Abstract:
The present invention is directed to an innovative, economical method or system disclosing a captive volume in a hydraulic circuit which can be selectively hydraulically or pneumatically locked, or vented in order to correspondingly maintain, or increase the total volume of the hydraulic circuit. The structural elements of an embodiment of the present invention may include an accumulator with or without a fixed solid stop, which limits its travel. The accumulator can either be a separate entity, or as an assembly within the master or slave pistons.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a division of Ser. No. 09/663,414 filed Sep. 15, 2000, now U.S. Pat. No. 6,415,752 and claims priority on U.S. Provisional Patent Application No. 60/154,473, filed Sep. 17, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a system and method for opening at least one valve in an internal combustion engine. More specifically the invention relates to a system and method, used both during positive power and engine braking engine operating conditions, for controlling the amount of “lost motion” between the at least one valve and an assembly for opening the at least one valve. 
     BACKGROUND OF THE INVENTION 
     Valve actuation in an internal combustion engine is required in order for the engine to produce positive power, as well as to produce engine braking. During positive power, intake valves may be opened to admit fuel and air into a cylinder for combustion. The exhaust valves may be opened to allow combustion gas to escape from the cylinder. 
     During engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, an internal combustion engine of compression-ignition type into an air compressor. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator with increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted compression release-type engine brake can develop retarding horsepower that is a substantial portion of the operating horsepower developed by the engine in positive power. 
     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 assembly. 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, transmit 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 and method is provided in Hu, U.S. Pat. Nos. 5,537,976 and 5,680,841, 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 shaft 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. 
     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. 
     The lost motion system and method 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, 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 and method used to obtain retarding and exhaust gas recirculation is provided by the Gobert, U.S. Pat. No. 5,146,890 (Sep. 15, 1992) for a Method And A Device For Engine Braking A Four Stroke Internal Combustion Engine, 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 system is not variable within an engine cycle. 
     The development of lost motion systems has also lead to the integration of such systems into existing engine components, as opposed to adding such systems aftermarket. One particular form of system integration that appears desirable is the integration of the lost motion system into an engine rocker arm, such as is shown in Hu, U.S. Pat. No. 5,680,841. By integrating the lost motion system into the engine rocker arm, savings in weight, size, and cost may be available. 
     All of the foregoing developments, such as high speed lost motion actuation, and rocker arm integration, have necessitated independently and collectively, smaller, faster, more robust, more controllable, and more compliant lost motion components. One such component that requires improvement to meet the needs of these new and advanced lost motion systems is the system accumulator. 
     Lost motion systems may require the use of an accumulator to absorb hydraulic fluid that is quickly shuttled into and out of the system, as well as to handle the rapid pressure changes (i.e. from high pressure to low pressure and visa-versa) that occur in the system as a result of high speed actuation. The very nature of accumulators dictates that they be sufficiently robust to withstand high and rapidly changing pressures. Compliance issues also require that the accumulators be located as closely as possible to the lost motion element with which they are in hydraulic communication. Compliance issues also mandate that the lost motion system, and to some degree, the accumulator, be adapted to bleed air from the working fluid thereby reducing the compressibility of the fluid. 
     Locating an accumulator near a lost motion element, particularly one integrated into an engine rocker arm, constrains the size and weight of the accumulator, which in turn affects the designers ability to make the accumulator robust. There is a natural inverse relationship between the robustness of an accumulator and its size and weight. The smaller and lighter the accumulator, the less robust it tends to be. Thus, the combination of loading and space requirements of accumulator pistons associated with integrated engine brakes provides a challenge to engine brake designers. In view of the foregoing, there is a need for an accumulator that is reduced in size, cost effective, sufficiently robust, capable of bleeding air, and controllable. 
     It has been determined that control over the amount of hydraulic fluid that the accumulator is designed to accumulate may be particularly important to the operation of the lost motion system. Without precise accumulator control, an engine valve may experience over-travel or under-travel. Moreover, imprecise accumulator control may have a negative impact on control and consistency of engine valve seating timing and velocities. 
     Engine valve over-travel during main events may result in valve to piston contact or the need for valve pockets in the piston. Neither valve to piston contact, nor valve pockets are desirable. Under-travel may lead to ineffective auxiliary valve events, such as compression-release events, or ineffective overlap between main intake and exhaust events. In order to reduce the likelihood of valve over-travel or under-travel, and to provide desirable valve seating timing and velocities, Applicant has developed an accumulator that absorbs a predetermined fixed volume of hydraulic fluid upon each actuation cycle of the engine brake. This accumulator provides the ability to lose the precise amount of motion provided by an engine brake lobe, or another auxiliary lobe on the exhaust cam. The loss of this precise amount of motion permits the engine valve to seat consistently, and the engine piston to be provided without pockets, while avoiding the likelihood of valve to piston contact. 
     Accumulator design must also take into account the undesired heating of the hydraulic fluid used in the lost motion system. Typically, engine oil is used as the working hydraulic fluid. Such engine oil enters the system already somewhat heated due to its use in the operation of the engine. The oil in the lost motion system is further heated as a result of flowing rapidly through the passages that make up the system. It would therefore be beneficial to provide accumulators with some means of cycling hydraulic fluid through the lost motion system so that there is a constant influx of fresh cool fluid into the system. 
     In order to provide an accumulator with all of the foregoing beneficial characteristics, Applicant has developed an accumulator that may be integrated into a lost motion piston, such as a slave piston. Such an integrated accumulator saves space and cost due to the use of the slave piston bore as the bore for the accumulator. The integrated accumulator is also capable of being quite robust because it may be manufactured of the same strength steel used for the slave piston. 
     Applicant has also developed an accumulator capable of providing a precise amount of lost motion clipping of a main engine valve event. Such precise clipping is attained through use of a fixed volume or fixed displacement accumulator. Clipping without a fixed volume may either result in too much, or too little engine valve travel being removed. The later may result in valve-to-piston contact, and the former may cause the valve to be seated at a higher velocity than desired. At a minimum, this may lead to increased engine valve seat wear, and possibly to some form of engine valve failure. 
     In accordance with embodiments of the present invention, it is contemplated that the accumulator system may be located in a master piston, a slave piston, or separate piston. It is further contemplated that in accordance with the present invention the accumulator system may be located within a rocker arm assembly of an engine rocker brake. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of the present invention to provide a lost motion system accumulator with improved robustness for its size. 
     It is another object of the present invention to provide a lost motion system accumulator that reduces accumulator bore wear. 
     It is another object of the present invention to reduce the package size of a lost motion system accumulator. 
     It is a further object of the present invention to provide a more cost effective method for packaging a lost motion system accumulator. 
     It is still another object of the present invention to reduce some of the variances of bleed rate for a lost motion system accumulator due to pressure differentials. 
     It is yet another object of the present invention to improve braking performance by improving compliance of a lost motion system accumulator. 
     It is still yet another object of the present invention to provide a lost motion system accumulator capable of venting and/or absorbing a fixed volume of hydraulic fluid to eliminate valve-to-piston clearance issues. 
     It is still another object of the present invention to provide a lost motion system accumulator with desirable air bleeding and hydraulic fluid circulation capabilities. 
     It is still another object of the present invention to provide a lost motion system accumulator that will reduce engine valve spring stresses as a result of fixed volume accumulator. 
     It is still another object of the present invention to provide a lost motion system accumulator that provides lower engine valve seating velocities. 
     It is still another object of the present invention to provide a lost motion system accumulator that provides more consistent valve seating timing and velocities. 
     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 challenge, Applicant has developed an innovative, economical method or system for providing a lost motion accumulator that uses a captive (fixed) volume that can be selectively hydraulically or pneumatically locked, or vented in order to maintain or increase the total volume of the lost motion system. 
     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 schematic view of a captive volume accumulator system in accordance with a first embodiment of the present invention. 
     FIG. 2 is a schematic view of a captive volume accumulator system in accordance with a second embodiment of the present invention. 
     FIG. 3 is a schematic view of a captive volume accumulator system in accordance with a third embodiment of the present invention. 
     FIG. 4 is a schematic view of a captive volume accumulator system in accordance with a fourth embodiment of the present invention. 
     FIG. 5 is a schematic view of a captive volume accumulator system in accordance with a fifth embodiment of the present invention. 
     FIG. 6 is a graphical representation of a valve lift profile according to an embodiment of the present invention. 
     FIG. 7 is a schematic view of an accumulator control valve in an “OFF” position in accordance with a sixth embodiment of the present invention. 
     FIG. 8 is a schematic view of the control valve of FIG. 7 in an “ON” position. 
     FIG. 9 is a schematic view of an accumulator control valve in an “OFF” position in accordance with a seventh embodiment of the present invention. 
     FIG. 10 is a schematic view of the control valve of FIG. 9 in an “ON” position. 
     FIG. 11 is a schematic view of an accumulator control valve in an “OFF” position in accordance with an eighth embodiment of the present invention. 
     FIG. 12 is a schematic view of the control valve of FIG. 11 in an “ON” position. 
     FIG. 13 is a detailed view of the slave piston and accumulator assembly shown in FIG.  1 . 
     FIG. 14 is a schematic view of a captive volume accumulator system in accordance with a ninth embodiment of the present invention. 
     FIG. 15 is a schematic view of a captive volume accumulator system in accordance with a tenth embodiment of the present invention in which the system is integrated into an engine rocker arm. 
     FIG. 16 is a view of the tenth embodiment shown in FIG. 15 along section C—C. 
     FIGS. 17-19 are illustrations of a captive volume accumulator system in accordance with an eleventh embodiment of the present invention in which the system is integrated into an engine rocker arm. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the present invention is shown as accumulator system  10  in FIG.  1 . The system  10  includes an energy source  100 , which provides the necessary energy to operate at least one engine valve  500 . The energy source  100  supplies energy to an energy transfer assembly  200 . The energy transfer assembly  200  transfers energy derived from the energy source  100  to an actuating assembly  300 , which activates the at least one engine valve  500 . A control assembly  400  may be provided to control the amount of energy and/or the amount of motion transferred by the energy transfer assembly  200  to the actuating assembly  300 . 
     With continued reference to FIG. 1, the energy source  100  may comprise a cam  110  as well as other typical valve train elements. The cam  110  may have at least one lobe  112  thereon to provide energy to perform a main engine valve event and at least one lobe  114  to provide energy to perform a secondary engine valve event. The main engine valve event may be a main exhaust event. The secondary engine valve event may include a compression-release braking event and/or an exhaust gas recirculation event. The present invention, however, is not limited to the use of a cam  110  as an energy source to operate the at least one engine valve  500 , rather, it is contemplated that other suitable sources of energy may be employed without departing from the scope of the invention. 
     The cam  110  may be in operational contact with a roller follower  122  provided on a master piston  120 . The master piston  120  may be slidably disposed in a master piston bore  210  and biased into contact with the cam  110  by the master spring  124 . The master piston bore  210  may be charged with hydraulic fluid from a low pressure supply passage  214 . Oil supplied by passage  214  flows into the system  10 , past a check valve  216 , and through a passage  212 . Oil from the passage  212  fills the master piston bore  210  and enters the slave piston bore  220 . 
     A slave piston  300  may be slidably disposed in the slave piston bore  220 . The slave piston  300  may include a slave piston body  310 , an accumulator piston  320 , and an accumulator spring  330 . A detailed illustration of the upper portion of the slave piston  300  is shown in FIG.  13 . As shown in FIG. 13, the travel of the accumulator piston  320  may be limited by an upper shoulder  314  and a lower shoulder  316 . The upper shoulder  314  may define a central opening  312  through which hydraulic fluid pressure can be applied to the accumulator piston  320 . The upper shoulder  314  may control the maximum volume of oil that may be contained in the accumulator chamber  315 . The arrangement shown in FIG. 13 provides for automatic lash take up between the slave piston  300  and the engine valve  500 . 
     The accumulator piston  320  may include a bleed passage  322  that may provide controlled or resultant leakage into the accumulator chamber  315 . The accumulator spring  330  may bias the accumulator piston  320  against the upper shoulder  314  when low pressure oil is provided to the slave piston bore  220 . The accumulator spring  330  may seat on an internal land  316 . 
     A passage  317  provides hydraulic communication between the chamber  315  containing the accumulator piston  320  and the sidewall of the slave piston body  310 . An annulus or recess  319  may be provided in the slave piston sidewall to facilitate a predetermined amount of hydraulic communication between the accumulator chamber  315  and the control valve bore  230  (shown in FIG.  1 ). 
     With renewed reference to FIG. 1, a control passage  222  provides hydraulic communication between the control valve bore  230  and the slave piston bore  220 . The control passage  222  may include an enlarge portion  224  that is designed to provide a predetermined amount of hydraulic communication between the slave piston and control valve bores. 
     A control valve  400  may be slidably disposed in the control valve bore  230 . The control valve may comprise a check valve body  410 , a check ball  420 , a check ball spring  430 , and a control valve spring  440 . A first end of the control valve bore  230  may connect to a control fluid supply passage  232  that selectively supplies hydraulic fluid to the control valve  400  under the control of a solenoid valve  234 . A second end of the control valve bore  230  may connect to a vent passage  226  that communicates with the atmosphere or a second accumulator (not shown). If the vent passage  226  connects to a second accumulator, the vented fluid may eventually be returned to the fluid supply, and thus the fluid supply passage  232 . The control valve  400  may either be a fast or slow acting mechanical, electro-mechanical, electro-magnetic, pneumatic, or hydraulic valve that controls the communication of the accumulator chamber  315  with the vent passage  226 . The check valve  410  portion of the control valve  400  can also supply low pressure oil to the system  10 . 
     With continued reference to FIG. 1, the control valve  400  is in an “off” position. The off position is defined as that in which the solenoid valve  234  does not have power supplied to it and the control valve body  410  is at the resting position. When in the off position, the control valve  400  permits hydraulic communication between the accumulator chamber  315  and the vent passage  226  by way of the passage  222 . The off position of the control valve  400  is used to provide positive power engine valve operation (i.e. no compression-release braking). 
     During positive power operation, the system  10  is charged with low pressure oil from the passage  214 . The check valve  216  prevents the oil provided to the master piston bore  210  and the slave piston bore  220  from flowing back towards the low pressure supply, and thus provides automatic lash take up. The oil provided from the passage  214  is not sufficiently pressurized to depress the accumulator spring  330 . Thus, the accumulator piston  320  remains biased against the upper shoulder  314  when the master piston  120  is at base circle (as shown). 
     As the cam  110  rotates, the master piston  120  is displaced upward by a secondary lobe  114 . The displacement of the master piston  120  causes the accumulator piston  320  to be correspondingly displaced downward against the bias of the accumulator spring  330  into the accumulator chamber  315  relative to the slave piston body  310 . From an observation point outside of the slave piston  300 , the accumulator piston  320  may move downward to some degree and the slave piston body  310  may move upward to some degree, in accordance with the hydraulic ratios of these elements that is dependent on the relative diameters of the slave piston bore  220  and the accumulator chamber  315 . Relative movement of the accumulator piston  320  and the slave piston body  310  causes the accumulator spring  330  to be depressed because as between it and the engine valve spring (not shown) it provides a lower biasing force. The volume of the accumulator chamber  315  is designed to fully absorb the oil displaced by the master piston  120  as a result of encountering the secondary lobe  114 . The lower shoulder  316  may be located such that the accumulator piston  320  engages the lower shoulder just as the maximum displacement produced by the secondary lobe  114  is applied to the master piston  120 . 
     After encountering the secondary lobe  114 , the master piston  120  is displaced further by the main event lobe  112 . The additional displacement of oil by the master piston  120  can no longer be absorbed by the accumulator piston  320  because it is already in contact with the lower shoulder  316  as a result of the displacement caused by the secondary lobe  114 . Thus, the additional displacement of hydraulic fluid by the master piston  120  causes the slave piston body  310  to slide downward in the slave piston bore  220  against the bias of the engine valve spring (not shown). In this manner, the main event lobe  112  may converted to a main event opening motion for the engine valve  500 . 
     Seating of the engine valve  500  occurs as the master piston  120  follows the cam  110  into the saddle of the second base circle (i.e. the secondary lobe  114 ). As the master piston  120  follows the cam  110  onto the first base circle, the slave piston  310  and the accumulator piston  320  return to their upper rest positions. 
     In the positive power mode, the bleed passage  322  is constantly operational. This passage provides system cooling by continuously replacing heated, worked oil with fresh, cooler oil from the supply passage  214 . 
     In order to place the engine in compression-release braking mode, the solenoid valve  234  may be actuated (or de-actuated, depending on whether the solenoid is arranged as normally open or normally closed). Actuation of the solenoid valve  234  causes low pressure hydraulic fluid to be applied to the control valve  400  through the passage  232 . The oil pressure applied to the control valve  400  causes it to be displaced downward against the bias of the control valve spring  440 . In this position the control valve  400  blocks hydraulic communication between the passage  222  and the vent passage  226 . The check ball  420  of the control valve  400 , however, permits the one way flow of oil into the high pressure circuit (passage  222  and slave piston bore  220 ), but not back out of the high pressure circuit. The check ball  420  allows oil to fill the accumulator chamber  315  as the accumulator piston  320  re-attains its upper most position when the cam  110  returns to base circle. 
     When the solenoid valve  234  is “on”, and the cam  110  is at base circle, the accumulator piston  320  is hydraulically locked into its upper position against the upper shoulder  314 . As the cam  110  rotates, the master piston  120  is first displaced upward by the secondary lobe  114 . Because the accumulator piston  320  is locked into position, the displacement of the master piston  120  by the secondary lobe  114  causes a corresponding downward displacement of the slave piston  310 . The downward motion of the slave piston  310  may in turn open the engine valve  560  for a compression-release event. 
     After the compression-release braking event occurs, the master piston may be further displaced by main event lobe  112  on the cam  110 . The main event lobe  112  cause the slave piston  320  to be further displaced, opening the engine valve  500  for its main event. At a certain point on the main event lobe profile, the recess  319  provided in the slave piston  310  comes into hydraulic communication with the vent passage  226 . When this communication occurs, the high pressure hydraulic fluid locking the accumulator piston  320  into its upper position is released to atmosphere or a second accumulator. This permits the accumulator piston  320  to move downward in the accumulator chamber  315  relative to the slave piston body  310  until it comes to rest on the lower shoulder  316 . Thus, communication of the recess  319  with the vent passage  226  permits the accumulator piston  320  to absorb the precise amount of additional motion provided by the secondary cam lobe  114 . In this manner, the main event motion provided to the engine valve  500  during engine braking operation is limited to the same amount of motion that is provided by a main event during positive power operation. Therefore, the present invention provides the same valve-to-piston clearance during positive power and engine braking operation. 
     It is appreciated that the afore-described process could be modified such that an exaggerated main exhaust event is provided by maintaining the control valve  400  in its positive power position during engine braking. 
     The bleed passage  322  provided in the accumulator piston  320  does not affect the ability of the accumulator piston to be hydraulically locked, which eliminates the variability of orifice bleeding that may ordinarily result from system pressure variations. When the accumulator chamber  315  is vented through the vent passage  226 , however, the bleed passage  322  is also able to vent. A certain amount of oil will be bled through the system each time the accumulator chamber  315  is placed in communication with the vent passage  226 . The position of the vent passage  226  may be selected so as to be anywhere in the range of valve lift for the main event, as long as it is less than the peak lift minus the lost motion portion of the lift. Oil for hydraulic lash adjustment and recovery from lost oil may be regained through the high-pressure check valve contained in the control valve  400 . 
     The engine valve  500  will seat as the master piston  120  follows the cam  110  back into the saddle of the second base circle (i.e. secondary event  114 ). As the master piston  120  begins to travel down the last ramp of the secondary event  114  to the first base circle, the accumulator piston  320  will reset to its upper position under the influence of oil provided through the control valve  400 . 
     FIG. 6 is a graphical representation of valve lift as disclosed in the present invention. In a lost motion system, where the cam profile has two events: one which can be suppressed, and the second is additive to the first (see FIG.  6 —Braking Lift). This leads to valve to piston clearance issues. A method of eliminating this over-travel, is to vent a fixed volume of oil. If the volume of oil is equal to the amount of lift of the first bump, then the valve will seat a shown. This process can be accomplished with any lost motion system and can use any means to enact the venting of the hydraulic volume. 
     A second embodiment of the present invention is shown in FIG. 2, in which like reference numerals refer to like elements. The operation of the system shown in FIG. 2 is similar to that of the system shown in FIG.  1 . In FIG. 2, a spool valve  412  that includes a check valve at one end serves as the control valve  400 . When the spool valve  412  is in the position shown, the accumulator piston  320  is free to be displaced in the accumulator chamber  315  as the result of high pressure received through the passage  212 . Displacement of the accumulator piston  320  causes the oil in the chamber  315  to be vented through the vent passage  226 . 
     The system shown in FIG. 2 may provide compression-release braking by actuating the solenoid valve  234 , which in turn causes oil to flow through the passage  232  and displace the spool valve  412  upward. This displacement of the spool valve  412  blocks communication between the passage  222  and the vent passage  226 , thereby hydraulically locking the accumulator piston  320  into its upper position. One way flow of oil into the accumulator chamber  315  is permitted by the check valve end  410  of the control valve  400 . Unlocking of the accumulator piston  320  during the main engine valve event may occur as a result of either communication between the slave piston passage  317  and the secondary vent passage  228 , or the high speed actuation of the spool valve  412  with an mechanical, electro-mechanical, electro-magnetic, pneumatic, or hydraulic actuator. The secondary vent passage  228  may communicate with the vent passage  226 . 
     The system shown in FIG. 2 (as well as that shown in FIGS. 3 and 5) may also benefit from the isolation of the spring  440  from the hydraulic pulses that may occur in the vent passage  226 . 
     A third embodiment of the present invention is shown in FIG. 3, in which like reference numerals refer to like elements. The operation of the system shown in FIG. 3 is similar to that of the system shown in FIG.  2 . In FIG. 3, a spool valve  412  serves as the control valve  400 . The spool valve  412  provides communication with the slave piston bore  220  alternatively with a vent passage  226  (during positive power operation) or with a constant checked supply of low pressure oil from a low pressure passage  214  (during engine braking operation). When the spool valve  412  is in the position shown, the accumulator piston  320  is free to be displaced in the accumulator chamber  315  as the result of high pressure received through the passage  212 . Displacement of the accumulator piston  320  causes the oil in the chamber  315  to be vented through the vent passage  226 . 
     With continued reference to FIG. 3, compression-release braking operation may be provided by actuating the solenoid valve  234 , which in turn causes oil to flow through the passage  232  and displace the spool valve  412  downward. This displacement of the spool valve  412  blocks communication between the passage  222  and the vent passage  226 , and opens communication between the supply passage  214  and the passage  222 , thereby hydraulically locking the accumulator piston  320  into its upper position. One way flow of oil into the accumulator chamber  315  is permitted by the check valve  216 . Unlocking of the accumulator piston  320  during the main engine valve event may occur as a result of either communication between the slave piston passage  317  and the secondary vent passage  228 , or the high speed actuation of the spool valve  412  via high speed actuation of the solenoid valve  234 . The secondary vent passage  228  may communicate with the vent passage  226 . The control valve  400  may either be a fast or slow acting mechanical, electro-mechanical, electro-magnetic, pneumatic, or hydraulic valve that controls the communication of the accumulator chamber  315  with the vent passage  226 . 
     A fourth embodiment of the present invention is shown in FIG. 4, in which like reference numerals refer to like elements. In this embodiment, the spool valve  412  alternatively connects the passage  222  (and thus the accumulator chamber  315 ) to either the vent passage  226  or a high pressure hydraulic fluid supply passage  212 . The solenoid valve  234  may control the position of the spool valve  412 . When the solenoid valve  234  blocks the flow of hydraulic fluid into the control valve bore  230 , the spool valve  412  is biased upward and provides communication between the passage  222  and the vent passage  226 . When the solenoid valve  234  supplies hydraulic pressure, the spool valve  412  is biased down into the position shown so that the vent passage  226  is closed and the high-pressure passage  212  is placed in communication with the accumulator chamber  315 . 
     A fifth embodiment of the present invention is shown in FIG. 5, in which like reference numerals refer to like elements. With reference to FIG. 5, a spool valve  412  with a bleed fill may be provided. During engine braking operation, the spool valve  412  is displaced upward against the bias of the control valve spring  440 . In this position, the accumulator chamber  315  is permitted to vent through the vent passage  226  to either the atmosphere, or a second accumulator that is connected back to the high-pressure circuit, to aid in re-fill. During positive power operation, the spool valve  412  is positioned as shown so that the vent passage  226  is blocked. The accumulator chamber  315  may be filled by leakage from the high-pressure passage  212  past the accumulator piston  320 . This leakage fill feature is further enhanced by the incorporation of a constant bleed passage  322  (shown in FIG. 1) into the accumulator piston  320 . 
     With reference to FIG. 7, an accumulator control valve  400  configured in accordance with a seventh embodiment of the present invention is shown, in which like reference numerals refer to like elements. With reference to FIG. 7, the spool valve  412  may be controlled via the application of low pressure hydraulic fluid from the passage  232 . The spool valve  412  may provide the passage  222  (connected to the accumulator chamber  315 ) with communication alternatively with the atmosphere through the vent plate  238  or with the checked low pressure supply via the check valve  216 . The passage  222  is offset from the passage  232  and the spool valve  412  is positioned so that the low pressure, supply passage does not ever communicate with the vent plate  238 . As a result of the foregoing arrangement, the application of low pressure hydraulic fluid in the passage  232  immediately causes the spool valve  412  to index upward and block communication between the passage  222  and the vent plate  238 . FIG. 7 shows the spool valve  412  in the position required for positive power operation (primary mode) of the lost motion system. FIG. 8 shows the same spool valve  412  as is shown in FIG. 7 in the position required for engine braking operation (secondary mode). The control valve  400  may either be a fast or slow acting mechanical, electro-mechanical, electro-magnetic, pneumatic, or hydraulic valve that controls the communication of the accumulator chamber  315  with the vent passage  226 . 
     With reference to FIG. 9, an accumulator control valve  400  configured in accordance with a sixth embodiment of the present invention is shown, in which like reference numerals refer to like elements. With reference to FIG. 9, the spool valve  412  may be controlled via the application of low pressure hydraulic fluid from the passage  232 . The spool valve  412  may provide the passage  222  (connected to the accumulator chamber  315 ) with communication alternatively with the atmosphere through the vent plate  238  or with the checked low pressure supply via the check valve  216 . The passage  222  is located directly across from the passage  232 , which simplifies manufacturing of the system. The spool valve  412  is positioned so that the passage  232  communicates with the vent plate  238  when the spool valve is in an “off” position. As a result of the foregoing arrangement, the application of low pressure hydraulic fluid in the passage  232  does not immediately cause the spool valve  412  to index upward and block communication between the passage  222  and the vent plate  238 . Spool valve  412  indexes upward only after the combined flow of oil past the check valve  216  and the vent plate  238  backs up sufficiently to allow hydraulic pressure to build underneath the spool valve. FIG. 9 shows the spool valve  412  in the position required for positive power operation of the lost motion system. FIG. 10 shows the same spool valve  412  as is shown in FIG. 9, in the position required for engine braking operation. The control valve  400  may either be a fast or slow acting mechanical, electro-mechanical, electro-magnetic, pneumatic, or hydraulic valve that controls the communication of the accumulator chamber  315  with the vent passage  226 . 
     With reference to FIG. 11, an accumulator control valve  400  configured in accordance with a seventh embodiment of the present invention is shown, in which like reference numerals refer to like elements. With reference to FIG. 11, the slug  414  may be controlled via the application of low pressure hydraulic fluid from the passage  232 . The slug  414  may selectively block the flow of hydraulic fluid from the accumulator chamber  315  to the atmosphere through the vent plate  238 . Actuation of the control valve  400  occurs due to the combination of the length of the passage  232  that connects to the accumulator bore  220  and the restriction provided by the check valve  216  being sufficient to delay the actuation of the slave piston body until after the slug  414  is indexed upward to block the vent plate  238 . FIG. 11 shows the slug  414  in the position required for positive power operation of the lost motion system. FIG. 12 shows the same slug  414  as is shown in FIG. 11, in the position required for engine braking operation. The control valve  400  may either be a fast or slow acting mechanical, electro-mechanical, electro-magnetic, pneumatic, or hydraulic valve that controls the communication of the accumulator chamber  315  with the vent passage  226 . 
     In accordance with variations of the present invention, an accumulator vent passage may be placed in communication with the high pressure circuit in the lost motion system  10  through the motion of the slave piston  310 , which contains a window to either the atmosphere, or a second accumulator that is connected back to the high-pressure circuit, to aid in re-fill. With reference to FIG. 14, in an alternative embodiment of the invention, an accumulator vent passage may be exposed through the motion of the master piston  120 , which contains a window to either the atmosphere or a second accumulator that is connected back to the high-pressure circuit, to aid in re-fill. This may effectively reset the engine valve  500 . 
     FIGS. 15 and 16 show the slave piston  300  and control valve  400  arrangement of FIG. 4 arranged in a rocker arm  600 . FIG. 15 also illustrates the use of a preferred accumulator piston  320  that includes a piston head  324  and a piston stem  326 , and dual accumulator springs  330  and  332 . The operation of the slave piston  300  and the control valve  400  is the same as that described in connection with FIG. 4 except that the downward force applied to the slave piston is provided by the rotation of the rocker arm  600  in the system shown in FIGS. 15 and 16, as opposed to the master piston  120  in the system of FIGS. 1 and 4. It is appreciated that any of the slave piston/control valve arrangements shown in FIGS. 1-5 and  7 - 14  may be integrated into a rocker arm as shown in FIGS. 15 and 16. The control valve  400  may either be a fast or slow acting mechanical, electro-mechanical, electro-magnetic, pneumatic, or hydraulic valve that controls the communication of the accumulator chamber  315  with the vent passage  226 . 
     FIGS. 17-19 show the slave piston  300  and control valve  400  arrangement of FIG. 1 arranged in a rocker arm  600 . 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the construction, configuration, and/or operation of the present invention without departing from the scope or spirit of the invention. For example, in the embodiments mentioned above, various changes may be made to the accumulator without departing from the scope and spirit of the invention. Further, it may be appropriate to make additional modifications or changes to the hydraulic system without departing from the scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of the invention provided they come within the scope of the appended claims and their equivalents.