Exhaust diagnostic control system and method with NH3 depletion cleansing

An exhaust diagnostic control system comprises a test enabling module, an exhaust gas temperature management module, and an exhaust diagnostic control system. The test enabling module is configured for executing a process for depleting a reductant load and subsequently establishing a known concentration of reductant on an after-treatment component following an occurrence of one or more trigger events. The exhaust gas temperature management module is configured for selectively adjusting a temperature of the after-treatment component to a predetermined temperature range using intrusive exhaust gas temperature management. The component management module is configured for executing a NOx reduction efficiency test following completion of the process for depleting a reductant load and subsequently establishing a known concentration of reductant on the after-treatment component. The NOx reduction efficiency test comprises determining a NOx reduction efficiency associated with the after-treatment component.

FIELD OF THE INVENTION

The subject invention relates to vehicle exhaust systems, and more particularly to exhaust diagnostic and control systems and methods that evaluate and control performance of after-treatment components and processes.

BACKGROUND

During combustion in a diesel engine, an air/fuel mixture is delivered through an intake valve to cylinders and is compressed and combusted therein. After combustion, the pistons force the exhaust gas in the cylinders into an exhaust system. The exhaust gas may contain oxides of nitrogen (NOx) and carbon monoxide (CO).

Exhaust gas treatment systems may employ catalysts in one or more components configured for accomplishing an after-treatment process such as reducing nitrogen oxides (NOx) to produce more tolerable exhaust constituents of nitrogen (N2) and water (H2O). Reductant may be added to the exhaust gas upstream from an after-treatment component, such as a selective catalyst reduction (SCR) component, and, for example only, the reductant may include anhydrous ammonia (NH3), aqueous ammonia or urea, any or all of which may be injected as a fine mist into the exhaust gas. When the ammonia, mixed with exhaust gases, reaches the after-treatment component, the NOx emissions are broken down. A Diesel Particulate Filter (DPF) may then capture soot, and that soot may be periodically incinerated during regeneration cycles. Water vapor, nitrogen and reduced emissions exit the exhaust system.

To maintain efficient NOx reduction in the after-treatment component, a control may be employed so as to maintain a desired quantity of the reductant (i.e., reductant load) in the after-treatment component. As exhaust gas containing NOx passes through the after-treatment component, the reductant is consumed, and the load is depleted. A model may be employed by the control to track and/or predict how much reductant is loaded in the after-treatment component and to inject additional reductant as required so as to maintain an appropriate reductant load for achieving a desired effect such as reduction of NOx in the exhaust stream.

Service regeneration of the DPF is often conducted at elevated exhaust temperatures. Because of these increased temperatures, a flow of reductant through the injector(s) may be maintained so as to prevent thermal damage of the injector. Unfortunately, it can be difficult to predict how much of the reductant injected for such purposes is oxidized or otherwise consumed in the after-treatment component and how much may have survived and accumulated so as to contribute to the loading of the after-treatment component.

As a consequence, model estimates of ammonia load may be inaccurate, and may thus be rendered unreliable. In particular, experience has shown that following the occurrence of certain events, such as a DPF service regeneration event, load estimates based on models may deviate substantially from observed levels of NH3 load on the after-treatment component. Hence, diagnostic processes based on measurement and evaluation of NOx reduction efficiencies in the after-treatment component may produce erroneous results such as where more reductant is actually loaded on the after-treatment component than the diagnostic system assumes based on the inaccuracies in the model. In such situations, NH3 “slip” can occur wherein NH3 is interpreted by downstream sensors as NOx. Such errors are particularly problematic when using sensors that are cross-sensitive to both NOx and NH3. Similarly, where an actual NH3 load is substantially lower than the model estimate, the incorrect NH3 load can cause a worse than expected NOx reduction efficiency to be assessed by the diagnostic system, potentially resulting in an incorrect diagnosis and invocation of remedial measures to be taken.

Accordingly, it is desirable to provide a system and method for more accurately predicting a quantity of reductant (i.e., the reductant load) present on after-treatment components and for managing the operations through which NOx are reduced in such after-treatment components with improved reliability following one or more trigger events.

SUMMARY OF THE INVENTION

In one exemplary embodiment of the invention, an exhaust diagnostic control system comprises a test enabling module, an exhaust gas temperature management module, and an exhaust diagnostic control system. The test enabling module is configured for executing a process for depleting a reductant load and subsequently establishing a known concentration of reductant on an after-treatment component following an occurrence of one or more trigger event. The exhaust gas temperature management module is configured for selectively adjusting a temperature of the after-treatment component to a predetermined temperature range using intrusive exhaust gas temperature management. The component management module is configured for executing a NOx reduction efficiency test following completion of the process for depleting a reductant load and subsequently establishing a known concentration of reductant on the after-treatment component. The NOx reduction efficiency test comprises determining a NOx reduction efficiency associated with the after-treatment component.

In another exemplary embodiment of the invention, a method for diagnosing an exhaust system comprises depleting a reductant load on an after-treatment component following an occurrence of one or more trigger event and subsequently establishing a known concentration of reductant on the after-treatment component. The method also comprises selectively adjusting a temperature of the after-treatment component to a predetermined temperature range using intrusive exhaust gas temperature management and executing a NOx reduction efficiency test comprising determining a NOx reduction efficiency associated with an after-treatment component.

DESCRIPTION OF THE EMBODIMENTS

As used herein, the term “module” refers to an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality.

While the following disclosure involves diesel engines, other types of engines such as gasoline engines, including direct injection engines, may benefit from the teachings herein.

In accordance with an exemplary embodiment of the invention, the present disclosure provides a system and method for resetting an exhaust diagnostic control system of a vehicle upon the occurrence of one or more trigger events or criteria. The control may be reset automatically or via an intrusive service test that may be initiated by a service test tool. A trigger event may comprise an assessment that a quality of a reductant exhibits inadequate quality, an occurrence of a recent service regeneration event, an indication of an elevated risk of an uncontrolled or unknown ammonia load on an after-treatment component, a determination that the ammonia load model may be inaccurate, or other events or criteria suggesting the need for testing the effectiveness of an after-treatment component, or the system as a whole, at reducing targeted constituents such as NOx.

For example, rather than waiting to see a NOx deviation before triggering a system reset, in an exemplary embodiment, reductant may be purged as soon as an elevated risk is identified of a potential future NOx deviation. Accordingly, rather than waiting for confirmation of such a NOx deviation, the after-treatment component may proactively be purged of ammonia and then reloaded with a known quantity of ammonia (reductant) so as to improve the reliability of control systems, which depend upon reliable knowledge of, and control over, the loading of ammonia on the after-treatment component. Thus, to improve the accuracy and/or reliability of reductant load predictions, an exemplary service procedure begins by re-calibrating the model responsible for creating those predictions. The service procedure accomplishes the calibration by establishing a known reductant load in or on the after-treatment component. In some embodiments, the service procedure achieves the calibration by executing a service regeneration test that is effective to reliably consume any reductant load in the after-treatment component.

Thus, upon the occurrence of criteria indicating that a sufficiently substantial risk exists that an actual reductant load in an after-treatment component fails to match a load predicted by a load model, such as may occur following detection/assessment of poor urea quality, an actual and reliably knowable reductant load is re-established in the after-treatment component, resulting in improved model accuracy. More specifically, after the service regeneration test, and provided that injector cooling via continuing injection of reductant has not occurred, the reductant load can reliably be assumed to be within an acceptable tolerance of a known level, e.g., zero. With the reductant load established, the model may be calibrated or otherwise re-set so that its prediction for reductant load matches the known level. With the model having been re-calibrated, normal control functions can be executed with improved accuracy and reliability.

More specifically, in an exemplary embodiment, upon the occurrence of one or more triggering criteria, the reductant load on the after-treatment component is intentionally depleted using reliable means such as execution of a regeneration event, so that the reductant load may reliably be at or below a pre-established threshold. This reductant-depleting, SCR-cleansing process may be performed as an initial step in the re-calibration of the load model. In a non-limiting exemplary embodiment, a cleansing process for an after-treatment component includes commanding dosing off until the load of NH3 or another reductant has been sufficiently depleted from the after-treatment component to a level below a predetermined threshold. An algorithm may be employed to evaluate the degree to which NOx are reduced in the after-treatment component so as to verify the extent to which reductant has been depleted.

The sufficient depletion of reductant can be confirmed by comparing information signals produced by the upstream and downstream NOx sensors so as to verify that any detectable difference between their outputs is within an acceptable level. In addition, or in the alternative, an algorithm based on empirical experience may be used to determine the extent of the depletion. For example, for a particular system, operation of the system may have been sufficiently characterized that rates of reductant consumption may be understood for certain operating conditions. Accordingly, the procedure provides assurance that the load of reductant is at a known level. After the NH3 load has been depleted, normal dosing can be commenced to re-establish a known (i.e., reliably predictable by the NH3 load model) NH3 load on the NOx catalyst.

As described above, the exhaust diagnostic control system according to the present disclosure first depletes the reductant load on the after-treatment component until the load is below a preset threshold. This may be accomplished by commanding, or maintaining, dosing of reductant at a level below a predetermined level until the reductant load has been sufficiently depleted from the after-treatment component. This predetermined level may be completely off, as described above, or may be set at one or more levels configured to result in depletion of reductant on the after-treatment component such as at levels less than the rate at which reductant is consumed in the SCR. It should be noted that it may be impractical to command dosing to be completely off. For example, in some situations, it is necessary to dose the exhaust stream with reductant so as to cool the reductant injection nozzles. Once the reductant has been sufficiently depleted, a condition that may be verified by an indication from the NOx sensors and/or from the model-predicted consumption of reductant in the SCR, the load of reductant will be at a reliably-knowable level, at or near zero.

As soon as the sufficient depletion of reductant has been confirmed, such as by comparing information signals produced by the upstream and downstream NOx sensors and verifying that any detectable difference between their outputs is within an acceptable level and/or by observing an indication from the load model that the after-treatment component is unloaded. The reductant load may reliably be considered to have been depleted, and normal dosing can be commenced to re-establish a known (i.e., reliably predictable by the reductant load model) load of reductant on the after-treatment catalyst.

Referring now toFIG. 1, a diesel engine system10is schematically illustrated. The diesel engine system10includes a diesel engine12and an exhaust treatment system13. The exhaust treatment system13further includes an exhaust system14and a dosing system16. The diesel engine12includes a cylinder18, an intake manifold20, a mass air flow (MAF) sensor22and an engine speed sensor24. Air flows into the diesel engine12through the intake manifold20and is monitored by the MAF sensor22. The air is directed into the cylinder18and is combusted with fuel to drive pistons (not shown). Although a single cylinder18is illustrated, it can be appreciated that the diesel engine12may include additional cylinders18. For example, diesel engines having 2, 3, 4, 5, 6, 8, 10, 12 and 16 cylinders are anticipated.

Exhaust gas is produced inside the cylinder18as a result of the combustion process. The exhaust system14treats the exhaust gas before the exhaust gas is released to atmosphere. The exhaust system14includes an exhaust manifold26and a diesel oxidation catalyst (DOC)28. The exhaust manifold26directs exhaust exiting the cylinder through the DOC28. The exhaust is treated within the DOC28to reduce the emissions. The exhaust system14further includes an after-treatment component30, a temperature sensor31, an inlet temperature sensor32, an outlet temperature sensor34and a particulate filter (PF)36. In an exemplary embodiment, after-treatment component30is a selective catalyst reduction (SCR) component.

The temperature sensor31may be positioned between the engine and the DOC18. The inlet temperature sensor32is located upstream from the after-treatment component30to monitor the temperature change at the inlet of the after-treatment component30. The outlet temperature sensor34is located downstream from the after-treatment component30to monitor the temperature change at the outlet of the after-treatment component30. Although the exhaust treatment system13is illustrated as including the inlet and outlet temperature sensors32,34arranged outside the after-treatment component30, the inlet and outlet temperature sensors32,34can be located inside the after-treatment component30to monitor the temperature change of the exhaust at the inlet and outlet of the after-treatment component30. The PF36further reduces emissions by trapping particulates (i.e., soot) in the exhaust gas.

The dosing system16includes a dosing injector40that injects reductant from a reductant supply38into the exhaust gas. The reductant mixes with the exhaust gas and further reduces the emissions when the mixture is exposed to the after-treatment component30. A mixer41may be used to mix the reductant with the exhaust gas upstream from the after-treatment component30. A control module42regulates and controls the operation of the engine system10.

An exhaust gas flow rate sensor44may generate a signal corresponding to the flow of exhaust in the exhaust system. Although the sensor is illustrated between the after-treatment component30and the PF36, various other locations within the exhaust system may be used for measurement including downstream from the exhaust manifold and upstream from the after-treatment component30. A temperature sensor46generates a particulate filter temperature corresponding to a measured particulate filter temperature. The temperature sensor46may be disposed on or within the PF36. The temperature sensor46may also be located upstream or downstream from the PF36.

Other sensors in the exhaust system may include an upstream NOx sensor50that generates a NOx signal based on a concentration of NOx present in the exhaust system. A downstream NOx sensor52may be positioned downstream from the PF36to measure a concentration of NOx leaving the PF36. In addition, an ammonia (NH3) sensor54generates a signal corresponding to the amount of ammonia within the exhaust gas. The NH3 sensor54is optional, but can be used to simplify the control system due to the ability to discern between NOx and NH3. Alternately and/or in addition, a hydrocarbon (HC) supply56and a HC injector58may be provided to supply HC in the exhaust gas reaching the DOC catalyst.

Referring now toFIG. 2, the control module42may include a component management module60that is used to monitor performance (e.g., conversion efficiency of NOx) of an after-treatment component30and/or to facilitate control over operation of the after-treatment component30. The control module42further includes an exhaust gas temperature management module62that intrusively controls a temperature of the after-treatment component30.

The component management module60includes a reset module70and a test initiation module72. As used herein, the term intrusive means that the control module42varies the control of the engine outside of the operating conditions to allow the test to occur. The test initiation module72initiates an intrusive NOx reduction efficiency test in the after-treatment component after the occurrence of a trigger event, such as a recent failure of a prior NOx reduction efficiency test or the passage of a prescribed period of time or another milestone, and/or the undertaking of other remedial action.

The intrusive test initiation module72sends a signal to the exhaust gas temperature management module62to initiate intrusive temperature control of the after-treatment component prior to a NOx reduction efficiency test in the after-treatment component. A test enabling module74ensures that enable conditions are met prior to initiation of testing or control functions.

The exhaust gas temperature management module62includes an after-treatment component temperature calculating module76that calculates a temperature of the after-treatment component. The temperature calculating module76may calculate the temperature of the after-treatment component based on the inlet temperature sensor32, the outlet temperature sensor34, a model or any other suitable method. For example only, the temperature calculating module76may calculate the temperature of the after-treatment component based on values from both the inlet and outlet temperature sensors32,34. For example only, the temperature calculating module76may calculate the temperature based on an average or a weighted average of the inlet and outlet temperature sensors32,34.

The control module42, the component management module60and/or the exhaust gas temperature management module62may include an operating parameter adjustment module78that adjusts other operating parameters prior to the intrusive NOx reduction efficiency test. For example, other operating parameters such as dosing, reductant load, EGR, and/or other conditions may also be adjusted within corresponding windows prior to the intrusive NOx reduction efficiency test.

The control module42includes a vehicle speed limiting module80that limits vehicle speed after the NOx reduction efficiency falls below a predetermined threshold. The control module42further includes a fueling control module82that determines fuel quantity, fuel injection timing, post injection, etc. When in the intrusive NOx reduction efficiency test mode, the exhaust gas temperature management module62adjusts fueling. The fueling adjustment increases a temperature of the after-treatment component. Alternately, a hydrocarbon injection module84injects fuel into the exhaust upstream from the DOC catalyst28to generate an exotherm to increase the temperature in the after-treatment component.

Thus, in an exemplary embodiment, an exhaust diagnostic control system68comprises a test enabling module74, an exhaust gas temperature management module62, and an exhaust diagnostic control system68. The test enabling module74is configured for executing a process for depleting a reductant load and subsequently establishing a known concentration of reductant on an after-treatment component30following an occurrence of one or more trigger events. The exhaust gas temperature management module62is configured for selectively adjusting a temperature of the after-treatment component30to a predetermined temperature range using intrusive exhaust gas temperature management. The component management module60is configured for executing a NOx reduction efficiency test following completion of the process for depleting a reductant load and subsequently establishing a known concentration of reductant on the after-treatment component30. The NOx reduction efficiency test comprises determining a NOx reduction efficiency associated with the after-treatment component30.

The trigger event may comprise detection of an inadequate NOx reduction efficiency associated with the after-treatment component30, an instance of a service regeneration event having occurred in the recent past, an assessment of an elevated risk of an uncontrolled or unknown quantity of ammonia loaded on the after-treatment component30, or a sensed deviation between sensed NOx concentration in the exhaust stream and predicted NOx concentration in the exhaust stream.

The process for depleting a reductant load may comprise executing a regeneration event. The process for depleting a reductant load may also comprise commanding dosing off until the load of reductant has been depleted from the after-treatment component30to a level below a predetermined threshold.

The test enabling module74may be configured for executing a process for evaluating the extent to which NOx are reduced in the after-treatment component30. The test enabling module74may be configured for executing a process for evaluating the extent to which reductant has been depleted from the after-treatment component30or for executing a process for comparing an information signal reflecting a NOx concentration upstream from the after-treatment component30to a NOx concentration downstream from the after-treatment component30and determining whether the difference between the NOx concentration upstream from the after-treatment component30and the NOx concentration downstream from the after-treatment component30is less than or equal to a predetermined limit.

Referring now toFIG. 3, control begins at100where it is determined whether an intrusive NOx reduction efficiency test, or another component diagnostic test, needs to be executed based on satisfaction of one or more prescribed conditions. For example only, the intrusive after-treatment component diagnostic test (which may also be used to deduce reductant quality) may be executed after the vehicle is placed in a speed-limited mode and/or other remedial action is taken following a failure to pass a prior NOx reduction efficiency test.

If100is false, control proceeds in a normal mode at102. If100is true, control continues at104and determines whether a first set of conditions are acceptable to run the test. For example only, the first set of conditions may include ensuring that regeneration of the PF36is not being performed. PF regeneration is typically performed when soot builds up in the PF36. Additionally, the first set of conditions may include ensuring that adaptation is not being performed. Adaptation occurs when there is a problem with the after-treatment component such that a difference between a downstream NOx sensor measurement and an expected NOx level based on a model exceeds a predetermined tolerance level. Still other conditions may be used in the first set of conditions instead of, or in addition to, these conditions.

If104is false, control returns to100. If104is true, control continues at106and optionally disables exhaust gas recirculation (EGR). At107, control activates a process for depleting a reductant load to establish a reliable reductant load on the after-treatment component. The process for depleting a reductant load includes commanding dosing at a reduced level (e.g., off) until the reductant load has been sufficiently depleted from the after-treatment component (i.e., the algorithm determines that the reductant load on the after-treatment component has been depleted to a level less than a predetermined threshold). Optionally, a regeneration test may be initiated so as to more quickly deplete the reductant load. The sufficient depletion of reductant can be confirmed by comparing information signals produced by the upstream and downstream NOx sensors so as to verify that any detectable difference between their outputs is within an acceptable level. In addition, the load model can be observed so as to ensure that it indicates that the after-treatment component is unloaded. In an exemplary embodiment, unloading may take up to 30 minutes. After the reductant load has been depleted, and/or depletion is predicted or confirmed, dosing can be re-commenced to re-establish a known (i.e., reliably predictable by the reductant load model) load on the after-treatment component. Thereafter, normal dosing may be resumed.

At108, control activates an intrusive NOx reduction efficiency test to achieve a predetermined temperature range for the after-treatment component. Control also turns dosing on at108. At112, control determines whether there is a sufficient reductant load on the after-treatment component (i.e., the catalyst)30. A time delay may be used to ensure that the sufficient reductant load has been re-established to provide acceptable NOx conversion.

If112is false, control waits until there is a sufficient reductant load on the after-treatment component. At114, control determines whether a second set of enable conditions have been met. For example only, the second set of enable conditions may include one or more of the following conditions: exhaust flow within a predetermined range; upstream NOx mass flow within a predetermined range; upstream NOx concentration within a predetermined range and/or NOx sensors ready. Still other conditions may be included in the second set of enable conditions.

At118, control measures an efficiency of the NOx conversion/reduction process in the after-treatment component. At120, control generates an efficiency of the NOx reduction (i.e., conversion) process as a function of upstream and downstream accumulated masses. At124, control generates an efficiency threshold as a function of upstream NOx and after-treatment component temperature. The efficiency threshold may be expressed as a percentage.

At128, control determines whether the efficiency of the NOx conversion process is greater than or equal to the efficiency threshold. If128is true, control declares an approval status (which may be interpreted as signaling acceptable reductant quality and/or after-treatment component operation based on acceptable NOx reduction efficiency) at130. If128is false, control declares an unacceptable condition (which may be interpreted as an unacceptable reductant quality and/or an unacceptable after-treatment component operation based on an unacceptable NOx reduction efficiency) at132. In the event of a declaration of an unacceptable condition, remedial measures may be undertaken such as illumination of a warning light or initiation of modifications to the manner in which the after-treatment component and/or the engine and/or the vehicle is operated. Upon declaration of an approval status, control continues from130with134and disables whichever failure mode may have caused the initiation of the intrusive test. For example, the vehicle speed limiting mode and/or other remedial measures are ended. Control continues from132and134with140where control ends intrusive exhaust gas temperature management and enables EGR (if previously disabled).

Referring now toFIG. 4, an intrusive exhaust gas temperature management method is shown. At146, control determines whether the intrusive NOx reduction efficiency test is running. If146is false, control returns to146. If146is true, control continues at148where control determines whether the after-treatment component temperature is within a predetermined temperature range (for example, between a minimum temperature TLo and a maximum temperature THi).

If148is true, control returns to146. If148is false, control determines whether the after-treatment component temperature is greater than the minimum temperature TLo at152. If152is false, control increases the exhaust temperature in any suitable manner. For example, the exhaust temperature can be increased by altering fueling (fuel quantity, fuel injection timing, post injection, etc.) and/or by starting or increasing HC injection at154. Control returns to146.

If148is false, control determines whether the after-treatment component temperature is less than the maximum temperature THi at156. If156is false, control decreases the exhaust temperature in any suitable manner. For example, the exhaust temperature can be decreased by altering fueling (fuel quantity, fuel injection timing, post injection, etc.) and/or by stopping or decreasing HC injection at158. Control returns to146.

Thus, a method for diagnosing an exhaust system comprises depleting a reductant load on an after-treatment component following an occurrence of one or more trigger events and subsequently establishing a known concentration of reductant on the after-treatment component (step107). The method also comprises selectively adjusting a temperature of the after-treatment component to a predetermined temperature range using intrusive exhaust gas temperature management (step108) and executing a NOx reduction efficiency test comprising determining a NOx reduction efficiency associated with an after-treatment component (step118). Exhaust gas temperature can be controlled, for example, by adjusting levels of fuel in the exhaust gas. The process for depleting a reductant load (step107) may comprise executing a regeneration event and/or may comprise commanding dosing off until the load of reductant has been depleted from the after-treatment component to a level below a predetermined threshold.

The method may also comprise evaluating the extent to which NOx are reduced in the after-treatment component and/or evaluating the extent to which reductant has been depleted from the after-treatment component (step118). The method may also comprise comparing an information signal reflecting a NOx concentration upstream from the after-treatment component to a NOx concentration downstream from the after-treatment component and determining whether the difference between the NOx concentration upstream from the after-treatment component and the NOx concentration downstream from the after-treatment component is less than or equal to a predetermined limit.

By completely, or nearly completely, depleting the load of NH3 of the after-treatment component, and subsequently re-establishing a reliably knowable load of NH3 on the after-treatment component, the control can reliably ensure that the NH3 load estimate is accurate and can ensure that after-treatment diagnostic tests are performed at times and under conditions that facilitate reliable knowledge of the NH3 load present on the after-treatment catalyst. This increases the robustness of the after-treatment efficiency diagnostic and avoids the inappropriate initiation of unnecessary remedial measures such as false warning light illumination or overzealous DEF Quality Inducement on vehicles after a DPF Service Regeneration process has been performed. As a result, better control over emission components, engine systems, and vehicles may be enabled, and customer satisfaction may be improved, warranty costs may be reduced, and confusion may be reduced.