EXHAUST AIR INJECTION

Abstract

Engine procedure which includes: Operating the engine (8) with lean combustion; and when the exhaust gas oxygen concentration is below a threshold value, injecting air into an exhaust passage (35) between a first exhaust gas purification device (70) and an SCR device (72), wherein - an amount of injected air is further controlled to maintain a temperature of the SCR device (72) within a threshold range, and / or - injecting air into the exhaust passage (35) further comprises directing supercharged intake air directly to the exhaust passage (35).

Description

FIELD OF THE INVENTIONThe present disclosure relates to an exhaust system of an engine.GENERAL STATE OF THE ART AND SUMMARY OF THE INVENTIONGasoline engines are typically controlled to operate substantially around a stoichiometric air-fuel ratio to balance fuel economy from emissions conversion in emission control devices. However, gasoline engines controlled to operate with relatively lean combustion may experience significant fuel economy benefits compared to stoichiometric combustion. For example, air-fuel ratios between 20:1 and 28:1 may provide optimal balancing between fuel economy, combustion stability, and emissions, although lean operation produces increased NOxemissions compared to stoichiometric operation. The excess NOx may be converted by providing a selective catalyst reduction (SCR) system in the exhaust system.WO 2010 / 108 083 A1 discloses an exhaust gas treatment system with a lean No x- trap.However, the inventors herein have identified potential problems in the above approach. For example, SCR systems may experience maximum NOx conversion efficiency when the exhaust oxygen concentration exceeds a lower limit, such as 8%. For example, in the lean operation with an air-fuel ratio of 25:1, the oxygen concentration may be below 8%, and thus the NOx conversion efficiency may be deteriorated.Thus, in one example, some of the above issues may be at least partially addressed by an engine method comprising: operating the engine with lean combustion; and if the exhaust oxygen concentration is below a threshold, injecting air into an exhaust passage between a first exhaust emission control device and an SCR device.In this way, exhaust oxygen concentration may be adjusted via introduction of secondary air into an exhaust passage between the upstream emission control device and the SCR device. The secondary air may be introduced based on feedback control to maintain an exhaust oxygen concentration at a desired value and / or maintain a temperature of the SCR device within a threshold range.The SCR device is configured to convert NOx using an injected reductant such as ammonia. By incorporating the SCR system and injecting secondary air at low oxygen concentration, efficient lean burn conversion of NOx may be provided, thereby balancing lean burn fuel economy benefits with exhaust purification.The above advantages and other advantages, as well as features of the present description, will be readily apparent from the following detailed description when taken alone or in conjunction with the accompanying drawings.It should be understood that the brief discussion above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not intended to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages mentioned above or in any part of this disclosure.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 shows a schematic illustration of a vehicle system having an engine and an associated exhaust aftertreatment system. FIG. 2 is a flowchart illustrating a method for controlling the air-fuel ratio of the exhaust gas according to an embodiment of the present disclosure. FIG. 3 is a flowchart illustrating a method of injecting air into an exhaust passage according to an embodiment of the present disclosure. FIG. 4 is a flowchart illustrating a method of controlling introduction of air during lean combustion according to an embodiment of the present disclosure. FIG. 5 is a flowchart illustrating a method for controlling introduction of air during stoichiometric combustion according to an embodiment of the present disclosure. FIG. 6 is a flowchart illustrating a method of controlling introduction of air during generation of ammonia according to an embodiment of the present disclosure. FIG. 7 is a flowchart illustrating a method for controlling introduction of air during catalyst cooling according to an embodiment of the present disclosure. FIGS. 8 and 9 show example plots of engine operating parameters during injection of air, according to embodiments of the present disclosure.DETAILED DESCRIPTIONAn air injection system may be provided in an exhaust system to maintain the exhaust oxygen concentration above a certain level prior to an SCR system during lean operating phases. Optimal conversion of NOx by the SCR system may occur at an exhaust oxygen concentration within a certain range, such as 8-10%. In gasoline engines configured to operate with lean combustion (e.g., air-fuel ratios about 25:1), oxygen concentrations are often below 8%. To increase oxygen concentration during lean operation without compromising fuel economy or the conversion of exhaust constituents in an upstream catalyst, air may be injected upstream of the SCR system. The amount and timing of air injection may be controlled via feedback from an oxygen sensor in the exhaust passage and / or via a temperature of the SCR catalyst.The air may be compressed intake air directed to the exhaust passage directly or via an EGR (Exhaust Recirculation) passage. The compressed intake air may be directed to the EGR passage when directed via the EGR passage before an EGR cooler to provide additional EGR cooler regeneration assistance. Thus, the injected air may improve emissions by supplying a lean exhaust gas to a downstream emission control device while increasing EGR cooler performance. FIG. 1 shows an engine including an air injection system, upstream and downstream emission control devices, and a controller configured to execute the methods of FIGS. 2-7.FIG. 1 shows a schematic illustration of a vehicle system 6. the vehicle system 6 includes an engine system 8 coupled to an exhaust aftertreatment system 22. the engine system 8 may include an engine 10 having multiple cylinders 30. Engine 10 includes an engine inlet 23 and an engine outlet 25, Engine inlet 23 includes a throttle 62 fluidly coupled to intake manifold 44 via an intake passage 42, Engine outlet 25 includes an exhaust manifold 48 that ultimately leads to an exhaust passage 35 that directs exhaust gas to the atmosphere. The throttle 62 may be located in the intake passage 42 behind a supercharger such as a turbocharger 50 or a supercharger. The turbocharger 50 may include a compressor 52 disposed between the intake passage 42 and the intake manifold 44. Compressor 52 may be at least partially driven by an exhaust turbine 54 disposed between exhaust manifold 48 and exhaust passage 35. The compressor 52 may be coupled to the exhaust turbine 54 via a shaft 56. Compressor 52 may also be driven at least partially by an electric motor 58. In the example shown, the electric motor 58 is shown coupled to the shaft 56. However, other suitable configurations of the electric motor are also possible. In one example, the electric motor 58 may be operated with stored electrical energy from a system battery, not shown, when the battery state of charge is above a charge threshold. By using the electric motor 58 to operate the turbocharger 50, for example at engine start-up, an electric boost (E-boost) may be provided to the intake charge air. In this way, the electric motor may provide motor assist to operate the supercharger. As such, after the engine has run a sufficient amount of time (e.g., a threshold time), the exhaust gas generated in the exhaust manifold may begin driving the exhaust turbine 54. Consequently, the motor assist of the electric motor can be reduced. That is, during turbocharger operation, the engine assist provided by the electric motor 58 may be adjusted in response to the operation of the exhaust turbine.Fuel system 18 may include a fuel tank 20 coupled to a fuel pump system 21. The fuel pump system 21 may include one or more pumps for pressurizing fuel supplied to the injection nozzles of the engine 10, such as the example injection nozzle 66 shown. While only a single injection nozzle 66 is shown, additional injection nozzles are provided for each cylinder. It should be appreciated that the fuel system 18 may be a return or no return fuel system or various other types of fuel system.Although not shown in FIG. 1, each cylinder 30 may include one or more intake valves and one or more exhaust valves for controlling intake of charge air and release of exhaust gas, respectively. The timing of opening and closing of the intake and exhaust valves may be fixed, or the opening and / or closing timings of the intake and exhaust valves may vary to provide positive or negative valve overlap, late intake valve closing, or various other examples.The engine outlet 25 may be coupled to an exhaust aftertreatment system 22 along the exhaust passage 35. The exhaust aftertreatment system 22 may include one or more emission control devices, such as emission control devices 70, 72, which may be mounted in a close-coupled position in the exhaust passage 35. The emission control devices may include a three-way catalyst, a lean NOx conversion device or particulate filter, a selective catalytic reduction (SCR) catalyst, and / or combinations thereof. In one embodiment, the emission control device 70 may be a three-way catalyst, and the emission control device 72 positioned behind the emission control device 70 may be a MNF (LNT-Lean NOx Trap). In another example, the emission control device 72 may be an SCR system or other underbody catalyst. For example, emission controller 72 may be an SCR catalyst configured to reduce exhaust NOx species to nitrogen upon reaction with a reductant such as ammonia or urea. The reductant injection nozzle 74 may inject the reductant 76 into the exhaust passage 35.The catalysts may allow toxic combustion byproducts generated in the exhaust gas, such as NOx species, unburned hydrocarbons, carbon monoxide, etc., to be catalytically converted to less toxic products prior to exhaust to the atmosphere. However, the catalytic efficiency of the catalyst may be largely affected by the temperature and oxygen content of the exhaust gas. For example, reduction of NOx species may utilize higher temperatures than oxidation of carbon monoxide. Undesirable side reactions may also occur at lower temperatures, such as the production of ammonia and N 2 O species, which may impair the efficiency of exhaust treatment and lower the quality of exhaust emissions. To improve the efficiency of exhaust gas aftertreatment while protecting the exhaust system components from high exhaust temperatures, it may be desirable to increase the oxygen content of the exhaust gas downstream of the first emission control device (e.g., emission control device 70).As further detailed herein with reference to FIGS. 2-7, an engine controller may be configured to inject a secondary air stream into the exhaust aftertreatment system downstream of the emission control device 70 to increase conversion of certain emissions during different operating conditions. As shown in FIG. 1, the secondary air may come from one or more of multiple sources. For example, the secondary air may include compressed intake air directed from the intake passage between the compressor 52 and the throttle 62 to the exhaust passage. An injection line 90 directs compressed intake air directly to the exhaust passage 35 behind the emission control device 70 and in front of the emission control device 72 The compressed intake air injected into the exhaust gas via the injection line 90 may be controlled via a valve 91, which may be controlled by the engine controller.Alternatively or additionally, air may be injected into the exhaust passage via an exhaust gas recirculation (EGR) passage. An air injection line 92 directs compressed air upstream of the throttle via the EGR passage 80 to the exhaust passage 35. The EGR passage 80 includes an EGR cooler 82 configured to cool the EGR (exhaust gas recirculate) before reaching the inlet. Further, the EGR passage 80 includes an EGR valve 84 that may be adjusted by the controller 12 to control the flow of EGR to the intake. As shown in FIG. 1, the EGR passage 80 is an LP EGR (Low Pressure EGR) passage because the EGR directed through the EGR passage 80 is low pressure EGR taken behind the exhaust turbine 54. The air injection line 92 is coupled to the EGR passage 80 between the EGR valve 84 and EGR cooler 82, and is controlled via the valve 93. When the injection of compressed intake air into the exhaust passage is indicated, the valve 93 is opened and the EGR valve 84 is closed. The compressed intake air is then directed through the air injection line 92 to the EGR passage 80 where it passes through the EGR cooler 82 before reaching the exhaust passage 35 behind the emission control device 70.Further, in some embodiments, an air pump 96 may be present to inject outside air (e.g., from the atmosphere) into the exhaust passage 35 under control of the valve 95 via the injection line 94.The exhaust aftertreatment system 22 may also include hydrocarbon retaining devices, particulate matter retaining devices, and other suitable exhaust aftertreatment devices, not shown. It should be appreciated that other components may be included in the engine, such as various valves and sensors.The vehicle system 6 may further include a control system 14. The control system 14 is shown receiving information from a plurality of sensors 16 (various examples of which are described herein) and transmitting control signals to a plurality of actuators 81 (various examples of which are described herein). As one example, sensors 16 may include an exhaust gas sensor 126 (disposed in exhaust manifold 48), upstream oxygen sensors 128, 130 (oxygen sensor 128 may be located upstream of emission control device 70, while oxygen sensor 130 may be located upstream of emission control device 72 and downstream of emission control device 70), downstream oxygen sensor 132 (located downstream of emission control device 72), and temperature sensor 134. Other sensors, such as pressure, temperature, air-fuel ratio, and composition sensors, may be coupled to various locations in the vehicle system 6, as will be discussed in more detail herein. As another example, the actuators may include fuel injection nozzles (such as fuel injection nozzle 66), a plurality of valves such as valves 91, 93, 95, pump 96, and throttle 62. The control system 14 may include a controller 12. The controller may receive input data from the various sensors, process the input data, and trigger the actuators in response to the processed input data based on an instruction or code programmed therein corresponding to one or more routines. An example control routine is described herein with reference to FIGS. 2-7.FIG. 2 is a flowchart illustrating a method 200 for controlling the air-fuel ratio of the exhaust gas. Method 200 may be executed by an engine controller, such as controller 12, in response to signals from one or more sensors and in accordance with instructions stored in the memory of the controller. Method 200 may be carried out in an engine including an exhaust system having at least two emission control devices, an upstream emission control device and a downstream emission control device. As used herein, terms "upstream" or "upstream" and "downstream" or "downstream" are to be understood with respect to the direction of exhaust flow from the engine and to the atmosphere, e.g., the upstream emission control device is closer to the engine and receives engine output exhaust gas before the exhaust gas reaches the downstream emission control device. In an embodiment described with respect to FIG. 2, the upstream emission control device is a three-way catalyst (TWC) and the downstream emission control device is a lean NOx trap (MNF); however, other emission control devices in other arrangements are also within the scope of this disclosure.Method 200 includes determining engine operating parameters at 202. Engine operating parameters may include engine speed, engine load, fuel injection amount and timing, exhaust gas air-fuel ratio, throttle position, EGR valve position, exhaust gas temperature, etc. Further, determining the air-fuel ratio may include the air-fuel ratio of the exhaust gas at multiple locations, such as the engine output air-fuel ratio, the air-fuel ratio at the inlets of one or more emission control devices, and the air-fuel ratio at the outlet of one or more emission control devices.At 204, the exhaust gas air-fuel ratio is adjusted to maintain the TWC (e.g., emission control device 70 of FIG. 1 ) at or below a threshold temperature. For example, the engine may be operated at a desired air-fuel ratio set for maximum beneficial fuel economy, torque, and / or emissions. However, during some conditions, such as high load conditions, the desired air-fuel ratio may result in exhaust temperatures at the inlet of the TWC exceeding a threshold temperature. The threshold temperature may be a maximum temperature above which degradation of exhaust components may occur, such as 950° C. In other embodiments, the threshold temperature may be below the maximum temperature to avoid the TWC reaching the maximum temperature. If the temperature of the exhaust gas at the inlet of the TWC exceeds the threshold, or if the actual temperature of the device exceeds the threshold, the air-fuel ratio of the combustion (and thus the air-fuel ratio of the exhaust gas before the TWC) may be adjusted to maintain the exhaust gas temperature at a desired temperature. For example, the engine may be operated with rich combustion to decrease exhaust temperatures.At 206, it is determined whether the engine is operating at an engine output exhaust air-fuel ratio below a threshold. The threshold may be an exhaust gas air-fuel ratio below which excess HC and CO are produced, such as stoichiometry. If the engine is operated with an air-fuel ratio of the exhaust gas below stoichiometry, otherwise known as rich combustion, excess HC and / or CO may be released into the exhaust gas, which may not be converted in the TWC and MNF at the current air-fuel ratio. As such, if the answer at 206 is yes, method 200 proceeds to 208 to inject air behind the TWC to provide additional oxygen in the exhaust gas before the MNF. As a result, the air-fuel ratio of the exhaust gas downstream (e.g., exhaust gas downstream of the TWC) is maintained at a higher (e.g., leaner) air-fuel ratio than the exhaust gas entering the TWC.The additional oxygen from the injected air is available for reaction with the HC in the exhaust gas, and the excess HC and CO may be oxidized in the MNF. The reaction with oxygen in the exhaust gas may raise the temperature of the exhaust gas at the MNF. Due to the additional path that the exhaust travels to reach the MNF (relative to the path that the exhaust travels to reach the TWC), the exhaust cools before reaching the MNF, such that the exotherm released as a result of the additional oxygen injected into the exhaust does not raise the temperature of the MNF above the maximum temperature. However, the amount of air injected behind the TWC may be controlled based on the MNF temperature at 210 to avoid an increase in the temperature of the MNF above the maximum temperature. Alternatively or additionally, the amount of air injected may be controlled based on the air-fuel ratio downstream of the MNF. For example, the air-fuel ratio may be maintained at stoichiometry after MNF. After controlling air injection downstream of the TWC, method 200 ends.Example plots illustrating engine operating parameters during a rich phase with air injection before an MNF and after a TWC are shown in FIG. 8. The TWC temperature is shown at 810, the upstream air-fuel ratio (e.g., upstream of the TWC and / or the MNF) is shown at 820, an air injection amount is shown at 830, and a downstream air-fuel ratio (downstream of the MNF) is shown at 840. For each graph, time is shown on the x-axis and each respective parameter is shown on the y-axis. For plots 820 and 840 representing the air-fuel ratio, the stoichiometric air-fuel ratio is indicated by the label 1. At time t 1 the TWC catalyst temperature reaches the threshold temperature, which in this embodiment is below the maximum temperature tolerated by the TWC. To prevent the TWC from reaching the maximum temperature, the engine is operated rich, as shown by the decreasing upstream air-fuel ratio. In response, air injection is activated. Since the amount of air injection is controlled via feedback from a downstream oxygen sensor, the downstream air-fuel ratio remains substantially stoichiometric. At time t 2 the rich phase ends, air injection is deactivated, and both the downstream and upstream air-fuel ratios are at stoichiometry.Referring again to 206 of FIG. 2, if the engine is not operating with rich combustion, method 200 proceeds to 212 to determine if the engine is operating with stoichiometric combustion under high load conditions. Under these conditions, the relatively high exhaust space velocity may disrupt NOx conversion in the TWC. To ensure storage of the excess NOx in the MNF, method 200 proceeds to 214 to inject air behind the TWC if the engine is operating at high load and stoichiometric combustion. At 216, the air injection amount may be controlled based on the MNF temperature and / or the downstream air-fuel ratio. For example, air may be injected such that a temperature of the MNF is maintained below a storage threshold, such as 450° C., and / or such that the downstream air-fuel ratio is maintained leaner than stoichiometric. After controlling air injection downstream of the TWC, method 200 ends.FIG. 8 also shows air injection during high load stoichiometric combustion conditions. At time t 3 the engine load increases due to, for example, a driver tip-in event. However, contrary to time t 1 the TWC catalyst temperature (shown at 810) remains below the threshold to initiate rich combustion. However, due to the high load stoichiometric combustion conditions (e.g., the upstream air-fuel ratio shown at 820 remains at about stoichiometry), air injection is activated at time t 3( shown at 830) to create a lean environment in the MNF for storage of NOx that may escape the TWC. As a result of the air injection, the downstream air-fuel ratio (shown at 840) increases until time t 4, when the load drops and the air injection is deactivated.Referring again to FIG. 2, if it is determined at 212 that the engine is not operating under high load with stoichiometric combustion, method 200 proceeds to 218 to determine if a lean combustion end (lean exit) is expected. An end of lean combustion may be a transition from lean combustion to stoichiometric or rich combustion and may be determined based on the controller commanded air-fuel ratio by engine operating parameters such as speed, load, etc., and / or by the exhaust gas air-fuel ratio. If an end of lean combustion is expected, air may be injected downstream of the TWC at 220. After a lean burn end, the NOx stored in the MNF may be released due to the lack of oxygen in the exhaust gas. To prevent this, air is injected before the MNF so that the exhaust gas is lean, and the NOx trapped in the MNF remains in the MNF until purge is indicated. Air injection may be controlled at 222 to maintain a lean air-fuel ratio at the MNF to maintain NOx storage. After controlling air injection, method 200 ends.If it is determined at 218 that a lean combustion end is not expected, method 200 proceeds to 224 to continue feedback control of the air-fuel ratio based on the sensor input from one or more exhaust gas sensors to maintain the desired combustion air-fuel ratio and TWC temperature without injecting air behind the TWC. Because air is not injected behind the TWC, the air-fuel ratio feedback control may include feedback from one or more exhaust gas sensors behind the TWC. In contrast, during operation where air is injected downstream of the TWC, the exhaust gas oxygen concentration may be omitted from the feedback control after being determined by the downstream exhaust gas sensors or may be adjusted to account for the additional oxygen present in the exhaust gas. The method 200 then ends.Thus, method 200 of FIG. 2 provides for increasing the air-fuel ratio of the exhaust gas after a TWC and before a MNF to maintain exhaust gas cleaning while maintaining the temperature of the TWC below a maximum temperature. Maintaining the TWC below a maximum temperature may include commanding the engine to operate rich to cool the exhaust gas. In some embodiments, if the engine is operating with positive valve overlap where the exhaust valve is open for a duration during which the intake valve is open, valve timing may be adjusted to avoid valve overlap during rich operation. Positive valve overlap may result in sufficient oxygen concentrations in the exhaust manifold to produce reactions with the rich exhaust gas, thereby heating the exhaust gas, and thus may be discontinued during rich operation.FIG. 3 is a flow chart illustrating a method 300 of injecting air behind an upstream emission control device, such as emission control device 70 of FIG. 1. Method 300 may be performed by controller 12 if air injection is indicated during execution of method 200 discussed with respect to FIG. 2.The method 300 includes determining whether an air injection is indicated at 302. Air injection may be indicated to provide a leaner air-fuel ratio downstream of the upstream emission control device than is provided by the engine. Air injection may be indicated based on the conditions discussed with respect to FIG. 2, for example at 208, 214, or 220, or based on another suitable parameter. For example, as described with respect to FIG. 1 and as embodied herein, air may be provided to the exhaust passage via an EGR passage including an EGR cooler. If the cooler effectiveness is low, air injection may be indicated to remove soot or other debris that may have accumulated on the cooler.If an injection is not indicated, method 300 returns to maintain monitoring for an air injection indication. If air injection is indicated, method 300 proceeds to 304 to direct compressed intake air to the exhaust passage downstream of the upstream emission control device. Directing compressed intake air may include opening a valve in an injection line coupled to the exhaust passage, or opening a valve in an injection line coupled to an EGR passage, at 306. As explained with reference to FIG. 1, the air injected into the exhaust passage may be taken from the intake passage behind the compressor and before the throttle. The intake air downstream of the compressor is compressed to a pressure above atmospheric pressure, for example. Opening the valve in the injection line allows air to enter the exhaust passage without the provision of a pump or other means for pressurizing the air. The air from the injection line may be directed to the exhaust passage or may be directed via an LP-EGR passage. However, in some embodiments, the air may be injected into the exhaust passage using an air pump. Such pumped air may not be drawn from the inlet, but may instead be, for example, atmospheric air.Directing compressed intake air to the exhaust passage may also include closing an EGR valve at 308 (if the injection line is coupled to an EGR passage). By closing the EGR valve, EGR flow from the outlet to the inlet is prevented, and the compressed air may instead travel through the EGR passage to the exhaust passage.At 310, one or more operating parameters may be adjusted to compensate for the reduced intake air pressure downstream of the compressor and / or to compensate for lost EGR flow. For example, the diversion of compressed intake air may reduce the pressure of the intake air before the throttle; as a result, the throttle position may be adjusted to provide a desired mass airflow to the engine. Further, the turbocharger may be controlled to increase boost pressure provided to the intake. A turbine wastegate may be adjusted to increase boost pressure, or a compressor bypass valve may be adjusted.If compressed intake air is directed through an EGR passage before reaching the exhaust passage, if EGR flow is desired to decrease combustion temperature or reduce engine pumping losses, one or more parameters may be adjusted to compensate for the loss of EGR flow. For example, if the engine includes an HP-EGR system, flow through the HP-EGR system may be increased. In another example, fuel injection timing may be adjusted to decrease the combustion temperature, or fuel injection may be split, which also acts to decrease the combustion temperature. After adjusting the engine operating parameters, method 300 ends.The methods 200 and 300 described above provide for injecting air into an exhaust passage in response to one or more conditions, such as rich combustion, to maintain the exhaust at a leaner air-fuel ratio than the exhaust before the TWC after a TWC. Method 200 controls injection of air based on feedback from an oxygen sensor and / or the temperature of a MNF behind the TWC. However, in engine systems where the MNF is replaced with a reduction catalyst, such as an SCR system, air may be injected upstream of the SCR system and downstream of the TWC to ensure efficient NOx conversion during engine operation at exhaust gas oxygen concentrations that are less than desired.Referring to FIG. 4, a method 400 for controlling air injection before a catalyst is illustrated. The method 400 may be performed by the controller 12. In some embodiments, method 400 may be performed in a gasoline engine configured to operate with lean burn combustion. As discussed above, lean combustion, which may include air-fuel ratios of about 20:1 or greater, produces higher amounts of NOx than stoichiometric combustion. To ensure conversion of the increased NOx species, gasoline lean engines may include a reduction catalyst, such as an SCR system, in the exhaust system. However, unlike these engines operating at substantially higher air-fuel ratios, gasoline lean burn engines may not generate exhaust gas with adequate oxygen concentration to ensure efficient NOx conversion. Thus, method 400 provides a mechanism for increasing the oxygen concentration prior to a reduction catalyst in a gasoline engine.At 402, engine operating parameters are determined. The engine operating parameters may include engine speed, load, air-fuel ratio, exhaust oxygen concentration, engine temperature, and SCR temperature. Other operating parameters that may be determined include mass airflow in the exhaust gas, exhaust NOx concentrations, injected reductant concentrations upstream of the SCR catalyst, and other parameters. At 404, it is determined whether the engine is operating in lean operation. Lean operation may include lean air-fuel ratios above stoichiometry and may include air-fuel ratios between 20:1 and 28:1, or other air-fuel ratios. The lean operation may be determined based on a measured exhaust air-fuel ratio, commanded fuel injection amounts, and / or operating parameters such as load. For example, the lean burn engine may be operated during low to medium engine loads (e.g., loads of 50% or less) and then operate at higher loads with stoichiometric combustion.If the engine is not operating with lean combustion, it is likely to operate with substantially stoichiometric combustion and method 400 proceeds to 406 to initiate a stoichiometric combustion routine, which is discussed in detail below with reference to FIG. 5. If it is determined that the engine is operating with lean combustion, method 400 proceeds to 408 to determine if an ammonia storage amount (NH 3) in the SCR catalyst is above a threshold. The SCR catalyst may store NH 3 to convert the NOx species entering the SCR catalyst. The stored ammonia may be oxidized by oxygen in the exhaust gas; thus, to maintain a baseline stored ammonia concentration sufficient for NOx conversion, the amount of stored ammonia must exceed a lower limit to compensate for the increased ammonia oxidation in response to the injected air. The stored NH 3- amount may be determined by a storage estimate that is a function of upstream NOx concentration, NH 3- consumption (e.g., determined by comparing upstream NOx concentrations to downstream NOx concentrations), SCR temperature, and NH 3- generation (e.g., determined by SCR temperature, oxygen consumption, and injected NH 3- concentrations). The lower limit for NH 3- storage may be a function of SCR temperature.If it is determined that the NH 3- storage amount does not exceed the lower limit, method 400 proceeds to 410 to initiate an ammonia generation routine discussed with reference to FIG. 6. If the NH 3- storage amount exceeds the threshold, method 400 proceeds to 412 to determine if the measured exhaust oxygen concentration is less than desired. The exhaust oxygen concentration may be determined by a sensor prior to the SCR catalyst, such as sensor 130 of FIG. 1. The desired oxygen concentration may be determined based on the SCR temperature, the exhaust space, and the exhaust NOx mass. Thus, the desired oxygen concentration is the amount of oxygen in the exhaust gas that results in almost complete conversion of NOx in the SCR catalyst. In one example, the desired oxygen concentration may be in the range of 8-10% and / or may be in a range of an air-fuel ratio of 25:1 to an air-fuel ratio of 30:1.If the exhaust oxygen concentration is not below the desired concentration, there is sufficient oxygen to convert NOxin the SCR, and method 400 returns to continue monitoring for conditions indicating that air should be injected. If the oxygen concentration is less than desired, method 400 proceeds to 414 to direct secondary air to the exhaust passage upstream of the SCR catalyst. In one example, the air may be directed behind an upstream emission control device, which may be, for example, a two-way or three-way catalyst. However, in other examples, the air may be directed upstream of the two-way or three-way catalyst. As explained with reference to FIGS. 1 and 3, the secondary air may be intake air directed to the exhaust passage from behind a turbocharger compressor or may be compressed intake air directed via an EGR passage. In other embodiments, the secondary air may be injected via an air pump. Directing the secondary air to the exhaust passage includes maintaining the exhaust oxygen concentration within a threshold range at 414. The secondary air may be introduced into the exhaust gas at an amount that maintains the exhaust gas below an upper limit (e.g., 10%) above which ammonia oxidation may reach undesirable levels, but at or above the desired amount of oxygen. The maintaining of the oxygen within the threshold range may be achieved by feedback control from the oxygen sensor in the exhaust passage.At 418, it is determined whether the SCR temperature is higher than desired. The SCR may achieve optimal NOx conversion within a temperature window, e.g., 200-450° C. If the temperature of the SCR is outside this window, the amount of secondary air directed to the outlet may be increased or decreased. For example, if the SCR temperature is greater than desired, method 400 may proceed to 420 to increase the secondary air to the outlet to cool the SCR catalyst. However, to ensure that ammonia oxidation does not occur, the amount of secondary air directed to the outlet is limited at 422 to maintain the exhaust oxygen concentration below a threshold. If the SCR temperature is not higher than desired, if possible, the amount of air introduced may be reduced or method 400 may return to 414 to continue introducing air based on feedback from the oxygen sensor. After introducing the secondary air and adjusting the amount of air based on feedback from an oxygen sensor and an SCR temperature sensor, method 400 returns.Thus, method 400 provides for injecting secondary air into an exhaust passage upstream of an SCR device. The air may be introduced when the exhaust gas oxygen is at a lower concentration than desired for optimal NOx conversion in the SCR catalyst, such as during operation with engine air-fuel ratios above 20:1, yet lower than typical diesel air-fuel ratios. By providing an SCR system in the outlet, introducing secondary air when indicated, and operating at air-fuel ratios of about 20:1 or 25:1, efficient conversion of NOx may occur in the SCR system, thereby reducing the need for a three-way catalyst prior to the SCR system. Thus, in some embodiments, the three-way catalyst may be replaced with a two-way catalyst.Example plots illustrating engine operating parameters during lean combustion with air injection before an SCR and after a TWC are shown in FIG. 9. Engine load is shown at 910, upstream air-fuel ratio (e.g., engine-off air-fuel ratio) is shown at 920, an air injection amount is shown at 930, and SCR temperature is shown at 940. For each graph, time is plotted on the x-axis and each respective parameter is plotted on the y-axis. For plot 920, a stoichiometric air-fuel ratio is indicated by mark 1. Prior to time t 1 the engine is operating with lean combustion because the load is relatively low. At time t 1 the upstream air-fuel ratio decreases and thus less oxygen is available in the outlet for converting NOx in the SCR. Thus, the air injection is activated to introduce secondary air into the exhaust passage upstream of the SCR catalyst. Air injection begins to cool the SCR and the amount of air injection may be increased to cool the SCR catalyst to a desired temperature. At time t 2 the air-fuel ratio increases and thus the air injection is disabled.Method 400 provides for introducing secondary air during lean combustion. However, during certain stoichiometric combustion operating events, such as acceleration events, high exhaust gas flows may allow some NOx to escape the TWC. To provide optimal conversion of NOxduring these events, method 500 of FIG. 5 may be performed as discussed below. Both methods 400 and 500 are based on having sufficient NH 3- storage levels in the SCR catalyst to avoid oxidizing all of the available NH 3 resulting in NOx leakage from the SCR catalyst. If it is determined that the NH 3- storage levels are too low, method 600 of FIG. 6 may be performed to generate NH 3 while providing for gas phase NOx conversion. Additionally, if it is determined that the temperature of the SCR catalyst is too high to store NH 3 method 700 of FIG. 7 may be executed to cool the SCR catalyst using introduction of secondary air.Referring now to FIG. 5, a method 500 for controlling the introduction of secondary air during stoichiometric combustion is illustrated. Method 500 may be executed by controller 12 in response to an indication that the engine is operating at stoichiometric combustion. At 502, method 500 includes determining if NH 3- storage levels are above a threshold, similar 408 to method 400 described above. If the elevations are not above the threshold, method 500 proceeds to 504 to initiate an ammonia generation routine, as described below with reference to FIG. 6. If the storage levels are above the threshold, method 500 proceeds to 506 to determine if the engine temperature exceeds a first temperature threshold T 1. The first temperature threshold may be the standard operating temperature or a light-off temperature of one or more catalysts in the exhaust system. If the engine is not above threshold T 1, method 500 returns until such time as the engine has reached the light-off temperature. Since the introduction of secondary air may cool the SCR catalyst, it may not be advantageous to direct the secondary air to the exhaust system while the engine temperature is low. While engine temperature is evaluated at 506, it is understood that other vehicle temperatures may be evaluated to determine whether air needs to be introduced, such as SCR catalyst temperature.If the engine temperature is above the threshold T 1, the method 500 proceeds to 508 to determine if the engine temperature is below a second temperature threshold T 2. The second temperature threshold may be a temperature above which the SCR catalyst cannot store ammonia and thus injection of secondary air may not improve NOx conversion efficiency. If the engine is above the second temperature threshold, method 500 proceeds to 510 to initiate a catalyst cooling routine discussed below with reference to FIG. 7. If the engine temperature is below the threshold, method 500 proceeds to 512 to determine if the measured exhaust oxygen concentration is less than desired. If not, method 500 returns. If so, method 500 proceeds to 514 to direct secondary air to the exhaust passage, adjusting the amount of introduced air to maintain the oxygen within a threshold range at 516 (e.g., based on feedback from the oxygen sensor), and adjusting based on the catalyst temperature at 518. Method 500 then returns.FIG. 9 also indicates air injection during stoichiometric combustion. For example, at time t 3 the engine is operating at a stoichiometric air-fuel ratio (shown in plot 920) due to the relatively high engine load (shown in plot 910). Air injection may be activated at time t 3( shown in plot 930) if the oxygen concentration is less than desired or if the engine experiences an acceleration event to convert any NOx that may leak past the upstream catalyst.FIG. 6 shows a method 600 for generating ammonia. Method 600 may be executed by controller 12 in response to an indication that ammonia storage levels in the SCR catalyst are below a threshold, such as at 410 and 504 of methods 400 and 500, respectively. The method 600 includes, at 602, determining whether the SCR catalyst temperature is below the second temperature threshold T 2, similar to the temperature threshold T 2 described above with reference to FIG. 5. The SCR catalyst temperature may be determined by a temperature sensor positioned in or near the catalyst, or may be estimated based on the engine temperature. If the catalyst temperature is not below the threshold T 2, method 600 proceeds to 604 to initiate a catalyst cooling routine discussed below. If the catalyst temperature is below the threshold, method 600 proceeds to 606 to determine if the measured oxygen concentration is below a desired concentration. The desired oxygen concentration before the ammonia generation mode is permitted may be different than the desired oxygen concentration for NOx conversion during lean or stoichiometric combustion. For example, the highest oxygen concentration during ammonia generation may be less than during lean or stoichiometric combustion. The NOxconversion in the SCR when the NH 3- storage is low occurs with gas phase NH 3 rather than with stored NH 3 and thus lower oxygen levels may be present to avoid oxidation of the gas phase NH 3. However, depending on the composition and temperature of the exhaust gas, NOx conversion using fuel as a reductant rather than ammonia may be more efficient; thus, the amount of oxygen for NOx conversion and NH 3- generation may vary based on whether the SCR acts as an HC-SCR or NH 3- SCR.If the measured oxygen concentration is not less than desired, method 600 returns. If less than desired, method 600 proceeds to 608 to direct secondary air to the exhaust passage. The amount of air directed may be balanced to provide the desired oxygen for NOx conversion and ammonia generation at 610, while maintaining the SCR catalyst at a target temperature for NH 3- storage at 612. Additionally, at 614, the engine may be operated during the rich combustion ammonia generation mode. Method 600 then returns.FIG. 7 is a flow chart illustrating a method 700 for cooling a catalyst using introduction of secondary air. Method 700 may be executed by controller 12 in response to an indication that the SCR catalyst is above an NH 3- storage temperature, such as at 510 and 604 of methods 500 and 600, respectively. Method 700 includes determining whether NOx conversion is possible with either fuel or NH 3 as the reductant (e.g., HC-SCR or NH 3- SCR) at 702. This may be based on conversion estimates determined for both HC and NH 3 which in turn may be based on SCR temperature, engine speed, and load. If NOx conversion is possible, method 700 proceeds to 704 to direct secondary air to the exhaust system for gas phase NOx conversion. The amount of air introduced may be adjusted to maintain the catalyst at a desired temperature at 706 to maintain the catalyst at a temperature range suitable for NH 3- generation / storage.At 708, method 700 includes determining if the exhaust oxygen concentration is above a storage maximum. If the oxygen concentration in the exhaust gas is not above a storage minimum, air may be introduced into the exhaust passage to promote NH 3- storage, and thus method 700 proceeds to 710 to initiate the ammonia generation routine of FIG. 6. If the exhaust oxygen concentration is above the storage minimum, method 700 returns to continue directing secondary air to cool the catalyst.Returning to 702, method 700 proceeds to 712 if NOx conversion is not possible due to the SCR temperature and exhaust gas composition at 702, to determine if the SCR temperature is above the gas phase conversion temperature limit. If not, method 700 returns. If so, method 700 proceeds to 714 to direct secondary air to the exhaust system to remove particulate accumulation from the SCR catalyst. Method 700 proceeds to 708 to determine if the oxygen concentration is above the storage maximum, as discussed above.Thus, the methods of FIGS. 4-7 may provide for injecting secondary air upstream of an SCR catalyst to provide desired oxygen concentrations for various functions such as NOx conversion, NH 3- storage, and / or aiding catalyst regeneration. According to the invention, an engine method comprises operating the engine with lean combustion, and when the exhaust oxygen concentration is below a threshold, injecting air into an exhaust passage between a first exhaust gas purifier and a second exhaust gas purifier, whereinan amount of the injected air is further controlled to maintain a temperature of the SCR device within a threshold range; and / orinjecting air into the exhaust passage further comprises directing boosted intake air directly to the exhaust passage. The first exhaust gas purification device may be a two-way or three-way catalyst, and the second exhaust gas purification device is an SCR system according to the invention. Operating with lean combustion may further include operating with air-fuel ratios between 20:1 and 30:1 or other lean air-fuel ratios.In some embodiments, the engine may be operated with lean combustion during a first condition and with stoichiometric combustion during a second condition. In each of the first and second conditions, the amount of air injected may be adjusted based on the exhaust oxygen concentration upstream of the SCR system and the temperature of the SCR system. The first condition may include low to medium engine load and the second condition may include medium to high engine load. Further, in some embodiments, adjusting the amount of injected air may include the air injection system being blocked from injecting air if the exhaust oxygen concentration exceeds a first threshold or if the temperature of the SCR system exceeds a second threshold.In another embodiment, an engine method includes, when a reductant storage level of a reduction catalyst is above a threshold, adjusting an amount of secondary air injected into an exhaust passage before the reduction catalyst to maintain the exhaust oxygen concentration in a first range, and when the reductant storage level is below the threshold, adjusting the amount of secondary air injected into the exhaust passage to maintain the exhaust oxygen concentration in a second, lower range. The second, lower range of exhaust oxygen concentration may be based on a temperature of the reduction catalyst and the reductant storage level.It should be understood that the configurations and methods disclosed herein are merely exemplary in nature and that these specific embodiments are not to be considered in a limiting sense because numerous variations are possible.For example, the above technology can be applied to V-6, I-4, I-6, V-12, Boxer-4 clock and other engine types. The subject matter of the present disclosure includes all novel and non-open combinations and sub-combinations of the various systems and configurations, and other features, functions, and / or characteristics disclosed herein.The following claims particularly emphasize certain combinations and sub-combinations that are considered novel and non-obvious. These claims may refer to "a" element or "a first" element or equivalent thereof. Such claims are to be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and / or characteristics may be claimed by altering the present claims or by providing novel claims in this or a related application. Such claims, whether more general, narrower, equal or different in scope to the original claims, are also considered to be included within the subject matter of the present disclosure.TABLE OF REFERENCE CHARACTERS6 Vehicle system 8 engine system 14 control system 18 fuel system 22 exhaust aftertreatment system 23 engine inlet 25 engine outlet 30 cylinder 35 exhaust passage 42 inlet passage 44 inlet manifold 48 exhaust manifold 50 turbocharger 52 compressor 54 exhaust turbine 56 shaft 58 electric motor 62 throttle 66 fuel injection nozzle exhaust emission control device, 70, 72 catalyst 72 SCR catalyst 74 reductant injection nozzle 80 EGR passage 82 EGR cooler 84 EGR valve 90, 92 injection line 91, 93, 95 valve 96 air pump 126, 128, 130, 132, 134 sensor

Claims

An engine method comprising: operating the engine (8) with lean combustion; and if the exhaust gas oxygen concentration is below a threshold, injecting air into an exhaust passage (35) between a first exhaust gas purifier (70) and an SCR device (72), wherein - an amount of the injected air is further controlled to maintain a temperature of the SCR device (72) within a threshold range, and / or - injecting air into the exhaust passage (35) further comprises directing boosted intake air directly to the exhaust passage (35).The engine method according to claim 1, wherein the first exhaust gas purifying device (70) is a three-way catalyst.The engine method of claim 1, wherein operating the lean burn engine (8) comprises operating the engine (8) at an air-fuel ratio between 20:1 and 30:1, and wherein the threshold is an oxygen concentration in the exhaust gas resulting from combustion at an air-fuel ratio of 25:1.The engine method according to claim 3, wherein the engine (8) is a gasoline engine.A system for a gasoline lean burn engine, comprising: an exhaust aftertreatment system (22) having an upstream catalyst (70) and a downstream SCR system (72); an air injection system coupled to an exhaust passage (35) between the catalyst (70) and the SCR system (72); and a controller (14) having instructions to: during a first condition, operate the engine (8) with lean combustion and adjust an injected air amount based on an exhaust oxygen concentration upstream of the SCR system (72) and a temperature of the SCR system (72).The engine system of claim 5, wherein the first condition comprises a low to moderate engine load.The engine system of claim 5, wherein the controller (14) includes further instructions to inject a first amount of air to maintain the exhaust oxygen concentration within a first range.The engine system of claim 7, wherein the controller (14) includes further instructions to adjust the first amount of air to maintain the temperature of the SCR system (72) within a second range.The engine system of claim 5, wherein the controller (14) includes further instructions to, during a second condition, operate the engine (8) at stoichiometric combustion and adjust the amount of air injected based on the exhaust oxygen concentration upstream of the SCR system (72) and the temperature of the SCR system (72).The engine system of claim 5, wherein the second condition comprises moderate to high engine load and an engine temperature above a threshold.The engine system of claim 5, wherein the air injection system couples the exhaust passage (35) directly to a portion of an intake passage (42) behind a compressor (52) of the turbocharger (50) and in front of a throttle (62), and wherein the controller (14) includes instructions for adjusting a valve (91, 93, 95) of the air injection system to provide compressed intake air as the injected air.The engine system of claim 5, wherein the controller (14) includes instructions to, if the exhaust oxygen concentration exceeds a first threshold or if the temperature of the SCR system (72) exceeds a second threshold, prevent the air injection system from injecting air.The engine system of claim 5, wherein the catalyst (70) is a two-way catalyst.An engine method comprising: when a reductant storage amount of a reduction catalyst (72) is above a threshold, adjusting an amount of secondary air injected into an exhaust passage (35) before the reduction catalyst (72) to maintain the exhaust oxygen concentration in a first range; and when the reductant storage amount is below the threshold, adjusting the amount of secondary air injected into the exhaust passage (35) to maintain the exhaust oxygen concentration in a second, lower range.The engine method of claim 14, further comprising operating the engine (8) with rich combustion when the reductant storage level is below the threshold.The engine method according to claim 14, further comprising: when the reductant storage level is above the threshold and the engine (8) is operating under low to medium load, operating the engine (8) with lean combustion; and when the reductant storage level is above the threshold and the engine (8) is operating under medium to high load, operating the engine (8) with stoichiometric combustion.The engine method of claim 14, wherein the second, lower range of exhaust oxygen concentration is based on a temperature of the reduction catalyst (72) and the reductant storage level.The engine method of claim 14, wherein the reduction catalyst (72) is a selective catalytic reduction catalyst.

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