Patent Publication Number: US-2006011159-A1

Title: System and method for controlling engine operation

Description:
TECHNICAL FIELD  
      The present invention is directed to a system and method for controlling the operation of an engine. More particularly, the present invention is directed to a system and method for controlling the actuation timing of engine valves.  
     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 oxides of nitrogen (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, exhaust gas recirculation, water injection, fuel injection timing, and fuel formulations, have been found to reduce the amount of emissions generated during the operation of an engine. Aftertreatments, such as, for example, traps and catalysts have been found to effectively remove emissions from an exhaust flow. 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, while valve actuation timing adjustments may provide efficiency benefits, these actuation timing adjustments may also result in detrimental engine performance under certain operating conditions. For example, a late intake Miller cycle may be inefficient when the engine is starting, operating under cold conditions, or experiencing a transient condition, such as a sudden increase in engine load. This detrimental engine performance is caused by a decrease in the mass of air flowing through the engine. Especially under cold ambient conditions, the delayed start of compression may lead to insufficient cylinder temperatures to support good combustions and startability.  
      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 type of arrangement is inflexible and may only be changed during the operation of the engine through the use of complex mechanical mechanisms.  
      The engine operation control system and method 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 a method of operating an engine that has a cylinder, an intake valve associated with the cylinder and moveable between a first position where the intake valve prevents a flow of fluid to the cylinder and a second position where the intake valve allows a flow of fluid to the cylinder, a cam assembly connected to the intake valve to move the intake valve between the first and second positions, and an actuator connected to the intake valve. At least one operating parameter of the engine is sensed. The engine is operated in a first mode in response to the sensed operating parameter being at one of a predetermined first set of conditions. In the first mode, the cam assembly begins to move the intake valve from the first position toward the second position when the piston is at or near a top dead center position of an intake stroke and the cam assembly returns the intake valve to the first position when the piston is at or near a bottom dead center position of the intake stroke. The engine is operated in a second mode in response to the sensed operating parameter being at one of a predetermined second set of conditions. In the second mode, the cam assembly begins to move the intake valve from the first position toward the second position when the piston is at or near a top dead center position of an intake stroke and the actuator prevents the intake valve from returning to the first position in response to the cam assembly.  
      In another aspect, the present invention is directed to an engine that includes an engine block defining a cylinder. A piston is slidably disposed within the cylinder and is moveable between a top dead center position and a bottom dead center position. An intake valve is operatively associated with the cylinder and is moveable between a first position where the intake valve prevents fluid from flowing to the cylinder and a second position where a flow of fluid is allowed to enter the cylinder. A cam assembly is connected to the intake valve to move the intake valve between the first and second positions. An actuator is configured to selectively prevent the intake valve from returning to the first position. A sensor is configured to sense at least one operating parameter of the engine. A controller is operable to selectively operate the engine in a first mode or a second mode based on the sensed operating parameter. In the first mode, the cam assembly begins to move the intake valve from the first position toward the second position when the piston is at or near the top dead center position of an intake stroke and the cam assembly returns the intake valve to the first position when the piston is at or near a bottom dead center position of the intake stroke. In the second mode, the cam assembly begins to move the intake valve from the first position toward the second position when the piston is at or near the top dead center position of an intake stroke and the actuator prevents the intake valve from returning to the first position in response to the cam assembly. 
    
    
     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  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. 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 intake valve actuation as a function of engine crank angle in accordance with the present invention; and  
       FIG. 6  is a flowchart illustrating an exemplary method of controlling the operation of an engine in accordance with 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  between a top dead center position and a bottom dead center position within cylinder  22 .  
      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  from a first, or closed, position where valve element  40  prevents a flow of fluid through the respective intake port  36  to a second, or open, position where valve element  40  allows a flow of fluid through the respective intake port  36 . The intake valves  32  for each cylinder  22  may be actuated in unison or independently.  
      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 lift valve element  48  to thereby open the respective exhaust port  38 . The exhaust valves  34  for each cylinder  22  may be actuated in unison or independently.  
       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 blocks 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. 1 , a series of valve actuation assemblies  44  are operatively associated with each intake valve  32  and exhaust valve  34 . Each valve actuation assembly  44  is operable to open the associated intake valve  32  or exhaust valve  34 . In the following exemplary description, valve actuation assembly  44  is driven by a combination of a cam assembly  52  and a fluid actuator  70 . One skilled in the art will recognize, however, that valve actuation assembly  44  may be driven by through other types of systems, such as, for example, a hydraulic actuation system, an electronic solenoid system, or any combination thereof.  
      In the exemplary embodiment of  FIG. 2 , 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 a first, or upper, position and a second, or 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  moves towards a bottom dead center position within cylinder  22  to allow air to flow from intake passageway  41  into cylinder  22 .  
      A similar valve actuation assembly  44  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 moving towards a top dead center position 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 (not shown) 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 low pressure fluid  84  is provided to draw fluid from a tank  87  and to supply pressurized fluid to fluid actuator  70 . Tank  87  may contain any type of fluid readily apparent to one skilled in the art, such as, for example, hydraulic fluid, fuel, or transmission fluid. Source of low pressure 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 410 KPa and 620 KPa (60 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 low pressure fluid  84  with fluid actuator  70 . In the exemplary embodiment of  FIG. 3 , source of low pressure 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 low pressure fluid  84  to fluid rail  86 . Control valve  82  may be closed to prevent pressurized fluid from flowing from source of low pressure fluid  84  to fluid rail  86 .  
      As illustrated in  FIG. 3 , fluid rail  86  supplies pressurized fluid from source of low pressure 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. 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 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 . A restrictive orifice  93  may be disposed in the inlet to accumulator  95 . As described in greater detail below, the combination of accumulator  95  and restrictive orifice  93  act to dampen oscillations in actuator chamber  76  and fluid line  80 , which may cause actuator piston  74  to oscillate.  
      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 low 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 low pressure 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 valve actuation assembly  44 , the fuel injection system, and any other engine function commonly controlled by an electronic control module. 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 one or more sensors may be configured to sense an operating parameter of engine  20 . For example, with reference to  FIG. 3 , 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 surface temperature of the engine, the ambient air temperature, the intake air temperature, the rotational speed of the engine, the load on the engine, the amount and/or rate of fuel supplied to the engine, the intake air pressure, the oil temperature, a combustion chamber pressure, the exhaust emissions, and the exhaust temperature.  
      Engine  20  may be further equipped with a sensor configured to monitor the crank angle of crankshaft  27 . The position of pistons  24  within their respective cylinders  22  may be determined by the crank angle of crankshaft  27 . As one skilled in the art will recognized, a piston in a conventional four-stroke diesel cycle reciprocates between a top dead center position and a bottom dead center position through a combustion stroke, an exhaust stroke, an intake stroke, and a compression stroke. Each piston stroke correlates to about 180° of crankshaft rotation. Thus, piston  24  may begin combustion stroke at about 0°, the exhaust stroke at about 180°, the intake stroke at about 360°, and the compression stroke at about 540°.  
      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 an intake valve actuation  104  and crankshaft  27  crank angle is illustrated in  FIG. 5 . As shown, intake valve  32  begins to open at about 360° of crankshaft rotation, i.e. when piston  24  is at or near a top dead center position of an intake stroke  106 .  
      Controller  100  may adjust the actuation timing of intake valves  32  based on information received from the sensors regarding the current operating conditions of engine  20 . An exemplary method of controlling the operation of engine  20  is illustrated in  FIG. 6 . It should be understood that various modifications may be made to the exemplary method without departing from the scope of the present invention.  
     INDUSTRIAL APPLICABILITY  
      Based on information provided by the engine sensors, controller  100  may operate engine  20  in either a first mode of operation or a second mode of operation. In the first mode of operation, intake valve  32  actuation is controlled to implement a conventional four-stroke diesel cycle. In the second mode of engine operation, intake valve  32  actuation is controlled to implement a “late intake” type Miller cycle.  
      An exemplary intake valve  32  actuation is illustrated in  FIG. 5 . As shown, cam assembly  52  begins to open intake valves  32  when piston  24  is starting intake stroke  106 , i.e. when piston  24  is at or near a top dead center position within cylinder  22 . As will be recognized by one skilled in the art, intake ports  36  will not be completely open until intake valves  32  are lifted a certain distance, for example, approximately 2 mm. During this portion of the lift of intake valves  32 , valve elements  40  begin to disengage valve seats  50  to start opening intake ports  36 . To ensure that intake ports  36  are completely open when piston  24  begins intake stroke  106 , the actuation or lift of intake valves  32  may begin slightly before the start of intake stroke  106 . In exemplary actuation of  FIG. 5 , intake valve actuation begins approximately 30° before the start of intake stroke  106 . This will ensure that intake ports  36  are completely open when piston  24  starts intake stroke  106 .  
      Controller  100  implements the second mode of operation by selectively actuating fluid actuator  70  to hold intake valve  32  open for at least a portion of the compression stroke  107  of piston  24 . This may be accomplished by moving control valve  82  and directional control valve  88  to the open positions before piston  24  starts intake stroke  106 . This allows pressurized fluid to flow from source of low pressure 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, fluid will flow through restricted orifice  93  into accumulator  95 . Restricted orifice  93  will limit the amount of fluid that may flow into accumulator  95 . In this manner, the combination of restricted orifice  93  and accumulator  95  acts to damper any oscillations that may result from the engagement of actuator rod  78  with rocker arm  64 .  
      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 .  
      Controller  36  may open directional control valve  88  to coordinate the closing of intake valves  32  with the motion of piston  24 . As illustrated in  FIG. 5 , a late intake closing  108  occurs when intake valves  32  remain open for at least a portion of a compression stroke  107  of piston  24 . The late closing allows some of the air in cylinder  22  to be forced out of cylinder  22  as piston  24  advances in cylinder  24  during compression stroke  107 . The amount of air allowed to escape cylinder  22  may be controlled by adjusting the crank angle at which intake valves  32  are closed. Closing intake valves  32  at a relatively earlier crank angle decreases the amount of escaping air, while closing intake valves  32  at a relatively later crank angle increases the amount of escaping air. As described in greater detail below, intake valves  32  may be held open for the entire compression stroke  107 .  
      In the first mode of operation, the intake valve  32  actuation is controlled to implement a conventional  4  stroke diesel cycle. Controller  100  may disengage the late intake Miller cycle by closing control valve  82 . Closing control valve  82  prevents fluid from flowing from source of low pressure 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. Thus, the actuation of intake valves  32  will be governed by the shape and orientation of cam  60 .  
      Thus, when control valve  82  is closed, intake valves  32  may 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 conventional closing  110 , the closing of intake valves  32  substantially coincides with the end of the intake stroke of piston  24 . One skilled in the art will recognize that valve elements  40  will begin to engage valve seats  50  and close intake ports  36  at approximately the end of intake stroke  106 . In conventional closing  110 , intake valves  32  may continue to move after piston  24  starts compression stroke  107 . However, this continued motion ensures that valve elements  40  are fully engaged with valve seats  50 . For the purposes of the present disclosure, intake valves  32  may be considered closed when valve elements  40  begin to engage valve seats  50 . It is expected that the initial engagement of valve elements  40  with valve seats  50  will occur within approximately 5° to 20° of a bottom dead center position of intake stroke  106 .  
      When intake valves  32  close at the end of intake stroke  106 , little or no air will be forced from cylinder  22  during the compression stroke. This results in an increased compression ratio in cylinder  22  and in an increased air flow through engine  20  relative to the second mode of engine operation. The increased compression ratio and the increased air flow allows for increased fuel rate which will result in increased power generated by engine  20 .  
      The flowchart of  FIG. 6  illustrates an exemplary method of controlling the operation of engine  20 . Controller  100  monitors the operating conditions of engine  20  based on sensory input from the sensors operatively engaged with engine  20 . Controller  100  will operate engine  20  in the first mode of operation when engine  20  is experiencing a first set of operating conditions and will operate engine  20  in the second mode of operation when engine  20  is experiencing a second set of operating conditions. As described in greater detail below, the first set of operating conditions may include, for example, engine starting and transient conditions. The second set of operating conditions may include, for example, engine cranking and steady state engine operation.  
      Referring to  FIG. 6 , controller  100  monitors the operating conditions of engine  20 . (Step  120 ). Controller  100  may determine that engine  20  is in the process of starting. (Step  122 ). Controller  100  then identifies the rotational speed of crankshaft  27 .  
      One skilled in the art will recognize that an external power source, such as, for example, a battery-powered starter motor, is used to accelerate crankshaft  27  to a certain rotational speed, such as, for example, 150 to 170 rpm, before fuel is introduced to cylinders  22  to start engine  20 . If controller  100  determines that crankshaft  27  is rotating at a speed less than this threshold (step  124 ), controller  100  may select the second mode of operation (step  126 ).  
      In the second mode of operation, intake valves  32  are opened for a portion of the compression stroke  107  and less work is required to rotate crankshaft  27 . Thus, by selecting the second mode of operation during the cranking or initial acceleration of crankshaft  27 , the amount of work required to accelerate crankshaft  27  is reduced. Accordingly, a smaller starter motor and/or battery may be required to crank engine  20 . When engine  20  is cranking, the amount of work required to accelerate crankshaft  27  may be further reduced by holding intake valves  32  open until the end of compression stroke  107 . In this manner, very little work will be required to accelerate crankshaft  27 .  
      If controller  100  determines that crankshaft  27  is rotating at an appropriate starting speed, controller  100  may initiate fuel delivery to cylinders  22  and operate engine  20  in the first mode of operation. (Step  128 ). In the first mode of operation, intake valves  32  are closed at the end of intake stroke  106 , which results in a greater compression ratio within cylinder  22 . The increased compression ration will facilitate the starting of engine  20 , particularly in cold conditions.  
      Controller  100  will continue to monitor engine  20  to determine when engine  20  has started and entered a steady-state operation. (Step  130 ). When engine  20  has achieved a steady-state operation, controller  100  will operate engine  20  in the second operating mode. (Step  131 ). In the second operating mode, engine  20  will operate on the late intake Miller cycle.  
      Controller  100  will continue to monitor the operating conditions of engine  20 . (Step  132 ). Controller  100  will identify a transient condition in engine operation. (Step  133 ). A transient condition may be experienced when the load on engine  20  is increased, such as, for example, during a rapid acceleration. Controller  100  may identify the transient condition by monitoring various engine parameters, as described previously.  
      When engine  20  is experiencing a transient condition, controller will operate engine  20  in the first operating mode. (Step  136 ). As described previously, in the first operating mode, engine  20  has a higher compression ratio and generates a greater flow of air through the engine. The greater air flow may allow engine  20  to generate additional power and may improve the function of auxiliary engine systems, such as, for example, a turbocharger. Thus, the first operating mode will allow engine  20  to meet the increased load. When the transient condition has been satisfied, controller  100  may return engine  20  to the second operating mode. (Step  134 ).  
      It should be noted that controller  100  may base the operating mode of engine  20  on other conditions and/or parameters in addition to those mentioned above. For example, controller  100  may operate engine  20  in the first operating mode, i.e. a conventional diesel cycle, when engine  20  is in a steady state operating condition but is experiencing a light load. When the engine load is light, such as, for example, less than approximately 25% load, the amount of emissions generated by engine  20  may be reduced by operating engine  20  in the first operating mode. One skilled in the art may recognize that the amount of emissions generated by engine  20  may be reduced by operating engine  20  in either the first or second operating modes based on other engine parameters and/or operating conditions. In addition, one skilled in the art may recognize that the performance of engine  20  may be improved by operating engine  20  in either the first or second operating modes based on other engine parameters and/or operating conditions.  
      As will be apparent from the foregoing description, the present invention provides a method and system for controlling an engine to meet the demands of different operating conditions. The engine may be operated in a first mode when the engine is subject to heavy loads during acceleration or when starting the engine. The first operating mode provides for maximum power output from the engine. The engine may further be operated in a second mode when the engine is experiencing steady state operating conditions. The second mode of operation provides for increased fuel efficiency.  
      It will be apparent to those skilled in the art that various modifications and variations can be made in the engine control 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.