Ammonia sensor control, with NO

1. A method, comprising:

determining an ammonia (NH 3 ) reference target comprising a target amount of NH 3 in an exhaust conduit between a first selective catalytic reduction (SCR) catalyst and a second SCR catalyst located in the exhaust conduit;

determining a present amount of NH 3 in the exhaust conduit between the first SCR catalyst and the second SCR catalyst with an ammonia sensor;

determining an NH 3 error term in response to the NH 3 reference target and the present amount of NH 3 ;

determining a reductant doser command in response to the NH 3 error term;

determining an amount of NO x downstream of the second SCR catalyst, and adjusting one of the NH 3 reference target and a reductant doser command in response to the amount of NO x downstream of the second SCR catalyst; and

injecting, with a reductant injector, an amount of reductant in response to the adjusted one of the NH 3 reference target and the reductant doser command.

2. The method of claim 1 , further comprising determining a rate of change of the amount of NO x downstream of the second SCR catalyst, and further adjusting the one of the NH 3 reference target and the reductant doser command in response to the rate of change of the amount of NO x downstream of the second SCR catalyst.

3. The method of claim 1 , further determining an amount of NH 3 downstream of the second SCR catalyst, and further adjusting the one of the NH 3 reference target and the reductant doser command in response to the amount of NH 3 downstream of the second SCR catalyst.

4. The method of claim 3 , further comprising determining an adjusted downstream NO x amount by subtracting the amount of NH 3 downstream of the second SCR catalyst from the amount of NO x downstream of the second SCR catalyst, and further adjusting the one of the NH 3 reference target and the reductant doser command in response to the adjusted downstream NO x amount.

5. The method of claim 4 , further comprising determining an excess downstream NO x amount in response to the adjusted downstream NO x amount and a NO x reference target, and further adjusting the one of the NH 3 reference target and the reductant doser command in response to the excess downstream NO x amount.

6. An apparatus, comprising:

an electronic controller operably connected to a reductant doser, the electronic controller including a plurality of modules that are implemented as at least one of hardware and instructions on a computer readable medium, the plurality of modules including:

an ammonia (NH 3 ) target module structured to determine an NH 3 reference target comprising a target amount of NH 3 in an exhaust conduit between a first selective catalytic reduction (SCR) catalyst and a second SCR catalyst, wherein the NH 3 reference target is the target amount of NH 3 that occurs at a mid-bed position between the first SCR catalyst and the second SCR catalyst in response to injection of a targeted amount of reductant upstream of the first SCR catalyst;

an NH 3 determination module structured to determine a present amount of NH 3 in the exhaust conduit between the first SCR catalyst and the second SCR catalyst in response to an output from an ammonia sensor that defines a separation point between the first SCR catalyst and the second SCR catalyst at the mid-bed position;

NH 3 an error module structured to determine an NH 3 error term in response to the NH 3 reference target and the present amount of NH 3 ;

a dosing control module structured to provide a reductant doser command in response to the NH 3 error term; and

an outer loop control module structured to determine an amount of NO x downstream of the second SCR catalyst, and to adjust one of the NH 3 reference target and the reductant doser command in response to the amount of NO x downstream of the second SCR catalyst and control the reductant doser to inject reductant in response to the adjusted one of the NH 3 reference target and the reductant doser command.

7. The apparatus of claim 6 , further comprising an NH 3 time response adjustment module structured to determine at least one of a rate of change of the present amount of NH 3 and a rate of change of the amount of NO x downstream of the second SCR catalyst, and to further adjust the one of the NH 3 reference target and the reductant doser command in response to the at least one rate of change.

8. The apparatus of claim 6 , further comprising an NH 3 time response adjustment module structured to determine a product of a rate of change of the present amount of NH 3 and a rate of change of the amount of NO x downstream of the second SCR catalyst, and to further adjust the one of the NH 3 reference target and the reductant doser command in response to the product.

9. the apparatus of claim 8 , wherein the NH 3 time response adjustment module is further structured to determine a rate-based adjustment amount as a function of the product, and to further adjust the one of the NH 3 reference target and the reductant doser command in response to the rate-based adjustment amount.

10. The apparatus of claim 9 , wherein the NH 3 time response adjustment module is further structured to limit the rate-based adjustment amount to a proportion of an amount of NO x upstream of the first SCR catalyst.

11. The apparatus of claim 9 , wherein the function of the product comprises a non-linear function having a low gain in a negative region of the product and a high gain in a positive region of the product.

12. The apparatus of claim 6 , further comprising a NO x trimming module structured to determine an adjusted downstream NO x amount in response to the amount of NO x downstream of the second SCR catalyst and an amount of NH 3 downstream of the second SCR catalyst, and to further adjust the one of the NH 3 reference target and the reductant doser command in response to the adjusted downstream NO x amount.

13. The apparatus of claim 12 , wherein the NO x trimming module is further structured to determine an excess downstream NO x amount in response to the adjusted downstream NO x amount and a NO x reference target, and to further adjust the one of the NH 3 reference target and the reductant doser command in response to the excess downstream NO x amount.

14. A method, comprising:

interpreting an NH 3 reference target comprising a target amount of NH 3 present at a mid-bed position between two selective catalytic reduction (SCR) catalysts, wherein the NH3 reference target is the target amount of NH 3 that occurs at a mid-bed position between the SCR catalysts in response to injection of a targeted amount of reductant upstream of the SCR catalysts;

interpreting an amount of NO x downstream of the SCR catalysts and a present amount of NH 3 in the exhaust conduit between the SCR catalysts in response to an output from ammonia sensor that defines a separation point between the SCR catalysts at the mid-bed position;

adjusting the NH 3 reference target in response to the amount of NO x downstream of the SCR catalysts, the present amount of NH 3 , and the NH 3 reference target; and

injecting, with a reductant injector, an amount of urea upstream of the SCR catalysts in response to the adjusted NH 3 reference target.

15. The method of claim 14 , further comprising interpreting an amount of NH 3 downstream of the SCR catalysts, determining an adjusted amount of NO x downstream of the SCR catalysts in response to the amount of NH 3 downstream of the SCR catalysts, and wherein the adjusting is further in response to the adjusted amount of NO x downstream of the SCR catalysts.

16. The method of claim 15 , further comprising interpreting a NO x reference target comprising a target amount of NO x present at a position downstream of the SCR catalysts, determining an excess downstream NO x amount in response to the adjusted amount of NO x downstream of the SCR catalysts and the target amount of NO x and wherein the adjusting is further in response to the excess downstream NO x amount.

17. The method of claim 16 , further comprising filtering the excess downstream NO x amount with a filter having a time constant of at least 10 seconds.

18. The method of claim 16 , further comprising filtering the excess downstream NO x amount with a filter having a time constant of at least 100 seconds.

19. The method of claim 16 , further comprising filtering the excess downstream NO x amount with a filter having a time constant of at least 1000 seconds.