Engine process and engine system

Abstract

Engine process, which includes the following: Adjusting the air-fuel ratio of the exhaust gas upstream of a first exhaust gas purification device (70) in order to keep the first exhaust gas purification device (70) at or below a threshold temperature; and If the upstream air-fuel ratio of the exhaust gas is below a threshold, air is injected into an exhaust gas passage (35) between the first exhaust gas purification device (70) and a second exhaust gas purification device (72) to maintain the exhaust gas downstream of the first exhaust gas purification device (70) at a different, higher air-fuel ratio, wherein if a reducing agent storage level of a reduction catalyst (72) is above a threshold, the amount of air injected into an exhaust gas passage (35) upstream of the reduction catalyst (72) is adjusted to maintain the exhaust gas oxygen concentration in a first range, and if the reducing agent storage level is below the threshold, the amount of air injected into the exhaust gas passage (35) is adjusted to maintain the exhaust gas oxygen concentration in a second, lower range.

Description

[0001] The present disclosure relates to an exhaust system of an engine. BACKGROUND OF THE INVENTION AND SUMMARY OF THE INVENTION

[0002] Turbocharging an engine allows the engine to deliver power similar to that of a larger-displacement engine while keeping the engine pumping work close to that of a non-turbocharged engine of similar displacement. Thus, turbocharging can expand an engine's operating envelope. However, in greatly downsized engines, the close coupling of the turbocharger and downstream exhaust components such as catalytic converters can cause difficulties during high-load engine operation. For example, the high exhaust temperatures of high-load operation can damage the turbocharger and / or catalytic converter. To avoid deterioration of the exhaust components, the engine can be operated with rich combustion under high-load conditions to lower exhaust temperatures.

[0003] EP 1 184 556 A2 discloses an exhaust gas purifier for an internal combustion engine. DE 198 16 276 A1 discloses a method for operating an internal combustion engine. DE 694 28 161 T2 discloses an engine exhaust gas purification device. US 2009 / 0 260 349 A1 discloses an air injection system.

[0004] However, the present inventors have identified potential problems with the above approach. For example, rich operation can generate significant amounts of CO and HC emissions. Furthermore, the reduced exhaust gas temperatures can lead to these emissions not being converted in the downstream catalysts.

[0005] Thus, in one example, some of the above issues may be at least partially addressed by an engine method comprising: adjusting the upstream exhaust air-fuel ratio to maintain a first emission control device at or below a threshold temperature, and when the upstream exhaust air-fuel ratio is below a threshold, injecting air into an exhaust passage between the first emission control device and a second emission control device to maintain the downstream exhaust air at a different, higher air-fuel ratio.

[0006] In this way, the temperature and air-fuel ratio of the downstream emission control device, which is located farther from the exhaust manifold and thus subject to lower exhaust temperatures than the upstream emission control device, can be adjusted independently of the upstream emission control device. This allows emissions that escape unconverted from the upstream emission control device to be converted in the downstream emission control device without exposing the upstream emission control device to high temperatures, which can contribute to component degradation.

[0007] In one example, the temperature and air-fuel ratio of the downstream emission control device may be adjusted via the introduction of secondary air into an exhaust passage between the upstream emission control device and the downstream emission control device. The secondary air may include compressed intake air directed from the inlet passage to the exhaust passage. Furthermore, in some embodiments, the compressed intake air may be directed to the exhaust passage via a low-pressure exhaust gas recirculation (LP EGR) passage coupled to the exhaust passage. The secondary air may be introduced based on feedback control to maintain a downstream air-fuel ratio at stoichiometry and / or maintain a temperature of the downstream emission control device within a threshold.

[0008] The above advantages and other advantages and features of the present description will be readily apparent from the following detailed description when taken alone or in conjunction with the accompanying drawings.

[0009] It should be understood that the above brief summary is presented to introduce, in simplified form, a selection of concepts further described in the detailed description. It is not intended to identify important or essential features of the claimed subject matter, the scope of which is defined exclusively by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that resolve any disadvantages mentioned above or in any part of this disclosure. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a schematic representation of a vehicle system with 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 shows a flowchart illustrating a method for injecting air into an exhaust passage according to an embodiment of the present disclosure. Fig. 4 shows a flowchart illustrating a method for controlling the introduction of air during lean combustion according to an embodiment of the present disclosure. Fig. 5 shows a flow diagram illustrating a method for controlling the introduction of air during stoichiometric combustion according to an embodiment of the present disclosure. Fig. 6 shows a flow diagram illustrating a method for controlling the introduction of air during the production of ammonia according to an embodiment of the present disclosure. Fig. 7 shows a flowchart illustrating a method for controlling the introduction of air during catalyst cooldown according to an embodiment of the present disclosure. Fig. 8 and Fig. 9 show example graphs of engine operating parameters during air injection according to embodiments of the present disclosure. DETAILED DESCRIPTION

[0010] An air injection system may be provided in an exhaust system to adjust the air-fuel ratio downstream of a first emission control device, such as a three-way catalyst. The three-way catalyst may be susceptible to deterioration due to high exhaust temperatures. To reduce exhaust temperatures, the engine may be operated rich, and air may be injected into the exhaust downstream of the three-way catalyst to provide additional oxygen to a downstream emission control device, such as a lean NOx trap. In other circumstances, such as when conditions disrupt effective NOx conversion in the three-way catalyst, air may be injected to ensure a lean air-fuel ratio at the lean NOx trap so that the NOx can be stored in the lean NOx trap.

[0011] The air can be compressed intake air, which is directed to the exhaust passage directly or via an EGR passage. The compressed intake air can be directed to the EGR passage upstream of an EGR cooler when directed via the EGR passage to provide additional EGR cooler regeneration support. Thus, the injected air can improve emissions by delivering a lean exhaust gas to a downstream emission control device while enhancing EGR cooler performance. Fig. 1 shows an engine with an air injection system, upstream and downstream emission control devices and a controller configured to carry out the methods of Fig. 2-7.

[0012] Fig. 1 shows a schematic representation 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 a plurality of cylinders 30. The engine 10 includes an engine intake 23 and an engine exhaust 25. The engine intake 23 includes a throttle 62 fluidly coupled to the engine intake manifold 44 via an intake passage 42. The 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 downstream of a boosting device 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.The compressor 52 may be at least partially driven by an exhaust turbine 54 disposed between the exhaust manifold 48 and the exhaust passage 35. The compressor 52 may be coupled to the exhaust turbine 54 via a shaft 56. The compressor 52 may also be at least partially driven by an electric motor 58. In the illustrated example, the electric motor 58 is shown coupled to the shaft 56. However, other suitable electric motor configurations are also possible. In one example, the electric motor 58 may be operated using stored electrical energy from a system battery (not shown) when the battery state of charge is above a charging threshold. By using the electric motor 58 to operate the turbocharger 50, for example, during engine start-up, an electric boost (E-boost) may be provided to the intake charge air.In this way, the electric motor can provide engine assistance to operate the boosting device. As such, after the engine has been running for a sufficient period of time (e.g., a threshold time), the exhaust gas generated in the exhaust manifold can begin driving the exhaust turbine 54. Consequently, the engine assistance of the electric motor can be reduced. That is, during turbocharger operation, the engine assistance provided by the electric motor 58 can be adjusted in response to the operation of the exhaust turbine.

[0013] The 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 delivered to the injectors of the engine 10, such as the exemplary injector 66 shown. While only a single injector 66 is shown, additional injectors are provided for each cylinder. It is understood that the fuel system 18 may be a return-type or returnless fuel system, or various other types of fuel systems.

[0014] Although in Fig. 1, each cylinder 30 may include one or more intake valves and one or more exhaust valves for controlling the intake of charge air and the release of exhaust gas, respectively. The timing of the opening and closing of the intake and exhaust valves may be fixed, or the opening and / or closing times of the intake and exhaust valves may vary to provide positive or negative valve overlap, late intake valve closing, or various other examples.

[0015] The engine exhaust 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 converter or particulate filter, an SCR (Selective Catalytic Reduction) 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 downstream of the emission control device 70 may be an MNF (LNT - Lean NOx Trap). In another example, the emission control device 72 may be an SCR system or other underbody catalyst.For example, emissions control device 72 may be an SCR catalyst configured to reduce exhaust NOx species to nitrogen upon reaction with a reductant such as ammonia or urea. Reductant injection nozzle 74 may inject reductant 76 into exhaust passage 35.

[0016] Catalysts can enable toxic combustion byproducts generated in the exhaust gas, such as NOx species, unburned hydrocarbons, carbon monoxide, etc., to be catalytically converted into less toxic products before being discharged to the atmosphere. However, the catalytic efficiency of the catalyst can be largely influenced by the temperature and oxygen content of the exhaust gas. For example, the reduction of NOx species may utilize higher temperatures than the oxidation of carbon monoxide. Undesirable side reactions can also occur at lower temperatures, such as the generation of ammonia and N2O species, which can impair the efficiency of exhaust treatment and reduce the quality of exhaust emissions.In order to improve the efficiency of exhaust 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 exhaust gas purification device (e.g., the exhaust gas purification device 70).

[0017] As referred to herein in Fig. 2-7, an engine controller may be configured to inject a secondary airflow into the exhaust aftertreatment system downstream of the emission control device 70 to increase the conversion of certain emissions during different operating conditions. As shown in Fig. 1, the secondary air may come from one or more of several 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 downstream of the emission control device 70 and upstream of the emission control device 72. The compressed intake air injected into the exhaust via the injection line 90 may be controlled via a valve 91, which may be controlled by the engine controller.

[0018] 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 is configured to redirect a portion of the exhaust gas back to the intake to reduce combustion temperatures and thus NOx emissions. The EGR passage 80 includes an EGR cooler 82 configured to cool the EGR (exhaust gas recirculation) before it reaches the intake. Furthermore, the EGR passage 80 includes an EGR valve 84 that can 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 passage (Low Pressure EGR passage) because the EGR directed through the EGR passage 80 is low-pressure EGR taken downstream of the turbine 54. The air injection line 92 is coupled to the EGR passage 80 between the EGR valve 84 and the EGR cooler 82 and is controlled by 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 downstream of the emission control device 70.

[0019] Furthermore, in some embodiments, an air pump 96 may be provided to inject outside air (e.g., from the atmosphere) into the exhaust passage 35 via the injection line 94, controlled by the valve 95.

[0020] The exhaust aftertreatment system 22 may also include hydrocarbon retention devices, particulate matter retention devices, and other suitable exhaust aftertreatment devices not shown. It is understood that other components may be included in the engine, such as various valves and sensors.

[0021] 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 sending control signals to a plurality of actuators 81 (various examples of which are described herein). As one example, the sensors 16 may include an exhaust gas sensor 126 (located in the exhaust manifold 48), upstream oxygen sensors 128, 130 (the oxygen sensor 128 may be located before the emission control device 70, while the oxygen sensor 130 may be located before the emission control device 72 and after the emission control device 70), a downstream oxygen sensor 132 (located after the emission control device 72), and a 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 discussed in more detail herein. As another example, the actuators may include fuel injectors (such as fuel injector 66), a variety 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, according to one or more routines. An example control routine is described herein with reference to FIG. Fig. 2-7 described.

[0022] Fig. 2 is a flowchart illustrating a method 200 for controlling the air-fuel ratio of the exhaust gas. The method 200 may be performed by an engine controller, such as the controller 12, in response to signals from one or more sensors and according to instructions stored in the memory of the controller. The method 200 may be performed in an engine including an exhaust system with at least two emission control devices, an upstream emission control device and a downstream emission control device. As used herein, the terms "upstream" or "before" and "downstream" or "after" 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 before the exhaust reaches the downstream emission control device. In a Fig. In the embodiment described in Figure 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.

[0023] Method 200 includes determining engine operating parameters at 202. The engine operating parameters may include engine speed, engine load, fuel injection amount and timing, exhaust air-fuel ratio, throttle position, EGR valve position, exhaust temperature, etc. Furthermore, determining the air-fuel ratio may include the exhaust air-fuel ratio at multiple locations, such as the engine exhaust 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.

[0024] At 204, the air-fuel ratio of the exhaust gas is adjusted to reduce the TWC (e.g., exhaust gas purification device 70 from Fig. 1) at or below a threshold temperature. For example, the engine may be operating 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 cause exhaust temperatures at the inlet of the TWC to exceed 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 prevent the TWC from reaching the maximum temperature.If the exhaust gas temperature at the TWC inlet exceeds the threshold, or if the actual temperature of the device exceeds the threshold, the combustion air-fuel ratio (and thus the exhaust gas air-fuel ratio before the TWC) can be adjusted to maintain the exhaust gas temperature at a target temperature. For example, the engine can be operated with rich combustion to reduce exhaust gas temperatures.

[0025] At 206, it is determined whether the engine is operating with an engine output exhaust air-fuel ratio below a threshold. The threshold may be an exhaust air-fuel ratio below which excess HC and CO are produced, such as stoichiometry. If the engine is operating with an exhaust air-fuel ratio below stoichiometry, otherwise known as rich combustion, excess HC and / or CO may be released into the exhaust, which at the current air-fuel ratio may not be converted in the TWC and MNF. As such, if the answer at 206 is yes, method 200 proceeds to 208 to inject air downstream of the TWC to provide additional oxygen in the exhaust upstream of the MNF. This maintains the downstream exhaust air-fuel ratio (e.g., exhaust downstream of the TWC) at a higher (e.g., leaner) air-fuel ratio than the exhaust entering the TWC.

[0026] The additional oxygen from the injected air is available to react with the HC in the exhaust, and the excess HC and CO can be oxidized in the MNF. The reaction with oxygen in the exhaust can raise the temperature of the exhaust at the MNF. Due to the additional distance the exhaust travels to reach the MNF (relative to the distance the exhaust travels to reach the TWC), the exhaust cools before reaching the MNF, so the exotherms released as a result of the additional oxygen injected into the exhaust do not raise the temperature of the MNF above the peak temperature. However, the amount of air injected downstream of the TWC can be controlled at 210 based on the MNF temperature to avoid increasing the temperature of the MNF above the peak temperature. Alternatively, or additionally, the amount of air injected can be controlled based on the air-fuel ratio downstream of the MNF.For example, the air-fuel ratio downstream of the MNF can be maintained at stoichiometry. After controlling air injection downstream of the TWC, method 200 ends.

[0027] Example diagrams showing engine operating parameters during a rich phase with air injection before a MNF and after a TWC are shown in Fig. 8. The TWC temperature is shown at 810, the upstream air-fuel ratio (e.g., before the TWC and / or the MNF) is shown at 820, an air injection amount is shown at 830, and a downstream air-fuel ratio (after the MNF) is shown at 840.

[0028] For each plot, time is plotted on the x-axis, and each respective parameter is plotted on the y-axis. For plots 820 and 840, which depict air-fuel ratio, the stoichiometric air-fuel ratio is indicated by the marker 1. At time t1, 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 run rich, as shown by the decreasing upstream air-fuel ratio. In response, air injection is activated. Because the amount of air injection is controlled via feedback from a downstream oxygen sensor, the downstream air-fuel ratio remains substantially stoichiometric.At time t2, the rich phase ends, air injection is deactivated, and both the downstream and upstream air-fuel ratios are at stoichiometry.

[0029] Again referring 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 downstream of the TWC if the engine is operating at high load with stoichiometric combustion. At 216, the amount of air injection 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 the air injection behind the TWC, procedure 200 ends.

[0030] Fig. Figure 8 also shows air injection during high-load stoichiometric combustion conditions. At time t3, the engine load increases, for example, due to a driver pedal input event. However, in contrast to time t1, the TWC catalyst temperature (shown at 810) remains below the threshold for initiating rich combustion. However, due to the high-load stoichiometric combustion conditions (e.g., the upstream air-fuel ratio shown at 820 remains approximately at stoichiometry), air injection is activated at time t3 (shown at 830) to create a lean environment in the MNF for storing NOx, which can escape past the TWC. As a result of air injection, the downstream air-fuel ratio (shown at 840) increases until time t4, when the load drops and air injection is deactivated.

[0031] Again with reference 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 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, engine operating parameters such as speed, load, etc., and / or the exhaust air-fuel ratio. If an end of lean combustion is expected, air may be injected downstream of the TWC at 220. Following an end of lean combustion, the NOx stored in the MNF may be released due to the lack of oxygen in the exhaust. To prevent this, air is injected upstream of the MNF so that the exhaust is lean, and the NOx trapped in the MNF remains in the MNF until purging 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.

[0032] If it is determined at 218 that an end to lean combustion is not expected, method 200 proceeds to 224 to continue feedback control of the air-fuel ratio based on sensor input from one or more exhaust gas sensors to maintain the desired combustion air-fuel ratio and the desired TWC temperature without injecting air downstream of the TWC. Because no air is injected downstream of the TWC, the feedback control of the air-fuel ratio may include feedback from one or more exhaust gas sensors downstream of the TWC. In contrast, during operation where air is injected downstream of the TWC, the exhaust gas oxygen concentration, as determined by the downstream exhaust gas sensors, may be omitted from the feedback control or may be adjusted to account for the additional oxygen present in the exhaust gas. Method 200 then ends.

[0033] Thus, the process ensures 200 of Fig. 2 for increasing the exhaust air-fuel ratio downstream of a TWC and upstream of an MNF to maintain emission control while keeping the TWC temperature 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 operates with positive valve overlap, where the exhaust valve is open for a duration during which the intake valve is open, the 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 create reactions with the rich exhaust gas, thereby heating the exhaust gas, and thus may be disrupted during rich operation.

[0034] Fig. 3 is a flowchart illustrating a method 300 for injecting air behind an upstream emission control device, such as the emission control device 70 of Fig. 1. The method 300 may be performed by the controller 12 if an air injection is required during the execution of method 200, with respect to Fig. 2, is indicated.

[0035] Method 300 includes determining at 302 whether air injection is indicated. Air injection may be indicated to provide a leaner air-fuel ratio behind the upstream emission control device than that provided by the engine. Air injection may be initiated based on the Fig. 2, for example, under the conditions discussed in 208, 214 or 220, or on the basis of another suitable parameter. For example, as with respect to Fig. 1 and elaborated upon herein, air may be delivered to the exhaust passage via an EGR passage containing an EGR cooler. If the cooler's effectiveness is low, air injection may be indicated to dislodge soot or other deposits that may have accumulated on the cooler.

[0036] If 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 emissions control device. Directing the compressed intake air may include, at 306, 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. As with respect to Fig. 1, the air injected into the exhaust passage may be taken from the intake passage downstream of the compressor and upstream of the throttle. The intake air downstream of the compressor is compressed, for example, to a pressure above atmospheric pressure. Opening the valve in the injection line allows the air to enter the exhaust passage without the provision of a pump or other device for pressurizing the air. The air from the injection line may be directed directly 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 diverted from the intake but may instead be, for example, atmospheric air.

[0037] 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). Closing the EGR valve prevents EGR flow from the exhaust to the intake, and the compressed air may instead move through the EGR passage to the exhaust passage.

[0038] 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, redirecting compressed intake air may reduce the pressure of the intake air upstream of the throttle; as a result, the throttle position may be adjusted to deliver a desired mass airflow to the engine. Furthermore, the turbocharger may be controlled to increase the boost pressure delivered to the intake. A turbine wastegate may be adjusted to increase boost pressure, or a compressor bypass valve may be adjusted.

[0039] If the compressed intake air is directed through an EGR passage before reaching the exhaust passage, if EGR flow is desired to lower 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 a high-pressure EGR system, flow through the high-pressure EGR system may be increased. In another example, fuel injection timing may be adjusted to lower combustion temperature, or fuel injection may be split, which also acts to lower combustion temperature. After adjusting the engine operating parameters, method 300 ends.

[0040] 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 downstream of a TWC at a leaner air-fuel ratio than the exhaust upstream of the TWC. Method 200 controls the injection of air based on feedback from an oxygen sensor and / or the temperature of an MNF downstream of the TWC. However, in engine systems where the MNF is replaced by 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 oxygen concentrations that are lower than desired.

[0041] With reference to Fig. 4, a method 400 for controlling air injection upstream of a catalyst is illustrated. The method 400 may be executed by the controller 12. In some embodiments, the method 400 may be executed in a gasoline engine configured to operate with lean combustion. As previously explained, lean combustion, which may include air-fuel ratios of approximately 20:1 or greater, produces higher amounts of NOx than stoichiometric combustion. To ensure conversion of the increased NOx species, lean-burn gasoline engines may include a reduction catalyst, such as an SCR system, in the exhaust system. However, unlike these engines, which operate at significantly higher air-fuel ratios, lean-burn gasoline engines may not produce exhaust with adequate oxygen concentration to ensure efficient NOx conversion.Thus, method 400 provides a mechanism for increasing the oxygen concentration upstream of a reduction catalyst in a lean-burn gasoline engine.

[0042] 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 exhaust mass air flow, 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-burn mode. Lean operation may include lean-burn air-fuel ratios and may include air-fuel ratios between 20:1 and 28:1, or other air-fuel ratios. 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 engine may operate with lean combustion during low to moderate engine loads (e.g.,loads of 50% or less) and then operate with stoichiometric combustion at higher loads.

[0043] If the engine is not operating with lean combustion, it is likely operating with substantially stoichiometric combustion, and method 400 proceeds to 406 to initiate a stoichiometric combustion routine, described below with reference to Fig. 5. If it is determined that the engine is operating lean-burn, method 400 proceeds to 408 to determine whether an ammonia storage amount (NH3) in the SCR catalyst is above a threshold. The SCR catalyst may store NH3 to convert NOx species entering the SCR catalyst. The stored ammonia may be oxidized by oxygen in the exhaust gas; thus, to maintain a baseline concentration of stored ammonia sufficient for NOx conversion, the amount of stored ammonia must exceed a lower threshold to compensate for the increased ammonia oxidation in response to the injected air.The amount of NH3 stored may be determined by a storage estimate that is a function of upstream NOx concentration, NH3 consumption (e.g., determined by comparing upstream NOx concentrations with downstream NOx concentrations), SCR temperature, and NH3 generation (e.g., determined by SCR temperature, oxygen consumption, and injected NH3 concentrations). The lower limit for NH3 storage may be a function of SCR temperature.

[0044] If it is determined that the NH3 storage amount does not exceed the lower limit, method 400 proceeds to 410 to initiate an ammonia generation routine, which, with reference to Fig. 6. If the NH3 storage amount exceeds the threshold, method 400 proceeds to 412 to determine whether the measured exhaust oxygen concentration is less than desired. The exhaust oxygen concentration may be measured by a sensor upstream of the SCR catalyst, such as sensor 130 of Fig. 1. The desired oxygen concentration can be determined based on the SCR temperature, exhaust volume, and 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 can be in the range of 8-10%.

[0045] If the exhaust oxygen concentration is not below the desired concentration, there is sufficient oxygen to convert NOx in the SCR, and method 400 returns to continue monitoring for conditions that indicate 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 downstream of 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 described with reference to Fig. 1 and Fig. 3, the secondary air may be intake air directed from behind a turbocharger compressor directly to the exhaust passage 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 at an amount that keeps the exhaust below an upper limit (e.g., 10%) above which ammonia oxidation may reach undesirable levels, but at or above the desired amount of oxygen. Maintaining the oxygen within the threshold range may be achieved through feedback control from the oxygen sensor in the exhaust passage.

[0046] At 418, it is determined whether the SCR temperature is higher than desired. The SCR can achieve optimal NOx conversion within a temperature window, e.g., 200-450°C. If the SCR temperature 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 higher 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, the amount of air introduced may be reduced, if possible, 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 air quantity based on feedback from an oxygen sensor and an SCR temperature sensor, method 400 returns.

[0047] Thus, method 400 provides for injecting secondary air into an exhaust passage upstream of an SCR device. The air may be introduced when exhaust oxygen is at a lower concentration than desired for optimal NOx conversion in the SCR catalyst, such as during operation at engine air-fuel ratios above 20:1, yet lower than typical diesel air-fuel ratios. By providing an SCR system in the exhaust, introducing secondary air when indicated, and operating at air-fuel ratios of approximately 20:1 or 25:1, efficient NOx conversion may occur in the SCR system, reducing the need for a three-way catalyst upstream of the SCR system. Thus, in some embodiments, the three-way catalyst may be replaced with a two-way catalyst.

[0048] Example diagrams showing engine operating parameters during lean combustion with air injection before an SCR and after a TWC are shown in Fig. 9. The engine load is shown at 910, the upstream air-fuel ratio (e.g., engine-off air-fuel ratio) is shown at 920, an air injection amount is shown at 930, and the SCR temperature is shown at 940. For each plot, 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 the mark 1. Before time t1, the engine is operating in lean combustion because the load is relatively light. At time t1, the upstream air-fuel ratio drops, and consequently, less oxygen is available in the exhaust to convert NOx in the SCR. Thus, 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 air injection rate can be increased to cool the SCR catalyst to a target temperature. At time t2, the air-fuel ratio increases, and thus air injection is deactivated.

[0049] Method 400 provides for the introduction of secondary air during lean combustion. However, during certain operating events with stoichiometric combustion, such as acceleration events, strong exhaust flows may allow some NOx to escape past the TWC. To provide optimal NOx conversion during these events, method 500 may Fig. 5 as explained below. Both methods 400 and 500 are based on sufficient NH3 storage levels in the SCR catalyst to prevent oxidation of all available NH3, which leads to NOx escape from the SCR catalyst. If it is determined that the NH3 storage levels are too low, method 600 of Fig. 6 to generate NH3 while providing gas-phase NOx conversion. If it is further determined that the temperature of the SCR catalyst is too high to store NH3, the method 700 of Fig. 7 to cool the SCR catalyst using the introduction of secondary air.

[0050] Now with reference to Fig. 5, a method 500 for controlling the introduction of secondary air during stoichiometric combustion is illustrated. The method 500 may be performed by the controller 12 in response to an indication that the engine is operating with stoichiometric combustion. At 502, the method 500 includes determining whether NH3 storage levels are above a threshold, similar to 408 of the method 400 described above. If the levels are not above the threshold, the 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 whether the engine temperature exceeds a first temperature threshold T1. 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 T1, method 500 returns to a time when the engine has reached the light-off temperature. Because introducing secondary air can cool the SCR catalyst, it may not be beneficial to direct secondary air to the exhaust system while the engine temperature is low. While the engine temperature is being assessed at 506, it is understood that other vehicle temperatures may be assessed to determine whether air needs to be introduced, such as the SCR catalyst temperature.

[0051] If the engine temperature is above threshold T1, method 500 proceeds to 508 to determine whether the engine temperature is below a second temperature threshold T2. 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, described 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 air introduced to maintain 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.

[0052] Fig. 9 also indicates air injection during stoichiometric combustion. For example, at time t3, 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 t3 (shown in plot 930) if the oxygen concentration is lower than desired or if the engine experiences an acceleration event to convert any NOx that may escape past the upstream catalyst.

[0053] Fig. 6 shows a method 600 for generating ammonia. The method 600 may be performed by the 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 determining at 602 whether the SCR catalyst temperature is below the second temperature threshold T2, similar to the method 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 threshold T2, 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 less than desired. The desired oxygen concentration before permitting ammonia generation mode may be different from 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.NOx conversion in the SCR, when NH3 storage is low, occurs with gas-phase NH3 rather than stored NH3, and thus lower oxygen levels may be present to avoid oxidation of the gas-phase NH3. However, depending on the exhaust gas composition and temperature, NOx conversion may be more efficient using fuel as a reductant instead of ammonia; thus, the amount of oxygen available for NOx conversion and NH3 generation may vary depending on whether the SCR is operating as an HC-SCR or NH3-SCR.

[0054] If the measured oxygen concentration is not less than desired, method 600 returns. If it is less than desired, method 600 proceeds to 608 to direct secondary air to the exhaust passage. The directed air amount 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 NH3 storage at 612. Additionally, at 614, the engine may be operated with rich combustion during the ammonia generation mode. Method 600 then returns.

[0055] Fig. 7 is a flowchart illustrating a method 700 for cooling a catalyst using introduction of secondary air. Method 700 may be performed by controller 12 in response to an indication that the SCR catalyst is above an NH3 storage temperature, such as at 510 and 604 of methods 500 and 600, respectively. Method 700 includes determining at 702 whether NOx conversion is possible using either fuel or NH3 as the reductant (e.g., HC-SCR or NH3-SCR). This may be based on conversion estimates determined for both HC and NH3, 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 can be adjusted to maintain the catalyst at a target temperature at 706 to maintain the catalyst at a temperature range suitable for NH3 generation / storage.

[0056] At 708, method 700 includes determining whether the exhaust oxygen concentration is above a storage maximum. If the oxygen concentration in the exhaust is not above a storage minimum, air may be introduced into the exhaust passage to promote NH3 storage, and thus, method 700 proceeds to 710 to execute 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.

[0057] Returning to 702, if at 702 NOx conversion is not possible due to the SCR temperature and exhaust gas composition, method 700 proceeds to 712 to determine if the SCR temperature is above the gas phase conversion temperature limit. If not, method 700 returns to 714 to direct secondary air to the exhaust system to remove particulate matter buildup from the SCR catalyst. Method 700 proceeds to 708 to determine if the oxygen concentration is above the storage maximum, as discussed above.

[0058] Thus, the procedures of Fig.4-7 provide for the injection of secondary air upstream of an SCR catalyst to deliver desired oxygen concentrations for various functions such as NOx conversion, NH3 storage, and / or assisting in catalyst regeneration. In one embodiment, 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 emission control device and a second emission control device. The first emission control device may be a two-way or three-way catalyst, and the second emission control device may be an SCR system. Operating with lean combustion may further include operating with air-fuel ratios between 20:1 and 28:1.

[0059] In some embodiments, the engine may operate with lean combustion during a first condition and with stoichiometric combustion during a second condition. In both the first and second conditions, the amount of injected air 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 involve a low to medium engine load, and the second condition may involve a medium to high engine load. Further, in some embodiments, adjusting the amount of injected air may include blocking the air injection system from injecting air if the exhaust oxygen concentration exceeds a first threshold or if the temperature of the SCR system exceeds a second threshold.

[0060] According to the invention, an engine method comprises, when a reductant storage level of a reduction catalyst is above a threshold, adjusting an amount of secondary air injected into an exhaust passage upstream of 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 the exhaust oxygen concentration may be based on a temperature of the reduction catalyst and the reductant storage level.

[0061] It is understood that the configurations and methods disclosed herein are merely exemplary in nature, and that these specific embodiments are not intended to be limiting, as numerous variations are possible. For example, the above technology may be applied to V-6, I-4, I-6, V-12, horizontally opposed 4-stroke, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and subcombinations of the various systems and configurations, and other features, functions, and / or characteristics disclosed herein.

[0062] The following claims particularly point out certain combinations and sub-combinations that are considered novel and non-obvious. These claims may refer to "an" element or "a first" element or the equivalent thereof. Such claims are to be construed as including the inclusion 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 properties may be claimed by amending the present claims or by presenting new claims in this or a related application. Such claims, whether more general, narrower, the same, or different in scope from the original claims, are also considered to be included within the subject matter of the present disclosure.

Claims

[1] Engine procedure which includes: Adjusting the air-fuel ratio of the exhaust gas upstream of a first exhaust gas purification device (70) to maintain the first exhaust gas purification device (70) at or below a threshold temperature; and if the upstream air-fuel ratio of the exhaust gas is below a threshold value, injecting air into an exhaust passage (35) between the first exhaust gas purification device (70) and a second exhaust gas purification device (72) in order to keep the exhaust gas downstream of the first exhaust gas purification device (70) at a different, higher air-fuel ratio, wherein if a reducing agent storage level of a reduction catalyst (72) is above a threshold value, the amount of air injected into an exhaust passage (35) upstream of the reduction catalyst (72) is adjusted in order to keep the exhaust gas oxygen concentration in a first range, and if the reducing agent storage level is below the threshold value, the amount of air injected into the exhaust passage (35) is adjusted in order to keep the exhaust gas oxygen concentration in a second, lower range. [2] The engine method of claim 1, wherein injecting air into the exhaust passage (35) further comprises directing boosted intake air directly to the exhaust passage (35). [3] The engine method of claim 1, wherein injecting air into the exhaust passage (35) further comprises directing boosted intake air to the exhaust passage (35) via a LP EGR system. [4] The engine method of claim 3, wherein directing boosted intake air to the exhaust passage (35) via the LP EGR system further comprises directing boosted intake air to an LP EGR passage (80) upstream of an LP EGR cooler (82), the LP EGR passage (80) being coupled to the exhaust passage. [5] The engine method of claim 1, wherein the upstream air-fuel ratio of the exhaust gas comprises the air-fuel ratio of the exhaust gas at an inlet of the first emission control device (70) and wherein the downstream air-fuel ratio comprises the air-fuel ratio of the exhaust gas at an outlet of the second emission control device (72). [6] The engine method of claim 2, wherein an amount of injected air is further controlled to maintain a temperature of the second exhaust gas purification device (72) within a threshold range. [7] The engine method of claim 1, wherein the threshold air-fuel ratio corresponds to stoichiometry and wherein the threshold temperature is a maximum temperature of the first emission control device (70). [8] Engine system (8) comprising: an exhaust system with a turbocharger (50), catalyst (70) and a lean NOx trap (72); an air injection system coupled to an exhaust passage (35) between the catalyst (70) and the lean NOx trap (72); and a controller (12) for executing an engine method according to one or more of the preceding claims with instructions for: Adjusting the air-fuel ratio upstream of the catalyst (70) to maintain a temperature of the catalyst (70) at or below a maximum temperature; and during selected conditions, directing compressed intake air to the exhaust passage (35) via the air injection system. [9] The engine system (8) of claim 8, wherein the air injection system couples the exhaust passage (35) directly to a portion of an intake passage (42) downstream of a compressor (52) and upstream of a throttle (62). [10] The engine system (8) of claim 9, further comprising a valve (91) positioned in the air injection system, and wherein the controller (12) has further instructions for opening the valve (91) during the selected conditions. [11] The engine system (8) of claim 8, wherein the air injection system couples the exhaust passage to a LP EGR passage (80) and wherein the compressed intake air is directed to the LP EGR passage (80) upstream of an EGR cooler (82). [12] The engine system (8) of claim 11, further comprising an EGR valve (84) disposed in the LP EGR passage (80), and wherein the controller (12) further includes instructions to close the EGR valve (84) at least during the selected conditions. [13] The engine system (8) of claim 12, further comprising adjusting fuel injection timing in response to closing the EGR valve (84). [14] Engine system (8) according to claim 8, wherein the selected conditions include a high engine load and stoichiometric combustion. [15] Engine system (8) according to claim 8, wherein the selected conditions comprise a transition from lean to stoichiometry. [16] An engine system (8) according to claim 8, wherein the selected conditions comprise rich combustion and wherein the compressed intake air is directed to the exhaust passage (35) to oxidize carbon monoxide produced during rich combustion. [17] Engine process comprising: during selected conditions, directing compressed intake air to a portion of an exhaust passage (35) between a three-way catalyst (70) and a lean NOx trap (72), the compressed intake air being directed via a LP EGR passage (80), wherein when a reductant storage level of the lean NOx trap (72) is above a threshold, an amount of compressed intake air injected into an exhaust passage (35) upstream of the lean NOx trap (72) is adjusted to maintain the exhaust gas oxygen concentration in a first range, and when the reductant storage level is below the threshold, the amount of compressed intake air injected into the exhaust passage is adjusted to maintain the exhaust gas oxygen concentration in a second, lower range. [18] The engine method of claim 17, wherein the selected conditions comprise rich combustion. [19] The engine method of claim 17, wherein the compressed intake air is directed to the LP EGR passage (80) prior to an EGR cooler (82). [20] The engine method of claim 19, wherein the selected conditions include a regeneration mode of the EGR cooler (82).

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