Abstract:
A lost motion valve actuation system for an internal combustion engine is disclosed. The system includes a motion feedback system ( 100 ) for detecting the motion and timing of a valve actuator ( 130 ) and an engine valve ( 140 ). By providing information on the condition of the engine during operation, the motion feedback system ( 100 ) permits adjustment of the valve actuation system that can optimize operation or prevent engine damage. The motion feedback system may be used in a common rail or a lost motion valve actuation system. In alternate embodiments, the valve actuation system includes an accumulator ( 300 ) with an accumulator piston ( 310 ) whose motion is limited by an accumulator stop ( 330 ) or a control valve ( 200 ). The limited accumulator ( 300 ) controls the amount of lost motion in the valve actuation system to provide fail-safe operation in the event of electrical failure. The valve actuation system is operable for engine positive power, compression release braking, and exhaust gas recirculation modes of operation.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     This application relates to and claims priority on Provisional Application Ser. No. 60/064,206, entitled “Fail-Safe, Fully Hydraulic Lost Motion Valve Actuation System,” filed Nov. 4, 1997; Provisional Application Ser. No. 60/065,815, entitled “Motion Feedback System for Valve Actuators,” filed Nov. 14, 1997; and Provisional Application Ser. No. 60/066,096, entitled “Exhaust Valve Operating System for Internal Combustion Engine Braking,” filed Nov. 17, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to engine valve actuation systems for internal combustion engines. In particular, the present invention relates to systems, used during positive power engine, braking, and exhaust gas recirculation, for providing a fail-safe, hydraulic control of the amount of “lost motion” between an engine valve and a means for opening the valve. In addition, the present invention relates to a system for detecting motion of a valve actuator of a common rail or lost motion valve actuation system. 
     BACKGROUND OF THE INVENTION 
     In many internal combustion engines the engine cylinder intake and exhaust valves may be opened and closed by fixed profile cams in the engine, and more specifically by one or more fixed lobes which may be an integral part of each of the cams. The use of fixed profile cams makes it difficult to adjust the timings and/or amounts of engine valve lift to optimize valve opening times and lift for various engine operating conditions, such as different engine speeds. 
     One method of adjusting valve timing and lift, given a fixed cam profile, has been to incorporate a “lost motion” device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage means. In a lost motion system, a cam lobe may provide the “maximum” (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve. 
     This variable length system (or lost motion system) may, when expanded fully, transmits all of the cam motion to the valve, and when contracted fully, transmit none or a minimum amount of the cam motion to the valve. An example of such a system is provided in U.S. Pat. No. 5,537,976 to Hu and U.S. Pat. No. 5,680,841, also to Hu, which are assigned to the same assignee as the present application, and which are incorporated herein by reference. 
     In the lost motion system of U.S. Pat. No. 5,680,841, an engine cam may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may be a solenoid valve and a check valve in communication with the hydraulic circuit including the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the motion of the master piston, which in turn displaces hydraulic fluid in direct response to the motion of a cam. When the solenoid is opened temporarily, the circuit may partially drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston. 
     In many electronically controlled valve actuation systems, there is a need to detect the motion and timing of a valve actuator so as to know the condition of the engine during operation. In some cases, knowing the phasing or timing of the event can be used to control the system and can compensate for changes in system operating conditions or other factors. In other cases, detecting the absence of valve motion allows the control system to shut off fuel injection or other valve motions for an affected cylinder so as to prevent engine damage. In the present invention, Applicants further disclose a system for detecting the motion of a valve actuator that may be used in a common rail or lost motion valve actuation system. A low-cost, on/off position sensor is used to detect whether or not a slave piston has moved, thus providing confirmation that the circuit is operational. By checking the time at which the slave piston moves to a certain distance (that at which the sensor changes state), the control module can compensate for system leads/lags versus desired timing. 
     In designing lost motion valve actuation systems, many different approaches have been considered. Hydromechanical systems allow for partial lost motion, while preserving mechanical valve actuation to some lesser extent than standard. These designs are somewhat complex, and experience difficult loading conditions during compression release retarding. Valve train designs employing a purely hydraulic system are flexible and conceptually simple to design, requiring only hydraulic connections between master pistons and slave pistons. For example, U.S. Pat. No. 4,278,233 to Zürner et al. discloses a hydraulic system for actuating gas-change valves in an internal combustion engine. Such systems are unlikely to achieve rapid acceptance in the conservative engine market due to their pronounced departure from conventional technology. These systems will not operate at all if oil pressure, fluid passage continuity or electrical element control is lost. 
     Previous lost motion systems have typically not utilized high speed mechanisms to rapidly vary the length of the lost motion system. Lost motion systems of the prior art have accordingly not been variable such that they may assume more than one length during a single cam lobe motion, or even during one cycle of the engine. By using a high speed mechanism to vary the length of the lost motion system, more precise control may be attained over valve actuation, and accordingly optimal valve actuation may be attained for a wide range of engine operating conditions. 
     Applicants have determined that the lost motion system of the present invention may be particularly useful in engines requiring valve actuation for both positive power and for compression release retarding and exhaust gas recirculation valve events. Typically, compression release and exhaust gas recirculation events involve much less valve lift than do positive power related valve events. Compression release and exhaust gas recirculation events may, however, require very high pressures and temperatures to occur in the engine. Accordingly, if left uncontrolled (which may occur with the failure of a lost motion system), compression release and exhaust gas recirculation could result in pressure or temperature damage to an engine at higher operating speeds. Therefore, Applicants have determined that it may be beneficial to have a lost motion system which is capable of providing control over positive power, compression release, and exhaust gas recirculation events, and which will provide only positive power or some low level of compression release and exhaust gas recirculation valve events, should the lost motion system fail. 
     An example of a lost motion system used to obtain retarding and exhaust gas recirculation is provided by U.S. Pat. No. 5,146,890 to Gobert, assigned to AB Volvo, and incorporated herein by reference. Gobert discloses a method of conducting exhaust gas recirculation by placing the cylinder in communication with the exhaust system during the first part of the compression stroke and optionally also during the latter part of the inlet stroke. Gobert uses a lost motion system to enable and disable retarding and exhaust gas recirculation, but such a system is not variable within an engine cycle. 
     The challenge addressed by the present invention is to employ lost motion valve actuation to achieve the benefits of variable valve actuation and the flexibility of hydraulic valve train design while preserving a predictable operating mode in the event of startup or failure conditions. In the present invention, Applicants disclose embodiments directed to both a fully hydraulic valve actuation system and a hydromechanical valve actuation system with electrical control. 
     Applicants&#39; method for implementing the flexible advantages of a fully hydraulic lost motion valve actuation system, while incorporating some measure of fail-safe operation, is accomplished by limiting the amount of motion which can be lost by designing the accumulator to accept less than a complete master piston stroke of working fluid. 
     In another embodiment of the present invention, Applicants disclose a system for valve actuation that employs a hydromechanical system with fail-safe features. It is known that internal combustion engines can be used to effect kinetic energy braking of a rolling vehicle by interrupting the engine&#39;s fuel flow, and operating the engine as an air compressor. In this mode, the rolling vehicle&#39;s kinetic energy is converted to potential energy (compressed air), and subsequently the potential energy is depleted by exhausting the compressed air into the atmosphere through the vehicle&#39;s exhaust system. Engine braking is described in detail in U.S. Pat. No. 3,220,392 to Cummins, which is incorporated herein by reference. 
     The effectiveness of engine compression braking can be improved further by recirculating exhaust gas into each cylinder at the time a cylinder&#39;s piston is at or near dead bottom at the beginning of the normal compression stroke. This process is commonly referred to as Exhaust Gas Recirculation or “EGR”. Including EGR in a compression braking cycle will result in the introduction of a greater volume of air to a given engine cylinder. Consequently, the engine works harder compressing the denser air volume and, as a result, more kinetic energy is converted into potential energy resulting in greater engine retardation. 
     EGR may also be used during normal positive power operation. The benefits derived from EGR during positive power operations are: (1) increased fuel-use efficiency due to the consumption of unburned combustibles in the exhaust gas; and (2) cleaner exhaust gas emissions. Details of EGR operating modes are provided in U.S. Pat. No. 5,787,859, which is assigned to the same assignee as the present application, and which is incorporated herein by reference. 
     Cylinder exhaust valves open at different times during engine braking and EGR operations than during positive power operations. For engine braking, the exhaust valves open at or near top dead center at the completion of a cylinder&#39;s compression stroke. For EGR events, the exhaust valve opens at or near the aforementioned dead bottom at or near the beginning of the compression stroke. The engine&#39;s conventional valve opening system associated with positive power operations holds a cylinder&#39;s exhaust valve closed at these times. Consequently, add-on systems that augment or modify the conventional exhaust valve opening system may be applied to internal combustion engines in order to permit engine braking and EGR operating modes. 
     Present engine braking and EGR systems derive the time for opening each cylinder&#39;s exhaust valve from a neighboring cylinder&#39;s intake or exhaust valve opening systems. The mechanical motion of the neighboring cylinder&#39;s main event valve opening system is transmitted to the selected cylinder&#39;s exhaust valve by add-on mechanical or hydro-mechanical systems. Engine braking and EGR exhaust valve opening derived in this fashion have certain disadvantages. For example, it may not be possible to open the exhaust valve at the optimum time for EGR and brake events. Also, the add-on systems add additional weight and size to an engine. As a result, there is a need for a system which provides optimum exhaust valve timing, opening duration, and lift for EGR and braking events. A system which provides independent control of each cylinder&#39;s valve(s) would be capable of provide optimum opening profiles (timing, duration, and lift) and result in increased braking energy, and improved engine efficiency. 
     Applicants&#39; alternative system for valve actuation replaces a conventional internal combustion engine&#39;s mechanical exhaust valve opening system with a hydro-mechanical system wherein auxiliary cam-actuated valve openings for engine braking can be inhibited or permitted by driver-initiated electrical control. Applicants&#39; present invention preserves normal positive power operation, and incorporates certain fail-safe features in the event of electrical control failure. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of the present invention to provide an internal combustion engine with an innovative and economical valve actuation system. 
     It is a further object of the present invention to provide valve motion detection. 
     It is a further object of the present invention to provide a system that provides feedback of actuator timing. 
     It is yet a further object of the present invention to provide a system that provides feedback of actuator motion. 
     It is still a further object of the present invention to provide a system that is capable of detecting failed valve events. 
     It is another object of the present invention to provide fully hydraulic valve actuation within practical limits. 
     It is a further object of the present invention to provide a fully hydraulic valve actuation system with a fail-safe operating condition. 
     It is yet another object of the invention to provide a fully hydraulic valve actuation system with a flexible system design. 
     It is still another object of the invention to provide a fully hydraulic valve actuation system with a limited lost motion capability. 
     It is also an object of the present invention to provide a fully hydraulic valve actuation system with limited accumulator motion. 
     It is another object of the present invention to provide controllable EGR and kinetic braking modes of an internal combustion engine. 
     It is yet another object of the present invention to provide optimum operation of an internal combustion engine&#39;s kinetic braking system. 
     It is a still another object of the present invention to provide an internal combustion engine with independently controlled valves. 
     It is also an object of the present invention to provide optimum control of engine valve timing. 
     It is a further object of the present invention to provide optimum control of engine valve lift. 
     It is a further object of the present invention to provide optimum control of the duration that the exhaust valve is open. 
     It is still a further object of the present invention to provide a fail-safe mode wherein positive power engine operation is not seriously impaired by electrical failure. 
     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 
     In response to the foregoing challenges, Applicants have developed an innovative, economical system for controlling engine valve operation in an internal combustion engine. The present invention is directed to a motion feedback system for detecting the motion and timing of an engine valve. The present invention is also directed to a system with a limited accumulator which controls the amount of lost motion in the valve actuation system to provide fail-safe operation in the event of electrical failure. 
     The present invention is directed to a valve actuation system for an internal combustion engine with at least one engine cylinder valve which is selectively openable. The valve actuation system may comprise actuating means for actuating at least one engine valve, detection means for detecting braking and exhaust motion of at least one engine valve in response to actuation of at least one engine valve by the actuating means and control means for controlling the actuating means in response to the detection means. 
     The valve actuation system of the present invention may further comprise means for imparting force from a force source to selectively operate at least one engine valve, a fluid system connected to the force imparting means for variably controlling the position of at least one engine valve, an accumulator connected to the fluid system, a control valve connected to the fluid system, means for actuating at least one engine valve in response to force imparted by the force imparting means through the fluid system, and a motion feedback system for detecting braking and exhaust motion of at least one engine valve in response to actuation of at least one engine valve by the actuating means. 
     The actuating means may comprise a slave piston slidably disposed within a slave piston bore in the brake housing, wherein the slave piston is connected to the fluid system and a valve actuator connected to the slave piston, wherein the valve actuator is in communication with at least one engine valve, upon displacement of the slave piston. 
     The motion feedback system may comprise at least one position sensor capable of detecting the motion of the valve actuator, and a control module electrically connected to at least one position sensor. The control module may be electrically connected to the control valve, to control the operation of the control valve in response to motion detected by at least one position sensor. The position sensor produces a position output signal in response to detecting the motion of the valve actuator past a switching point. The switching point of the valve actuation system of the present invention is adjustable. 
     The present invention is further directed to a valve actuation system which may comprise means for imparting force from a force source to selectively operate at least one engine valve, a fluid system which links at least one valve to the force imparting means such that a force derived from the force imparting means is transferred to at least one valve to operate the valve, an accumulator connected to the fluid system for selectively absorbing fluid within the fluid system to vary the operation of at least one valve, the accumulator having means for limiting the amount of an accumulated fluid within the accumulator, a control valve connected to the fluid system, and means for actuating at least one engine valve, in response to force imparted by the force imparting means through the fluid system. The accumulator may further include an accumulator piston. The accumulator limiting means limits the maximum stroke of the accumulator piston. The force imparting means may include a master piston for delivering motion from a cam, and the limiting means of the accumulator may limits the maximum stroke of the accumulator piston to a distance less than the maximum master piston lift. 
     In another embodiment, the present invention is directed to a valve actuation system which includes an alternate means of limiting the accumulator from absorbing the full volume of fluid displaced by the force imparting means during positive power operation of the engine. The accumulator limiting means may include a control valve connected by the fluid system to the accumulator. In this embodiment, the control valve limits displacement of the accumulator piston in the accumulator when the control valve is closed. In addition, the accumulator is capable of absorbing the full volume of fluid displaced by the master piston during predetermined operating conditions. 
    
    
     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 
     The invention will now be described in conjunction with the following drawings in which like reference numerals designate like elements and wherein: 
     FIG. 1 is a cross-sectional schematic diagram of a valve actuation system with motion feedback system according to a preferred embodiment of the present invention; 
     FIG. 2 is a graph depicting sensing signals in connection with valve actuation events; 
     FIG. 3 is a cross-sectional schematic diagram of a valve actuation system according to an alternate embodiment of the present invention; and 
     FIG. 4 is a cross-sectional schematic diagram of a valve actuation system according to a second alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to an embodiment of the present invention, an example of which is illustrated in the accompanying drawings. An embodiment of a valve actuation system with motion feedback system for an internal combustion engine is shown in FIG. 1 as  10 . Valve actuation system  10  is provided with a force imparting system  400 . Force imparting system  400  is connected to fluid system  500 , provided in brake housing  501 . Fluid system  500  is connected to control valve  200 , accumulator  300 , and slave piston  110 . Slave piston  110  is connected to valve actuator  130 , which is in communication at least one exhaust valve  140 . In a preferred embodiment, valve actuation system  10  is also provided with motion feedback system  100 . Motion feedback system  100  comprises valve actuation control assembly  600 , slave piston  110 , and valve actuator  130 . Valve actuation control assembly  600  is electrically connected to control valve  200 . 
     Force imparting system  400  includes cam  410  in communication with roller-follower  420 . Roller-follower  420  is connected to push-tube  430 . Push-tube  430  is connected to master piston  440 . 
     As embodied herein, fluid system  500  comprises conduits in brake housing  501 , including first fluid passage  510 , second fluid passage  530 , third fluid passage  550 , and fourth fluid passage  560 . First fluid passage  510  includes first check valve  520 . Second fluid passage includes second check valve  540 . Accumulator  300  is connected to fluid system  500  by fourth fluid passage  560 . Control valve  200  is connected to fluid system  500  by first fluid passage  510  and second fluid passage  530 . Control valve  200  further includes first port  210  connected to first fluid passage  510  and second port  220  connected to second fluid passage  530 . 
     With continuing reference to FIG. 1, system  10  includes slave piston  110 , slave piston spring  120  and valve actuator  130 . Slave piston  110  is slidably disposed in bore  112 , and is urged toward engine valve  140  by spring  120 . Valve actuator  130  is connected to slave piston  110  and may come into communication with engine valve  140  when slave piston  110  is displaced. 
     Valve actuation control assembly  600  includes position sensor  610  and control module  620 . Position sensor  610  may be a Hall-effect position sensor. Position sensor  610  is electrically connected to control module  620  through first electrical connection  630 . Control module  620  is electrically connected to control valve  200  through second electrical connection  640 . 
     Accumulator  300  is provided with accumulator piston  310  and accumulator spring  320 . Accumulator piston  310  is slidably disposed in accumulator  300 , and is urged toward fluid source end of accumulator  300  by accumulator spring  320 . 
     With continuing reference to FIG. 1, valve actuation system  10 , as embodied herein, operates as follows: motion of cam  410  is transferred to engine valve  140  through fluid system  500 . Fluid system  500  is preferably filled with low pressure (nominally 30-60 psi) engine lubricating oil from the engine crank case (not shown), however, other fluids are contemplated to be within the scope of the present invention. When control valve  200  is open, working fluid is taken into accumulator  300  until accumulator piston  310  is driven against some limit, shown in this preferred embodiment as accumulator spring  320 . After accumulator  300  is driven against its spring  320 , additional motion of master piston  440  will result in slave piston  110  displacement, regardless of the condition of control valve  200 . Displacement of slave piston  110  moves valve actuator  130  into communication with engine valve  140 , opening engine valve  140 . 
     When control valve  200  is open, working fluid moves freely to and from accumulator  300 . As cam  410  rotates, master piston  440  moves, thereby displacing a volume of the working fluid. 
     As embodied herein, motion feedback system  100  operates as follows: position sensor  610  is used to change or switch the state of a high/low output signal to control module  620  when valve actuator  130  moves past a certain point. By choosing the “switching” point of sensor  610 , it is possible to determine whether or not specific valve events created by a flexible valve actuation system  10  have occurred. By comparing the time at which position sensor  610  did in fact change state against an expected time, control module  620  senses whether the timing of valve actuation system  10  needs to be changed, or whether system  10  is functioning properly. 
     In response to signals generated by control module  620 , control valve  200  may be operated to control the operation of accumulator  300  to adjust the amount of motion transferred from master piston  440  to valve actuator  130 . Motion feedback system  100 , as shown in FIG. 1, is used in connection with lost motion valve actuation system  10 , however, it is contemplated by the present invention that motion feedback system  100  is capable of being used in numerous valve actuation systems, including but not limited to common rail electrohydraulic systems. 
     Referring now to FIG. 2, a graph is shown depicting the sensing signals for several valve actuation events. As shown in FIG. 2, the valve motions are compared with several typical sensor outputs. Distance Δ represents the amount of motion of valve actuator  130  required to activate position sensor  610 , causing position sensor  610  to change state from “off” to “on” or from “on” to “off.” Three conditions are described. In condition I, position sensor  610  changes state during braking lift and return motion as illustrated by curve A, as well as during lift and return of the main exhaust event as illustrated by curve B. Control module  620  receives the output signal from the sensor  610  to determine whether or not valve actuation is operating properly. Condition I illustrates normal valve operation. For condition I, the crank angle degree is shown where position sensor  610  changes state: at braking lift (−10°) and return (+30°) along curve A, and at exhaust lift (180°) and return (−360) along curve B. 
     In condition II, the sensor  610  detects “late” valve opening exhaust event as illustrated by Curve C, relative to the normal opening illustrated by curve B, as well as the relatively “early” closing also illustrated by Curve C. For condition II, the crank angle degree is shown where position sensor  610  changes state: at exhaust lift (200°) and return (−330°) along curve B. In response to condition II, control module  620  may generate the necessary signals to operate control valve  200  of valve actuation system  10 . Adjustments to the system can be made to produce valve opening for a normal exhaust event as shown in condition I. 
     In condition III, no valve motion occurs and sensor  610  does not change state, as illustrated by curve D. If valve motion were expected, an error condition is generated by control module  620 . In response, control module  620  may shut off fuel injection or other valve motions for an affected cylinder in order to prevent engine damage. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the construction and configuration of the present invention without departing from the scope or spirit of the invention. For example, it is contemplated that the control module  620  can “poll” the input from the sensor  610  as needed to verify a condition, or “wait” for a change in state to determine an actual switching time. Furthermore, it is contemplated that multiple sensors can be used from different sources of motion on one controller input for a “superimposed” signal. Multiple sensors can be used on the same actuator for redundancy or varying levels of detection. The system can be used to detect the presence or absence of an event, the timing of the event, or any combination thereof. Furthermore, it is contemplated that the motion feedback system may be located within the manifold for the valve actuation system  10 , as shown in FIG.  1 . Similarly, the motion feedback system may be exteriorly attached to the manifold. Accordingly, it is possible to retrofit existing valve actuation systems. Thus, it is intended that the present invention cover the modifications and variations of the invention. 
     Referring now to FIG. 3, another embodiment is shown as  20 . In this embodiment, master piston  440 , located axially within brake housing  501 , is connected to one or more slave pistons  110  via fluid system  500 . Control valve  200  allows isolation from or connection to accumulator  300 . Control valve  200  is provided with first port  210  for fluid intake from fluid system  500  and second port  220  for fluid outflow back to fluid system  500 . Control valve  200  is preferably a high-speed, normally open, solenoid valve. 
     Accumulator  300  is provided with accumulator piston  310  and accumulator spring  320 . Accumulator piston  310  is slidably disposed in accumulator  300 , and is urged toward fluid source end of accumulator  300  by accumulator spring  320 . Accumulator piston  310  is provided with accumulator stop  330 , which limits motion of accumulator piston  310  in accumulator  300 . 
     Fluid system  500  includes fluid intake port  502  for inflow of fluid from a fluid source (not shown), first fluid passage  510  connecting fluid intake port  502  to first port  210  of control valve  200 , second fluid passage  530  for outflow of fluid from second port  220  of control valve  200 , third fluid passage  550  connecting master piston  440  to slave piston  110 , fourth fluid passage  560  connecting accumulator  300  to fluid system  500  and check valve  520  for restricting the flow of fluid back to the fluid source. 
     Alternate embodiment  20 , as shown in FIG. 3, operates as follows: cam  410  motion is transferred to engine valve  140  by means of fluid system  500 . When control valve  200  is open, working fluid is taken into accumulator  300  until accumulator piston  310  is driven against some limit, shown in this embodiment as solid stop  330 . After accumulator piston  310  is driven against stop  330 , additional motion of master piston  440  will result in slave piston  110  displacement regardless of the condition of control valve  200 . This will occur as long as: 
     
       
         δ acc &lt;(δ cam   * A   mp ) /A   acc . 
       
     
     Where: 
     δ acc =the maximum stroke of accumulator  300 ; 
     δ cam =the maximum master piston lift due to cam  410 ; 
     A mp =the cross-sectional area of master piston  440 ; and 
     A acc =the cross-sectional area of accumulator  300 . 
     Referring now to FIG. 4, another embodiment of the present invention is shown as valve actuation system  30 . For simplicity, valve actuation system  30  is shown in connection with a single engine cylinder (not shown). In practice the invention could be applied to all engine cylinders. FIG. 4 depicts a multi-valve cylinder which includes two exhaust valves per cylinder. Multi-valve cylinders are common in contemporary internal combustion engines. 
     As embodied herein, valve actuation system  30  replaces an engine&#39;s mechanical exhaust valve opening system (normally consisting of combinations of camshafts, push-rods or push-tubes, rocker arms, and valve lifters) with an electrically controlled, hydromechanical system. Valve actuation system  30  comprises replacement cam  410  connected to master piston  440 , slave pistons  110  connected to master piston  440  by means of fluid system  500 , and accumulator  300  and control valve  200  connected to fluid system  500 . 
     Accumulator  300  is preferably a limited accumulator. Lost motion valve actuation systems of the known art typically have control valves located on the main fluid passage connecting the master piston and the slave piston. In contrast, embodiment  30  of the present invention has control valve  200  connected to fluid system  500  downstream from accumulator  300 . Intake of fluid into control valve  200  through first port  210  and fifth fluid passage  570  occurs when fluid flows out from accumulator  300 . Control valve  200  may be a low-speed trigger valve. Hydraulic or other fluid may enter fluid system  500  through fluid intake port  502  and first fluid passage  510 . First check valve  520  is located in first fluid passage  510 . Accumulator  300  comprises accumulator piston  310  and accumulator spring  320 . First fluid passage  510  is connected to accumulator  300  which may accept inflow of fluid from first fluid passage  510 . Continued inflow of fluid from first fluid passage  510  displaces accumulator piston  310  until it reaches the end of accumulator  300 . Fluid system  500  further comprises second fluid passage  530  for fluid outflow from control valve  200 , third fluid passage  550  connecting master piston  440  with slave pistons  110 , fifth fluid passage  570  for fluid outflow from accumulator  300  into control valve  200 , and sixth fluid passage  580  for fluid inflow into accumulator  300 . The present invention may supplement an otherwise conventional internal combustion engine with driver-initiated engine braking and EGR operating modes. 
     Replacement cam  410  includes exhaust valve cam lobes  412  (one per cylinder) that are machined to correspond with exhaust valve opening profiles optimized for positive power operation, engine braking, and EGR. Certain operating modes of the system, described below, permit exhaust valves  140  to replicate entirely the motions induced by the profile of cam  410 . 
     As embodied herein, valve actuation system  30  as shown in FIG. 4 operates as follows: fluid system  500  is initially filled with fluid. Such fluid may be low-pressure (nominally 30-60 psi) engine lubricating oil from the engine crankcase (not shown) but other types of fluid are within the scope of and contemplated by the present invention. The initial filling and supply for maintaining low-pressure oil in fluid system  500  may be augmented by an additional low-pressure accumulator  302 , located upstream from fluid intake port  502  in engine supply oil passage (not shown). Control valve  200  is normally open (de-energized). When the control valve  200  is open, low pressure oil moves freely to and from the chamber of accumulator  300 . As cam  410  rotates, master piston  440  moves, displacing a volume of oil. The oil volume displaced by master piston  440  varies according to the profile of cam  410 . The chamber of accumulator  300  is designed to absorb all of the oil displaced by master piston  440  in response to an engine braking or EGR lobe. Consequently, when control valve  200  is open or de-energized, slave pistons  110  do not move and exhaust valves  140  do not open in response to master piston motion generated by either an engine braking lobe or an EGR lobe. However, during positive power operation the oil volume displaced by master piston  440  is greater than the oil volume displaced during engine braking or EGR operation. Limited accumulator  300  cannot absorb all of the oil displaced when master piston  440  moves in response to a positive power cam lobe  412 . During positive power operations with control valve  200  open, once accumulator  300  is full, master piston  440  and slave pistons  110  become hydraulically linked allowing slave pistons  110  to replicate the balance of cam lobe  412  displacement. Operation of valve actuation system  30  with control valve  200  open results in “lost motion,” since all of master piston  440  motion is not transferred to slave piston  110 . During positive power operation exhaust valves  140  will open some amount, regardless of the position of control valve  200 . Since positive power operation may be maintained without electrical power, valve actuation system  30  includes a fail-safe operating mode. 
     As embodied herein, valve actuation system  30  is activated by closing control valve  200 . Control valve  200  is closed upon receipt of a signal from the engine&#39;s electrical control system (not shown). With control valve  200  closed, oil cannot move into or out of accumulator  300 . A full hydraulic link is established between slave pistons  110  and master piston  440 . “Lost motion” is eliminated, and slave pistons  110  replicate master piston  440  motion, causing exhaust valves  140  to open for engine braking, EGR, and positive power operation. 
     Another embodiment of the present invention is a partial authority system. Valve actuation system  30  of the present invention, shown in FIG. 4, may be converted into a partial authority system by the following: (1) replacing limited accumulator  300  with an unlimited accumulator; and (2) replacing control valve  200  with a high-speed (nominally, a 2 millisecond response latency) trigger valve. The unlimited accumulator has sufficient capacity to absorb all of the oil displaced by master piston  440  when high-speed valve is open. In this embodiment, exhaust valve opening is controlled electronically for all modes of operation (positive power, engine braking, and EGR). The ability to electronically control the exhaust valve opening provides the operator with fine control of the system, since cycling the high speed trigger valve will result in exhaust valve opening. This embodiment allows dynamic optimization over the operating range (RPM) of the engine. The partial authority system does not provide a fail-safe mode, however, since an electrical signal must shut the high-speed trigger valve in order for the engine valves to operate. 
     Another embodiment of the valve actuation system  30  present invention includes a high-speed trigger valve and electronic timing control. This embodiment includes the elements shown in FIG. 4, with the exception of control valve  200  which is replaced by a high-speed trigger valve. Limited accumulator  300  remains part of the system. This embodiment provides dynamic optimization for the engine braking and EGR while retaining normal positive power operation and providing a fail-safe mode. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the construction and configuration of the present invention without departing from the scope or spirit of the invention. For example, with reference to valve actuation system  20 , any means may be used to stop the accumulator after some displacement δ acc , including but not limited to a hydraulic cushion, mechanical stop, flow occluded by accumulator displacement, hydraulic lock, etc. Furthermore, it is contemplated that any additional system elements may be added without changing the scope of the invention, such as lash adjustment, valve seating, or other control devices. In addition, either a high or low speed solenoid may be used in alternate embodiment  20 . Further, with reference to valve actuation systems  10 ,  20  and  30 , it is contemplated that any suitable fluid may be used as the working fluid (including oil or fuel), and that valve actuation system systems  10 ,  20  and  30  may be used to control any type of engine valve (exhaust or intake) or injector. Thus, it is intended that the present invention cover the modifications and variations of the invention.