NOX SENSOR DIAGNOSTICS IN HEAVY-DUTY MOTOR VEHICLE ENGINES

Abstract

A heavy-duty truck has a diesel engine, an exhaust after-treatment system, and an engine control unit. The exhaust after-treatment system includes a close-coupled selective catalytic reduction system and an underbody selective catalytic reduction system, a first NO.sub.x sensor upstream of the close-coupled selective catalytic reduction system, a second NO.sub.x sensor between the two selective catalytic reduction systems, and a third NO.sub.x sensor downstream of the underbody selective catalytic reduction system. The engine control unit may perform methods allowing intrusive diagnostics to be performed on exhaust gas NO.sub.x sensors using the selective catalytic reduction systems during normal operation of the heavy-duty truck.

Claims

1. A method, comprising: operating a diesel engine of a heavy-duty truck such that the diesel engine generates an exhaust gas flow and the exhaust gas flow flows into an exhaust after-treatment system of the heavy-duty truck, the exhaust after-treatment system including a close-coupled selective catalytic reduction system and an underbody selective catalytic reduction system downstream of the close-coupled selective catalytic reduction system with respect to the exhaust gas flow; controlling a DEF injector upstream of the close-coupled selective catalytic reduction system to increase a rate at which the DEF injector injects DEF into the exhaust gas flow; measuring a first level of NO.sub.x gases with a first NO.sub.x sensor upstream of the close-coupled selective catalytic reduction system and a second level of NO.sub.x gases with a second NO.sub.x sensor downstream of the close-coupled selective catalytic reduction system as the rate at which the DEF injector injects DEF increases; and using the measurements to determine a level of error in measurements provided by the first NO.sub.x sensor.

2. The method of claim 1, further comprising controlling NO.sub.x tailpipe emissions using the underbody selective catalytic reduction system.

3. The method of claim 1, further comprising adjusting future measurements provided by the first NO.sub.x sensor based on the determined level of error.

4. The method of claim 1 wherein operating the diesel engine includes controlling the diesel engine such that the exhaust gas flow has a constant flow rate, a constant level of NO.sub.x gases, and a constant temperature as it enters the exhaust after-treatment system.

5. The method of claim 1 wherein determining the level of error includes: determining measured ammonia-to-NO.sub.x ratios in the close-coupled selective catalytic reduction system as the rate at which the DEF injector injects DEF increases based on the measurements provided by the first NO.sub.x sensor; and determining measured percentage efficiencies of the close-coupled selective catalytic reduction system as the rate at which the DEF injector injects DEF increases based on the measurements provided by the first and second NO.sub.x sensors.

6. The method of claim 5 wherein determining the level of error further includes determining a difference between a measured ammonia-to-NO.sub.x ratio at a peak in the measured percentage efficiencies and a known ammonia-to-NO.sub.x ratio at a peak in efficiency of the close-coupled selective catalytic reduction system.

7. The method of claim 1, further comprising: while operating the diesel engine, operating the exhaust after-treatment system to eliminate ammonia present in the close-coupled selective catalytic reduction system; after operating the exhaust after-treatment system to eliminate ammonia present in the close-coupled selective catalytic reduction system, controlling the diesel engine such that the exhaust gas flow has a variable level of NO.sub.x gases as it enters the exhaust after-treatment system; measuring NO.sub.x gases with the first NO.sub.x sensor and the second NO.sub.x sensor as the level of NO.sub.x gases in the exhaust gas flow as it enters the exhaust after-treatment system varies; and using the measurements taken as the level of NO.sub.x gases in the exhaust gas flow as it enters the exhaust after-treatment system varies to determine a level of error in measurements provided by the second NO.sub.x sensor.

8. The method of claim 7, further comprising adjusting future measurements provided by the second NO.sub.x sensor based on the determined level of error in measurements provided by the second NO.sub.x sensor.

9. The method of claim 7 wherein operating the exhaust after-treatment system of the heavy-duty truck to eliminate ammonia present in the close-coupled selective catalytic reduction system includes controlling the DEF injector to stop injecting DEF into the exhaust after-treatment system upstream of the close-coupled selective catalytic reduction system with respect to the exhaust gas flow.

10. The method of claim 7 wherein controlling the diesel engine such that the exhaust gas flow has a variable level of NO.sub.x gases as it enters the exhaust after-treatment system includes adjusting an injection timing of the diesel engine.

11. The method of claim 7 wherein controlling the diesel engine such that the exhaust gas flow has a variable level of NO.sub.x gases as it enters the exhaust after-treatment system includes adjusting an exhaust gas recirculation through the diesel engine.

12. The method of claim 7 wherein determining a level of error in measurements provided by the second NO.sub.x sensor includes performing a linear regression using the measurements taken by the first and second NO.sub.x sensors as the level of NO.sub.x gases in the exhaust gas flow as it enters the exhaust after-treatment system varies.

13. A method, comprising: operating a diesel engine of a heavy-duty truck such that the diesel engine generates an exhaust gas flow and the exhaust gas flow flows into an exhaust after-treatment system including a close-coupled selective catalytic reduction system and an underbody selective catalytic reduction system downstream of the close-coupled selective catalytic reduction system with respect to the exhaust gas flow; controlling a DEF injector downstream of the close-coupled selective catalytic reduction system and upstream of the underbody selective catalytic reduction system to increase a rate at which the DEF injector injects DEF into the exhaust gas flow; measuring a first level of NO.sub.x gases with a first NO.sub.x sensor downstream of the close-coupled selective catalytic reduction system and upstream of the underbody selective catalytic reduction system and a second level of NO.sub.x gases with a second NO.sub.x sensor downstream of the underbody selective catalytic reduction system as the rate at which the DEF injector injects DEF increases; and using the measurements to determine a level of error in measurements provided by the first NO.sub.x sensor.

14. The method of claim 13, further comprising controlling NO.sub.x tailpipe emissions using the close-coupled selective catalytic reduction system.

15. The method of claim 13 wherein determining the level of error includes: determining measured ammonia-to-NO.sub.x ratios in the underbody selective catalytic reduction system as the rate at which the DEF injector injects DEF increases based on the measurements provided by the first NO.sub.x sensor; and determining measured percentage efficiencies of the underbody selective catalytic reduction system as the rate at which the DEF injector injects DEF increases based on the measurements provided by the first and second NO.sub.x sensors.

16. The method of claim 15 wherein determining the level of error further includes determining a difference between a measured ammonia-to-NO.sub.x ratio at a peak in the measured percentage efficiencies and a known ammonia-to-NO.sub.x ratio at a peak in efficiency of the underbody selective catalytic reduction system.

17. The method of claim 13, further comprising: while operating the diesel engine, operating the exhaust after-treatment system to eliminate ammonia present in the underbody selective catalytic reduction system; after operating the exhaust after-treatment system to eliminate ammonia present in the underbody selective catalytic reduction system, controlling the diesel engine and the close-coupled selective catalytic reduction system such that the exhaust gas flow has a variable level of NO.sub.x gases as it enters the underbody selective catalytic reduction system; measuring NO.sub.x gases with the first NO.sub.x sensor and the second NO.sub.x sensor as the level of NO.sub.x gases in the exhaust gas flow as it enters the underbody selective catalytic reduction system varies; and using the measurements taken as the level of NO.sub.x gases in the exhaust gas flow as it enters the underbody selective catalytic reductions system varies to determine a level of error in measurements provided by the second NO.sub.x sensor.

18. The method of claim 17 wherein determining a level of error in measurements provided by the second NO.sub.x sensor includes performing a linear regression using the measurements taken by the first and second NO.sub.x sensors as the level of NO.sub.x gases in the exhaust gas flow as it enters the underbody selective catalytic reduction system varies.

19. A heavy-duty truck, comprising: a diesel engine; an exhaust after-treatment system having an upstream end and a downstream end opposite the upstream end, the upstream end coupled to the diesel engine, the exhaust after-treatment system including: a first NO.sub.x sensor; a close-coupled selective catalytic reduction system downstream of the first NO.sub.x sensor; a second NO.sub.x sensor downstream of the close-coupled selective catalytic reduction system; an underbody selective catalytic reduction system downstream of the second NO.sub.x sensor; and a third NO.sub.x sensor downstream of the underbody selective catalytic reduction system.

20. The heavy-duty truck of claim 19, further comprising an engine control unit configured to: operate the diesel engine such that the diesel engine generates an exhaust gas flow; control the diesel engine such that the exhaust gas flow has a constant level of NO.sub.x gases as it enters the exhaust after-treatment system; control a DEF injector upstream of the close-coupled selective catalytic reduction system to increase a rate at which the DEF injector injects DEF into the exhaust gas flow; measure a first level of NO.sub.x gases with the first NO.sub.x sensor and a second level of NO.sub.x gases with the second NO.sub.x sensor as the rate at which the DEF injector injects DEF increases; and use the measurements to determine a level of error in measurements provided by the first NO.sub.x sensor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 illustrates a diagram of an exhaust after-treatment system including a DOC, a DPF, and dual SCR systems.

[0014] FIG. 2 illustrates performance differences between two NO.sub.x sensors in the exhaust after-treatment system of FIG. 1.

[0015] FIG. 3 illustrates additional performance differences between two NO.sub.x sensors in the exhaust after-treatment system of FIG. 1.

[0016] FIG. 4 illustrates performance of a NO.sub.x sensor in the exhaust after-treatment system of FIG. 1.

[0017] FIG. 5 illustrates a flow chart of a method of using the systems described herein.

DETAILED DESCRIPTION

[0018] In the following description, certain specific details are set forth in order to provide a thorough understanding of various disclosed embodiments. However, one skilled in the relevant art will recognize that embodiments may be practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures associated with the technology have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments.

[0019] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. Also, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the context clearly dictates otherwise.

[0020] The use of ordinals such as first, second and third does not necessarily imply a ranked sense of order, but rather may only distinguish between multiple instances of an act or structure.

[0021] Terms of geometric alignment may be used herein. Any components of the embodiments that are illustrated, described, or claimed herein as being aligned, arranged in the same direction, parallel, or having other similar geometric relationships with respect to one another have such relationships in the illustrated, described, or claimed embodiments. In alternative embodiments, however, such components can have any of the other similar geometric properties described herein indicating alignment with respect to one another. Any components of the embodiments that are illustrated, described, or claimed herein as being not aligned, arranged in different directions, not parallel, perpendicular, transverse, or having other similar geometric relationships with respect to one another, have such relationships in the illustrated, described, or claimed embodiments. In alternative embodiments, however, such components can have any of the other similar geometric properties described herein indicating non-alignment with respect to one another.

[0022] Various examples of suitable dimensions of components and other numerical values may be provided herein. In the illustrated, described, and claimed embodiments, such dimensions are accurate to within standard manufacturing tolerances unless stated otherwise. Such dimensions are examples, however, and can be modified to produce variations of the components and systems described herein. In various alternative embodiments, such dimensions and any other specific numerical values provided herein can be approximations wherein the actual numerical values can vary by up to 1, 2, 5, 10, 15 or more percent from the stated, approximate dimensions or other numerical values.

[0023] Traditionally, heavy-duty vehicles included many components of exhaust after-treatment systems “underbody,” that is, underneath the engine, cab, or another portion of the vehicle, where space is relatively freely available and these components can therefore generally be larger than would otherwise be practical. Some modern heavy-duty vehicles, however, have begun to include a “close-coupled,” “up-close,” or “light-off” SCR unit much closer to the engine and exhaust ports thereof (e.g., adjacent to a turbine outlet of a turbocharger) and upstream of the traditional underbody exhaust after-treatment system, which can provide certain advantages in that the temperature of the engine exhaust may be higher when it is closer to the engine, although locating an SCR unit nearer the engine limits the available space and thus its practical size. Thus, some modern heavy-duty vehicles have included both a “close-coupled” SCR unit upstream with respect to the flow of the exhaust, such as adjacent to a turbine outlet of a turbocharger, to take advantage of the higher exhaust temperatures, as well as an “underbody” SCR unit downstream with respect to the flow of the exhaust, such as under the engine or cab of the vehicle, to take advantage of the greater available space.

[0024] FIG. 1 illustrates a diagram of an exhaust after-treatment system 100 that has a first, upstream end 102 and a second, downstream end 104 opposite to the first, upstream end 102. The exhaust after-treatment system 100 is a component of a vehicle, such as a large, heavy-duty, diesel truck, and in use carries exhaust from the diesel engine of the truck to a tailpipe of the truck. For example, the first, upstream end 102 of the exhaust after-treatment system 100 may be coupled directly to an exhaust port or an outlet port of the diesel engine, such as a turbine outlet of a turbocharger thereof, and the second, downstream end 104 may be coupled directly to an inlet port of a tailpipe or muffler of the truck. Thus, when the engine is running and generating exhaust, the exhaust travels along the length of the exhaust after-treatment system 100 from the first, upstream end 102 thereof to the second, downstream end 104 thereof.

[0025] As illustrated in FIG. 1, the exhaust after-treatment system 100 includes, at its first, upstream end 102, or proximate or adjacent thereto, a first temperature sensor 106, which may be a thermocouple, to measure the temperature of the exhaust gas flow as it leaves the engine and enters the exhaust after-treatment system 100, before heat begins to be lost through the exhaust after-treatment system 100 to the environment. The exhaust after-treatment system 100 also includes, at its first, upstream end 102, or proximate or adjacent thereto, or just downstream of the first temperature sensor 106, a first NO.sub.x sensor 108, to measure the content of NO.sub.x gases in the exhaust gas flow as it leaves the engine and enters the exhaust after-treatment system 100. The exhaust after-treatment system 100 also includes, at its first, upstream end 102, or proximate or adjacent thereto, or just downstream of the first NO.sub.x sensor 108, a first DEF injector 110, to inject DEF into the exhaust gas flow as it leaves the engine and enters the exhaust after-treatment system 100.

[0026] The exhaust after-treatment system 100 also includes, proximate or adjacent its first, upstream end 102, or just downstream of the first DEF injector 110, a heater 112, which may be an electrically-powered resistive heater or heating element, a burner, or any other suitable heater, to inject heat energy into the exhaust gas flow and the injected DEF as they flow through the exhaust after-treatment system 100. The exhaust after-treatment system 100 also includes, just downstream of the heater 112, a second temperature sensor 114, which may be a thermocouple, to measure the temperature of the exhaust gas flow as it leaves the heater 112 and just before or just as it enters a first, close-coupled SCR system 116, or at the inlet to the close-coupled SCR system 116. The exhaust after-treatment system 100 also includes, just downstream of the heater 112 and the second temperature sensor 114, the first, close-coupled SCR system 116, which is configured to reduce oxides of nitrogen (NO.sub.x) in the exhaust gas flow.

[0027] The exhaust after-treatment system 100 also includes, just downstream of the first SCR system 116, a second NO.sub.x sensor 118, to measure the content of NO.sub.x gases in the exhaust gas flow as it leaves the first SCR system 116. In practice, the first NO.sub.x sensor 108 and the second NO.sub.x sensor 118 can be used together to monitor, assess, or measure the performance of the first SCR system 116. Together, the first temperature sensor 106, the first NO.sub.x sensor 108, the first DEF injector 110, the heater 112, the second temperature sensor 114, the first, close-coupled SCR system 116, and the second NO.sub.x sensor 118 can be referred to as a close-coupled portion of the exhaust after-treatment system 100, as they can be collectively located at or adjacent to the engine of the vehicle.

[0028] The exhaust after-treatment system 100 also includes, downstream of the first SCR system 116 and the second NO.sub.x sensor 118, a DOC component 120, to oxidize unburned fuel and carbon monoxide in the exhaust gas flow. The exhaust after-treatment system 100 also includes, downstream of the DOC component 120, a DPF 122, to reduce or otherwise control particulate matter in the exhaust gas flow. The exhaust after-treatment system 100 also includes, downstream of the DPF 122, a second DEF injector 124, to inject DEF into the exhaust gas flow as it leaves the DPF 122. The exhaust after-treatment system 100 also includes, downstream of the second DEF injector 124, a second, underbody SCR system 126, which is configured to reduce oxides of nitrogen (NO.sub.x) in the exhaust gas flow. The exhaust after-treatment system 100 also includes, just downstream of the second SCR system 126 and at its second, downstream end 104, or proximate or adjacent thereto, a third NO.sub.x sensor 128, to measure the content of NO.sub.x gases in the exhaust gas flow as it leaves the second SCR system 126.

[0029] In practice, the second NO.sub.x sensor 118 and the third NO.sub.x sensor 128 can be used together to monitor, assess, or measure the performance of the second SCR system 126. Together, the DOC component 120, the DPF 122, the second DEF injector 124, the second SCR system 126, and the third NO.sub.x sensor 128 can be referred to as an underbody portion of the exhaust after-treatment system 100, as they can be collectively located underneath the engine, cab, or another portion of the vehicle.

[0030] As noted previously, performance of exhaust after-treatment systems, and of SCR systems in particular, is dependent upon accurate operation of various sensors, including NO.sub.x sensors, as well as other parameters. More specifically, DEF dosing levels and injection by the injectors 110 and 124 into exhaust after-treatment systems is often determined or controlled based on measurements of NO.sub.x levels provided by the NO.sub.x sensors 108, 118, and/or 128. Thus, accurate operation of the NO.sub.x sensors 108, 118, and/or 128 can be critical to maintaining compliance with emissions regulations. Nevertheless, injection of excess DEF is wasteful and costly, and therefore represents a reduction of overall system efficiency, and can result in ammonia slip (a condition where ammonia not utilized in the NO.sub.x reduction reaction is emitted from the catalyst), which is also limited by emissions mandates. Thus, it is critical to ensure accurate and precise performance of the sensors 108, 118, and/or 128, to ensure compliance with emissions standards without unduly reducing overall system efficiency.

[0031] It has been found that the performance of NO.sub.x sensors used in exhaust after-treatment systems varies from sensor to sensor, such as a result of production variation, and degrades or drifts over time, due to a variety of factors such as aging, abuse through over-heating, and/or failure of other components. Thus, in order to ensure that such sensors are performing adequately, that is, providing accurate and precise measurements of actual NO.sub.x levels in the exhaust gas flow, the importance of monitoring sensor performance has been recognized. Nevertheless, it can be difficult to perform accurate diagnostics on SCR systems or NO.sub.x sensors associated therewith during normal operation of the motor vehicle of which they are a part without affecting the performance of the SCR systems or NO.sub.x sensors under test (sometimes referred to as “intrusive diagnostics”). Intrusive diagnostics can change, and in some cases increase, tailpipe emissions, which is prohibited by certain regulatory bodies. To monitor the performance of NO.sub.x sensors in exhaust after-treatment systems, such as the sensors 108, 118, and 128 in the exhaust after-treatment system 100, it has therefore been found to be advantageous to monitor performance of the sensors in accordance with the techniques described herein.

[0032] In a first embodiment, the second SCR system 126 is used to control NO.sub.x tailpipe emissions from the vehicle, with DEF dosing provided by the second DEF injector 124 controlled by a measurement of NO.sub.x levels in the exhaust gas flow provided by either the first NO.sub.x sensor 108 or the second NO.sub.x sensor 118 upstream of the second DEF injector 124 and the second SCR system 126. While the second SCR system 126 is being used to control NO.sub.x tailpipe emissions to within acceptable ranges, performance of the first SCR system 116 can be modified as needed in order to perform diagnostics on the first NO.sub.x sensor 108 and/or the second NO.sub.x sensor 118.

[0033] In a first portion of the diagnostic testing, an engine control unit (ECU) of the vehicle controls the first DEF injector 110 to stop injecting DEF into the exhaust after-treatment system and the exhaust gas flow therein. The engine control unit may then allow the first SCR system 116 to stabilize over a period of time, until any residual ammonia present in the first SCR system 116 has reacted with NO.sub.x gases therein or otherwise been consumed or left the first SCR system 116. Once sufficient time has passed and no residual ammonia remains present in the first SCR system 116, or an amount of ammonia present in the first SCR system 116 is less than a threshold amount, the engine control unit of the vehicle can adjust operating parameters of the diesel engine to change actual levels of NO.sub.x gases in the exhaust gas flow travelling through the exhaust after-treatment system 100. Such operating parameters may include injection timing of the diesel engine, rate of fuel injection into the diesel engine, and/or exhaust gas recirculation through the diesel engine.

[0034] Measurements of the levels of the NO.sub.x gases in the exhaust gas flow travelling through the exhaust after-treatment system 100 are provided by the first NO.sub.x sensor 108 and the second NO.sub.x sensor 118 and recorded, such as in a database, by the engine control unit. A linear regression is then performed between the set of measurements provided by the first NO.sub.x sensor 108 and the second NO.sub.x sensor 118. Such linear regression provides a calculated slope and a calculated position of a best fit line between the two sets of data, which may be referred to as a gain and an offset, respectively, between the measurements provided by the NO.sub.x sensors 108 and 118. For example, FIG. 2 illustrates three examples of best fit lines calculated based on data provided by the first NO.sub.x sensor 108 (illustrated along the Y-axis) and the second NO.sub.x sensor 118 (illustrated along the X-axis), with the same units along the respective axes. In particular, FIG. 2 illustrates that the measurements provided by the two sensors may have a slope or a gain with respect to one another of greater than 1, equal to 1, or less than 1. As another example, FIG. 3 illustrates three examples of best fit lines calculated based on data provided by the first NO.sub.x sensor 108 (illustrated along the Y-axis) and the second NO.sub.x sensor 118 (illustrated along the X-axis), with the same units along the respective axes. In particular, FIG. 3 illustrates that the measurements provided by the two sensors may have an offset with respect to one another of greater than 0, equal to 0, or less than 0.

[0035] In a second portion of the diagnostic testing, the engine control unit of the vehicle controls operation of the vehicle and its diesel engine to ensure that the exhaust gas flow has a constant or substantially constant flow rate, temperature, and levels of NO.sub.x gases. The engine control unit and/or EAS then controls the first DEF injector 110 to slowly and gradually increase the rate at which DEF is injected into the exhaust after-treatment system and the exhaust gas flow therein. Such increase may be linear with time or follow any other established pattern. As the rate at which DEF is injected increases, measurements of the levels of the NO.sub.x gases in the exhaust gas flow travelling through the exhaust after-treatment system 100 are provided by the first NO.sub.x sensor 108 and the second NO.sub.x sensor 118 and recorded, such as in a database, by the engine control unit.

[0036] Measured ammonia-to-NO.sub.x levels or ratios (ANR) present in the first SCR system 116 over the course of the testing are determined based on the rate of injection of DEF and the measurements taken by the first NO.sub.x sensor 108, such as by calculating ratios of the rate of DEF injection to the measurements taken by the first NO.sub.x sensor 108 at various times over the course of the testing. Measured reductions in NO.sub.x levels produced by the first SCR system 116 over the course of the testing are also determined by calculating the difference between the measurements taken by the first NO.sub.x sensor 108 and the second NO.sub.x sensor 118, or by calculating a percentage of the NO.sub.x levels measured by the first NO.sub.x sensor represented by the NO.sub.x levels measured by the second NO.sub.x sensor, at various times over the course of the testing. The measured ammonia-to-NO.sub.x levels and the measured reductions (such as in units of percentage efficiency of the first SCR system 116) may be recorded, such as in the database by the engine control unit and/or EAS.

[0037] Efficiencies of SCR system performances have well-established relationships with ammonia-to-NO.sub.x levels present in the SCR systems during operation. Generally, efficiency increases with increased ammonia-to-NO.sub.x levels, but increases in efficiency generally taper off as they reach 100%, may reach a peak, and do not exceed 100%. As illustrated in FIG. 4, when such efficiencies are plotted against ANR, the resulting curves typically have three well-established phases or portions: a first portion at lower levels of ANR and resulting efficiency, a second portion at intermediate levels of ANR and resulting efficiency, and a third portion at higher levels of ANR and resulting efficiency, which may include a peak in efficiency, with well-defined and visible transitions or corners between the first and second portions and between the second and third portions. In particular, the first portion of the curve typically has a first slope, the second portion of the curve typically has a second slope, and the third portion of the curve typically has a third slope, where the slope of the second portion of the curve is greater or steeper than the slope of the first portion of the curve and the slope of the first portion of the curve is greater or steeper than the slope of the third portion of the curve, and the third portion of the curve may include a peak of the efficiency of the first SCR system 116.

[0038] Because the shapes of these curves are consistent and have been well-established, they can be used to determine the accuracy of the first NO.sub.x sensor 108. For example, the engine control unit can determine a difference between a measured ammonia-to-NO.sub.x ratio at a peak of the SCR system efficiency, or at the transition between the second and third portions of the curves described herein, and a known ammonia-to-NO.sub.x ratio at a peak of the SCR system efficiency, or at the transition between the second and third portions of the curves described herein, respectively, such as by subtracting the measured ratio from the known ratio. FIG. 4 illustrates that such a calculation may reveal that the measurements of NO.sub.x levels provided by the first NO.sub.x sensor 108 are less than actual NO.sub.x levels, equal to actual NO.sub.x levels, or greater than actual NO.sub.x levels.

[0039] Once such a difference is calculated, it can be used to adjust or correct future measurements provided by the first NO.sub.x sensor 108, thereby compensating for errors in such measurements. For example, a subsequent measurement provided by the first NO.sub.x sensor 108 can be adjusted by dividing the uncorrected, unadjusted measured NO.sub.x levels by one minus the calculated difference to get the corrected or adjusted measured NO.sub.x levels at the location of the first NO.sub.x sensor 108. Once the error in the first NO.sub.x sensor 108 has been determined and corrected in this manner, such determination can be used in combination with the linear regression performed in the first portion of the diagnostic testing to similarly determine and correct errors in the measurements provided by the second NO.sub.x sensor 118. In some embodiments, future operation of the vehicle, its engine, and the exhaust after-treatment system 100, including any of the components and parameters described herein, such as the rate at which the first DEF injector 110 injects DEF or parameters of operation of the heater 112, may be operated or selected based on such corrected or adjusted measured NO.sub.x levels at the locations of the first and second NO.sub.x sensors 108 and 118, rather than on the uncorrected, unadjusted measured NO.sub.x levels.

[0040] The techniques described herein may be referred to as “intrusive diagnostics” because they require variation in the performance of NO.sub.x emissions-controlling components during normal operation of the motor vehicle of which they are a part and a diesel engine thereof. In some embodiments, the first portion of the diagnostic testing may be performed before the second portion of the diagnostic testing, while in other embodiments, the second portion of the diagnostic testing may be performed before the first portion of the diagnostic testing.

[0041] In a second embodiment, the first SCR system 116 is used to control NO.sub.x tailpipe emissions from the vehicle, with DEF dosing provided by the first DEF injector 110 controlled by a measurement of NO.sub.x levels in the exhaust gas flow provided by the first NO.sub.x sensor 108 upstream of the first DEF injector 110 and the first SCR system 116. While the first SCR system 116 is being used to control NO.sub.x tailpipe emissions to within acceptable ranges, performance of the second SCR system 126 can be modified as needed in order to perform diagnostics on the second NO.sub.x sensor 118 and/or the third NO.sub.x sensor 128. In some implementations, the operation of the second SCR system 126 in such an embodiment may only improve (that is, decrease) NO.sub.x emissions levels within the exhaust after-treatment system 100, such that these intrusive diagnostics may be allowed to be performed without additional exhaust treatment downstream. In some implementations, the operation of the first SCR system 116 may be configured to reduce NO.sub.x emissions levels to below regulated levels (e.g., such as by 1%, 2%, 5%, 10%, or more) such that slight increases in emissions levels resulting from operation of the second SCR system 126 do not increase tailpipe emissions to above regulated levels, such that these intrusive diagnostics may be performed without crossing regulated emission level thresholds.

[0042] In a first portion of the diagnostic testing, an engine control unit (ECU) of the vehicle controls the second DEF injector 124 to stop injecting DEF into the exhaust after-treatment system and the exhaust gas flow therein. The engine control unit may then allow the second SCR system 126 to stabilize over a period of time, until any residual ammonia present in the second SCR system 126 has reacted with NO.sub.x gases therein or otherwise been consumed or left the second SCR system 126. Once sufficient time has passed and no residual ammonia remains present in the second SCR system 126, or an amount of ammonia present in the second SCR system 126 is less than a threshold amount, the engine control unit of the vehicle can adjust operating parameters of the diesel engine and/or the first SCR system 116 to change actual levels of NO.sub.x gases in the exhaust gas flow travelling through the exhaust after-treatment system 100. Such operating parameters may include injection timing of the diesel engine, rate of fuel injection into the diesel engine, and/or exhaust gas recirculation through the diesel engine, as well as a rate of DEF injection provided by the first DEF injector 110.

[0043] Measurements of the levels of the NO.sub.x gases in the exhaust gas flow travelling through the exhaust after-treatment system 100 are provided by the second NO.sub.x sensor 118 and the third NO.sub.x sensor 128 and recorded, such as in a database, by the engine control unit. A linear regression is then performed between the set of measurements provided by the second NO.sub.x sensor 118 and the third NO.sub.x sensor 128. Such linear regression provides a calculated slope and a calculated position of a best fit line between the two sets of data, which may be referred to as a gain and an offset, respectively, between the measurements provided by the NO.sub.x sensors 118 and 128. For example, FIG. 2 illustrates three examples of best fit lines calculated based on data provided by the second NO.sub.x sensor 118 (illustrated along the Y-axis) and the third NO.sub.x sensor 128 (illustrated along the X-axis), with the same units along the respective axes. In particular, FIG. 2 illustrates that the measurements provided by the two sensors may have a slope or a gain with respect to one another of greater than 1, equal to 1, or less than 1. As another example, FIG. 3 illustrates three examples of best fit lines calculated based on data provided by the second NO.sub.x sensor 118 (illustrated along the Y-axis) and the third NO.sub.x sensor 128 (illustrated along the X-axis), with the same units along the respective axes. In particular, FIG. 3 illustrates that the measurements provided by the two sensors may have an offset with respect to one another of greater than 0, equal to 0, or less than 0.

[0044] In a second portion of the diagnostic testing, the engine control unit of the vehicle controls operation of the vehicle, its diesel engine, and the first SCR system 116 to ensure that the exhaust gas flow has constant or substantially constant flow rate, temperature, and levels of NO.sub.x gases. The engine control unit and/or EAS then controls the second DEF injector 124 to slowly and gradually increase the rate at which DEF is injected into the exhaust after-treatment system and the exhaust gas flow therein. Such increase may be linear with time or follow any other established pattern. As the rate at which DEF is injected increases, measurements of the levels of the NO.sub.x gases in the exhaust gas flow travelling through the exhaust after-treatment system 100 are provided by the second NO.sub.x sensor 118 and the third NO.sub.x sensor 128 and recorded, such as in a database, by the engine control unit.

[0045] Measured ammonia-to-NO.sub.x levels or ratios (ANR) present in the second SCR system 126 over the course of the testing are determined based on the rate of injection of DEF and the measurements taken by the second NO.sub.x sensor 118, such as by calculating ratios of the rate of DEF injection to the measurements taken by the second NO.sub.x sensor 118 at various times over the course of the testing. Measured reductions in NO.sub.x levels produced by the second SCR system 126 over the course of the testing are also determined by calculating the difference between the measurements taken by the second NO.sub.x sensor 118 and the third NO.sub.x sensor 128, or by calculating a percentage of the NO.sub.x levels measured by the second NO.sub.x sensor 118 represented by the NO.sub.x levels measured by the third NO.sub.x sensor, 128 at various times over the course of the testing. The measured ammonia-to-NO.sub.x levels and the measured reductions (such as in units of percentage efficiency of the second SCR system 126) may be recorded, such as in the database by the engine control unit and/or EAS.

[0046] The engine control unit can then determine a difference between a measured ammonia-to-NO.sub.x ratio at a peak of the efficiency of the second SCR system 126, or at the transition between the second and third portions of the curves described herein, and a known ammonia-to-NO.sub.x ratio at a peak of the efficiency of the second SCR system 126, or at the transition between the second and third portions of the curves described herein, respectively, such as by subtracting the measured ratio from the known ratio. FIG. 4 illustrates that such a calculation may reveal that the measurements of NO.sub.x levels provided by the second NO.sub.x sensor 118 are less than actual NO.sub.x levels, equal to actual NO.sub.x levels, or greater than actual NO.sub.x levels.

[0047] Once such a difference is calculated, it can be used to adjust or correct future measurements provided by the second NO.sub.x sensor 118, thereby compensating for errors in such measurements. For example, a subsequent measurement provided by the second NO.sub.x sensor 118 can be adjusted by dividing the uncorrected, unadjusted measured NO.sub.x levels by one minus the calculated difference to get the corrected or adjusted measured NO.sub.x levels at the location of the second NO.sub.x sensor 118. Once the error in the second NO.sub.x sensor 118 has been determined and corrected in this manner, such determination can be used in combination with the linear regression performed in the first portion of the diagnostic testing to similarly determine and correct errors in the measurements provided by the third NO.sub.x sensor 128. In some embodiments, future operation of the vehicle, its engine, and the exhaust after-treatment system 100, including any of the components and parameters described herein, such as the rate at which the second DEF injector 124 injects DEF or parameters of operation of a heater, may be operated or selected based on such corrected or adjusted measured NO.sub.x levels at the locations of the second and third NO.sub.x sensors 118 and 128, rather than on the uncorrected, unadjusted measured NO.sub.x levels.

[0048] The techniques described herein may be referred to as “intrusive diagnostics” because they require variation in the performance of NO.sub.x emissions-controlling components during normal operation of the motor vehicle of which they are a part and a diesel engine thereof. In some embodiments, the first portion of the diagnostic testing may be performed before the second portion of the diagnostic testing, while in other embodiments, the second portion of the diagnostic testing may be performed before the first portion of the diagnostic testing.

[0049] FIG. 5 illustrates a flow chart 200 of a summarized version of a method in accordance with the present disclosure. As illustrated in FIG. 5, the method includes, at 202, operating a diesel engine such that the diesel engine generates an exhaust gas flow. The method further includes, at 204, controlling the diesel engine such that the exhaust gas flow has a constant level of NO.sub.x gases as it enters an exhaust after-treatment system, and at 206, controlling a DEF injector upstream of a close-coupled selective catalytic reduction system to increase a rate at which the DEF injector injects DEF into the exhaust gas flow. The method further includes, at 208, measuring a first level of NO.sub.x gases with a first NO.sub.x sensor and a second level of NO.sub.x gases with a second NO.sub.x sensor as the rate at which the DEF injector injects DEF increases, and at 210, using the measurements to determine a level of error in measurements provided by the first NO.sub.x sensor.

[0050] The techniques described herein may include controlling operation of the vehicle and its diesel engine to ensure that the exhaust gas flow has a constant or substantially constant flow rate, temperature, and levels of NO.sub.x gases. In some alternative embodiments, however, the techniques described herein may be carried out under transient NO.sub.x, temperature, and/or exhaust flow rate conditions. In other embodiments, the exhaust after-treatment system 100 may include three, four, or any other number of independent SCR systems, together with respective heaters, temperature sensors, and NO.sub.x sensors. Each of the SCR systems and respective NO.sub.x sensors can have features corresponding to those described herein for the close-coupled SCR system 116, the underbody SCR system 126, and/or the NO.sub.x sensors 108, 118, and/or 128.

[0051] The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.