System and method for controlling ammonia levels in a selective catalytic reduction catalyst using a nitrogen oxide sensor

1. A system comprising:

a first electronic circuit configured to determine an air/fuel ratio based on an input from an air/fuel ratio sensor positioned downstream from a three-way catalyst that is positioned upstream from a selective catalytic reduction (SCR) catalyst;

a second electronic circuit configured to:

determine an ammonia level based on an input received from at least one nitrogen oxide (NOx) sensor positioned downstream from the three-way catalyst when the air/fuel ratio is rich; and

determine that the ammonia level is equal to a predetermined value when the air/fuel ratio is lean; and

a third electronic circuit configured to control an actuator of an engine to adjust the air/fuel ratio based on the ammonia level, wherein:

the at least one NOx sensor includes a first NOx sensor positioned at an inlet of the SCR catalyst and a second NOx sensor positioned downstream from the inlet of the SCR catalyst;

the second electronic circuit is configured to determine a first ammonia level and a second ammonia level based on an input received from the first NOx sensor and the second NOx sensor, respectively,

the third electronic circuit is configured to adjust the air/fuel ratio to one of stoichiometric and rich when a ratio of the second ammonia level to the first ammonia level is less than a first predetermined value; and

the third electronic circuit is configured to adjust the air/fuel ratio to lean when the ratio of the second ammonia level to the first ammonia level is greater than or equal to a second predetermined value.

2. The system of claim 1 wherein the predetermined value is approximately zero.

3. The system of claim 1 wherein the second NOx sensor is positioned at an outlet of the three-way catalyst.

4. The system of claim 3 further comprising a fourth electronic circuit configured to estimate an ammonia storage level in the SCR catalyst based on the ammonia level, an exhaust temperature, and an exhaust flow rate.

5. The system of claim 4 wherein the third electronic circuit is configured to adjust the air/fuel ratio to one of stoichiometric and rich when the ammonia storage level is less than a first predetermined level and adjusts the air/fuel ratio to lean when the ammonia storage level is greater than or equal to a second predetermined level.

6. The system of claim 1 further comprising a fourth electronic circuit configured to determine an ammonia storage level in the SCR catalyst based on the first ammonia level and the second ammonia level.

7. The system of claim 1 further comprising a fourth electronic circuit configured to determine an ability of the SCR catalyst to store ammonia based on the first ammonia level and the second ammonia level.

8. The system of claim 1 wherein the first, second, and third electronic circuits include at least one of an Application Specific Integrated Circuit (ASIC), a combinational logic circuit, a field programmable gate array (FPGA), a processor, and memory.

9. A method comprising:

determining an air/fuel ratio based on an input from an air/fuel ratio sensor positioned downstream from a three-way catalyst that is positioned upstream from a selective catalytic reduction (SCR) catalyst;

determining an ammonia level based on an input received from at least one nitrogen oxide (NOx) sensor positioned downstream from the three-way catalyst when the air/fuel ratio is rich, wherein the at least one NOx sensor includes a first NOx sensor positioned at an inlet of the SCR catalyst and a second sensor NOx positioned downstream from the inlet of the SCR catalyst;

determining that the ammonia level is equal to a predetermined value when the air/fuel ratio is lean;

controlling an actuator of an engine to adjust the air/fuel ratio based on the ammonia level;

determining a first ammonia level and a second ammonia level based on an input received from the first NOx sensor and the second NOx sensor, respectively;

adjusting the air/fuel ratio to one of stoichiometric and rich when a ratio of the second ammonia level to the first ammonia level is less than a first predetermined value; and

adjusting the air/fuel ratio to lean when the ratio of the second ammonia level to the first ammonia level is greater than or equal to a second predetermined value.

10. The method of claim 9 wherein the predetermined value is approximately zero.

11. The method of claim 9 wherein the second NOx sensor is positioned at an outlet of the three-way catalyst.

12. The method of claim 11 further comprising estimating an ammonia storage level in the SCR catalyst based on the ammonia level, an exhaust temperature, and an exhaust flow rate.

13. The method of claim 12 further comprising:

adjusting the air/fuel ratio to one of stoichiometric and rich when the ammonia storage level is less than a first predetermined level; and

adjusting the air/fuel ratio to lean when the ammonia storage level is greater than or equal to a second predetermined level.

14. The method of claim 9 further comprising determining an ammonia storage level in the SCR catalyst based on the first ammonia level and the second ammonia level.

15. The method of claim 9 further comprising determining an ability of the SCR catalyst to store ammonia based on the first ammonia level and the second ammonia level.

16. A system comprising:

a first electronic circuit configured to:

determine a first ammonia level based on an input received from a first nitrogen oxide (NOx) sensor positioned at an inlet of a selective catalytic reduction (SCR) catalyst; and

determine a second ammonia level based on an input received from a second NOx sensor positioned downstream from the inlet of the SCR catalyst;

a second electronic circuit configured to:

determine an ammonia storage efficiency of the SCR catalyst based on the first and second ammonia levels; and

determine an ability of the SCR catalyst to store ammonia based on the ammonia storage efficiency of the SCR catalyst and a temperature of the SCR catalyst; and

a third electronic circuit configured to control an actuator of an engine to adjust an air/fuel ratio based on at least one of the first and second ammonia levels.

17. The system of claim 16 wherein the second electronic circuit determines the ammonia storage ability of the SCR catalyst based on a current value of the ammonia storage efficiency at the temperature of the SCR catalyst and a previous value of the ammonia storage efficiency at the same temperature of the SCR catalyst.

18. The system of claim 16 further comprising a fourth electronic circuit configured to estimate the ammonia storage efficiency of the SCR catalyst based on the temperature of the SCR catalyst, wherein the second electronic circuit determines the ammonia storage ability of the SCR catalyst based on the determined and estimated ammonia storage efficiencies.

19. The system of claim 16 wherein the first and second electronic circuits include at least one of an Application Specific Integrated Circuit (ASIC), a combinational logic circuit, a field programmable gate array (FPGA), a processor, and memory.

20. A method comprising:

determining a first ammonia level based on an input received from a first nitrogen oxide (NOx) sensor positioned at an inlet of a selective catalytic reduction (SCR) catalyst;

determining a second ammonia level based on an input received from a second NOx sensor positioned downstream from the inlet of the SCR catalyst;

determining an ammonia storage efficiency of the SCR catalyst based on the first and second ammonia levels;

determining an ability of the SCR catalyst to store ammonia based on the ammonia storage efficiency of the SCR catalyst and a temperature of the SCR catalyst; and

controlling an actuator of an engine to adjust an air/fuel ratio based on at least one of the first and second ammonia levels.

21. The method of claim 20 further comprising determining the ammonia storage ability of the SCR catalyst based on a current value of the ammonia storage efficiency at the temperature of the SCR catalyst and a previous value of the ammonia storage efficiency at the same temperature of the SCR catalyst.

22. The method of claim 20 further comprising:

estimating the ammonia storage efficiency of the SCR catalyst based on the temperature of the SCR catalyst; and

determining the ammonia storage ability of the SCR catalyst based on the determined and estimated ammonia storage efficiencies.