Method and algorithm for performing an NH3 sensor rationality diagnostic
An example method includes interpreting an NH 3 composition value at a position upstream of a selective reduction catalyst (SCR) element fluidly disposed in the exhaust conduit of an engine, interpreting a NO x composition value at a position downstream of the SCR element, and determining an NH 3 sensor rationality threshold in response to the upstream NH 3 composition value. The method further includes determining an NH 3 sensor health value as indicating a sensor failure in response to the downstream NO x composition value exceeding the NH 3 sensor rationality threshold.
1. A system, comprising:
an internal combustion engine producing an exhaust stream as a byproduct of operation; an exhaust conduit fluidly coupled to the engine, and structured to receive the exhaust stream; a selective reduction catalyst (SCR) element fluidly disposed in the exhaust conduit, the SCR catalyst having an upstream side and a downstream side;
an NH 3 sensor operationally coupled to the exhaust conduit and structured to provide an upstream side NH 3 composition value;
a NO x sensor operationally coupled to the exhaust conduit at a position downstream of the SCR element and structured to provide a downstream side NO x composition value; and
a controller, comprising;
a composition module structured to interpret the upstream NH 3 composition value and the downstream NO x composition value;
an NH 3 sensor threshold module structured to determine an NH 3 sensor rationality threshold in response to the upstream NH 3 composition value; and
an NH 3 sensor diagnostic module structured to determine an NH 3 sensor health value for the NH 3 sensor in response to a comparison of the downstream NO x composition value and the NH 3 sensor rationality threshold.
2. The system of claim 1 , wherein the NH 3 sensor diagnostic module is further structured to determine the NH 3 sensor health value as indicating a sensor failure in response to the downstream NO x composition value exceeding the NH 3 sensor rationality threshold.
3. The system of claim 2 , wherein the NH 3 sensor diagnostic module is further structured to determine the NH 3 sensor health value as indicating an in-range failure in response to the upstream NH 3 composition value being an in-range value.
4. The system of claim 1 , further comprising a second SCR element disposed in the exhaust conduit at a position upstream of the NH 3 sensor.
5. The system of claim 1 , further comprising an ammonia oxidation catalyst (AMOX) disposed in the exhaust conduit at a position downstream of the NH 3 sensor and upstream of the NO x sensor.
6. The system of claim 5 , further comprising a second SCR element disposed in the exhaust conduit at a position upstream of the NH 3 sensor.
7. The system of claim 6 , wherein the NH 3 sensor diagnostic module is Wither structured to determine the NH 3 sensor health value as failed in response to the euuation
NOx
(
AMOX
downstream
)
>
x
ANR
(
1
-
η
2
)
+
(
x
-
x
ANR
η
2
)
(
1
-
η
3
)
;
wherein NOx(AMOX downstream )comprises the downstream side NO x composition value, wherein x comprises the upstream side NH 3 composition value, wherein η 2 comprises a NO x conversion efficiency on the SCR catalyst, wherein η 3 comprises an AMOX conversion efficiency, and wherein ANR omprises an NH 3 to NO x ratio.
8. The system of claim 7 , wherein the NH 3 sensor diagnostic. module is further structured to determine the NH 3 sensor health value as failed in-range in response to the NH 3 sensor health value being failed, and the value x being within operational limits of the NH 3 sensor.
9. The system of claim 6 , wherein the NH 3 sensor diagnostic module is further structured to determine the NH3 sensor health value as failed in response to the equation
NOx
(
AMOX
downstream
)
>
x
ANR
(
1
-
η
2
)
+
(
x
-
x
ANR
η
2
)
;
wherein NOx(AMOX downstream ) comprises the downstream side NOx composition value, wherein x comprises the upstream side NH 3 composition value, wherein η 2 comprises a NO x conversion efficiency on the SCR catalyst, and wherein ANR comprises an NH 3 to NO x ratio.
10. The system of claim 9 , wherein the NH 3 sensor diagnostic module is further structured to determine the NH 3 sensor health value as failed in-range in response to the NH 3 sensor health value being failed, and the value x being within operational limits of the NH 3 sensor.
11. The system of claim 1 , wherein the NH 3 sensor rationality threshold comprises the downstream NO x composition value.
12. The system of claim 1 , wherein the NH 3 sensor rationality threshold comprises a multiple of the downstream NO x composition value, wherein the multiple comprises a value between 0.9 and 8.0.
13. The system of claim 1 , wherein the NH 3 sensor diagnostic module is further structured to determine the NH 3 sensor health value as failed in response to the equation
NOx
(
SCR
downstream
)
>
x
ANR
(
1
-
η
2
)
+
(
x
-
x
ANR
η
2
)
;
wherein NOx(SCR downstream ) comprises the downstream side NO x composition value, wherein x comprises the upstream side NH 3 composition value, wherein η 2 comprises a NOx conversion efficiency on the SCR catalyst, wherein ANR comprises an NH 3 to NO x ratio, and wherein k is a cross-sensitivity of the NO x sensor to NH 3 .
14. The system of claim 13 , wherein the NH 3 sensor diagnostic module is further structured to determine the NH 3 sensor health value as failed in-range in response to the NH 3 sensor health value being failed, and the value x being within operational limits of the NH 3 sensor.
15. An electronic controller with instructions on a non-transitory computer readable medium operable to execute a method, comprising:
interpreting an NH 3 composition value from an output of an NH 3 sensor at a position upstream of a selective reduction catalyst (SCR) element fluidly disposed in the exhaust conduit of an engine;
interpreting a NO x composition value at a position downstream of the SCR element;
determining an NH 3 sensor rationality threshold in response to the upstream NH 3 composition value; and
determining an NH 3 sensor health value for the NH 3 sensor as indicating a sensor failure in response to the downstream NO x composition value exceeding the NH 3 sensor rationality threshold.
16. The method of claim 15 , further comprising determining the NH 3 sensor health value in response to determining that the SCR catalyst element temperature is within a NOx conversion efficiency range.
17. The method of claim 15 , further comprising determining the NH 3 sensor health value in response to determining that an ammonia oxidation catalyst (AMOX) positioned downstream of the SCR element is within an NH 3 conversion efficiency range.
18. The method of claim 15 , further comprising determining the NH 3 sensor health value in response to determining that the SCR catalyst element temperature is greater than a storage capacity threshold temperature.
19. The method of claim 15 , further comprising estimating a stored NH 3 variation value, and wherein the determining an NH 3 sensor health value further comprises compensating for the stored NH 3 variation value.
20. The method of claim 15 , further comprising determining a NO x conversion efficiency value of the SCR catalyst element, and wherein determining the NH 3 sensor rationality threshold is further in response to the NO x conversion efficiency value.
21. The method of claim 15 , further comprising determining a NO x conversion efficiency value of the SCR catalyst element, determining an NH 3 conversion efficiency value of an ammonia oxidation catalyst (AMOX) positioned downstream of the SCR element, and wherein determining the NH 3 sensor rationality threshold is further in response to the NO x conversion efficiency value and the NH 3 conversion efficiency value.
22. A system for diagnosing a sensor in an exhaust conduit fluidly coupled to an internal combustion engine operable to produce an exhaust stream as a byproduct of operation, the system comprising:
a selective reduction catalyst (SCR) element fluidly disposed in the exhaust conduit, the SCR catalyst having an upstream side and a downstream side;
an NH 3 sensor operationally coupled to the exhaust conduit and structured to provide an upstream side NH 3 composition value;
a NO x sensor operationally coupled to the exhaust conduit at a position downstream of the SCR element and structured to provide a downstream site NO x composition value; and
an electronic controller to determine an NH 3 sensor health value for the NH 3 sensor in response to the upstream NH 3 composition value and the downstream NO x composition value.
23. The system of claim 22 , wherein the electronic controller is further configured to determine an NH 3 sensor rationality threshold in response to the upstream NH 3 composition value.
24. The system of claim 23 , wherein the electronic controller is further configured to indicate a sensor failure in response to the downstream NO x composition value exceeding the NH 3 sensor rationality threshold.
25. The system of claim 24 , wherein the electronic controller is further configured to determine the NH 3 sensor health value indicates an in-range failure.
26. The system of claim 24 , wherein the NH 3 sensor rationality threshold comprises at least one value selected from the values consisting of: the downstream NO x composition value, a multiple of the downstream NO x composition value, wherein the multiple comprises a value between 0.9 and 8.0.
27. The system of claim 24 , wherein the multiple comprises a value selected from the values consisting of: 0.4, 0.5, 0.8, 0.9, 1.0, 2.0, 2.5, 3.0, and 8.0.
28. The system of claim 22 , further comprising an ammonia oxidation catalyst disposed in the exhaust conduit at a position downstream of the NH 3 sensor and upstream of the NO x sensor.
29. The system of claim 28 , wherein the electronic controller is further configured to determine whether NH 3 sensor check conditions are present, and prevent determining the NH 3 sensor health value in response to the NH 3 sensor check conditions not being present.
30. The system of claim 29 , wherein the NH 3 sensor check conditions comprise at least one condition selected from the conditions consisting of: an SCR catalyst element storage capacity being lower than a threshold value, the SCR catalyst element being saturated with adsorbed NH 3 , a temperature of the SCR catalyst element exceeding a threshold, and an exhaust flow value exceeding a threshold.
31. The system of claim 29 , wherein the controller is further configure to perform a sensor failure verification operation.
32. The system of claim 31 , wherein the sensor failure verification operation comprises at least one operation selected from the operations consisting of averaging a plurality of sensor test results, incrementing a fault counter in response to a sensor test indicating a failed sensor, decrementing a fault counter in response to a sensor test indicating a passed sensor, integrating the upstream NH 3 composition value and the downstream NO x composition value over a predetermined period of time and comparing the integrated values to the NH 3 sensor rationality threshold, and modeling NH 3 storage on the SCR catalyst and accounting for the storage in the sensor failure verification operation.