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, wherein the exhaust conduit includes a first selective catalytic reduction (SCR) catalyst and a second SCR catalyst, and 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;

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 that defines a separation point between the first SCR catalyst and the second SCR catalyst at the mid-bed position;

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

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

determining an amount of NO x downstream of the second SCR catalyst, and further comprising determining a rate of change of the present amount of NH 3 and adjusting one of the NH 3 reference target and the reductant closer command in response to the rate of change; 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 closer 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 closer 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. A method, comprising:

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

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 that defines a separation point between the first SCR catalyst and the second SCR catalyst at the mid-bed position;

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;

determining a rate of change of the present amount of NH 3 and the amount of NO x downstream of the second SCR catalyst;

determining a product of the rate of change of the present amount of NH 3 and the rate of change of the amount of NO x downstream of the second SCR catalyst, and further adjusting one of the NH 3 reference target and the reductant doser command in response to the product; 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.

7. The method of claim 6 , further comprising determining a rate-based adjustment amount as a function of the product, and further adjusting the one of the NH 3 reference target and the reductant doser command in response to the rate-based adjustment amount.

8. The method of claim 7 , further comprising limiting the rate-based adjustment amount to a proportion of an amount of NO x upstream of the first SCR catalyst.

9. The method of claim 8 , wherein the proportion comprises an amount less than one-half of the amount of NO, upstream of the first SCR catalyst.

10. A method, comprising:

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

interpreting, with an ammonia sensor that defines a separation point between the SCR catalysts at the mid-bed position, an amount of NOx 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 the ammonia sensor;

interpreting a rate of change of the amount of NO x downstream of the SCR catalysts;

interpreting a rate of change of an amount of NH 3 between the SCR catalysts;

determining a product of the rate of change of the amount of NO x downstream of the SCR catalysts and the rate of change of the amount of NH 3 between the SCR catalysts;

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, wherein the adjusting further comprises adjusting the NH 3 reference target in response to the product; and

injecting, with a reductant injector, an amount of urea upstream of the SCR catalysts in response to the adjusted NH3 reference target.

11. The method of claim 10 , wherein the adjusting comprises reducing the NH 3 reference target in response to the product being negative, and increasing the NH 3 reference target in response to the product being positive.

12. The method of claim 11 , wherein the increasing has a higher gain than the decreasing.

13. The method of claim 12 , further comprising limiting the adjusting to a proportion of an amount of NO x upstream of the SCR catalysts.

14. The method of claim 10 , 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.

15. The method of claim 14 , further comprising interpreting a NO x reference target comprising a target amount of NO present at a position downstream of the SCR catalysts, determining an excess downstream NO x amount in response to the adjusted amount of NO 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.

16. The method of claim 15 , further comprising filtering the excess downstream NO x amount with a filter having a time constant of at least 10 seconds.

17. The method of claim 15 , further comprising filtering the excess downstream NO x amount with a filter having a time constant of at least 100 seconds.

18. The method of claim 15 , further comprising filtering the excess downstream NO amount with a filter having a time constant of at least 1000 seconds.