Variable valve actuation control for operation at altitude

A method of controlling a variable valve actuation system for an engine is provided. A cam assembly is operated to move an intake valve between a first position and a second position. A parameter indicative of an altitude at which the engine is operating is sensed. A first lookup map is accessed to determine a desired air-to-fuel ratio when the sensed parameter indicates that the engine is operating at an altitude below a first predetermined value. A second lookup map is accessed to determine a desired air-to-fuel ratio when the sensed parameter indicates that the engine is operating at an altitude above the first predetermined value. A desired valve actuation period is determined based on the determined air-to-fuel ratio. The intake valve is prevented from returning to the first position until the end of the determined valve actuation period.

TECHNICAL FIELD

The present invention is directed to a system and method for controlling a variable valve actuation system and, more particularly, to a system and method for controlling a variable valve actuation system to account for altitude operating conditions.

BACKGROUND

The performance of an internal combustion engine, such as, for example, a diesel, gasoline, or natural gas engine may be impacted by the conditions under which the engine is operated. For example, the performance of an internal combustion engine may change as the altitude at which the engine is operated increases. In particular, the operation of the engine at higher altitudes may cause a decrease in fuel efficiency and/or an increase in the generation of undesirable emissions.

The impact of altitude on engine performance results from the decrease in air density and air pressure at higher altitudes. The decrease in air density and air pressure at higher altitudes causes a reduction in the air-fuel ratio provided to the engine, a reduction in the efficiency of an associated turbocharger system, and a reduction in the combustion efficiency within the engine. The reduction in each of these parameters may result in a decreased fuel efficiency and/or increased emission generation.

Generally, an internal combustion engine operates on a selected air-to-fuel ratio regardless of the altitude at which the engine is operating. The operating air-to-fuel ratio is selected to meet certain fueling and power requirements and may depend upon the current engine speed and load. The selected air-to-fuel ratio may be achieved by actuating the engine valve for a certain period of time and by injecting a certain amount of fuel into a cylinder. However, when the engine is operating at a high altitude where the air density and pressure is reduced, less air will pass by the engine valves during a given time period. Accordingly, the air-to-fuel ratio supplied to the engine will decrease as the altitude of operation increases.

The air-to-fuel ratio is a critical component of an internal combustion engine, such as, for example, a diesel engine. A reduction in the air-to-fuel ratio typically translates to a reduction in the efficiency of combustion. Usually, the reduced air-to-fuel ratio reduces the rate of combustion and also reduces the amount of the combustion energy that may be translated to mechanical work. When less combustion energy is translated to work, the fuel efficiency of the engine decreases and the temperature of the exhaust gas increases.

A turbocharger system may be added to the internal combustion engine to improve the performance of the engine. The turbocharger system recovers energy from the exhaust stream and uses the recovered energy to increase the pressure of the air in the intake stream. The increased intake air pressure may result in more air being pushed into the combustion chamber and thereby increase the air-to-fuel ratio.

However, under standard operating conditions, a typical turbocharger system is approximately 60-65% effective, which means that only 60-65% of the recovered energy is applied to the intake air flow. The lower density of the air at high altitudes further reduces the efficiency of the turbocharger. Thus, not all of the increased exhaust gas energy is translated to increased intake manifold pressure. Accordingly, the turbocharger will not compensate for all losses associated with operating at higher altitudes.

The 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 controlling a variable valve actuation system. A can assembly is operated to move an intake valve between a first position where the intake valve blocks a flow of fluid and a second position where the intake valve allows a flow of fluid. A parameter indicative of an altitude at which the engine is operating is sensed. A first lookup map is accessed to determine a desired air-to-fuel ratio when the sensed parameter indicates that the engine is operating at an altitude below a first predetermined value. A second lookup map is accessed to determine a desired air-to-fuel ratio when the sensed parameter indicates that the engine is operating at an altitude above the first predetermined value. A desired valve actuation period is determined based on the determined air-to-fuel ratio. The valve actuator prevents the intake valve from returning to the first position in response to operation of the cam assembly. The valve actuator is released to allow the intake valve to return to the first position at the end of the determined valve actuation period.

In another aspect, the present invention is directed to an intake valve actuation system for an engine. An intake valve is moveable between a first position where the intake valve prevents a flow of fluid and a second position where the intake valve allows a flow of fluid. A cam assembly is connected to the intake valve to move the intake valve between the first position and the second position. A valve actuator selectively engages the intake valve and prevent the intake valve from returning to the first position. A sensor senses a parameter indicative of an altitude at which the engine is operating. A controller that has a memory is adapted to store a first lookup map and a second lookup map. The controller accesses the first lookup map to determine a desired air-to-fuel ratio when the sensed parameter indicates that the engine is operating at an altitude below a first predetermined value and accesses the second lookup map to determine a desired air-to-fuel ratio when the sensed parameter indicates that the engine is operating at an altitude above the first predetermined value. The controller determines a desired valve actuation period based on the determined air-to-fuel ratio and prevents the intake valve from returning to the first position until the end of the determined valve actuation period.

DETAILED DESCRIPTION

An exemplary embodiment of an engine system10is illustrated in FIG.1. Engine system10includes an intake air passageway13that leads to an engine20. One skilled in the art will recognize that engine system10may include various components, such as, for example, a turbocharger12and an aftercooler14. An exhaust air passageway15may lead from engine20to turbocharger12.

Engine20may be an internal combustion engine as illustrated in FIG.2. For the purposes of the present disclosure, engine20is depicted and described as a four stroke diesel engine. One skilled in the art will recognize, however, that engine20may be any other type of internal combustion engine, such as, for example, a gasoline or natural gas engine.

As illustrated inFIG. 2, engine20includes an engine block28that defines a plurality of cylinders22. A piston24is disposed for sliding movement between a top dead center position and a bottom dead center position within each cylinder22. In the illustrated embodiment, engine20includes six cylinders22and six associated pistons24. One skilled in the art will recognize that engine20may include a greater or lesser number of pistons24and that pistons24may be disposed in an “in-line” configuration, a “V” configuration, or any other conventional configuration.

As also shown inFIG. 2, engine20includes a crankshaft27that is rotatably disposed within engine block28. A connecting rod26connects each piston24to crankshaft27. Each piston24is coupled to crankshaft27so that a sliding motion of piston24within the respective cylinder22results in a rotation of crankshaft27. Similarly, a rotation of crankshaft27will result in a sliding motion of piston24.

Engine20also includes a cylinder head30. Cylinder head30defines an intake passageway41that leads to at least one intake port36for each cylinder22. Cylinder head30may further define two or more intake ports36for each cylinder22.

An intake valve32is disposed within each intake port36. Intake valve32includes a valve element40that is configured to selectively block intake port36. As described in greater detail below, each intake valve32may be actuated to lift valve element40to thereby open the respective intake port36. The intake valves32for each cylinder22may be actuated in unison or independently.

Cylinder head30also defines at least one exhaust port38for each cylinder22. Each exhaust port38leads from the respective cylinder22to an exhaust passageway43. Cylinder head30may further define two or more exhaust ports38for each cylinder22.

An exhaust valve34is disposed within each exhaust port38. Exhaust valve34includes a valve element48that is configured to selectively block exhaust port38. As described in greater detail below, each exhaust valve34may be actuated to lift valve element48to thereby open the respective exhaust port38. The exhaust valves34for each cylinder22may be actuated in unison or independently.

FIG. 3illustrates an exemplary embodiment of one cylinder22of engine20. As shown, cylinder head30defines a pair of intake ports36connecting intake passageway41to cylinder22. Each intake port36includes a valve seat50. One intake valve32is disposed within each intake port36. Valve element40of intake valve32is configured to engage valve seat50. When intake valve32is in a closed position, valve element40engages valve seat50to close intake port36and block fluid flow relative to cylinder22. When intake valve32is lifted from the closed position, intake valve32allows a flow of fluid relative to cylinder22.

Similarly, cylinder head30may define two or more exhaust ports38(only one of which is illustrated inFIG. 2) that connect cylinder22with exhaust passageway43. One exhaust valve34is disposed within each exhaust port38. A valve element48of each exhaust valve34is configured to close exhaust port38when exhaust valve34is in a closed position and block fluid flow relative to cylinder22. When exhaust valve34is lifted from the closed position, exhaust valve32allows a flow of fluid relative to cylinder22.

As shown inFIG. 2, engine20includes a series of valve actuation assemblies44. One valve actuation assembly44may be operatively associated with each pair of intake valves32for each cylinder22. Each valve actuation assembly44is operable to move or “lift” the associated intake valve32or exhaust valve34from a first, or closed, position to a second, or open, position.

In the exemplary embodiment ofFIG. 3, valve actuation assembly44includes a bridge54that is connected to each valve element40through a pair of valve stems46. A spring56may be disposed around each valve stem46between cylinder head30and bridge54. Spring56acts to bias both valve elements40into engagement with the respective valve seat50to thereby close each intake port36.

Valve actuation assembly44may also include a rocker arm64. Rocker arm64is configured to pivot about a pivot66. One end68of rocker arm64is connected to bridge54. The opposite end of rocker arm64is connected to a cam assembly52. In the exemplary embodiment ofFIG. 3, cam assembly52includes a cam60having a cam lobe and mounted on a cam shaft, a push rod61, and a cam follower62. One skilled in the art will recognize that cam assembly52may have other configurations, such as, for example, where cam60acts directly on rocker arm64.

Valve actuation assembly44may be driven by cam60. Cam60is connected to crankshaft27so that a rotation of crankshaft27induces a corresponding rotation of cam60. Cam60may be connected to crankshaft27through 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 cam60will cause cam follower62and associated push rod61to periodically reciprocate between an upper and a lower position.

The reciprocating movement of push rod61causes rocker arm64to pivot about pivot66. When push rod61moves in the direction indicated by arrow58, rocker arm64will pivot and move bridge54in the opposite direction. The movement of bridge54causes each intake valve32to lift and open intake ports36. As cam60continues to rotate, springs56will act on bridge54to return each intake valve32to the closed position.

In this manner, the shape and orientation of cam60controls the timing of the actuation of intake valves32. As one skilled in the art will recognize, cam60may be configured to coordinate the actuation of intake valves32with the movement of piston24. For example, intake valves32may be actuated to open intake ports36when piston24is moving from a top-dead-center position to a bottom-dead-center position during an intake stroke to allow air to flow from intake passageway41into cylinder22.

A similar valve actuation assembly44may be connected to exhaust valves34. A second cam (not shown) may be connected to crankshaft27to control the actuation timing of exhaust valves34. Exhaust valves34may be actuated to open exhaust ports38when piston24is moving from a bottom-dead-center position to a top-dead-center position in an exhaust stroke to allow exhaust to flow from cylinder22into exhaust passageway43.

As shown inFIG. 3, valve actuation assembly44also includes a valve actuator70. Valve actuator70includes an actuator cylinder72that defines an actuator chamber76. An actuator piston74is slidably disposed within actuator cylinder72and is connected to an actuator rod78. A return spring (not shown) may act on actuator piston74to return actuator piston74to a home position. Actuator rod78is engageable with an end68of rocker arm64.

A fluid line80is connected to actuator chamber76. Pressurized fluid may be directed through fluid line80into actuator chamber76to move actuator piston74within actuator cylinder72. Movement of actuator piston74causes actuator rod78to engage end68of rocker arm64.

Fluid may be introduced to actuator chamber76when intake valves32are in the open position to move actuator rod78into engagement with rocker arm64to thereby hold intake valves32in the open position. Alternatively, fluid may be introduced to actuator chamber76when intake valves32are in the closed position to move actuator rod78into engagement with rocker arm64and pivot rocker arm64about pivot66to thereby open intake valves32.

As illustrated inFIGS. 2 and 4, engine system10may include a source of fluid84to draw fluid from a tank87that holds a supply of fluid, which may be, for example, a hydraulic fluid, a lubricating oil, a transmission fluid, or fuel. Source of fluid84may increase the pressure of the fluid and direct the fluid into a main gallery83. Source of fluid84and main gallery83may be part of a lubrication system, such as typically accompanies an internal combustion engine. Main gallery83may contain 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).

As shown inFIG. 4, a fluid supply system79connects main gallery83with valve actuator70. A restrictive orifice75may be positioned in fluid line85between main gallery83and a first end of fluid rail86. A control valve82may be connected to an opposite end of fluid rail86and lead to tank87. Control valve82may be opened to allow a flow of fluid through restrictive orifice75and fluid rail86to tank87. Control valve82may be closed to allow a build up of pressure in the fluid within fluid rail86.

As illustrated inFIG. 4, fluid rail86supplies pressurized fluid to a series of valve actuators70. Each valve actuator70may be associated with either the intake valves32or the exhaust valves34of a particular engine cylinder22(referring to FIG.2). Fluid lines80direct pressurized fluid from fluid rail86into the actuator chamber76of each valve actuator70.

A directional control valve88may be disposed in each fluid line80. Each directional control valve88may be opened to allow pressurized fluid to flow between fluid rail86and actuator chamber76. Each directional control valve88may be closed to prevent pressurized fluid from flowing between fluid rail86and actuator chamber76. Directional control valve88may be normally biased into a closed position and actuated to allow fluid to flow through directional control valve88. Alternatively, directional control valve88may be normally biased into an open position and actuated to prevent fluid from flowing through directional control valve88. One skilled in the art will recognize that directional control valve88may be any type of controllable valve, such as, for example a two coil latching valve.

One skilled in the art will also recognize that fluid supply system79may have a variety of different configurations and include a variety of different components. For example, fluid supply system79may include one or more check valves (not shown). A first check valve may be placed in parallel with directional control valve88between restrictive orifice75and valve actuator70. A second check valve may be placed in fluid line85between main gallery83and fluid rail86. In addition, fluid supply system79may include a source of high pressure fluid. Fluid supply system79may also include a snubbing valve that controls the rate of fluid flow from valve actuator70and a damping system, which may include an accumulator and a restricted orifice, that prevents pressure oscillations in actuator chamber76and fluid line80.

As shown inFIGS. 1 and 2, engine system10includes a controller100that is connected to each valve actuation assembly44and to control valve82. Controller100may include an electronic control module that has a microprocessor and a memory101. 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.

Controller100may be programmed to control one or more aspects of the operation of engine20. For example, controller100may be programmed to control valve actuation assembly44, the fuel injection system, and any other engine function commonly controlled by an electronic control module. Controller100may control engine20based on the current operating conditions of the engine and/or instructions received from an operator.

Controller100may control valve actuation assembly44by transmitting a signal, such as, for example, a current, to directional control valve88. The transmitted signal may result in the selective opening and/or closing of directional control valve88. If directional control valve88is a normally closed valve, the transmitted signal may cause directional control valve88to open for a certain period of time. If directional control valve88is a normally open valve, the transmitted signal may cause directional control valve to close for a certain period of time. By controlling the opening and closing of directional control valve88, controller may control the flow of fluid to and from valve actuator70and thereby control the engagement of actuator rod78with rocker arm64to delay the closing of intake valve32for a predetermined period. An exemplary intake valve actuation104is illustrated in FIG.5.

As illustrated inFIGS. 1-4, engine system10may include a series of sensors, which are described in greater detail below. Each sensor is configured to monitor a particular operating parameter of engine20. One skilled in the art may recognize that alternative sensors may be used with engine system10to monitor other operating parameters of engine20.

As shown inFIG. 1, an intake sensor16may be disposed in intake passageway13. Intake sensor16may be configured to sense the pressure of the intake air and/or the mass flow rate of the intake air. Intake sensor16may 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 passageway13.

Engine system10may also include a pressure sensor17. Pressure sensor17may be configured to sense a pressure representative of the ambient air pressure. Pressure sensor17may be any type of sensor readily apparent to one skilled in the art as capable of providing an indication of the ambient air pressure. Controller100may use the sensed air pressure to approximate the operating altitude of engine system10. For example, an air pressure reading of approximately 83 kPa corresponds to an altitude of approximately 1,700 m (5,500 ft) and an air pressure reading of 70 kPa corresponds to an altitude of approximately 3,000 m (10,000 ft). One skilled in the art will recognize that engine system10may be equipped with any type of sensor adapted to provide an indication of the operating altitude of engine system10.

At least one engine sensor18may also be operatively connected with engine20. Engine sensor18may be any type of sensor commonly used to monitor an operating parameter of engine20. For example, engine sensor18may be configured to sense the load on engine20, the amount of fuel being supplied to engine20, the rotational speed of engine20, the pressure within one or more cylinders22, the rotational angle of crankshaft27, or any other commonly sensed operating parameter. Engine sensor18may be any type of sensor readily apparent to one skilled in the art as capable of sensing these types of engine operating parameters.

Memory101of controller100may store information related to the operation of engine20in the form of a “map.” For the purposes of the present disclosure, the term “map” is intended to include any electronic storage structure for storing information related to the operation of the engine, such as, for example, data tables, look-up tables, graphs, or any other electronic storage format readily apparent to one skilled in the art. These maps may define optimal engine operating characteristics as a function of engine operating parameters. For example, memory101may store a map that defines an optimal air-to-fuel ratio for a particular engine speed and fuel injection quantity. Similarly, memory101may store a map that defines an optimal fuel delivery rate for a particular engine speed and load. Memory101may other maps, such as, for example, a map that defines limits on the valve actuation period for a particular engine speed and engine load.

Memory101may store different versions or variations on each of these maps. For example, memory101may store several air-to-fuel ratio maps. In particular, memory101may store air-to-fuel ratio maps that identify the optimal air-to-fuel ratio as a function of engine speed and fuel injection quantity for: (1) steady-state conditions at low altitude; (2) transient conditions at low altitude; (3) steady-state conditions at high altitude; (4) transient conditions at high altitude; (5) steady-state conditions at very high altitude; and (6) transient conditions at very high altitude. For the purposes of the present disclosure, low altitude may be considered to include elevations below approximately 1,700 m (5,500 ft), high altitude may be considered to include elevations between approximately 1,700 m (5,500 ft) and 3,000 m (10,000 ft), and very high altitude may be considered to include elevations above approximately 3,000 m (10,000 ft). One skilled in the art will recognize that other elevations may be used to differentiate between these maps.

Controller100may use the information provided by the sensors to access the maps stored in memory101to identify an optimal air-to-fuel ratio and an optimal intake valve actuation period for the current engine operating conditions. The flowcharts ofFIGS. 6aand6billustrate an exemplary method118of determining an optimal air-to-fuel ratio and an intake valve actuation period.

Industrial Applicability

Controller100may selectively operate valve actuator70to implement a late intake type Miller cycle in engine20. When operating under the late intake Miller cycle, controller100operates valve actuator70to delay the closing of intake valve32from a conventional closing, where the closing substantially coincides with the end of an intake stroke, to a delayed closing, where intake valve32is held open for a predetermined portion of a compression stroke. The duration of the intake valve actuation period may be determined based on the current operating conditions of engine20.

As described above, cam assembly52controls the initial actuation timing of intake valves32. As cam60and push rod61start to pivot rocker arm64, controller100ensures control valve82and directional control valve88are in an open position. This allows pressurized fluid to flow from source of hydraulic fluid84through fluid rail86and into actuator chamber76. The force of the fluid entering actuator chamber76moves actuator piston74so that actuator rod78follows end68of rocker arm64as rocker arm64pivots to open intake valves32. The distance and rate of movement of actuator rod78will depend upon the configuration of actuator chamber76and fluid supply system79. Fluid supply system79may be configured to provide a sufficient flow of fluid to actuator chamber76to ensure that actuator chamber76is filled with fluid before cam60returns intake valve32to the closed position.

Controller100may actuate valve actuator70by closing directional control valve88. This prevents fluid from escaping from actuator chamber76. As cam60continues to rotate and springs56urge intake valves32towards the closed position, actuator rod78will engage end68of rocker arm and prevent intake valves32from closing. As long as directional control valve88remains in the closed position, the trapped fluid in actuator chamber76will prevent springs56from returning intake valves32to the closed position. Thus, valve actuator70will hold intake valves32in the open position, independently of the action of cam assembly52.

Controller100may disengage valve actuator70to allow intake valves32to close by opening directional control valve88. This allows the pressurized fluid to flow out of actuator chamber76. The force of springs56forces the fluid from actuator chamber76, thereby allowing actuator piston74to move within actuator cylinder72. This allows rocker arm64to pivot so that intake valves32are moved to the closed position.

As illustrated inFIG. 5, operation of valve actuator70may extend intake valve actuation104from a conventional closing110to a delayed closing108. The period, or duration, of the extended intake valve actuation may be measured in terms of the angle of rotation of crankshaft27, as a function of time, or in any other manner readily apparent to one skilled in the art. When implementing a late intake type Miller cycle, the extended intake valve actuation period may be between about 0° and 120° crankshaft rotation. One skilled in the art will recognize, however, that valve actuator70may be used to implement other types of valve actuation timing variations.

Controller100may vary the intake valve actuation period to achieve optimum engine performance based upon the current operating conditions of engine20and/or the altitude at which engine20is operating. For example, when engine20is operating at a low elevation, the optimal duration of the valve actuation period may be shorter than when engine20is operating at a higher elevation. The flowchart ofFIGS. 6aand6billustrate one exemplary method of determining the intake valve actuation period based on the operating elevation of engine20.

Controller100may receive information about the current operating conditions of engine20from the various sensors. For example, controller100may receive an indication of the current engine speed, the current engine load, and the ambient air pressure. (Step120). Controller100may also receive information regarding additional operating parameters of engine20, such as, for example, an intake manifold pressure, an in-cylinder pressure, or an operating fluid temperature.

Controller100may then determine a fuel injection quantity. (Step122). Controller100may use the current engine load and speed to access a lookup map that stores the fuel injection quantity as a function of engine load and speed. The fuel injection quantity may represent the total quantity of fuel that is injected into cylinder22during a particular operating cycle, including any “pilot injection.”

Controller100may also determine the operating altitude of engine20. (Step124). Controller100may use the ambient air pressure to determine the operating altitude of engine20. For example, an ambient air pressure of about 83 kPa indicates that engine20is operating at approximately 1,700 m (5,500 ft) and an ambient air pressure of about 70 kPa indicates that engine20is operating at approximately 3,000 m (10,000 ft). Controller100may use any conversion routine readily apparent to one skilled in the art to translate the sensed ambient air pressure, or other such sensed operating parameter, into an approximate operating altitude.

Controller100may also determine if engine20is operating in a steady-state condition or a transient condition. (Step126). Controller100may make this determination based on a comparison between the current values of the sensed operating parameters and previous values of the operating parameters. For example, an increase in the engine speed or engine load may indicate that engine20is experiencing a transient condition. One skilled in the art will recognize that various parameters and/or analysis may be used to make this determination.

If engine20is operating in steady-state conditions, controller100determines if the operating altitude of engine20is below a first predetermined value that indicates that engine20is operating at a low altitude. (Step128) For example, a low altitude may be considered to be altitudes between approximately sea level and approximately 1,700 m (5,500 ft). If the operating altitude is below the first predetermined value, controller100may access a lookup map for low altitude steady-state conditions to determine the desired air-to-fuel ratio. (Step130). The lookup map may store the desired air-to-fuel ratio as a function of engine speed and fuel injection quantity.

If the operating altitude of engine20is above the first predetermined value, controller100may compare the operating altitude to a second predetermined value. (Step132). The second predetermined value may be set to indicate a very high operating altitude. For example, a very high altitude may be an altitude above approximately 3,000 m (10,000 ft). If engine20is operating above the first predetermined value and below the second predetermined value, controller100may access a lookup map for high altitude steady-state conditions to determine the desired air-to-fuel ratio. (Step134) If engine20is operating above the first and second predetermined values, controller100may access a lookup map for very high altitude steady-state conditions to determine the desired air-to-fuel ratio. (Step136)

If the operating altitude of engine20is between sea level and the first predetermined value or between the first predetermined value and the second predetermined value, controller100may interpolate the desired air-to-fuel ratio from the corresponding lookup maps. For example, if engine20is operating at approximately 2,500 m (8,200 ft) controller100may obtain the air-to-fuel ratio for the current engine speed and fuel quantity from both the high altitude steady-state map and the very high altitude steady state map. Controller100may interpolate between the two air-to-fuel ratio values using the assumption that the desired air-to-fuel ratio varies linearly between the high altitude value and the very high altitude value. It should be noted that controller100may use another approach to interpolating the desired air-to-fuel ratio such as, for example, any type of numerical or statistical analysis or model.

Controller100may determine the intake valve actuation period. (Step137). The intake valve actuation period may be expressed as a function of the engine speed (ES), the intake air pressure (IP), and the desired airflow (AF). For example, the intake valve actuation period (P) maybe determined by the following equation:
P=A+B(ES)+C(ES)2+D(IP)+E(IP)2+F(AF)+G(AF)2+H(ES)(IP)(AF)

The above formula will yield an intake valve actuation period, P, that is expressed in terms of an engine crank angle. The determined crank angle may represent the angle at which the current to directional control valve88should be terminated to open directional control valve88and release valve actuator70. Alternatively, the determined crank angle may represent the angle at which intake valve actuator70should be returned to the closed position. In the latter example, controller100may then determine the engine crank angle at which to terminate the current to directional control valve88based on a constant that is indicative of the time required for the intake valve32to close after the current to directional control valve88has been terminated. One skilled in the art may recognize that different formulas and/or constants may be developed to present different representations of the valve actuation period. For example, valve actuation period may be expressed as an amount of a rotation of crankshaft or a time period.

If controller100determines that engine20is operating under transient conditions (referring to Step126inFIG. 6a), controller100may access lookup maps that store air-to-fuel ratios for transient conditions. The operating altitude of engine20may be compared to the first predetermined value indicating a low altitude operation. (Step138,FIG. 6b) The first predetermined value may be equivalent to the first predetermined value used in the steady-state process described above. If the operating altitude is below the first predetermined value, controller100may access a lookup map for low altitude transient conditions to determine the desired air-to-fuel ratio. (Step140).

If the operating altitude of engine20is above the first predetermined value, controller100may compare the operating altitude to a second predetermined value. (Step142). The second predetermined value may be set to indicate a very high operating altitude and may be equivalent to the second predetermined value used in the steady-state process described above. If engine20is operating above the first predetermined value and below the second predetermined value, controller100may access a lookup map for high altitude transient conditions to determine the desired air-to-fuel ratio. (Step144) If engine20is operating above the first and second predetermined values, controller100may access a lookup map for very high altitude steady-state conditions to determine the desired air-to-fuel ratio. (Step146).

If the operating altitude of engine20is between sea level and the first predetermined value or between the first predetermined value and the second predetermined value, controller100may interpolate the desired air-to-fuel ratio from the corresponding lookup maps. Controller100may perform the interpolation process as described above. Alternatively, controller100may use another approach to interpolating the desired air-to-fuel ratio such as, for example, any type of numerical or statistical analysis or model.

Controller100may determine the intake valve actuation period for transient conditions. (Step147). The intake valve actuation period may be determined in terms of a crank angle as described above.

Controller100may then control directional control valve88to actuate valve actuator70to achieve the desired valve actuation period by closing intake valves32at the determined crank angle. Controller100may continuously monitor the operating parameters and altitude of engine20and adjust the intake valve actuation period accordingly. In this manner, controller100may optimize the air-to-fuel ratio based on the current operating conditions and altitude of engine20.

As will be apparent, the above-described method provides for the control of a variable valve actuation assembly for an internal combustion engine to account for performance variations due to changes in altitude. The described method provides for the optimization of the air-to-fuel ratio supplied to the engine based on the operating conditions and altitude of the engine. The air-to-fuel ratio may be optimized based on the current operating altitude to improve the performance of the engine and/or reduce the amount of emissions generated by the engine.