Abstract:
An internal combustion engine may include a hydraulic linkage used to transfer motion from a valve train element, such as a cam, to an engine valve. Method and apparatus for selectively limiting the motion transferred by the hydraulic linkage from the valve train element to the engine valve are disclosed. The motion transferred by the hydraulic linkage may be limited by a means for resetting or clipping that is integrated into the rocker arm/shaft assembly provided in the valve train.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    The application relates to and claims priority on U.S. Provisional Patent Application Ser. No. 60/172,581, filed on Dec. 20, 1999. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to valve actuation in internal combustion engines that include compression release-type engine retarders. In particular, it relates to methods and apparatus for controlling valve lift and duration for compression release valve events and main exhaust valve events.  
         BACKGROUND OF THE INVENTION  
         [0003]    Engine retarders or brakes of the compression release-type are well-known in the art. Engine retarders are designed to convert, at least temporarily, an internal combustion engine of compression-ignition type into an air compressor. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted compression release-type engine retarder can develop retarding horsepower that is a substantial portion of the operating horsepower developed by the engine in positive power.  
           [0004]    Functionally, compression release-type retarders supplement the braking capacity of the primary vehicle wheel braking system. In so doing, it extends substantially the life of the primary (or wheel) braking system of the vehicle. The basic design for a compression release engine retarding system of the type involved with this invention is disclosed in Cummins, U.S. Pat. No. 3,220,392 (November 1965) for a Vehicle Engine Braking And Fuel Control System.  
           [0005]    The compression release-type engine retarder disclosed in the Cummins &#39;392 patent employs a hydraulic system or linkage. The hydraulic linkage of a typical compression release-type engine retarder may be linked to the valve train of the engine. When the engine is under positive power, the hydraulic linkage may be disabled from providing valve actuation. When compression release-type retarding is desired, the hydraulic linkage is enabled such that valve actuation is provided by the hydraulic linkage responsive to an input from the valve train.  
           [0006]    Among the hydraulic linkages that have been employed to control valve actuation (both in braking and positive power), are so-called “lost-motion” systems. Lost-motion, per se, is not new. It has been known that lost-motion systems are useful for variable valve control for internal combustion engines for decades. In general, lost-motion systems work by modifying the hydraulic or mechanical circuit connecting the actuator (typically the cam shaft) and the valve stem to change the length of that circuit and lose a portion or all of the cam actuated motion that would otherwise be delivered to the valve stem to produce a valve opening event. In this way lost-motion systems may be used to vary valve event timing, duration, and the valve lift.  
           [0007]    Compression release-type engine retarders may employ a lost motion system in which a lash piston is included in the valve train (e.g. a linkage of a push tube, cam, and/or rocker arm) of the engine. When the retarder is engaged, the lash piston is hydraulically extended to cause the exhaust valve of the internal combustion engine to open at a point near the end of a piston&#39;s compression stroke. In doing so, the work that is done in compressing the intake air cannot be recovered during the subsequent expansion (or power) stroke of the engine. Instead, it is dissipated through the exhaust and radiator systems of the engine. By dissipating energy developed from the work done in compressing the cylinder gases, the compression release-type retarder dissipates the kinetic energy of the vehicle, which may be used to slow the vehicle down.  
           [0008]    Regardless of the specific actuation means chosen, inherent limits were imposed on operation of the compression release-type retarder based on engine parameters. One such engine parameter is the physical relationship of an engine cylinder valve used for compression release braking and the piston in the same cylinder. If the extension of the valve into the cylinder was unconstrained during compression release braking, the valve could extend so far down into the cylinder that it impacts with the piston in the cylinder.  
           [0009]    There may be a significant risk of valve-to-piston contact when a unitary cam lobe is used to impart the valve motion for both the compression release valve event and the main exhaust valve event. Use of a unitary cam lobe for both events means that the relatively large main exhaust lobe motion will be imparted to the hydraulic linkage, or more particularly to the slave piston. Because there is typically little or no lash between the lash piston and the exhaust valve during engine braking, input of the main exhaust event motion to the lash piston may produce a greater than desired main exhaust event. A means for limiting the downward stroke of an exhaust valve for its main exhaust event during engine braking is needed.  
           [0010]    Some systems do not use a unitary cam lobe for both the compression release valve event and the main exhaust valve event. These systems may operate using a dedicated braking cam lobe to drive a dedicated braking rocker arm, and a dedicated main exhaust cam lobe to drive a dedicated main exhaust rocker arm. The braking and main exhaust rocker arms may actuate different or the same exhaust valves using one or more bridges or similar arrangements to convey the rocker arm motions to the selected exhaust valves. Although these “dedicated” systems do not run the same risks of valve-to-piston contact as the “unitary cam” systems, they may also benefit from inclusion of a means to limit the downward stroke of the exhaust valves.  
           [0011]    One way of limiting the downward stroke of an exhaust valve used for compression release valve events and/or main exhaust valve events is to limit the extension of the hydraulic lash piston that is responsible for pushing the valve into the cylinder during compression release braking. A device that may be used to limit piston extension or motion is disclosed in Cavanagh, U.S. Pat. No. 4,399,787 (Aug. 23, 1983) for an Engine Retarder Hydraulic Reset Mechanism, which is incorporated herein by reference. Another device that may be used to limit piston motion is disclosed in Hu, U.S. Pat. No. 5,201,290 (Apr. 13, 1993) for a Compression Relief Engine Retarder Clip Valve, which is also incorporated herein by reference. Both of these (reset valves and clip valves) may comprise means for blocking a passage in a lash piston during the downward movement of the lash piston (such as the passage  344  of the slave piston  340  of FIG. 6). After the lash piston reaches a threshold downward displacement, the reset valve or clip valve may unblock the passage through it and allow the oil displacing it to drain there through, causing the lash piston to return to its upper position under the influence of a return spring.  
           [0012]    A reset valve, such as the one disclosed in Cavanagh, may be provided as part of a lash adjuster or a lash piston. A reset valve may comprise a hydraulically actuated means for unblocking a passage through the lash piston to limit its displacement. In Cavanagh, compression release retarding is carried out by opening one of two valves connected by a crosshead member or bridge. A purpose of the reset valve used in Cavanagh is to reseat the exhaust valve used for the compression release event before a subsequent main exhaust valve event so that the rocker arm will not push down on an unbalanced crosshead during the main exhaust event and transmit a bending force to the crosshead guide pin or to the non-braking valve stem.  
           [0013]    A clip valve, such as the one disclosed in Hu, may comprise a mechanically actuated means for unblocking the passage through a hydraulically extendable piston to limit its extension.  
           [0014]    As evident from the foregoing, compression release retarding systems have historically been implemented as bolt-on systems added to an existing engine as an optional or after-market item. As the market for compression release-type engine retarders has developed and matured, the direction of technological development has moved away from bolt-on systems towards compact, cost-efficient integrated engine braking systems. More and more engine manufacturers have expressed an interest in incorporating or integrating the engine brake components into their fundamental engine designs in order to achieve their cost and performance goals. It is believed that incorporation of the engine brake into the engine will ultimately provide the needed cost, weight, performance, and efficiency benefits.  
           [0015]    One method of engine brake integration is disclosed in Cartledge, U.S. Pat. No. 3,809,033 (May 7, 1974) for a Rocker Arm Engine Brake System. With reference to FIGS.  6 - 8  of Cartledge, a rocker arm  16  incorporates a lash piston  31  that may be hydraulically extended from the rocker arm for braking operation. The rocker arm transfers braking motion from a cam (not shown) to an exhaust valve  15 . The lash piston  31  takes up the lash between the rocker arm  16  and its associated exhaust valve during engine braking. The elimination of this lash during braking allows a small braking lobe on the exhaust cam to produce a compression release opening of the exhaust valve near the top of the piston compression stroke.  
           [0016]    A more recent development of the rocker arm brake is disclosed in McCarthy, U.S. Pat. No. 5,975,251 (Nov. 2, 1999) for a Rocker Brake Assembly With Hydraulic Lock, which is incorporated herein by reference. With reference to FIG. 1 of McCarthy, a rocker arm assembly  10  having a brake rocker arm  100  mounted on a rocker shaft  200  is shown. The brake rocker arm  100  pivots about the rocker shaft  200  and includes a first end  110  and a second end  120 . The first end  110  of the brake rocker arm  100  includes a brake cam lobe follower  111 . The brake cam lobe follower  111  may include a roller  112  that is in contact with a brake cam lobe, not shown. The second end  120  of the brake rocker arm  100  includes an actuator assembly  121 . The actuator assembly  121  is spaced from the crosshead of an exhaust rocker arm, not shown. When activated, the brake rocker arm  100  and the actuator assembly  121  contact the crosshead pin, not shown, of the crosshead to open the at least one exhaust valve to perform a braking operation. The brake rocker arm  100  also includes a fluid passageway  130  that extends from the actuator assembly  121 . Hydraulic fluid from a passageway  210  in the shaft  200  may be supplied to the fluid passageway  130  to operate the actuator assembly  121 .  
           [0017]    Furthermore, both current and expected environmental restrictions have forced engine manufacturers to explore a variety of new ways to improve the efficiency of their engines. These changes have forced a number of engine modifications. Engines have become smaller and more fuel efficient, increasing the need for weight saving integration of engine brakes. Yet, the demands on retarder performance have often increased, requiring the compression release-type engine retarder to generate greater amounts of retarding horsepower under more limiting conditions.  
           [0018]    In view of the foregoing, there is a need for an integrated engine braking system and method of operation therefor, that includes a lash piston that may be hydraulically reset and/or clipped. In particular, there is a need for an engine braking system having a lash piston and a means for resetting or clipping the lash piston integrated into a rocker arm assembly.  
         OBJECTS OF THE INVENTION  
         [0019]    It is therefore an object of the present invention to provide an actuation means for engine braking that optimizes engine retarding performance.  
           [0020]    It is another object of the present invention to provide a system and method for avoiding valve-to-piston contact during a main exhaust valve event.  
           [0021]    It is a further object of the present invention to provide a system and method for limiting the stroke of a lash piston during an engine valve opening event.  
           [0022]    It is yet another object of the present invention to provide a system and method for resetting a lash piston following an engine valve opening event.  
           [0023]    It is still another object of the present invention to provide a system and method for clipping the motion of a lash piston during an engine valve opening event.  
           [0024]    It is still a further object of the present invention to provide a system and method of engine braking that is integrated into the rocker arm/shaft assembly.  
           [0025]    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.  
         SUMMARY OF THE INVENTION  
         [0026]    In response to this challenge, Applicants have developed an innovative and reliable engine braking system, for providing a compression release valve event in an internal combustion engine, comprising: a rocker arm shaft; a rocker arm having a central bore adapted to receive the rocker arm shaft; means for pivoting the rocker arm on the rocker arm shaft to provide a compression release valve event; an hydraulically extendable lash piston disposed in a piston bore in the rocker arm, said lash piston being adapted to open an engine valve for the compression release event; means for providing hydraulic fluid to the piston bore; an hydraulic relief port provided on the rocker arm, said relief port having hydraulic communication with the piston bore; and means for selectively unblocking the relief port responsive to pivoting of the rocker arm.  
           [0027]    Applicants have also developed an engine braking system, for providing a compression release valve event in an internal combustion engine, comprising: a rocker arm shaft; an hydraulic relief passage formed in the rocker arm shaft, said relief passage communicating with an outer surface of the rocker arm shaft; a rocker arm having a central bore adapted to receive the rocker arm shaft; means for pivoting the rocker arm on the rocker arm shaft to provide a compression release valve event; an expandable hydraulic tappet disposed in a piston bore in the rocker arm, said tappet being adapted to open an engine valve for the compression release event; means for providing hydraulic fluid to the tappet; and means for providing selective hydraulic communication between the relief passage and the tappet responsive to pivoting of the rocker arm.  
           [0028]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated herein by reference, and which constitute a part of this specification, illustrate certain embodiments of the invention and, together with the detailed description, serve to explain the principles of the present invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    FIGS.  1 - 19  and  21 - 22  are cross-sectional views in elevation and top plan of eleven related alternative embodiments of the invention.  
         [0030]    [0030]FIGS. 20 and 23 are schematic drawings illustrating fundamental elements of the embodiments of the invention shown in FIGS.  13 - 16 , and FIGS.  21 - 22 , respectively.  
         [0031]    FIGS.  24 - 29  are cross-sectional views in elevation and top plan of three related alternative embodiments of the invention.  
         [0032]    FIGS.  30 - 33  are cross-sectional views in elevation and top plan of two related alternative embodiments of the invention.  
         [0033]    FIGS.  34 - 37  are cross-sectional views in elevation and top plan of two related alternative embodiments of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    Reference will now be made in detail to the various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and in which like reference numerals refer to like elements. A first embodiment of the present invention is shown in FIGS. 1 and 2 as engine braking system  100 . Generally, the engine braking system  100 , exemplified by the system shown in FIGS. 1 and 2, may include an operative arrangement of a rocker arm  200 , a rocker arm shaft  300 , a means for imparting motion to the rocker arm  400 , and an engine valve assembly  500 . A lash piston  210  may be formed in an end of the rocker arm  200 . The arrangement of one or more hydraulic passages formed in the rocker arm  200  provide for the selective relief of hydraulic fluid from the lash piston  210  responsive to pivoting of the rocker arm on the rocker arm shaft  300 . When the lash piston  210  is in contact with the engine valve assembly  500 , the relief of hydraulic fluid from the lash piston may be used to clip or reset the motion of the engine valve.  
         [0035]    A detailed explanation of the embodiment of the invention shown in FIGS. 1 and 2, and its operation, will now be provided. The rocker arm  200  includes a lash piston  210  at a first end and a cam follower  250  at a second end. The cam follower  250  is rotatable so that the rotary motion of the cam  400  may be converted into a pivoting motion by the rocker arm  200  with minimal friction. The means for imparting motion to the rocker arm is a cam  400  in the system shown in FIG. 1. When the engine valve  500  is implemented as an exhaust valve or dedicated braking valve, the cam  400  may have fixed compression release, main exhaust, and/or EGR lobes formed thereon.  
         [0036]    The means for imparting motion may include a push tube, or other valve train element between the cam  400  and the rocker arm  200  without departing from the scope of the invention. While preferred, the cam  400  is not critical to the invention, and it is within the scope of the invention for the means for imparting motion to the rocker arm  200  to be implemented without a cam.  
         [0037]    The lash piston  210  may be implemented as a hydraulic tappet having an outer piston  212  and an inner piston  214 . The outer and inner pistons may be biased apart by a spring  216  so that an interior hydraulic chamber  218  is formed. Hydraulic communication with the interior hydraulic chamber  218  may be made through one or more openings  220  and  222  in the walls of the outer and inner pistons  212  and  214 , respectively.  
         [0038]    The lash piston  210  is slidably disposed in a piston bore  224 . An upper hydraulic chamber  226  is formed between the end of the piston bore  224  and the lash piston  210 . The lash piston  210  may be biased into the piston bore  224  by the valve spring associated with the engine valve assembly  500 .  
         [0039]    The rocker arm  200  is pivotally mounted on a rocker arm shaft  300 . The rocker arm shaft  300  is disposed in a central bore  260  formed in the rocker arm  200 . A first hydraulic passage  230  formed in the rocker arm  200  connects the central bore  260  with the upper hydraulic chamber  226 . A second hydraulic passage  232  connects the central bore  260  with a control valve bore  270 . A third hydraulic passage  234  connects the control valve bore  270  with a port  228  in the wall of the piston bore  224 . A fourth hydraulic passage  236  connects the central bore  260  with the third hydraulic passage  234 . The fourth hydraulic passage  236  may be sealed from the atmosphere by a plug  238 . The end of the fourth hydraulic passage  236  that intersects with the central bore  260  may be enlarged to provide an opening into the central bore of a predetermined size. A check valve  240  is disposed in the first hydraulic passage  230  so as to prevent back flow from the upper hydraulic chamber  226  to the central bore  260 . A second check valve  242  is disposed in the fourth hydraulic passage  236  so as to prevent hydraulic flow from the central bore  260  to the third hydraulic passage  234 .  
         [0040]    With reference to FIG. 2, a control valve  272  is slidably disposed within the control valve bore  270 . The control valve comprises a spool  274  biased towards the second hydraulic passage  232  by a spring  276 . The spool  274  includes an internal hydraulic passage and check valve arrangement  278  that enables one way hydraulic flow from the second hydraulic passage  232  through the spool. One or more drain passages  280  may be provided in the end of the control valve bore  270 .  
         [0041]    The rocker arm shaft  300  may include multiple hydraulic passages adapted to provide hydraulic fluid to, and receive hydraulic fluid from, the passages in the rocker arm  200 . A control passage  310  formed in the rocker arm shaft  300  provides hydraulic fluid to the second hydraulic passage  232  and the control valve  272 . Hydraulic fluid may be provided to the control passage  310  under the control of a remotely located solenoid valve (not shown). A relief passage  312  formed in the rocker arm shaft  300  provides for selective relief of hydraulic pressure from the fourth hydraulic passage  236 , the third hydraulic passage  234 , and the tappet  210 . A lash passage  314  formed in the rocker arm shaft  300  provides hydraulic fluid to the first hydraulic passage  230  and the upper hydraulic chamber  226 .  
         [0042]    With continued reference to FIGS. 1 and 2, the engine braking system  100  may be operated preferably with a cam  400  that includes at least a main exhaust lobe and a compression release lobe. During positive power operation of the engine in which the engine braking system  100  resides, low pressure hydraulic fluid in the lash passage  314  of the rocker arm shaft  300  is provided to the first hydraulic passage  230 , past the check valve  240 , and into the upper hydraulic chamber  226 . The low pressure fluid in the upper hydraulic chamber  226  is prevented from escaping from the chamber by the check valve  240 . The low pressure in the upper hydraulic chamber  226  is sufficient to cause the tappet  210  to extend downward as a unit until it contacts the engine valve assembly  500 . The low pressure fluid in the upper hydraulic chamber  226  is not sufficient to open the engine valve assembly  500  against the force of the engine valve spring included therewith, nor is it sufficient to compress the spring  216  separating the inner piston  214  from the outer piston  212  in the tappet  210 . In this manner, any lash space between the tappet  210  and the engine valve assembly  500  is automatically taken up without the need for mechanical adjustment.  
         [0043]    With continued reference to operation during positive power, there is little or no hydraulic pressure provided in the control passage  310  in the rocker arm shaft  300  during positive power. The absence of significant pressure in the control passage  310  results in the continued biasing of the spool  274  into a “brake off” position by the spring  276 , as shown in FIG. 2. When the spool  274  is in a “brake off” position, the hydraulic pressure within the interior hydraulic chamber  218  of the tappet  210  is free to dissipate through the third hydraulic passage  234  and out of the drain passages  280  to the atmosphere.  
         [0044]    The absence of hydraulic fluid pressure in the tappet  210  results in the loss of the relatively small motion imparted to the rocker arm  200  by the compression release lobe of the cam  400  during positive power operation. The loss of pressure in the interior chamber  218  causes the inner piston  214  and the outer piston  212  to collapse and engage each other mechanically via the internal spring  216 . The tappet  210  is dimensioned such that when it is collapsed the tappet is still of a size to transfer the main exhaust motion imparted by the cam  400  to the engine valve assembly  500 . The tappet  210  is not of sufficient size in its collapsed state, however, to deliver the smaller compression release valve motion imparted by the cam  400 . The compression release valve motion is “lost” by the compression of the spring  216  within the interior hydraulic chamber  218 . In order for the compression release motion to be completely lost, the separation of the inner piston  214  from the outer piston  212  provided by the spring  216  must be at least as great as the magnitude of the compression release motion.  
         [0045]    With continued reference to FIGS. 1 and 2, low pressure hydraulic fluid is provided to the control passage  310  in the rocker arm shaft  300  in order to institute engine braking. The low pressure fluid is provided to the control passage  310  under the control of a remote solenoid valve (not shown). Low pressure fluid from the control passage  310  flows through the second hydraulic passage  232  into the control valve bore  270  and displaces the spool  274  against the bias of the spring  276 . Displacement of the spool  274  into a “brake on” position blocks the hydraulic communication between the third hydraulic passage  234  and the drain passage  280 .  
         [0046]    At the same time, displacement of the spool  274  places the third hydraulic passage  234  in hydraulic communication with second hydraulic passage  232 . The low pressure fluid from the second hydraulic passage  232  flows through the internal hydraulic passage and check valve arrangement  278  in the spool  274 , through the third hydraulic passage  234 , and into the interior hydraulic chamber  218  of the tappet  210 . The check valve  278  prevents the back flow of hydraulic fluid from the tappet  210  to the second hydraulic passage  232 . Thus the length of the tappet  210  becomes hydraulically locked when the spool  274  is displaced into the “brake on” position and the cam  400  is at base circle.  
         [0047]    The cam  400  does not remain at base circle for the entire engine cycle. As referenced above, the cam  400  may first impart a relatively small compression release pivoting motion to the rocker arm  200 . This pivoting motion causes the rocker arm  200  to rotate relative to the fixed position of the rocker arm shaft  300 . As the rocker arm rotates, the angular separation of the fourth hydraulic passage  236  and the relief passage  312  decreases. Rotation of the rocker arm  200  for compression release is not sufficient, however, to establish hydraulic communication between the fourth hydraulic passage  236  and the relief passage  312 . The tappet  210  remains hydraulically locked at a fixed length throughout the compression release event, and accordingly, the entire compression release valve motion is transferred by the tappet to the engine valve assembly  500 .  
         [0048]    In addition to the compression release event, the cam  400  may also provide a main exhaust event. The pivoting motion imparted to the rocker arm  200  during the main exhaust event is larger than that for the compression release event. As the rocker arm  200  rotates for the main exhaust event, the angular separation of fourth hydraulic passage  236  and the relief passage  312  again decreases. Rotation of the rocker arm  200  for the main exhaust event, however, is sufficient to establish hydraulic communication between the fourth hydraulic passage  236  and the relief passage  312 . Due to the high pressure on the tappet  210 , the hydraulic communication between the fourth hydraulic passage  236  and the relief passage  312  causes the tappet  210  to collapse. The timing of the pressure release to the relief passage  312  determines whether the collapse of the tappet  210  will result in the engine valve motion being clipped or reset. The release of this pressure prior to the main exhaust event (i.e., at the end of the compression release event) results in a resetting (i.e. engine valve reseating) event; the release of this pressure during the main exhaust event results in a clipping event.  
         [0049]    The hydraulic fluid collected by the relief passage  312  during the clipping or resetting event may be accumulated in an accumulator in the rocker arm shaft  300  or the rocker arm  200 , or vented to atmosphere. Following the clipping or resetting event, the rocker arm  200  pivots in the reverse direction as it returns to the base circle of the cam  400 . When the rocker arm  200  returns to base circle, the tappet  210  may refill with hydraulic fluid through the internal hydraulic passage and check valve arrangement  278  in the control valve  272 .  
         [0050]    The system  100  may be returned to its positive power configuration by actuating (or de-actuating, as the case may be) the remote solenoid to block the supply of low pressure hydraulic fluid to control valve  272 . Hydraulic leakage past the spool  274  and out of the drain passage  280  allows the spool to return to its “brake off” position shown in FIG. 2.  
         [0051]    With reference to FIGS. 3 and 4, in which like reference numerals refer to like elements, the configuration of the system  100  is varied in an alternative embodiment of the invention as follows. An air vent passage  282  is provided between the control valve bore  270  and the atmosphere. Furthermore, the check valve and hydraulic passage arrangement is eliminated from the spool  274 . Hydraulic fluid is supplied to the tappet  210  as the result of leakage past the spool  274  when the control valve  272  is in a “brake on” position, as shown in FIG. 4. In other respects, the system  100  shown in FIGS. 3 and 4 operates in substantially the same way as the system  100  shown in FIGS. 1 and 2.  
         [0052]    With reference to FIGS. 5 and 6, in which like reference numerals refer to like elements, the configuration of the system  100  is varied in another alternative embodiment of the invention as follows The tappet  210  is provided with a check valve  229 . Lash adjustment of the tappet  210  is achieved by the flow of hydraulic fluid past the check valve  229  into the upper hydraulic chamber  226 . The addition of the check valve  229  eliminates the need for a first hydraulic passage and a lash passage (shown in FIG. 1).  
         [0053]    The hydraulic fluid used to accomplish lash adjustment is provided from the lash passage  314  to the fifth hydraulic passage  244 . The fifth hydraulic passage  244  provides hydraulic communication between the central bore  260  and the control valve bore  270 . During positive power operation, the spool  274  permits the flow of hydraulic fluid from the fifth hydraulic passage  244  to the third hydraulic passage  234  for lash adjustment. During engine braking operation, the spool  274  blocks the flow of hydraulic fluid from the fifth hydraulic passage  244 , but permits the flow of hydraulic fluid through the internal hydraulic passage and check valve arrangement  278  for lash adjustment.  
         [0054]    With reference to FIGS. 7 and 8, in which like reference numerals refer to like elements, the configuration of the system  100  is varied from that shown in FIGS. 5 and 6, in another alternative embodiment of the invention as follows. A sixth hydraulic passage  246  in the rocker arm  200  provides selective hydraulic communication between the central bore  260  and the control valve bore  270 . During positive power operation, the control valve  272  blocks the sixth hydraulic passage  246  from communicating with the control valve bore  270 . The hydraulic fluid required for lash adjustment is provided from the fifth hydraulic passage  244  during positive power.  
         [0055]    During engine braking, the spool  274  blocks the fifth hydraulic passage  244 , and places the sixth hydraulic passage  246  in communication with the third hydraulic passage  234 . The hydraulic fluid needed for lash adjustment is supplied through the internal hydraulic passage and check valve arrangement  278 . Rotation of the rocker arm  200  for the main exhaust event results in hydraulic communication between the sixth hydraulic passage  246  and the relief passage  312 .  
         [0056]    With reference to FIGS. 9 and 10, in which like reference numerals refer to like elements, the configuration of the system  100  is varied from that shown in FIGS. 1 and 2 in another embodiment of the invention as follows. The arrangement of the tappet  210  is the same as that shown in FIGS.  5 - 8 . A seventh hydraulic passage  231  is provided between the central bore  260  and the third hydraulic passage  234 . A check valve  241  is provided in the seventh hydraulic passage  231  to prevent the back flow of hydraulic fluid from the third hydraulic passage  234  to the central bore  260 . The seventh hydraulic passage  231  provides hydraulic fluid to the tappet  210  for lash adjustment during positive power and engine braking operation.  
         [0057]    An accumulator bore  284  is provided in the rocker arm  284 . An eighth hydraulic passage  286  provides hydraulic communication between the accumulator bore  284  and the central bore  260 . A ninth hydraulic passage  288  provides hydraulic communication between the accumulator bore  284  and the control valve bore  270 . An accumulator piston  290  is biased by a spring  292  towards the end of the accumulator bore  284  that connects with the eighth and ninth hydraulic passages,  286  and  288 .  
         [0058]    During positive power operation, the spool  274  allows hydraulic communication between the third hydraulic passage  234  and ninth hydraulic passage  288 . The accumulator piston  290  is free to absorb the flow of hydraulic fluid from the tappet  210 , which accordingly, collapses to lose the compression release motion imparted to the rocker arm  200  by the cam  400 . During engine braking operation, the spool  274  is moved into a “brake on” position under the influence of hydraulic fluid from the control passage  310 . The spool  274  blocks the flow of hydraulic fluid between the third hydraulic passage  234  and the ninth hydraulic passage  288 . Release of the hydraulic fluid in the tappet  210  can only occur through the fourth hydraulic passage  236  when the spool  274  is in its “brake on” position. However, the fourth hydraulic passage  236  only communicates with the accumulator piston  290  when the rocker arm  200  pivots during a main exhaust event such that hydraulic communication is established between the fourth hydraulic passage  236  and the lash passage  314 . When this communication is established, the hydraulic pressure in the tappet  210  can be relieved through the fourth hydraulic passage, the lash passage  314 , and the eighth hydraulic passage  286 , into the accumulator bore  284 .  
         [0059]    With reference to FIG. 10, the phantom lines illustrate that excess material  202  may be removed from the rocker arm  200  to reduce its mass.  
         [0060]    With reference to FIGS. 11 and 12, in which like reference numerals refer to like elements, the configuration of the system  100  is varied from that shown in FIGS. 9 and 10 in another embodiment of the invention as follows. The self-adjusting lash piston  210  shown in FIG. 9 is replaced by a solid piston  210 . The lash of the solid piston  210  may be manually adjusted using the screw  204 .  
         [0061]    With reference to FIGS. 13 and 14, in which like reference numerals refer to like elements, the configuration of the system  100  is varied from that shown in FIGS. 11 and 12 in another embodiment of the invention as follows. During engine braking operation, hydraulic fluid communication between the upper hydraulic chamber  226  and the accumulator piston bore  284  is established through the combination of the fourth hydraulic passage  236 , the relief passage  312 , and a tenth hydraulic passage  289 . A check valve  287  is disposed in the eighth hydraulic passage  286  to prevent back flow from the accumulator bore  284  to the lash passage  314 . A check valve  291  is provided in the tenth hydraulic passage  289  to prevent hydraulic back flow directly from the third hydraulic passage  234  to the accumulator bore  284 . During both positive power and engine braking operation, the upper hydraulic chamber  226  is filled with hydraulic fluid from the lash passage  314 .  
         [0062]    With reference to FIGS. 15 and 16, in which like reference numerals refer to like elements, the configuration of the system  100  is varied from that shown in FIGS.  13  and  14  in another embodiment of the invention by elimination of the lash adjustment screw  204 .  
         [0063]    [0063]FIG. 20, in which like reference numerals refer to like elements, is a schematic representation of the system  100  as shown in FIGS.  13 - 16 .  
         [0064]    With reference to FIGS. 17 and 18, in which like reference numerals refer to like elements, the configuration of the system  100  is varied from that shown in FIGS. 11 and 12 in another embodiment of the invention as follows. The control valve  272  is eliminated. Lash adjustment of lash piston  210  is made under the influence of the spring  217  and screw  204 . During positive power operation, the remote solenoid (not shown) blocks the flow of hydraulic fluid in the control passage  310 . Accordingly, during positive power operation, there is no hydraulic pressure in the upper hydraulic chamber  226 .  
         [0065]    During engine braking operation, low pressure hydraulic fluid is provided in the control passage  310 . The low pressure hydraulic fluid fills the upper hydraulic chamber  226  through the seventh hydraulic passage  231  and the third hydraulic passage  234 / 236 . The reverse flow of hydraulic fluid through the seventh hydraulic passage  231  is prevented by the check valve  241 . Reverse flow to the control passage  310  from the third hydraulic passage  234  may occur when the rocker arm  200  pivots sufficiently to place the third hydraulic passage  234 / 236  in hydraulic communication with the control passage  310 . The hydraulic pressure released to the control passage  310  during the main exhaust event is transferred via the eleventh passage  311  to the accumulator bore  284 .  
         [0066]    With reference to FIG. 19, in which like reference numerals refer to like elements, the configuration of the system  100  is varied from that shown in FIGS. 17 and 18 in another embodiment of the invention by the placement of the accumulator remote from the rocker arm  200 . The accumulator may be placed at the end of the rocker arm shaft, in the rocker arm pedestal, in another rocker arm, or in any other remote location.  
         [0067]    With reference to FIGS. 21 and 22, in which like reference numerals refer to like elements, the configuration of the system  100  is varied from that shown in FIGS. 13 and 14 in another embodiment of the invention as follows. The fourth hydraulic passage  236  is eliminated. The tenth hydraulic passage  289  provides hydraulic communication between the ninth hydraulic passage  288  and the control valve bore  270 . The control valve  272  is mounted upright in a distal end of the rocker arm  200 . The bottom of the control valve  272  includes an extension  279  which may be used in conjunction with an external stop  600  to trigger the control valve  272  to provide hydraulic communication between the third hydraulic passage  234  and the ninth hydraulic passage  288 .  
         [0068]    More specifically, the system  100  shown in FIGS.  21 - 22  operates as follows. During positive power operation, no significant hydraulic pressure is provided in the control passage  310 . The absence of significant hydraulic pressure in the control passage  310  permits the spring  276  to bias the spool  274  upward into a position that provides hydraulic communication between the upper hydraulic chamber  226  and the ninth hydraulic passage  288 , which in turn communicates with the accumulator piston  290 . Hydraulic communication between the upper hydraulic chamber  226  and the accumulator piston  290  permits the lash piston  210  to translate upward in its bore  224  when the rocker arm  200  rotates downward toward a valve stem (not shown).  
         [0069]    The upward motion of the lash piston  210  forces hydraulic fluid in the upper chamber  226  and the ninth passage  288  to be absorbed by the accumulator piston  290 . The lash piston  210  may translate upward until it seats against the upper end of the bore  224  or until it cuts off hydraulic communication with the third hydraulic passage  234 . The point at which the lash piston  210  stops its upward movement may be designed to result in the absorption of the all the motion provided to the rocker arm  200  by the engine braking cam lobe. As a result, the lash piston  210  may provide only the main exhaust event associated with the main exhaust cam lobe when there is no hydraulic pressure in the control passage  310 .  
         [0070]    With continued reference to FIGS. 21 and 22, hydraulic pressure is supplied to the control passage  310  to institute engine braking operation. The presence of hydraulic pressure in the control passage  310  causes the spool  274  to translate downward against the bias of the spring  276 . In this position, the spool  274  cuts off communication between the upper hydraulic chamber  226  and the ninth passage  288 , and provides hydraulic communication between the upper hydraulic chamber and the tenth hydraulic passage  289 . The flow of hydraulic fluid out of the upper hydraulic chamber  226 , however, is blocked by the check valve  291  during the initial downward movement of the rocker arm  200  under the influence of the engine braking cam lobe. As a result, the engine braking valve event is transmitted by the rocker arm  200  to the engine valve (not shown).  
         [0071]    As the rocker arm  200  continues to move downward under the influence of the main exhaust cam lobe, the spool extension  279  may contact the external stop  600 . This contact forces the spool  274  upward until hydraulic communication is reestablished between the upper hydraulic chamber  226  and the accumulator  290  through the ninth hydraulic passage  288 . This hydraulic communication allows the upper hydraulic chamber  226  to vent and the lash piston  210  to collapse upward into its bore  224 . As a result the motion of the engine valve during the main exhaust event may be reset or clipped, depending upon the point at which the upper hydraulic chamber  226  is vented. The movement of the spool  274  to reset or clip the engine valve motion may be repeated with each revolution of the cam during engine braking operation.  
         [0072]    [0072]FIG. 23, in which like reference numerals refer to like elements, is a schematic representation of the system  100  as shown in FIGS.  21 - 22 .  
         [0073]    With reference to FIGS. 24 and 25, in which like reference numerals refer to like elements, the configuration of system  100  is varied in yet another embodiment of the invention as follows. The rocker arm shaft  300  pivotally supports an exhaust rocker arm  200  and an intake rocker arm  750 . The exhaust rocker arm  200  is driven by an exhaust/compression release cam  400 , which includes a main exhaust lobe  410 . The intake rocker arm is driven by an intake cam  700 , which includes a main intake lobe  710 .  
         [0074]    A follower arm  800  is disposed on the rocker arm shaft  300  between the intake rocker  750  and the exhaust rocker  200 . The follower arm  800  includes a sleeve  850  that extends laterally from the follower arm between the exhaust rocker  200  and the rocker arm shaft  300 . The sleeve  850  may form a pivotal seal between the rocker arm shaft  300  and the central bore  260  in the rocker arm  200 . The intake cam  700  is slightly wider than normal in order to drive the follower arm  800 .  
         [0075]    The exhaust rocker  200  includes one or more hydraulic passages (as shown in FIGS.  1 - 23 ) that provide hydraulic communication between the lash piston  210  and the central bore  260 . Opening  298  is provided at the intersection of the central bore  260  and the hydraulic passage(s) connecting the central bore with the lash piston  210 . A relief passage  312  is provided in the rocker arm shaft  300 . Sleeve  850  includes a window  852  that provides selective communication between the relief passage  312  and the opening  298 . Alignment of the window  852  with the relief passage  312  and the opening  298  may occur when the follower arm  800  is pivoted by the intake cam  700 . The length and orientation of the follower arm  800  may be selected to produce alignment of the window  852  with the relief passage  312  and the opening  298  at the point in the engine cycle at which clipping or resetting of the lash piston  210  is desired. Furthermore, the selection of the size and shape of the window  852 , the relief passage  312 , and the opening  298  may be used to control the clipping or resetting event.  
         [0076]    As illustrated in the embodiments of the invention shown in FIGS.  1 - 23 , the embodiment of the invention shown in FIGS.  24 - 25  may include an accumulator to receive the hydraulic fluid released from the lash piston  210  during the clipping/resetting event. The accumulator may be provided in the exhaust rocker arm  200 , or at a remote location such as the end of the rocker arm shaft  300 . Furthermore, the exhaust rocker arm  200  may also include a control valve, such as those shown in FIGS.  1 - 23 , to place the exhaust rocker arm in a “brake on” mode in the same manner as described for the other embodiments of the invention.  
         [0077]    With reference to FIGS. 26, 26A, and  27 , in which like reference numerals refer to like elements, the configuration of the system  100  is varied from that shown in FIGS. 24 and 25 in another embodiment of the invention as follows. In addition to the exhaust rocker arm  200  and the intake rocker arm  750 , the rocker arm shaft  300  pivotally supports an injector rocker arm  950  between the exhaust and intake rocker arms. The injector rocker arm  950  is driven by an injector cam  900  which includes one or more lobes synchronized to produce a fuel injection event in the engine cylinder serviced by the exhaust, intake, and injector rocker arms. The system  100  shown in FIGS.  26 - 27  differs from that shown in FIGS.  24 - 25  primarily by the substitution of the injector rocker arm  950  in the system shown in the later figures for the follower arm  800  shown in the former figures. The variations possible with the system  100  shown in FIGS.  26 - 27  are comparable to those possible with the system shown in FIGS.  1 - 25 .  
         [0078]    With reference to FIGS. 28 and 29, in which like reference numerals refer to like elements, the configuration of the system  100  is varied from that shown in FIGS. 24 and 25 in another embodiment of the invention as follows. The follower arm  800  is driven by a dedicated follower cam  860  which includes one or more lobes synchronized to produce alignment of the window  852  with the relief passage  312  and the opening  298  at the point in the engine cycle at which clipping or resetting of the lash piston  210  is desired. The system  100  shown in FIGS.  28 - 29  differs from that shown in FIGS.  24 - 25  primarily by the substitution of the dedicated follower cam  860  in the system shown in the later figures for the intake cam  700  shown in the former figures. The variations possible with the system  100  shown in FIGS.  28 - 29  are comparable to those possible with the system shown in FIGS.  1 - 27 .  
         [0079]    With reference to FIGS. 30 and 31, in which like reference numerals refer to like elements, the configuration of the system  100  is varied from that shown in FIGS. 24 and 25 in another embodiment of the invention as follows. The follower arm  800  includes an extension  810 , so that it is L-shaped. The exhaust rocker arm  200  includes a clip/reset actuator  299 . The length and shape of the follower arm  800  may be selected to produce contact between the extension  810  and the actuator  299  at the point in the engine cycle at which clipping or resetting of the lash piston  210  is desired. This contact triggers the release of hydraulic fluid from the lash piston  210 .  
         [0080]    In a variation of the system  100  shown in FIGS. 30 and 31, the length and shape of the follower arm  800  may be selected to remove contact between the extension  810  and the actuator  299  at the point in the engine cycle at which clipping or resetting of the lash piston  210  is desired. This removal of contact triggers the release of hydraulic fluid from the lash piston  210 . The variations possible with the system  100  shown in FIGS.  30 - 31  are comparable to those possible with the system shown in FIGS.  1 - 29 .  
         [0081]    With reference to FIGS. 32 and 33, in which like reference numerals refer to like elements, the configuration of the system  100  is varied from that shown in FIGS. 30 and 31 in another embodiment of the invention as follows. In addition to the exhaust rocker arm  200  and the intake rocker arm  750 , the rocker arm shaft  300  pivotally supports an injector rocker arm  950  between the exhaust and intake rocker arms. The injector rocker arm  950  is driven by an injector cam  900  which includes one or more lobes synchronized to produce a fuel injection event in the engine cylinder serviced by the exhaust, intake, and injector rocker arms. The system  100  shown in FIGS.  32 - 33  differs from that shown in FIGS.  30 - 31  primarily by the substitution of the injector rocker arm  950  in the system shown in the later figures for the follower arm  800  shown in the former figures. The contact (or removal of contact) used to trigger the clip or reset event occurs between the injector rocker arm  950  and the actuator  299 , rather than between a follower arm and the actuator.  
         [0082]    With reference to FIGS. 34 and 35, in which like reference numerals refer to like elements, the configuration of system  100  is varied in still another embodiment of the invention as follows. The rocker arm shaft  300  pivotally supports an exhaust rocker arm  200  and an intake rocker arm  750 . The exhaust rocker arm  200  is driven by an exhaust/compression release cam  400 , which includes a main exhaust lobe  410 . The intake rocker arm is driven by an intake cam  700 , which includes a main intake lobe  710 .  
         [0083]    A follower arm  800  is disposed on the rocker arm shaft  300  between the intake rocker  750  and the exhaust rocker  200 . The follower arm  800  includes a ring  854  that forms a pivotal seal between the exhaust rocker arm  200  and the intake rocker arm  750 . The follower arm  800  may be driven by the intake rocker cam  700 .  
         [0084]    The exhaust rocker  200  includes one or more hydraulic passages  234  that provide hydraulic communication between the lash piston  210  and the side of the exhaust rocker arm  200  that is sealed against the ring  854 . Opening  298  is provided in the exhaust rocker arm  200  at the intersection of the side of the exhaust rocker arm and the ring  854 . Ring  854  includes a window passage  852  offset from the opening  298  such that the window passage and the opening are selectively placed in hydraulic communication. Alignment of the window passage  852  with the opening  298  may occur when the follower arm  800  is pivoted by the intake cam  700  in one direction and the exhaust rocker arm  200  is pivoted by the exhaust cam  400  in the opposite direction. Alignment of the window passage  852  and the opening  298  allows the hydraulic fluid in the lash piston  210  to vent to atmosphere or a remotely located accumulator. The length and orientation of the follower arm  800 , as well as the size and shape of the window passage  852  and the opening  298 , may be selected to produce alignment of the window  852  with the opening  298  at the point in the engine cycle at which clipping or resetting of the lash piston  210  is desired.  
         [0085]    With reference to FIGS. 36 and 37, in which like reference numerals refer to like elements, the configuration of the system  100  is varied from that shown in FIGS. 34 and 35 in another embodiment of the invention as follows. The follower arm  800  is eliminated. A window passage  752  is provided in the intake rocker arm  750  (or alternatively in an injector rocker arm). The exhaust rocker arm  200  and the intake rocker arm  750  each include a boss that forms a pivotal seal with the boss on the other rocker arm. Alignment of the window passage  752  with the opening  298  may occur when the intake rocker arm  750  is pivoted by the intake cam  700  in one direction and the exhaust rocker arm  200  is pivoted by the exhaust cam  400  in the opposite direction. Alignment of the window passage  752  and the opening  298  allows the hydraulic fluid in the lash piston  210  to vent to atmosphere or a remotely located accumulator.  
         [0086]    It will be apparent to those skilled in the art that variations and modifications of the present invention can be made without departing from the scope or spirit of the invention. For example, the lash pistons, tappets, rocker arms, rocker arm shafts, and hydraulic passages therein, contemplated as being within the scope of the invention include those of any shape or size so long as the elements in combination provide the functions described in the specification. Furthermore, it is contemplated that the scope of the invention extends to variations of the hydraulic passages shown in the drawing figures, and that it should be appreciated that each passage may have an enlarged end opening as may be needed to perform the described functions of the passage. It is further contemplated that any hydraulic fluid may be used in a system configured in accordance with the invention. It is still further contemplated that the various embodiments of the invention may be used in either a unitary cam engine braking arrangement or a dedicated cam engine braking arrangement. Furthermore, each embodiment of the invention may be varied to include or not include, as desirable, a control valve and/or an accumulator piston, located in the rocker arms described, or remotely. The control valves that utilize a spool and a check valve incorporated therein, may be provided as a separate spool and check valve. It is also contemplated and understood that all of the embodiments of the invention may be used outside of the engine braking field. For example, the system may be used for internal EGR. 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.