Method and system for improved dilution tolerance

Methods and systems are provided for expediting purging of LP-EGR from an air intake system during conditions of decreasing engine load. During a tip-out, an EGR valve is closed while the engine is shifted to a VDE mode of operation to improve engine tolerance to EGR at lower engine loads and until the EGR is sufficiently purged. A non-VDE mode is resumed if higher engine loads are restored, else, a deceleration fuel shut-off mode is selected if the tip-out continues to zero load.

TECHNICAL FIELD

The present application relates to methods and systems for improving exhaust gas recirculation dilution tolerance of an engine during operation at light loads.

BACKGROUND AND SUMMARY

Exhaust gas recirculation (EGR) systems recirculate a portion of exhaust gas from an engine exhaust to an engine intake system to improve fuel economy and vehicle emissions by reducing throttling losses and combustion temperatures. In turbo-charged direct injection engines, a low-pressure EGR (LP-EGR) circuit may be implemented. The LP-EGR circuit recirculates exhaust gases from an exhaust passage downstream of a turbine to an intake passage upstream of a turbocharger compressor.

In order to provide EGR over a wide-range of operating conditions, LP-EGR systems may utilize a specialized EGR schedule. One example EGR schedule is shown by Fujita et al. in US 20070246028. Therein, a fixed amount of EGR is delivered at all EGR conditions, the EGR amount delivered as one or more of low pressure EGR and high pressure EGR. Another example schedule is shown by Styles et al. in US 20120023937. Therein, LP-EGR is delivered at a fixed EGR rate (or percentage) relative to fresh airflow at all engine loads except high loads.

However, the inventors herein have identified potential issues with such schedules. As an example, delivering EGR as per the schedules of Styles or Fujita may lead to combustion instability and engine misfire events during transient operations due to the presence of excess EGR dilution. This is largely due to the pre-compressor location of EGR delivery. Particularly, in turbo-charged systems, providing EGR though the LP-EGR circuit may cause long transport delays as the exhaust gases have to travel though the turbocharger compressor, high-pressure air induction plumbing, charge air cooler, and intake manifold before reaching the combustion chamber. As a result of the transport delay, during conditions when EGR needs to be rapidly reduced, such as during a tip-out operation where the engine goes from a high load and high EGR rate condition to a low load and low EGR rate condition, EGR may not be purged from the air intake system fast enough. As a result, there may be elevated intake-air EGR dilution during the low load condition until the EGR is purged from the air intake system. The presence of increased intake-air dilution at low loads can increase combustion stability issues and the propensity for engine misfires.

While the flat schedule of Styles may reduce the likelihood of high EGR amounts at lower engine loads, the schedule may also limit the fuel economy benefits of LP-EGR. For example, the flat EGR schedule may result in LP-EGR being provided at some low load points where no fuel economy benefits from EGR are achieved. In some cases, there may even be a fuel penalty associated with the delivery of LP-EGR at the low load point. As another example, the lower EGR at the lower load points may limit the peak EGR rates achievable during subsequent higher load engine operation. The delayed purging of EGR requiring EGR in the engine intake system at low engine loads can also render the intake compressor susceptible to corrosion and condensation. Furthermore, increased condensation may occur at the charge air cooler of a boosted engine system due to the flow of EGR through the cooler. The increased condensation may necessitate additional counter-condensation measures which further reduce engine efficiency and fuel economy.

In one example, some of the above issues can be at least partly addressed by a method for an engine comprising: operating an engine with all cylinders combusting while flowing low pressure EGR; and responsive to decreasing engine load, disabling EGR and operating the engine with one or more cylinders deactivated until EGR is below a threshold. In this way, the EGR tolerance of the engine at low loads is improved.

As an example, during medium to high engine loads, an engine may be operating with low pressure EGR (LP-EGR) flowing to provide fuel economy and emissions benefits. The LP-EGR flow may be adjusted to be at a fixed rate relative to airflow. In response to a decrease in engine load to low load conditions, such as due to an operator pedal tip-out, air flow may be decreased (e.g., by adjusting an intake throttle) while also decreasing LP-EGR flow (e.g., adjusting an LP-EGR valve). For example, an EGR valve may be adjusted to provide lower engine dilution at the lower load conditions. However, the purging of the EGR from the engine system may occur slower than desired due to transport delays in the LP-EGR system. Specifically, more EGR may be present in the air induction system, specifically at a pre-compressor location, than desired, degrading combustion stability and potentially inducing misfires.

In order to improve the low load engine combustion stability and EGR tolerance, in response to the decreasing engine load, while EGR is reduced, one or more cylinders may be selectively deactivated. For example, fuel to the one or more cylinders may be cut off while intake and exhaust valves are deactivated. A number of cylinders deactivated may be based on the decrease in engine load. In one example, the engine may have two banks of cylinders and in response to the decreasing engine load, all cylinders of a first engine bank may be deactivated while all cylinders of the second bank are maintained active. As a result, for the same engine torque, the cylinders of the second bank may be working at a higher average cylinder load. The higher load operation of the active cylinders allows for engine operation at the lingering EGR with reduced likelihood of misfires and slow burn issues. In addition, by isolating the air volume of the deactivated bank, the effective boosted volume of the engine is decreased, and LP-EGR depletion is expedited. The engine may continue to be operated with one or more cylinders deactivated until LP-EGR has been depleted to a threshold level. In response to a subsequent tip-in, the previously deactivated engine cylinders may be reactivated. In addition, during the reactivation, fueling may be adjusted so as to purge an exhaust catalyst coupled to the deactivated group of cylinders. As such, the fuel economy gain from the higher cylinder load operation and expedited EGR purging may balance or outweigh the fuel economy associated with the purging of the exhaust catalyst.

In this way, EGR purging from an engine intake can be expedited. By selectively deactivating one or more engine cylinders during decreasing engine load and decreasing EGR conditions, the average cylinder load can be increased, improving cylinder EGR tolerance and combustion stability. By isolating the air volume of the deactivated cylinders, the effective boosted volume of the engine is decreased and LP EGR depletion is expedited. As such, this enables EGR levels in the air induction system and intake manifold to be reduced faster (e.g., up to half the time) than would have been otherwise possible. By rapidly reducing the intake EGR level at low load conditions and by increasing EGR dilution tolerance at these low engine load conditions via cylinder deactivation, higher EGR rates can be achieved when the engine is subsequently restarted. As such, this substantially improves engine efficiency, particularly in medium to high engine speed-load regions. By replacing the EGR with fresh air, evaporation of water and hydrocarbon condensates is increased, reducing their concentration in the engine, and the need for counter-condensation measures. In addition, the reduction in condensation reduces compressor and charge air cooler corrosion and degradation. Overall, boosted engine performance is improved.

DETAILED DESCRIPTION

The following description relates to systems and methods for selectively deactivating one or more cylinders of an engine system (such as the engine system ofFIGS. 1-3) to improve combustion EGR tolerance and expedite purging of EGR from the engine at low load conditions. While operating the engine with EGR flowing, in response to decreasing engine load, EGR delivery to the engine may need to be rapidly reduced. During such conditions, a controller may be configured to perform a control routine, such as the example routine ofFIG. 4, to selectively deactivate fuel and the intake and exhaust valves to one or more engine cylinders, while operating the active cylinders at a higher average cylinder load. The engine may be operated with the one or more cylinders deactivated for a duration until EGR is sufficiently purged from the engine's intake manifold. In one example, the engine may be operated in a VDE mode with all cylinders of one engine bank deactivated until the EGR is sufficiently purged. Example adjustments are shown with reference toFIGS. 6-7. In this way, dilution tolerance of an engine at low engine loads is improved.

FIG. 1shows an example variable displacement engine (VDE)10having a first bank15aand a second bank15b. In the depicted example, engine10is a V6 engine with the first and second banks each having three cylinders. Engine10has an intake manifold16, with throttle20, and an exhaust manifold18coupled to an emission control system30. Emission control system30includes one or more catalysts and air-fuel ratio sensors, such as described with regard toFIG. 2. As one non-limiting example, engine10can be included as part of a propulsion system for a passenger vehicle.

During selected conditions, such as when the full torque capability of the engine is not needed, one of a first or a second cylinder group may be selected for deactivation (herein also referred to as a VDE mode of operation). Specifically, one or more cylinders of the selected group of cylinders may be deactivated by shutting off respective fuel injectors while maintaining operation of the intake and exhaust valves such that air may continue to be pumped through the cylinders. While fuel injectors of the disabled cylinders are turned off, the remaining enabled cylinders continue to carry out combustion with fuel injectors active and operating. To meet the torque requirements, the engine produces the same amount of torque on those cylinders for which the injectors remain enabled. In other words, the remaining active cylinders are operated at higher average cylinder loads. This requires higher manifold pressures, resulting in lowered pumping losses and increased engine efficiency. Also, the lower effective surface area (from only the enabled cylinders) exposed to combustion reduces engine heat losses, improving the thermal efficiency of the engine.

In alternate examples, engine system10may have cylinders with selectively deactivatable intake and/or exhaust valves. Therein, the cylinder valves may be selectively deactivated via hydraulically actuated lifters (e.g., lifters coupled to valve pushrods), or via a cam profile switching mechanism in which a cam lobe with no lift is used for deactivated valves.

Cylinders may be grouped for deactivation in a bank-specific manner. For example, inFIG. 1, the first group of cylinders may include the three cylinders of the first bank15awhile the second group of cylinders may include the three cylinders of the second bank15b. In an alternate example, instead of one or more cylinders from each bank being deactivated together, two cylinders from each bank of the V8 engine may be selectively deactivated together.

Engine10may operate on a plurality of substances, which may be delivered via fuel system8. Engine10may be controlled at least partially by a control system including controller12. Controller12may receive various signals from sensors4coupled to engine10(described in further detail with reference toFIG. 2), and send control signals to various actuators22coupled to the engine and/or vehicle (as also described with reference toFIG. 2). The various sensors may include, for example, various temperature, pressure, and air-fuel ratio sensors. In addition, controller12may receive an indication of cylinder knock or pre-ignition from one or more knock sensors distributed along the engine block. When included, the plurality of knock sensors may be distributed symmetrically or asymmetrically along the engine block. Further, the one or more knock sensors may include accelerometers, ionization sensors or in cylinder pressure transducers.

Fuel system8may be further coupled to a fuel vapor recovery system (not shown) including one or more canisters for storing refueling and diurnal fuel vapors. During selected conditions, one or more valves of the fuel vapor recovery system may be adjusted to purge the stored fuel vapors to the engine intake manifold to improve fuel economy and reduce exhaust emissions. In one example, the purge vapors may be directed near the intake valve of specific cylinders. For example, during a VDE mode of operation, purge vapors may be directed only to the cylinders that are firing. This may be achieved in engines configured with distinct intake manifolds for distinct groups of cylinders. Alternatively, one or more vapor management valves may be controlled to determine which cylinder gets the purge vapors.

FIG. 2depicts an example embodiment of a combustion chamber or cylinder of engine10(ofFIG. 1). Engine10may receive control parameters from a control system including controller12and input from a vehicle operator130via an input device132. In this example, input device132includes an accelerator pedal and a pedal position sensor134for generating a proportional pedal position signal PP. Cylinder (herein also “combustion chamber”)30of engine10may include combustion chamber walls136with piston138positioned therein. Piston138may be coupled to crankshaft140so that reciprocating motion of the piston is translated into rotational motion of the crankshaft. Crankshaft140may be coupled to at least one drive wheel of the passenger vehicle via a transmission system. Further, a starter motor may be coupled to crankshaft140via a flywheel to enable a starting operation of engine10.

Cylinder30can receive intake air via a series of intake air passages142,144, and146. Intake air passage146can communicate with other cylinders of engine10in addition to cylinder30. In some embodiments, one or more of the intake passages may include a boosting device such as a turbocharger or a supercharger. For example,FIG. 2shows engine10configured with a turbocharger including a compressor174arranged between intake passages142and144, and an exhaust turbine176arranged along exhaust passage148. Compressor174may be at least partially powered by exhaust turbine176via a shaft180where the boosting device is configured as a turbocharger. However, in other examples, such as where engine10is provided with a supercharger, exhaust turbine176may be optionally omitted, where compressor174may be powered by mechanical input from a motor or the engine. A charge air cooler (CAC)177may be positioned in intake air passage144, downstream of compressor174, for cooling the boosted aircharge prior to delivery to the engine cylinders. A throttle20including a throttle plate164may be provided along an intake passage of the engine for varying the flow rate and/or pressure of intake air provided to the engine cylinders. For example, throttle20may be disposed downstream of compressor174as shown inFIG. 2, or alternatively may be provided upstream of compressor174.

Exhaust passage148can receive exhaust gases from other cylinders of engine10in addition to cylinder30. Exhaust gas sensor128is shown coupled to exhaust passage148upstream of emission control device178. Sensor128may be selected from among various suitable sensors 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 (as depicted), a HEGO (heated EGO), a NOx, HC, or CO sensor, for example. Emission control device178may be a three way catalyst (TWC), NOx trap, various other emission control devices, or combinations thereof.

Exhaust temperature may be estimated by one or more temperature sensors (not shown) located in exhaust passage148. Alternatively, exhaust temperature may be inferred based on engine operating conditions such as speed, load, air-fuel ratio (AFR), spark retard, etc. Further, exhaust temperature may be computed by one or more exhaust gas sensors128. It may be appreciated that the exhaust gas temperature may alternatively be estimated by any combination of temperature estimation methods listed herein.

Each cylinder of engine10may include one or more intake valves and one or more exhaust valves. For example, cylinder30is shown including at least one intake poppet valve150and at least one exhaust poppet valve156located at an upper region of cylinder30. In some embodiments, each cylinder of engine10, including cylinder30, may include at least two intake poppet valves and at least two exhaust poppet valves located at an upper region of the cylinder.

Intake valve150may be controlled by controller12by cam actuation via cam actuation system151. Similarly, exhaust valve156may be controlled by controller12via cam actuation system153. Cam actuation systems151and153may each include 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 that may be operated by controller12to vary valve operation. The position of intake valve150and exhaust valve156may be determined by valve position sensors155and157, respectively. In alternative embodiments, the intake and/or exhaust valve may be controlled by electric valve actuation. 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 systems. In still other embodiments, the intake and exhaust valves may be controlled by a common valve actuator or actuation system, or a variable valve timing actuator or actuation system.

In some embodiments, each cylinder of engine10may be configured with one or more injectors for providing a knock or pre-ignition suppressing fluid thereto. In some embodiments, the fluid may be a fuel, wherein the injector is also referred to as a fuel injector. As a non-limiting example, cylinder30is shown including one fuel injector166. Fuel injector166is shown coupled directly to cylinder30for injecting fuel directly therein in proportion to the pulse width of signal FPW received from controller12via electronic driver168. In this manner, fuel injector166provides what is known as direct injection (hereafter also referred to as “DI”) of fuel into combustion cylinder30. WhileFIG. 2shows injector166as a side injector, it may also be located overhead of the piston, such as near the position of spark plug192. Such a position may improve mixing and combustion when operating the engine with an alcohol-based fuel due to the lower volatility of some alcohol-based fuels. Alternatively, the injector may be located overhead and near the intake valve to improve mixing.

Fuel may be delivered to fuel injector166from a high pressure fuel system8including fuel tanks, fuel pumps, and a fuel rail. Alternatively, fuel may be delivered by a single stage fuel pump at lower pressure, in which case the timing of the direct fuel injection may be more limited during the compression stroke than if a high pressure fuel system is used. Further, while not shown, the fuel tanks may have a pressure transducer providing a signal to controller12. It will be appreciated that, in an alternate embodiment, injector166may be a port injector providing fuel into the intake port upstream of cylinder30.

As described above,FIG. 2shows only one cylinder of a multi-cylinder engine. As such each cylinder may similarly include its own set of intake/exhaust valves, fuel injector(s), spark plug, etc.

Fuel tanks in fuel system8may hold fuel with different qualities, such as different compositions. These differences may include different alcohol content, different octane, different heat of vaporizations, different fuel blends, and/or combinations thereof etc. In one example, fuels with different alcohol contents could include one fuel being gasoline and the other being ethanol or methanol. In another example, the engine may use gasoline as a first substance and an alcohol containing fuel blend such as E85 (which is approximately 85% ethanol and 15% gasoline) or M85 (which is approximately 85% methanol and 15% gasoline) as a second substance. Other alcohol containing fuels could be a mixture of alcohol and water, a mixture of alcohol, water and gasoline etc.

Further, in the disclosed embodiments, an EGR system may route a desired portion of exhaust gas from exhaust passage148to intake manifold142.FIG. 2shows Engine10may include an LP-EGR system (shown atFIG. 3) wherein LP-EGR is routed through LP-EGR passage from downstream of turbine176to upstream of compressor174. The amount of LP-EGR provided to intake passage144may be varied by controller12via an LP-EGR valve. Likewise, there may be an HP-EGR system (shown atFIG. 3) wherein HP-EGR is routed through an HP-EGR passage from upstream of turbine176to downstream of compressor174. The amount of HP-EGR provided to intake passage144may be varied by controller12via a dedicated HP-EGR valve. The HP-EGR system may include an HP-EGR cooler (seeFIG. 3) and the LP-EGR system may include LP-EGR cooler to 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 or induction passages142or144and may provide an indication of one or more of mass flow, pressure, temperature, concentration of O2, and concentration of the exhaust gas. In some embodiments, one or more sensors may be positioned within the LP-EGR passage or induction passages142or144to provide an indication of one or more of a flow rate, pressure, temperature, and concentration of O2 or another species to determine LP EGR flow rate and concentration relative to air flow rate. Exhaust gas diverted through the LP-EGR passage may be diluted with fresh intake air at a mixing point located at the junction of the LP-EGR passage and intake passage142. Specifically, by adjusting the LP-EGR valve in coordination with a low pressure AIS throttle in duct142, 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 sensor in the engine intake gas stream. Specifically, the sensor may be positioned downstream of the LP-EGR valve, such that the LP-EGR dilution may be accurately determined. The sensor may be, for example, a hot film or hot wire anemometer mass flow meter or a delta pressure orifice or venturi. Alternatively an oxygen sensor such as a UEGO sensor can be used in duct144to calculate LP EGR rate.

Controller12is shown inFIG. 2as a microcomputer, including microprocessor unit106, input/output ports108, an electronic storage medium for executable programs and calibration values shown as read only memory chip110in this particular example, random access memory112, keep alive memory114, 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 sensor122; engine coolant temperature (ECT) from temperature sensor116coupled to cooling sleeve118; a profile ignition pickup signal (PIP) from Hall effect sensor120(or other type) coupled to crankshaft140; throttle position (TP) from a throttle position sensor; absolute manifold pressure signal (MAP) from sensor124, cylinder AFR from EGO sensor128, and abnormal combustion from a knock sensor. 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.

Storage medium read-only memory110can be programmed with computer readable data representing instructions executable by processor106for performing the methods described below as well as other variants that are anticipated but not specifically listed.

Now turning toFIG. 3, an example embodiment300of an engine system10including a plurality of cylinder banks and an exhaust gas recirculation system is illustrated. In one embodiment, engine10may comprise a turbocharger including compressor174and turbine176, throttle363upstream of compressor174, and a low-pressure exhaust gas recirculation (LP-EGR) system. The LP-EGR system may route EGR from downstream of turbine176to upstream of compressor174and downstream of throttle363. The engine system may further comprise a HP-EGR system which routes EGR from upstream of turbine176to downstream of throttle362.

Turning toFIG. 3, air may enter engine10through an air filter310. Air filter310may be configured to remove solid particulates from the air so a clean air mass may enter engine10. The clean air mass flow may be measured as it flows past mass air flow sensor320and then through intake throttle363. The clean air mass flow measured by mass air flow sensor320may be communicated to controller12. In one embodiment, the clean air mass may be split between the different cylinder banks of engine10downstream of intake throttle363and upstream of turbocharger compressor174. An EGR system may inject exhaust gas upstream of turbocharger compressor174so that a combination of clean air and exhaust gas can be compressed by turbocharger compressor174. In one embodiment, turbocharger compressor174may include a first compressor174afor a first cylinder bank and a second compressor174bfor a second cylinder bank.

The compressed combination of clean air and exhaust gas downstream of turbocharger compressor174may be cooled by a charge air cooler (CAC)354upstream of a second throttle362. In one embodiment, the oxygen content of the airflow downstream from turbocharger compressor174a-bmay be measured by a sensor345upstream of CAC354. In an alternate embodiment, the oxygen content of the airflow downstream from turbocharger compressor174a-bmay be measured by a sensor347downstream of CAC354. Measurements from sensors345and/or347may be communicated to controller12.

In one embodiment, high pressure exhaust gas may be combined with the compressed combination of clean air and exhaust gas downstream of throttle362and upstream of intake manifold344. The combination of gases may be routed to one or more cylinder banks by intake manifold344. For example, gases may be routed to a first cylinder bank344aand a second cylinder bank344b. After combustion in the cylinders, exhaust gas may be routed through exhaust passage348. In one embodiment, exhaust passage348includes an exhaust manifold for each bank of cylinders, such as exhaust manifold348afor a first cylinder bank and exhaust manifold348bfor a second cylinder bank.

During selected conditions, such as when the full torque capability of the engine is not needed, cylinders of one of first cylinder bank344aand second cylinder bank344bmay be selected for deactivation (herein also referred to as a VDE mode of operation). Specifically, one or more cylinders of the selected cylinder bank may be deactivated by shutting off respective fuel injectors while maintaining operation of the intake and exhaust valves such that air may continue to be pumped through the cylinders. Alternatively, the intake and exhaust valves on the deactivated cylinders can also be disabled to prevent air from being pumped through these cylinders. While fuel injectors of the disabled cylinders are turned off, the remaining enabled cylinders continue to carry out combustion with fuel injectors active and operating. To meet the torque requirements, the engine produces the same amount of torque on those cylinders for which the injectors remain enabled by operating the active cylinders at higher average cylinder loads, allowing for increased engine efficiency. As also elaborated atFIG. 4, during conditions of decreasing load, cylinders of one of bank344aand344bmay be deactivated, EGR may be disabled, and the engine may be operated in the VDE mode until EGR is sufficiently purged from the air induction system. Thereafter, based on the engine load, the engine may resume operation in a non-VDE mode by reactivating the cylinders.

At least a portion of the exhaust gasses may drive a turbine176of the turbocharger. In one embodiment, turbine176may include a first turbine176afor a first cylinder bank and a second turbine176bfor a second cylinder bank. In one embodiment, at least a portion of the exhaust gasses may be routed through an HP-EGR system. For example, an HP-EGR system may include HP-EGR cooler346and valve342for routing cooled exhaust gasses upstream of intake manifold344. In one embodiment, a HP-EGR system may include a first HP-EGR cooler346aand valve342afor a first cylinder bank and a second HP-EGR cooler346band valve342bfor a second cylinder bank.

Downstream from turbine176a-b, at least a portion of the exhaust gasses may flow downstream through emission control device371and muffler320. In one embodiment, emission control device371may include a first light-off catalyst or three-way catalyst (TWC)371afor a first cylinder bank and a second light-off catalyst or three-way catalyst371bfor a second cylinder bank. Muffler320may be configured to dampen exhaust noise from engine10.

At least a portion of the exhaust gasses from downstream of turbine176a-bmay be routed upstream of turbocharger compressor174a-bby an LP-EGR system. For example, an LP-EGR system may include LP-EGR cooler246and valve242for routing cooled exhaust gasses upstream of compressor174. In one embodiment, an LP-EGR system may include a first LP-EGR cooler246aand valve242afor a first cylinder bank and a second LP-EGR cooler246band valve242bfor a second cylinder bank.

An amount of LP-EGR delivered may be adjusted based on engine speed-load conditions. For example, delivery of LP-EGR at mid to high load conditions allows for improved fuel economy and exhaust emissions. In one example, at all loads down from a mid-load, LP-EGR may be delivered according to a flat schedule wherein the EGR rate is kept constant relative to airflow. This may require the opening of LP-EGR valves242aand242bto be coordinated with the opening of intake throttle363. Thus, during increasing loads, such as during a tip-in, as airflow is increased, LP-EGR delivery may be correspondingly increased. Likewise, during decreasing loads, such as during a tip-out, as airflow is decreased, LP-EGR delivery may be correspondingly decreased.

However, due to a large transport delay between the LP-EGR valve and the combustion chamber, EGR may not be decreased as fast as required, in particular in the air induction system from the EGR outlet to the compressor inlet (on each bank). Specifically, since the LP-EGR passage takes off exhaust gas after the turbine and injects exhaust gas before the compressor, there is a large delay in purging of the EGR from the air intake system, which in turn leads to combustion stability risks.

As elaborated here atFIG. 5, during low load conditions where EGR dilution is higher than desired, such as due to delays in EGR purging, dilution tolerance is improved by deactivating cylinders of one engine bank while operating the engine in a VDE mode until EGR is sufficiently purged. The higher average cylinder load increases the cylinder's dilution tolerance and reduces the combustion stability risk. For example, cylinder bank344amay be deactivated while cylinder bank344bcontinues to operate. As such, during the VDE mode, exhaust catalyst371acoupled downstream of the deactivated bank344amay become oxygen saturated and catalytically inefficient. Thus, during cylinder reactivation, when exiting the VDE mode, fueling to cylinder bank344amay be adjusted so that exhaust catalyst371acan be purged and catalytic efficiency improved. For example, during cylinder reactivation, cylinder bank344amay be operated richer than stoichiometry for a duration until exhaust catalyst371ahas been purged while cylinder bank344bis operated at stoichiometry.

In addition, the reduced effective boosted volume at the deactivated bank (if intake and exhaust valves are deactivated) expedites the replacement of EGR in the air induction system with fresh air when VDE mode is activated, further improving the ability of VDE to achieve higher EGR rates at medium and higher loads while purging the LP EGR at lower engine loads.

Now turning toFIG. 4, an example routine400is shown for selectively deactivating one or more engine cylinders during decreasing engine load conditions so as to improve combustion EGR tolerance and expedite low pressure EGR purging from an engine intake.

At402, the routine includes estimating and/or measuring engine operating conditions. These may include, for example, engine speed, torque demand, engine temperature, ambient conditions (such as ambient temperature, barometric pressure, and ambient humidity), etc. At404, based on the estimated operating conditions, a target EGR rate may be determined. The target EGR rate may include a target rate of high pressure EGR (HP-EGR), low pressure-EGR (LP-EGR), or a combination of high and low pressure EGR. In one example, based on the engine operating conditions, such as the engine speed-load conditions, the engine may be operating boosted with fuel delivered to the boosted engine via direct injection. In addition, the engine may be operating with LP-EGR flowing. By flowing LP-EGR during engine operation, fuel economy is improved via pumping work reduction, knock mitigation, combustion efficiency improvement, and enrichment reduction.

In one example, the LP-EGR rate and schedule may be a flat schedule wherein LP-EGR is delivered at a fixed rate relative to airflow. The LP-EGR includes cooled exhaust residuals recirculated from an engine exhaust manifold, downstream of an exhaust turbine, to an engine intake manifold, upstream of an intake compressor.

As such, the amount of EGR (HP-EGR, LP-EGR, or a combination of the two) delivered to the intake may be based on an engine-speed load map stored in the memory of controller12. In one example, the engine speed-load map may include at least two LP-EGR operating modes, including a fixed and a variable operating mode. The fixed mode range may comprise all engine loads from higher loads down to minimum load, and/or engine speeds lower than a threshold, such as at or below 3500 RPM. At very high engine loads and speeds up to full engine load, LP-EGR may be reduced to avoid excessive heat transfer or engine performance limitations. Minimum engine load as used herein comprises the lowest possible load allowable for current operating conditions, e.g. the lowest load that sustains combustion for current engine speed, temperature, etc., and may correspond to a closed throttle engine load for current engine speed conditions. In some conditions, the minimum load may be lower than the load at idle. Thus, the minimum load may be encountered during non-idle conditions and may include the most challenging operating point for avoiding engine misfire during an engine tip-out with a fixed EGR schedule, the fixed EGR scheduled required during tip-out conditions by the large transport delays and boosted volumes inherent to LP EGR. In comparison, the variable EGR mode may be enabled to allow higher EGR rates in the mid and higher engine load ranges for improved fuel economy benefit, with reduced EGR rates in other areas of the engine map, such as lower loads, to avoid engine misfire or combustion stability issues due to high dilution levels.

At406, the routine includes adjusting a position of the EGR valve to flow EGR at the determined target EGR rate and schedule. For example, the opening of an LP-EGR valve in an LP-EGR passage and/or the opening of an HP-EGR valve in an HP-EGR passage may be adjusted. In some embodiments, the opening of the EGR valve may be adjusted based on the target EGR rate relative to a current EGR estimate. EGR may be estimated using one or more sensors coupled to the EGR passage. Alternatively, LP-EGR may be estimated based on the output of an intake oxygen sensor coupled downstream of the compressor and upstream of an intake throttle. A controller may infer the current EGR dilution based on a change (e.g., decrease) in oxygen concentration at the sensor.

At408, it may be determined if there is decreasing engine load. For example, it may be determined if the engine load has been decreased to below a threshold load. In one example, the decreasing engine load is responsive to an operator pedal tip-out from medium or high load to low load conditions. If there is no decrease in engine load, EGR control may be maintained at410. This includes adjusting the EGR rate based on engine operating conditions so as to continue to provide LP-EGR at a desired rate relative to engine speed/load conditions.

At412, while operating the engine with all cylinders combusting and while flowing low pressure EGR, responsive to the decreasing engine load, EGR may be decreased. This may include decreasing LP-EGR while maintaining LP-EGR at a fixed rate relative to intake airflow so as to provide or maintain a flat LP-EGR schedule at the lower load. In another example, decreasing EGR includes disabling EGR, for example, by closing an LP-EGR valve in an LP-EGR passage, the LP-EGR passage delivering exhaust residuals from an exhaust manifold, downstream of an exhaust turbine, to an intake manifold, upstream of an intake compressor. Disabling EGR may alternatively include closing an HP-EGR valve in an HP-EGR passage.

Also responsive to the decreasing engine load, at414, the routine includes selectively deactivating one or more engine cylinders and operating the engine with the one or more cylinders deactivated until EGR is below a threshold. Operating the one or more cylinders deactivated includes selectively disabling or deactivating fuel to the one or more cylinders. For example, the cylinders may be configured with selectively deactivatable fuel injectors and disabling fuel to the cylinders may include disabling fuel injectors of the one or more cylinders while maintaining intake and exhaust valve operation so that air continues to be pumped through the deactivated cylinders. In another example, the cylinders may be configured with selectively deactivatable intake and/or exhaust valves and disabling the cylinders may include deactivating the intake and/or exhaust valve of the one or more cylinders so that air is not pumped through the deactivated cylinders. As such, by deactivating one or more cylinders, the remaining active cylinders may need to be operated at a higher average cylinder load to maintain engine torque output.

A number of cylinders that are deactivated responsive to the decreasing engine load is variable and depends on various factors such as the number of cylinders in the engine configuration, engine displacement, engine smoothness or NVH requirements, exhaust catalyst or emission after-treatment device conditions, desired VDE switching frequency and cylinder reactivation issues. For example, based on the various factors, the deactivation may include deactivating 2 cylinders per bank on a V8 engine, one bank or one cylinder per bank on a V6 engine, 1 or 2 cylinders on an I4 engine or one cylinder on an I3 engine, although other scenarios could be considered. Thus, for example, the engine may be operating with all cylinders combusting and with LP-EGR flowing at a first EGR rate (which is a first percentage of EGR in the intake manifold). Then, during a decrease in engine load, the EGR valve may be closed and one or more cylinders may be deactivated. The engine may then be operated with the EGR valve closed and with the remaining active cylinders combusting until the EGR rate in the intake manifold has dropped from the first EGR rate to a minimum threshold rate.

In one example, the engine may include a first group of cylinders on a first engine bank and a second group of cylinders on a second engine bank. The first group of cylinders may be coupled to a first exhaust catalyst located downstream of the first bank while the second group of cylinders may be coupled to a second exhaust catalyst located downstream of the second bank. In response to the decreasing engine load, all the cylinders of the first bank may be selectively deactivated. The engine may then be operated with the first group of cylinders deactivated and the second group of cylinders activated. In other words, where the engine is a VDE engine, in response to the decreasing engine load, the engine may be transitioned from a non-VDE mode (with all cylinders combusting) to a VDE mode (with half the cylinders combusting).

The engine may continue to be operated with the one or more cylinders deactivated until EGR is sufficiently purged from the engine. As such, during the decreasing engine load, airflow to the engine may be decreased and correspondingly, LP-EGR to the engine may also be decreased. However, due to a large transport delay between the LP-EGR valve and the combustion chamber, EGR may not be decreased as fast as required, in particular in the air induction system from the EGR outlet to the compressor inlet. Specifically, since the LP-EGR passage takes off exhaust gas after the turbine and injects exhaust gas before the compressor, there is a large delay in purging of the EGR from the air intake system. The delay is exacerbated by the presence of a large boosted volume (e.g., up to six times the engine displacement volume) in some engine configurations. The delay in purging leads to combustion stability risks. For example, the presence of more dilution than required in the air induction system can increase the likelihood of misfires.

The adoption of the flat EGR schedule, wherein the EGR rate is kept constant relative to airflow, helps in alleviating some of the issues associated with the delayed purging. However, the use of a flat schedule results in operating LP-EGR at some lower load points where no fuel economy benefit is achieved. In fact, in some lower engine speed-load points, operating LP-EGR results in a fuel penalty. In addition, the compressor may be exposed to EGR at the low load conditions, necessitating corrosion and condensation countermeasures. As EGR flows through the charge air cooler, additional condensation may arise that may also need to be addressed. At some low load conditions, a low pressure air intake throttle may also need to be operated to drive the EGR flow. Furthermore, the lower load points limit the flat EGR schedule at higher load points as these are the points where the combustion system is the most dilution (EGR) limited. For example, the flat schedule limits the peak EGR rate achievable at higher engine speed-load conditions. As such, this limits the fuel economy benefit of LP-EGR.

The inventors herein have recognized that purging of LP-EGR from the air induction system can be expedited by selectively deactivating one or more engine cylinders. Specifically, by deactivating some cylinders and operating the remaining cylinders at a higher effective cylinder load, the dilution tolerance of the cylinder is improved. That is, the higher load operation of the cylinder allows for operation in the presence of higher EGR amounts (that are lingering due to the transport delay in purging EGR) with reduced risk of misfire, combustion instability, and slow burn issues.

At416, it may be determined if the EGR has been sufficiently purged from the engine air induction system. For example, it may be determined if EGR (flow, amount, concentration, etc.) in the intake is lower than a threshold. In one example, an intake oxygen sensor may be used to estimate the concentration of EGR in the intake. Therein, an increase in intake oxygen concentration may be used to infer a decrease in EGR dilution delivery.

The EGR threshold may be based on the engine load, the threshold decreased as the engine load decreases. The EGR threshold may be further based on a number of cylinders that are deactivated and an EGR tolerance of the active cylinders at the low engine load conditions. For example, as the number of cylinders that are deactivated increases and the EGR tolerance of the remaining active cylinders increases, the threshold may be increased.

If the EGR has not been sufficiently purged and the EGR level is still above the threshold, then at417, the engine may continue to be operated with the one or more cylinders deactivated. If the EGR has been sufficiently purged and the EGR level is below the threshold, then at418, the routine includes determining if the engine load is still low or in there is a further drop in engine load.

If after EGR is below the threshold, no further drop in engine load is confirmed at418, the routine proceeds directly to427where the routine includes reactivating the one or more deactivated cylinders. For example, the fuel injectors of the deactivated cylinders may be reactivated and cylinder fueling may be re-enabled. In addition, EGR may be re-enabled. For example, an LP-EGR valve may be opened and EGR may be provided at an optimal LP-EGR rate relative to engine speed/load conditions. In one example, LP-EGR may be provided as per a flat schedule, with EGR adjusted to be at a fixed rate relative to airflow.

From427, the routine proceeds to428where during cylinder reactivation, an exhaust catalyst (e.g., a three-way catalyst) may be purged. Catalyst purging includes adjusting a fuel of the previously deactivated engine cylinders so as to reduce the oxygen saturation of an exhaust catalyst coupled downstream of the deactivated engine cylinders. For example, in embodiments where a first engine bank is selectively deactivated at414while a second engine bank continues combusting, catalyst purging may include, during reactivation of the first bank, adjusting a fueling of the first bank so as to reduce the oxygen saturation of a first exhaust catalyst coupled to the first bank (and not the second engine bank). As such, during operation in the VDE mode, due to operation of the intake and exhaust valves, air may have been pumped through the first exhaust catalyst, causing catalyst oxygen saturation with a resulting drop in catalyst efficiency. Thus, during the reactivation, the catalyst may need to be purged or regenerated. As discussed with reference to the example ofFIGS. 6-7, during the reactivation, the first bank may be operated richer than stoichiometry for a duration, while the second bank is operated at stoichiometry, until the first exhaust catalyst is sufficiently regenerated. A degree of richness and/or a duration of the purging may be adjusted based on the degree of catalyst purging required, which is based on the duration of operation in the VDE mode during the immediately preceding bank deactivation (at414). Herein, the exhaust catalyst may be a three-way catalyst. In alternate examples, such as where the first exhaust catalyst is an SCR catalyst or a reduction catalyst, the exhaust catalyst may be purged by increasing injection of reductant from a reductant injector located upstream of the first exhaust catalyst in the exhaust manifold.

While the above example details exhaust catalyst purging in engine embodiments where the first group of cylinders is coupled to a first exhaust catalyst and the second group of cylinders is coupled to a second exhaust catalyst, the exhaust manifold branching upstream of first and second exhaust catalysts and recombining at a location downstream of each of the first and second exhaust catalysts, in alternate embodiments, such as where the first group of cylinders and the second group of cylinders are coupled to a common exhaust catalyst, or where the banks are coupled to respective exhaust catalysts but the exhaust manifold is combined at a location upstream of the first and second exhaust catalyst, catalyst purging may not be required. Alternatively, less purging may be required. This may be due to the exposure of the exhaust catalyst to stoichiometric exhaust from the second active group of cylinders during engine operation.

Returning to418, if a drop in engine load is confirmed, then at420, it may be determined if the drop in engine load includes a tip-out to zero load conditions. If yes, then after EGR is below the threshold (that is sufficiently purged), in response to drop in engine load to zero load conditions, at422, the routine includes shutting off fuel to all engine cylinders while maintaining intake valve and exhaust valve operation of all the engine cylinders. For example, in response to the drop in engine load after EGR has been sufficiently purged, the engine may be automatically moved into a deceleration fuel shut off (DFSO) mode of operation. The engine may then remain in the DFSO mode with fueling disabled to all engine cylinders until engine load increases.

If at tip-out to zero load is not confirmed at420, then a drop in engine load to low load conditions is determined and at424, the selectively deactivated engine cylinders are maintained deactivated and EGR is maintained disabled. For example, the VDE mode of engine operation is continued with the LP-EGR valve closed. The engine may then remain in the VDE mode with the one or more cylinders selectively deactivated and EGR disabled until there is an increase in engine load.

At425, it may be determined if there is an increase in engine load. For example, it may be determined if there is a tip-in. If an increase in engine load is not confirmed, then at426, the routine includes maintaining engine operation in the existing operating mode. For example, the engine may continue to be operated in the DFSO mode with all fueling of all engine cylinders deactivated and EGR disabled (at422) or in the VDE mode with fueling of selected engine cylinders deactivated and EGR disabled (at424).

If an increase in engine load is confirmed, the routine proceeds to427to reactivate the previously deactivated engine cylinders and re-enable EGR. For example, in response to an increase in engine load while the engine is in the DFSO mode, the routine includes exiting the DFSO mode by re-enabling fueling to all engine cylinders and resuming combustion in all engine cylinders. In addition, the LP-EGR valve may be opened to allow for exhaust gas recirculation (e.g., as per a flat schedule). As another example, in response to an increase in engine load while the engine is in the VDE mode, the routine includes exiting the VDE mode by re-enabling fueling to the selectively deactivated engine cylinders, resuming cylinder intake and exhaust valve operation, and resuming combustion in all engine cylinders. In addition, the LP-EGR valve may be opened to allow for exhaust gas recirculation (e.g., as per a flat schedule). The routine then proceeds to428to purge exhaust catalysts during the reactivation, as discussed above.

In one example, during reactivation following a DFSO event, each of the first and second exhaust catalyst may be purged in embodiments where each of the first and second group of cylinders are coupled to respective downstream exhaust catalysts. Alternatively, if the engine is configured with a common exhaust catalyst, catalyst purging may be required following a DFSO event.

In this way, by deactivating some engine cylinders and increasing an average cylinder load of remaining active cylinders during conditions when EGR levels are higher than desired, high dilution tolerance of cylinders at low loads can be improved.

Now turning toFIG. 5, an example cylinder deactivation routine500is shown for expediting low pressure EGR purging from an engine induction system.

At502, as at402, engine operating conditions may be estimated and/or measured. At504, as at404, a target EGR rate may be determined based on the estimated operating conditions. For example, a target LP-EGR rate may be determined. At506, as at406, an LP-EGR valve opening may be adjusted based on the target EGR rate to provide the desired EGR. For example, an LP-EGR valve opening may be increased at mid to high load conditions to provide the optimal LP-EGR rate relative to engine speed/load conditions (e.g., according to a flat schedule, with EGR provided at a fixed rate relative to airflow).

At508, it may be determined if an operator pedal tip-out event has occurred. If a tip-out is not confirmed, at510, EGR control is maintained with EGR continuing to be provided at the fixed rate relative to airflow. If comparison at512, in response to a tip-out from higher engine load while flowing low pressure EGR, the routine includes deactivating fuel to a first group of engine cylinders on a first engine bank while operating a second group of cylinders on a second engine bank with higher average cylinder load. Additionally, the intake and exhaust valves are closed to isolate the volume on the de-activated bank to effectively reduce the boosted volume and EGR purge times. In one example, where the engine is a variable displacement engine (VDE), the deactivating includes transitioning the engine from operating in a non-VDE mode (with all cylinders combusting) to operating in a VDE mode (with half the engine cylinders combusting).

As discussed atFIG. 4(specifically at414), a number of cylinders that are deactivated responsive to the decreasing engine load may be adjusted based on various parameters such as the number of cylinders in the engine configuration, engine displacement, engine smoothness or NVH requirements, exhaust catalyst or emission after-treatment device conditions, desired VDE switching frequency and cylinder reactivation issues.

Also, at514, the routine includes disabling EGR, such as by closing an LP-EGR valve in an LP-EGR passage. The engine may continue to be operated with the first group of engine cylinders deactivated and the second group of cylinders active until EGR has been sufficiently purged and the EGR in the air induction system is below a threshold.

It will be appreciated that while the routine ofFIG. 5(as well asFIG. 4) teaches disabling EGR while also deactivating engine cylinders to expedite EGR purging, in alternate examples, the controller may first deactivate the engine cylinders (e.g., in a bank specific manner by operating in a VDE mode) by shutting off fuel and deactivating the intake and exhaust valves on the deactivated cylinders so as to decrease the effective boosted volume and EGR purging time. As discussed previously, by operating the engine in the VDE mode with the EGR valve closed, EGR purging from the engine intake system can be expedited. By deactivating the first group of cylinders while operating the second group of cylinders at a higher average cylinder load, the dilution tolerance of the second group of cylinders at low load and higher than requested EGR levels is improved, without reducing engine torque output. By improving the low load high dilution tolerance of the cylinders, misfires and combustion stability issues are reduced. In addition, the decreased effective boosted volume by deactivating the intake and exhaust valves on the deactivated cylinders expedites replacement of EGR in the induction system with fresh air.

Returning toFIG. 5, at516, as at416, it may be determined if EGR is less than the threshold. The threshold may be based on the tip-out from the higher load. An intake oxygen sensor may be used to estimate the concentration of EGR in the intake, for example. Alternatively, a hot film or wire anemometer or a delta pressure orifice or venturi coupled to the EGR passage may be used to estimate the concentration of EGR in the intake.

If the EGR has not been sufficiently purged and the EGR level is still above the threshold, then at517, as at417, the engine may continue to be operated in the VDE mode with the first group of cylinders deactivated and with the second group of cylinders operating at higher average cylinder load.

If the EGR has been sufficiently purged and the EGR level is below the threshold, then at518, the routine includes confirming if there is a further drop in engine load. If after EGR is below the threshold (that is sufficiently purged), no further drop in engine load is confirmed at518, the routine proceeds directly to528wherein the routine includes reactivating the first bank of engine cylinders. This includes resuming fueling to the first engine bank while also re-enabling cylinder intake and exhaust valves. Also at528, EGR may be re-enabled. For example, the LP-EGR valve may be opened and EGR may be provided at an optimal LP-EGR rate relative to engine speed/load conditions. In one example, LP-EGR may be provided as per a flat schedule, with EGR adjusted to be at a fixed rate relative to airflow.

Returning to518, if a drop in engine load is confirmed, then at520, it may be determined if the drop in engine load includes a tip-out to zero load conditions. If yes, then after EGR is below the threshold (that is sufficiently purged), in response to drop in engine load to zero load conditions, at522, the routine includes shutting off fuel to all engine cylinders while maintaining intake valve and exhaust valve operation of all the engine cylinders. For example, in response to the drop in engine load after EGR has been sufficiently purged, the engine may be automatically moved into a deceleration fuel shut off (DFSO) mode of operation. The engine may then remain in the DFSO mode with fueling disabled to all engine cylinders until engine load increases.

If at tip-out to zero load is not confirmed at520, then a drop in engine load to low load conditions is determined and at524, the routine includes, continuing engine operation in the VDE mode with the first group of cylinders deactivated and with the second group of cylinders operating at higher average cylinder load. That is, after EGR is depleted from the air induction system, the engine may continue to run in the bank shut-off mode (or VDE mode) with EGR disabled until a DFSO mode is entered or there is an increase in engine load. DFSO conditions may be considered met if the torque demand remains below a threshold and the vehicle speed falls below a threshold (such as during vehicle cruising), if a brake pedal is applied and engine braking is requested (such as during a deceleration), or if there is a tip-out to zero load.

At525, it may be determined if there is an increase in engine load. For example, it may be determined if there is a tip-in. If an increase in engine load is not confirmed, then at526, the routine includes maintaining engine operation in the existing operating mode. For example, the engine may continue to be operated in the DFSO mode with all fueling of all engine cylinders deactivated and EGR disabled (at522) or in the VDE mode with fueling of the first engine bank deactivated and EGR disabled (at524).

If an increase in engine load is confirmed, the routine proceeds to528to reactivate the previously deactivated engine cylinders and re-enable EGR. For example, in response to an increase in engine load while the engine is in the DFSO mode, the routine includes exiting the DFSO mode by re-enabling fueling to all engine cylinders and resuming combustion in all engine cylinders. In addition, the LP-EGR valve may be opened to allow for exhaust gas recirculation (e.g., as per a flat schedule). As another example, in response to an increase in engine load while the engine is in the VDE mode, the routine includes exiting the VDE mode by re-enabling fueling to the first bank of engine cylinders, resuming cylinder intake and exhaust valve operation in the first bank of cylinders, and resuming combustion in all engine cylinders. In addition, the LP-EGR valve may be opened to allow for exhaust gas recirculation (e.g., as per a flat schedule). As such, during the reactivation from VDE mode, exhaust catalyst purging on the deactivated cylinder bank may not be required since during the shutdown, exhaust flow for the deactivated cylinder bank is also shutdown. During reactivation from the DFSO mode, the exhaust catalysts on both cylinder banks may need to be purged, as described previously inFIG. 4. Likewise, in examples where following operation in the VDE mode, the engine enters a DFSO mode, during reactivation from the DFSO mode, exhaust catalysts coupled to both groups of cylinders may need to be purged.

In one example, an engine system comprises an engine, a selectively deactivatable fuel injector coupled to a cylinder of the engine, a system for deactivating the intake and exhaust valves of the deactivated cylinders, a turbocharger including an intake compressor driven by an exhaust turbine and an EGR system including a passage for recirculating cooled exhaust residuals from downstream of the turbine to upstream of the compressor. The engine system may further include a controller with computer readable instructions for operating the engine with EGR enabled. Then, in response to decreasing engine load, the controller is configured with further instructions for disabling EGR, deactivating fuel and intake and exhaust valves to a first group of engine cylinders, the first group of engine cylinders selected based on the decreasing engine load and other criteria such as NVH/engine smoothness, desired frequency of VDE switching and cylinder re-activation requirements; and maintaining the number of engine cylinders deactivated until EGR is below a threshold level. The engine system may further comprise an oxygen sensor coupled downstream of the compressor and upstream of an air intake throttle, and the controller may include further instructions for estimating the EGR based on an output of the oxygen sensor. The controller may also be configured to, after EGR is below the threshold level, deactivate fuel to all engine cylinders. In comparison, after EGR is below the threshold level, in response to increasing engine load, the controller may reactivate the first group of engine cylinders and external EGR.

Now turning toFIGS. 6-7, example engine deactivation operations are shown for expediting EGR purging. Map600ofFIG. 6depicts change in engine load at plot602, change in average cylinder load at plot604, change in LP-EGR amount at plot606, fueling of a first engine bank at plot608, fueling of a second engine bank at plot610, exhaust air-fuel ratio at the first bank at plot612, and oxygen saturation of an exhaust catalyst coupled downstream of only the first bank at plot614.

Prior to t1, the engine may be operating at mid to high engine load (plot602), with cylinders on both a first and second engine bank combusting (plots608,610). In particular, both banks may be operating at or around stoichiometry (plot612). Due to all the cylinders operating to provide engine torque, an average cylinder torque may be lower (plot604). During the engine operation at mid-high loads, the engine may be operating with LP-EGR flowing (plot606). In the depicted example, prior to t1, the engine may be operating with a desired LP-EGR schedule, wherein LP-EGR is delivered at the optimal level versus engine speed and load. In addition, due to the flow of stoichiometric exhaust gas through the exhaust catalyst, the oxygen saturation of an exhaust catalyst coupled downstream of the first engine bank (as well as an exhaust catalyst coupled downstream of a second engine bank) may be lower (plot614).

At t1, an operator pedal tip-out may occur, resulting in a drop in engine load to low load conditions. In response to the decreasing engine load, LP-EGR and airflow may also be decreased. However, due to the transport delay in LP-EGR, the actual decrease in LP-EGR (plot606, solid line) may be substantially slower than the desired decrease in LP-EGR (as shown by dashed segment605). Consequently, there may be high dilution in the cylinders at low load conditions, rendering the cylinders prone to misfires and slow burn related issues. To improve the high dilution tolerance of the cylinders, at t1, in response to the decreasing engine load, while LP-EGR is decreased, a VDE mode may be entered and fueling to cylinders on the first engine bank may be selectively disabled, for example, via selectively deactivatable fuel injectors. At the same time, fueling of cylinders on the second engine bank may be continued. As a result of a drop in engine load (to about half) and a corresponding deactivation of one bank of engine cylinders, an average cylinder load of the active cylinders may remain roughly constant, even through engine load has dropped, improving their dilution tolerance. However, the increased pumping of fresh air through the first bank increases the oxygen saturation of a first exhaust catalyst coupled downstream of the first bank (but not second bank).

Between t1 and t2, the VDE mode of operation may be maintained while LP-EGR is purged towards Thr_EGR. At t2, LP-EGR may be at the minimum threshold. Herein, by operating the cylinders on the second engine bank with reduced effective boosted volume, EGR purging to the minimum threshold is expedited. As such, if the engine cylinders were not deactivated, LP-EGR would reach the minimum threshold later, for example, substantially after t2.

While LP-EGR is at the minimum threshold at t2, due to engine load remaining low, the VDE mode may be continued. At t3, an operator tip-in is confirmed. In response to the operator tip-in, and the resultant increase in engine load, the engine may exit the VDE mode and reenter a non-VDE mode with fueling to the first bank resumed and with all cylinders combusting. However, due to elevated oxygen saturation of the first exhaust catalyst coupled to the first engine bank, the first exhaust catalyst may need to be purged during reactivation of the first engine bank.

Accordingly, at t3, while reactivating the cylinders of the first engine bank, fueling to the cylinders of the first bank may be adjusted so as to operate the cylinders richer than stoichiometry for a duration until the oxygen saturation of the catalyst is sufficiently reduced (that is, the catalyst is sufficiently purged). At t4, when the first exhaust catalyst is sufficiently purged, fueling of cylinders on the first engine bank may be returned to stoichiometry. As such, while the first exhaust catalyst is purged, combustion at the second engine bank may be maintained at or around stoichiometry.

In this way, by deactivating a bank of engine cylinders and disabling EGR in response to the decreasing load, the active cylinders can be operated with reduced effective boosted volume and EGR purging can be made faster. In particular, EGR purging achieved with one of more engine cylinders deactivated is faster than would be possible if no engine cylinders were deactivated.

Now turning toFIG. 7, another example deactivation is shown. Map700ofFIG. 6depicts change in engine load at plot702, change in average cylinder load at plot704, change in LP-EGR amount at plot706, fueling of a first engine bank at plot708, fueling of a second engine bank at plot710, exhaust air-fuel ratio at the first bank at plot712and at the second bank at plot716, oxygen saturation of an exhaust catalyst coupled downstream of only the first bank at plot714, and oxygen saturation of an exhaust catalyst coupled downstream of only the second bank at plot718.

Prior to t1, the engine may be operating at mid to high engine load (plot702), with cylinders on both a first and second engine bank combusting (plots708,710). In particular, both banks may be operating at or around stoichiometry (plots712,716). Due to all the cylinders operating to provide engine torque, an average cylinder torque may be lower (plot704). During the engine operation at mid-high loads, the engine may be operating with LP-EGR flowing (plot706). In the depicted example, prior to t1, the engine may be operating with a flat LP-EGR schedule, wherein LP-EGR is delivered at a fixed rate relative to airflow. In addition, due to the flow of stoichiometric exhaust gas through the exhaust catalyst, the oxygen saturation of an exhaust catalyst coupled downstream of the first engine bank as well as an exhaust catalyst coupled downstream of a second engine bank may be lower (plots714,718).

At t1, an operator pedal tip-out may occur, resulting in a drop in engine load to low load conditions. In response to the decreasing engine load, LP-EGR and airflow may also be decreased. However, due to the transport delay in LP-EGR, the actual decrease in LP-EGR (plot706, solid line) may be substantially slower than the desired decrease in LP-EGR (as shown by dashed segment705). Consequently, there may be high dilution in the cylinders at low load conditions, rendering the cylinders prone to misfires and slow burn related issues. To improve the high dilution tolerance of the cylinders, at t1, in response to the decreasing engine load, while LP-EGR is decreased, a VDE mode may be entered and fueling to cylinders on the first engine bank may be selectively disabled, for example, via selectively deactivatable fuel injectors. At the same time, fueling of cylinders on the second engine bank may be continued. As a result, even though there is a drop in engine load, there may not be a drop in the average cylinder load of the active cylinders, improving their dilution tolerance. In addition, by operating the first bank with fuel deactivated but with intake and/or exhaust valve operation maintained, fresh air may continue to be pumped through the first bank. As such, this increases the replacement of LP-EGR in the air induction system with fresh air, expediting the drop in LP-EGR amount to a minimum threshold Thr_EGR. However, the increased pumping of fresh air through the first bank also increases the oxygen saturation of a first exhaust catalyst coupled downstream of the first bank (but not second bank).

Between t1 and t2, the VDE mode of operation may be maintained while LP-EGR is purged towards Thr_EGR. Herein, by operating the cylinders on the second engine bank with reduced effective boosted volume, EGR purging to the minimum threshold is expedited and achieved by t2. As such, if the engine cylinders were not deactivated, LP-EGR would reach the minimum threshold later, for example, substantially after t2.

At t2, LP-EGR may be at the minimum threshold. However, due to engine load remaining low, the VDE mode may be continued. Between t2 and t3, DFSO conditions may be confirmed and responsive to the DFSO conditions, fueling to the second bank may also be deactivated. Following deactivation of fuel to the second bank, pumping of fresh air through the second bank also increases the oxygen saturation of a second exhaust catalyst coupled downstream of the second bank (but not the first bank). As such, the oxygen saturation incurred at the second exhaust catalyst (between t2 and t3) may be smaller than the oxygen saturation incurred at the first exhaust catalyst (between t1 and t3).

At t3, an operator tip-in is confirmed. In response to the operator tip-in, and the resultant increase in engine load, the engine may exit the DFSO mode and reenter a non-VDE mode with fueling to the first bank and second bank resumed and with all cylinders combusting. However, due to elevated oxygen saturation of the first exhaust catalyst coupled to the first engine bank and the second exhaust catalyst coupled to the second engine bank, each of the first exhaust catalyst and the second exhaust catalyst may need to be purged during reactivation of the first and second engine banks.

Accordingly, at t3, while reactivating the cylinders of the first engine bank, fueling to the cylinders of the first bank may be adjusted so as to operate the cylinders richer than stoichiometry for a first duration d1 until the oxygen saturation of the first exhaust catalyst is sufficiently reduced (that is, the catalyst is sufficiently purged) while fueling to the cylinders of the second bank is also adjusted so as to operate the cylinders richer than stoichiometry for a second duration d2 until the oxygen saturation of the second exhaust catalyst is sufficiently reduced. As shown, first duration d1 may be longer than second duration d2. In addition, the degree of richness required to purge the first exhaust catalyst may be larger than the degree of richness required to purge the second exhaust catalyst.

Between t3 and t4, when the second exhaust catalyst is sufficiently purged, fueling of cylinders on the second engine bank may be returned to stoichiometry. Likewise, at t4, when the first exhaust catalyst is sufficiently purged, fueling of cylinders on the first engine bank may be returned to stoichiometry.

In this way, running of optimal LP EGR rates in mid to higher loads can be achieved while allowing for LP EGR to be purged form the air intake system when there is a tip-out to lower engine loads. By deactivating one or more engine cylinders during low load, higher than desired EGR dilution conditions, the higher average load of the active cylinders can be advantageously used to improve cylinder EGR tolerance and combustion stability. If the intake and exhaust valves are also deactivated on the deactivated cylinders, effective boosted volume can also be reduced and EGR purging is improved. By expediting EGR purging, combustion stability issues and misfire events are reduced. In addition, condensation and related issues are reduced. Furthermore, higher EGR rates can be achieved during subsequent high load engine operations. Overall, boosted engine performance is improved.