Control system for mobile NOx SCR applications

• US 20040128982A1
• Filed: 12/17/2003
• Published: 07/08/2004
• Est. Priority Date: 10/16/2000
• Status: Active Grant

• Current Assignee: Engelhard Corporation (BASF SE)
• Original Assignee: Engelhard Corporation (BASF SE)
• Inventors: Genderen, Mathus Van, Patchett, Joseph A., Calabrese, John Lawrence, Rice, Gary Wayne, Verbeek, Rudolfus Petrus, Grimston, Karl Richard
• Granted Patent: US 7,150,145 B2
• Time in Patent Office: Days
• US Class Current: 60/274
• CPC Class Codes: B01D 53/90 Injecting reactants
• CPC Class Codes: B01D 53/9431 Processes characterised by a specific device
• CPC Class Codes: B01D 53/9495 Controlling the catalytic process
• CPC Class Codes: F01N 13/009 having two or more separate purifying devices arranged in series
• CPC Class Codes: F01N 2570/18 Ammonia
• CPC Class Codes: F01N 2610/02 the substance being ammonia or urea
• CPC Class Codes: F01N 2610/08 with prior mixing of the substances with a gas, e.g. air
• CPC Class Codes: F01N 2610/12 the substance being in solid form, e.g. pellets or powder
• CPC Class Codes: F01N 2900/0404 using a data filter
• CPC Class Codes: F01N 2900/14 said parameters being related to the exhaust gas
• CPC Class Codes: F01N 2900/1402 Exhaust gas composition
• CPC Class Codes: F01N 2900/1622 Catalyst reducing agent absorption capacity or consumption amount
• CPC Class Codes: F01N 2900/1808 Pressure
• CPC Class Codes: F01N 3/103 Oxidation catalysts for HC and CO only
• CPC Class Codes: F01N 3/208 Control of selective catalytic reduction [SCR], e.g. dosing of reducing agent
• CPC Class Codes: F01N 5/02 the devices using heat
• CPC Class Codes: F01N 9/005 using models instead of sensors to determine operating characteristics of exhaust systems, e.g. calculating catalyst temperature instead of measuring it directly
• CPC Class Codes: Y02A 50/20 Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
• CPC Class Codes: Y02T 10/12 Improving ICE efficiencies
• CPC Class Codes: Y02T 10/40 Engine management systems

Abstract

A diesel powered vehicle is provided with an SCR system which uses an external reducing reagent to convert NOx emissions in a manner which accounts for the effects of NOx transient emissions on the reducing catalyst. Actual NOx emissions produced by the engine are filtered using a variable NOx time constant in turn correlated to the reductant/NOx storage capacity of the reducing catalyst at its current temperature to account for changes in the SCR system attributed to NOx transient emissions. Catalyst temperature is filtered using a variable catalyst time, constant corresponding to current space velocity of the exhaust gas to account for changes in the catalyst temperature attributed to NOx transient emissions. The reductant is metered on the basis of the filtered, corrected NOx concentration applied at a NSR ratio based, in turn, on the filtered, corrected reducing catalyst temperature.

Claims

1. ) A method for reducing NOx emissions produced in mobile internal combustion engine applications by an external reductant supplied to an SCR system having a reducing catalyst comprising the steps of:

a) sensing one or more engine operating parameters to predict a concentration of NOx emissions indicative of the actual quantity of NOx emissions produced by the engine;

b) when the actual concentration of NOx emissions changes and the temperature of said reducing catalyst is within a set range, varying the actual concentration of NOx emissions, by a time constant to produce a calculated concentration of NOx emissions different than the actual concentration of NOx emissions; and

,

c) metering the external reductant to the reducing catalyst in said SCR system at a rate sufficient to cause the reducing catalyst to reduce said calculated concentration of NOx emissions whereby metering of the reductant accounts for the effects on said SCR system attributed to transient NOx emissions.

2. ) The method of claim 1 wherein when said actual NOx emissions increase, said time constant results in a calculated NOx emission concentration which is less than the increased actual NOx emissions produced by said internal combustion engine.

3. ) The method of claim 2 wherein when said actual NOx emissions decrease, said time constant results in a calculated NOx emission concentration which is greater than the decreased actual NOx emissions produced by said internal combustion engine.

4. ) The method of claim 3 wherein said time constant corresponds to the capacity of any given reducing catalyst to at least store the external reductant at said given reducing catalyst'"'"'s current temperature and varies for different catalyst temperatures within a set temperature range.

5. ) The method of claim 4 wherein the change in actual emissions is positively determined to be increasing or decreasing by considering the rate of change of the temperature of the catalyst with respect to time according to the relationship dT/dt where T is the reducing catalyst temperature and t is time.

6. ) The method of claim 1 further including the step of ceasing reductant metering when the temperature of said reducing catalyst is below said range or when the engine is decelerating.

7. ) The method of claim 1 wherein said time constant is a variable correlated to the time any given catalyst can store and/or release a given quantity of reductant at a given catalyst temperature within said set catalyst temperature range, said storage time decreasing as said catalyst temperature increases within said set temperature range.

8. ) The method of claim 7 wherein said reductant is ammonia and the storage and release capacity of said reducing catalyst includes the surface area of said catalyst over which said exhaust gases flow and the number and strength of ammonia adsorption/absorption sites on said surface area.

9. ) The method of claim 8 wherein said storage capacity of said catalyst also includes the ability of said catalyst to store NOx at a given temperature within said catalyst temperature range.

10. ) The method of claim 1 wherein said time constant is implemented through an NOx filter.

11. ) The method of claim 10 wherein said NOx filter includes two first order filters in series with one another.

12. ) The method of claim 10 wherein said NOx filter in a continuous time domain filters the actual concentration of NOx emissions in accordance with the transfer function

13. ) The method of claim 12 wherein is a function of the temperature of said reducing catalyst correlated to the capacity of said reducing catalyst to at least store said reductant at said catalyst temperature and designated τ

_NOX.

14. ) The method of claim 13 wherein said NOx filter is in discreet form and implemented by a microprocessor, said NOx filter in said discrete form including two first order filters in series with one another.

15. ) The method of claim 14 wherein the first NOx filter in discrete form is represented by difference equation

16. ) The method of claim 13 further including the step of determining the temperature of said reducing catalyst by sensing or calculating the temperature of the exhaust gas and filtering the temperature of the exhaust gas by a catalyst temperature filter as a function of the changing space velocity of the exhaust gas to establish a functional catalyst temperature, said functional catalyst temperature used to determine said calculated concentration of NOx emissions.

17. ) The method of claim 16 wherein said catalyst temperature filter in the continuous time domain is represented by the transfer function

18. ) The method of claim 17 wherein said catalyst temperature filter is in discreet form and implemented by a microprocessor, said filter in said discrete form including two first order filters in series with one another.

19. ) The method of claim 18 wherein the first catalyst filter in discrete form is represented by difference equation

20. ) The method of claim 19 wherein

21. ) The method of claim 18 further including the steps of sensing ambient temperature and upon shut down of the engine in said mobile application, filtering said functional catalyst temperature by the effect of said ambient temperature as a function of the time said engine has been shut down to determine a functional cool down catalyst temperature and using said cool down temperature as said functional catalyst temperature upon restart of said engine.

22. ) The method of claim 21 wherein said step of filtering said functional catalyst temperature to produce said cool down catalyst temperature is represented by a first order cool down filter, said cool down filter implemented in the continuous time domain by the transfer function

23. ) The method of claim 22 wherein said cool down filter is implemented in a discrete form by said microprocessor.

24. ) The method of claim 23 wherein said cool down filter is effected in said catalyst filter by switching said first and second catalyst filters into a parallel arrangement upon shut down of said engine and switching said first and second catalyst filters back to said series relationship upon engine start.

25. ) The method of claim 24 wherein said cool down filter in discrete form is represented by the difference equation

26. ) The method of claim 16 further including the step of determining a normalized stoichiometric ratio (NSR) of reductant to NOx at which ratio a set quantity of reductant is to be injected into the exhaust stream for a set quantity of NOx emissions and factoring said calculated NOx emissions by said NSR to determine the rate at which said external reductant is to be metered to said reducing catalyst.

27. ) The method of claim 26 wherein said step of determining said NSR includes the step of using said functional catalyst temperature to select a NSR value representing a desired stoichiometric based relationship between said external reductant and said calculated NOx emissions.

28. ) The method of claim 12 further including the step of adjusting said NSR ratio by an additional factor correlated to the ageing of said reducing catalyst whereby the rate of injection is reduced as said reducing catalyst ages.

29. ) The method of claim 12 wherein said reducing catalyst is a zeolite or a mixture of titanium, vanadium, tungsten, and/or molybdenum oxide.

30. ) A method for determining the functional temperature of a catalyst in an exhaust system of a vehicle comprising the steps of

i) determining, by sensing or calculating, the temperature of the exhaust, gases and the space velocity of the exhaust gases through the catalyst;

ii) filtering the exhaust gas temperature by a catalyst filter to generate the functional temperature of the catalyst, the catalyst filter implementing a time constant determined as a function of changing space velocity to filter the exhausts gas temperature.

31. ) The method of claim 30 wherein said exhaust gas temperature is the temperature of the exhaust gas, sensed or calculated, at the inlet of the reducing catalyst.

32. ) The method of claim 31 wherein said operating parameters sensed to determine the temperature of said exhaust gas upstream of said SCR includes one or more or the following:

i) the engine coolant temperature with and without reference to the ambient temperature;

ii) fueling and combustion air temperature; and

,

iii) the temperature of the exhaust gas upstream of the SCR catalyst.

33. ) The method of claim 30 wherein said catalyst filter in the continuous time domain is represented by the transfer function

34. ) The method of claim 33 wherein said catalyst filter is in discreet form and implemented by a microprocessor, said filter in said discrete form including two first order filters in series with one another.

35. ) The method of claim 34 wherein the first catalyst filter in discrete form is represented by difference equation

36. ) The method of claim 35 wherein

37. ) The method of claim 30 further including the steps of sensing ambient temperature and upon shut down of the engine in said mobile application, filtering said functional catalyst temperature by the effect of said ambient temperature as a function of the time said engine has been shut down or the difference between ambient and the cooled down temperature to determine a functional cool down catalyst temperature and using said cool down temperature as said functional catalyst temperature upon restart of said engine.

38. ) The method of claim 37 wherein said step of filtering said functional catalyst temperature to produce said cool down catalyst temperature is represented by a first order cool down filter, said cool down filter implemented in the continuous time domain by the transfer function

39. ) The method of claim 30 further including the steps of sensing ambient temperature after shut down of the engine and filtering the catalyst temperature by a first order filter using a cool down time constant established as a function of time elapsed from engine shut down at any given catalyst temperature at shut down or as a function of the difference in temperature between ambient temperature and catalyst shut down temperature whereby the catalyst temperature upon engine restart is determined.

40. ) A system for metering an external reductant to a reducing catalyst in an SCR system applied to a vehicle powered by an internal combustion engine comprising:

a) means for sensing operating conditions of the vehicle and engine to generate, by calculation and/or measurement, signals indicative of the actual quantity of NOx emissions emitted by the engine, the temperature of the exhaust gas and the space velocity of the exhaust gas;

b) means for filtering the actual NOx emission signal by an NOx time constant to produce a calculated NOx signal different than the actual NOx signal when the NOx signal is changing and when the temperature of the catalyst is within a set temperature range;

c) means for filtering the exhaust gas temperature signal by a catalyst time constant to produce a functional catalyst temperature signal different than the exhaust gas temperature when the space velocity signal changes;

d) means for factoring the functional catalyst temperature signal and the space velocity signal to generate a NSR signal indicative of a normalized stoichiometric ratio of reductant to NOx emissions; and

,

e) means for metering the reductant to the reducing catalyst by factoring the calculated NOx signal by the NSR signal to produce a metering signal controlling a metering device for the external reductant.

41. ) The system of claim 40 wherein said means for filtering said NOx emissions includes an NOx filter and said means for filtering said exhaust gas temperature includes a catalyst filter;

both said NOx and catalyst filters operable in a continuous time domain to perform a transfer function represented by the differential equation

42. ) The system of claim 41 wherein both NOx and catalyst filters are implemented in discrete form as second order filters, each including a first order filter in series with a second first order filter.

43. ) The system of claim 42 wherein the first NOx filter in discrete form is represented by difference equation

44. ) The system of claim 43 wherein the first catalyst filter in discrete form is represented by difference equation

45. ) The system of claim 44 wherein said reducing catalyst is a zeolite or a mixture of titanium, vanadium, tungsten, and/or molybdenum oxide.

46. ) A method for controlling dosage of an external reductant supplied to an SCR system including a reducing catalyst in a mobile application having an IC engine to account for transient NOx emissions produced by the engine, said method comprising the steps of:

a) determining the actual NOx emissions produced by the engine to generate an NOx actual signal;

b) filtering the NOx actual signal when the reducing catalyst is within a set temperature change to produce a variable delay in the NOx actual signal; and

c) metering the reductant in real time at the value of the delayed NOx actual signal.

47. ) The method of claim 46 where the filtering step is effected by an NOx filter having an NOx time constant determined as a function of the capacity of the reducing catalyst at the temperature of the catalyst.

48. ) The method of claim 47 wherein said time constant is a variable correlated to the time any given catalyst can store and/or release a given quantity of reductant at a given catalyst temperature within said set catalyst temperature range, said storage time decreasing as said catalyst temperature increases within said set temperature range.

49. ) The method of claim 47 wherein said NOx filter in a continuous time domain filters the actual concentration of NOx emissions in accordance with the transfer function

50. ) The method of claim 49 wherein said NOx filter is in discreet form and implemented by a microprocessor, said NOx filter in said discrete form including two first order filters in series with one another.

51. ) The method of claim 50 wherein the first NOx filter in discrete form is represented by difference equation

52. ) The method of claim 51 further including the step of determining the temperature of said reducing catalyst by sensing or calculating the temperature of the exhaust gas and filtering the temperature of the exhaust gas by a catalyst filter when the space velocity of the exhaust gas changes to establish a functional catalyst temperature.

53. ) The method of claim 52 wherein said catalyst filter in the continuous time domain is represented by the transfer function

54. ) The method of claim 53 wherein said catalyst filter is in discreet form and implemented by a microprocessor, said filter in said discrete form including two first order filters in series with one another.

55. ) The method of claim 54 wherein the first catalyst filter in discrete form is represented by difference equation

56. ) The method of claim 55 wherein

57. ) The method of claim 46 further including the steps of sensing ambient temperature after shut down of the engine and filtering the catalyst temperature by a first order filter using a cool down time constant established as a function of time elapsed from engine shut down at any given catalyst temperature at shut down or as a function of the difference in temperature between ambient temperature and catalyst shut down temperature whereby the catalyst temperature upon engine restart is determined.