METHOD FOR BOOST PRESSURE CONTROL

Abstract

Method for a supercharged piston engine (100), comprising: while a downstream compressor (114) is spinning up, accelerating an upstream compressor (110), wherein a bypass valve (62) coupled in a first bypass (60) across the upstream compressor (110) is closed to provide a flow of compressed air to the piston engine (100); and opening the bypass valve (62) in response to an overshoot of the boost pressure.

Description

Field of InterestThe present description relates generally to methods and systems for regulating engine boost pressure in multi-stage boosted engine systems.Background / SummaryEngines may be operated using superchargers such as turbochargers or compressors to increase mass air flow into a combustion chamber. Turbochargers and compressors compress the intake air entering the engine using an intake compressor. Further, one or more intake charge devices may be arranged in series or in parallel in a stepwise manner to improve the boost response of the engine.An example of a multistage boosted engine is shown by Kawamura et al. in U.S. Pat. No. 6,938,420 B2. Therein, an electric compressor driven by an electric motor and an electric supercharger bypass valve (ESBPV) are disposed stepwise downstream of a turbocharger. During conditions where the turbocharger compressor has not spun up, the ESBPV may be closed and the electric compressor may be rotated to provide a transient boost positive pressure to reduce turbo lag. If the turbocharger compressor is then sufficiently spun up, the ESBPV may be opened and the electric compressor may be deactivated so that the turbocharger may provide the desired boost pressure.US 2004 / 0 182 371 A1 shows an arrangement with an upstream electric compressor, turbocharger with a downstream compressor and bypass valve which is coupled in a bypass via the upstream compressor. The opening of the bypass valve is disclosed when the set setpoint boost pressure is reached or exceeded.U.S. Pat. No. 7,210,296 B2 describes how boost pressure fluctuations can be avoided during operation of an additional compressor. For this purpose, the working point of the compressor is constantly adapted to the prevailing pressure conditions, so that the use of a bypass valve is superfluous.However, the present inventors have recognized potential problems with systems as described in U.S. Pat. No. 6,938,420 B2. For example, if the electric compressor is aggressively operated to reduce turbo lag, electric boost overshoot may occur, which may be difficult to control. In particular, due to hardware constraints, it may be impossible to brake the electric motor and provide a negative torque to slow the speed of the shaft of the electric compressor in response to an electric boost overshoot. Instead, the engine may be deactivated, whereby the high speeds of the electric compressor may be reduced using natural decay that includes drag effects of friction and drag. Meanwhile, however, the actual boost pressure may continue to overshoot, resulting in excessive engine torque output. Meanwhile, the engine intake throttle may not have the bandwidth for responding to the rapid pressure build-up. As a result, adjustments to the throttle may result in the actual manifold pressure overshoot the desired manifold pressure, which further contributes to overshoot of boost. To exploit the natural decay of compressor speed to begin the overshoot of boost, it may be necessary to command the deactivation of the electric compressor before the desired boost pressure or desired torque is reached. However, this increases the time to reach the maximum torque and also leads to a boost hole during the operation of the electric compressor. Thus, the excessive boost and torque may lead to driveability issues.In one example, the issues described above may be addressed by a method for a boosted engine, comprising: while a downstream compressor is cranking, accelerating an upstream compressor, wherein a bypass valve coupled in a bypass via the first compressor is closed to provide a flow of compressed air to a reciprocating engine, and responsive to a boost pressure overshoot, opening the bypass valve. Thus, the overshoot of the boost pressure can be more accurately controlled while shortening a time to reach the maximum torque.For example, an electric compressor (EK) including an electric motor driven compressor may be disposed stepwise upstream of a turbocharger (TL) including an exhaust turbine driven compressor. An electric compressor bypass valve (ESBPV) may be coupled in a bypass around the EK. To reduce turbo lag when the turbocharger compressor is spun up, the ESBPV may be closed while the electric compressor is temporarily operated via the electric motor to provide positive pressure. In response to the boost pressure overshoot downstream of the EK compressor still during TL compressor cranking (i.e., during an electric boost overshoot condition), the ESBPV may be temporarily opened to quickly reduce the electric boost pressure provided by the electric compressor. Additionally, the electric compressor may be deactivated or decelerated simultaneously. Further, the compressor settings in the complementary frequency band may be provided as settings of a wastegate coupled to the TL's exhaust turbine, enabling a more rapid and accurate regulation of the TL compressor speed. In particular, the wastegate control loop may be more aggressively tuned because the fast dynamics of the ESBPV would be able to dampen any oscillations and reduce the overshoot of the boost pressure.Thus, airflow through an electric compressor may be substantially immediately constrained, allowing for more rapid control of electric boost overshoot. The technical effect of opening a bypass valve around the electric compressor while a downstream turbocharger is cranking is that boosted air flow to the engine may be more quickly reduced to a level desired based on driver demand. This enables a more rapid regulation of the pressure of boosted air and a reduction of excessive output of engine torque. By slowing the compressor simultaneously, boost pressure may not be affected by delays due to compressor speed droop. In addition, if a sudden operator re-decision occurs while the turbocharger is cranking up, the ESBPV may be closed and the EK may be accelerated to quickly meet the driver demand without degrading the time to reach maximum torque. Further, adjustments to the EK and ESBPV may be provided in coordination with throttle adjustments and in the complementary frequency band as wastegate adjustments, enabling more rapid and accurate boost pressure regulation. Furthermore, the wastegate control loop can be matched more aggressively.It should be understood that the summary above is provided to introduce in a simplified manner a selection of concepts that are further described in the detailed description. It is not intended to identify key or key features of the claimed subject matter, the scope of which is uniquely defined in the claims that follow the detailed description. Moreover, the claimed subject matter is not limited to implementations that eliminate disadvantages noted above or in any part of this disclosure.Brief Description of the DrawingsFIG. 1 shows an example embodiment for a boosted engine system including multiple stepped boost devices. FIG. 2 shows a high level flow chart depicting a routine that may be implemented for regulating engine boost pressure in an engine system including an electric compressor and a turbocharger. FIG. 3 is a flow chart illustrating a routine that may be implemented to regulate the overshoot of boost to an electric compressor or turbocharger. FIG. 4 shows a compressor characteristic diagram of the compressor pressure ratio over the air mass flow. FIG. 5 shows adjustment patterns that may be used to reduce EK overshoot and TL overshoot.Detailed DescriptionThe following description relates to systems and methods for improving boost control in an engine system having stepped boost devices, such as in the boosted engine system of FIG. 1, where a turbocharger is disposed stepwise downstream of an electric compressor. A controller may be configured to execute a routine, such as the example routines of FIGS. 2-3, to use the electric compressor to reduce turbo lag while regulating boost pressure overshoot using adjustments to a speed of the compressor engine and opening of a bypass valve coupled via the compressor. These adjustments may be used at a complementary frequency for wastegate valve adjustments that regulate boost pressure. During conditions of boost pressure overshoot downstream of the turbocharger, compressor speed of the electric compressor may be regulated to throttle air flow through the turbocharger, the compressor speed adjusted based on a compressor map, such as the map of FIG. 4. Example operation of boost control to coordinate adjustments across the compressor and the turbocharger is shown with reference to FIG. 5. Thus, overshoot of the boost pressure can be more effectively controlled.FIG. 1 schematically illustrates aspects of an example engine system 100 that includes an engine 10. In the depicted embodiment, engine 10 is a boosted engine that includes multiple stepped boost devices. Specifically, the engine 10 includes a first supercharger 15 disposed stepwise upstream of a second supercharger 13. The configuration results in a second compressor 114 (of the second boosting device) being disposed in the intake pipe 42 of the engine downstream of a first compressor 110. In the present example, the second boost device is a turbocharger 13, while the first boost device is an electric compressor 15. The downstream turbocharger 13 may be capable of generating a higher boost pressure than the compressor 15. In the present invention, the downstream compressor is larger than the upstream compressor (for example, the downstream turbocharger compressor may have larger inertia, flow area, etc., and accordingly may be capable of providing a larger amount of boost pressure over a longer period of time as compared to the upstream compressor of the compressor.The electric compressor 15 includes a first compressor 110 driven by an electric motor 108. The electric motor 108 is operated at an engine speed based on a desired boost pressure. The first compressor 110 may be decelerated by reducing the engine speed of the electric motor 108. The motor 108 is operated by a built-in energy storage device, such as system battery 106. The second compressor 114 is driven by an exhaust turbine 116. Fresh air is introduced into the engine 10 along the intake pipe 42 via the air cleaner 112 and flows to the first compressor 110. Air compressed by the first compressor 110 is then supplied to the second compressor 114. During selected conditions, as set forth below, the air may bypass compressor 15 and be directed to turbocharger 13 through first compressor bypass 60 by adjusting the opening of electric compressor bypass valve (ESBPV) 62.The turbocharger 13 includes the second compressor 114 driven by an exhaust turbine 116. The second compressor 114 is shown as a turbocharger compressor mechanically coupled to the turbine 116 via a shaft 19, the turbine 116 being driven by expanding engine exhaust gases. In one embodiment, the turbocharger may be a twin scroll device. In another embodiment, the turbocharger may be a variable turbine geometry (VTG) supercharger, where the turbine geometry is actively varied as a function of engine operating conditions. Fresh air received at the compressor inlet of the second compressor 114 is fed into the engine 10. Air compressed by the turbocharger 13 may be recirculated from the outlet to the inlet of the compressor 114 through the second compressor bypass 70 by adjusting the opening of a compressor recirculation valve (CRV) 72. The CRV 72 may be a continuously variable valve, and increasing the opening of the recirculation valve may include actuating (or driving) a solenoid of the valve. As shown in FIG. 1, the second compressor 114 is coupled to the throttle valve 20 through the charge-air cooler (CAC) 18 (also referred to herein as an intercooler). The throttle valve 20 is coupled to the intake manifold 22 of the engine. From the second compressor, the compressed air charge flows through the charge air cooler 18 and the throttle valve to the intake manifold. The charge air cooler may be, for example, an air-to-air or water-to-air heat exchanger. In the embodiment shown in FIG. 1, the pressure of the air charge within the intake manifold is sensed by manifold air pressure (MAP) sensor 124.It should be noted that the first compressor as used herein refers to the upstream of the staged compressors and the second compressor as used herein refers to the downstream of the staged compressors. In one non-limiting example, as shown, the second downstream compressor is a turbocharger compressor, while the first upstream compressor is a supercharger compressor. However, other combinations and configurations of boost devices may be possible.During selected conditions, such as tip-in, turbo lag may occur during the transition from un-boosted engine operation to boosted engine operation. This is due to delays in spinning the turbine of the second compressor 114. To reduce this turbo lag, compressor 15 and turbocharger 13 may both be activated during these selected conditions. In particular, boost pressure may be provided by the upstream supercharger compressor 110 during the cranking of the turbine 116. Activating the compressor includes drawing energy from the battery 106 to rotate the motor 108 to thereby accelerate the first compressor 110. Additionally, bypass valve 62 may be closed to allow a greater amount of air to be compressed by first compressor 110. Due to its smaller size and limited charging of the system battery, the first compressor 110 may quickly start up and be rotated for a short time, enabling the provision of a short duration charge over pressure in response to a sudden increase in the charge demand. If the turbine is then sufficiently spun up and is capable of driving the second compressor 114, the first compressor may be decelerated by disabling the engine 108. Additionally, bypass valve 62 may be opened so that a larger portion of air may bypass first compressor 110. As set forth herein, by coordinating opening of the ESBV 62 with engine speed adjustments, electric boost overshoot may be addressed downstream of the first compressor and upstream of the second compressor.During selected conditions, such as tip-in from boosted engine operation to higher boosted engine operation, boost pressure may overshoot downstream of the turbocharger compressor (also referred to herein as turbocharger overshoot). The turbocharger overshoot includes a higher input pressure at the throttle than desired because the turbocharger compressor 114 rotates faster than desired, which may result in excessive engine torque output. As embodied herein, to mitigate turbocharger overshoot, the first compressor 110 may be accelerated to a speed corresponding to a desired throttle flow of air charge through the second compressor, as determined based on the compressor map (FIG. 4 ). Additionally, the ESBPV 62 may be closed. As a result, the input pressure at the throttle valve can be reduced substantially directly to a desired pressure.The second compressor 114 may be further decelerated by increasing the opening of the wastegate 90 by opening the wastegate valve 92 via a wastegate actuator to allow exhaust air to bypass the turbine 116. During operation of the first compressor 110, the turbocharger wastegate 90 may be actuated at a higher than the predetermined boost tuning. The faster electric compressor speed settings may reduce the boost error of turbocharger 13 more quickly than by actuation of turbocharger wastegate 90, such that wastegate 90 may be more aggressively tuned during operation of electric compressor 15.In some embodiments, the ESBPV 62 may be normally partially opened during operation of the first compressor 110. In response to electric compressor overshoot, the opening of ESBPV 62 may be increased, resulting in a substantially immediate reduction in boost pressure provided by the electric compressor. For example, the valve may be shifted from the predetermined partially opened position to a fully opened position. An opening degree of the valve during these conditions may be based on the electric compressor boost pressure output failure. The ESBPV 62 may be maintained closed while the electric compressor is deactivated.One or both of the valves 62 and 72 may be continuously adjustable valves, wherein a position of the valve is continuously adjustable from a fully closed position to a fully open position. Alternatively, the compressor recirculation valve 72 may be a continuously variable valve while the compressor bypass valve 62 is an open / close valve.One or more sensors may be coupled to an inlet of the second compressor 114 (as shown) and / or the first compressor 110 (not shown). For example, a temperature sensor 55 for estimating a compressor inlet temperature may be coupled to the inlet. As another example, a pressure sensor 56 for estimating a pressure of the air charge entering the compressor may be coupled to the inlet. Still other sensors may include, for example, combustion air ratio sensors, humidity sensors, etc. In further examples, one or more of the compressor inlet conditions (such as humidity, temperature, etc.) may be inferred based on engine operating conditions. The sensors may estimate a condition of intake air drawn from the intake manifold at the compressor inlet and air charge recirculated from upstream of the CAC. One or more sensors may be further coupled to intake manifold 42 upstream of compressor 114 and compressor 110 to determine a composition and condition of air charge entering the compressor. These sensors may include, for example, the manifold air quantity sensor 57.Intake manifold 22 is coupled to a series of combustion chambers 30 through a series of intake valves (not shown). The combustion chambers are further coupled to the exhaust manifold 36 via a series of exhaust valves (not shown). In the illustrated embodiment, a single exhaust manifold 36 is shown. However, in other embodiments, the exhaust manifold may include a plurality of exhaust manifold sections. Configurations having a plurality of exhaust manifold sections may allow waste water from different combustion chambers to be directed to different locations in the engine system.In one embodiment, each of the exhaust and intake valves may be electronically actuated or regulated. In another embodiment, each of the exhaust and intake valves may be cam actuated or controlled. Whether electronically actuated or cam actuated, the timing of the opening and closing of the exhaust and intake valves may be adjusted as required for desired performance in combustion and emissions control.The combustors 30 may be supplied with one or more fuels such as gasoline, alcohol-fuel mixtures, diesel, biodiesel, compressed natural gas, etc. Fuel may be delivered to the combustion chambers via direct injection, port injection, throttle valve body injection, or a combination thereof. Combustion may be initiated in the combustion chambers via spark ignition and / or compression ignition.As shown in FIG. 1, exhaust from one or more exhaust manifold sections is directed to turbine 116 to drive the turbine. If reduced turbine torque is desired, some exhaust may instead be directed through wastegate 90 and thereby bypass the turbine. Wastegate valve 92 may be actuated to open to exhaust at least some exhaust pressure from upstream of the turbine via wastegate 90 to a location downstream of the turbine. Turbine speed may be further reduced by reducing exhaust pressure upstream of the turbine. Similarly, during turbo lag, turbine speed may be increased by increasing exhaust pressure upstream of the turbine, such as by activating an upstream electric compressor while maintaining the turbocharger wastegate closed. Accordingly, during turbo lag, the electric compressor may be used to provide the desired boost pressure because the turbocharger accelerates while simultaneously increasing the rate at which the turbocharger accelerates by increasing the exhaust pressure upstream of the turbine.The combined flow from the turbine and wastegate then flows through emission control 170. Generally, one or more emission control devices 170 may include one or more exhaust aftertreatment catalysts configured to catalytically treat the exhaust stream and thereby reduce an amount of one or more substances in the exhaust stream. For example, an exhaust aftertreatment catalyst may be configured to store NO x from the exhaust flow when the exhaust flow is lean and reduce the stored NO x when the exhaust flow is rich. In further examples, an exhaust aftertreatment catalyst may be configured to disproportionate NO x or selectively reduce NO x using a reductant. In still further examples, an exhaust aftertreatment catalyst may be configured to oxidize hydrocarbon and / or carbon monoxide residues in the exhaust stream. Different catalytic converters for exhaust gas aftertreatment with such functionality can be arranged either separately or jointly in washcoats or elsewhere in the exhaust gas aftertreatment stages. In some embodiments, the exhaust aftertreatment stages may include a regeneratable soot filter configured to store and oxidize soot particles in the exhaust stream.The treated exhaust gas from the emission control 170 may be discharged to the atmosphere wholly or partly via the exhaust pipe 35. However, depending on operating conditions, some exhaust gas may instead be diverted to the intake manifold via an EGR pipe (not shown) including an EGR cooler and an EGR valve. The EGR may be recirculated to the inlet of the first compressor 110, the second compressor 114, or both.The engine system 100 may further include the control system 14. Control system 14 is shown receiving information from a plurality of sensors 16 (various examples of which are described in this disclosure) and transmitting control signals to a plurality of actuators 81 (various examples of which are described in this disclosure). For example, sensors 16 may include exhaust gas sensor 126, MAP sensor 124, exhaust temperature sensor 128, exhaust pressure sensor 129, compressor inlet temperature sensor 55, compressor inlet pressure sensor 56, and LMM sensor 57 located upstream of the emissions control device. Other sensors, such as additional pressure, temperature, combustion air ratio, and composition sensors, may be coupled to various locations in engine system 100. Actuators 81 may include, for example, actuators for throttle valve 20, compressor recirculation valve 72, ESBPV 62, electric motor 108, wastegate valve 92, and injector 66. The control system 14 may include a controller 12. The controller may receive input data from the various sensors, process the input data, and employ the various actuators based on the received signals and instructions stored on a memory of the controller. The controller may implement the actuators in response to the processed input data based on instruction or code programmed therein according to one or more routines, such as example control routines as described in this disclosure with respect to FIGS. 2-3.For example, the controller 12 may include computer readable instructions stored on non-transitory memory to flow compressed air to the engine by operating the first compressor 110 while the bypass valve 62 and the wastegate valve 90 are each closed until the speed of the turbine 116 dexceeds a threshold turbine speed, responsive to tip-in by an operator, and intermittently open the bypass valve 62 while the wastegate valve 90 is maintained closed, responsive to boost pressure overshoot while the first compressor 110 is operated. The controller 12 may further include computer readable instructions stored on non-transitory memory to, responsive to a tip-in by an operator from boosted engine operation as the second compressor 114 rotates, close the wastegate valve 90 to adjust a speed of the second compressor 114 and adjust a speed of the first compressor 110 to limit airflow into the second compressor 114.Thus, the components of FIG. 1 enable an example engine system, comprising: an engine having a suction side; a first intake compressor powered by an electric motor, the engine operated by a battery; a second intake compressor powered by an exhaust turbine, the second compressor disposed downstream of the first compressor along the suction side; a bypass including a bypass valve coupled via the first compressor; a wastegate including a wastegate valve coupled via the exhaust turbine; a compressor recirculation valve coupled via the downstream compressor; and a controller. The controller may be configured with computer readable instructions stored on non-transitory memory to: responsive to a tip-in by an operator, flow compressed air to the engine by operating the first compressor while the bypass valve and the wastegate valve are each closed (and the recirculation valve is closed) until a turbine speed exceeds a threshold turbine speed; and responsive to a boost pressure overshoot while the first compressor is operated, intermittently open the bypass valve while the wastegate valve (and the recirculation valve) is maintained closed. The controller may include further instructions for: flowing compressed air to the engine by operating the second compressor while bypassing the second compressor, the bypass valve closed and the wastegate valve partially opened after the turbine speed exceeds the threshold turbine speed; and responsive to a boost pressure overshoot while operating the second compressor, intermittently increasing an opening of the wastegate valve (and / or the recirculation valve) while maintaining the bypass valve closed. The controller may include further instructions to actuate the wastegate valve at a higher than the predetermined trim when the bypass valve is not degraded, and actuate the wastegate valve at the predetermined trim in response to an indication of bypass valve degradation.The components of FIG. 1 may further enable another example engine system, wherein the controller, with computer readable instructions stored on non-transitory memory, is configured to: responsive to a pedal operation by an operator as the second compressor rotates, close the wastegate valve to adjust a speed of the second compressor; and simultaneously adjust a speed of the first compressor to limit airflow into the second compressor. A degree of wastegate valve closure and a speed of the first compressor may be adjusted to provide a desired pressure ratio at the second compressor, the desired pressure ratio based on driver demand. The controller may include further instructions for, upon reaching the desired pressure ratio, slowing the first compressor while increasing an opening of the wastegate valve (and / or the recirculation valve). The controller may include further instructions for: responsive to a tip-in by an operator while the second compressor is not rotating, closing the wastegate valve (and / or the recirculation valve) to increase the speed of the second compressor; and simultaneously increasing the speed of the first compressor to increase airflow through the second compressor while increasing an opening of the bypass valve based on a boost fault.FIG. 2 shows an example routine 200 for operating a compressor of an upstream boosting device (e.g., compressor) and a downstream boosting device (e.g., turbocharger) in response to changes in engine boosting demand. Instructions for carrying out method 200 and the remaining methods included herein may be executed by a controller based on instructions stored on a memory of the controller and in conjunction with signals received from sensors of the engine system, such as the sensors described above with reference to FIG. 1. The controller may employ engine actuators of the engine system to adjust engine operation according to the methods described below. The engine system herein includes a first compressor disposed stepwise upstream of a second compressor along an air intake pipe. Further, the first compressor is driven by an electric motor, while the second compressor is driven by an exhaust turbine. In one example, as shown with reference to FIG. 1, the first compressor is a supercharger compressor, while the second compressor is a turbocharger compressor.At 202, the method includes determining engine operating conditions such as engine speed, pedal position, operator torque request, ambient conditions (ambient temperature, pressure, humidity), engine temperature, etc. At 204, operating parameters are determined based on engine operating conditions. These may include, for example, EGR values, desired boost pressure, spark ignition timing, wastegate opening, throttle opening, etc. At 206, the method includes determining if charging is required. In one example, boost may be required at mid-high engine loads. In another example, boost may be required in response to operator tip-in or driver torque demand increase.If boost is not required, such as when engine load is low or driver torque demand is low, the method proceeds to 208, where the engine is operated with natural aspiration and the method ends.If boost is required, then at 210, the method includes activating the first upstream compressor and closing a bypass valve (the electric compressor bypass valve or ESBV) of a bypass coupled via the first upstream compressor. The electric compressor is used to provide transient boost pressure upon tip-in to reduce turbo lag caused by the slow acceleration of the turbocharger compressor. Because of its smaller size, the electric compressor may be rapidly accelerated to provide a desired boost pressure until the turbocharger is able to provide the desired boost pressure. The use of the electric compressor may be limited by the capacity of the battery used to rotate the electric motor of the electric compressor. Accordingly, the electric supercharger may be used to provide a fast but temporarily available amount of boost positive pressure as the turbocharger turbine spins up, and may drive the larger turbocharger compressor.Therefore, in response to an increase in driver demand torque, the first compressor is accelerated and the flow of compressed air to the engine is increased. Herein, accelerating the first compressor includes rotating the first compressor via the electric motor using energy drawn from a battery. For example, the first compressor may be rotated by adjusting an electromechanical actuator coupled to an electric motor of the compressor to rotate the motor at a higher speed by sending a control signal from the controller to the actuator. The first compressor is accelerated at a speed responsive to an increase in boost demand, and an ESBPV coupled via the electric compressor is maintained closed to allow all intake air to flow through the first compressor. Therefore, compressed air is provided to the engine via the first compressor. At this time, boost demand may be met solely via the supercharger compressor.At 212, it is determined if there is an electric compressor boost overshoot. That is, it is determined whether the boost pressure downstream of the electric compressor and upstream of the turbocharger compressor is higher than a desired boost pressure. If not, as boost continues to be provided via the electric compressor, a wastegate coupled via the exhaust turbine of the turbocharger is closed (or opening is reduced) to propel a greater portion of exhaust through the turbocharger turbine at 216, thereby accelerating turbine spin-up.The inventors of the present invention have recognized that aggressive use of the electric compressor can be advantageously used to reduce the time to achieve maximum torque and turbo lag. However, the same aggressive use may also result in the actual boost pressure overshoot the desired boost pressure. This overshoot can be addressed by adjusting the opening of the ESBV. If the boost pressure of the electric compressor is greater than the desired pressure and there is an overshoot in the boost of the electric compressor, the boost pressure of the electric compressor is regulated by adjusting (e.g., slowing) the speed of the electric compressor and simultaneously increasing the opening of the ESBV at 214, as further described in FIG. 3. In one example, the ESBV is momentarily moved to a fully open position to reduce the overshoot of the charge. In an alternative example, the ESBV is partially opened. Thus, because of hardware constraints, it may be impossible to brake the electric motor coupled to the compressor. Accordingly, by increasing the opening of the ESBPV in response to the boost pressure overshoot, airflow through the electric supercharger compressor may be rapidly reduced, whereby boost pressure provided by the electric supercharger may be substantially immediately reduced. While the ESBPV is opened to reduce the boost pressure of the electric compressor to the desired boost pressure, the electric compressor may also be decelerated, for example, to a compressor speed capable of providing the desired boost pressure without less overshoot. Thus, using a combination of adjustments to the speed of the electric supercharger compressor and the opening of the ESBV, a desired boost pressure may be achieved quickly and with less boost errors.In an alternative example where the electric compressor includes hardware for braking, the first compressor may be decelerated in response to the boost overshoot by applying a negative torque by the electric motor, such as by using magnetic brakes. From 214, the method proceeds to 216, where the turbocharger wastegate is maintained closed and the turbocharger turbine is accelerated.At 218, it is determined if the turbocharger turbine speed exceeds a threshold speed, such as a threshold speed at which the turbocharger is able to meet the boost demand. If not, then as the turbine continues to spin up, operation of the first compressor (compressor) is maintained at 220, with the electric supercharger compressor continuing to meet boost demand. Additionally, the ESBPV is closed (e.g., fully closed) and the routine ends.If the turbocharger turbine speed is above the threshold speed, then the electric compressor is decelerated at 222 and the opening of the ESBPV is increased. In one example, the ESBV is moved to a fully open position such that intake airflow may be directed to the turbocharger compressor while bypassing the supercharger compressor. Because the turbocharger is able to meet the boost demand, the electric compressor may be deactivated to avoid discharging the battery of the electric compressor. By opening the ESBPV, the electric compressor may be bypassed, allowing the electric compressor to be decelerated without restricting airflow through the intake path.At 224, the method includes flowing intake air compressed by the second, downstream compressor (the supercharger compressor) into the engine while bypassing the first, upstream compressor (the turbocharger compressor). At this time, boost demand may be met solely via the turbocharger compressor.At 226, it is determined whether the boost pressure provided by the turbocharger exceeds a requested boost pressure (i.e., whether there is a turbocharger boost overshoot). If not, the boost actuator settings are maintained at 228 and the method ends. If there is a boost pressure overshoot downstream of the turbocharger compressor, one or more of the boost actuators may be adjusted at 230 according to the methods further outlined in FIG. 3. The actuators may include an exhaust wastegate 232, the electric compressor 234, a compressor recirculation valve 236, and an electric compressor bypass valve 238. As embodied at FIG. 3, boost pressure may be reduced via adjustments to one or more of a wastegate valve (coupled via the exhaust turbine) and a compressor recirculation valve (coupled via the turbocharger compressor). Additionally, boost pressure overshoot may be addressed by reducing turbocharger compressor flow via adjustments to the ESBV coupled via the supercharger compressor and the speed of the supercharger compressor (via adjustments to the corresponding electric motor).FIG. 3 shows an example routine 300 for addressing boost pressure overshoot in a multi-stage boosted engine system. The method uses upstream compressor adjustments to reduce boost pressure overshoot at both the upstream compressor and the downstream compressor. In one example, the routine of FIG. 3 may be executed as part of the routine of FIG. 2, such as at step 214 and / or 230.At 302, electric charge overshoot conditions are confirmed. Herein, it may be determined whether boost pressure provided by the upstream electric supercharger compressor exceeds a desired boost pressure. As such, the electric compressor may be used to provide the demanded boost pressure while a downstream turbocharger is cranking. In one example, the electric compressor boost pressure overshoot may be determined based on a boost pressure estimated downstream of the first compressor and upstream of the second compressor relative to the requested boost pressure.If an electric compressor boost overshoot is confirmed, then at 304, the electric compressor is slowed and the opening of the ESBPV is simultaneously increased and the routine ends. Increasing the opening of the ESBV includes moving the ESBV from a current bypass valve position based on the desired boost pressure (which may be fully open). Additionally, each of a degree and duration of increasing the opening of the bypass valve may be based on the boost error. Herein, as the boost error (the amount of overshoot relative to the requested boost pressure) increases, the opening degree may be increased and / or the opening duration may be extended. In one example, the ESBV immediately transitions to a fully open position (from the fully closed position). Increasing the opening of the ESBPV may allow airflow to bypass the first compressor, resulting in a substantially immediate reduction in boost pressure. Meanwhile, the electric compressor may be decelerated, such as to a modified compressor speed based on the desired boost pressure. The electric compressor may be decelerated by adjusting the speed of the electric motor from a first engine speed based on the desired boost pressure to a second engine speed based on the boost error. If the overshoot condition has then decayed, the opening of the ESBV may be reduced (e.g., the ESBV may resume a fully closed position) and the supercharger compressor may be accelerated if desired.Thus, if the boost pressure overshoot were addressed by merely slowing or disabling the electric motor coupled to the compressor, the speed of the electric compressor would be reduced using natural decay that includes drag effects of friction and drag. Meanwhile, however, the actual boost pressure would continue to overshoot, resulting in excessive engine torque output. Meanwhile, the engine intake throttle may not have the bandwidth to respond to the rapid pressure build-up. As a result, any adjustments to the throttle would result in the actual manifold pressure overshoot the desired manifold pressure, which further contributes to overshoot of boost. To take advantage of the natural decay of the compressor speed to begin the overshoot of the boost, the controller may need to deactivate the electric compressor before the desired boost pressure or desired torque has been reached. However, this would increase the time to reach the maximum torque and also lead to a boost hole during operation of the electric compressor. In other words, if the ESBPV were not opened and only the supercharger compressor was decelerated (by decelerating the corresponding engine), the resulting drop in boost pressure would not occur fast enough and the overshoot state of boost would be prolonged. Thus, regulating the opening of the ESBPV in response to boost errors may enable more accurate control of electric compressor boost pressure.Additionally or optionally, while the ESBPV is opened, a throttle angle of the engine may be decreased to reduce input pressure at the throttle. However, in other examples, the intake throttle may be maintained fully open during boost control to prevent the wastegate control loop from being degraded. Further, during electric boost overshoot, each of the wastegate_valve and the CRV may be maintained in a closed or partially open position.Thus, as a downstream compressor spins up, a controller may accelerate an upstream compressor, wherein a bypass valve coupled in a bypass via the first compressor is closed to provide a flow of compressed air to a piston engine; and responsive to boost pressure overshoot, open the bypass valve. The downstream compressor herein is larger than the upstream compressor (for example, the downstream turbocharger compressor may have larger inertia, larger flow area, etc. Accelerating the upstream compressor includes operating an electric motor at an engine speed based on the desired boost pressure. Opening the bypass valve in response to the boost pressure overshoot may include opening the bypass valve momentarily from a fully closed position, each of an opening degree of the bypass valve and an opening duration of the bypass valve based on the boost pressure overshoot, the boost pressure overshoot occurring downstream of the downstream compressor, the boost pressure overshoot including the actual boost pressure exceeding a desired boost pressure. Additionally, the intake throttle is maintained open while the ESBV is opened. One or more of the degree of opening of the bypass valve and the duration of opening of the bypass valve is increased when the actual boost pressure exceeds the desired boost pressure, the bypass valve being closed when the actual boost pressure is at the desired boost pressure. In response to the boost pressure overshoot as compressed air flows over the upstream compressor and as the downstream compressor spins up, the controller may slow the upstream compressor by reducing engine speed of the electric motor. In response to an exhaust turbine speed exceeding a threshold speed, the upstream compressor may be decelerated, the bypass valve opened, and compressed air may flow to the reciprocating engine via the downstream compressor while bypassing the upstream compressor. Referring again to FIG. 3, if an electric supercharger boost overshoot is not confirmed, at 306, the routine determines whether a downstream turbocharger boost overshoot is present. Accordingly, it should be noted that an overshoot of the boost pressure at the upstream electric supercharger compressor and the downstream turbocharger compressor occurs at different, non-overlapping times of boosted engine operation. In particular, an overshoot in boost pressure at the electric supercharger may occur during lower airflow conditions when the electric supercharger is used to meet driver demand and when the turbocharger compressor is not compressing intake air. In comparison, a turbocharger boost overshoot may occur during higher airflow conditions when the turbocharger is used to meet driver demand and when the supercharger compressor is not compressing intake air.Confirming the overshoot of boost pressure at the turbocharger includes determining whether boost pressure provided by the downstream turbocharger compressor exceeds a desired boost pressure. In one example, an electric compressor boost pressure overshoot may be determined based on a boost pressure estimated downstream of the second compressor relative to the requested boost pressure. Boost pressure may also overshoot if the turbocharger turbine speed is above a threshold and the air flow through the turbocharger compressor is above a threshold flow rate. If the turbocharger is not providing excessive boost pressure, then at 308, the boost actuator settings may be maintained and the routine ends.If a turbocharger boost overshoot is confirmed, at 310, the opening of one or more of an exhaust wastegate and a compressor recirculation valve (CRV) may be increased. In one example, one or both of the wastegate and the CRV may be moved to a fully open position. By opening the exhaust wastegate, exhaust flow through the turbine may be reduced, slowing the exhaust turbine and, as a result, the turbocharger compressor. By opening the CRV, at least some of the air compressed by the turbocharger compressor may be recirculated to upstream of the compressor, resulting in a substantially immediate reduction in intake pressure at the throttle.In one example, opening the exhaust wastegate valve includes adjusting an opening degree of the wastegate valve responsive to the boost error at a higher than the predetermined boost tuning. Herein, the electric supercharger settings (including opening of the ESBV and using a supercharger-based throttled airflow) may be provided in coordination with throttle settings and in the complementary frequency band as wastegate settings, thereby enabling a more rapid and accurate boost pressure regulation. This allows the wastegate control loop to be tuned more aggressively, as the fast dynamics of the electric compressor are able to dampen any oscillations resulting from the wastegate and reduce boost pressure overshoot.The wastegate may be operated at higher than the predetermined gain tuning during boost control and resume the predetermined gain tuning in response to actual boost tracking according to the desired boost profile or the boost error being less than a threshold (e.g., no boost error). Alternatively, the predetermined gain tuning may be resumed in response to a system FMEM mode that may be triggered in response to degradation of a boosted engine component. In one example, the wastegate may resume the predetermined gain tuning in response to an indication of ESBV degradation.However, due to relatively slow wastegate dynamics, wastegate opening may result in a slower drop in actual boost pressure, which extends the overshoot condition. Additionally, while CRV adjustment may result in a rapid drop in intake pressure at the throttle, a more severe drop in boost performance may also occur. Thus, if the boost overshoot condition is due to an operator tip-out event and relying on wastegate and / or CRV adjustment alone, it may result in a slower time to reach maximum torque if the operator briefly actuates the pedal (such as due to a re-decision). Therefore, as set forth below, further engine boost control may be performed using electric compressor based settings.At 312, it may be determined if throttle flow is required based on the electric compressor. In one example, throttle flow may be required if boost overshoot error is greater. In another example, throttle flow may be required if wastegate opening is limited due to other restrictions. In yet another example, the controller may determine whether throttle flow may be provided based on a state of charge of a battery coupled to the electric motor of the supercharger compressor. Specifically, throttle flow may be activated only if the battery state of charge is greater than a threshold. If throttle flow through the electric compressor is not required, the electric compressor settings are maintained as at 314 and the routine ends.The inventors herein have recognized that operating the electric compressor at a given rotational speed of the supercharger compressor may affect an amount of air flowing through the supercharger compressor into the downstream turbocharger compressor. This amount may be further affected by the intake air flow levels. Thus, during lower (e.g., idle) airflow levels, operating the supercharger compressor may increase airflow through the supercharger compressor into the downstream turbocharger compressor. This is exploited during conditions where the turbine is not sufficiently cranking to rapidly accelerate the supercharger compressor and direct boosted air flow to the engine. However, at higher airflow levels, such as when the turbine has started and the engine is receiving boosted air via the turbocharger compressor, operating the supercharger compressor may reduce airflow through the supercharger compressor into the downstream turbocharger compressor. As a result, the flow of air into the turbocharger compressor is restricted or throttled, which provides a substantially immediate reduction in boost pressure. By adjusting the speed of the supercharger compressor via adjustments to the speed of the electric motor, the flow of air into the turbocharger compressor may be decreased, thereby regulating the output boost pressure by the turbocharger compressor. In other words, the supercharger compressor may be operated as a flow restrictor for the turbocharger compressor, and throttled flow into the turbocharger compressor may be used to control the overshoot of boost pressure. Thus, this may be used in addition to wastegate adjustments in complementary frequency bands to enable more accurate boost control.If throttle flow is required, at 316, an amount of airflow (throttle flow) to be provided by the electric supercharger compressor into the turbocharger compressor may be determined based on the boost pressure error. In one example, as boost error increases (where the actual boost pressure overshoot the desired boost pressure), a lower airflow into the turbocharger compressor may be requested (i.e., a lower absolute airflow level resulting from a higher amount of throttling of the airflow). In another example, the amount of airflow may be selected to provide a desired pressure ratio across the downstream turbocharger compressor.At 318, a speed of the electric supercharger compressor corresponding to the desired amount of throttle flow may be determined. As such, the required supercharger compressor speed required for desired throttle flow may vary based on operating conditions including an intake throttle position and an intake air flow (e.g., manifold air flow levels). For example, at a given manifold airflow level, the airflow into the turbocharger compressor may be increased as the speed of the supercharger compressor increases. Accordingly, the greater the rotational speed of the compressor compressor, the higher the throttle flow occurs. As shown in FIG. 4, at different compressor speeds, throttle flow is where the constant speed line intersects the x-axis (or the pressure ratio is 1). Airflow into the turbocharger compressor is reduced as the ESBV has been closed. The lower the compressor speed, the less current passes through. As another example, for a given supercharger compressor speed, a lower airflow may be directed into the downstream turbocharger compressor as the intake manifold airflow level increases. The controller may refer to a compressor map, such as the example map of FIG. 4, to determine a desired speed of the supercharger compressor corresponding to the desired throttle flow.At 320, the electric compressor may be accelerated to the desired compressor speed determined at 318 to reduce the downstream compressor airflow to the desired throttle flow. Simultaneously, the ESBPV may be closed (e.g., fully closed) to direct airflow through the electric supercharger compressor and throttle airflow into the turbocharger compressor, thereby reducing boost pressure. Then, the routine ends. Herein, the upstream compressor is accelerated while maintaining an intake throttle open until airflow through the downstream compressor is below a threshold. Thereafter, the upstream compressor may be decelerated (e.g., to zero or a minimum speed).Thus, in response to boost pressure overshoot, a speed of the first, upstream compressor is adjusted. In one example, adjusting the speed of the electric supercharger compressor includes an increase from zero speed (such as when the compressor was deactivated and the compressor was reactivated and rotated from zero speed to a speed corresponding to throttle flow). In an alternative example, the adjusting includes an increase from a minimum speed, wherein an amount of increase is based on a degree of overshoot. In either case, the speed of the electric supercharger compressor is adjusted to limit flow through the downstream compressor to a threshold airflow. Herein, the speed of the electric compressor is adjusted by rotating an electric motor coupled to the supercharger compressor at an engine speed based on the airflow.In one example, responsive to boost pressure overshoot as compressed air flows over a downstream compressor, a controller may increase an opening of one or more of a wastegate valve coupled via the exhaust turbine and a compressor recirculation valve coupled in bypass via the downstream compressor. In comparison, responsive to the boost pressure overshoot as compressed air flows across the upstream compressor, the controller may maintain each of the wastegate valve and the compressor recirculation valve closed. Alternatively, the opening of the wastegate valve and the CRV may be maintained, such as in a partially open position. Herein, the wastegate valve may be actuated at a higher than the predetermined gain setting while compressed air flows across the upstream compressor and at the predetermined gain setting while compressed air flows across the downstream compressor.Thus, an upstream electrically actuated compressor may be accelerated during some conditions to increase airflow through a downstream compressor, whereas during other conditions, the electrically actuated compressor may be accelerated to decrease airflow through the downstream compressor. By using compressor and bypass valve speed adjustments along with wastegate adjustments in complementary frequency bands, boost pressure overshoot downstream of the turbocharger compressor may be better handled and quickly regulated. Thus, this enables boost pressure to be regulated without degrading the time to reach the maximum torque.FIG. 4 shows an example compressor map 400 for an electric supercharger compressor. The compressor map 400 may be stored in the memory of a controller and used to determine a speed of the supercharger compressor required to provide a throttled flow of air into a downstream turbocharger compressor. The vertical axis (y-axis) of map 400 shows the compressor pressure ratio, which is defined as the ratio of the air pressure at the outlet of the electric supercharger compressor to the ambient air pressure (atmospheric pressure). The horizontal axis (x-axis) shows the flow rate of the mass air flow through the electric compressor, wherein the electric compressor is arranged stepwise upstream of a turbocharger. Line 402 (solid) shows an surge limit for the supercharger compressor. Operating the supercharger compressor under conditions to the left of this line may result in compressor surge, throttling flow through the supercharger compressor. The solid lines 404 (only 2 are labeled) represent constant speed lines of the supercharger compressor. The direction of increasing the compressor speed is shown.As described with reference to FIG. 3, an upstream electric supercharger compressor may be accelerated to a desired compressor speed in response to a boost pressure overshoot at a downstream turbocharger compressor to throttle airflow through the turbocharger compressor. As a result, the intake pressure at the throttle valve and the boost pressure can be substantially directly reduced. Operation of the electric supercharger produces effective throttle flow by limiting the flow of air that may pass through the electric supercharger compressor at a given compressor speed and pressure ratio. The electric compressor may regulate the flow restriction to boost a particular volume of air through the turbocharger, resulting in more precise and substantially immediate regulation of boost pressure.In one example, an engine control may determine a desired airflow into the turbocharger compressor based on the boost error. Then, based on the manifold air flow, the controller may determine a corresponding pressure ratio across the upstream supercharger compressor. Thus, the desired pressure ratio across the compressor may correspond to a ratio of the pressure at the inlet of the supercharger compressor (corresponding to manifold air flow) relative to the pressure at the outlet of the supercharger compressor (corresponding to desired throttle flow). Based on the desired pressure ratio and the given manifold air flow, the controller may determine a corresponding compressor speed. The controller may then actuate the electric motor of the electric compressor to rotate the compressor at the determined compressor speed. In another example, the compressor map may be extrapolated to a pressure ratio of less than 1. Throttle flow may begin when the flow is on a line of constant speed across the x-axis (or pressure ratio=1). A desired output rotational speed of the compressor can then be looked up on the basis of the desired air mass flow. The feedback control may be added based on the current pressure ratio (<1) and the desired mass air flow. For example, an outlet pressure of the compressor may be below ambient pressure (resulting in a pressure ratio outside the range of pressure ratios depicted in the MSG map of FIG. 4 ), such as at a time just after the ESBPV is closed and while the electric compressor is spun up. The controller may extrapolate the compressor map of FIG. 4 to the lower pressure ratio to estimate a desired compressor speed. As the compressor speed increases and the compressor pressure ratio likewise increases, feedback control may be used to determine a desired compressor speed.FIG. 5 shows an example sequence 500 illustrating boost pressure control by actuating the wastegate (WG) and the ESBV in coordination with the electric compressor speed control. The horizontal axis (x-axis) indicates time and the vertical markings t1-t8 indicate essential points in time for the boost pressure regulation. The first plot from the top shows the change in pedal position (line 502) over time. The second plot (line 504) shows the change in boost pressure over time. The third curve (line 508) shows the change in speed of an electric supercharger compressor. The fourth plot (line 512) shows the change in speed of a turbocharger turbine over time. The fifth plot (line 516) shows changes in wastegate opening over time. In the open state, a wastegate allows exhaust gas to bypass the turbine of the turbocharger, thereby decreasing the speed of the turbocharger turbine. The sixth plot (line 518) shows changes in position of an electric compressor bypass valve (ESBV) coupled via the electric compressor. The ESBV is opened to allow intake air to bypass the electric compressor, or closed to direct air through the electric compressor.Prior to time t 1, the engine is operated at a substantially constant speed without boosting. At time t 1, the operator actuates the pedal out of the closed pedal, transitioning the engine from natural intake engine operation to boosted engine operation, as seen by the increase at line 502. The engine boost pressure may be increased in response to the pedal actuation event by actuating an electric motor to increase the speed of the electric compressor. Simultaneously, the ESBV is closed to direct more air through the supercharger compressor. Simultaneously, the wastegate opening is reduced to allow more exhaust gas to flow through the turbocharger turbine and to speed up the turbine spin. By operating the smaller electric supercharger compressor in response to the tip-in event, boost pressure may be quickly increased to meet driver demand while the turbine is cranking. Thus, if the electric supercharger compressor was not spun, turbo lag may have occurred due to the lag in spinning the turbine up (lag in the actual boost pressure reaching the desired boost pressure), as shown by dashed curve 506. Specifically, the desired boost pressure is provided by operating the electric compressor until t 2, whereas in turbo lag, the desired boost pressure is provided at about t 4.At time t 2, as compressed air flows to the engine via the electric compressor, boost pressure may overshoot. In particular, aggressive operation of the electric compressor that reduces turbo lag may further result in the actual boost pressure downstream of the supercharger compressor briefly exceeding the desired boost pressure. To address this boost pressure overshoot, the ESBPV is opened (fully in the illustrated example) for a duration from t2to t3to direct the air flow to the engine while bypassing the compressor. Opening the ESBPV may reduce the amount of airflow passing therethrough and compressed by the supercharger compressor, thereby substantially immediately reducing boost pressure provided by the electric supercharger. At the same time, the speed of the electric compressor is reduced to better match the desired boost pressure. Since adjusting the opening of the ESBV may result in a substantially immediate decrease in boost pressure during compressor overshoot, the ESBV may be actuated at higher than the predetermined gain tuning when the electric compressor is accelerated (from time t 1 to t 2). Due to actuation of the ESBPV at higher than the predetermined gain tuning, the overshoot of the compressor is limited in degree and duration, as seen at line 508. Thus, in the absence of ESBV adjustments, a greater and longer boost pressure overshoot may occur, as seen at dotted line 510. This longer and more overshoot of boost pressure could lead to excessive torque being delivered, leading to driveability issues.At time t 3, electric compressor overshoot has been reduced by ESBPV actuation, thus the ESBPV is closed. Additionally, because the turbine has still not been sufficiently spun up, flow of compressed air via the electric compressor to the engine via the electric compressor is resumed.It should be noted that while the ESBV is shown as an open / close valve movable between fully open and fully closed positions, in an alternative example, the ESBV may be a continuously variable valve whose position may be adjusted to any position therebetween and including the fully open and fully closed positions. In such a case, the opening of the ESBV may be increased in response to the boost pressure overshoot, such that a particular amount of air flow may bypass the electric compressor during the overshoot to have a resulting boost pressure equal to the desired boost pressure. The speed of the electric supercharger compressor may be adjusted to the desired compressor speed in response to the boost error, and the opening of the ESBV may be adjusted to maintain the resulting boost pressure at the desired boost pressure.At time t 4, the turbocharger turbine speed reaches a desired turbine speed at which it may provide the desired boost pressure without assistance from the electric compressor. Once the turbine has been sufficiently spun up, the wastegate opening may be adjusted with a higher gain tuning to maintain boost pressure. To conserve battery charge of the electric compressor, the ESBPV is opened once the turbine has started, as seen at line 518, and the electric compressor is decelerated, as seen at line 508. Thereafter, compressed air may flow into the engine to meet the driver's boost demand via the turbocharger compressor.At time t 5, the operator re-actuates the pedal from partial pedal depression, resulting in a transition of the engine from a lower boosted state to a higher boosted state. In response to the boost demand being increased, the speed of the turbocharger turbine is increased by decreasing the wastegate opening, actuating the wastegate at a higher than the predetermined boost tuning, as seen at line 516. Since the turbine has already started at this time, the supercharger compressor may remain deactivated.At time t 6, as compressed air flows to the engine via the turbocharger, boost pressure may overshoot. In response to the overshoot, the wastegate opening is increased (e.g., the wastegate is fully opened) to slow the turbine and reduce turbocharger performance, as seen at line 516. To further speed up the boost pressure reduction, the electric compressor is accelerated to a desired speed to throttle airflow through the turbocharger compressor. In particular, the electric supercharger is accelerated to a speed that allows a desired pressure ratio to be provided at the downstream turbocharger compressor. During acceleration of the compressor, the ESBV is closed to direct airflow through the supercharger compressor, actuating the ESBV at the predetermined gain tuning. By throttling the flow of air to the supercharger compressor, the electric supercharger reduces boost pressure more quickly. Thus, in the absence of compressor induced throttle flow due to the relatively slower wastegate dynamics, the turbine speed and boost pressure may have decreased more slowly, as seen at dotted lines 506 and 514. The resulting output of excessive torque would have caused driveability problems.At time t 7, the boost error is reduced. Thus, at t 7, the ESBV is opened as seen at line 518 and the electric compressor is decelerated as seen at line 508. The speed of the turbocharger compressor is maintained by reducing the wastegate opening, as seen at line 516. Thereafter, the discharge of compressed air to the engine via the turbocharger compressor is resumed.At time t 8, the operator releases the pedal, as seen at line 502. The turbocharger boost pressure and turbine speed are reduced to establish a desired boost pressure, as seen at lines 504 and 512, by opening the turbocharger wastegate, as seen at line 516. The wastegate is closed when the boost pressure at the turbocharger is equal to the desired boost pressure, as seen at line 516.Thus, during a first boost state, a controller may increase the opening of a bypass valve coupled in a bypass via a first upstream compressor; and during a second boost state, the controller may increase the opening of a wastegate valve coupled in a bypass via an exhaust turbine, the turbine driving a second downstream compressor, wherein the boost state overshoot occurs downstream of the second compressor during both the first and second states. In one example, responsive to the first state of boost pressure overshoot, flow of compressed air may be provided to the engine via the first compressor, the second compressor deactivated and the first compressor driven by an electric motor, whereas during the second state of overshoot, flow of compressed air of boost pressure may be provided to the engine via the second compressor while bypassing the first compressor, the second compressor driven by an exhaust turbine. During the first boost pressure overshoot condition, turbine speed may be below a threshold speed and air flow through the second compressor may be below a threshold flow rate, whereas during the second boost pressure overshoot condition, turbine speed may be above the threshold speed and air flow through the second compressor may be above the threshold flow rate (such as an idle flow rate). Further, the controller may maintain a position of the exhaust wastegate valve during the first boost pressure overshoot condition, maintain a position of the bypass valve during the second boost pressure overshoot condition, and maintain an opening of an intake throttle during both the first and second boost pressure overshoot conditions. Herein, each of the first and second overshoot conditions of boost pressure may include a boost error between the actual boost pressure and the desired boost pressure, whereas during the first overshoot condition of boost pressure, the position of the bypass valve is based on the desired boost pressure and each of a degree and duration of increasing the opening of the bypass valve is based on the boost error, whereas during the second condition, the position of the wastegate valve is based on the desired boost pressure and a degree of increasing the opening of the wastegate valve is based on the boost error. The controller may further reduce a speed of the electric motor from a first engine speed based on the desired boost pressure to a second engine speed based on the boost error during the first overshoot state of boost pressure. Increasing the opening of the bypass valve during the first boost pressure overshoot condition includes increasing the opening of the bypass valve until the boost error is reduced, and then closing the bypass valve while the first compressor is further accelerated.Thus, boost pressure is more accurately and quickly regulated by coordinating adjustments to a speed of the electric compressor and an associated bypass valve with adjustments to a wastegate valve.The technical effect of coordinating adjustments to an electric compressor gradually disposed upstream of a turbocharger with adjustments to an exhaust wastegate valve and a compressor recirculation valve on the suction side is that boost pressure may be more accurately regulated. By relying on an electric supercharger bypass valve to reduce boost pressure overshoot, an electric supercharger may be operated more aggressively to reduce turbo lag as the downstream turbocharger spins up. By slowing the compressor while bypassing flow to the compressor via the bypass valve, boost pressure may not be affected by delays associated with compressor speed droop. By using the electric compressor as a flow restrictor during conditions when the turbocharger is operating, the surge of boost pressure may be quickly regulated, thereby reducing driveability issues associated with excessive torque output. By improving the time to reach the maximum torque, the boost response of the engine is improved, including during a sudden event of a re-decision by the operator. Further, by coordinating the compressor speed and bypass valve settings with throttle and complementary frequency band settings as wastegate settings, a wastegate control loop may be more aggressively tuned without degrading boost accuracy.In one example, a method for a boosted engine includes: while a downstream compressor is cranking, accelerating an upstream compressor, wherein a bypass valve coupled in a bypass via the first compressor is closed to provide a flow of compressed air to a reciprocating engine, and responsive to a boost pressure overshoot, opening a bypass valve. In the preceding example, the downstream compressor is additionally or optionally larger than the upstream compressor. The larger size of the downstream compressor results in greater inertia, boost capability, etc. In any or all of the preceding examples, additionally or optionally, the opening of the bypass valve includes opening the bypass valve momentarily from a fully closed position, each of an opening degree of the bypass valve and an opening duration of the bypass valve based on the overshoot of boost pressure, the overshoot of boost pressure occurring downstream of the downstream compressor, the overshoot of boost pressure including the actual boost pressure exceeding a desired boost pressure, the method further comprising maintaining an intake throttle open. In any or all of the preceding examples, additionally or optionally, one or more of the degree of opening of the bypass valve and the duration of opening of the bypass valve is increased when the actual boost pressure exceeds the desired boost pressure, the bypass valve being closed when the actual boost pressure is at the desired boost pressure. In any or all of the preceding examples, additionally or optionally, the upstream compressor is accelerated via an electric motor and the downstream compressor is driven by an exhaust turbine, and accelerating the upstream compressor includes operating the electric motor at an engine speed based on the desired boost pressure. The upstream compressor may be controlled by an electric motor that is decoupled from engine operation and may be independently controlled. In any or all of the preceding examples, additionally or optionally, the method further comprises: responsive to the boost pressure overshoot as compressed air flows over the upstream compressor, slowing the upstream compressor by reducing engine speed of the electric motor. In any or all of the preceding examples, additionally or optionally, the method further comprises: responsive to an exhaust turbine speed exceeding a threshold speed, slowing the upstream compressor, opening the bypass valve, and flowing compressed air via the downstream compressor to the reciprocating engine bypassing the upstream compressor. In any or all of the preceding examples, additionally or optionally, the method further comprises: responsive to a boost pressure overshoot as compressed air flows over the downstream compressor, accelerating the upstream compressor, wherein the bypass valve is closed to limit airflow into the downstream compressor. In any or all of the preceding examples, additionally or optionally, the method further comprises: responsive to the boost pressure overshoot while compressed air flows over the downstream compressor, increasing an opening of one or more of the wastegate valve and a compressor recirculation valve coupled in a bypass over the downstream compressor, and responsive to the boost pressure overshoot while compressed air flows over the upstream compressor, maintaining each of the wastegate valve and the compressor recirculation valve closed. In any or all of the preceding examples, additionally or optionally, the wastegate valve is actuated at a higher than predetermined gain tuning while compressed air flows across the upstream compressor and at the predetermined gain tuning while compressed air flows across the downstream compressor.In another example, an engine method includes: during a first boost pressure overshoot condition, increasing an opening of a bypass valve coupled in a bypass via a first upstream compressor, and during a second boost pressure overshoot condition, increasing an opening of a wastegate valve coupled in a bypass via an exhaust turbine, the turbine driving a second downstream compressor, wherein the boost pressure overshoot occurs downstream of the second compressor during both the first and second conditions. The above example method additionally or optionally includes: during the first boost pressure overshoot condition, providing a flow of compressed air to the engine via the first compressor, the second compressor deactivated and the first compressor driven by an electric motor, and during the second boost pressure overshoot condition, providing the flow of compressed air to the engine via the second compressor while bypassing the first compressor, the second compressor driven by an exhaust turbine. In any or all of the preceding examples, additionally or optionally, during the first state of boost pressure, the turbine speed is below a threshold speed and the air flow through the second compressor is below a threshold flow rate, and during the second state of boost pressure, the turbine speed is above the threshold speed and the air flow through the second compressor is above the threshold flow rate. In any or all of the preceding examples, additionally or optionally, the method further comprises: maintaining a position of the exhaust wastegate valve during the first boost pressure overshoot condition, maintaining a position of the bypass valve during the second boost pressure overshoot condition, and maintaining an opening of an intake throttle during both the first and second boost pressure overshoot conditions. In any or all of the preceding examples, additionally or optionally, each of the first and second boost overshoot conditions includes a boost error between the actual boost pressure and the desired boost pressure, during the first boost overshoot condition, the position of the bypass valve is based on the desired boost pressure and each of a degree and duration of increasing the opening of the bypass valve is based on the boost error, and during the second condition, the position of the wastegate valve is based on the desired boost pressure and a degree of increasing the opening of the wastegate valve is based on the boost error. In any or all of the preceding examples, additionally or optionally, the method further comprises: reducing a speed of the electric motor during the first overshoot state of boost pressure from a first engine speed based on the desired boost pressure to a second engine speed based on the boost error. In any or all of the preceding examples, additionally or optionally, increasing the opening of the bypass valve during the first boost pressure overshoot condition includes increasing the opening of the bypass valve until the boost error is reduced, and then closing the bypass valve while the first compressor is further accelerated.In yet another example, an engine system includes an engine having a suction side, a first intake compressor powered by an electric motor, the engine operated by a battery, a second intake compressor powered by an exhaust turbine, the second compressor disposed downstream of the first compressor along the suction side, a bypass including a bypass valve coupled via the first compressor, a wastegate including a wastegate valve coupled via the exhaust turbine, and a controller with computer readable instructions stored on non-transitory memory for: responsive to operator pedal operation, flowing compressed air to the engine by operating the first compressor while the bypass valve and wastegate valve are each closed until a turbine speed exceeds a threshold turbine speed, and responsive to a boost pressure overshoot while the first compressor is operating, intermittently opening the bypass valve while the wastegate valve is maintained closed. The above example system may additionally or optionally include further instructions for, in the controller: flowing compressed air to the engine by operating the second compressor while bypassing the second compressor by closing the bypass valve and partially opening the wastegate valve after turbine speed exceeds the threshold turbine speed; and in response to a boost pressure overshoot while operating the second compressor, intermittently increasing an opening of the wastegate valve while maintaining the bypass valve closed. In any or all of the preceding examples, additionally or optionally, the controller includes further instructions to actuate the wastegate valve at greater than the predetermined trim when the bypass valve is not degraded, and actuate the wastegate valve at the predetermined trim in response to an indication of bypass valve degradation.It should be appreciated that the example rules and estimation routines included herein may be used with various engine and / or vehicle system configurations. The control methods and routines disclosed herein may be stored as executable instructions in non-transitory memories and executed by the control system including the control in combination with the various sensors, actuators, and other engine hardware. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various illustrated acts, acts, and / or functions may be performed in the order illustrated, in parallel, or in some cases omitted. Also, the order of processing is not necessarily required to achieve the features and advantages of the embodiments described herein, but is provided for ease of processing and description. One or more of the illustrated acts, acts, and / or functions may be repeatedly performed, depending on the particular strategy being employed. Further, the described acts, operations, and / or functions may graphically represent code to be programmed into non-transitory memories of the computer readable storage medium in the engine control system, in which the described acts are performed by executing the instructions in a system including the various engine hardware components in combination with the electronic controller.It should be noted that the configurations and routines disclosed herein are exemplary in nature and that these specific embodiments are not to be considered in a limiting sense, as numerous variations are possible. For example, the above technology may be applied to V6, I4, I6, V12, 4-cylinder horizontally opposed, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and / or characteristics disclosed herein.The following claims set forth in particular certain combinations and sub-combinations which are considered novel and not obvious. These claims may refer to "a" element or "a first" element or the equivalent thereof. Such claims should be understood to include inclusion of one or more such elements and neither require nor exclude two or more such elements. Further combinations and sub-combinations of the disclosed features, functions, elements and / or characteristics may be claimed by altering the present claims or by filing novel claims in this or a related application. Such claims, whether broader, narrower, equal or different in scope from the original claims, are further considered to be within the scope of the present disclosure.

Claims

A method for a boosted reciprocating engine (100), comprising: while a downstream compressor (114) is cranking, accelerating an upstream compressor (110), wherein a bypass valve (62) coupled in a first bypass (60) via the upstream compressor (110) is closed to provide a flow of compressed air to the reciprocating engine (100); and responsive to a boost pressure overshoot, opening the bypass valve (62).The method of claim 1, wherein the downstream compressor (114) is larger than the upstream compressor (110).The method of claim 1, wherein opening the bypass valve (62) includes opening the bypass valve (62) momentarily from a fully closed position, each of an opening degree of the bypass valve (62) and an opening duration of the bypass valve (62) based on the boost pressure overshoot, the boost pressure overshoot occurring downstream of the downstream compressor (114), the boost pressure overshoot including the actual boost pressure exceeding a desired boost pressure, the method further comprising maintaining an intake throttle (20) open.The method of claim 3, wherein one or more of the opening degree of the bypass valve (62) and the opening duration of the bypass valve (62) is increased when the actual boost pressure exceeds the desired boost pressure, the bypass valve (62) being closed when the actual boost pressure is at the desired boost pressure.The method of claim 3, wherein the upstream compressor (110) is accelerated via an electric motor (108) and the downstream compressor (114) is driven by an exhaust turbine (116), and wherein accelerating the upstream compressor (110) includes operating the electric motor (108) at an engine speed based on the desired boost pressure.The method of claim 5, further comprising, in response to the boost pressure overshoot as compressed air flows across the upstream compressor (110), slowing the upstream compressor (110) by reducing engine speed of the electric motor (108).The method of claim 5, further comprising, in response to a speed of the exhaust turbine (116) being greater than a threshold speed, slowing the upstream compressor (110), opening the bypass valve (62), and flowing compressed air via the downstream compressor (114) to the reciprocating engine (100) bypassing the upstream compressor (110).The method of claim 7, further comprising, in response to a boost pressure overshoot as compressed air flows across the downstream compressor (114), accelerating the upstream compressor (110), wherein the bypass valve (62) is closed to limit airflow into the downstream compressor (114).The method of claim 8, further comprising, in response to the boost pressure overshoot as compressed air flows across the downstream compressor (114), increasing an opening of one or more of a wastegate valve (92) and a compressor recirculation valve (72) coupled in a second bypass (70) across the downstream compressor (114), and, in response to the boost pressure overshoot as compressed air flows across the upstream compressor (110), maintaining each of the wastegate valve (92) and the compressor recirculation valve (72) closed.The method of claim 9, wherein the wastegate valve (92) is actuated at a higher than the predetermined gain setting while compressed air flows across the upstream compressor (110), and at the predetermined gain setting while compressed air flows across the downstream compressor (114).

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