Upstream NOx estimation

Abstract

A method for controlling operation of an internal combustion engine determines an estimated NOx value as a function of at least one engine operating parameter. The method also determines an actual NOx value using a NOx sensor positioned in an exhaust gas stream of the internal combustion engine. The method detects at least one condition indicative of whether or not the actual NOx value is accurate. The actual NOx value is used for controlling engine operation when the at least one condition indicates that the actual NOx value is accurate, while the estimated NOx value is used for controlling engine operation when the at least one condition indicates that the actual NOx value is inaccurate.

Claims

1. A method for controlling operation of an internal combustion engine, comprising: determining an estimated NO.sub.x value as a function of at least one engine operating parameter; determining an actual NO.sub.x value using a NO.sub.x sensor positioned in an exhaust gas stream of the internal combustion engine; detecting at least one condition indicative of whether or not the actual NO.sub.x value is accurate; controlling engine operation using the actual NO.sub.x value when the at least one condition indicates that the actual NO.sub.x value is accurate; and controlling engine operation using the estimated NO.sub.x value when the at least one condition indicates that the actual NO.sub.x value is inaccurate.

2. The method of claim 1, wherein the estimated NO.sub.x value is determined as a function of at least engine speed and torque.

3. The method of claim 1, wherein the at least one condition comprises one or more of exhaust gas temperature, dew point, humidity, system voltage, and oxygen concentration of the exhaust gas.

4. The method of claim 1, wherein the at least one condition comprises dew point and wherein: engine operation is controlled using the actual NO.sub.x value when the dew point as at or above a first predetermined level; and engine operation is controlled using the estimated NO.sub.x value when the dew point is below the first predetermined level.

5. The method of claim 1, wherein the at least one condition comprises oxygen concentration in the exhaust gas stream and wherein: engine operation is controlled using the actual NO.sub.x value when the oxygen concentration is at or above a predetermined level; and engine operation is controlled using the estimated NO.sub.x value when the oxygen concentration is below the predetermined level.

6. A method for controlling operation of an internal combustion engine having a NOx level output, the method comprising: determining an actual NO.sub.x value using a NO.sub.x sensor positioned in an exhaust gas stream of the internal combustion engine; determining a steady state NO.sub.x estimate as a function of at least engine speed and torque, the steady state NO.sub.x corresponding to the NO.sub.x level output by the engine during a steady state operation where engine speed and power are constant; determining a transitory NO.sub.x estimate as a function of at least engine speed and torque, the transitory NO.sub.x estimate corresponding to the NO.sub.x level output by the engine during a transitory operation where engine power is increasing; determining a compensation factor based on intake manifold pressure; applying the compensation factor to the steady state and transitory NO.sub.x estimates to arrive at a final estimated NO.sub.x value, wherein the compensation factor weights the final estimated NO.sub.x value towards the actual NO.sub.x estimate with decreasing intake manifold pressure; detecting at least one condition indicative of whether or not the actual NO.sub.x value is accurate; controlling engine operation using the actual NO.sub.x value when the at least one condition indicates that the actual NO.sub.x value is accurate; and controlling engine operation using the final estimated NO.sub.x value when the at least one condition indicates that the actual NO.sub.x value is inaccurate.

7. The method of claim 6, wherein the estimated NO.sub.x value is determined as a function of at least engine speed and torque.

8. The method of claim 6, wherein the at least one condition comprises one or more of exhaust gas temperature, dew point, system voltage, and oxygen concentration of the exhaust gas.

9. The method of claim 6, wherein the at least one condition comprises dew point and wherein: engine operation is controlled using the actual NO.sub.x value when the dew point as at or above a predetermined level; and engine operation is controlled using the estimated NO.sub.x value when the dew point is below the predetermined level.

10. The method of claim 6, wherein the at least one condition comprises oxygen concentration in the exhaust gas stream and wherein: engine operation is controlled using the actual NO.sub.x value when the oxygen concentration is at or above a predetermined level; and engine operation is controlled using the estimated NO.sub.x value when the oxygen concentration is below the predetermined level.

11. A method as set forth in claim 6, wherein the step of determining a compensation factor further comprises: determining an estimated intake manifold pressure as a function of at least engine speed and torque; sensing the actual intake manifold pressure; and determining the compensation factor as a function of a difference between the actual and estimated intake manifold pressures.

12. A method as set forth in claim 11, wherein the compensation factor is also a function one or more of exhaust manifold pressure, mass air flow, turbocharger boost, exhaust flow, and combinations thereof.

13. A method as set forth in claim 6, wherein the compensation factor has a value ranging from 0 to 1 and wherein the final NO.sub.x estimate is determined in accordance with the following formula:
NO.sub.x.sub._.sub.OUT.sub._.sub.EST=(CF.Math.NO.sub.x.sub._T)+((1CF).Math.NO.sub.x.sub._SS) where CF is the compensation factor, NO.sub.x.sub._T is the transient NO.sub.x estimate and NO.sub.x.sub._SS is the steady state NO.sub.x estimate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic illustration of an internal combustion engine with an exhaust gas SCR system.

(2) FIG. 2 is a flow diagram of an exemplary method for determining the NO.sub.x level in an engine's exhaust according to certain embodiments of the present technology.

(3) FIG. 3 is a schematic of exemplary control logic for determining the NO.sub.x level in an engine's exhaust according to certain embodiments of the present technology.

(4) FIG. 4 is a schematic illustration of exemplary control logic for determining the NO.sub.x level in an engine's exhaust according to certain embodiments of the present technology.

(5) FIG. 5 is a flow diagram of an exemplary method for controlling operation of an internal combustion engine according to certain embodiments of the present technology.

(6) FIG. 6 is a schematic illustration of exemplary control logic for controlling operation of an internal combustion engine according to certain embodiments of the present technology.

DETAILED DESCRIPTION

(7) Various examples of embodiments of the present technology will be described more fully hereinafter with reference to the accompanying drawings, in which such examples of embodiments are shown. Like reference numbers refer to like elements throughout. Other embodiments of the presently described technology may, however, be in many different forms and are not limited solely to the embodiments set forth herein. Rather, these embodiments are examples representative of the present technology. Rights based on this disclosure have the full scope indicated by the claims.

(8) FIG. 1 shows an exemplary schematic depiction of an internal combustion engine 10 and an SCR system 12 for reducing NO.sub.x from the engine's exhaust. The engine 10 can be used, for example, to power a vehicle such as an over-the-road vehicle (not shown). The engine 10 can be a compression ignition engine, such as a diesel engine, for example. Generally speaking, the SCR system 12 includes a catalyst 20, a reductant supply 22, a reductant injector 24, an electronic control unit 26, and one or more parameters sensors.

(9) The ECU 26 controls delivery of a reductant, such as ammonia, from the reductant supply 22 and into the exhaust system 28 through the reductant injector 24. The reductant supply 22 can include canisters (not shown) for storing ammonia in solid form. In most systems, a plurality of canisters will be used to provide greater travel distance between recharging. A heating jacket (not shown) is typically used around the canister to bring the solid ammonia to a sublimation temperature. Once converted to a gas, the ammonia is directed to the reductant injector 24. The reductant injector 24 is positioned in the exhaust system 28 upstream from the catalyst 20. As the ammonia is injected into the exhaust system 28, it mixes with the exhaust gas and this mixture flows through the catalyst 20. The catalyst 20 causes a reaction between NO.sub.x present in the exhaust gas and a NO.sub.x reducing agent (e.g., ammonia) to reduce/convert the NO.sub.x into nitrogen and water, which then passes out of the tailpipe 30 and into the environment. While the SCR system 12 has been described in the context of solid ammonia, it will be appreciated that the SCR system could alternatively use a reductant such as pure anhydrous ammonia, aqueous ammonia or urea, for example.

(10) According to at least some embodiments, the ECU 26 controls engine operation and operation of the SCR system 12, including operation of the reductant injector 24, based on a plurality of operating parameters. In the exemplary embodiment, the operating parameters include intake manifold pressure (IMP), engine speed (N) (i.e., rotational speed), engine load or torque (TQ) and the level of NO.sub.x in engine's exhaust (Engine Out NO.sub.x). The intake manifold pressure (IMP) can be determined via a pressure sensor 52 positioned to sense the pressure in the engine's intake manifold and produce a responsive output signal. The engine speed (N) can be determined using a sensor 54 to detect the rotation speed of the engine, e.g., crankshaft rpm. Engine load (TQ) can be based on accelerator pedal position as measured by a sensor 58 or fuel setting, for example.

(11) As explained in greater detail, the ECU 26 may estimate the level of NO.sub.x in engine's exhaust based on one or more engine operating parameters. For example, in at least some embodiments, the ECU 26 can determine an estimated NO.sub.x value based on the engine speed (N), load (TQ) and intake manifold pressure (IMP). In addition, the ECU 26 may determine an actual level of NO.sub.x value using a NO.sub.x sensor 60 positioned in the engine's exhaust gas stream, e.g., between the engine 10 and the catalyst 20. The ECU 26 may also detect one or more conditions indicative of whether or not the actual NO.sub.x value is accurate. For example, the ECU may monitor one or more of exhaust gas temperature (T) via a temperature sensor 62, dew point (DP) via a dew point sensor 64, oxygen concentration (O.sub.2) in the exhaust system via an oxygen sensor 65, and system voltage (V) via a voltage sensor 66. In some embodiments, the ECU 26 controls engine operation using the actual NO.sub.x value when the at least one condition indicates that the actual NO.sub.x value is accurate, but uses the estimated NO.sub.x value to control engine operation when the at least one condition indicates that the actual NO.sub.x value may be inaccurate.

(12) In addition to controlling the dosing or metering of ammonia, the ECU 26 can also store information such as the amount of ammonia being delivered, the canister providing the ammonia, the starting volume of deliverable ammonia in the canister, and other such data which may be relevant to determining the amount of deliverable ammonia in each canister. The information may be monitored on a periodic or continuous basis. When the ECU 26 determines that the amount of deliverable ammonia is below a predetermined level, a status indicator (not shown) electronically connected to the controller 26 can be activated.

(13) FIG. 2 is a flow chart of an exemplary method 200 for determining the NO.sub.x level in an engine's exhaust in accordance with certain aspects of the present technology. The method 200 begins in step 202. Control is then passed to the step 205, where the exemplary method determines the engine speed (N), the engine load (TQ) and the actual intake manifold pressure (IMP_ACT), e.g., by reading the output from the sensors 52, 54, 58.

(14) Control is then passed to step 210, where the method 200 determines a first NO.sub.x value or estimate (NO.sub.x.sub._SS) as a function of engine speed (N) and engine load (TQ). The first NO.sub.x estimate (NO.sub.x.sub._SS) corresponds to the NO.sub.x output by engine under a first engine operating condition (and at a given speed (N) and load (TQ) combination). In some embodiments, the first operating condition corresponds to substantially steady state operation of the engine, i.e., at constant or slowly changing engine speed. In some embodiments, the method 200 determines the first NO.sub.x estimate (NO.sub.x.sub._SS) by accessing a look-up table or map that provides an estimate of the NO.sub.x level produced by the engine at the given engine speed (N) and load (TQ) during the first operating condition (e.g., steady state operation). The look-up table can, for example, be empirically constructed by operating the engine in the first operating condition and measuring actual NO.sub.x level, i.e., with a NO.sub.x sensor, at different engine speed and load combinations.

(15) Control is then passed to step 215 where the method determines a second NO.sub.x value or estimate (NO.sub.x.sub._T) as a function of engine speed (N) and engine load (TQ). The second NO.sub.x estimate (NO.sub.x.sub._T) corresponds to the NO.sub.x output by the engine during a second operating condition (and at a given engine speed (N) and load (TQ) combination). In some embodiments, the second operating condition corresponds to transient operation where engine power is increasing, e.g., during acceleration of a vehicle. In some embodiments, the method 200 determines the second NO.sub.x value (NO.sub.x.sub._T) by accessing a look-up table or map that provides an estimate of the NO.sub.x level produced by the engine at the given engine speed (N) and load (TQ) under the second operating condition (e.g., transient operation).

(16) Next, in step 220 the method 200 determines an estimated intake manifold pressure (IMP_EST) as a function of at least engine speed (N) and torque (TQ). In the exemplary embodiment, the estimated intake manifold pressure (IMP_EST) corresponds to the engine's intake manifold pressure when the engine is under the first operating condition (and at a given engine speed (N) and load (TQ) combination). In some embodiments, the method determines the estimated intake manifold pressure (IMP_EST) by accessing a look-up table or map that provides an estimate of the intake manifold pressure (IMP) at the given engine speed (N) and load (TQ) during the first operating condition (e.g., steady state operation). The look-up table can, for example, be empirically constructed by operating the engine in the first mode and measuring actual intake manifold pressure, i.e., with a sensor, at different engine speed and load combinations.

(17) Control is then passed to step 225 where the method 200 determines a pressure difference (IMP_) between the estimated intake manifold pressure (IMP_EST) and the actual intake manifold pressure (IMP_ACT). Control is then passed to step 230 where the method determines a compensation factor (CF) based on the pressure difference (IMP_) between the estimated and actual intake manifold pressures. According to some embodiments, the compensation factor ranges from 0 when the pressure difference is at first threshold and 1 when the pressure difference is at a second threshold.

(18) Control is then passed to step 235 where the method 200 determines the estimated NOx level being output from the engine (NO.sub.x.sub._OUT_EST). In some embodiments, the NO.sub.x output by the engine is determined as a function of the compensation factor and the first and second NO.sub.x estimates. According to at least some as embodiments of the present technology, the estimated engine out NO.sub.x (NO.sub.x.sub._OUT_EST) can be determined in accordance with the following equation.
NO.sub.x.sub._OUT_EST=(CF.Math.NO.sub.x.sub._T)+((1CF).Math.NO.sub.x.sub._SS)
The estimated engine at NO.sub.x (NO.sub.x.sub._OUT_EST) can be used by the ECU in controlling the SCR system, including controlling the reductant value in order to control dosing of reductant into the exhaust system 28.

(19) FIG. 3 is a schematic of exemplary control logic 300 for determining the NO.sub.x level in an engine's exhaust in accordance with certain aspects of the present technology. The control logic includes a first block 305 that determines a first NO.sub.x value (or estimate) (NO.sub.x.sub._SS) as a function of at least engine speed (N) and engine load (TQ). The first NO.sub.x estimate (NO.sub.x.sub._SS) output by the first logic block 305 corresponds to the NO.sub.x output by engine under a first engine operating condition (and at a given speed (N) and load (TQ) combination). In some embodiments, the first operating condition corresponds to substantially steady state operation of the engine, i.e., at constant or slowly changing engine speed. In at least some embodiments, the control logic 300 determines the first NO.sub.x value (NO.sub.x.sub._SS) by accessing a look-up table or map that provides an estimate of the NO.sub.x level produced by the engine at the given engine speed (N) and load (TQ) during the first operating condition (e.g., steady state operation). The look-up table can, for example, be empirically constructed by operating the engine in the first operating condition and measuring actual NO.sub.x level, i.e., with a NO.sub.x sensor, at different engine speed and load combinations.

(20) The control logic 300 also includes a second logic block 310 that determines a second NO.sub.x value (or estimate) (NO.sub.x.sub._T) as a function of at least engine speed (N) and engine load (TQ). The second NO.sub.x estimate (NO.sub.x.sub._T) output by the second logic block 310 corresponds to the NO.sub.x output by the engine during a second operating condition (and at a given engine speed (N) and load (TQ) combination). In at least some embodiments, the second operating condition corresponds to transient operation where engine power is increasing, e.g., during acceleration of a vehicle. In some embodiments, the control logic 300 determines the second NO.sub.x value (NO.sub.x.sub._T) by accessing a look-up table or map that provides an estimate of the NO.sub.x level produced by the engine at the given engine speed (N) and load (TQ) under the second operating condition (e.g., transient operation). The look-up table can be empirically constructed by operating the engine under the second condition and measuring the actual NO.sub.x level, i.e., with a sensor, output from the engine at different speed and load combinations.

(21) Control logic 300 also includes a third logic block 315 that determines an estimated intake manifold pressure (IMP_EST) as a function of at least engine speed (N) and torque (TQ). In at least one embodiment, the estimated intake manifold pressure (IMP_EST) corresponds to the engine's intake manifold pressure when the engine under the first operating condition (and at a given engine speed (N) and load (TQ) combination). According to some embodiments, the estimated intake manifold pressure (IMP_EST) corresponds to the engine's intake manifold pressure when the engine is operating at steady state (and at a given engine speed (N) and load (TQ) combination). In some embodiments, the control logic determines the estimated intake manifold pressure (IMP_EST) by accessing a look-up table or map that provides an estimate of the intake manifold pressure (IMP) at the given engine speed (N) and load (TQ) during the first operating condition (e.g., steady state operation). The look-up table can, for example, be empirically constructed by operating the engine in the first operating condition (e.g., steady state operation) and measuring actual intake manifold pressure, i.e., with a sensor, at different engine speed and load combinations.

(22) Control logic includes logic 320 for calculating a pressure difference (IMP_) between the estimated intake manifold pressure (IMP_EST) and the actual intake manifold pressure (IMP_ACT). A fourth logic block 325 determines a compensation factor (CF) as a function of the pressure difference (IMP_) between the estimated and actual intake manifold pressures. According to some embodiments, the compensation factor (CF) ranges from 0 when the pressure difference is at first threshold and 1 when the pressure difference is at a second threshold. The control logic also includes logic 330 for estimating NO.sub.x level being output from the engine (NO.sub.x.sub._OUT_EST) as a function of the compensation factor (CF), the first NO.sub.x estimate (NO.sub.x.sub._SS) and the second NO.sub.x estimate (NO.sub.x.sub._T). According to at least some embodiments of the present technology, the estimated engine output NO.sub.x (NO.sub.x.sub._OUT_EST) can be determined in accordance with the following equation.
NO.sub.x.sub._OUT_EST=(CF.Math.NO.sub.x.sub._T)+((1CF).Math.NO.sub.x.sub._SS)

(23) FIG. 4 is a schematic illustrating control logic for determining NO.sub.x level according to certain aspects of at least one embodiment of the present technology. The control logic of FIG. 4 includes a plurality of logic blocks configured to provide NO.sub.x estimates as a function of the engine's operating mode. In the illustrated example, the control logic includes a Normal Operating Mode NO.sub.x estimator 402, a Regeneration Operating Mode NO.sub.x estimator 404 and an OFR Mode NO.sub.x estimator 406. Each of the estimators 402-406 determines a NO.sub.x estimate corresponding to level of NO.sub.x produced by the engine during a respective operating mode. A selector 408 sets a final NO.sub.x estimate to the output of one of the estimators 402-406 in dependence on the engine's current operating mode, e.g., as provided by the ECU 26. For example, when the engine is operating in a regeneration mode, the selector 408 uses the output of Regeneration Operating Mode NO.sub.x estimator 404 as the final estimated the NO.sub.x value.

(24) Although not shown in detail, each of the estimators 402-404 can include control logic similar to the control logic 300 shown in FIG. 3. In this regard, each of the estimators 402-206 may include logic that determines a first or steady state NO.sub.x value (NO.sub.x.sub._SS) corresponding to the NO.sub.x produced at a given engine operating condition, e.g., steady state (and at a given engine speed (N) and load (TQ) combination) when the engine is operating in a respective mode, e.g., normal, regeneration or OFR. Similarly, each estimator 402-406 may include logic that determines a second or transient NO.sub.x value (NO.sub.x.sub._T) corresponding to the NO.sub.x produced at a given engine operating condition, e.g., transient operation (and at a given engine speed (N) and load (TQ) combination) when the engine is operating in a respective mode, e.g., normal, regeneration or OFR. The estimators 402-406 may also include logic (not shown) that determines a compensation factor based on intake manifold pressure and applies the compensation factor to the steady state and transitory NO.sub.x estimates to arrive at a final NO.sub.x estimate. As explained above, in some embodiments, the compensation factor weights the final NO.sub.x estimate towards the transitory NO.sub.x estimate with decreasing intake manifold pressure. The final NO.sub.x estimates from the estimators 402-406 are supplied to the selector 408, which in turn sets the final estimated NOx value to the output of one of the estimators 402-406 in dependence on the engine operating mode, e.g., as provided by the ECU 26.

(25) FIG. 5 is a flow diagram of an exemplary method 500 for controlling operation of an internal combustion engine according to certain embodiments of the present technology. The method begins in step 505. Control is then passed to step 510 where the method determines an estimated NO.sub.x value based on the engine speed (N), load (TQ) and intake manifold pressure (IMP). In at least some embodiments, the method 200 of FIG. 2 may be used to determine the estimated NO.sub.x value in step 510. Control is then passed to step 515 where the method 500 determines an actual NO.sub.x value using the NO.sub.x sensor 60. Control is then passed to step 520 where the method determines whether the actual NO.sub.x value is accurate. If the actual NO.sub.x value is determined to be accurate, control is passed to step 525, causing the engine to be controlled using the actual NO.sub.x value. Conversely, if the actual NO.sub.x value is determined to be inaccurate, control is passed to step 525, causing the engine to be controlled using the estimated NO.sub.x value.

(26) In some embodiments, the method 500 may determine the accuracy of the actual NO.sub.x value by monitoring one or more conditions indicative of whether or not the NO.sub.x sensor 60 is functioning properly. For example, the method can monitor one or more of exhaust gas temperature (T), dew point (DP), oxygen concentration (O.sub.2) in the exhaust system, system voltage (V) and any other environmental or operating conditions that could adversely affect the accuracy of the NO.sub.x sensor 60.

(27) Some NO.sub.x sensors may not provide satisfactory accuracy unless the exhaust gas is above a threshold temperature. Accordingly, in some embodiments, engine operation may be controlled using the actual NO.sub.x value when the exhaust gas temperature is at or above a temperature threshold, while engine operation may be controlled using the actual NO.sub.x value when the exhaust gas temperature is below the temperature threshold. Likewise, some NO.sub.x sensors may not provide satisfactory accuracy unless the oxygen concentration of the exhaust gas is above a threshold level. Accordingly, in some embodiments engine operation may controlled using the actual NO.sub.x value when the exhaust gas oxygen concentration is as at or above an oxygen concentration threshold, while engine operation may be controlled using the estimated NO.sub.x value when the exhaust gas oxygen concentration is below the oxygen concentration threshold.

(28) Further, some NO.sub.x sensors may not provide satisfactory accuracy when the dew point is below a threshold level. Accordingly, in some engine operation may be controlled using the actual NO.sub.x value when the dew point is at or above a dew point threshold, while engine operation may be controlled using the estimated NO.sub.x value when dew point is below the dew point threshold.

(29) FIG. 6 is a schematic illustration of exemplary control logic 600 according to certain embodiments of the present technology. The control logic 600 includes a logic block 602 that produces an estimated .sub.NO.sub.x value as a function of at least one engine operating parameter. In at least some embodiments, the logic block 602 may be constructed generally in accordance with the control logic 300 of FIG. 3. Briefly, the logic block 602 may include logic that determines a first or steady state .sub.NO.sub.x value (.sub.NO.sub.x.sub._SS) corresponding to the .sub.NO.sub.x produced at a given engine operating condition, e.g., steady state (and at a given engine speed (N) and load (TQ) combination). Likewise, the logic block 602 may include logic that determines a second or transient .sub.NO.sub.x value (.sub.NO.sub.x.sub._T) corresponding to the .sub.NO.sub.x produced at a given engine operating condition, e.g., transient operation (and at a given engine speed (N) and load (TQ) combination). The logic block 602 may also include logic (not shown) that determines a compensation factor based on intake manifold pressure and applies the compensation factor to the steady state and transitory .sub.NO.sub.x estimates to arrive at a final .sub.NO.sub.x estimate, in the manner described above in connection with FIG. 3. Further, as explained above, in some embodiments, the compensation factor weights the final .sub.NO.sub.x estimate towards the transitory .sub.NO.sub.x estimate with decreasing intake manifold pressure.

(30) The final NO.sub.x estimate from logic block 602 is supplied to the selection block 610. The selection block 610 also receives the actual NO.sub.x value from the NO.sub.x sensor 60. The selection block 610 determines whether to use the actual NO.sub.x value from the sensor 60 or the estimated NO.sub.x value from the logic block 602 based on one or more parameters or conditions. For example, in some embodiments, the selection block 610 determines whether the actual NO.sub.x value is accurate based on one or more environmental and/or operating conditions. If the actual NO.sub.x value is determined to be accurate, the selection block 610 causes the engine to be controlled using the actual NO.sub.x value. Conversely, if the actual NO.sub.x value is determined to be inaccurate, the selection block 610, causing the engine to be controlled using the estimated NO.sub.x value. In some embodiments, the control logic 610 may determine the accuracy of the actual NO.sub.x value by monitoring one or more conditions indicative of whether or not the NO.sub.x sensor 60 is functioning properly. For example, the method can monitor one or more of exhaust gas temperature (T), dew point (DP), oxygen concentration (O.sub.2) in the exhaust system, system voltage (V) and any other environmental or operating conditions that could adversely affect the accuracy of the NO.sub.x sensor 60.

(31) While this disclosure has been described as having exemplary embodiments, this application is intended to cover any variations, uses, or adaptations using the general principles set forth herein. It is envisioned that those skilled in the art may devise various modifications and equivalents without departing from the spirit and scope of the disclosure as recited in the following claims. Further, this application is intended to cover such departures from the present disclosure as come within the known or customary practice within the art to which it pertains. While this disclosure has been described as having exemplary embodiments, this application is intended to cover any variations, uses, or adaptations using the general principles set forth herein. It is envisioned that those skilled in the art may devise various modifications and equivalents without departing from the spirit and scope of the disclosure as recited in the following claims. Further, this application is intended to cover such departures from the present disclosure as come within the known or customary practice within the art to which it pertains.