Abstract:
The present invention provides a method and apparatus for controlling an intake engine valve capable of variable closing timing. A condition indicative of white smoke production is determined. An intake engine valve is closed at a first crank angle for a given engine operating condition when the condition indicative of white smoke production does not exist. The intake valve is closed at a second crank angle for the given engine operating condition when the condition indicative of white smoke production exists. The second crank angle is less than the first crank angle.

Description:
TECHNICAL FIELD 
   The present invention is directed to a system and method for actuating an engine valve. More particularly, the present invention is directed to a system and method for actuating the valves in an internal combustion engine. 
   BACKGROUND 
   An internal combustion engine, such as, for example, a diesel, gasoline, or natural gas engine, typically includes a series of intake and exhaust valves. These valves may be actuated, or selectively opened and closed, to control the amount of intake and exhaust gases that flow to and from the combustion chambers of the engine. Typically, the actuation of the engine valves is timed to coincide with the reciprocating movement of a series of pistons. For example, the intake valves associated with a particular combustion chamber may be opened when the respective piston is moving through an intake stroke. The exhaust valves associated with the particular combustion chamber may be opened when the respective piston is moving through an exhaust stroke. 
   The combustion process of an internal combustion engine may generate undesirable emissions, such as, for example, white smoke, particulates and oxides of nitrogen (NOx). These emissions are generated when a fuel, such as, for example, diesel, gasoline, or natural gas, is combusted within the combustion chambers of the engine. If no emission reduction systems are in place, the engine will exhaust these undesirable emissions to the environment. 
   An engine may include many different types of emission reduction systems to reduce the amount of emissions exhausted to the environment. For example, the engine may include an engine gas recirculation system and/or an aftertreatment system. Unfortunately, while these emission reduction systems may effectively reduce the amount of emissions exhausted to the environment, these systems typically result in a decrease in the efficiency of the engine. 
   Efforts are currently being focused on improving engine efficiency to counterbalance the effect of emission reduction systems. One such approach to improving 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, 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. 
   One problem with implementing a Miller cycle in an engine is that the resulting reduced air flow and compression ratio may negatively impact the performance of the engine under certain operating conditions, such as, for example, to create white smoke. In these types of conditions, engine performance may be enhanced by switching the operation of the engine to a convention diesel cycle. This may be accomplished with a variable valve actuation system, such as the system described in U.S. Pat. No. 6,237,551. As described, the variable valve actuation system may include a valve that is operable to selectively enable and disable a Miller cycle. This technique of switching to a conventional diesel cycle, however, removes any engine performance benefit obtained by using a Miller cycle. 
   SUMMARY OF THE INVENTION 
   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. 
   A method and apparatus for controlling an intake engine valve capable of variable closing timing. A condition indicative of white smoke production is determined. An intake engine valve is closed at a first crank angle for a given engine operating condition when the condition indicative of white smoke production does not exist. The intake valve is closed at a second crank angle for the given engine operating condition when the condition indicative of white smoke production exists. The second crank angle is less than the first crank angle. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and together with the description, serve to explain the principles of the invention. In the drawings: 
       FIG. 1  is a diagrammatic and schematic representation of an engine system in accordance with an exemplary embodiment of the present invention; 
       FIG. 2  is a diagrammatic cross-sectional view of an internal combustion engine in accordance with an exemplary embodiment of the present invention; 
       FIG. 3  is a diagrammatic cross-sectional view of a cylinder and valve actuation assembly in accordance with an exemplary embodiment of the present invention; 
       FIG. 4  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. 5  is a flow chart according to one embodiment of the invention; and 
       FIG. 6  is a graph according to one embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to exemplary embodiments of the invention, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
   An exemplary embodiment of an engine system  10  is illustrated in  FIG. 1 . Engine system  10  includes an intake air passageway  13  that leads to an engine  20 . One skilled in the art will recognize that engine system  10  may optionally include various components, such as, for example, a turbocharger  12  and an aftercooler  14 , that are disposed in intake air passageway  13 . An exhaust air passageway  15  may lead from engine  20  to turbocharger  12 . 
   Engine  20  may be an internal combustion engine as illustrated in  FIG. 2 . 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. 2 , 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” or “V” type configuration. 
   As also shown in  FIG. 2 , 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 cause 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 lift valve element  40  to thereby open 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. 3  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 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. 2 ) 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 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 or “lift” 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, a piezoelectric actuation system, or any other way known to those skilled in the art. 
   In the exemplary embodiment of  FIG. 3 , 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. 3 , 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  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 advancing within cylinder  22  to allow exhaust to flow from cylinder  22  into exhaust passageway  43 . 
   As shown in  FIG. 3 , 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. 2 and 4 , a source of hydraulic fluid  84  is provided to draw fluid from a tank  87  and to supply pressurized fluid to fluid actuator  70 . Source of hydraulic fluid  84  may be part of a lubrication system, such as typically accompanies an internal combustion engine. Such a lubrication system may provide pressurized fluid 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 hydraulic fluid may be a pump configured to provide fluid 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 hydraulic fluid  84  with fluid actuator  70 . In the exemplary embodiment of  FIG. 4 , source of hydraulic 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 hydraulic fluid  84  to fluid rail  86 . Control valve  82  may be closed to prevent pressurized fluid from flowing from source of hydraulic fluid  84  to fluid rail  86 . 
   As illustrated in  FIG. 4 , fluid rail  86  supplies pressurized fluid from source of hydraulic 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. 2 ). 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 and include a variety of different components. For example, fluid supply system  79  may include a check valve (not shown) placed in parallel with directional control valve  88  between control valve  82  and fluid actuator  70 . In addition, fluid supply system  79  may include a source of high-pressure fluid. Fluid supply system  79  may also include a snubbing valve to control the rate of fluid flow from fluid actuator  70  and a damping system, which may include an accumulator and a restricted orifice, to prevent pressure oscillations in actuator chamber  76  and fluid line  80 . 
   As shown in  FIGS. 1 and 2 , engine system  10  includes a controller  100 , such as an engine valve controller. 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. 
   The transmitted signal may result in the selective opening and closing of directional control valve  88 . If directional control valve  88  is a normally closed valve, the transmitted signal may open the valve to allow hydraulic fluid to flow to and/or from fluid actuator  70 . If directional control valve  88  is a normally opened valve, the transmitted signal may close the valve to prevent fluid from flowing to and/or from fluid actuator  70 . 
   As illustrated in  FIGS. 1–4 , a variety of sensors known to those skilled in the art may be operatively engaged with engine  20  and/or valve actuation assemblies  44 . Each sensor is configured to monitor a particular parameter of the performance of engine  20  or valve actuation assemblies  44 . Some examples of sensors include an intake manifold temperature sensor, an intake manifold pressure sensor, and an engine speed sensor. One skilled in the art may recognize that alternative sensors may be used with engine system  10  to monitor the performance of engine  20  or valve actuation assemblies  44 . 
   As also shown in  FIG. 1 , at least one engine sensor  18  is operatively connected with engine  20 . Engine sensor  18  may be any type of sensor commonly used to monitor engine performance. For example, engine sensor  18  may be configured to measure one or more of the following: a rotational speed of the engine, a delivered torque of the engine, a temperature of the engine, a pressure within one or more of cylinders  22 , and a rotational angle of crankshaft  27 . 
   As further shown in  FIG. 1 , at least one intake sensor  16  may be disposed in intake passageway  13 . Intake sensor(s)  16  may be configured to sense the temperature and/or pressure of the intake air and/or the mass flow rate of the intake air. Intake sensor  16 ( s ) may be any type of sensor readily apparent to one skilled in the art as capable of sensing these types of parameters and may be disposed at any point along intake passageway  13 . 
   As further shown in  FIG. 1 , a turbocharger sensor  17  may be operatively connected with turbocharger  12 . Turbocharger sensor  17  may be configured to sense the speed of the turbocharger. Turbocharger sensor  17  may also be configured to any other operational parameter of turbocharger  12 . 
   INDUSTRIAL APPLICABILITY 
   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. In other operating condtions, the controller  100  may operate the engine  20  in a normal Miller cycle. Further, when implementing a normal Miller cycle, under operating conditions indicative of white smoke production, such as, for example, a low intake manifold temperature or a low intake manifold pressure, or both, the controller  100  may operate each valve actuation assembly  44  to implement a shortened late intake Miller cycle, as will be described below. 
   When engine  20  is operating under a first set of predetermined operating conditions, controller  100  may implement a normal or shortened 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 transmitting a signal to move 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 hydraulic 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 . 
   When actuator chamber  76  is filled with fluid, directional control valve  88  may be closed. 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 . 
   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. A snubbing valve 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 . 
   When the engine operating conditions indicate that white smoke production is likely to exist, controller  100  may implement a shortened late intake Miller cycle by selectively actuating fluid actuator  70  to hold intake valve  32  open for a second portion of the compression stroke of piston  24 , the second portion being less than the first portion. That is, the controller  100  closes the intake valve  32  earlier, e.g., at a lower crank angle, than it would have under the non-shortened late intake Miller cycle. Typically this crank angle will still be greater than the crank angle at which the intake valve  32  closes during a conventional diesel cycle. 
     FIG. 5  is a flow chart  120  showing one technique for controlling an engine valve  32  according to one embodiment of the invention. In block  122 , the controller  100  determines various engine operating characteristics/conditions. For example, the controller may determine the intake manifold pressure, intake manifold temperature, and engine speed, by ways known to those skilled in the art. Other engine characteristics/conditions known to those skilled in the art could also be determined. 
   In block  124 , the controller determines whether the determined engine characteristics indicate that white smoke conditions are likely to exist. For example, a low intake manifold pressure, a low intake manifold temperature, and an excessive amount of fuel (e.g., more than needed for stoichiometric combustion) are all conditions that are more likely to produce white smoke from the engine  20 . Other engine characteristics, or combinations thereof, known to those skilled in the art could also be used. 
   If the controller  100  determines that white smoke conditions are not likely to exist, control passes to block  126 . In block  126 , the controller  100  closes the intake engine valve  32  at the conventional Miller cycle crank angle. 
   If the controller  100  determines that white smoke conditions are likely to exist, control passes to block  128 . In block  128 , the controller  100  closes the intake engine valve  32  at a shortened Miller cycle crank angle, e.g., sooner than the crank angle at which a conventional Miller cycle would close the intake engine valve  32 . 
     FIG. 6  is a graph  130  of intake valve position vs. crank angle according to one embodiment of the invention. At the beginning of the combustion cycle (intake stroke) the intake valve opens (point A). If the controller  100  determines that the engine  20  should be operated in a conventional diesel cycle, the position of the intake valve  32  follows the curve profile  132  shown and closes at point B. If the controller  100  determines that the engine  20  should be operated in a conventional Miller cycle, the position of the intake valve  32  follows the curve profile shown and closes at point C. If the controller  100  determines that the engine  20  should be operated in a Miller cycle, but white smoke conditions are likely to exist, the position of the intake valve  32  follows the curve profile shown and closes at point D. 
   For a conventional diesel cycle, the intake valve  32  may close at a crank angle of approximately 160° before top dead center (“BTDC”). For a normal Miller cycle, the intake valve  32  may close at some crank angle greater than approximately 160° before top dead center (“BTDC”). For a shortened Miller cycle, the intake valve  32  may close at a crank angle anywhere between the crank angle for a conventional diesel cycle and a normal Miller cycle. 
   The particular crank angle for a shortened Miller cycle may be a function of various engine-operating conditions. In particular, experimentation has shown that the engine speed may be a pertinent factor in determining this crank angle. The exact relationship between engine speed and the crank angle for closure of the intake valve  32  may vary depending on the particular characteristics of the engine design, and may be determined through experimentation. 
   Thus, according to one embodiment of the invention, an engine  20  may operate in a normal Miller cycle when conditions of the engine do not indicate that white smoke production is likely, and may operate in a shortened Miller cycle when conditions indicate that white smoke production is likely. Because in a normal Miller cycle the intake valve  32  closes after bottom dead center (“ABDC”), the quantity of air in the cylinders  22  is less than that if the engine  20  was operating in a conventional diesel cycle (where the intake valve  32  closes closer to or at bottom dead center. By closing the intake valve  32  earlier in the shortened Miller cycle, more air is present in the cylinder  22 , leading to a higher pressure and temperature within the cylinder  22  during the combustion cycle. This, in turn, may tend to reduce the production of white smoke. 
   Although some examples herein describe an engine  20  capable of operating in a conventional diesel cycle, a normal Miller cycle, and a shortened Miller cycle, the invention may have application to engines that operate only in the normal Miller cycle, to thereby switch between the normal and shortened Miller cycle as conditions indicate. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the engine valve actuation system and method of the present invention without departing from the scope or spirit 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 and spirit of the invention being limited only by the following claims and their equivalents.