Patent Publication Number: US-6907851-B2

Title: Engine valve actuation system

Description:
RELATED APPLICATIONS 
   This application is a continuation-in-part of co-pending application Ser. No. 10/283,373 filed on Oct. 30, 2002 for Engine Valve Actuation System, which is a CIP of co-pending application Ser. No. 10/144,062 filed on May 14, 2002 for Engine Valve Actuation System. 

   TECHNICAL FIELD 
   The present invention is directed to an engine valve actuation system for an internal combustion engine. More particularly, the present invention is directed to variable valve actuation system for an internal combustion engine. 
   BACKGROUND 
   The operation of an internal combustion engine, such as, for example, a diesel, gasoline, or natural gas engine, may cause the generation of undesirable emissions. These emissions, which may include particulates and nitrous oxide (NOx), are generated when fuel is combusted in a combustion chamber of the engine. An exhaust stroke of an engine piston forces exhaust gas, which may include these emissions from the engine. If no emission reduction measures are in place, these undesirable emissions will eventually be exhausted to the environment. 
   Research is currently being directed towards decreasing the amount of undesirable emissions that are exhausted to the environment during the operation of an engine. It is expected that improved engine design and improved control over engine operation may lead to a reduction in the generation of undesirable emissions. Many different approaches, such as, for example, engine gas recirculation and aftertreatments, have been found to reduce the amount of emissions generated during the operation of an engine. Unfortunately, the implementation of these emission reduction approaches typically results in a decrease in the overall efficiency of the engine. 
   Additional efforts are being focused on improving engine efficiency to compensate for the efficiency loss due to the emission reduction systems. One such approach to improving the engine efficiency involves adjusting the actuation timing of the engine valves. For example, the actuation timing of the intake and exhaust valves may be modified to implement a variation on the typical diesel or Otto cycle known as the Miller cycle. In a “late intake” type Miller cycle, the intake valves of the engine are held open during a portion of the compression stroke of the piston. 
   The engine valves in an internal combustion engine are typically driven by a cam arrangement that is operatively connected to the crankshaft of the engine. The rotation of the crankshaft results in a corresponding rotation of a cam that drives one or more cam followers. The movement of the cam followers results in the actuation of the engine valves. The shape of the cam governs the timing and duration of the valve actuation. As described in U.S. Pat. No. 6,237,551 to Macor et al., issued on May 29, 2001, a “late intake” Miller cycle may be implemented in such a cam arrangement by modifying the shape of the cam to overlap the actuation of the intake valve with the start of the compression stroke of the piston. 
   However, a late intake Miller cycle may be undesirable under certain operating conditions. For example, a diesel engine operating on a late intake Miller cycle will be difficult to start when the engine is cold. This difficulty arises because diesel fuel combustion is achieved when an air and fuel mixture is pressurized to a certain level. Implementation of the late intake Miller cycle reduces the amount of air and the amount of compression within each combustion chamber. The reduced compression combined with the reduced temperature of the engine results in a lower maximum pressure level of the air and fuel mixture. Thus, achieving combustion in a cold engine operating on a late intake Miller cycle may prove difficult. 
   As noted above, the actuation timing of a valve system driven by a cam arrangement is determined by the shape of the driving cam. Because the shape of the cam is fixed, this arrangement is inflexible and may not be changed during the operation of the engine. In other words, a conventional cam driven valve actuation system may not be modified to account for different operating conditions of the engine. 
   One possible solution to controlling the timing of the valves is the use of a hydraulic system or at least a partially hydraulic system. However, the use of hydraulic systems can lead to other problems. In order for hydraulic systems to provide consistent, reliable performance, hydraulic pressure must be controlled or maintained at same minimum pressure. Further, fluid viscosity must be taken into account to properly operate the system over a wide range of temperatures. 
   The intake valve actuation system of the present invention solves one or more of the problems set forth above. 
   SUMMARY OF THE INVENTION 
   In one aspect, the present invention is directed to an engine valve actuation system that includes an intake valve that is moveable between a first position to prevent a flow of fluid and a second position to allow a flow of fluid. A cam assembly is configured to move the intake valve between the first position and the second position. A fluid actuator is configured to selectively prevent the intake valve from moving to the first position. A source of fluid is in fluid communication with the fluid actuator. A directional control valve is configured to control a flow of fluid between the source of fluid and the fluid actuator. A fluid passageway connects the directional control valve with the fluid actuator. An accumulator is in fluid communication with the fluid passageway. A restricted orifice is disposed between the accumulator and the fluid passageway to restrict a flow of fluid between the accumulator and the fluid passageway. 
   In another aspect, the present invention is directed to a method of controlling an engine having a piston moveable through an intake stroke followed by a compression stroke. A cam is rotated to move an intake valve between a first position to prevent a flow of fluid and a second position to allow a flow of fluid during the intake stroke of the piston. Fluid is directed through a directional control valve and a fluid passageway to a fluid actuator associated with the intake valve when the intake valve is moved from the first position. The directional control valve is actuated to selectively prevent fluid from flowing through the fluid passageway from the fluid actuator to thereby prevent the intake valve from moving to the first position during at least a portion of the compression stroke of the piston. Fluid is directed from the fluid passageway through a restricted orifice to an accumulator to inhibit oscillations in the fluid actuator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic cross-sectional view of an exemplary embodiment of an internal combustion engine; 
       FIG. 2  is a diagrammatic cross-sectional view of a cylinder and valve actuation assembly in accordance with an exemplary embodiment of the present invention; 
       FIG. 3   a  is a schematic and diagrammatic representation of a fluid supply system for a fluid actuator for an engine valve in accordance with an exemplary embodiment of the present invention; 
       FIG. 3   b  is a schematic and diagrammatic representation of another embodiment of a fluid supply system for a fluid actuator for an engine valve in accordance with an exemplary embodiment of the present invention; 
       FIG. 3   c  is a schematic and diagrammatic representation of another embodiment of a fluid supply system for a fluid actuator for an engine valve in accordance with an exemplary embodiment of the present invention; 
       FIG. 4   a  is a schematic and diagrammatic representation of a fluid supply system for a fluid actuator in accordance with another exemplary embodiment of the present invention; 
       FIG. 4   b  is a schematic and diagrammatic representation of a fluid supply system for a fluid actuator in accordance with another exemplary embodiment of the present invention; 
       FIG. 5  is a graphic illustration of an exemplary valve actuation as a function of engine crank angle for an engine operating in accordance with the present invention; 
       FIG. 6  is a cross-sectional view of an exemplary embodiment of a check valve for a fluid actuator in accordance with an embodiment of the present invention; 
       FIG. 7  is a cross-sectional view of an exemplary embodiment of an accumulator for a fluid actuator in accordance with an embodiment of the present invention; 
       FIG. 8  is a side sectional view of a fluid actuator in accordance with an exemplary embodiment of the present invention. 
       FIG. 9  is a graphic illustration of rail pressures of engine valve actuation systems with and without an exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   An exemplary embodiment of an internal combustion engine  20  is illustrated in FIG.  1 . For the purposes of the present disclosure, engine  20  is depicted and described as a four stroke diesel engine. One skilled in the art will recognize, however, that engine  20  may be any other type of internal combustion engine, such as, for example, a gasoline or natural gas engine. 
   As illustrated in  FIG. 1 , engine  20  includes an engine block  28  that defines a plurality of cylinders  22 . A piston  24  is slidably disposed within each cylinder  22 . In the illustrated embodiment, engine  20  includes six cylinders  22  and six associated pistons  24 . One skilled in the art will readily recognize that engine  20  may include a greater or lesser number of pistons  24  and that pistons  24  may be disposed in an “in-line” configuration, a “V” configuration, or any other conventional configuration. 
   As also shown in  FIG. 1 , engine  20  includes a crankshaft  27  that is rotatably disposed within engine block  28 . A connecting rod  26  connects each piston  24  to crankshaft  27 . Each piston  24  is coupled to crankshaft  27  so that a sliding motion of piston  24  within the respective cylinder  22  results in a rotation of crankshaft  27 . Similarly, a rotation of crankshaft  27  will result in a sliding motion of piston  24 . 
   Engine  20  also includes a cylinder head  30 . Cylinder head  30  defines an intake passageway  41  that leads to at least one intake port  36  for each cylinder  22 . Cylinder head  30  may further define two or more intake ports  36  for each cylinder  22 . 
   An intake valve  32  is disposed within each intake port  36 . Intake valve  32  includes a valve element  40  that is configured to selectively block intake port  36 . As described in greater detail below, each intake valve  32  may be actuated to move or “lift” valve element  40  to thereby open the respective intake port  36 . In a cylinder  22  having a pair of intake ports  36  and a pair of intake valves  32 , the pair of intake valves  32  may be actuated by a single valve actuation assembly or by a pair of valve actuation assemblies. 
   Cylinder head  30  also defines at least one exhaust port  38  for each cylinder  22 . Each exhaust port  38  leads from the respective cylinder  22  to an exhaust passageway  43 . Cylinder head  30  may further define two or more exhaust ports  38  for each cylinder  22 . 
   An exhaust valve  34  is disposed within each exhaust port  38 . Exhaust valve  34  includes a valve element  48  that is configured to selectively block exhaust port  38 . As described in greater detail below, each exhaust valve  34  may be actuated to move or “lift” valve element  48  to thereby open the respective exhaust port  38 . In a cylinder  22  having a pair of exhaust ports  38  and a pair of exhaust valves  34 , the pair of exhaust valves  34  may be actuated by a single valve actuation assembly or by a pair of valve actuation assemblies. 
     FIG. 2  illustrates an exemplary embodiment of one cylinder  22  of engine  20 . As shown, cylinder head  30  defines a pair of intake ports  36  connecting intake passageway  41  to cylinder  22 . Each intake port  36  includes a valve seat  50 . One intake valve  32  is disposed within each intake port  36 . Valve element  40  of intake valve  32  is configured to engage valve seat  50 . When intake valve  32  is in a closed position, valve element  40  engages valve seat  50  to close intake port  36  and block fluid flow relative to cylinder  22 . When intake valve  32  is lifted from the closed position, intake valve  32  allows a flow of fluid relative to cylinder  22 . 
   Similarly, cylinder head  30  may define two or more exhaust ports  38  (only one of which is illustrated in  FIG. 1 ) that connect cylinder  22  with exhaust passageway  43 . One exhaust valve  34  is disposed within each exhaust port  38 . A valve element  48  of each exhaust valve  34  is configured to close exhaust port  38  when exhaust valve  34  is in a closed position and block fluid flow relative to cylinder  22 . When exhaust valve  34  is lifted from the closed position, exhaust valve  32  allows a flow of fluid relative to cylinder  22 . 
   As also shown in  FIG. 2 , a valve actuation assembly  44  is operatively associated with intake valves  32 . Valve actuation assembly  44  includes a bridge  54  that is connected to each valve element  40  through a pair of valve stems  46 . A spring  56  may be disposed around each valve stem  46  between cylinder head  30  and bridge  54 . Spring  56  acts to bias both valve elements  40  into engagement with the respective valve seat  50  to thereby close each intake port  36 . 
   Valve actuation assembly  44  also includes a rocker arm  64 . Rocker arm  64  is configured to pivot about a pivot  66 . One end  68  of rocker arm  64  is connected to bridge  54 . The opposite end of rocker arm  64  is connected to a cam assembly  52 . In the exemplary embodiment of  FIG. 2 , cam assembly  52  includes a cam  60  having a cam lobe and mounted on a cam shaft, a push rod  61 , and a cam follower  62 . One skilled in the art will recognize that cam assembly  52  may have other configurations, such as, for example, where cam  60  acts directly on rocker arm  64 . 
   Valve actuation assembly  44  may be driven by cam  60 . Cam  60  is connected to crankshaft  27  so that a rotation of crankshaft  27  induces a corresponding rotation of cam  60 . Cam  60  may be connected to crankshaft  27  through any means readily apparent to one skilled in the art, such as, for example, through a gear reduction assembly (not shown). As one skilled in the art will recognize, a rotation of cam  60  will cause cam follower  62  and associated push rod  61  to periodically reciprocate between an upper and a lower position. 
   The reciprocating movement of push rod  61  causes rocker arm  64  to pivot about pivot  66 . When push rod  61  moves in the direction indicated by arrow  58 , rocker arm  64  will pivot and move bridge  54  in the opposite direction. The movement of bridge  54  causes each intake valve  32  to lift and open intake ports  36 . As cam  60  continues to rotate, springs  56  will act on bridge  54  to return each intake valve  32  to the closed position. 
   In this manner, the shape and orientation of cam  60  controls the timing of the actuation of intake valves  32 . As one skilled in the art will recognize, cam  60  may be configured to coordinate the actuation of intake valves  32  with the movement of piston  24 . For example, intake valves  32  may be actuated to open intake ports  36  when piston  24  is withdrawing within cylinder  22  to allow air to flow from intake passageway  41  into cylinder  22 . 
   A similar valve actuation assembly may be connected to exhaust valves  34 . A second cam (not shown) may be connected to crankshaft  27  to control the actuation timing of exhaust valves  34 . Exhaust valves  34  may be actuated to open exhaust ports  38  when piston  24  is advancing within cylinder  22  to allow exhaust to flow from cylinder  22  into exhaust passageway  43 . 
   As shown in  FIG. 2 , valve actuation assembly  44  also includes a fluid actuator  70 . Fluid actuator  70  includes an actuator cylinder  72  that defines an actuator chamber  76 . An actuator piston  74  is slidably disposed within actuator cylinder  72  and is connected to an actuator rod  78 . A return spring  75  (referring to  FIG. 8 ) may act on actuator piston  74  to return actuator piston  74  to a home position. Actuator rod  78  is engageable with an end  68  of rocker arm  64 . 
   A fluid line  80  is connected to actuator chamber  76 . Pressurized fluid may be directed through fluid line  80  into actuator chamber  76  to move actuator piston  74  within actuator cylinder  72 . Movement of actuator piston  74  causes actuator rod  78  to engage end  68  of rocker arm  64 . Fluid may be introduced to actuator chamber  76  when intake valves  32  are in the open position to move actuator rod  78  into engagement with rocker arm  64  to thereby hold intake valves  32  in the open position. Alternatively, fluid may be introduced to actuator chamber  76  when intake valves  32  are in the closed position to move actuator rod  78  into engagement with rocker arm  64  and pivot rocker arm  64  about pivot  66  to thereby open intake valves  32 . 
   As illustrated in  FIGS. 1 and 3 , a source of fluid  84 , which is connected to a tank  87 , supplies pressurized fluid to fluid actuator  70 . Tank  87  may store any type of fluid readily apparent to one skilled in the art, such as, for example, hydraulic fluid, fuel, or transmission fluid. Source of fluid  84  may be part of a lubrication system, such as typically accompanies an internal combustion engine. Such a lubrication system may provide pressurized oil having a pressure of, for example, less than 700 KPa (100 psi) or, more particularly, between about 210 KPa and 620 KPa (30 psi and 90 psi). Alternatively, the source of fluid may be a pump configured to provide oil at a higher pressure, such as, for example, between about 10 MPa and 35 MPa (1450 psi and 5000 psi). 
   A fluid supply system  79  connects source of fluid  84  with fluid actuator  70 . In the exemplary embodiment of  FIG. 3   a , source of fluid  84  is connected to a fluid rail  86  through fluid line  85 . A control valve  82  is disposed in fluid line  85 . Control valve  82  may be opened to allow pressurized fluid to flow from source of fluid  84  to fluid rail  86 . Control valve  82  may be closed to prevent pressurized fluid from flowing from source of fluid  84  to fluid rail  86 . 
   As illustrated in  FIG. 3   a , fluid rail  86  supplies pressurized fluid from source of fluid  84  to a series of fluid actuators  70 . Each fluid actuator  70  may be associated with either the intake valves  32  or the exhaust valves  34  of a particular engine cylinder  22  (referring to FIG.  1 ). Fluid lines  80  direct pressurized fluid from fluid rail  86  into the actuator chamber  76  of each fluid actuator  70 . 
   A directional control valve  88  may be disposed in each fluid line  80 . Each directional control valve  88  may be opened to allow pressurized fluid to flow between fluid rail  86  and actuator chamber  76 . Each directional control valve  88  may be closed to prevent pressurized fluid from flowing between fluid rail  86  and actuator chamber  76 . Directional control valve  88  may be normally biased into a closed position and actuated to allow fluid to flow through directional control valve  88 . Alternatively, directional control valve  88  may be normally biased into an open position and actuated to prevent fluid from flowing through directional control valve  88 . One skilled in the art will recognize that directional control valve  88  may be any type of controllable valve, such as, for example a two coil latching valve. 
   One skilled in the art will recognize that fluid supply system  79  may have a variety of different configurations. For example, as illustrated in  FIG. 3   b , a restrictive orifice  83  may be positioned in fluid line  85  between source of fluid  84  and a first end of fluid rail  86 . Control valve  82  may be connected to an opposite end of fluid rail  86  and lead to tank  87 . Control valve  82  may be opened to allow a flow of fluid through restrictive orifice  83  and fluid rail  86  to tank  87 . Control valve  82  may be closed to allow a build up of pressure in the fluid within fluid rail  86 . 
   In  FIG. 3   c , another exemplary embodiment is shown. A main gallery  89  is a fluid rail positioned between the source of fluid  84  and the fluid rail  86 . The main gallery  89  may provide fluid to various other engines systems (not shown). Fluid line  85  fluidly connects the main gallery  89  and the fluid rail  86 . Further, a fluid line check valve  91  and a bleed orifice  97  are disposed in parallel in the fluid line  85 , between the main gallery  89  and fluid rail  86 . The fluid line check valve  91  allows fluid flow to travel from the main gallery  89  to the fluid rail  86  but prevents fluid flow in the opposite direction. It should be noted that fluid line check valve  91  could also be an integral port of either the main gallery  89  or the fluid rail  86 . 
   In addition, as illustrated in  FIG. 4   a , fluid supply system  79  may include a check valve  94  placed in parallel with directional control valve  88  between control valve  82  and fluid actuator  70 . Check valve  94  may be configured to allow fluid to flow in the direction from control valve  82  to fluid actuator  70 . 
   As shown in  FIG. 6 , check valve  94  may be a poppet style check valve. Check valve  94  includes a housing  121  that defines an inlet passageway  123  and includes a seat  124 . A poppet  122  is adapted to sealingly engage seat  124 . A spring  120  acts on poppet  122  to engage poppet  122  with seat  124 . Poppet  122  may be disengaged with seat  124  to create a fluid passage between inlet passageway  123  and a fluid outlet  125 . 
   Check valve  94  will open when poppet  122  is exposed to a pressure differential that is sufficient to overcome the force of spring  120 . Poppet  122  will disengage from seat  124  when a force exerted by pressurized fluid in inlet passageway  123  is greater than the combination of a force exerted by fluid in fluid outlet  125  and the force of spring  120 . If, however, the combination of the force exerted by fluid in fluid outlet  125  and the force of spring  120  is greater than the force exerted by the pressurized fluid in inlet passageway  123 , poppet  122  will remain engaged with seat  124 . In this manner, check valve  94  may ensure that fluid flows only from control valve  82  to fluid actuator  70 , i.e. from inlet passageway  123  to fluid outlet  125 . One skilled in the art will recognize that other types of check valves, such as, for example, a ball-type check valve or a plate-type check valve, may also be used. 
   As also shown in  FIG. 4   a , fluid supply system  79  may include an air bleed valve  96 . Air bleed valve  96  may be any device readily apparent to one skilled in the art as capable of allowing air to escape a hydraulic system. For example, air bleed valve  96  may be a spring biased ball valve that allows air to flow through the valve, but closes when exposed to fluid pressure. 
   In addition, a snubbing valve  98  may be disposed in fluid line  81  leading to actuator chamber  76 . Snubbing valve  98  may be configured to restrict the flow of fluid through fluid line  81 . For example, snubbing valve  98  may be configured to decrease the rate at which fluid exits actuator chamber  76  to thereby slow the rate at which intake valve  32  closes. 
   Fluid supply system  79  may also include an accumulator  95 . An exemplary embodiment of accumulator  95  is illustrated in FIG.  7 . As shown, accumulator  95  includes a housing  126  that defines a chamber  128 . A piston  130  is slidably disposed in chamber  128 . A spring  132  is disposed in housing  126  and acts on piston  130  to move piston  130  relative to housing  126  to minimize the size of chamber  128 . One skilled in the art may recognize that other types of accumulators, such as for example, a bladder-type accumulator, may also be used. 
   As also shown in  FIG. 7. a  restrictive orifice  93  may be disposed in an inlet  134  to accumulator  95 . Restrictive orifice  93  is configured to restrict the rate at which fluid may flow between accumulator chamber  128  and inlet  134 . As described in greater detail below, the combination of accumulator  95  and restrictive orifice  93  may act to dampen pressure oscillations in actuator chamber  76  and fluid line  80 , which may cause actuator piston  74  to oscillate. 
   The components of fluid actuator  70  may be contained within a single housing that is mounted on engine  20  to allow actuator rod  78  to engage rocker arm  64 . Alternatively, the components of fluid actuator  70  may be contained in separate housings. One skilled in the art will recognize that space considerations will impact the location of the components of fluid actuator  70  relative to engine  20 . 
   An exemplary embodiment of a housing  140  for fluid actuator  70  is illustrated in FIG.  8 . Housing  140  includes an inlet  144 . In the illustrated embodiment, inlet  144  includes a first opening  146  that leads to a first fluid passageway  148  and a second opening  150  that leads to a second fluid passageway  152 . Each of the first and second fluid passageways  148  and  152  lead from inlet  144  to a third fluid passageway  154 . One skilled in the art will recognize that inlet  144  may have alternative configurations. For example, inlet  144  may include a single opening that leads to a single passageway that subsequently divides into first and second passageways  148  and  152 . 
   Check valve  94  may be disposed in first fluid passageway  148  between inlet  144  and third fluid passageway  154 . As discussed previously, check valve  94  may allow fluid to flow from inlet  144  to third fluid passageway  154 . Check valve  94  may prevent fluid from flowing from third fluid passageway  154  to inlet  144 . 
   Directional control valve  88  (referring to  FIG. 4   a ) may be disposed proximate second opening  150 . Direction control valve  88  controls the flow of fluid through second fluid passageway  152 . Directional control valve  88  may be opened to allow fluid to flow in either direction through second fluid passageway  152 . 
   Accumulator  95  may be disposed proximate third fluid passageway  154  so that inlet  134  of accumulator  95  opens to third fluid passageway  154 . This allows fluid from either first or second fluid passageway  148  or  152  to flow through inlet  134  to accumulator  95 . Restricted orifice  93  restricts the amount of fluid that may flow from third fluid passageway  154  into accumulator  95 . 
   As also illustrated in  FIG. 8 , snubbing valve  98  is positioned between third fluid passageway  154  and actuator chamber  76 . Snubbing valve  98  controls the rate at which fluid may flow into and out of actuator chamber  76 . Snubbing valve  98  may be configured to allow a high rate of fluid flow into actuator chamber  76  when piston  74  moves away from a home position. Similarly, snubbing valve  98  may allow a high rate of fluid flow from actuator chamber  76  when piston  74  starts moving from the end position towards the home position. Snubbing valve  98  may slow the rate of fluid flow from actuator chamber  76  when piston  74  approaches the home position. In this manner, snubbing valve  98  may reduce the impact speed of intake valve  32  with valve seat  50 . 
   Another exemplary embodiment of a fluid supply system  79  is illustrated in  FIG. 4   b . As shown, fluid supply system  79  includes a source of high pressure fluid  92 . Directional control valve  88  is configured to selectively connect either source of pressure fluid  84  or source of high pressure fluid  92  with fluid line  81 . In this manner, either low or high pressure fluid may be directed to fluid actuator  70  to meet the needs of the current operating conditions. Directional control valve  88  may be normally biased into a position where source of fluid  84  is connected with fluid line  81 . 
   As shown in  FIG. 1 , a controller  100  is connected to each valve actuation assembly  44  and to control valve  82 . Controller  100  may include an electronic control module that has a microprocessor and a memory. As is known to those skilled in the art, the memory is connected to the microprocessor and stores an instruction set and variables. Associated with the microprocessor and part of electronic control module are various other known circuits such as, for example, power supply circuitry, signal conditioning circuitry, and solenoid driver circuitry, among others. 
   Controller  100  may be programmed to control one or more aspects of the operation of engine  20 . For example, controller  100  may be programmed to control the valve actuation assembly, the fuel injection system, and any other function readily apparent to one skilled in the art. Controller  100  may control engine  20  based on the current operating conditions of the engine and/or instructions received from an operator. 
   Controller  100  may be further programmed to receive information from one or more sensors operatively connected with engine  20 . Each of the sensors may be configured to sense one or more operational parameters of engine  20 . For example, with reference to  FIG. 3   a , a sensor  90  may be connected with fluid supply system  79  to sense the temperature of the fluid within fluid supply system  79 . One skilled in the art will recognize that many other types of sensors may be used in conjunction with or independently of sensor  90 . For example, engine  20  may be equipped with sensors configured to sense one or more of the following: the temperature of the engine coolant, the temperature of the engine, the ambient air temperature, the engine speed, the load on the engine, and the intake air pressure. 
   Engine  20  may be further equipped with a sensor configured to monitor the crank angle of crankshaft  27  to thereby determine the position of pistons  24  within their respective cylinders  22 . The crank angle of crankshaft  27  is also related to actuation timing of intake valves  32  and exhaust valves  34 . An exemplary graph  102  indicating the relationship between valve actuation timing and crank angle is illustrated in FIG.  5 . As shown by graph  102 , exhaust valve actuation  104  is timed to substantially coincide with the exhaust stroke of piston  24  and intake valve actuation  106  is timed to substantially coincide with the intake stroke of piston  24 . 
   Industrial Applicability 
   Based on information provided by the engine sensors, controller  100  may operate each valve actuation assembly  44  to selectively implement a late intake Miller cycle for each cylinder  22  of engine  20 . Under normal operating conditions, implementation of the late intake Miller cycle will increase the overall efficiency of the engine  20 . Under some operating conditions, such as, for example, when engine  20  is cold, controller  100  may operate engine  20  on a conventional diesel cycle. 
   The following discussion describes the implementation of a late intake Miller cycle in a single cylinder  22  of engine  22 . One skilled in the art will recognize that the system of the present invention may be used to selectively implement a late intake Miller cycle in all cylinders of engine  22  in the same or a similar manner. In addition, the system of the present invention may be used to implement other valve actuation variations on the conventional diesel cycle, such as, for example, an exhaust Miller cycle. 
   When engine  20  is operating under normal operating conditions, controller  100  implements a late intake Miller cycle by selectively actuating fluid actuator  70  to hold intake valve  32  open for a first portion of the compression stroke of piston  24 . This may be accomplished by moving control valve  82  and directional control valve  88  to the open positions when piston  24  starts an intake stroke. This allows pressurized fluid to flow from source of fluid  84  through fluid rail  86  and into actuator chamber  76 . The force of the fluid entering actuator chamber  76  moves actuator piston  74  so that actuator rod  78  follows end  68  of rocker arm  64  as rocker arm  64  pivots to open intake valves  32 . The distance and rate of movement of actuator rod  78  will depend upon the configuration of actuator chamber  76  and fluid supply system  79 . When actuator chamber  76  is filled with fluid and rocker arm  64  returns intake valves  32  from the open position to the closed position, actuator rod  78  will engage end  68  of rocker arm  64 . 
   Fluid supply system  79  may be configured to supply a flow rate of fluid to fluid actuator  70  to fill actuator chamber  76  before cam  60  returns intake valves  32  to the closed position. In the embodiment of fluid supply system  79  illustrated in  FIG. 4   a , pressurized fluid may flow through both directional control valve  88  and check valve  94  into actuator chamber  76 . Alternatively, directional control valve  88  may remain in a closed position and fluid may flow through check valve  94  into actuator cylinder  76 . 
   When actuator chamber  76  is filled with fluid, controller  100  may close directional control valve  88 . This prevents fluid from escaping from actuator chamber  76 . As cam  60  continues to rotate and springs  56  urge intake valves  32  towards the closed position, actuator rod  78  will engage end  68  of rocker arm and prevent intake valves  32  from closing. As long as directional control valve  88  remains in the closed position, the trapped fluid in actuator chamber  76  will prevent springs  56  from returning intake valves  32  to the closed position. Thus, fluid actuator  70  will hold intake valves  32  in the open position, independently of the action of cam assembly  52 . 
   When actuator rod  78  engages rocker arm  64  to prevent intake valves  32  from closing, the force of springs  56  acting through rocker arm  64  may cause an increase in the pressure of the fluid within fluid system  79 . In response to the increased pressure, a flow of fluid will be throttled through restricted orifice  93  into chamber  128  of accumulator  95 . The throttling of the fluid through restricted orifice  93  will dissipate energy from the fluid within fluid system  79 . 
   The force of the fluid entering accumulator  95  will act to compress spring  132  and move piston  130  to increase the size of chamber  128 . When the pressure within fluid system  79  decreases, spring  130  will act on piston  130  to force the fluid in chamber  128  back through restricted orifice  93 . The flow of fluid through restricted orifice  93  into third fluid passageway  154  will also dissipate energy from fluid system  79 . 
   Restricted orifice  93  and accumulator  95  will therefore dissipate energy from the fluid system  79  as fluid flows into and out of accumulator  95 . In this manner, restricted orifice  93  and accumulator may absorb or reduce the impact of pressure fluctuations within fluid system  79 , such as may be caused by the impact of rocker arm  64  on actuator rod  78 . By absorbing or reducing pressure fluctuations, restricted orifice  93  and accumulator  95  may act to inhibit or minimize oscillations in actuator rod  78 . 
   Controller  100  may close intake valves  32  by opening directional control valve  88 . This allows the pressurized fluid to flow out of actuator chamber  76 . The force of springs  56  forces the fluid from actuator chamber  76 , thereby allowing actuator piston  74  to move within actuator cylinder  72 . This allows rocker arm  64  to pivot so that intake valves  32  are moved to the closed position. Snubbing valve  98  may restrict the rate at which fluid exits actuator chamber  76  to reduce the velocity at which intake valves  32  are closed. This may prevent valve elements  40  from being damaged when closing intake ports  36 . 
   An exemplary late intake closing  108  is illustrated in FIG.  5 . As shown, the intake valve actuation  106  is extended into a portion of the compression stroke of piston  24 . This allows some of the air in cylinder  22  to escape. The amount of air allowed to escape cylinder  22  may be controlled by adjusting the crank angle at which directional control valve  88  is opened. Directional control valve  88  may be closed at an earlier crank angle to decrease the amount of escaping air or at a later crank angle to increase the amount of escaping air. 
   As noted previously, certain operating conditions may require that engine  20  be operated on a conventional diesel cycle instead of the late intake Miller cycle described above. These types of operating conditions may be experienced, for example, when engine  20  is first starting or is otherwise operating under cold conditions. The described valve actuation system  44  allows for the selective disengagement of the late intake Miller cycle. 
   In the exemplary embodiment of  FIG. 3   a , controller  100  may disengage the late intake Miller cycle by closing control valve  82 . Control valve  82  may be closed when controller  100  receives sensory input indicating that engine  20  is starting or is operating under cold conditions. Closing control valve  82  prevents fluid from flowing from source of fluid  84  into actuator chamber  76 . Without the introduction of fluid to actuator chamber  76 , fluid actuator  70  will not prevent intake valves  32  from returning to the closed position in response to the force of springs  56 . 
   Thus, when control valve  82  is closed, intake valves  32  will follow a conventional diesel cycle as governed by cam  60 . As shown in  FIG. 5 , intake valve actuation  106  will follow a conventional closing  110 . In the conventional closing  110 , the closing of intake valves  32  substantially coincides with the end of the intake stroke of piston  24 . When intake valves  32  close at the end of the intake stroke, no air will be forced from cylinder  22  during the compression stroke. This results in piston  24  compressing the fuel and air mixture to a higher pressure, which will facilitate diesel fuel combustion. This is particularly beneficial when engine  20  is operating in cold conditions. 
   In the exemplary embodiment of  FIG. 3   b , controller  100  may disengage the Miller cycle by opening control valve  82 . Control valve  82  may be opened when controller  100  receives sensory input indicating that engine  20  is starting or is operating under cold conditions. Opening control valve  82  allows fluid to flow through restrictive orifice  83  and fluid rail  86  to tank  87 . Opening control valve  82  may therefore reduce the pressure of the fluid within fluid rail  86 . The decreased pressure of the fluid within fluid rail  86  may not generate a force having a force great enough to move actuator piston  74 . Thus, fluid actuator  70  will not engage intake valve  32  to prevent intake valve from closing. Accordingly, engine  20  will operate on a conventional diesel cycle as governed by cam  60 . 
   Opening control valve  82  may also increase the responsiveness of valve actuator  70  when engine  20  is starting or operating under cold conditions. If the fluid within fluid rail  86  is cold, the fluid will have an increased viscosity. The increased viscosity of the fluid may decrease the rate at which the fluid may flow into and out of actuator chamber  76  and thereby impact the operation of valve actuator  70 . By opening control valve  82 , the cold fluid may be replaced by warmer fluid from source of fluid  84 . This may decrease the viscosity of the fluid within fluid rail  86 , which may increase the responsiveness of valve actuator  70  when control valve  82  is closed to operate engine  20  on the Miller cycle. 
   Restrictive orifice  83  may ensure that the pressure of the fluid upstream of restrictive orifice  83 , i.e. between source of fluid  84  and restrictive orifice  83 , does not decrease when control valve  82  is opened. Restrictive orifice  83  may create a smaller opening than is created by the opening of control valve  82 . In other words, the opening of control valve  82  allows fluid to flow out of fluid rail  86  at a faster rate than restrictive orifice  83  allows fluid to flow into fluid rail  86 . This creates a pressure drop over restrictive orifice  83  where the pressure of the fluid on the upstream side of restrictive orifice  83  will be greater that the pressure of the fluid in fluid rail  86 . Thus, opening control valve  82  will not impact the pressure of fluid upstream of restrictive orifice  83 . 
   The exemplary embodiment illustrated in  FIG. 3   c  helps to control the hydraulic pressure within fluid rail  86 . Generally, the rail pressure of fluid rail  86  varies as the intake valves  32  are actuated. Specifically, actuator chamber  76  is filled with fluid, thereby drawing fluid from fluid rail  86 , when intake valve  32  is actuated as a result of cam  60 . This draw down the fluid in fluid rail  86  and causes a slight decrease in pressure as the source of fluid  84  works to make up the “lost” volume. When intake valve  32  is closed, regardless of whether the closing was delayed or not, fluid is expelled from actuator chamber  76 , as actuator rod  78  returns to its original position. The fluid exiting actuator chamber  76  returns to fluid rail  86 , which causes a temporary increase in pressure of the fluid rail  86 . However, due to the fact that the fluid supply system  79  is a low pressure system, a “momentum” phenomenon occurs in which the fluid being expelled from actuator chamber  76  actually causes a fluid flow from fluid rail  86  back towards main gallery  89 . This can result in a substantial decrease in pressure in fluid rail  86 . The higher the engine speed and load, the worse the potential pressure drop. At high engine speed and load, the pressure in fluid rail  86  can actually go below zero, creating a momentary vacuum effect. An average pressure of fluid rail  86  is shown in  FIG. 9   a  as represented by first pressure curve  158 . The pressure drop in the fluid rail  86  can prevent a following fluid actuator  76  from getting completely filled with fluid meaning that the actuator rod  78  does not follow the end of rocker arm  68  down as it is actuated by cam  60 . This can significantly impact proper operation of the fluid actuator  70 . The pressure drop may be further exasperated during cold conditions, due to the viscous nature of the fluid at cold temperatures. 
   In order to control the pressure drops within fluid rail  86 , the fluid line check valve  91  prevents the back flow of fluid from fluid rail  86  to main gallery  89 , resulting in a substantial increase in the average fluid pressure within fluid rail  86 . The pressure increase can be seen in  FIG. 9   b , as represented by second pressure curve  160 . Preferably fluid line check valve  91  is positioned closer to main gallery  89  than to fluid rail  86 . By positioning fluid line check valve in such a manner, the effective volume of fluid rail  86  is essentially increased to include the part of fluid line  85  after fluid line check valve  91 . The increased volume helps absorb the pressure waves created by the movement of fluid in to and out of fluid actuator  76 . 
   The bleed orifice  97  may be added to help with cold start conditions. The bleed orifice  97  allows fluid to drain from fluid supply system  79  when the engine is off. By draining fluid supply system  79 , cold, viscous fluid is not caught within fluid supply system  79  at engine start, allowing the engine to fill the fluid supply system  79  with warmer, less viscous fluid quicker. Bleed orifice  97  must be sized appropriately so that it does not allow any significant back flow from fluid rail  86  to main gallery  89 , thereby defeating the utility of the fluid line check valve  91 . 
   By implementing fluid line check valve  91 , other costlier alternatives are avoided. For example, a higher pressure fluid system could be implemented but this would decrease the engine&#39;s fuel economy. Further, a separate pump loop, dedicated to the fluid supply system  79 , could be utilized but this would add parts and substantial cost to the engine. The fluid line check valve provides cheap and effective way to limit the impact of pressure waves within the fluid rail  86 . 
   As will be apparent from the foregoing description, the present invention provides an engine valve actuation system that may selectively alter the timing of the intake and/or exhaust valve actuation of an internal combustion engine. The actuation of the engine valves may be based on sensed operating conditions of the engine. For example, the engine valve actuation system may implement a late intake Miller cycle when the engine is operating under normal operating conditions. The late intake Miller cycle may be disengaged when the engine is operating under adverse operating conditions, such as when the engine is cold. Thus, the present invention provides a flexible engine valve actuation system that provides for both enhanced cold starting capability and fuel efficiency gains. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the engine valve actuation system of the present invention without departing from the scope of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the invention being indicated by the following claims and their equivalents.