Patent Publication Number: US-7712449-B1

Title: Lost motion variable valve actuation system for engine braking and early exhaust opening

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a system for actuating one or more engine valves in an internal combustion engine. In particular, the present invention relates to a lost motion system for providing variable valve actuation (VVA) for engine braking and early exhaust opening (EEO). 
     BACKGROUND OF THE INVENTION 
     Internal combustion engines typically use either a mechanical, electrical or hydro-mechanical valve actuation system to actuate the engine valves. These systems may include a combination of camshafts, rocker arms and push rods that are driven by the engine&#39;s crankshaft rotation. When a camshaft is used to actuate the engine valves, the timing of the valve actuation may be fixed by the size and location of the lobes on the camshaft. 
     For each 360 degree rotation of the camshaft, the engine completes a full cycle made up of four strokes, i.e., expansion, exhaust, intake, and compression. Both the intake and exhaust valves may be closed, and remain closed, during most of the expansion stroke, when the piston is traveling away from the cylinder head and the volume between the cylinder head and the piston head is increasing. During positive power operation, fuel is burned during the expansion stroke and positive power is delivered by the engine. The expansion stroke ends at the bottom dead center (BDC) point, at which time the piston reverses direction. The exhaust valve may be opened for a main exhaust event prior to BDC. A lobe on the camshaft may be synchronized to open the exhaust valve for the main exhaust event as the piston travels upward and forces combustion gases out of the cylinder. Near the end of the exhaust stroke, another lobe on the camshaft may open the intake valve for the main intake event at which time the piston travels away from the cylinder head. The intake valve closes and the intake stroke ends when the piston is near bottom dead center. Both the intake and exhaust valves are closed as the piston again travels upward for the compression stroke. 
     The main intake and main exhaust valve events are required for positive power operation of an internal combustion engine. Additional auxiliary valve events, while not required, may be desirable. For example, it may be desirable to actuate the intake and/or exhaust valves during positive power or other engine operation modes for compression-release engine braking, bleeder engine braking, exhaust gas recirculation (EGR), brake gas recirculation (BGR), or other auxiliary intake and/or exhaust valve events. 
     With respect to auxiliary valve events, flow control of exhaust gas through an internal combustion engine has been used in order to provide vehicle engine braking. One or more exhaust valves may also be selectively opened to convert, at least temporarily, the engine into an air compressor for engine braking operation. This air compressor effect may be accomplished by either opening one or more exhaust valves near piston top dead center position for compression-release type braking, or by maintaining one or more exhaust valves in a relatively constant cracked open position during much or all of the piston motion, for bleeder type braking. In both types of braking, the engine may develop a retarding force that may be used to help slow a vehicle down. This braking force may provide the operator with increased control over the vehicle, and may also substantially reduce the wear on the service brakes. Compression-release type engine braking has been long known and is disclosed in Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is hereby incorporated by reference. 
     Generally, engine braking systems may control the flow of exhaust gas to incorporate the principles of compression-release braking, bleeder braking, exhaust gas recirculation, and/or brake gas recirculation. During compression-release engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, a power producing internal combustion engine into a power absorbing air compressor. As a piston travels upward during its compression stroke, the gases that are trapped in the cylinder may be compressed. The compressed gases may oppose the upward motion of the piston. As the piston approaches the top dead center position, at least one exhaust valve may be opened to release the compressed gases in the cylinder to the exhaust manifold, preventing the energy stored in the compressed gases from being returned to the engine on the subsequent expansion down-stroke. In doing so, the engine may develop retarding power to help slow the vehicle down. 
     During bleeder engine braking, in addition to, and/or in place of, the main exhaust valve event, which occurs during the exhaust stroke of the piston, the exhaust valve(s) may be held slightly open during the remaining three engine cycles (full-cycle bleeder brake) or during a portion of the remaining three engine cycles (partial-cycle bleeder brake). The bleeding of cylinder gases in and out of the cylinder may act to retard the engine. Usually, the initial opening of the braking valve(s) in a bleeder braking operation is in advance of the compression top dead center, i.e., early valve actuation, and then lift is held constant for a period of time. As such, a bleeder type engine brake may require lower force to actuate the valve(s) due to early valve actuation, and generate less noise due to continuous bleeding instead of the rapid blow-down of a compression-release type brake. 
     Exhaust gas recirculation (EGR) systems may allow a portion of the exhaust gases to flow back into the engine cylinder during positive power operation. EGR may be used to reduce the amount of NO created by the engine during positive power operations. An EGR system can also be used to control the pressure and temperature in the exhaust manifold and engine cylinder during engine braking cycles. Generally, there are two types of EGR systems, internal and external. External EGR systems recirculate exhaust gases back into the engine cylinder through an intake valve(s). Internal EGR systems recirculate exhaust gases back into the engine cylinder through an exhaust valve(s) and/or an intake valve(s). 
     Brake gas recirculation (BGR) systems may allow a portion of the exhaust gases to flow back into the engine cylinder during engine braking operation. Recirculation of exhaust gases back into the engine cylinder during the intake stroke, for example, may increase the mass of gases in the cylinder that are available for compression-release braking. As a result, BGR may increase the braking effect realized from the braking event. 
     Many different actuation systems may be used to selectively actuate engine valves to produce brake gas recirculation and compression-release events. One known type of actuation system is a lost motion system, described in the above-referenced Cummins patent. Another example of a lost motion system for variable valve actuation is disclosed in Vanderpoel, et al., U.S. Pat. No. 7,152,576 (Dec. 26, 2006), which is hereby incorporated by reference. An example of a system with primary and offset actuator rocker arms for engine valve actuation is disclosed in Janak, et al., U.S. Pub. No. 2006/0005796 (Jan. 12, 2006), which is hereby incorporated by reference. 
     In many internal combustion engines, the intake and exhaust valves may be actuated by fixed profile cams, and more specifically, by one or more fixed lobes or bumps that are an integral part of each cam. The cams may include a lobe for each valve event that the cam is responsible for providing. The size and shape of the lobes on the cam may dictate the valve lift and duration which result from the lobe. For example, an exhaust cam profile for a system may include a lobe for a brake gas recirculation event, a lobe for a compression-release event, and a lobe for a main exhaust event. 
     It may also be desirable to increase the exhaust back pressure in the exhaust manifold during engine braking. Higher exhaust back pressure may increase gas mass and pressure in the engine cylinder available for engine braking, and thereby increase braking power. Increased exhaust back pressure, however, may undesirably increase the force required to open the exhaust valve for a compression-release event because the opening force applied to the exhaust valve must exceed the increased pressure in the engine cylinder resulting from the increased exhaust back pressure. To some extent the increased exhaust back pressure may also increase the pressure applied to the back of the exhaust valve, which may counter-balance the increased pressure in the cylinder and thus reduce the loading on the exhaust valve opening mechanism used for the compression-release event. 
     Increasing the pressure of gases in the exhaust manifold may be accomplished by restricting the flow of gases through the exhaust manifold. Exhaust manifold restriction may be accomplished through the use of any structure that may, upon actuation, restrict all or partially all of the flow of exhaust gases through the exhaust manifold. The exhaust restrictor may be in the form of an exhaust engine brake, a turbocharger, a variable geometry turbocharger, a variable geometry turbocharger with a variable nozzle turbine, and/or any other device which may limit the flow of exhaust gases. 
     Exhaust brakes generally provide restriction by closing off all or part of the exhaust manifold or pipe, thereby preventing the exhaust gases from escaping. This restriction of the exhaust gases may provide a braking effect on the engine by providing a back pressure when each cylinder is on the exhaust stroke. For example, Meneely, U.S. Pat. No. 4,848,289 (Jul. 18, 1989); Schaefer, U.S. Pat. No. 6,109,027 (Aug. 29, 2000); Israel, U.S. Pat. No. 6,170,474 (Jan. 9, 2001); Kinerson et al., U.S. Pat. No. 6,179,096 (Jan. 30, 2001); and Anderson et al., U.S. Pat. Appl. Pub. No. US 2003/0019470 (Jan. 30, 2003) disclose exhaust brakes for use in retarding engines. 
     Turbochargers may similarly restrict exhaust gas flow from the exhaust manifold. Turbochargers often use the flow of high pressure exhaust gases from the exhaust manifold to power a turbine. A variable geometry turbocharger (VGT) may alter the amount of the high pressure exhaust gases that it captures in order to drive a turbine. For example, Arnold et al., U.S. Pat. No. 6,269,642 (Aug. 7, 2001) discloses a variable geometry turbocharger where the amount of exhaust gas restricted is varied by modifying the angle and the length of the vanes in a turbine. An example of the use of a variable geometry turbocharger in connection with engine braking is disclosed in Faletti et al., U.S. Pat. No. 5,813,231 (Sep. 29, 1998), Faletti et al., U.S. Pat. No. 6,148,793 (Nov. 21, 2000), and Ruggiero et al., U.S. Pat. No. 6,866,017 (Mar. 15, 2005), which are hereby incorporated by reference. 
     Over the years there have been improvements to lost motion systems for engine braking and there continues to be a need for improvements as technology evolves and new problems are discovered. Improvements are needed for many reasons, including providing a mechanically-driven exhaust main event for cold start and failsafe modes, meeting loading limits, (e.g., cam Hertz stress), avoiding separation and impact loading between cams and rollers, avoiding bridge tilt, meeting exhaust valve seating velocity limits, and protecting against valve-piston contact. There is a risk of valve-piston contact in many electronically-controlled variable valve actuation (VVA) systems. For example, lost motion VVA systems that provide early valve opening and spill oil near peak lifts have an increased risk of valve piston contact if the spill does not function, which may occur, for example, due to a clogged spill port or a broken valve spring. The valve/cam lift ratio of a rocker-actuated VVA system is more limited by the valve-train layout than that of a master-slave system, where the valve/cam lift ratio is governed by hydraulic piston diameters. 
     SUMMARY OF THE INVENTION 
     Responsive to the foregoing challenges, Applicant has developed an innovative system for actuating an internal combustion engine exhaust valve to provide compression release actuation during an engine braking mode of engine operation and early exhaust valve opening actuation during a positive power mode of engine operation, said system comprising: a first cam having a compression release lobe, an early exhaust valve opening lobe, and optionally a BGR lobe; a hydraulic lost motion system operatively contacting said first cam, said hydraulic lost motion system including a first rocker arm; a hydraulically actuated piston extending from said hydraulic lost motion system, said hydraulically actuated piston adapted to provide said exhaust valve with compression release actuation during an engine braking mode of engine operation and early exhaust valve opening actuation during a positive power mode of operation; a second cam having a main exhaust lobe; and a main exhaust rocker arm operatively contacting said second cam and adapted to provide a main exhaust actuation to said exhaust valve. 
     Applicant has further developed an innovative system for actuating an internal combustion engine exhaust valve comprising: a first means for imparting motion for a compression release engine braking actuation optionally including BGR actuation, and an early exhaust valve opening actuation; a hydraulic lost motion system operatively contacting said first means for imparting motion, said hydraulic lost motion system including a first rocker arm; a hydraulically actuated piston extending from said hydraulic lost motion system, said hydraulically actuated piston adapted to selectively provide said exhaust valve with compression release engine braking actuation and early exhaust valve opening actuation; a second means for imparting motion for a main exhaust actuation; a main exhaust rocker arm operatively contacting said second means for imparting motion; and means for controlling said hydraulic lost motion system to selectively provide the compression release engine braking actuation and the early exhaust valve opening actuation. 
     Applicant has further developed an innovative method of actuating an internal combustion engine exhaust valve to selectively provide compression release engine braking actuation and early exhaust valve opening actuation using a cam with a compression release engine braking lobe and a early exhaust valve opening lobe, and with optional BGR actuation, said method comprising: imparting compression release engine braking actuation motion and early exhaust valve opening actuation motion from said cam to a hydraulic lost motion system including a first rocker arm; determining whether the internal combustion engine is in an engine braking mode of operation; selectively hydraulically locking and unlocking a hydraulically actuated piston in said hydraulic lost motion system to provide said exhaust valve with compression release engine braking actuation when the internal combustion engine is in the engine braking mode of operation; determining whether the internal combustion engine is in a positive power mode of operation and early exhaust valve opening is desired; and selectively hydraulically locking and unlocking the hydraulically actuated piston in said hydraulic lost motion system to provide said exhaust valve with early exhaust valve opening actuation when the internal combustion engine is in the positive power mode of operation and early exhaust valve opening is desired. 
     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 
       In order to assist the understanding of this invention, reference will now be made to the appended drawings, in which like reference numerals refer to like elements. The drawings are exemplary only, and should not be construed as limiting the invention. 
         FIG. 1  is a schematic block diagram illustrating a valve actuation system in accordance with an embodiment of the present invention. 
         FIG. 2  is a side view of a motion imparting means or cam in accordance with an exemplary embodiment of the present invention. 
         FIG. 3  is a schematic top view in partial cross-section of a variable valve actuation system including two half rocker arms and a lost motion system housing in accordance with an embodiment of the present invention. 
         FIG. 4  is a schematic side view in partial cross-section of a first of the rocker arms and the lost motion system housing shown in  FIG. 3 . 
         FIG. 5  is a schematic side view in partial cross-section of a second of the rocker arms and a portion of the lost motion system housing shown in  FIG. 3 . 
         FIG. 6  is a schematic top view in partial cross-section of a variable valve actuation system including a half rocker arm and a full rocker arm and a lost motion system housing in accordance with an alternative embodiment of the present invention. 
         FIG. 7  is a schematic front view in partial cross-section of a of the variable valve actuation system of  FIG. 6  that illustrates contact by the full rocker arm and a forked slave piston with a valve bridge. 
         FIG. 8  is a schematic top view in partial cross-section of a half rocker arm which includes a flange for contact with either a slave piston or actuator piston in accordance with an alternative embodiment of the present invention. 
         FIG. 9  is a schematic front view in partial cross-section of the half rocker arm shown in  FIG. 8  with a slave piston positioned above the half rocker arm flange. 
         FIG. 10  is a schematic front view in partial cross-section of the half rocker arm shown in  FIG. 8  with an actuator piston positioned above the half rocker arm flange. 
         FIG. 11  is a schematic side view in partial cross-section of a rocker arm that includes an actuator piston such as shown in  FIG. 10  which may be used in conjunction with the half rocker arm shown in  FIG. 8   
         FIG. 12  is a schematic top view in partial cross-section of a shuttle valve which may be used in the rocker arm shown in  FIG. 11 . 
         FIG. 13  illustrates the cam lift profiles of a main exhaust cam and an auxiliary cam with a compression-release/EEO lobe and a BGR lobe in accordance with an embodiment of the present invention. 
         FIG. 14  illustrates the compression-release, main exhaust, and BGR exhaust valve lifts that may be obtained using the cam lobe profiles of  FIG. 13  with a lost motion system and specified trigger valve operation in accordance with an embodiment of the present invention. 
         FIG. 15  illustrates the early exhaust valve opening and the main exhaust valve lifts that may be obtained using the cam lobe profiles of  FIG. 13  with a lost motion system and a second specified trigger valve operation in accordance with an embodiment of the present invention. 
         FIG. 16  illustrates the early exhaust valve opening and the main exhaust valve lifts that may be obtained using the cam lobe profiles of  FIG. 13  with a lost motion system and a third specified trigger valve operation in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Embodiments of the present invention may be used to provide variable valve actuation for compression-release engine braking and brake gas recirculation during an engine braking mode of engine operation, as well as early exhaust valve opening during a positive power mode of engine operation. 
       FIG. 1  is a block diagram that illustrates a valve actuation system  100  in accordance with a first embodiment of the present invention. The valve actuation system  100  may include a first motion imparting means  102 , such as a cam with one or more lobes or bumps, operatively contacting a lost motion system  104 , which in turn may be operatively connected to one or more engine valves  106 . The valve actuation system may further include a second motion imparting means  103  operatively connected to the one or more engine valves  106 . It is appreciated that any number of valve train elements, such as push tubes, rocker arms, and or valve bridges may be provided between or as part of the first and second motion imparting means  102  and  103 , the lost motion system  104 , and the engine valves  106 , without departing from the intended scope of the present invention. The first motion imparting means  102  may preferably include both a early exhaust valve opening lobe or bump, a compression release lobe or bump, and optionally a BGR lobe or bump, which may provide input motion to the lost motion system  104 . The motion selectively transferred to the engine valves  106  by the lost motion system  104  may be used to produce various valve actuation events, such as, but not limited to, a compression release braking event, a bleeder braking event, an exhaust gas recirculation event, a brake gas recirculation event, an early exhaust valve opening event, an early intake valve closing event, and/or a centered lift valve event. Preferably, the lost motion system  104  may be switched on or off to selectively transfer either no motion, compression release motion or early exhaust valve opening motion to the engine valves  106  by a controller  108 . The second motion imparting means  103  may also provide valve actuation events, such as a main exhaust event. The engine valves  106  may be exhaust valves, intake valves, or auxiliary valves. The first motion imparting means  102  and the second motion imparting means  103  may actuate the engine valves  106  independently of each other. 
     The first motion imparting means  102  may comprise any combination of cams, push tubes, and/or rocker arms or their equivalents. The lost motion system  104  may comprise any structure that connects the motion imparting means  102  to the engine valves  106  and selectively transfers motion from the motion imparting means  102  to the engine valves  106 . In one sense, the lost motion system  104  may be any structure capable of selectively attaining more than one fixed length. For example, the lost motion system  104  may comprise a mechanical linkage, a hydraulic circuit, a hydro-mechanical linkage, an electromechanical linkage and/or any other linkage adapted to connect to the motion imparting means  102  to the engine valves  106  and attain more than one operative length. When the lost motion system  104  incorporates a hydraulic circuit, the lost motion system  104  may include pressure-adjusting means to adjust the pressure or amount of fluid in a circuit, such as, for example, trigger valves, check valves, accumulator, and/or other devices for releasing hydraulic fluid from or adding hydraulic fluid to the circuit. The lost motion system  104  may be located at any point in the valve train connecting the motion imparting means  102  with the engine valves  106 . 
     The controller  108  may comprise any electronic, mechanical, or hydraulic device for communicating with and controlling the lost motion system  104 . The controller  108  may include a microprocessor, which is linked to other engine components, to determine and select the appropriate instantaneous length of the lost motion system  104 . Valve actuation may be optimized at a plurality of engine speeds and conditions by controlling the instantaneous length of the lost motion system  104  based upon information collected by the microprocessor from engine components. Preferably, the controller  108  may be adapted to operate the lost motion system  104  at high speed (i.e., one or more times per engine cycle) using a high speed hydraulic trigger valve. 
       FIG. 2  is a schematic diagram of a cam  200  which may serve as all or part of the motion imparting means  102 . The motion imparting means  102  of  FIG. 1  may comprise the cam  200 , a rocker arm, and/or a push tube. The cam  200  may include one or more lobes corresponding to various valve actuation events, such as a compression release lobe  202 , an early exhaust valve opening lobe  204 , and a brake gas recirculation lobe  208 . The depictions (e.g., number, size, shape, location) of the lobes on the cam  200  are intended to be illustrative only and not limiting. Several embodiments of the invention contemplate using separate cams for the main valve opening event (i.e., main exhaust or main intake) and the auxiliary valve opening events such as compression release, early valve opening, and/or brake gas recirculation. 
       FIG. 3  is a schematic top view in partial cross section of variable valve actuation system  700  in accordance with a second embodiment of the present invention. The variable valve actuation system  700  may include first and second valve train assemblies extending between the first and second motion imparting means,  102  and  103 , respectively, and the exhaust valves  716 . The first valve train assembly may include a first half (or main exhaust) rocker arm  701  pivotally mounted on a rocker shaft  718 , and a second half rocker arm  702  pivotally mounted on the rocker shaft directly behind the first half rocker arm which together comprise an articulated rocker arm. The first half rocker arm  701  may include an elephant foot or other valve bridge contacting portion  712  adapted to apply a valve actuation motion to the engine valves, preferably exhaust valves,  716  through a valve bridge  714 . 
     The second half rocker arm  702  may include a valve end portion  703  adapted to apply a pivoting motion to the first half rocker arm  701  so as to actuate the exhaust valves  716 . The second half rocker arm  702  may be biased towards the second motion imparting means  103  by a spring  705 , which may create such bias force by pushing against a flange or contact surface  760  provided on the second half rocker arm from a fixed stop or flange  762  provided on a fixed engine part so that a cam roller  708  provided with the second half rocker arm remains in relatively constant contact with the second motion imparting means  103 . 
     The second valve train assembly of the variable valve actuation system  700  may further include a third half rocker arm  704  pivotally mounted on the rocker shaft  718  adjacent to the first and second half rocker arms  701  and  702 . The third half rocker arm  704  may be biased by a second spring  707  through a master piston  730  and a rod  711  that acts on a contact surface  713  provided on the third half rocker arm so that a second cam roller  710  provided with the third half rocker arm remains in relatively constant contact with the first motion imparting means  102 . The rod  711  may include a contact surface to act on a master piston  730  which is slidably disposed in a master piston bore  732  provided in a lost motion system housing  706 . Hydraulic fluid may be provided to the master piston bore  732 . The lost motion system housing  706  may be fixed by bolts or other connection means to the internal combustion engine that includes the exhaust valves  716 . The master piston bore  732  may be connected to a high-speed trigger valve  736  and optionally to an accumulator  722 , and a slave piston  720  by a hydraulic fluid circuit or passages  734 . 
     The interaction of the third half rocker arm  704  and the lost motion system  706  are illustrated in  FIG. 4 . As shown in  FIG. 4 , a control valve or high-speed trigger valve  736  may be provided in the hydraulic circuit  734  such that it may control the supply of hydraulic fluid to and from the hydraulic circuit. Further, the accumulator  722  may include an accumulator piston  724  that biases an accumulator piston  726  into an accumulator bore. Similarly, a slave piston spring  802  may bias the slave piston  720  upward into the slave piston bore  728  towards an adjustable lash screw  804 . With continued reference to  FIGS. 3 and 4 , when the trigger valve  736  is maintained closed, hydraulic fluid may be trapped in the hydraulic circuit  734  and prevented from flowing into or out of the accumulator  722 , and conversely, when the trigger valve is maintained open, hydraulic fluid may flow freely out of the hydraulic circuit and into and out of the accumulator  722 . When the first motion imparting means  102  (shown in  FIG. 3 ) causes the third half rocker arm  704  to pivot about the rocker shaft  718 , the contact surface on the third half rocker arm may push the master piston  730  into the master piston bore, which in turn may force the slave piston  720  downward and into contact with the first half rocker arm  701 . In turn the half rocker arm  701  may act through the valve bridge  714  to actuate or open the exhaust valves  716 . Use of a first motion imparting means  102 , such as a cam with a compression release engine braking lobe, a early exhaust valve opening lobe, and/or a brake gas recirculation lobe, coupled with selective operation of the trigger valve  736  may enable selective provision of compression release engine braking and brake gas recirculation during an engine braking mode of engine operation and variable degrees of early exhaust valve opening during a positive power mode of engine operation. 
     The interaction of the first and second half rocker arms  701  and  702  with each other and the lost motion system  706  is illustrated in  FIG. 5 . As shown in  FIG. 5 , the spring  705  may bias the second half rocker arm  702  away from the first half rocker arm  701  and into contact with the second motion imparting means  103  (shown in  FIG. 3 ) by acting on flange  760 . Variations in the angle in which the spring  705  meets the flange  760  are contemplated to be within the scope of the present invention. For example, in some embodiments, the spring  705  may act almost directly downward on the second half rocker arm  702  over the cam roller  708  to save space by reducing the height of the spring-flange arrangement. When the second motion imparting means  103  provides a valve actuation motion, such as a main exhaust event actuation, to the second half rocker arm  702 , the second half rocker arm may, in turn, act on the first half rocker arm  701  to actuate the exhaust valves  716  for a main exhaust event. Because the first half rocker arm  701  is free to pivot away from the second half rocker arm  702 , the lost motion system  706  may also act on the first half rocker arm  701  to provide valve actuation events such as compression release engine braking, brake gas recirculation, and/or early exhaust valve opening independent of the pivoting of the second half rocker arm  702 . 
       FIG. 6  is a schematic top view of a variable valve actuation system  700  in accordance with a third embodiment of the present invention, in which like reference characters refer to like elements. The first valve train assembly of the variable valve actuation system  700  may include a first (or main exhaust) rocker arm  1002  pivotally mounted on a rocker shaft  718 . The first rocker arm  1002  may include a cam roller  708  biased by a spring  705  which acts by pushing from a fixed stop  762  against a contact surface  760  provided on the first rocker arm so that the cam roller is maintained in relatively constant contact with a second motion imparting means  103 , such as a cam provided on a camshaft. The first rocker arm  1002  may include a valve actuation end  1004  adapted to contact and act on a valve bridge  714  which in turn may actuate engine valves such as exhaust valves  716 . 
     The second valve train assembly of the variable valve actuation system  700  shown in  FIG. 6  may further include a third half rocker arm  704  pivotally mounted on the rocker shaft  718  adjacent to the first rocker arm  1002 . The third half rocker arm  704  and the lost motion system  706  may include the same elements as the variable valve actuation system described in connection with  FIGS. 3-5  above, save for the design of the slave piston  720 . 
     The slave piston  720  shown in  FIG. 6  may be designed as illustrated in  FIG. 7 , for example. With reference to  FIG. 7 , the slave piston  720  may include two forks  721  which may extend downward from the slave piston on either side of the valve end  1004  of the first rocker arm  1002  and into contact with the valve bridge  714 . The forked slave piston  720  may apply valve actuation motions for compression release engine braking, early exhaust valve opening, and/or brake gas recirculation, for example, without interfering with and independent of the operation of the first rocker arm  1002 . 
     Further variable valve actuation system embodiments of the present invention are illustrated by  FIGS. 8-12 . With reference to  FIGS. 8 and 10 , in a fourth embodiment of the present invention, the first valve train assembly of the variable valve actuation system  700  may include a first half rocker arm  701  pivotally disposed on a rocker shaft  718 , similar to that described in connection with  FIG. 3 . The first half rocker arm  701  may be acted upon by a second half rocker arm ( 702  in  FIG. 8 ) to provide a main exhaust valve event in the same manner as the system described in connection with  FIG. 3 . The first half rocker arm  701  may include a valve-side end with an elephant foot or other contact surface  712  adapted to provide exhaust valve actuation motion for exhaust valves  716  through a valve bridge  714 . The first half rocker arm  701  may further include a side flange  709  which is adapted to receive exhaust valve actuation motion from a lost motion rocker arm. 
     With continued reference to  FIG. 8  and in connection with the fourth embodiment of the present invention, the second valve train assembly of the variable valve actuation system  700  may include a lost motion rocker arm  900  pivotally mounted on the rocker shaft  718  adjacent to the first half rocker arm  701 . The lost motion rocker arm  900  may include a cam roller  910  adapted to receive exhaust valve actuation motions from a first motion imparting means  102  which may provide valve actuation motions such as those required for a compression release engine braking event, an early exhaust valve opening event, and a brake gas recirculation event. The lost motion rocker arm  900  may be biased toward the first motion imparting means  102  by a spring  972  (shown in  FIG. 11 ) which may create such bias force by pushing against a flange or contact surface  970  provided on the lost motion rocker arm from a fixed stop or flange  974  provided on a fixed engine part so that cam roller  910  may remain in relatively constant contact with the first motion imparting means  102 . A hydraulic actuator piston  960  may be provided in one end of the lost motion rocker arm  900 . The hydraulic actuator piston  960  may be selectively extended to engage a side flange  709  provided on the first half rocker arm  701 . A hydraulic circuit may be provided in the lost motion rocker arm  900  so that hydraulic fluid may be selectively supplied to and drained from the hydraulic actuator piston  960 . The hydraulic circuit may include a first hydraulic passage  770  connecting the hydraulic actuator piston  960  with the high-speed trigger valve  736  located in the adjacent rocker shaft pedestal  719  via hydraulic passages  772  and  774  provided in the rocker shaft  718  and rocker shaft pedestal, respectively. In turn, the trigger valve  736  may be connected to the accumulator  722  via a hydraulic passage  776 . 
     The hydraulic actuator piston  960  may be slidably disposed within a bore in the lost motion rocker arm  900 . The hydraulic actuator piston  960  may be sized to slide within its bore  926  while maintaining a relatively secure hydraulic seal with the wall of its bore. A vertically adjustable lash member or screw  962  (see  FIG. 10 ) may be slidably received within the actuator piston  960 . The stroke of the hydraulic actuator piston  960  may be limited by contact with a stop  964  on lash member  962  to provide travel slightly greater than the maximum valve lift due to the first motion imparting means  102 . 
     With reference to  FIGS. 10 and 11 , in a fifth embodiment of the present invention, the lost motion rocker arm  900  may include a central opening  920  for receipt of the rocker shaft, a first bore  922  for receipt of an accumulator  722 , a second bore  924  for receipt of a control valve  950 , and a third bore  926  for receipt of a hydraulic actuator piston  960  with a stop  964  to limit the maximum stroke of the hydraulic actuator piston. A hydraulic circuit may be provided in the full rocker arm  900 . The hydraulic circuit may include a first passage  930  connecting the central opening  920  with the second bore  924 , a second passage  932  connecting the central opening  920  with the first bore  922 , a third passage  934  connecting the first bore  922  with the second bore  924 , and a fourth passage  936  connecting the second bore  924  with the third bore  926 . As a result of the hydraulic circuit, hydraulic fluid that may be provided to the central opening from one or more hydraulic fluid passages (not shown) in the rocker shaft  718  may be provided to the accumulator  722 , the control valve  950  and the hydraulic actuator piston  960 . 
     The hydraulic actuator piston  960  may be slidably disposed within the third bore  926 . The hydraulic actuator piston  960  may be sized to slide within the third bore  926  while maintaining a relatively secure hydraulic seal with the wall of the third bore. A vertically adjustable lash member or screw  962  may be slidably received within the actuator piston  960  with a stop  964  to limit the maximum stroke of the hydraulic actuator piston. 
     The lost motion rocker arms  900  shown in both  FIGS. 8 and 11  may act on the flange  709  shown in  FIG. 10  to transfer valve actuation motion through the first half rocker arm  701  to one or more engine valves  716 . 
     An example of the control valve described above in connection with  FIG. 11  is illustrated in  FIG. 12 . With reference to  FIG. 12 , the control valve  950  may include a control valve piston  952  and a control valve spring  954  which biases the control valve piston into the second bore  924 . The second bore  924  in which the control valve piston is disposed may be connected with the first, third and fourth passages  930 ,  934  and  936 , respectively. The selective supply of hydraulic fluid to the first passage  930  may cause the control valve piston  952  to shuttle back towards the spring  954  so that an annular recess  956  in the control valve piston places the third passage  934  in hydraulic communication with the fourth passage  936 . Thus, selective supply of hydraulic fluid by a high-speed trigger valve to the first passage  930  can be used to selectively provide hydraulic fluid from the accumulator  722  to the actuator piston  960  and/or to selectively drain hydraulic fluid from the actuator piston  960  to the accumulator  722 . When hydraulic fluid pressure is decreased in the first passage  930 , the control valve piston  952  may be forced towards the first passage by the spring  954  so that the hydraulic fluid communication between the third and fourth passages  934  and  936  is cut off and the actuator piston  960  is hydraulically locked into a fixed position. 
     The interaction of the second half rocker arm  702  and the first half rocker arm  701  is illustrated by reference to  FIGS. 8 ,  10  and  11 . A spring may bias the second half rocker arm  702  away from the first half rocker arm  701  and into contact with the second motion imparting means  103 . When the second motion imparting means  103  provides a valve actuation motion, such as a main exhaust event actuation, to the second half rocker arm  702 , the second half rocker arm may, in turn, act on the first half rocker arm  701  to actuate the exhaust valves  716  for a main exhaust event. Because the first half rocker arm  701  is free to pivot away from the second half rocker arm, the full rocker arm  900  may also act on the first half rocker arm  701  through the flange  709  to provide valve actuation events such as compression release engine braking, brake gas recirculation, and/or early exhaust valve opening independent of the pivoting of the second half rocker arm. 
     The interaction of the full rocker arm  900 , the hydraulic actuator piston  960  and the first half rocker arm  701  is illustrated by reference to  FIGS. 8 ,  10 - 12 . A spring  972  may bias the full rocker arm  900  into contact with the first motion imparting means  102 . As shown in  FIGS. 11 and 12 , a control valve  950 , which may operate under the control of a high-speed solenoid trigger valve (not shown) may be provided in the hydraulic circuit including passages  930 ,  932 ,  934  and  936  such that the control valve may control the supply of hydraulic fluid to and from the actuator piston  960 . Further, the accumulator  722  may include an accumulator piston spring  724  ( FIG. 4 ) that biases an accumulator piston  726  ( FIG. 4 ) into an accumulator bore. When the control valve  950  is maintained closed, hydraulic fluid may be trapped in the third bore  926  so that the actuator piston  960  is prevented from being pushed into the third bore. Conversely, when the control valve  950  is maintained open, hydraulic fluid may flow freely out of the third bore  926  and into and out of the accumulator  722 . When the control valve  950  is maintained open and the first motion imparting means  102  ( FIG. 8 ) causes the full rocker arm  900  to pivot about the rocker shaft  718 , the actuator piston  960  is forced against the flange  709 . Because the control valve  950  is open, however, the actuator piston  960  is forced into the third bore  926  and the hydraulic fluid in the third bore is pushed back in the hydraulic circuit and absorbed by the accumulator  722 . When the control valve  950  is closed, however, and the first motion imparting means  102  ( FIG. 3 ) causes the full rocker arm  900  to pivot about the rocker shaft  718 , the actuator piston  960  is hydraulically locked into position and the motion from the motion imparting means is transferred through the actuator piston to the flange  709  and from the flange to the first half rocker arm  701 . In turn the half rocker arm  701  may act through the valve bridge  714  to actuate or open the exhaust valves  716 . Use of the first motion imparting means  102 , such as a cam with a compression release engine braking lobe, an early exhaust valve opening lobe, and/or a brake gas recirculation lobe, coupled with selective operation of the control valve  950  may enable selective provision of compression release engine braking and brake gas recirculation during an engine braking mode of engine operation and variable degrees of early exhaust valve opening during a positive power mode of engine operation. 
     The sixth embodiment of the present invention consists of a variation of the second embodiment wherein the fixed housing lost motion system  706  described in connection with  FIGS. 3-5  is substituted for the lost motion rocker arm  900  described above. The sixth embodiment is illustrated in  FIG. 9 , in which like reference characters correspond to like elements described in connection with the foregoing embodiments. In the sixth embodiment of the present invention, the slave piston  720  may selectively provide exhaust valve actuation motions, such as those for a compression release engine braking event, an early exhaust valve opening event, and/or a brake gas recirculation event to a side flange  709  extending from the side of the first half rocker arm  701 , as shown in  FIG. 9 . 
       FIG. 13  illustrates a first cam profile  302 ,  304 ,  306  and  308  corresponding to the lobes on the cam  200 , and a second cam profile  300  with a main exhaust lobe. The first cam profile may be used to provide the first motion imparting means with one or more auxiliary exhaust valve actuation motions and the second cam profile may be used to provide the second motion imparting means with a main exhaust valve actuation motion. In a preferred embodiment, the cam  200  may include a compression release lobe  302 , an early exhaust valve opening lobe  304  with a closing ramp  306 , and a brake gas recirculation lobe  308 . The first cam profile in  FIG. 3  may include, starting from the left, the compression release lobe  302  leading into the early exhaust valve opening lobe  304 , followed by a flat cam segment and a closing ramp  306  which meets the cam base circle  307  before the end of the main exhaust cam lobe  300 . The first cam profile may further include a brake gas recirculation (BGR) or exhaust gas recirculation (EGR) lobe  308 . 
       FIGS. 14-16  illustrate the exhaust valve lifts that may be provided using the first and second cam profiles illustrated in  FIG. 13  in combination with the variable valve actuation systems described in connection with FIGS.  1  and  3 - 12  which may selectively transfer motion from the cam  200  to the exhaust valve(s). The trigger valve or control valve operation to provide three different sets of exhaust valve actuations are also illustrated in  FIGS. 14-16 . In  FIGS. 14-16 , a state of “0” indicates that the trigger or control valve is closed and the lost motion master-slave system ( FIGS. 3-7  and  9 ) or the lost motion rocker arm system (FIGS.  8  and  10 - 12 ) is in a state in which valve actuation motion is transferred to the exhaust valves from the first motion imparting means. A state of “1” indicates that the trigger or control valve is open and the lost motion master-slave system or the lost motion rocker arm system is in a state in which the valve actuation motion applied by the first motion imparting means is absorbed. 
     With reference to  FIG. 14 , during an engine braking mode of engine operation and when the trigger or control valve is initially in state “0”, or closed, the lost motion master-slave or rocker arm system may transfer motion from the compression release cam lobe  302  ( FIG. 13 ) to produce a compression release valve event  302 ′. When the compression release valve event is completed, at about (0) crank angle degrees (i.e., top dead center compression) the trigger or control valve may be opened to release hydraulic pressure in the master-slave circuit or actuator piston and close the exhaust valves. Thereafter the exhaust valves may be opened for the main exhaust event  300 ′ by the second motion imparting means. At about (450) crank angle degrees (i.e., after the end of cam segment  306 ) the trigger or control valve may be closed again so that the lost motion master-slave or rocker arm system may transfer motion from the brake gas recirculation lobe  308  ( FIG. 13 ) to produce a brake gas recirculation valve event  308 ′. Refill of the lost motion hydraulic circuit may occur between about (130) and (450) crank angle degrees, depending on whether a master-slave system or a rocker arm system is being used. 
     With reference to  FIG. 15 , during a first positive power mode of engine operation and when the trigger or control valve is initially in state “1”, or open, the lost motion master-slave or rocker arm system may absorb the motion received from the compression release cam lobe  302  ( FIG. 13 ) of the first motion imparting means so that the exhaust valves remain initially closed. After the compression release cam lobe is passed, at about (0) crank angle degrees the trigger or control valve may be closed so that the lost motion master-slave or rocker arm system may transfer motion from the early exhaust valve opening lobe  304  ( FIG. 13 ) to produce an early exhaust valve opening event  304 ′. Thereafter, the main exhaust valve motion from the second motion imparting means may take over to provide the remainder of the actuation required for the main exhaust valve event  300 ′. After about (130) or about (270) crank angle degrees, depending on whether a master-slave or rocker arm system is used, the trigger or control valve may be opened again for hydraulic circuit refill so that the lost motion master-slave or rocker arm system may absorb the motion from the brake gas recirculation lobe  308  ( FIG. 13 ) of the first motion imparting means. 
     With reference to  FIG. 16 , during a second positive power mode of engine operation and when the trigger or control valve is initially in state “1”, or open, the lost motion master-slave or rocker arm system may absorb the motion received from the compression release cam lobe  302  ( FIG. 13 ) of the first motion imparting means so that the exhaust valves remain initially closed. After the compression release cam lobe is passed, at about (45) crank angle degrees the trigger or control valve may be closed so that the lost motion master-slave or rocker arm system may transfer motion from the early exhaust valve opening lobe  304  ( FIG. 13 ) to produce an abbreviated early exhaust valve opening event  304 ″. Thereafter, the main exhaust valve motion from the second motion imparting means may take over to provide the remainder of the actuation required for the main exhaust valve event  300 ″. After about (130) or about (270) crank angle degrees, depending on whether a master-slave or rocker arm system is used, the trigger or control valve may be opened again for hydraulic circuit refill and so that the lost motion master-slave or rocker arm system may absorb the motion from the brake gas recirculation lobe  308  ( FIG. 13 ) of the first motion imparting means. 
     Embodiments of the present invention may have many advantages, including providing variable engine braking, brake gas recirculation, and variable early exhaust valve opening for exhaust gas temperature control for emissions after-treatment and/or turbo stimulation for improved transient torque. Additional advantages may include a mechanically-driven exhaust main event for cold start and failsafe, meeting loading limits, especially cam Hertz stress, avoiding separation and impact loading between cams and rollers, avoiding valve bridge tilt, meeting exhaust valve seating velocity limits, and protecting against valve-piston contact. 
     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, it is appreciated that selective control of the trigger valve or control valve operation may produce engine valve actuations with timing other than those illustrated in  FIGS. 14-16 . Further, it is appreciated that the variable valve actuation systems described in connection with  FIGS. 1-12  may be used to actuate not only exhaust valves, but also intake and/or auxiliary engine valves. Thus, it is intended that the present invention cover all such modifications and variations of the invention, provided they come within the scope of the appended claims and their equivalents.