Cylinder to cylinder balancing using fully flexible valve actuation and cylinder pressure feedback

A control system for an engine includes an valve actuator, a cylinder pressure module, and a valve control module. The valve actuator opens a valve of a cylinder at a first target opening timing during a first combustion cycle of the cylinder. The cylinder pressure module receives a cylinder pressure measured by a cylinder pressure sensor of the cylinder and, at a predetermined crankshaft angle after the valve opens during the first combustion cycle, sets a valve opening pressure equal to the cylinder pressure. The valve control module receives a reference cylinder pressure and generates a second target opening timing for a second combustion cycle of the cylinder based on the valve opening pressure and the reference cylinder pressure. The second combustion cycle is after the first combustion cycle. During the second combustion cycle, the valve actuator opens the valve at the second target opening timing.

FIELD

The present disclosure relates to internal combustion engines and more particularly to valve control systems and methods.

BACKGROUND

Air is drawn into an engine through an intake manifold. A throttle valve and/or intake valve timing controls airflow into the engine. The air mixes with fuel from one or more fuel injectors to form an air/fuel mixture. The air/fuel mixture is combusted within one or more cylinders of the engine. Combustion of the air/fuel mixture may be initiated by, for example, injection of the fuel or spark provided by a spark plug.

Combustion of the air/fuel mixture produces torque and exhaust gas. Torque is generated via heat release and expansion during combustion of the air/fuel mixture. The engine transfers torque to a transmission via a crankshaft, and the transmission transfers torque to one or more wheels via a driveline. The exhaust gas is expelled from the cylinders to an exhaust system.

An engine control module (ECM) controls the torque output of the engine. The ECM may control the torque output of the engine based on driver inputs and/or other inputs. The driver inputs may include, for example, accelerator pedal position, brake pedal position, and/or one or more other suitable driver inputs. The other inputs may include, for example, cylinder pressure measured using a cylinder pressure sensor, one or more variables determined based on the measured cylinder pressure, and/or one or more other suitable values.

SUMMARY

In an example, a control system for an engine includes an intake valve actuator, a cylinder pressure module, and a valve control module. The intake valve actuator opens an intake valve of a cylinder at a first target opening timing during a first combustion cycle of the cylinder. The cylinder pressure module receives a cylinder pressure measured by a cylinder pressure sensor of the cylinder and, at a predetermined crankshaft angle after the intake valve opens during the first combustion cycle, sets a valve opening pressure equal to the cylinder pressure. The valve control module receives a reference cylinder pressure and generates a second target opening timing for a second combustion cycle of the cylinder based on the valve opening pressure and the reference cylinder pressure. The second combustion cycle is after the first combustion cycle. During the second combustion cycle, the intake valve actuator opens the intake valve at the second target opening timing.

In an example, a control system for an engine includes an exhaust valve actuator, a cylinder pressure module, and a valve control module. The exhaust valve actuator opens an exhaust valve of a cylinder at a first target opening timing during a first combustion cycle of the cylinder. The cylinder pressure module receives a cylinder pressure measured by a cylinder pressure sensor of the cylinder and, at a predetermined crankshaft angle after the exhaust valve opens during the first combustion cycle, sets a valve opening pressure equal to the cylinder pressure. The valve control module receives a reference cylinder pressure and generates a second target opening timing for a second combustion cycle of the cylinder based on the valve opening pressure and the reference cylinder pressure. The second combustion cycle is after the first combustion cycle. During the second combustion cycle, the exhaust valve actuator opens the exhaust valve at the second target opening timing.

DETAILED DESCRIPTION

An engine control module (ECM) controls opening and closing of intake valves and exhaust valves of an engine. Using a fully flexible valve actuation (FFVA) system, the ECM can control opening and closing of each intake and exhaust valve independently of each other intake and exhaust valve.

A cylinder pressure sensor is provided for each cylinder of the engine. The ECM samples the cylinder pressure measured by a cylinder pressure sensor after each intake valve opening of the cylinder, each intake valve closing of the cylinder, each exhaust valve opening of the cylinder, and each exhaust valve closing of the cylinder. The ECM similarly samples the cylinder pressure of each other cylinder after each intake valve opening, intake valve closing, exhaust valve opening, and exhaust valve closing.

One of the cylinders of the engine may be selected as a reference cylinder. For example, a first cylinder in a predetermined firing order of the cylinders may be selected as the reference cylinder. The ECM may set a cylinder's intake valve opening timing, intake valve closing timing, exhaust valve opening timing, and/or exhaust valve closing timing to adjust that cylinder's pressures toward or to the reference cylinder's pressures, respectively. The ECM may perform the adjustment for each of the cylinders to balance each of the cylinders' pressures with the reference cylinder's pressures, respectively. This may decrease one or more of noise, vibration, and harshness (NVH) experienced, provide better cylinder-to-cylinder air/fuel imbalance, and/or provide one or more other benefits.

Referring now toFIG. 1, a functional block diagram of an example engine system100is presented. The engine system100includes an engine102that combusts an air/fuel mixture to produce drive torque for a vehicle. While the engine102will be discussed as a spark ignition direct injection (SIDI) engine, the engine102may include another suitable type of engine that operates or selectively operates using homogenous charge compression ignition (HCCI). One or more electric motors and/or motor generator units (MGUs) may be used with the engine102.

Air is drawn into an intake manifold106through a throttle valve108. The throttle valve108may vary airflow into the intake manifold106. For example only, the throttle valve108may include a butterfly valve having a rotatable blade. An engine control module (ECM)110controls a throttle actuator module112(e.g., an electronic throttle controller or ETC), and the throttle actuator module112controls opening of the throttle valve108.

Air from the intake manifold106is drawn into cylinders of the engine102. While the engine102may include more than one cylinder, only a single representative cylinder114is shown. Air from the intake manifold106is drawn into the cylinder114through one or more intake valves, such as intake valve118. One or more intake valves may be provided with each cylinder. Timing of opening and closing of the intake valve(s) may affect flow into and out of the cylinder.

The ECM110controls a fuel actuator module120, and the fuel actuator module120controls fuel injection (e.g., amount and timing) by a fuel injector121. The fuel injector121injects fuel into the cylinder114. Fuel is provided to the fuel injector121by a low pressure fuel pump and a high pressure fuel pump (not shown). The low pressure fuel pump draws fuel from a fuel tank and provides fuel at low pressures to the high pressure fuel pump. The high pressure fuel pump selectively further pressurizes the fuel, for example, for direct injection into the cylinders of the engine102. A fuel injector may be provided for each cylinder.

The injected fuel mixes with air and creates an air/fuel mixture in the cylinder114. A piston (not shown) within the cylinder114compresses the air/fuel mixture. Based upon a signal from the ECM110, a spark actuator module122may energize a spark plug124in the cylinder114. Spark generated by the spark plug124ignites the air/fuel mixture during spark ignition (SI) operation of the engine102. The timing of the spark may be specified relative to the time when the piston is at its topmost position, referred to as top dead center (TDC). During homogenous charge compression ignition (HCCI) operation of the engine102, heat generated by compression causes ignition. The ECM110may control whether the engine102operates using SI or HCCI.

Combustion of the air/fuel mixture drives the piston away from the TDC position, and the piston drives rotation of a crankshaft (not shown). After reaching a bottom most position, referred to as bottom dead center (BDC), the piston begins moving toward the TDC position again and expels contents of the cylinder114through one or more exhaust valves, such as exhaust valve126. One or more exhaust valves may be provided for each cylinder. The byproducts of combustion are exhausted from the vehicle via an exhaust system128. Timing of opening and closing of the exhaust valve(s) may also affect flow into and out of the cylinder.

An intake valve actuator130controls actuation of the intake valve118. An exhaust valve actuator132controls actuation of the exhaust valve126. A valve actuator module134controls the intake and exhaust valve actuators130and132based on signals from the ECM110.

The intake and exhaust valve actuators130and132control opening and closing of the intake and exhaust valves118and126, respectively. The intake and exhaust valve actuators130and132are fully flexible valve actuators. The intake and exhaust valve actuators130and132may include, for example, electro-hydraulic actuators, electro-mechanical actuators, or another suitable type of fully flexible valve actuator. Fully flexible valve actuators may be camshaft based valve actuators or camless valve actuators. One fully flexible valve actuator may be provided for each intake valve of the engine102, and one fully flexible valve actuator may be provided for each exhaust valve of the engine102.

Fully flexible intake and exhaust valve actuators enable actuation of each intake valve and exhaust valve of the engine102to be controlled independently of each other valve. The intake and exhaust valve actuators provide what may be referred to as fully flexible valve actuation (FFVA). Using FFVA, the flow of gasses into and out of each cylinder can be individually regulated (via control of intake and exhaust valve opening and closing) to control the flow into and out of the cylinders and, therefore, the combustion conditions within each cylinder.

A crankshaft position sensor142monitors rotation of the crankshaft and generates a crankshaft position signal based on the rotation of the crankshaft. For example only, the crankshaft position sensor142may include a variable reluctance (VR) sensor or another suitable type of crankshaft position sensor. The crankshaft position signal may include a pulse train. A pulse may be generated in the crankshaft position signal as a tooth of a P-toothed wheel (not shown) that rotates with the crankshaft passes the crankshaft position sensor142, where P is an integer greater than one. Accordingly, each pulse corresponds to an angular rotation of the crankshaft by an amount approximately equal to 360° divided by P teeth. The P-toothed wheel may also include a gap of one or more missing teeth, and the gap may be used as an indicator of one complete revolution of the crankshaft (i.e., 360° of crankshaft rotation).

A cylinder pressure sensor150measures pressure within the cylinder114and generates a cylinder pressure signal based on the pressure. A cylinder pressure sensor is provided for each cylinder of the engine102. One or more other sensors158may also be provided. For example, the other sensors158may include a mass air flowrate (MAF) sensor, a manifold absolute pressure (MAP) sensor, an intake air temperature (IAT) sensor, a coolant temperature sensor, and/or one or more other suitable sensors.

Referring now toFIG. 2, a functional block diagram of an example valve control system including a portion of the ECM110is presented. A valve control module204may include an intake valve control module208and an exhaust valve control module212. The intake valve control module208generates a target opening timing and a target closing timing for the intake valve118. The target opening timing and the target closing timing for the intake valve118will be referred to as target intake valve opening (IVO) timing216and target intake valve closing (IVC) timing220, respectively. Generation of the target intake valve opening and closing timings216and220is discussed further below. During the next combustion cycle of the cylinder, the valve actuator module134opens the intake valve118at the target IVO timing216and closes the intake valve118at the target IVC timing220. An intake valve opening may refer to when the intake valve begins to open. An intake valve closing may refer to when the intake valve becomes closed.

The exhaust valve control module212generates a target opening timing and a target closing timing for the exhaust valve126. The target opening timing and the target closing timing for the exhaust valve126will be referred to as target exhaust valve opening (EVO) timing224and target exhaust valve closing (EVC) timing228, respectively. Generation of the target exhaust valve opening and closing timings224and228is also discussed further below. During the next combustion cycle of the cylinder, the valve actuator module134opens the exhaust valve126at the target EVO timing224and closes the exhaust valve126at the target EVC timing228. An exhaust valve opening may refer to when the exhaust valve begins to open. An exhaust valve closing may refer to when the exhaust valve becomes closed.

A cylinder pressure module232receives a cylinder pressure236measured using the cylinder pressure sensor150. The cylinder pressure module232selectively sets a plurality of cylinder pressures for a present combustion cycle of the cylinder based on the cylinder pressure236.

The cylinder pressure module232sets the cylinder pressures when triggered by a triggering module240. The triggering module240triggers the cylinder pressure module232based on a crankshaft position244measured using the crankshaft position sensor142. The triggering module240triggers the cylinder pressure module232after each opening and after each closing of the intake valve118. The triggering module240also triggers the cylinder pressure module232after each opening and after each closing of the exhaust valve126.

The triggering module240generates an intake valve opening (IVO) trigger248after each opening timing of the intake valve118. For example only, the triggering module240may generate the IVO trigger248a predetermined rotational distance (angle) after the target IVO timing216set for the present combustion cycle of the cylinder114. For another example only, the triggering module240may generate the IVO trigger248when the crankshaft position244reaches a predetermined intake opening angle during the present combustion cycle of the cylinder114. The predetermined intake opening angle may be, for example, approximately 5-10 crank angle degrees (CAD) after the TDC position of the exhaust stroke or another suitable angle. The predetermined intake closing angle may be a fixed, calibrated value or may be a variable value determined, for example, based on engine speed and engine load. In response to the generation of the IVO trigger248, the cylinder pressure module232sets an IVO pressure252(P-IVO) equal to the cylinder pressure236.

The triggering module240generates an intake valve closing (IVC) trigger256after each closing timing of the intake valve118. For example only, the triggering module240may generate the IVC trigger256a predetermined rotational distance after the target IVC timing220set for the present combustion cycle of the cylinder114. For another example only, the triggering module240may generate the IVC trigger256when the crankshaft position244reaches a predetermined intake closing angle during the present combustion cycle of the cylinder114. The predetermined intake closing angle may be, for example, approximately 90 CAD after the BDC position of the intake stroke or another suitable angle. The predetermined intake closing angle may be a fixed, calibrated value or may be a variable value determined, for example, based on engine speed and engine load. In response to the generation of the IVC trigger256, the cylinder pressure module232sets an IVC pressure260(P-IVC) equal to the cylinder pressure236.

The triggering module240generates an exhaust valve opening (EVO) trigger264after each opening timing of the exhaust valve126. For example only, the triggering module240may generate the EVO trigger264at a predetermined rotational distance after the target EVO timing224set for the present combustion cycle of the cylinder114. For another example only, the triggering module240may generate the EVO trigger264when the crankshaft position244, for example, reaches a predetermined exhaust opening angle during the present combustion cycle of the cylinder114. The predetermined exhaust opening angle may be, for example, approximately the BDC position of the expansion stroke of a combustion cycle or another suitable angle. The predetermined exhaust opening angle may be a fixed, calibrated value or may be a variable value determined, for example, based on engine speed and engine load. In response to the generation of the EVO trigger264, the cylinder pressure module232sets an EVO pressure268(P-EVO) equal to the cylinder pressure236.

The triggering module240generates an exhaust valve closing (EVC) trigger272after each closing timing of the exhaust valve126. For example only, the triggering module240may generate the EVC trigger272a predetermined rotational distance after the target EVC timing228set for the present combustion cycle of the cylinder114. For another example only, the triggering module240may generate the EVC trigger272when the crankshaft position244reaches a predetermined exhaust closing angle during the present combustion cycle of the cylinder114. The predetermined exhaust closing angle may be, for example, approximately 5-10 CAD before the TDC position of the exhaust stroke of a combustion cycle or another suitable angle. The predetermined exhaust closing angle may be a fixed, calibrated value or may be a variable value determined, for example, based on engine speed and engine load. In response to the generation of the EVC trigger272, the cylinder pressure module232sets an EVC pressure276(P-IVC) equal to the cylinder pressure236.

As stated above, a cylinder pressure sensor is provided with each cylinder of the engine102. Thus, an IVO pressure, an IVC pressure, an EVO pressure, and an EVC pressure, like the IVO pressure252, the IVC pressure260, the EVO pressure268, and the EVC pressure276, respectively, can be obtained for each cylinder and each combustion cycle. The pressures of other cylinders may be provided by the cylinder pressure module232in response to triggers generated by the triggering module240for the other cylinders, respectively, or one or more other modules may provide the pressures. In various implementations, one cylinder pressure module and one triggering module may be provided for each cylinder.

One of the cylinders of the engine102may be selected as a reference cylinder. For example, a first cylinder in a predetermined firing order of the cylinders may be selected as a reference cylinder. A reference pressure module280outputs the reference cylinder's IVO pressure, IVC pressure, EVO pressure, and EVC pressure a reference IVO pressure284, a reference IVC pressure288, a reference EVO pressure292, and a reference EVC pressure296, respectively. For example, if the cylinder114is the reference cylinder, the reference pressure module280outputs the IVO pressure252, the IVC pressure260, the EVO pressure268, and the EVC pressure276as the reference IVO pressure284, the reference IVC pressure288, the reference EVO pressure292, and the reference EVC pressure296, respectively.

Referring back to the intake and exhaust valve control modules208and212, the intake valve control module208determines a desired IVO timing and a desired IVC timing (not shown). The exhaust valve control module212determines a desired EVO timing and a desired EVC timing (not shown). The desired IVO timing, the desired IVC timing, the desired EVO timing, and the desired EVC timing may be determined as a function of, for example, engine speed, engine load, and/or one or more other suitable parameters.

The intake valve control module208sets the target IVO timing216to adjust the IVO pressure252of the next control loop equal to the reference IVO pressure284. The intake valve control module208may determine the target IVO timing216based on the desired IVO timing, the reference IVO pressure284, and the IVO pressure252. For example, the intake valve control module208may determine a first timing adjustment as a function of a difference between the reference IVO pressure284and the IVO pressure252and determine the target IVO timing216as a function of the first timing adjustment (e.g., in CAD) and the desired IVO timing. For example only, the intake valve control module208may set the target IVO timing216equal to a sum of the first timing adjustment and the desired IVO timing. The intake valve control module208may determine the first timing adjustment based on the difference, for example, using proportional (P) control, proportional integral (PI) control, proportional integral derivative (PID) control, or another suitable type of feedback control.

The intake valve control module208sets the target IVC timing220to adjust the IVC pressure260of the next control loop equal to the reference IVC pressure288. The intake valve control module208may determine the target IVC timing220based on the desired IVC timing, the reference IVC pressure288, and the IVC pressure260. For example, the intake valve control module208may determine a second timing adjustment as a function of a difference between the reference IVC pressure288and the IVC pressure260and determine the target IVC timing220as a function of the second timing adjustment (e.g., in CAD) and the desired IVC timing. For example only, the intake valve control module208may set the target IVC timing220equal to a sum of the second timing adjustment and the desired IVC timing. The intake valve control module208may determine the second timing adjustment based on the difference, for example, using P control, PI control, PID control, or another suitable type of feedback control.

The exhaust valve control module212sets the target EVO timing224to adjust the EVO pressure268of the next control loop equal to the reference EVO pressure292. The exhaust valve control module212may determine the target EVO timing224based on the desired EVO timing, the reference EVO pressure292, and the EVO pressure268. For example, the exhaust valve control module212may determine a third timing adjustment as a function of a difference between the reference EVO pressure292and the EVO pressure268and determine the target EVO timing224as a function of the third timing adjustment (e.g., in CAD) and the desired EVO timing. For example only, the exhaust valve control module212may set the target EVO timing224equal to a sum of the third timing adjustment and the desired EVO timing. The exhaust valve control module212may determine the third timing adjustment based on the difference, for example, P control, PI control, PID control, or another suitable type of feedback control.

The exhaust valve control module212sets the target EVC timing228to adjust the EVC pressure276of the next control loop equal to the reference EVC pressure296. The exhaust valve control module212may determine the target EVC timing228based on the desired EVC timing, the reference EVC pressure296, and the EVC pressure276. For example, the exhaust valve control module212may determine a fourth timing adjustment as a function of a difference between the reference EVC pressure296and the EVC pressure276and determine the target EVC timing228as a function of the fourth timing adjustment (e.g., in CAD) and the desired EVC timing. For example only, the exhaust valve control module212may set the target EVC timing228equal to a sum of the fourth timing adjustment and the desired EVC timing. The exhaust valve control module212may determine the fourth timing adjustment based on the difference, for example, using P control, PI control, PID control, or another suitable type of feedback control.

Setting the target IVO, IVC, EVO, and EVC timings216,220,224, and228of the cylinder to adjust the IVO, IVC, EVO, and EVC pressures252,260,268, and276to the reference IVO, IVC, EVO, and EVC pressures284,288,292, and296, respectively, may balance torque production of the cylinders. This setting of the target timings may equalize indicated mean effective pressure (IMEP) and pumping mean effective pressure (PMEP) of the cylinders, thereby equalizing net mean effective pressure (NMEP) of the cylinders. More specifically, airflow into and exhaust flow out of each cylinder may be equalized. The balancing of the cylinders may provide better air fuel imbalance (AFIM) across the cylinders and reduce noise, vibration, and/or harshness (NVH) experienced by users of the vehicle.

Referring now toFIG. 3, a flowchart depicting an example method of controlling intake and exhaust valve opening and closing timing using fully flexible valve actuation is presented. Control may begin with304where control obtains the IVO pressure, the IVC pressure, the EVO pressure, and the EVC pressure for a cylinder. For example, control may obtain the IVO pressure252, the IVC pressure260, the EVO pressure268, and the EVC pressure276for the cylinder114. WhileFIG. 3will be discussed in conjunction with the cylinder114, control may perform the method ofFIG. 3for each other cylinder and for each combustion cycle.

Control obtains the IVO pressure252from a measurement of the cylinder pressure sensor150taken after the IVO timing of the cylinder114. Control obtains the IVC pressure260from a measurement of the cylinder pressure sensor150taken after the IVC timing of the cylinder114. Control obtains the EVO pressure268from a measurement of the cylinder pressure sensor150taken after the EVO timing of the cylinder114. Control obtains the EVC pressure276from a measurement of the cylinder pressure sensor150taken after the EVC timing of the cylinder114.

At308, control obtains the reference IVO pressure284, the reference IVC pressure288, the reference EVO pressure292, and the reference EVC pressure296. If the cylinder114is selected as the reference cylinder, the IVO pressure252, the IVC pressure260, the EVO pressure268, and the EVC pressure276may be equal to the reference IVO pressure284, the reference IVC pressure288, the reference EVO pressure292, and the reference EVC pressure296, respectively. If another cylinder is selected as the reference cylinder, the reference IVO pressure284, the reference IVC pressure288, the reference EVO pressure292, and the reference EVC pressure296may be equal to the IVO pressure, the IVC pressure, the EVO pressure, and the EVC pressure obtained for the other cylinder.

At312, control generates the target IVO timing216, the target IVC timing220, the target EVO timing224, and the target EVC timing228for the cylinder114. Control may determine the target IVO timing216, the target IVC timing220, the target EVO timing224, and the target EVC timing228to adjust the IVO pressure252, the IVC pressure260, the EVO pressure268, and the EVC pressure276toward or to the reference IVO pressure284, the reference IVC pressure288, the reference EVO pressure292, and the reference EVC pressure296, respectively, during a next combustion cycle of the cylinder. For example, control may determine the target IVO timing216, the target IVC timing220, the target EVO timing224, and the target EVC timing228based on a difference between the IVO pressure252and the reference IVO pressure284, a difference between the IVC pressure260and the reference IVC pressure288, a difference between the EVO pressure268and the reference EVO pressure292, and a difference between the EVC pressure276and the reference EVC pressure296.

Control opens the intake valve118of the cylinder114at the target IVO timing216, control closes the intake valve118of the cylinder114at the target IVC timing220, control opens the exhaust valve126of the cylinder114at the target EVO timing224, and control closes the exhaust valve126of the cylinder114at the target EVC timing228. Control controls opening and closing of the intake valve118at the target IVO timing216and the target IVC timing220via the intake valve actuator130. Control controls opening and closing of the exhaust valve126at the target EVO timing224and the target EVC timing228via the exhaust valve actuator132. While control is shown as ending after312, as described above, control may performFIG. 3for each cylinder and for each combustion cycle.

Referring now toFIG. 4, another flowchart depicting an example method of controlling intake and exhaust valve opening and closing timing using fully flexible valve actuation is presented. Control may begin with404where control obtains the IVO pressure for a cylinder. For example, control may obtain the IVO pressure252for the cylinder114. WhileFIG. 4will be discussed in conjunction with the cylinder114, control may perform the method ofFIG. 4for each other cylinder and for each combustion cycle. Control obtains the IVO pressure252from a measurement of the cylinder pressure sensor150taken after the IVO timing of the cylinder114.

At408, control obtains the reference IVO pressure284. Control may generate the target IVO timing216for the cylinder114at412. Control generates the target IVO timing216for the cylinder114based on a difference between the IVO pressure252and the reference IVO pressure284. Control opens the intake valve118of the cylinder114at the target IVO timing216via the intake valve actuator130.

At416, control obtains the IVC pressure260for the cylinder114. At420, control obtains the reference IVC pressure288. Control may generate the target IVC timing220for the cylinder114at424. Control generates the target IVC timing220for the cylinder114based on a difference between the IVC pressure260and the reference IVC pressure288. Control closes the intake valve118of the cylinder114at the target IVC timing220via the intake valve actuator130.

Control obtains the EVO pressure268for the cylinder114at428. At432, control obtains the reference EVO pressure292. Control may generate the target EVO timing224for the cylinder114at436. Control generates the target EVO timing224for the cylinder114based on a difference between the EVO pressure268and the reference EVO pressure292. Control opens the exhaust valve126of the cylinder114at the target EVO timing224via the exhaust valve actuator132.

At440, control obtains the EVC pressure276for the cylinder114. At444, control obtains the reference EVC pressure296. Control may generate the target EVC timing228for the cylinder114at448. Control generates the target EVC timing228for the cylinder114based on a difference between the EVC pressure276and the reference EVC pressure296. Control closes the exhaust valve126of the cylinder114at the target EVC timing228via the exhaust valve actuator132. While control is shown as ending after448, as described above, control may performFIG. 4for each cylinder and for each combustion cycle.