Exhaust pressure estimation from wastegate motor current

In one embodiment, a method for an engine comprises adjusting an engine operating parameter based on exhaust pressure, the exhaust pressure estimated based on wastegate actuator motor current.

FIELD

The present disclosure relates to an internal combustion engine.

BACKGROUND AND SUMMARY

Turbocharged engines are configured to compress ambient air entering the engine in order to increase power. A wastegate may control the amount of exhaust energy provided to the turbine of a turbocharger, thereby affecting boost pressure. Changes in wastegate position affect the exhaust pressure, which in turn affect engine breathing. Because of a lack of low-cost, reliable exhaust manifold pressure sensors, the exhaust pressure is often estimated in an engine controller to assist the speed-density calculations for estimating engine flow.

Typically, the exhaust pressure is estimated based in part on wastegate position. Traditional pneumatic wastegates lack position measurement, and thus wastegate position may be estimated using a force balance between the pneumatic, the spring, and the exhaust forces. However, the inventors herein have recognized that such wastegate position estimation may result in significant variability in the exhaust pressure estimation, leading to inaccurate engine air flow determinations.

Accordingly, a method is provided to at least partly address the above issues. In one embodiment, a method for an engine comprises adjusting an engine operating parameter based on exhaust pressure, the exhaust pressure estimated based on wastegate actuator motor current.

In this way, exhaust pressure may be estimated from the amount of current drawn by the motor of an electrical wastegate actuator. The current of the wastegate actuator motor may be proportional to the exhaust forces acting on the wastegate, and thus the motor current may provide an accurate mechanism for estimating the exhaust pressure and thus the engine breathing.

DETAILED DESCRIPTION

In boosted engines, electronic wastegate actuators may provide precise output to achieve delivery of a desired boost to the engine. The output of an electric actuator may be a function of the current supplied to the actuator motor and the magnetic field generated by its magnets, if present. The force produced by an electric actuator may be a function of its magnetic flux multiplied by the current flowing through its windings (hereinafter referred to as “motor current”). Thus, the actuator motor current is proportional to the force acting on the wastegate by the exhaust. By measuring the motor current over time, an average exhaust pressure may be estimated. The estimated exhaust pressure may be used to determine the engine air charge, estimate the turbine power, adjust fuel injection quantity, determine if a particulate filter soot load has reached a regeneration threshold, and other parameters.

If the camshaft timing has been set to include positive valve overlap, the use of average exhaust pressure in calculation of engine air charge may not be fully accurate, as the exhaust pressure during the valve overlap period may be different than the exhaust pressure during non-overlap periods. The exhaust pressure during the valve overlap period may be determined by using motor current measurements. Specifically, the sampling of the motor current may be timed to measure the current during the overlap period, as well as times between the valve events. Using these measurements, the exhaust pressure may be determined during the valve overlap period. The exhaust pressure during overlap can then be used to provide a more accurate air charge estimation rather than using only the average exhaust pressure.

An example engine including a turbocharger and electrically-actuated wastegate is depicted inFIG. 1. The engine also includes a controller configured to carry out the method depicted inFIG. 2according to the map depicted inFIG. 3.

Referring now toFIG. 1, it shows a schematic diagram of one cylinder of multi-cylinder engine10, which may be included in a propulsion system of an automobile. Engine10may be controlled at least partially by a control system including controller12and by input from a vehicle operator132via an input device130. In this example, input device130includes an accelerator pedal and a pedal position sensor134for generating a proportional pedal position signal PP. Combustion chamber (i.e., cylinder)30of engine10may include combustion chamber walls32with piston36positioned therein. In some embodiments, the face of piston36inside cylinder30may have a bowl. Piston36may be coupled to crankshaft40so that reciprocating motion of the piston is translated into rotational motion of the crankshaft. Crankshaft40may be coupled to at least one drive wheel of a vehicle via an intermediate transmission system. Further, a starter motor may be coupled to crankshaft40via a flywheel to enable a starting operation of engine10.

Combustion chamber30may receive intake air from intake manifold44and may exhaust combustion gases via exhaust passage48. Intake manifold44is supplied via intake passage42. Intake manifold44and exhaust passage48can selectively communicate with combustion chamber30via respective intake valve52and exhaust valve54. In some embodiments, combustion chamber30may include two or more intake valves and/or two or more exhaust valves.

Intake valve52may be controlled by controller12via electric valve actuator (EVA)51. Similarly, exhaust valve54may be controlled by controller12via EVA53. Alternatively, the variable valve actuator may be electro hydraulic or any other conceivable mechanism to enable valve actuation. During some conditions, controller12may vary the signals provided to actuators51and53to control the opening and closing of the respective intake and exhaust valves. The position of intake valve52and exhaust valve54may be determined by valve position sensors55and57, respectively. In alternative embodiments, one or more of the intake and exhaust valves may be actuated by one or more cams, and may utilize one or more of cam profile switching (CPS), variable cam timing (VCT), variable valve timing (VVT) and/or variable valve lift (VVL) systems to vary valve operation. For example, cylinder30may alternatively include an intake valve controlled via electric valve actuation and an exhaust valve controlled via cam actuation including CPS and/or VCT.

Fuel injector66is shown coupled directly to combustion chamber30for injecting fuel directly therein in proportion to the pulse width of signal FPW received from controller12via electronic driver68. In this manner, fuel injector66provides what is known as direct injection of fuel into combustion chamber30. The fuel injector may be mounted in the side of the combustion chamber or in the top of the combustion chamber, for example. Fuel may be delivered to fuel injector66by a fuel system (not shown) including a fuel tank, a fuel pump, and a fuel rail.

Ignition system88can provide an ignition spark to combustion chamber30via spark plug92in response to spark advance signal SA from controller12, under select operating modes. Though spark ignition components are shown, in some embodiments, combustion chamber30or one or more other combustion chambers of engine10may be operated in a compression ignition mode, with or without an ignition spark.

Intake passage42may include throttles62and63having throttle plates64and65, respectively. In this particular example, the positions of throttle plates64and65may be varied by controller12via signals provided to an electric motor or actuator included with throttles62and63, a configuration that is commonly referred to as electronic throttle control (ETC). In this manner, throttles62and63may be operated to vary the intake air provided to combustion chamber30among other engine cylinders. The positions of throttle plates64and65may be provided to controller12by throttle position signals TP. Pressure, temperature, and mass air flow may be measured at various points along intake passage42and intake manifold44. For example, intake passage42may include a mass air flow sensor120for measuring clean air mass flow entering through throttle63. The clean air mass flow may be communicated to controller12via the MAF signal.

Engine10may further include a compression device such as a turbocharger including at least a compressor162arranged upstream of intake manifold44. For a turbocharger, compressor162may be at least partially driven by a turbine164(e.g., via a shaft) arranged along exhaust passage48. Thus, the amount of compression provided to one or more cylinders of the engine via a turbocharger may be varied by controller12. Further, exhaust passage48may include wastegate26for diverting exhaust gas away from turbine164. Wastegate26may be operated with an actuator28, which, for example, may be an electric actuator including permanent magnets. In some embodiments, actuator28may be an electric motor. Wastegate26and/or a compressor bypass valve (not shown inFIG. 1) may be controlled by controller12via actuators (e.g., actuator28) to be opened when a lower boost pressure is desired, for example.

A charge air cooler154may be included downstream from compressor162and upstream of intake valve52. Charge air cooler154may be configured to cool gases that have been heated by compression via compressor162, for example. In one embodiment, charge air cooler154may be upstream of throttle62. Pressure, temperature, and mass air flow may be measured downstream of compressor162, such as with sensor145or147. The measured results may be communicated to controller12from sensors145and147via signals148and149, respectively. Pressure and temperature may be measured upstream of compressor162, such as with sensor153, and communicated to controller12via signal155.

Further, in the disclosed embodiments, an exhaust gas recirculation (EGR) system may route a desired portion of exhaust gas from exhaust passage48to intake manifold44.FIG. 1shows an HP-EGR system and an LP-EGR system, but an alternative embodiment may include only an LP-EGR system, or only an HP-EGR system. The HP-EGR is routed through HP-EGR passage140from upstream of turbine164to downstream of compressor162. The amount of HP-EGR provided to intake manifold44may be varied by controller12via HP-EGR valve142. The LP-EGR is routed through LP-EGR passage150from downstream of turbine164to upstream of compressor162. The amount of LP-EGR provided to intake manifold44may be varied by controller12via LP-EGR valve152. The HP-EGR system may include HP-EGR cooler146and the LP-EGR system may include LP-EGR cooler158to reject heat from the EGR gases to engine coolant, for example.

Under some conditions, the EGR system may be used to regulate the temperature of the air and fuel mixture within combustion chamber30. Thus, it may be desirable to measure or estimate the EGR mass flow. EGR sensors may be arranged within EGR passages and may provide an indication of one or more of mass flow, pressure, temperature, concentration of O2, and concentration of the exhaust gas. For example, an HP-EGR sensor144may be arranged within HP-EGR passage140.

In some embodiments, one or more sensors may be positioned within LP-EGR passage150to provide an indication of one or more of a pressure, temperature, and air-fuel ratio of exhaust gas recirculated through the LP-EGR passage. Exhaust gas diverted through LP-EGR passage150may be diluted with fresh intake air at a mixing point located at the junction of LP-EGR passage150and intake passage42. Specifically, by adjusting LP-EGR valve152in coordination with first air intake throttle63(positioned in the air intake passage of the engine intake, upstream of the compressor), a dilution of the EGR flow may be adjusted.

A percent dilution of the LP-EGR flow may be inferred from the output of a sensor145in the engine intake gas stream. Specifically, sensor145may be positioned downstream of first intake throttle63, downstream of LP-EGR valve152, and upstream of second main intake throttle62, such that the LP-EGR dilution at or close to the main intake throttle may be accurately determined. Sensor145may be, for example, an oxygen sensor such as a UEGO sensor.

Exhaust gas sensor126is shown coupled to exhaust passage48downstream of turbine164. Sensor126may be any suitable sensor for providing an indication of exhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), a two-state oxygen sensor or EGO, a HEGO (heated EGO), a NOX, HC, or CO sensor.

Emission control devices71and72are shown arranged along exhaust passage48downstream of exhaust gas sensor126. Devices71and72may be a selective catalytic reduction (SCR) system, three way catalyst (TWC), NOXtrap, various other emission control devices, or combinations thereof. For example, device71may be a TWC and device72may be a particulate filter (PF). In some embodiments, PF72may be located downstream of TWC71(as shown inFIG. 1), while in other embodiments, PF72may be positioned upstream of TWC72(not shown inFIG. 1).

Controller12is shown inFIG. 1as a microcomputer, including microprocessor unit102, input/output ports104, an electronic storage medium for executable programs and calibration values shown as read only memory chip106in this particular example, random access memory108, keep alive memory110, and a data bus. Controller12may receive various signals from sensors coupled to engine10, in addition to those signals previously discussed, including measurement of inducted mass air flow (MAF) from mass air flow sensor120; engine coolant temperature (ECT) from temperature sensor112coupled to cooling sleeve114; a profile ignition pickup signal (PIP) from Hall effect sensor118(or other type) coupled to crankshaft40; throttle position (TP) from a throttle position sensor; and absolute manifold pressure signal, MAP, from sensor122. Engine speed signal, RPM, may be generated by controller12from signal PIP. Manifold pressure signal MAP from a manifold pressure sensor may be used to provide an indication of vacuum, or pressure, in the intake manifold. Note that various combinations of the above sensors may be used, such as a MAF sensor without a MAP sensor, or vice versa. During stoichiometric operation, the MAP sensor can give an indication of engine torque (through airflow estimates). Further, this sensor, along with the detected engine speed, can provide an estimate of charge (including air) inducted into the cylinder. In one example, sensor118, which is also used as an engine speed sensor, may produce a predetermined number of equally spaced pulses every revolution of the crankshaft.

Storage medium read-only memory106can be programmed with computer readable data representing instructions executable by processor102for performing the methods described below as well as other variants that are anticipated but not specifically listed.

As described above,FIG. 1shows only one cylinder of a multi-cylinder engine, and that each cylinder may similarly include its own set of intake/exhaust valves, fuel injector, spark plug, etc.

Turning now toFIG. 2, a method200for determining exhaust pressure is illustrated. Method200may be carried out by controller12according to instructions stored thereon. Method200may determine the exhaust pressure from an engine based on current from a wastegate actuator motor, for example based on current drawn from electric wastegate actuator28ofFIG. 1.

At202, engine operating parameters are determined. The engine operating parameters may include, but are not limited to, engine speed and load, intake manifold pressure, exhaust flow, exhaust temperature, air-fuel ratio, camshaft position, valve timing, and other parameters. At204, it is determined if the engine is operating with intake and exhaust valve overlap. During valve overlap, the exhaust valve and intake valve of a given cylinder may be open at the same time for a portion of the engine cycle. For example, the exhaust valve closing timing may be delayed and/or the intake valve opening time may be advanced such that the exhaust valve is closing as the opening of the intake valve commences, resulting in a period of overlap where both valves are open.

As a result, there are two things that can happen. One possibility is that the amount of exhaust remaining in the cylinders during combustion (referred to as internal EGR) may be increased, improving engine efficiency and reducing emissions during some conditions. This occurs when the pressure in the exhaust during overlap is greater than the pressure inside the cylinder. This trapping of the air affects the air charge.

The other possibility when the exhaust valve is open during a portion of the time when the intake valve opens, is that some of the intake air drawn into the cylinder may be immediately expelled out to the exhaust system, further affecting the exhaust pressure. This effect is also known as scavenging, and affects the relationship between airflow trapped inside the engine and the total airflow through the engine. This effect is a function of the exhaust pressure during overlap.

The calculated average exhaust pressure may not correlate well with the exhaust pressure during overlap. As a result, if the wastegate actuator motor current is sampled arbitrarily with respect to valve timing, it may not accurately reflect the exhaust pressure during overlap.

Thus, if it is determined at204that the engine is operating with valve overlap, method200proceeds to218, which will be explained below. If the engine is not operating with valve overlap, method200proceeds to206to measure the current drawn by the wastegate actuator. The motor current may be measured, for example, with a current probe or sensor. Alternatively, the current may be calculated based on Ohm's law as the ratio of actuator voltage (e.g., terminal voltage) and actuator resistance, if these two quantities are known or can be measured and when a resistance/temperature lookup table is available. The current drawn by the wastegate actuator (otherwise referred to as the motor current) may be measured periodically over a given duration of time, for example it may be measured 10 times every engine cycle, once every millisecond, etc., for one second or 10 engine cycles or other suitable time duration. The motor current may then be averaged to provide an average current over the given duration.

At208, an average exhaust pressure is determined based on the average motor current. As the exhaust pressure fluctuates due to the exhausting of combustion gases following each engine combustion event, the average exhaust pressure over the given duration may be calculated to provide a more stable representation of the overall exhaust pressure. The motor current signal may or may not be filtered when determining the average motor current. The average current drawn by the wastegate actuator corresponds to the force the wastegate exerts to overcome the average exhaust forces. Therefore, the actuator current can be translated into the pressure difference across the turbine.FIG. 3shows an example map300of turbine pressure difference vs. actuator current. The vertical axis is the difference between pre-turbine and post-turbine pressure. Map300may be stored in the memory of the controller, and used to look up the turbine pressure difference for a given average current. As shown by map300, the pressure difference across the turbine increases proportionally to the change in wastegate actuator current. The exhaust pressure can then be estimated from the turbine pressure difference and the measured or estimated post-turbine pressure.

The exhaust pressure may be determined based on additional operating parameters. For example, as indicated at210, the exhaust pressure may be estimated based on actuator motor current and further based on wastegate position. The inclusion of the wastegate position provides improved flexibility to account for changes in linkage angles and the wastegate poppet surface orientation with wastegate position, which may not be reflected in the exhaust pressure determination with the motor current.

The wastegate position may be determined in a suitable manner. In one example, the wastegate position may be determined based on the wastegate motor. In an example, a sensor may measure linear displacement of a rod actuated by the motor. Alternatively, the motor may include a rotary encoder housed internally in the motor. The encoder may be coupled to the slowest rotating element in the motor which is coupled to an actuating rod. Such an encoder may collect measurements across the entire range through which the element rotates, which may be for example 180 degrees. In this case, the output of the encoder varies as the motor rotates. In another example, the motor includes a screw (e.g., a ball screw), the rotation of which may be measured and used to determine the position of the wastegate valve. However, a different positional encoder may be used, as the ball screw or other rotating element may rotate through a range greater than 180 and/or 360 degrees. Various suitable encoders may be used which, for example, detect changes in angular position as opposed to absolute position.

In other examples, as indicated at212, the exhaust pressure may be estimated based on motor current and further based on exhaust flow. The inclusion of exhaust flow provides flexibility to account for gas dynamics due to change in flow directions around the wastegate poppet valve. Exhaust flow may be determined by a sensor in the exhaust, may be estimated based on intake mass flow and combustion conditions, or by other suitable mechanisms.

At214, the engine air charge is determined based on the average exhaust pressure calculated at218. The air charge may be a function of the intake manifold pressure and the exhaust pressure. At215, the turbine power is determined based on the air charge and exhaust pressure. Turbine power may describe the power output by the turbine, and in one example may be based on the torque applied to the shaft of the turbocharger and the angular velocity of the shaft. Turbine power may be calculated based on the pressure ratio across the turbine, which may be used to calculate the torque applied to the shaft by the turbine, and thus the turbine power. Based on the engine air charge, turbine power, and/or exhaust pressure, various engine operating parameters may be adjusted at216. Example operating parameters that may be adjusted include fuel injection quantity, fuel injection timing, EGR valve position, throttle position, spark timing, wastegate position, etc. For example, to maintain a desired air-fuel ratio, the amount of fuel injected to the engine may be adjusted based on the engine air charge. In another example, a desired EGR rate may be maintained by adjusting the position of an EGR valve, such as the LP-EGR valve and/or HP-EGR, based on the engine air charge. Further, the exhaust pressure determined at208may be used to adjust various operating parameters. For example, if the exhaust pressure is greater than a threshold, it may indicate that the soot load on a particulate filter in the exhaust passage has reached a threshold level. The controller may then initiate a regeneration of the particulate filter. In another example, the difference between the exhaust pressure and intake manifold pressure, along with the EGR valve position, may dictate how much EGR actually flows to the engine; the position of the EGR valve may be adjusted based on the exhaust pressure to maintain a desired amount of EGR at the engine. In another example, as explained above, the turbine power may be determined, and the wastegate position may be adjusted based on the turbine power. For example, if the turbine power is less than a desired turbine power, the wastegate may be moved to a more closed position.

Returning to204, if it is determined that the engine is operating with valve overlap, method200proceeds to218to determine if the valve overlap period is greater than a first threshold. The valve overlap period may include a period of time in which both the exhaust valve and intake valve for a given cylinder are open. As explained above, during valve overlap, the exhaust pressure may differ from the pressure without valve overlap. However, if the valve overlap period is relatively small (e.g., less than the first threshold), the effect on the exhaust pressure may be minimal. The threshold overlap period may be a suitable threshold below which minimal effect on the overall exhaust pressure is observed, such as five degrees crank angle. Thus, if the valve overlap period is less than a first threshold, method200proceeds back to206to measure the wastegate motor current and calculate the exhaust pressure without regard for the valve timing events, as explained above. If the valve overlap period is greater than or equal to the first threshold, method200proceeds to220to determine if the engine speed is less than a second threshold.

If the engine speed is relatively high (e.g., greater than the second threshold), it may not be possible to accurately sample the motor current specifically during the valve overlap period. The second threshold may be a suitable engine speed, such as 2000 RPMs. If the engine speed is greater than or equal to the second threshold, sampling during the valve overlap period may be inaccurate, so method200proceeds back to206, as explained above. If the engine speed is below the second threshold, method200proceeds to222to measure the current drawn by the wastegate actuator motor. This measurement may be similar to the measurement explained above at206. However, this measurement also includes, at224, sampling the current during the valve overlap period. The current sampling may be a profile ignition pickup (PIP) based sampling of the actuator current, which provides more localized information related to the exhaust pressure during valve overlap. The sampling during the overlap period may be performed such that the motor current is sampled once every combustion event near top dead center, such as over the 150-180° C. A duration, or at intake valve opening. Additionally, motor current may be sampled during other times of the engine cycle. This sampling results in more representative air charge calculations during the valve overlap period. Additional detail about sampling the motor current during the valve overlap period is presented below with respect toFIG. 4.

At226, the exhaust pressure during the valve overlap period is determined based on the average current sampled during the valve overlap period. The exhaust pressure may be further modified using information from current sampled at other portions of the engine cycle to adjust for other pressure wave propagation effects, sensor dynamics and dynamic effect of pressure parts of engine cycle. The exhaust pressure during overlap determined at226may be determined in a similar manner to the average exhaust pressure determined at208. For example, as indicated at228, the exhaust pressure may be determined based on the current during overlap and further based on wastegate position. Also, as indicated at230, the exhaust pressure during overlap may be determined based on the average current and further based on exhaust flow. Also, as indicated at231, the exhaust pressure during overlap may be determined based on the current during overlap and further based on average current over the engine cycle.

At232, it is determined if a cylinder air charge imbalance is detected. The cylinder air charge imbalance may be indicated if the exhaust pressure during the valve overlap period for each cylinder is not equal. For example, the motor current sampled during the valve overlap period for each cylinder may be stored, and the exhaust pressure during the overlap periods determined. If the exhaust pressure from one cylinder is different than the exhaust pressure from the other cylinders (e.g., if the exhaust pressure differs by more than 5%), it may be determined that a cylinder is out of balance. For example, the intake and/or exhaust valve timing may not be optimally set for the imbalanced cylinder, resulting in differential exhaust flow through the cylinder as compared to other cylinders in the engine.

If a cylinder imbalance is detected, method200proceeds to234to adjust the valve overlap of the imbalanced cylinder. This may include adjusting exhaust and/or intake valve closing timing, exhaust and/or intake valve lift, etc. Method200then proceeds to214to determine the air charge and/or turbine power from the exhaust pressure, as explained above. Similarly, if a cylinder imbalance is not detected at232, method200also proceeds to214to calculate air charge and/or turbine power. As explained previously, one or more operating parameters may be adjusted based on the air charge and/or turbine power at216. Method200then returns.

Thus, method200determines an average exhaust pressure by sampling wastegate actuator motor current periodically during one or more engine cycles. If the engine is operating with valve overlap, the timing of the sampling of the motor current may be set to correspond to the valve overlap period. That is, at least one sample of the motor current per engine cycle may be synchronous with an intake valve opening event of a given cylinder.

FIG. 4is a map400depicting example valve timing events for a plurality of cylinders and engine exhaust pressure. In the example depicted inFIG. 4, the valve timing events for three cylinders of a four cylinder engine are illustrated (cylinders 1, 3, and 4), assuming an engine firing order of 1-3-4-2 (cylinder 2 is not illustrated inFIG. 4). However, it is to be understood that other engine arrangements are possible, such as six-cylinder engines.

Curves402and404depict valve timings for an exhaust valve (dashed curve402) and an intake valve (solid curve404) for cylinder 1 during normal engine operation at part load. As illustrated, an exhaust valve may be opened near the time that the piston bottoms out at the end of the power stroke. The exhaust valve may then close as the piston completes the exhaust stroke, remaining open at least until a subsequent intake stroke has commenced. In the same way, an intake valve may be opened at or before the start of an intake stroke, and may remain open at least until a subsequent compression stroke has commenced.

As a result of the timing differences between exhaust valve closing and intake valve opening, for a short duration, before the end of the exhaust stroke and after the commencement of the intake stroke, both intake and exhaust valves may be open. This period, during which both valves may be open, is referred to as a positive intake to exhaust valve overlap406(or simply, valve overlap), represented by a hatched region at the intersection of curves402and404. In one example, the valve overlap406may be a default cam position of the engine.

Curves408and410depict valve timings for an exhaust valve (curve408) and an intake valve (curve410) of cylinder 3. The valve overlap for cylinder 3 is illustrated by valve overlap412. Curves414and416depict valve timings for an exhaust valve (curve414) and an intake valve (curve416) of cylinder 4. The valve overlap for cylinder 4 is illustrated by valve overlap418.

During each exhaust event, exhaust is expelled to the exhaust manifold, raising the exhaust pressure. Thus, the exhaust pressure may pulsate as each exhaust valve opens. As such, to determine an overall exhaust pressure, the motor current of the wastegate actuator may be sampled periodically, and the average motor current used to calculate the average exhaust pressure. Further, because the engine is operating with intake and exhaust valve overlap, the sampling of the motor current may be timed to overlap with the valve overlap period.

Curve420illustrates exhaust pressure in the exhaust manifold and/or exhaust passage downstream of the manifold. The dashed boxes indicate periods of time in which the wastegate motor current is sampled to calculate the average exhaust pressure. Additionally, the hatched-line boxes are motor current samples that overlap with the valve overlap periods of the cylinders. Thus, as shown inFIG. 4, the motor current is sampled four times every exhaust event, with one motor current sample per exhaust event being taken during the valve overlap period. The samples during the overlap period may be timed equally with the other samples of the motor current, as shown, or may be additional samples that are not necessarily timed equally with the other samples.