Method and system for reducing lean-burn vehicle emissions using a downstream reductant sensor

• US 6,487,853 B1
• Filed: 06/19/2001
• Issued: 12/03/2002
• Est. Priority Date: 06/19/2001
• Status: Expired due to Fees

• Current Assignee: Ford Global Technologies, LLC (Ford Motor Company)
• Original Assignee: Ford Global Technologies, LLC (Ford Motor Company)
• Inventors: Hepburn, Jeffrey Scott, Dearth, Mark Allen, Temple, JoAnne
• Primary Examiner(s): Denion, Thomas
• Assistant Examiner(s): TRAN, BINH Q
• Application Number:
  US09/884,760
• Publication Number:
  US 20020189246A1
• Time in Patent Office: 532 Days
• Field of Search: 602/74, 602/77, 602/85, 602/95, 602/97, 603/01, 602/86, 602/76
• US Class Current: 60/295
• CPC Class Codes: F01N 13/009 having two or more separate purifying devices arranged in series
• CPC Class Codes: F01N 13/0093 the purifying devices are of the same type
• CPC Class Codes: F01N 2570/04 Sulfur or sulfur oxides
• CPC Class Codes: F01N 2570/16 Oxygen
• CPC Class Codes: F01N 3/0842 Nitrogen oxides
• CPC Class Codes: F02D 2041/389 for injecting directly into the cylinder
• CPC Class Codes: F02D 2200/0806 NOx storage amount, i.e. amount of NOx stored on NOx trap
• CPC Class Codes: F02D 2200/0808 NOx storage capacity, i.e. maximum amount of NOx that can be stored on NOx trap
• CPC Class Codes: F02D 41/028 Desulfurisation of NOx traps or adsorbent
• CPC Class Codes: F02D 41/1441 Plural sensors
• CPC Class Codes: F02D 41/1446 the characteristics being exhaust temperatures
• CPC Class Codes: F02D 41/1456 with sensor output signal being linear or quasi-linear with the concentration of oxygen
• CPC Class Codes: F02D 41/1475 Regulating the air fuel ratio at a value other than stoichiometry
• CPC Class Codes: F02D 41/187 using a hot wire flow sensor

Abstract

A method and system for controlling the operation of a lean-burn engine whose exhaust gas is directed through an emission control device and a downstream reductant-concentration sensor, wherein a stored value for the device'"'"'s instantaneous capacity to store a selected exhaust gas constituent, such as NO_x, is periodically adaptively updated when the sensor'"'"'s output signal falls outside a predetermined range during a device purge event. A device purge event is scheduled when an accumulated measure of instantaneous feedgas NO_x concentration during lean engine operation exceeds the stored NO_x-storage capacity value. The purge event is discontinued when the sensor'"'"'s output signal exceeds the upper limit of the predetermined range, or when a determined value representing a cumulative amount of excess fuel supplied to the engine during the purge event exceeds a threshold value calculated based upon previous values for stored NO_x and stored oxygen.

Claims

1. A method for controlling an engine operating over a range of operating conditions characterized by combustion of air-fuel mixtures that are lean and rich of a stoichiometric air-fuel ratio to generate exhaust gas, wherein the exhaust gas is directed through an upstream emission control device and a downstream sensor that generates an output signal representing a concentration of reductants in the exhaust gas exiting the device, the method comprising:

determining, during a lean operating condition characterized by combustion of an air-fuel mixture having a lean air-fuel ratio, a first value representing a cumulative amount of a selected constituent of the exhaust gas being generated by the engine;

comparing the first value to a previously stored second value representing an instantaneous capacity of the device to store the selected constituent, wherein the second value is periodically updated as a function of an amplitude of the output signal generated by the reductant sensor and at least one predetermined reference value;

and selecting an engine operating condition as a function of the first and second values.

2. The method of claim 1, wherein determining the first value includes estimating an instantaneous amount of the selected constituent generated by the engine as a function of at least one of the group consisting of an engine speed, an engine load, an ignition timing, an air-fuel ratio, and EGR.

3. The method of claim 1, wherein periodically updating the second value includes:

comparing, during a rich operating condition characterized by combustion of an air-fuel mixture having a rich air-fuel ratio, the output signal with a predetermined maximum reference value; and

increasing the second value by a predetermined amount based upon the comparison of the output signal with the predetermined maximum reference value.

4. The method of claim 3, wherein increasing includes increasing the second value by the predetermined amount when an amplitude of the output signal exceeds the predetermined maximum reference value.

5. The method of claim 4, wherein the selecting step includes discontinuing the rich operating condition when the amplitude of the output signal exceeds the predetermined maximum reference value.

6. The method of claim 1, wherein selecting includes comparing, during a lean operating condition characterized by combustion of an air-fuel mixture having a lean air-fuel ratio, the first value to the second value;

and

discontinuing the lean operating condition when the first value exceeds the second value.

7. The method of claim 1, wherein selecting includes:

calculating, during a rich operating condition characterized by combustion of an air-fuel mixture having a rich air-fuel ratio, a third value representing an amount of fuel, in excess of a stoichiometric amount of fuel sufficient to provide an air-fuel mixture having a stoichiometric air-fuel ratio, required to release stored selected constituent and stored oxygen from the device as a function of the second value and a previously stored fourth value representing an amount of excess fuel required to release only stored oxygen from the device;

determining a fifth value representing a cumulative amount of fuel, in excess of the stoichiometric amount, supplied to the engine during the rich operating condition; and

discontinuing the rich operating condition when the fifth value exceeds the third value.

8. The method of claim 7, wherein determining the fifth value includes:

comparing, during a lean operating condition characterized by combustion of an air-fuel mixture having a lean air-fuel ratio, the output signal to the predetermined maximum reference value; and

increasing or decreasing the fifth value by a predetermined amount based upon the comparison of the output signal with the first predetermined reference value.

9. The method of claim 8, wherein increasing the fifth value includes increasing the fifth value by the predetermined amount when an amplitude of the output signal exceeds the predetermined maximum reference value.

10. The method of claim 8, wherein decreasing the fifth value includes decreasing the fifth value by the predetermined amount when an amplitude of the output signal is less than a predetermined minimum reference value.

11. The method of claim 7, wherein determining the fifth value includes:

comparing, during a rich operating condition characterized by combustion of an air-fuel mixture having a rich air-fuel ratio, the output signal to a set of reference values including the predetermined maximum reference value; and

if the fifth value does not exceed the third value, decreasing the fifth value by the predetermined amount based upon the comparison of the output signal with the set of reference values.

12. The method of claim 10, wherein decreasing the fifth value includes decreasing the fifth value by the predetermined amount when an amplitude of the output signal is less than the predetermined maximum reference value.

13. The method of claim 1, wherein selecting includes:

comparing, during a lean operating condition characterized by combustion of an air-fuel mixture having a lean air-fuel ratio, the second value to a minimum device capacity value; and

selecting a device-desulfating engine operating condition when the first value exceeds the second value, and the second value falls below the minimum device capacity value.

14. The method of claim 13, further including indicating device deterioration if a predetermined number of device-desulfating engine operating conditions are performed without a significant increase in a maximum value for the first value.

15. A system for controlling an engine, wherein the engine operates over a range of operating conditions characterized by combustion of air-fuel mixtures that are lean and rich of a stoichiometric air-fuel ratio to generate exhaust gas, wherein the exhaust gas is directed through an upstream emission control device and a downstream sensor that generates an output signal representing a concentration of reductants in the exhaust gas exiting the device, the system comprising:

a controller including a microprocessor arranged to determine, during a lean operating condition characterized by combustion of an air-fuel mixture having a lean air-fuel ratio, a first value representing a cumulative amount of a selected constituent of the exhaust gas being generated by the engine, and wherein the controller is further arranged to compare the first value to a previously stored second value representing an instantaneous capacity of the device to store the selected constituent, wherein the second value is periodically updated as a function of an amplitude of the output signal generated by the reductant sensor and at least one predetermined reference value; and

to select an engine operating condition as a function of the first and second values.

16. The system of claim 15, wherein the controller is further arranged to compare, during a rich operating condition characterized by combustion of an air-fuel mixture having a rich air-fuel ratio, the output signal with a predetermined maximum reference value, and to increase the second value by a predetermined amount when an amplitude of the output signal exceeds the predetermined maximum reference value.

17. The system of claim 16, wherein the controller is further arranged to discontinue the rich operating condition when the amplitude of the output signal exceeds the predetermined maximum reference value.

18. The system of claim 16, wherein the controller is further arranged to calculate, during the rich operating condition, a third value representing an amount of fuel, in excess of a stoichiometric amount of fuel sufficient to provide an air-fuel mixture having a stoichiometric air-fuel ratio, required to release stored selected constituent and stored oxygen from the device as a function of the second value and a previously stored fourth value representing an amount of excess fuel required to release only stored oxygen from the device, and to determine a fifth value representing a cumulative amount of fuel, in excess of the stoichiometric amount, supplied to the engine during the rich operating condition;

and wherein the controller is further arranged to discontinue the rich operating condition when the fifth value exceeds the third value.

19. The system of claim 18, wherein the controller is further arranged to compare, during a lean operating condition characterized by combustion of an air-fuel mixture having a lean air-fuel ratio, the output signal to the predetermined maximum reference value, and to increase or decrease the fifth value by a predetermined amount based upon the comparison of the output signal with the first predetermined reference value.

20. The system of claim 18, wherein the controller is further arranged to compare, during the rich operating condition, the output signal to a set of reference values including the predetermined maximum reference value;

and

if the fifth value does not exceed the third value, decreasing the fifth value by the predetermined amount based upon the comparison of the output signal with the set of reference values.

21. The system of claim 15, wherein the controller is further arranged to compare, during a lean operating condition characterized by combustion of an air-fuel mixture having a lean air-fuel ratio, the first value to the second value, and to discontinue the lean operating condition when the first value exceeds the second value.

22. The system of claim 15, wherein the controller is further arranged to indicate device deterioration if a predetermined number of device-desulfating engine operating conditions are performed without a significant increase in a maximum value for the first value.