assembly diverts hydraulic fluid between support and non-support of a piston slidably disposed in an elongated housing and terminating in a ball head. The geometry for the valve is such that the valve pin is force-balanced, thus reducing significantly the required strength and size of solenoid relative to the prior art solenoid and valving. 
     The preferred hydraulic fluid is diesel fuel which is readily available in a diesel engine from the low-pressure fuel supply pump for the engine fuel rail, which fuel preferably is used to lubricate the entire “top end” of the engine. Relatively dirty, carbon-contaminated engine lubricating oil is relegated to lubricating only the engine crankshaft and cylinder bores. The continuously refreshed and very much cleaner diesel fuel is used to lubricate the camshaft bearings and valve train, with conventional seals providing segregation of the two fluids. It is understood that the lubricity of diesel fuel is less than that of lubricating oil, however there is a clearly defined minimum standard for lubricity against which the bearing surfaces may be designed. Use of diesel fuel rather than engine lubricating oil allows for substantially smaller diameter passages and cleaner operation than in prior art systems. The use of smaller passages reduces very significantly the amount of fluid to be diverted between operating modes, eliminating the need for an accumulator chamber and accumulator piston as in the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is an elevational cross-sectional view of a prior art valve deactivation system substantially as disclosed in U.S. Pat. No. 6,883,492; 
         FIG. 2  is an elevational cross-sectional view of a typical prior art hydraulic lash adjuster; 
         FIG. 2   a  is an elevational cross-sectional view of a prior art Type 2 valve train employing the hydraulic lash adjuster shown in  FIG. 2 ; 
         FIG. 3  is an elevational cross-sectional view of a Type 2 valve train employing a first embodiment of a Type 2 valve deactivation hydraulic lash adjuster in accordance with the invention; 
         FIG. 4  is an elevational cross-sectional view of the DHLA shown in  FIG. 3 , taken orthogonal to the view therein; 
         FIG. 5  is an elevational cross-sectional view of the a second embodiment of a DHLA in accordance with the invention; 
         FIG. 6  is an elevational cross-sectional view of a Type 3 valve train employing an articulated rocker arm and conventional hydraulic lash adjuster in accordance with the invention; 
         FIG. 6   a  is an elevational cross-sectional view of a Type 3 valve train employing a first embodiment of a Type 3 valve deactivation rocker arm (VDRA) in accordance with the invention, shown in valve-activation mode; 
         FIG. 6   b  is a cross-sectional view of an embodiment showing a pushrod between the DHLA and buttress of  FIG. 6   a;    
         FIG. 7  is a view like that shown in  FIG. 6 , showing the VDRA in valve-deactivation lost-motion mode; and 
         FIG. 8  is an elevational cross-sectional view of an alternate version of a Type 3 valve train employing a VDRA, in accordance with the invention, shown in valve-activation mode. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The benefits and advantages of a valve deactivating electro-hydraulic lash adjuster in accordance with the invention may be better appreciated by first considering a prior art variable valve deactivation system. 
     Referring to  FIG. 1 , a prior art variable valve activation/deactivation lost-motion system  10  is substantially as disclosed in FIG. 7 of U.S. Pat. No. 6,883,492. The valve train includes an engine cam shaft  12 ; a rocker arm  14  rotational upon a rocker shaft  16  and comprising a cam roller follower  18  and an actuation arm  20 ; first and second paired engine valves  22  jointly operated by a valve bridge  24 ; a master piston  26  disposed in a main bore  28  in engine head  29  and biased by a return spring  31 ; and a slave piston  30  disposed in a slave bore  32  in communication with the main bore. The master piston is energized by a push tube  34  connected to actuation arm  20 . Hydraulic fluid  36  in the form of engine oil from the engine crankcase is supplied to the main and slave bores via a check valve  38  and a solenoid-operated trigger valve  40  disposed in a feed passage  42 . A lost-motion accumulator chamber  44  in communication with main bore  28  includes an accumulator piston  46 . 
     In operation, rotation of camshaft  12  causes oscillation of rocker arm  14  which drives master piston  26  within main bore  28 . If trigger valve  40  is closed (valve-enabling mode), slave piston  30  is displaced by the motion of master piston  26 , displacing bridge  24  and actuating valves  22 . If trigger valve  40  is open (valve-disabling mode), the valve springs keep valves  22  closed, and accumulator piston  46  is displaced in lost motion within accumulator chamber  44  by the motion of master piston  26 . 
     As noted above, known VVA/VVT systems such as prior art system  10  have several serious drawbacks, which the present invention overcomes. First, they employ engine lubricating oil as the hydraulic medium, which tends to be dirty and relatively high viscosity, requiring relatively large passageways to prevent flow failure. Furthermore, the dirty fluid encourages the use of relatively low hydraulic pressures to minimize abrasive wear, which in turn requires large diameter pistons. To minimize parasitic loss of hydraulic fluid, an accumulator piston device is necessary to store and recuperate the displaced fluid. Second, they employ a relatively bulky, powerful solenoid and non-force-balanced control valve which because of their size have a relatively slow speed of response; further, they occupy an undesirably large region within and adjacent to the cylinder head. Third, they introduce significant additional complexity to the cylinder head that requires extensive and expensive redesign and modification of engine manufacturing lines. 
     Referring to  FIG. 2 , a typical prior art hydraulic lash adjuster  50 , substantially as disclosed in U.S. Pat. No. 6,039,017, comprises a cylindrical adjuster body  52  having a bore  54  for slidably receiving a domed hollow plunger  56  retained in bore  54  by a retainer  58 . A low pressure reservoir  60  within plunger  56  receives oil from an engine gallery (not shown) in communication with a first annular distribution groove  62  formed in body  52 , a first supply port  64  through body  52  to a second annular distribution groove  66  formed in plunger  56 , and a second supply port  68  through plunger  56  to reservoir  60 . A lash-adjusting spring  70  disposed between body  52  and plunger  56  urges plunger  56  outwards of bore  54  to remove lash in the associated valve train (not shown). As plunger  56  moves outwards of bore  54 , a vacuum is created in high-pressure chamber  72 , drawing oil into chamber  72  from reservoir  60  via check valve  74  by compressing check valve spring  76 . 
     The objective of an HLA such as HLA  50  is to eliminate the lash that is an essential feature of traditional mechanical valve train linkages. This lash, or clearance, changes due to engine temperature, initial setting errors, and valve train wear. The lash adjuster compensates for these effects through a limited range of automatic adjustment, typically up to 5.0 mm maximum, so that intended engine operation is continuously maintained. 
     Referring to  FIG. 2   a,  in a prior art Type 2 valve train  78 , HLA  50  is disposed in a well  80  formed in engine head  29 . Oil is supplied via an engine oil gallery  82 . A roller finger follower (RFF)  84  comprises a first arm  86  having a hemispherical seat  88  for receiving head  90  of plunger  56  and a second arm  92  for engaging and actuating valve stem  94 . A roller  96  disposed between first and second arms  86 ,  92  follows camshaft  12 , the eccentric motion of which is translated into axial reciprocal motion of valve stem  94  because HLA  50  is incompressible. 
     Selective disabling of the motion of valve stem  94 , to selectively deactivate the associated engine valve in accordance with the invention, may be achieved by providing an improved hydraulic lash adjuster means that is selectively compressible in lost motion in lieu of motion of valve stem  94 . Provision of such lost-motion to the valve train requires a larger range of controlled adjustment than is required for HLA  50 , and this must be equal to or greater than that of either the cam lift or the full valve lift, depending upon the system geometry. Thus a lost-motion valve train mechanism must incorporate this extended range of collapsible travel to negate the cam lift; must provide additionally the adjustment range of a lash adjuster; and also must provide a control mechanism having a speed of response high enough to provide resolution of valve event control to within ±5 crank degrees of a target value. Moreover, this functionality must be packaged in a manner that makes it compatible with the constraints of modern engines, which implies that it should be dimensioned substantially similar to existing valve train components. Thus replacing a conventional HLA for selective valve deactivation is an excellent and novel strategy that can meet these criteria. 
     Referring to  FIGS. 3 and 4 , in a Type 2 deactivation valve train  178  in accordance with the invention, a first embodiment  100  of a deactivation hydraulic lash adjuster (DHLA) is disposed in a well  180  in head  29  similar to prior art well  80  except that well  180  is somewhat deeper to accommodate the necessary added length of DHLA  100  and a cross bore  102  at the distal end  104  accommodates an electrical connector (not shown) for spade lug  106 . 
     A steel body  108  close-fitting into well  180  includes a snout  110  having a bore  112  at its outer end into which a close-fitting piston  114  is slidably disposed. A requirement for minimal fluid leakage between piston and bore suggests that these items are preferably match-ground during manufacture as is common practice in manufacture of fuel injectors. A sleeve  116  having a spherical end  118  and a well  120  is slidably disposed on snout  110 . Spherical end  118  engages with hemispherical seat  88 , and piston  114  has an exposed length sufficient to accommodate both valve lost-motion travel and lash adjustment travel. Piston  114  is preferably not attached to sleeve  116  but rather engages sleeve  116  via a pressure plate  122  defined by the bottom of well  120  over which piston  114  is free to wander. Thus, side loads applied to DHLA  100  from RFF  84  are carried by sleeve  116  and snout  110 , leaving piston  114  free to reciprocate within bore  112  without bending loads being imposed upon it. An external bias spring  123  is disposed around sleeve  116  between body  108  and head  118  to urge DHLA  100  into continuous contact with RFF  84 . 
     Body  108  is preferably sealed into well  180  by first and second O-rings  124  disposed on either side of a hydraulic fluid gallery  126  formed in head  29  for supplying hydraulic fluid to DHLA  100 . As discussed hereinabove, a preferred hydraulic fluid in accordance with the invention is diesel fuel which may be conveniently supplied to gallery  126  parasitically from a low-pressure pump (not shown) in an existing engine system for supplying diesel fuel to a fuel injector rail (not shown). The low-pressure stage in a typical diesel engine fuel supply system operates at about 6 bar, which pressure is quite suitable for operation of DHLA  100 . 
     Within body  108  and communicating with bore  112  via first and second drillings  128 ,  130  is a spring-loaded inlet check or non-return valve  132  which may be of the ball or flute-guided conical seat type. Preferably, valve seat  134  is pressed into place to retain the valve components, although alternative constructions and orientations are fully contemplated by the invention, including but not limited to a reversed valve wherein the seat is machined directly into the body and a blanking plug is used to retain the spring; and a valve bore drilled at an angle which may allow packaging in a smaller diameter body. A shallow annular groove  136  permits access of hydraulic fluid from gallery  126  to check valve  132  at any installed rotational orientation of DHLA  100 . 
     Further within body  108  is a lost-motion control valve  138  disposed in an axial bore  140  having a valve seat  142 . Valve  138  which is spring-biased closed, is of a two-way two-position “balanced” spool design wherein opening and closing hydraulic forces are substantially equal at all times. A reduced-diameter central portion  144  of valve pin  139  is in communication with check valve  132  via third and fourth drillings  146 ,  148 , drilling  146  being blanked by a plug  150 . Hydraulic pressure created within body  108  acts upon control valve  138 , but since the upper piston portion of the valve is substantially the same diameter as the diameter of seat  142 , there is no resultant axial force due to pressure on the control valve; thus a low-force actuator is adequate to provide rapid valve motion. 
     An armature  152  disposed in an armature cavity  154  formed in body  108  is connected to valve pin  138 . In opening, valve  138  is withdrawn off of seat  142  into cavity  154 . Hydraulic fluid flows across seat  142  into armature cavity  154  and into a drilling  156  that breaks out into the under valve cover area  158  of the engine. Preferably, a cross drilling  160  breaks into bore  140  above valve  138  so that both ends of the valve are pressure balanced and in communication with area  158  pressure. 
     Within well  180  in head  29 , a solenoid stator assembly  162  is disposed adjacent body  108 . Assembly  162  includes a steel shell  164  providing a load path for the valve train forces into cylinder head  29 . Shell  164  receives a molded-in stator  166 , solenoid windings  168 , and a valve return spring  170  surrounding armature  152 . Electrical leads  172  extend to spade lug  106 . Preferably, stator assembly  162  is assembled and tested prior to attachment to adjuster body  108  as by electron-beam or laser welding. 
     In normal operation, diesel fuel oil is supplied to the gallery  126  in the cylinder head  29  that feeds the deactivation hydraulic lash adjuster  100  (and all other such lash adjusters in a bank of engine cylinders). At a nominal feed pressure of 6 bar, this fuel is readily able to overcome the light spring load of check valve  132  and therefore to fill the interior drillings  128 ,  130 ,  146 ,  148  and thereby assist bias spring  123  in loading RFF  84  against camshaft  12 . With lost-motion piston  114  extended so that the RFF is in contact with the cam on its base circle portion, all mechanical lash in the valve train is taken up, and check valve  132  then seats under coercion of its own spring. As camshaft  12  rotates in operation of the engine, an axial compressive load is applied to piston  114  that is in proportion to the dynamic load from the engine valve and spring. This load is reacted by hydraulic fluid captive within DHLA  100  and therefore, except for any undesired leakage past piston  114 , check valve  132 , and control valve  138 , the piston and therefore the fulcrum for RFF  84  remains stable. By keeping internal fluid volume to a minimum, piston depression due to fluid compressibility is very low. In this situation, no activation of control valve  138  occurs and engine valve motion is determined by the cam profile. 
     If an early valve closing event is desired during a normal open cycle of an associated engine valve as just described, control valve  138  is energized via controller  139 , causing release of hydraulic fluid from bore  112  which results in controlled collapse of piston  114  as the engine valve rapidly returns to its seat. Prior to the engine valve reaching its seat, the control valve may be re-energized, thus stopping further fluid release and arresting valve train motion. Engine valve seating can be closely controlled by this means in regions where the cam velocity is at or below an acceptable threshold value, typically about 0.2 m/sec at low engine idle speed. If lost-motion functionality is required over a wider range of timing authority including areas of high cam velocity, a valve seating snubber is required, as described further hereinbelow. Energizing of the control valve is managed by a computerized Engine Control Module (ECM) (not shown) in known fashion, with input from a crankshaft encoder and potentially with feedback from an accelerometer on the engine to give an indication of valve seating impact. 
     If total valve deactivation is desired, such as is the case if cylinder deactivation is desired, control valve  138  is energized for the whole cam lift event so that all motion of the cam and RFF is absorbed by lost-motion displacement of piston  114  within bore  112 . 
     If it is desired to delay opening of an engine valve, control valve  138  may be energized while the RFF is on the base circle portion of the cam. Initial cam motion of the RFF on the cam eccentric portion serves to depress piston  114  in lost motion until such time as valve lift is desired, at which point control valve  138  is de-energized allowing it to close, and the resulting internal hydraulic lock provides a stable fulcrum in a new and lower position for the RFF, resulting in a later opening and lowered, centered lift event. 
     If, during normal operation there is a tendency for DHLA  100  to “pump up” as is known to happen occasionally for prior art HLAs due to dynamic fluctuations in the valve train, control valve  138  can be energized momentarily to normalize the internal pressure prior to a scheduled valve event, or prior to TDC if there is concern for a possible piston/valve collision. 
     Referring now to  FIG. 5 , a second embodiment  200  of a DHLA in accordance with the invention is identical in valving and actuation to first embodiment  100 ; however, sleeve  216  doubles as the piston ( 114 ) and lash spring  223  is disposed within sleeve/piston  216 . An advantage of embodiment  200  is that the overall length is shorter than embodiment  100 ; however, a disadvantage is that the piston is not isolated from side loads imposed by RFF  84  during its normal rotary motion. 
     Referring now to  FIG. 6 , a novel Type 3 engine valve train  378  comprises a conventional hydraulic lash adjuster  300  disposed in an articulated rocker arm assembly  314 . Rocker arm assembly  314  is rotatably mounted on a rocker arm shaft  316  and includes a roller follower  317  for following the surface of a lobe of camshaft  12 , and an actuating arm  320  for actuating valve stem  94 . 
     In the prior art, a comparable Type  3  rocker arm is an inflexible unit wherein rotary motion of the cam is translated faithfully into reciprocal motion of the valve stem. Prior art lash adjustment typically is provided by either a screw head (not shown) on valve stem  94  or a hydraulic valve lifter (HVL) assembly (not shown) disposed between arm  320  and valve stem  94 . In HLA embodiment  300 , rocker arm assembly  314  is provided as first and second arms  320 ,  322  independently and rotatably mounted on shaft  316 . First arm  320  is adapted to engage valve stem  94 . 
     A well  380  is provided in second arm  322 . HLA  300  is disposed in well  380  and is provided with hydraulic fluid, for example, engine oil, via an axial gallery  324  in shaft  316  and a radial passage  325  extending to a connector drilling  326  in arm  322  that communicates with HLA  300 . As shown in the exemplary assembly in  FIG. 6 , HLA  300  is substantially identical in arrangement with HLA  50  shown in  FIG. 2 , and those components need not be repeated here save to note that the overall size of HLA  300  may be significantly smaller than HLA  50  as needed. 
     First arm  320  is provided with a buttress  328  having a wear surface  330  for receiving the spherical head  318  of HLA  300 . It will be seen that as HLA  300  expands in accordance with the prior art arm  322  is urged away from buttress  328 , causing arm  322  and/or arm  320  to counter-rotate on shaft  316 , thus changing the angular relationship between the two elements until all mechanical lash in valve train  378  between camshaft  12  and valve stem  94  is eliminated. 
     Referring now to  FIGS. 6   a  and  7 , a third embodiment  400  of a DHLA in accordance with the invention is adapted for use with a Type 3 engine valve train  478 . A rocker arm assembly  414  is rotatably mounted on a rocker arm shaft  416  and includes a roller follower  417  for following the surface of a lobe of camshaft  12 , and an actuating arm  420  for actuating valve stem  94 . 
     In DHLA embodiment  400 , rocker arm assembly  414  is provided as first and second arms  420 ,  422  independently and rotatably mounted on shaft  416 . First arm  420  is adapted to engage valve stem  94  as described further below, and second arm  422  is supportive of cam follower roller  417 . A well  480  in second arm  422  is analogous to well  380  in  FIG. 6 . DHLA  300  is disposed in well  480  and is provided with hydraulic fluid, preferably in the form of diesel fuel oil, via an axial gallery  424  in shaft  416  and a radial passage  425  extending to a connector drilling  426  in arm  422  that communicates with DHLA  400 . As shown in the exemplary assembly in  FIG. 6   a,  DHLA  400  is substantially identical in arrangement and function with DHLA  200  shown in  FIG. 5 , and those components need not be repeated here save to note that the overall size of DHLA  400  may be significantly smaller than DHLA  200 . 
     First arm  420  is provided with a buttress  428  having a wear surface  430  for receiving the spherical head  418  of DHLA  400 . In an alternative embodiment shown in  FIG. 6   b,  a short pushrod  410  is interposed between the spherical head  418  of DHLA  400  and a spherical thrust socket  412  located in the buttress  428 . Thus, relative motion between the DHLA and buttress, as the rocker arm articulates, is taken up in the thrust socket. 
     Electrical connection of a pigtail to DHLA  400  may be provided via a tail bore  402  (analogous to bore  102  in  FIG. 3 ). Other forms of electrical connection, for example, via mating slip rings (not shown) on shaft  416  and arm  422 , are fully comprehended by the invention. 
     In operation in valve-activating mode, as DHLA  400  expands like a conventional HLA, arm  422  is urged away from buttress  428 , causing arm  422  and/or arm  420  to counter-rotate on shaft  416 , thus changing the angular relationship between the two elements until all mechanical lash in valve train  478  between camshaft  12  and valve stem  94  is eliminated. Because rocker arm assembly  478  is thus hydromechanically rigid, rotary action of camshaft  12  is faithfully translated into reciprocal action of valve stem  94 . 
     If in normal operation there is a tendency for DHLA  400  to “pump up” as is known to occur occasionally in prior art HLAs due to dynamic fluctuations in valve train  478 , control valve  138  may be energized momentarily to normalize internal pressure prior to a scheduled valve lift event or prior to engine piston TDC if there is concern for a potential piston/valve collision. 
     Referring now to  FIG. 7  in conjunction with  FIG. 5 , in operation in valve-deactivating lost-motion mode of DHLA  400 , control valve  138  is opened by energizing of solenoid assembly  162 , permitting hydraulic fluid to be forced from bore  212  thereby allowing collapse of piston  216  into body  208  in lost motion in response to raising of roller follower  417  by camshaft  12 . Valve stem  94  is not actuated and the associated engine valve is not opened. 
     In some applications, it may be desirable to allow the engine valve to open fully in valve-activation mode but to close the valve early by changing assembly  478  to valve-deactivation mode on the closing slope of the valve lift cycle. Energizing of solenoid assembly  162  at this point causes immediate collapse of DHLA  400 , resulting in the full closing force of the compressed engine valve spring being brought to bear on the engine valve. Such abrupt closing can cause objectionable valve clatter as well as excessive valve wear and premature failure. Accordingly, a snubber assembly  500  is preferably included in valve train  478  to arrest the motion of valve stem  94  before the valve fully closes and to provide a graduated valve closing. 
     A problem with typical prior art hydraulic valve seating snubbers is that they have a fixed snubbing characteristic irrespective of operating conditions other than fluid viscosity. In the present case, snubbing is desired over only the final half millimeter or so of engine valve head travel before engagement with the valve seat, and this can be very difficult to arrange for all valves In an engine under all conditions of valve seat wear, recession, and assembly tolerances. 
     To cater to these variables and to ensure that seating velocity is controlled over only the final portion of travel, a currently preferred embodiment of variably controllable snubber assembly  500  incorporates an eccentric snail cam adjuster  502  at the rocker tip  504  whereby the base circle portion  505  of the snail cam contacts the tip  506  of valve stem  94 . The snail cam adjuster is rotated during engine assembly to engage the snubber piston  508  by a desired amount with the engine valve seated. This sets the range of action of the snubber in absorbing travel of the valve stem during the act of valve closing. Snail cam adjuster  502  is formed of a wear-resistant material such as hardened steel and is retained in a slot in rocker tip  504  by a through bolt  510 . 
     Snubber  500  comprises a body  512  having a stepped bore therethrough comprising a first diameter region  516  and a narrower diameter region  518 . A stepped snubber piston comprises a large-diameter portion  508   a  and a small diameter portion  508   b  and is slidably disposed in bores  516 / 518 . Bore  516  is closed as by a screw plug  520 , forming a closed chamber  522  above piston  508   a.  Bore region  516  is hydraulically pressurized dynamically to a predetermined pressure, preferably by connection to the same source of hydraulic fluid supplying fluid to gallery  424  for DHLA  400 . The pressure within the snubber assembly may be controllably varied in known fashion as a function of engine speed or other desired parameter to controllably modulate the valve seating velocity. 
     Referring now to  FIG. 8 , a fourth embodiment  600  of a DHLA in accordance with the invention is adapted for use with the alternate version of a Type 3 engine valve train  678  described above. A rocker arm assembly  614  is pivotably positioned, at its midpoint, against spherical end  618  of DHLA  600  and includes a first arm  601  for receiving an end of pushrod  603 , a second arm  604  for engaging and actuating valve stem  94  and valley  605  for receiving end  618  of DHLA as described further below. 
     DHLA  600  is disposed in well  680  formed in head  629 . Well  680  is open ended at its distal end to accommodate an electrical connector (not shown) for spade lug  606 . 
     Body  608  of DHLA  600  is close-fitting into well  680  and includes a snout  610  having a bore  612  at its outer end into which a close-fitting piston  616  is slidably disposed. Piston  616  includes spherical end  618  which engages valley  605  as described above. Piston  616  has a length sufficient to accommodate both valve lost-motion travel and lash adjustment travel. 
     Body  608  is preferably sealed into well  680  by first and second O-rings  624  disposed on either side of a hydraulic fluid gallery  626  formed in head  629  for supplying hydraulic fluid, preferably diesel fuel, to DHLA  600 . Within body  608  and communicating with bore  612  via first and second drillings  628 ,  630  is a spring-loaded inlet check valve  632  which may be of the ball or flute-guided conical seat type. Preferably, valve seat  634  is pressed into place to retain the valve components. A shallow annular groove  636  permits access of hydraulic fluid from gallery  626  to check valve  632  at any installed rotational orientation of DHLA  600 . 
     The details of lost-motion control valve  638 , armature  652  and solenoid stator assembly  662  are identical to valve, armature and stator assembly  138 ,  152  and  162 , as described in the first embodiment, and need not be described again. 
     In operation, in valve-activating mode, as DHLA  600  expands like a conventional HLA to provide a pivot point on which rocker arm assembly  614  pivots to cause the engine valve to open, all mechanical lash in valve train  678  between camshaft pushrod  603  and valve stem  94  is eliminated. In operation in valve-deactivating lost-motion mode of DHLA  600 , control valve  638  is opened by energizing of solenoid assembly  662 , allowing collapse of piston  616  into body  608  in lost motion in response to raising of pushrod  604  by the camshaft. Valve stem  94  is not actuated and the associated engine valve is not opened. 
     Snubber assembly  500  may be similarly provided, as described above with respect to the third embodiment, to arrest the motion of valve stem  94  before the valve fully closes and to provide a graduated valve closing. 
     While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.