METHOD AND APPARATUS FOR REGENERATING AN AFTERTREATMENT DEVICE FOR A SPARK-IGNITION DIRECT-INJECTION ENGINE

Abstract

The disclosure sets forth operating a spark-ignition, direct-fuel injection internal combustion engine equipped with an exhaust aftertreatment system including a lean-NOx adsorber device. The engine is operated substantially un-throttled and at a lean air/fuel ratio and a first fuel pulse is injected to meet an engine output torque during a compression stroke of each engine cycle prior to a spark-ignition event. When regeneration of the lean-NOx adsorber device is commanded, a second fuel pulse is injected during a second engine stroke of each engine cycle.

Description

TECHNICAL FIELD

This disclosure is related to control of spark-ignition direct injection internal combustion engines.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Known spark-ignition engines function by introducing a fuel/air mixture into a combustion chamber and igniting the mixture using an ignition source such as a spark plug. A spark-ignition engine can operate at an air/fuel ratio that is at or near stoichiometry, or at a lean air/fuel ratio. A spark-ignition engine can operate at the lean air/fuel ratio, including operating in a stratified charge combustion mode which includes operating substantially un-throttled with fuel directly injected into each combustion chamber during a compression stroke, just prior to initiation of spark. Known aftertreatment systems for spark-ignition engines operating lean of stoichiometry can include a lean-NOx adsorber device, which can be used in concert with other exhaust aftertreatment devices including three-way catalytic converters. A lean-NOx adsorber device requires regeneration to desorb and reduce adsorbed NOx elements. Known regenerative techniques include operating the spark-ignition engine at an air/fuel ratio that is at stoichiometry or rich of stoichiometry.

It is known to transition a spark-ignition engine from a stratified charge combustion mode to a homogeneous mode to effect regeneration of a lean-NOx adsorber device. Operating a spark-ignition engine in a homogeneous charge mode includes operating at stoichiometric air/fuel ratio, with an engine throttle valve controlled to a predetermined position, and with fuel directly injected in each combustion chamber during an intake stroke prior to the compression stroke and spark ignition. It is known that cylinder pressures reached during operation in the stratified charge combustion mode are substantially greater than those reached during operation in the homogeneous mode, and any transition between the modes has an effect upon engine vibration. It is known that a portion of the exhaust gas feedstream generated during the homogeneous mode can be converted to inert gases in a three-way catalytic converter placed upstream of the NOx adsorber device, affecting regeneration of the NOx adsorber device.

SUMMARY

A method for controlling operation of a spark-ignition, direct-fuel injection internal combustion engine equipped with an exhaust aftertreatment system including a lean-NOx adsorber device includes operating the engine substantially un-throttled and at a lean air/fuel ratio. A first fuel pulse is injected sufficient to power the engine to achieve an engine output torque during a compression stroke of each engine cycle prior to a spark-ignition event. A regeneration of the lean-NOx adsorber device is commanded and a second fuel pulse is injected during a second engine stroke of each engine cycle during the commanded regeneration.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:

FIGS. 1 and 2 are schematic diagrams of an engine and exhaust aftertreatment system, in accordance with the present disclosure; and,

FIGS. 3, 4, and 5 are control datagraphs, in accordance with the present disclosure.

DETAILED DESCRIPTION

Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same, FIG. 1 schematically illustrates an internal combustion engine 10 and accompanying control module 5 that have been constructed in accordance with an embodiment of the disclosure. The engine 10 comprises a multi-cylinder spark-ignition, direct-injection four-stroke internal combustion engine having reciprocating pistons 14 slidably movable in cylinders 15 which define variable volume combustion chambers 16. Each piston 14 includes a bowl portion at the top of the piston 14 into which fuel is injected. Each piston 14 is connected to a rotating crankshaft 12 by which linear reciprocating piston travel is translated to rotational motion. A single one of the cylinders 15 is shown in FIG. 1. The engine 10 is selectively operative in a stratified charge combustion mode and a homogeneous charge combustion mode. The stratified charge combustion mode includes of operating at an air/fuel ratio that is lean of stoichiometry, for example an air/fuel ratio ranging from 17:1 to 60:1, with single-injection fueling comprising a single fuel pulse which occurs late in a compression stroke, and a high dilution EGR mass. A high dilution EGR mass can be an EGR mass which is greater than 40% of a cylinder charge. An engine throttle valve 34 is maintained at or near a substantially wide-open-throttle position. The homogeneous charge combustion mode includes operating at an air/fuel ratio that is at or near stoichiometry, preferably with single-injection fueling comprising a single fuel pulse which occurs during an intake stroke, and a low dilution EGR mass, e.g., less than 5% of the cylinder charge.

Engine operation is controlled to achieve an engine output torque based upon an engine load including an operator torque request, including controlling the throttle valve 34. The engine 10 operates in the stratified charge combustion mode under light to medium engine loads. The engine 10 operates in the homogeneous charge combustion mode under heavier engine loads. The engine 10 further can be controlled to operate to regenerate an exhaust aftertreatment system 50.

The engine 10 includes an air intake system 30 which channels and distributes intake air to each combustion chamber 16. The air intake system 30 is made up of air flow channels between the throttle valve 34 and engine intake valves 20, and preferably includes ductwork, an intake manifold 31, and intake passages 29. The air intake system 30 includes devices for monitoring and controlling the intake air flow therethrough. The devices for controlling the intake air flow preferably comprise the throttle valve 34 in this embodiment. The devices for monitoring the intake air flow preferably include a pressure sensor 36 adapted to monitor manifold absolute pressure and barometric pressure in the intake manifold 31. A mass air flow sensor 32 is preferably located upstream of the throttle valve 34 to monitor mass of the intake air flow and intake air temperature. The throttle valve 34 preferably comprises an electronically controlled device adapted to control the intake air flow to the engine 10 in response to a control signal (‘ETC’) from the control module 5. An external flow passage (not shown) recirculates exhaust gases from an exhaust manifold 40 to the air intake system 30, controlled by an exhaust gas recirculation (hereafter ‘EGR’) control valve 38. The control module 5 controls mass flow of exhaust gas to the air intake system 30 by controlling opening of the EGR control valve 38.

Engine valves, including intake valve(s) 20 and exhaust valve(s) 18 control flow into and out of each combustion chamber 16. The intake air flow from the intake passage 29 into the combustion chamber 16 is controlled by the intake valve(s) 20. Exhaust gas flow out of the combustion chamber 16 is controlled by the exhaust valve(s) 18 to the exhaust manifold 40 via exhaust passages 39. Openings and closings of the intake and exhaust valves 20 and 18 are preferably controlled with a dual camshaft (as depicted), the rotations of which are linked and indexed with rotation of the crankshaft 12. The intake and exhaust valves 20 and 18 may be controlled by devices 22 and 24. Device 22 preferably comprises a controllable mechanism operative to variably control valve lift (‘VLC’) and variably control cam phasing (‘VCP’) of the intake valve(s) 20 for each cylinder 15 in response to a control signal (‘INTAKE’) from the control module 5. Device 24 preferably comprises a controllable mechanism operative to variably control valve lift (‘VLC’) and variably control cam phasing (‘VCP’) of the exhaust valve(s) 18 for each cylinder 15 in response to a control signal (‘EXHAUST’) from the control module 5. Devices 22 and 24 each preferably comprises a controllable two-step valve lift mechanism operative to control magnitude of valve lift, or opening, to one of two discrete steps, e.g., a low-lift valve open position (typically about 4-6 mm) for load speed, low load operation, and a high-lift valve open position (typically about 8-10 mm) for high speed and high load operation. Devices 22 and 24 further comprise variable cam phasing mechanisms to control phasing, i.e., relative timing of opening and closing of the intake valve(s) 20 and the exhaust valve(s) 18 respectively, measured in crank angle degrees. The variable cam phasing mechanisms shift valve opening time relative to crankshaft and piston position. The VCP system has a range of phasing authority of preferably 40°-90° of crank rotation, thus permitting the control module 5 to advance or retard opening and closing of one of the intake valves 20 and the exhaust valves 18 relative to position of the piston 14. The range of phasing authority is defined and limited by the devices 22 and 24. Devices 22 and 24 are actuated using one of electro-hydraulic, hydraulic, and electric control force, controlled by the control module 5.

A fuel injection system comprises a plurality of high-pressure fuel injectors 28 which directly inject fuel into the combustion chamber 16. A fuel pulse is a mass of fuel injected into the combustion chamber 16 in response to a control signal (‘INJ_PW’) from the control module 5. The control signal from the control module 5 preferably comprises timing for a start of each fuel pulse relative to a crank angle which defines a position of the piston 14 in the cylinder 15, and duration of a pulsewidth to inject a predetermined fuel mass from the injector 28 into the cylinder 15. The fuel injectors 28 are supplied pressurized fuel from a fuel distribution system (not shown). Fuel can be injected during single-injection fueling for each cylinder 15 for each combustion cycle. There can be multiple fueling events for each cylinder 15 for each combustion cycle, as described hereinbelow.

The fuel injector 28 comprises a high-pressure solenoid-controlled fuel injector. Operating parameters include a minimum operating pulsewidth at which the solenoid-controlled fuel injector 28 can be controlled, thus establishing a minimum fuel mass delivered for a fuel pressure level. Alternatively, a fuel injector 28 may comprise a high-pressure fuel injector utilizing an alternative actuation technology, e.g., piezoelectric actuation. The alternative fuel injector 28 is controllable to deliver a minimal fuel mass for the fuel pressure level.

A spark-ignition system provides electrical energy to a spark plug 26 for igniting cylinder charges in each combustion chamber 16, in response to a control signal (‘IGN’) from the control module 5. The control signal IGN is controlled to achieve a preferred spark-ignition timing based upon a crank angle which defines the position of the piston 14 in the cylinder 15 during each engine cycle.

Various sensing devices monitor engine operation, including a rotational speed sensor 13 adapted to monitor rotational speed of the crankshaft 12 and a wide range air/fuel ratio sensor 42 adapted to monitor exhaust gas air/fuel ratio. The engine 10 may include a combustion sensor 44 adapted to monitor in-cylinder combustion in real-time during ongoing operation of the engine 10. The combustion sensor 44 comprises a sensor device operative to monitor a state of a combustion parameter and is depicted as a cylinder pressure sensor operative to monitor in-cylinder combustion pressure. Alternatively, other sensing systems can be used to monitor real-time in-cylinder combustion parameters which can be translated into combustion phasing, e.g., ion-sense ignition systems and non-intrusive pressure sensors.

The exhaust aftertreatment system 50 is fluidly connected to the exhaust manifold 40, preferably comprising one or more catalytic and/or trap devices operative to oxidize, adsorb, desorb, and reduce combustion elements of the exhaust gas feedstream. As depicted in FIG. 2, the exhaust aftertreatment system 50 preferably includes one or more three-way catalytic converters (‘TWC’) 48 upstream of a lean-NOx reduction catalyst (‘LNT’) 52, and preferably a selective catalyst reduction device (‘SCR’) 53. One or more exhaust gas sensor(s) 55 monitor the exhaust gas feedstream downstream of the lean-NOx reduction catalyst 52 or downstream of the exhaust aftertreatment system 50. The output(s) of the exhaust gas sensor(s) 55 is monitored by the control module 5 for control and diagnostic purposes.

The lean-NOx reduction catalyst 52 comprises an adsorber device which is operative to adsorb nitrates in the exhaust gas feedstream, with the amount of adsorption based upon temperature, flowrate, and air/fuel ratio of the exhaust gas feedstream and amount of nitrates already adsorbed thereon.

The lean-NOx reduction catalyst 52 preferably comprises a NOx adsorber device comprising a substrate having a washcoat containing catalytically active material. The substrate preferably comprises a monolithic element formed from cordierite with a cell density that is typically 400 to 600 cells per square inch, and a wall thickness of three to seven mils. The cells of the substrate comprise flow passages through which exhaust gas flows to contact the catalytically active materials of the washcoat to effect adsorption and desorption of nitrates, oxygen storage, and oxidization and reduction of constituents of the exhaust gas feedstream. The washcoat preferably contains alkali and/or alkali earth metal compounds, e.g., Ba and K, operative to store NOx as nitrates that are generated during engine operation that is lean of stoichiometry. The washcoat can also contain catalytically active materials, i.e., platinum-group metals comprising Pt, Pd, and Rh, and additives (e.g., Ce, Zr, La). When the exhaust gas feedstream is rich of stoichiometry there are excess reductants and adsorbed nitrates are not stable and decompose to release stored NOx. The reductants in the exhaust gas feedstream preferably comprise HC molecules, hydrogen molecules, and CO which are generated when the engine is operated at a rich air/fuel ratio. The washcoat adsorbs nitrates during lean engine operation, and desorbs and reduces nitrates during engine operation that generates a rich exhaust gas feedstream. The desorbed nitrates are reduced by the excess reductants at PGM catalyst sites. The lean-NOx reduction catalyst 52 can saturate with adsorbed nitrates, thus reducing its effectiveness. The lean-NOx reduction catalyst 52 can be regenerated by desorbing the adsorbed nitrates in the presence of the aforementioned reductant by reacting with the reductant to reduce to nitrogen and other inert elements.

During operation in the stratified charge combustion mode, the engine 10 preferably operates un-throttled, i.e., the throttle valve 34 is at a substantially wide-open position, on gasoline or similar fuel blends over a range of engine speeds and loads. The throttle valve 34 can be slightly closed to generate a vacuum in the intake manifold 31 to effect flow of EGR gas through the EGR control valve 38. A first fuel pulse is injected during the compression stroke of each engine cycle. The engine 10 operates in the homogeneous charge combustion mode with the throttle valve 34 controlled for stoichiometric operation, under conditions not conducive to the stratified charge combustion mode operation, and to achieve engine power to meet the operator torque request. Widely available grades of gasoline and light ethanol blends thereof are preferred fuels; however, alternative liquid and gaseous fuels such as higher ethanol blends (e.g. E80, E85), neat ethanol (E99), neat methanol (M100), natural gas, hydrogen, biogas, various reformates, syngases, and others may be used in the implementation of the present disclosure.

The control module 5 preferably comprises a general-purpose digital computer generally comprising a microprocessor or central processing unit, storage mediums comprising non-volatile memory including read only memory (ROM) and electrically programmable read only memory (EPROM), random access memory (RAM), a high speed clock, analog to digital (A/D) and digital to analog (D/A) circuitry, and input/output circuitry and devices (J/O) and appropriate signal conditioning and buffer circuitry. The control module 5 has a set of control algorithms, comprising resident program instructions and calibrations stored in the non-volatile memory and executed to provide the respective functions of each computer. The algorithms are executed during preset loop cycles such that each algorithm is executed at least once each loop cycle. Algorithms are executed by the central processing unit and are operable to monitor inputs from the aforementioned sensing devices and execute control and diagnostic routines to control operation of the actuators, using preset calibrations. Loop cycles are executed at regular intervals, for example each 3.125, 6.25, 12.5, 25 and 100 milliseconds during ongoing engine and vehicle operation. Alternatively, algorithms may be executed in response to occurrence of an event.

In operation, the control module 5 monitors inputs from the aforementioned sensors to determine states of engine parameters. The control module 5 executes algorithmic code stored therein to control the aforementioned actuators to form the cylinder charge, including controlling throttle position, spark-ignition timing, fuel injection mass and timing, EGR valve position to control flow of recirculated exhaust gases, and intake and/or exhaust valve timing and phasing on engines so equipped. The control module 5 can operate to turn the engine on and off during ongoing vehicle operation, and can operate to selectively deactivate a portion of the combustion chambers through control of fuel and spark and valve deactivation.

In operation, the engine 10 can be commanded to regenerate the lean-NOx reduction catalyst 52, preferably an exhaust gas feedstream that is rich of stoichiometry, preferably at elevated exhaust gas temperatures, to generate the reductants. The engine operation includes operating in the stratified charge combustion mode, with the throttle valve 34 is substantially at wide open and the first fuel pulse is injected into the combustion chamber 16 during the compression stroke coordinated to immediately precede the spark-ignition timing, effecting stratified ignition thereof. The mass of fuel injected during the first fuel pulse is determined based upon an amount sufficient to operate the engine 10 to meet the operator torque request. While operating in the stratified charge combustion mode, subsequent fuel pulses are injected to the combustion chamber 16. The subsequent fuel pulses are injected during other strokes of the combustion cycle to generate an exhaust gas feedstream having an air/fuel ratio which is stoichiometric or rich to act as a reductant to regenerate the lean-NOx reduction catalyst 52.

FIG. 3 graphically shows operating the exemplary engine 10 for a single engine cycle, including a measure of cylinder pressure and occurrence of fuel pulses depicted in crank angle degrees. The engine cycle includes the compression stroke, an expansion stroke, an exhaust stroke, and an intake stroke. The engine is operating in the stratified charge combustion mode. A first fuel pulse 110 (‘Power Fuel Pulse’) injected into the combustion chamber 16 during the compression stroke to generate a stratified charge air/fuel distribution in the combustion chamber 16 which is coordinated to immediately precede the spark-ignition timing, effecting stratified ignition thereof. The first fuel pulse 110 preferably injects a mass of fuel sufficient to power the engine 10 to achieve the engine output torque based upon the engine load. A second fuel pulse 120 (‘Regen Fuel Pulse’) is injected during the intake stroke, generating a partially homogeneous air/fuel charge which passes uncombusted into the exhaust gas feedstream and is sufficient to break through the three-way catalytic converter 48 to the lean-NOx reduction catalyst 52. Such operation reduces a need for a mode transition to homogeneous operation with a rich air/fuel ratio to achieve regeneration of the lean-NOx reduction catalyst 52. Alternatively, the second fuel pulse can be injected during the expansion stroke or the exhaust stroke (not shown).

FIG. 4 graphically shows operating the exemplary engine 10 for a single engine cycle, including a measure of cylinder pressure and occurrence of fuel pulses depicted in crank angle degrees. The engine is operating in the stratified charge combustion mode. The first fuel pulse 110 (‘Power Fuel Pulse’) is injected into the combustion chamber 16 during the compression stroke to generate a stratified charge air/fuel distribution in the combustion chamber 16 and coordinated to immediately precede the spark-ignition timing, effecting stratified ignition thereof. A third fuel pulse 130 (‘Regen Fuel Pulse’) is injected during the expansion stroke, and the second fuel pulse 120 (‘Regen Fuel Pulse’) is injected during the intake stroke to generate the stratified charge air/fuel distribution which passes into the exhaust gas feedstream and is of sufficient mass to at least partially break through the three-way catalytic converter 48 to the lean-NOx reduction catalyst 52. The second fuel pulse 120 during the intake stroke generates an air/fuel distribution in the combustion chamber 16 that is substantially homogeneous, and the first fuel pulse 110 during the compression stroke generates the stratified charge air/fuel distribution in the combustion chamber 16 coordinated to immediately precede the spark-ignition timing. The third fuel pulse 130 during the end of the expansion stroke enriches the air/fuel ratio in the exhaust gas feedstream, preferably to an air/fuel ratio rich of stoichiometry to facilitate NOx reduction. A portion of the mass of fuel injected during the third fuel pulse 130 may generate power and contribute to the torque output of the engine 10, depending upon timing the injection. This can be determined and accounted for by adjusting the mass of fuel injected during the first fuel pulse 110 to eliminate effects on the engine output torque.

FIG. 5 graphically shows operating the exemplary engine 10 for a single engine cycle, including a measure of cylinder pressure and occurrence of fuel pulses depicted in crank angle degrees. The engine is operating in the stratified charge combustion mode. The first fuel pulse 110 (‘Power Fuel Pulse’) injected into the combustion chamber 16 during the compression stroke to generate the stratified charge air/fuel distribution in the combustion chamber 16, coordinated to immediately precede the spark-ignition timing, effecting stratified ignition thereof. The third fuel pulse 130 (‘Regen Fuel Pulse’) is injected during the expansion stroke, the second fuel pulse 120 (‘Regen Fuel Pulse’) is injected during the intake stroke and the fourth fuel pulse 140 (‘Regen Fuel Pulse’) is injected during the exhaust stroke to generate a stratified charge air/fuel distribution which passes into the exhaust gas feedstream and is sufficient to at least partially break through the three-way catalytic converter 48 to the lean-NOx reduction catalyst 52. The second fuel pulse 120 during the intake stroke generates an air/fuel distribution in the combustion chamber 16 that is substantially homogeneous, and the first fuel pulse 110 during the compression stroke generates the stratified charge air/fuel distribution in the combustion chamber 16 coordinated to immediately precede the spark-ignition timing. The third and fourth fuel pulses 130 and 140 during the expansion stroke and during the exhaust stroke enrich the air/fuel ratio in the exhaust gas feedstream, preferably to an air/fuel ratio rich of stoichiometry to facilitate NOx reduction. This operation more fully optimizes the performance of the stratified charge engine operation and generates an optimized rich exhaust-gas distribution and content during the regeneration process. For each of the aforementioned fuel injection strategies, a range of injection timings and delivered fuel masses can be calibrated to optimize the performance of a particular engine and exhaust aftertreatment system constructed in accordance with the disclosure.

The amounts of fuel injected during first and third fuel pulses 110 and 130 are calibrated based upon engine fueling to achieve the engine output torque to meet the engine load. The first fuel pulse 110 delivers a substantial amount of the fuel to achieve the engine output torque. The second fuel pulse 120 generates a partially homogeneous air/fuel charge in the exhaust gas feedstream and enriching the air/fuel charge during regeneration of the exhaust aftertreatment system 50. Preferably, a lean combustion charge from the second fuel pulse 120 in combination with rich combustion from the first and third fuel pulses 110 and 130 generate preferred exhaust products to stratify the air/fuel charge, thus transporting a portion of the rich exhaust gases through the three-way catalyst 48 and generating a higher H2/CO ratio for regeneration of the lean-NOx reduction catalyst 52.

The disclosure has described certain preferred embodiments and modifications thereto. Further modifications and alterations may occur to others upon reading and understanding the specification. Therefore, it is intended that the disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.

Claims

1. Method for controlling operation of a spark-ignition, direct-fuel injection internal combustion engine, comprising:

equipping the engine with an exhaust aftertreatment system including a lean-NOx adsorber device;

operating the engine substantially un-throttled and at a lean air/fuel ratio;

injecting a first fuel pulse sufficient to power the engine to achieve an engine output torque during a compression stroke of each engine cycle prior to a spark-ignition event;

commanding a regeneration of the lean-NOx adsorber device; and

injecting a second fuel pulse during a second engine stroke of each engine cycle during the commanded regeneration of the lean-NOx adsorber device.

2. The method of claim 1, comprising injecting the second fuel pulse during an intake stroke of each engine cycle during the commanded regeneration of the lean-NOx adsorber device.

3. The method of claim 2, further comprising injecting a third fuel pulse during an expansion stroke of each engine cycle during the commanded regeneration of the lean-NOx adsorber device.

4. The method of claim 3, further comprising the first and third fuel pulses sufficient to power the engine to achieve the engine output torque.

5. The method of claim 3, further comprising injecting a fourth fuel pulse during an exhaust stroke of each engine cycle during the commanded regeneration of the lean-NOx adsorber device.

6. The method of claim 1, comprising injecting the second fuel pulse during an exhaust stroke of each engine cycle during the commanded regeneration of the lean-NOx adsorber device.

7. The method of claim 1, comprising injecting the second fuel pulse during an expansion stroke of each engine cycle during the commanded regeneration of the lean-NOx adsorber device.

8. The method of claim 1, wherein the exhaust aftertreatment system includes a three-way catalytic converter upstream of the lean-NOx adsorber device.

9. The method of claim 8, comprising injecting the second fuel pulse sufficient to generate an exhaust gas feedstream having a rich air/fuel ratio sufficient for a portion of exhaust gas reductants to break through the three-way catalytic converter upstream of the lean-NOx adsorber device.

10. Method for operating a spark-ignition, direct-fuel injection internal combustion engine equipped with a lean-NOx adsorber device for exhaust gas aftertreatment, comprising:

operating the engine in a stratified charge combustion mode including a first fuel pulse sufficient to power the engine to achieve an engine output torque;

commanding a regeneration of the lean-NOx adsorber device; and

continuing operating the engine in the stratified charge combustion mode and injecting a second fuel pulse during a second engine stroke of each engine cycle during the commanded regeneration of the lean-NOx adsorber device, the second fuel pulse sufficient to generate an exhaust gas feedstream having a rich air/fuel ratio.

11. The method of claim 10, wherein operating the engine in the stratified charge combustion mode comprises operating substantially un-throttled and injecting the first fuel pulse during a compression stroke of each engine cycle prior to a spark-ignition event.

12. The method of claim 11, comprising injecting the second fuel pulse during an intake stroke of each engine cycle during the commanded regeneration of the lean-NOx adsorber device.

13. The method of claim 12, further comprising injecting a third fuel pulse during an expansion stroke of each engine cycle during the commanded regeneration of the lean-NOx adsorber device.

14. The method of claim 13, further comprising injecting a fourth fuel pulse during an exhaust stroke of each engine cycle during the commanded regeneration of the lean-NOx adsorber device.

15. The method of claim 10, comprising injecting the second fuel pulse during an exhaust stroke of each engine cycle during the commanded regeneration of the lean-NOx adsorber device.

16. The method of claim 10, comprising injecting the second fuel pulse during the expansion stroke of each engine cycle during the commanded regeneration of the lean-NOx adsorber device.

17. The method of claim 10, wherein the exhaust aftertreatment system includes a three-way catalytic converter upstream of the lean-NOx adsorber device.

18. The method of claim 17, comprising injecting the second fuel pulse sufficient to generate an exhaust gas feedstream having a rich air/fuel ratio sufficient for a portion of exhaust gas reductants to break through the three-way catalytic converter upstream of the lean-NOx adsorber device.

19. Method for operating an internal combustion engine, comprising:

equipping the engine with an exhaust aftertreatment system including a lean-NOx adsorber device;

operating the engine substantially un-throttled and at a lean air/fuel ratio;

directly injecting a first fuel pulse into a combustion chamber during a compression stroke of each engine cycle immediately prior to a spark-ignition event, wherein mass of fuel injected during the first fuel pulse is sufficient to power the engine to achieve an engine output torque; and

directly injecting a second fuel pulse during a second engine stroke of each engine cycle for a period of time, wherein mass of fuel injected during the second fuel pulse is determined based upon regenerating the lean-NOx adsorber device.

20. The method of claim 19, comprising:

directly injecting the first fuel pulse during the compression stroke to generate a stratified charge air/fuel distribution prior to initiating the spark-ignition event; and

directly injecting the second fuel pulse during the second engine stroke to generate a mixture of fuel and air in the exhaust gas feedstream that is uncombusted.

21. The method of claim 20, further comprising directly injecting the second fuel pulse during the exhaust stroke and directly injecting a third fuel pulse during one of an intake stroke and the compression stroke to generate the uncombusted mixture of fuel and air in the exhaust gas feedstream.