SCR Catalyst Diagnosis

Abstract

Methods for an engine, including: Operating the engine with an upstream exhaust gas sensor (125), an intermediate exhaust gas sensor (126) and a downstream exhaust gas sensor (127), each displaying substoichiometric readings; Initiating a heat event to cause NH3 to be released from an SCR catalyst (72) prior to oxidation of the SCR catalyst (72) and a three-way catalyst (71); Setting the engine operation to operate the engine with the upstream exhaust gas sensor (125), the intermediate exhaust gas sensor (16) and the downstream exhaust gas sensor (127), each indicating overstoichiometric readings; Adjusting the engine operation to run the engine with the upstream exhaust gas sensor (125) indicating substoichiometrically, and the intermediate exhaust gas sensor (126) and the downstream exhaust gas sensor (127) indicating superstoichiometrically; and Indication of a fault of the SCR catalyst (72) based on when the intermediate and downstream exhaust gas sensors (126, 127) switch from superstoichiometric to substoichiometric, wherein the upstream exhaust gas sensor (125) is located upstream of a three-way catalyst (71), the intermediate exhaust gas sensor (126) is located downstream of the three-way catalyst (71) and upstream of the SCR catalyst (72), and the downstream exhaust gas sensor (127) is located downstream of the SCR catalyst (72).

Description

[0001] The present disclosure relates to an internal combustion engine.

[0002] To reduce the release of exhaust emissions into the atmosphere, vehicles can be equipped with various exhaust aftertreatment devices. For example, three-way catalysts can reduce the levels of various emissions, including carbon monoxide and unburned hydrocarbons, while selective catalyst reduction systems can be used to reduce NOx levels. To ensure that the aftertreatment devices operate optimally, various sensors can be installed upstream and / or downstream of the devices, and feedback from the sensors can be used to determine whether emissions are slipping past the devices.

[0003] JP 2001-124 730 A discloses a system and method for controlling lean nitrogen oxide emissions. DE 10 2010 050 055 A1 discloses a method for diagnosing an exhaust catalyst and / or an exhaust sensor of a motor vehicle internal combustion engine. DE 10 2009 038 948 A1 discloses a system and method for controlling lean nitrogen oxide emissions. US 5 280 707 A discloses a device for detecting catalyst deterioration.

[0004] To determine if NOx is slipping past a catalyst, a NOx sensor located downstream of the catalyst can be monitored. If the sensor detects NOx in the exhaust, operating parameters can be adjusted to reduce NOx emissions in the exhaust and / or alert the vehicle operator that a catalyst failure is present. However, particularly when used in gasoline engines, NOx sensors may not be sensitive enough to detect the lower levels of NOx produced by the engine. Furthermore, NOx sensors can be expensive, limiting their use.

[0005] The inventors have recognized the problems of the above approach and provide a method that at least partially addresses them. In one embodiment, a method for an engine comprises operating the engine with an upstream exhaust gas sensor, an intermediate exhaust gas sensor, and a downstream exhaust gas sensor each indicating understoichiometric, initiating a heat event to cause NH3 to be released from an SCR catalyst prior to oxidizing the SCR catalyst and a three-way catalyst, adjusting engine operation to operate the engine with the upstream exhaust gas sensor indicating understoichiometric and with the intermediate exhaust gas sensor and the downstream exhaust gas sensor each indicating overstoichiometric, and indicating a fault of the SCR catalyst based thereon,when the intermediate and downstream exhaust gas sensors switch from over-stoichiometric to under-stoichiometric, wherein the upstream exhaust gas sensor is arranged upstream of a three-way catalyst, the intermediate exhaust gas sensor is arranged downstream of the three-way catalyst and upstream of the SCR catalyst, and the downstream exhaust gas sensor is arranged downstream of the SCR catalyst.

[0006] In this way, an exhaust gas oxygen sensor such as a HEGO can be used to diagnose an SCR catalyst. The engine may be operating to store oxygen in the three-way catalyst, and during a heat event, ammonia is released into the SCR catalyst. Upon returning to slightly substoichiometric operation, the oxygen stored in the three-way catalyst (TWC) can react with the reductants in the exhaust gas from the engine to prevent the intermediate sensor located upstream of the SCR catalyst from detecting reductants. The downstream sensor, located downstream of the SCR catalyst, thereby detects only NH3 released from the SCR catalyst.Based on the time at which the downstream sensor switches from lean to lean (indicating the amount of ammonia released by the SCR catalyst), SCR catalyst failure may be indicated. Alternatively, if the time difference for the intermediate and downstream sensors to switch from lean to lean falls below a threshold, SCR catalyst failure may be indicated.

[0007] The above advantages and other advantages and features of the present description will become apparent from the following detailed description taken alone or in conjunction with the accompanying drawings.

[0008] It will be understood that the above summary is provided to provide, in simplified form, a selection of concepts that are further described in the detailed description. It is not intended to identify principal or essential features of the claimed subject matter, the scope of which is defined solely in the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that eliminate potential disadvantages mentioned above or in any part of this disclosure.

[0009] They show: Fig. 1 a schematic representation of an engine. Fig. 2 an exemplary method for diagnosing an SCR catalyst. Fig. 3 exemplary sensor readings during the execution of the procedure from Fig. 2.

[0010] To diagnose a malfunction of a selective catalyst reduction (SCR) catalyst, the release of ammonia from the SCR catalyst can be monitored by exhaust gas oxygen sensors located upstream and downstream of the SCR catalyst. Specifically, oxygen sensors such as HEGO (heated exhaust gas oxygen) sensors exhibit a sharp voltage transition when the gas composition changes between sub- and super-stoichiometric conditions. These sensors switch from super-stoichiometric to sub-stoichiometric with any combination of reductants such as hydrocarbons (HC), carbon monoxide (CO), hydrogen (H2), and ammonia (NH3).By utilizing the oxygen storage capacity of a three-way catalyst upstream of the SCR catalyst, conditions can be created where no HC, CO, and H2 are present at the SCR catalyst (and upstream of the HEGO). Afterward, the NH3 stored in the SCR catalyst can be thermally released to the downstream sensor. Therefore, during these conditions, the downstream sensor may be exposed to NH3 but not other reductants. The timing at which the downstream sensor switches from lean to lean as a result of NH3 release can indicate whether a catalyst failure is present and / or provide an estimate of the catalyst's NH3 storage capacity. Fig. Figure 1 shows an engine diagram with an SCR catalyst, an upstream three-way catalyst and exhaust gas sensors. Fig. 1 also comprises a controller which provides instructions for carrying out the method Fig. 2 includes. Fig. 3 shows an exemplary output of the exhaust gas sensors during the execution of the method from Fig. 2.

[0011] With reference to Fig. 1 includes a schematic diagram showing one cylinder of a multi-cylinder internal combustion engine 10. The engine 10 may be controlled at least in part by a control system including a controller 12 and by input from a vehicle operator 132 via an input device 130. In this example, the input device 130 includes an accelerator pedal and a pedal position sensor 134 for generating a proportional pedal position signal PP.

[0012] The combustion cylinder 30 of engine 10 may include combustion cylinder walls 32 with the piston 36 disposed therein. The piston 36 may be coupled to the crankshaft 40 such that the reciprocating motion of the piston is translated into a rotational motion of the crankshaft. The crankshaft 40 may be coupled to at least one drive wheel of a vehicle via an intermediate transmission system. Furthermore, a starter motor may be coupled to the crankshaft 40 via a flywheel to activate a starting operation of the engine 10.

[0013] Combustion cylinder 30 may receive intake air from intake manifold 44 via intake passage 42 and expel combustion gases via exhaust port 48. Intake manifold 44 and exhaust port 48 may be selectively in communication with combustion cylinder 30 via the respective intake valve 52 or exhaust valve 54. In some embodiments, combustion cylinder 30 may include two or more intake valves and / or two or more exhaust valves.

[0014] In this example, intake valve 52 and exhaust valves 54 may be controlled by cam actuation of the respective cam actuation systems 51 and 53. Cam actuation systems 51 and 53 may each include one or more cams and utilize one or more of cam profile control (CPS), variable camshaft timing (VCT), variable valve timing (VVT), and / or variable valve lift (VVL) systems operable by controller 12 to vary valve operation. The position of intake valve 52 and exhaust valve 54 may be determined by position sensors 55 and 57, respectively. In alternative embodiments, intake valve 52 and / or exhaust valve 54 may be controlled by electrical valve actuation.For example, cylinder 30 may alternatively include an intake valve controlled via electric valve actuation and an exhaust valve controlled via cam actuation with CPS and / or VCT systems.

[0015] The fuel injector 66 is directly coupled to the combustion cylinder 30 for injecting fuel directly therein, depending on the pulse width of the FPW signal received from the controller 12 via the electric drive 68. In this way, the fuel injector 66 provides what is known as direct injection of fuel into the combustion cylinder 30. The fuel injector can be mounted, for example, on the side of the combustion cylinder or on the top of the combustion cylinder. The fuel can be supplied to the fuel injector 66 via a fuel delivery system (not shown) that includes a fuel tank, a fuel pump, and a fuel rail.In some embodiments, the combustion cylinder 30 may alternatively or additionally include a fuel injector disposed in the intake passage 42 in a configuration that provides what is commonly known as port injection of fuel into the intake manifold upstream of the combustion cylinder 30.

[0016] The intake passage 42 may include a load movement control valve (CMCV) 84 and a CMCV poppet 82, and may also include a throttle body 62 with a throttle plate 64. In this particular example, the position of the throttle body 64 may be varied by a controller 12 using a signal provided to an electric motor or actuator included in the throttle body 62, in a configuration commonly referred to as electronic throttle control (ETC). In this manner, the throttle body 62 may be operated to vary the intake air provided to the combustion cylinder 30 and other engine cylinders. The intake passage 42 may further include a mass air flow sensor 120 and a manifold air pressure sensor 122 for providing the corresponding MAF and MAP signals to the controller 12.

[0017] The ignition system 88 may, in select operating modes, provide an ignition spark to the combustion chamber 30 using the spark plug 92 in response to an advance spark signal SA from the controller 12. Although ignition components are depicted in some embodiments, the combustion chamber 30 or one or more other combustion chambers of the engine 10 may be operated in a compression ignition mode with or without an ignition spark.

[0018] Emission control devices 71, 72, and 76 are shown arranged along the exhaust passage 48. In the illustrated embodiment, device 71 may be a three-way catalyst (TWC), device 72 may be a selective reduction catalyst (SCR), while device 76 may be a diesel oxidation catalyst (DOC), diesel particulate filter (DPF), three-way catalyst (TWC), NOx trap, various other emission control devices, or combinations thereof. Alternative arrangements are also possible in some embodiments, such as only device 71 and device 72 being arranged in the exhaust passage. Reductant (e.g., NH3) may be generated for the SCR catalyst (e.g., device 72) via the upstream TWC. However, in some embodiments, a reductant tank 73 may be provided for storing reductant such as urea or NH3.Tank 73 may be coupled to an injector 75 for injecting reductant into the exhaust gas upstream of device 72 or into device 72 to reduce NOx in device 71. A mixer 74 may also be provided to ensure adequate mixing of the reductant with the exhaust stream. Ammonia may be injected proportionally to an amount of raw engine exhaust NOx entering the SCR.

[0019] The exhaust gas sensors are shown coupled to the exhaust passage 48. Sensors 125, 126, and 127 may each be any suitable sensor for providing an indication of an exhaust air / fuel ratio, such as a linear oxygen sensor or UEGO (universal or wideband exhaust gas oxygen sensor), a two-stage oxygen sensor or EGO, a HEGO (heated EGO), a NOx, HC, or CO sensor. Sensor 125 may be an upstream sensor provided upstream of the emission control devices 71, 72, and 76, while sensor 126 may be an intermediate sensor provided downstream of the emission control device 71 and upstream of the emission control device 72. Sensor 127 may be a downstream sensor provided downstream of the emission control device 72.

[0020] The control 12 is in Fig. 1 as a microcomputer including the microprocessor unit 102, input / output ports 104, an electronic storage medium for executable programs and calibration values, shown in this example as a read-only memory chip 106, random access memory 108, keep-alive memory 110, and a data bus. The controller 12, in addition to the previously described signals and information, may receive various signals from the sensors coupled to the engine 10, including the measurement of induced mass air flow (MAF) from the mass air flow sensor 120; the engine coolant temperature (ECT) from the temperature sensor 112 coupled to the cooling sleeve 114; the profile ignition pickup (PIP) signal from the Hall effect sensor 118 (or other type) coupled to the crankshaft 40; and the throttle position (TP) from a throttle position sensor, and the absolute manifold pressure signal MAP from sensor 122.The write-protected storage medium 106 may be programmed with computer-readable data executable by the processor 102 to perform the methods described below, as well as other variations thereof.

[0021] The read-only storage medium 106 may be programmed with computer-readable data representing instructions executable by the processor 102 for performing the methods described below, as well as other variations that may be contemplated but are not specifically listed.

[0022] As described above, Fig. 1 only one cylinder of a multi-cylinder engine, and each cylinder may similarly have its own set of intake and exhaust valves, fuel injector, spark plug, etc.

[0023] Therefore, the system described above provides a system comprising: an engine coupled to an exhaust duct; a three-way catalyst disposed in the exhaust passage; an SCR catalyst arranged in the exhaust passage downstream of the three-way catalyst; an upstream exhaust gas sensor arranged upstream of the three-way catalyst, an intermediate exhaust gas sensor arranged downstream of the three-way catalyst, and a downstream exhaust gas sensor arranged downstream of the SCR catalyst; and a controller having the following instructions: operating the engine with the upstream, intermediate, and downstream exhaust gas sensors each indicating lean of stoichiometric; Adjusting engine operation to operate the engine with the upstream exhaust gas sensor measuring substoichiometric and the intermediate exhaust gas sensor and the downstream exhaust gas sensor each measuring superstoichiometric; and indicating a fault of the SCR catalyst based on when the downstream exhaust gas sensor switches from lean to stoichiometric.

[0024] The controller may include instructions to stop fuel injection to the engine to operate with the upstream, intermediate, and downstream exhaust gas sensors each indicating lean of stoichiometric. The controller may include instructions to adjust engine operation by resuming fuel injection and operating at lean of stoichiometric combustion to operate the engine with the upstream exhaust gas sensor indicating lean of stoichiometric and the intermediate exhaust gas sensor and the downstream exhaust gas sensor each indicating lean of stoichiometric. The controller may include instructions to indicate SCR catalyst failure when an amount of time from adjusting engine operation to the downstream exhaust gas sensor switching from lean of stoichiometric to lean of stoichiometric is greater than a threshold.Alternatively, the controller may indicate a fault of the SCR catalyst when the time difference for the intermediate and downstream sensors to switch from lean to stoichiometric is less than a threshold. The controller may further include instructions for determining a reductant storage capacity of the SCR catalyst based on when the downstream exhaust gas sensor switches from lean to stoichiometric.

[0025] With reference to Fig. 2 illustrates a method 200 for diagnosing an SCR catalyst. The method 200 may be performed by the controller 12 according to instructions stored thereon to diagnose an SCR catalyst (such as device 72) based on feedback received from an upstream exhaust oxygen sensor such as a UEGO (e.g., exhaust sensor 125), a first, intermediate HEGO (e.g., sensor 126), and a second, downstream HEGO (such as sensor 127). The upstream UEGO may be located upstream of a three-way catalyst (TWC), the intermediate HEGO may be located downstream of the TWC but upstream of the SCR catalyst, and the downstream HEGO may be located downstream of the SCR catalyst.

[0026] At 202, method 200 includes determining the engine operating parameters. The operating parameters may include engine speed and load, engine temperature, time since the previous SCR diagnostic routine was performed, catalyst efficiency (e.g., as determined based on output from the exhaust sensors), etc. At 204, it is determined whether an appropriate time exists to perform an SCR diagnostic routine. The SCR diagnostic routine may be indicated when a threshold amount of time has elapsed since a previous routine was performed. Additionally or alternatively, the SCR diagnostic routine may be performed when certain operating conditions are met, such as engine or catalyst temperature being below a threshold (as the routine may increase the engine and / or exhaust temperature). If the SCR diagnostic routine is not indicated, method 200 returns.

[0027] If the routine is indicated, method 200 proceeds to 206 to optionally initiate a thermal event to purge NH3 from the SCR catalyst. Before determining whether NH3 is being released from the SCR catalyst at a particular rate and / or before determining the storage of the SCR catalyst, the SCR catalyst may first be purged of any stored NH3. This prevents excessive NH3 from being inadvertently released to the atmosphere when the SCR catalyst is subsequently filled with NH3. The thermal event may include vehicle acceleration, which may be performed at a stoichiometric air-fuel ratio, which may serve to increase the temperature of the exhaust gas and thus the SCR catalyst, thereby causing NH3 to be released from the SCR catalyst. However, in some embodiments, the SCR diagnostic may be initiated without performing an initial purge of NH3 from the SCR.

[0028] At 208, the engine operates at substoichiometric combustion, generating NH3 in a three-way catalyst (TWC) upstream of the SCR catalyst. As indicated at 210, the NH3 generated by the TWC is transported downstream of the SCR catalyst where it may be stored. The engine may operate at substoichiometric combustion until the SCR catalyst reaches saturation. As a result of the substoichiometric combustion, all three sensors in the purge system may indicate substoichiometric combustion, as indicated at 212.

[0029] At 213, another heat event is initiated to warm the SCR catalyst so that NH3 can be desorbed from the SCR catalyst. Similar to the heat event described above, this heat event may be caused by stoichiometric vehicle acceleration. At 214, the catalysts may be oxidized by operating at lean combustion or by introducing secondary air into the TWC. In one example, lean combustion may be provided by stopping fuel injection to the engine, for example, by initiating a deceleration fuel shut-off (DFSO) event. During DFSO, fuel injection to the engine may be stopped while intake air is still being pumped through the cylinders. Therefore, the exhaust gas reaching the catalysts may be lean of stoichiometric. Other mechanisms for providing oxygen to the TWC are also possible.As a result of additional oxygen in the exhaust during DFSO operation, the oxygen may be stored in the TWC, as indicated at 216. Additionally, the UEGO and HEGO may each indicate lean of stoichiometry, as indicated at 218. Furthermore, the unburned air may be cooler than the burned air, thus lowering the temperature of the catalysts.

[0030] After oxidizing the TWC and SCR catalyst, the process proceeds from 200 to 220 to start fuel injection and operate at substoichiometric combustion. Substoichiometric combustion can be merely stoichiometrically rich, such as a lambda of 0.990 to 0.999. The more substoichiometric the air-fuel ratio, the faster the penetration of reductants through the TWC and the faster the switching of the intermediate HEGO occurs. Therefore, by maintaining only a slightly substoichiometric air-fuel ratio, the period for monitoring the switch from lean to substoichiometric HEGO can be maximized, as described below, increasing the precision and reliability of the diagnostics. As indicated at 222, the upstream UEGO indicates substoichiometric due to substoichiometric combustion.The intermediate and downstream HEGO may each continue to read lean for a short period of time after the start of lean combustion, as indicated at 224. The oxygen stored in the TWC reacts with the reductants in the exhaust, preventing the release of reductant into the SCR catalyst. As a result, the intermediate HEGO continues to read lean until the stored oxygen has been depleted and the reductants permeate through the TWC to the sensor. Due to the lack of reductants in the exhaust, the downstream HEGO also reads lean until NH3 release from the SCR catalyst, caused by the temperature increase during acceleration, reaches a sufficient concentration to cause the downstream HEGO to switch to lean.

[0031] Due to the previous acceleration event, which initiates NH3 desorption, the SCR temperature only decreases during DFSO and the return to slightly substoichiometric operation. Therefore, NH3 is still released from the SCR after DFSO because a period of time is needed for NH3 to be released while the heat moves through the SCR catalyst. During this time, DFSO and the return to slightly substoichiometric operation can be performed, and then NH3 release from the SCR catalyst can be monitored.

[0032] At 226, the switching of the HEGO from lean to lean is monitored. As described above, the upstream UEGO may remain lean as long as the engine is operating with lean combustion. The downstream HEGO may switch from lean to lean once a threshold amount of NH3 is released from the SCR catalyst. The intermediate HEGO may switch from lean to lean once the reductants permeate the TWC. The amount of time it takes for the reductants to permeate the TWC may be longer than the amount of time it takes for the NH3 to be released from the SCR catalyst, which may be relatively rapid for a catalyst without failure.Therefore, the time to switch from lean to stoichiometric of the downstream HEGO can be compared with the time to switch from lean to stoichiometric of the intermediate HEGO, and if the difference is less than expected, a fault may be indicated.

[0033] At 228, it is determined whether the amount of time between the switching of the intermediate and downstream HEGO is greater than a threshold. The threshold time may be based on an expected time determined from previous operations; for example, a fresh catalyst may be monitored and the time difference between the switching of the intermediate HEGO and downstream HEGO may be stored.

[0034] If the time between switches is greater than a threshold, method 200 proceeds to 232 to indicate no fault while the SCR catalyst is storing an appropriate amount of NH3. If the time between switches is not greater than the threshold time, method 200 proceeds to 230 to indicate an SCR catalyst fault and take remedial action. Therefore, if the downstream HEGO takes longer than expected to switch from lean to stoichiometric (i.e., a smaller amount of time between the intermediate HEGO switching from lean to stoichiometric and the downstream HEGO switching from lean to stoichiometric), less NH3 may be stored in the SCR catalyst than expected. This may indicate a fault with the catalyst or a reduced NH3 storage capacity.The standard corrective action may include notifying a vehicle operator (by illuminating a malfunction lamp, for example), setting a diagnostic code, and / or adjusting engine operating parameters (such as increasing the exhaust gas recirculation rate to reduce NOx levels in the exhaust). Method 200 then returns.

[0035] While the method described above monitors the time between HEGO switching, the time for the downstream HEGO to switch from lean to stoichiometric can be monitored and compared to an expected threshold time from previous engine operations. If the downstream HEGO switch occurs after a threshold time has elapsed since the start of lean combustion, a fault may be indicated. Furthermore, by monitoring both the HEGO downstream of the SCR and the HEGO upstream of the TWC, a possible TWC fault may be indicated if the intermediate HEGO (downstream of the TWC) switches faster than expected.

[0036] The amount of time required for the intermediate and downstream sensor to switch from lean to stoichiometric can depend on the exhaust flow rate, the stoichiometry level of the air-fuel ratio after DFSO operation, and the oxygen storage capacity of the TWC, which can change as the TWC ages. To normalize HEGO switch times for these variables, the difference between the intermediate and downstream HEGO switch times can be divided by the amount of time required for the intermediate HEGO to switch from lean to stoichiometric.

[0037] Fig. 3 shows a diagram 300 showing exemplary readings of the three exhaust gas sensors during the execution of the method from Fig.2 and the NH3 release from the SCR as a function of time. The reading of the upstream UEGO is represented by line 302, the reading of the first, intermediate HEGO is represented by line 304, the reading of the second, downstream HEGO is represented by line 306, and the NH3 release from the SCR is represented by line 308. For the UEGO readout, a stoichiometric reading is represented by the dashed horizontal line, with a substoichiometric reading shown below stoichiometry in the graph and a superstoichiometric reading shown above stoichiometry. The HEGO readings are the voltage output from the sensors, with a higher voltage indicating a substoichiometric reading (e.g. 0.8 V) and a lower voltage indicating a superstoichiometric reading (e.g. 0.1 V).

[0038] Before time t1, the SCR diagnostic routine was not initiated and the engine is operating at a stoichiometric air-fuel ratio. Additionally, little or no NH3 is released from the SCR catalyst. At time t1, the diagnostic routine is initiated and the SCR catalyst warms to release NH3. As represented by line 308, NH3 release from the SCR increases and, as represented by line 306, the downstream HEGO sensor indicates substoichiometric as a result of the released NH3. While the upstream UEGO and intermediate HEGO are shown as indicating substoichiometric, they may also indicate substoichiometric if the heating event initiated to release NH3 includes a decrease in the air-fuel ratio.After releasing NH3 from the SCR, the engine operates with lean combustion at time t2 to produce NH3 in the TWC for storage in the SCR. Therefore, as represented by lines 302, 304, and 306, each of the sensors indicates lean. Once the SCR reaches saturation, a heat event is initiated to start the release of NH3 from the SCR, immediately before time t3. DFSO is initiated at t3 or fuel injection to the engine is otherwise terminated. The resulting lean air causes the UEGO and each HEGO sensor to switch to lean. The oxygen in the exhaust gas oxidizes the catalysts, leading to oxygen storage in the TWC and a temperature drop in the SCR.After a predetermined period of time, DFSO operation is terminated. At time t4, fuel injection starts, and the engine operates with slightly substoichiometric combustion. The upstream UEGO (line 302) switches back to substoichiometric, reflecting the engine's off-gas air-fuel ratio. However, the intermediate and downstream HEGO remain superstoichiometric due to the lack of reductants leaving the TWC.

[0039] Once the SCR reaches a sufficiently high temperature, NH3 is released from the SCR, as represented by line 308. After reaching a threshold amount of NH3 in the exhaust gas, the downstream HEGO switches from lean to lean. For a catalyst without fault, this occurs relatively quickly, and before the intermediate HEGO switches from lean to lean, which is represented at time t5. Therefore, for a catalyst without fault, the downstream HEGO switches from lean to lean before the intermediate HEGO, and a relatively long time elapses between switches. However, with a defective catalyst, little or no NH3 can be stored in the SCR, delaying the downstream HEGO's switch from lean to lean, as represented by dashed line 310.Even if the SCR catalyst is malfunctioning, the downstream HEGO will switch from lean to stoichiometric due to the penetration of reductants from the intermediate HEGO. This switch may occur slightly later than the intermediate HEGO due to the oxygen storage capacity of the SCR. With a defective catalyst, the time between the downstream HEGO switching and the intermediate HEGO switching may be shorter than with a catalyst without a malfunction.

[0040] Therefore, the methods and systems described herein provide a method for an engine comprising: Operating the engine with an upstream exhaust gas sensor, an intermediate exhaust gas sensor, and a downstream exhaust gas sensor each indicating over-stoichiometric; adjusting engine operation to operate the engine with the upstream exhaust gas sensor indicating substoichiometric and the intermediate exhaust gas sensor and the downstream exhaust gas sensor each indicating superstoichiometric; and indicating a fault of an SCR catalyst based on when the intermediate and downstream exhaust gas sensors switch from lean to stoichiometric.

[0041] The upstream exhaust gas sensor may be located upstream of a three-way catalyst, the intermediate exhaust gas sensor may be located downstream of the three-way catalyst and upstream of the SCR catalyst, and the downstream exhaust gas sensor may be located downstream of the SCR catalyst. Adjusting engine operation to operate with the upstream exhaust gas sensor indicating lean of stoichiometric and with the intermediate and downstream exhaust gas sensors each indicating lean of stoichiometric may occur immediately after operating the engine with the upstream exhaust gas sensor, the intermediate exhaust gas sensor, and the downstream sensor each indicating lean of stoichiometric.

[0042] Operating the engine with the upstream, intermediate, and downstream exhaust gas sensors each indicating lean may include operating the engine with fuel injection to the engine discontinued, and ceasing engine operation may include starting fuel injection and operating the engine with lean combustion. After starting fuel injection with lean combustion, failure of an SCR catalyst may be indicated if a difference between the intermediate exhaust gas sensor switching from lean to lean and the downstream exhaust gas sensor switching from lean to lean is less than a threshold.

[0043] The method may further include indicating no fault if the difference between the intermediate exhaust gas sensor switching from lean to lean and the downstream exhaust gas sensor switching from lean to lean is greater than the threshold. The intermediate exhaust gas sensor switching from lean to lean may occur due to the permeation of reductants from the three-way catalyst, and the downstream exhaust gas sensor switching from lean to lean may occur due to the release of ammonia from the SCR catalyst.

[0044] In another example, a method includes oxidizing an SCR catalyst and three-way catalyst located downstream of the SCR catalyst by terminating all engine fuel injection; then oxidizing the SCR and three-way catalyst, thereby combusting a substoichiometric engine air-fuel ratio; and indicating the SCR catalyst fault based on a time difference for an intermediate oxygen sensor downstream of the three-way catalyst and a downstream oxygen sensor downstream of the SCR catalyst to switch from lean to stoichiometric.

[0045] The method may further comprise, prior to oxidizing the SCR and three-way catalyst, initiating a thermal event to cause NH3 to be released from the SCR catalyst. The method may further comprise, prior to initiating the thermal event, producing reductant in the three-way catalyst by operating it with substoichiometric combustion, wherein the reductant is stored in the SCR catalyst.

[0046] The SCR catalyst failure may be indicated if a difference between the downstream exhaust gas oxygen sensor switching from lean to lean and the intermediate oxygen sensor switching from lean to lean is less than a threshold. The method may further include indicating no failure if the time difference between the intermediate exhaust gas oxygen sensor switching from lean to lean and the downstream oxygen sensor switching from lean to lean is greater than the threshold.

[0047] The downstream oxygen sensor may switch from lean to stoichiometric when a threshold level of reductant is released from the SCR catalyst, and the intermediate oxygen sensor may switch from lean to stoichiometric when a threshold level of reductant is released from the three-way catalyst.

[0048] It will be appreciated that the configurations and methods disclosed herein are exemplary, and that these specific embodiments are not to be considered limiting, as numerous variations thereof are possible. For example, the technology described above may be applied to V-6, V-8, I-4, I-6, V-12, horizontally opposed four, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations and other features, functions, and / or characteristics disclosed herein.

[0049] The following claims highlight certain combinations and subcombinations that are considered novel and non-obvious. These claims may refer to "a" element or "a first" element, or an equivalent thereof. Such claims are to be construed as including the inclusion of one or more such elements and neither requiring nor excluding two or more such elements. Other combinations and subcombinations 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 that are broader, narrower, the same, or different in scope with respect to the original claims are intended to be included within the subject matter of the present disclosure.

Claims

[1] A method for an engine, comprising: Operating the engine with an upstream exhaust gas sensor (125), an intermediate exhaust gas sensor (126) and a downstream exhaust gas sensor (127), each indicating substoichiometric; initiating a heat event to cause NH3 to be released from an SCR catalyst (72) prior to oxidizing the SCR catalyst (72) and a three-way catalyst (71); Adjusting engine operation to operate the engine with the upstream exhaust gas sensor (125), the intermediate exhaust gas sensor (16), and the downstream exhaust gas sensor (127) each indicating lean of stoichiometric; Adjusting engine operation to operate the engine with the upstream exhaust gas sensor (125) indicating substoichiometric and the intermediate exhaust gas sensor (126) and the downstream exhaust gas sensor (127) each indicating overstoichiometric; and Indicating a fault of the SCR catalyst (72) based on when the intermediate and downstream exhaust gas sensors (126, 127) switch from lean to stoichiometric, wherein the upstream exhaust gas sensor (125) is arranged upstream of a three-way catalyst (71), the intermediate exhaust gas sensor (126) is arranged downstream of the three-way catalyst (71) and upstream of the SCR catalyst (72), and the downstream exhaust gas sensor (127) is arranged downstream of the SCR catalyst (72). [2] The method of claim 1, wherein adjusting engine operation to operate with the upstream exhaust gas sensor (125) indicating under-stoichiometric and the intermediate and downstream exhaust gas sensors (126, 127) each indicating over-stoichiometric occurs immediately after operating the engine with the upstream exhaust gas sensor (125), the intermediate exhaust gas sensor (126), and the downstream sensor (127) each indicating over-stoichiometric. [3] The method of claim 1, wherein operating the engine with the upstream, intermediate, and downstream exhaust gas sensors (125, 126, 127) each indicating lean of stoichiometric includes operating the engine with fuel injection into the stopped engine. [4] The method of claim 2, wherein adjusting engine operation includes commencing fuel injection and operating the engine with substoichiometric combustion. [5] The method of claim 4, wherein after the start of the fuel injection with substoichiometric combustion, which indicates a fault of the SCR catalyst (72), when a difference between the switching of the intermediate exhaust gas sensor (126) from lean to lean and the switching of the downstream exhaust gas sensor (127) from lean to lean is less than a threshold value. [6] The method of claim 5, further comprising indicating no fault if the difference between the intermediate exhaust gas sensor (126) switching from lean to stoichiometric and the downstream exhaust gas sensor (127) switching from lean to stoichiometric is greater than the threshold. [7] Method according to claim 5, wherein the switching from over-stoichiometric to under-stoichiometric of the intermediate exhaust gas sensor (126) takes place due to a penetration of reducing agents from the three-way catalyst (71). [8] The method of claim 5, wherein the switching from over-stoichiometric to under-stoichiometric of the downstream exhaust gas sensor (127) occurs due to the release of ammonia from the SCR catalyst (72). [9] Method comprising: Initiating a heat event to cause NH3 to be released from an SCR catalyst (72) prior to oxidizing the SCR catalyst (72); oxidizing the SCR catalyst (72) and a three-way catalyst (71) located upstream of the SCR catalyst (72) by terminating all engine fuel injection; after oxidizing the SCR catalyst (72) and the three-way catalyst (71), burning a substoichiometric engine air-fuel ratio; and indicating an SCR catalyst fault based on a time difference for an intermediate oxygen sensor (126) downstream of the three-way catalyst and for a downstream oxygen sensor downstream of the SCR catalyst (127) to switch from lean to lean. [10] The method of claim 9, further comprising, prior to initiating the heat event, producing a reductant in the three-way catalyst (71) by operating it with substoichiometric combustion, the reductant being stored in the SCR catalyst (72). [11] The method of claim 9, wherein a fault of the SCR catalyst (72) is indicated when the time difference between the switching of the downstream exhaust gas oxygen sensor (125) from lean to lean and the switching of the intermediate oxygen sensor (126) from lean to lean is less than a threshold value. [12] The method of claim 10, further comprising indicating no fault if the time difference between the downstream exhaust gas oxygen sensor (127) switching from lean to lean and the intermediate oxygen sensor (126) switching from lean to lean is greater than the threshold. [13] The method of claim 9, wherein the downstream oxygen sensor (125) switches from lean to stoichiometric when a threshold level of reductant is released from the SCR catalyst (72), and wherein the intermediate oxygen sensor (126) switches from lean to stoichiometric when a threshold level of reductant is released from the three-way catalyst (71). [14] System comprising: an engine coupled to an exhaust duct (48); a three-way catalyst (71) arranged in the exhaust passage (48); an SCR catalyst (72) arranged in the exhaust passage (48) downstream of the three-way catalyst (71); an upstream exhaust gas sensor (125) arranged upstream of the three-way catalyst (71), an intermediate exhaust gas sensor (126) arranged downstream of the three-way catalyst (71), and a downstream exhaust gas sensor (127) arranged downstream of the SCR catalyst (72); and a controller that has the following instructions: Operating the engine with the upstream, intermediate and downstream exhaust gas sensors (125, 126, 127) each indicating over-stoichiometric; adjusting engine operation to operate the engine with the upstream exhaust gas sensor (125) measuring substoichiometrically and the intermediate exhaust gas sensor (126) and the downstream exhaust gas sensor (127) each measuring overstoichiometrically; wherein the controller further comprises instructions for determining a Reductant storage capacity of the SCR catalyst (72) based on when the downstream exhaust gas sensor (127) switches from lean to stoichiometric; and Indicating a fault of the SCR catalyst (72) based on when the downstream exhaust gas sensor (127) switches from lean to stoichiometric. [15] The system of claim 14, wherein the controller includes instructions to terminate fuel injection into the engine to operate with the upstream, intermediate, and downstream exhaust gas sensors (125, 126, 127) each indicating lean of stoichiometric. [16] The system of claim 14, wherein the controller includes instructions for adjusting engine operation by resuming fuel injection and operating at substoichiometric combustion to operate the engine with the upstream exhaust gas sensor (125) indicating substoichiometric and the intermediate exhaust gas sensor (126) and downstream exhaust gas sensor (127) each indicating superstoichiometric. [17] The system of claim 14, wherein the controller includes instructions for indicating a fault of the SCR catalyst (72) when an amount of time from cessation of engine operation to switching of the downstream exhaust gas sensor (127) from lean to stoichiometric is greater than a threshold.

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