Methods for vehicle emissions control

1. A method for an engine, comprising:

storing emissions including at least HC and NH 3 during a lower, first temperature of engine operation in a hydrocarbon trap, the hydrocarbon trap including a zeolite molecular adsorbent material;

releasing the stored emissions once exhaust gas temperature has reached a higher, second desorption temperature in which downstream catalysts and/or a sensor is active; and

indicating degradation of the hydrocarbon trap based on at least a first NOx sensor output and alignment of the second desorption temperature to a functional state of the zeolite.

2. The method of claim 1 , wherein indicating the degradation of the hydrocarbon trap based on at least the first NO x , sensor output includes:

determining a highest NH 3 amount over a predetermined temperature window based on the first NO x sensor output; and

indicating the degradation of the hydrocarbon trap based on the highest NH 3 amount;

wherein the first NO x sensor output is output by a first NO x sensor disposed downstream of the hydrocarbon trap in an exhaust passage coupled to the engine.

3. The method of claim 2 , wherein indicating the degradation of the hydrocarbon trap based on the highest NH 3 amount includes:

determining an exhaust gas temperature at which the highest NH 3 amount is determined; and

responsive to the exhaust gas temperature being lower than a temperature threshold, indicating the degradation of the hydrocarbon trap;

wherein the exhaust gas temperature is measured via a temperature sensor disposed adjacent to the first NO x sensor in the exhaust passage.

4. The method of claim 1 , further comprising:

providing exhaust gas at a predetermined air-fuel ratio prior to reaching a predetermined temperature window.

5. The method of claim 4 , wherein indicating the degradation of the hydrocarbon trap based on at least the first NO x sensor output includes:

determining a difference between the first NO x sensor output and a second NO x sensor output; and

responsive to the difference being less than a difference threshold, indicating the degradation of the hydrocarbon trap;

wherein the second NO x sensor output is output by a second NO x sensor disposed upstream of the hydrocarbon trap in an exhaust passage.

6. The method of claim 5 , wherein the difference threshold is determined based on an amount of NH 3 stored or released by the hydrocarbon trap over the predetermined temperature window.

7. The method of claim 1 , wherein indicating the degradation of the hydrocarbon trap based on at least the first NO x sensor output comprises only indicating the degradation of the hydrocarbon trap based on at least the first NO x sensor output responsive to a plurality of diagnostic entry conditions being met, the plurality of diagnostic entry conditions comprising:

an exhaust gas air-fuel ratio being less than an air-fuel ratio threshold;

the engine having been in operation over an initial duration immediately following an engine cold start; and

an exhaust gas temperature being greater than a temperature threshold.

8. A method for an engine, comprising:

storing emissions including at least HC, NH 3 , and water during a lower, first temperature of engine operation in a hydrocarbon trap, the hydrocarbon trap including a zeolite molecular adsorbent material;

releasing the stored emissions once exhaust gas temperature has reached a higher, second desorption temperature in which downstream catalysts and/or a sensor is active; and

responsive to a plurality of diagnostic entry conditions being met:

determining a first NH 3 amount based on a First NOx sensor output; and

indicating degradation of the hydrocarbon trap based on at least the first NH 3 amount;

wherein the plurality of diagnostic entry conditions comprises the engine having been in operation over a first duration immediately following an engine cold start; and

wherein the first NOx sensor output is output by a first NOx sensor disposed in an exhaust passage coupled to the engine.

9. The method of claim 8 , further comprising:

responsive to initiation of the engine cold start, flowing exhaust gas from the engine through the exhaust passage;

wherein the plurality of diagnostic entry conditions further comprises:

an exhaust gas air-fuel ratio lower than an air-fuel ratio threshold; and

an exhaust gas temperature greater than a temperature threshold.

10. The method of claim 8 , wherein the first NH 3 amount is a highest NH 3 amount over a predetermined temperature window.

11. The method of claim 8 , wherein indicating the degradation of the hydrocarbon trap based on at least the first NH 3 amount includes:

determining an exhaust gas temperature at which the first NH 3 amount is determined; and

responsive to the exhaust gas temperature being lower than a temperature threshold, indicating the degradation of the hydrocarbon trap;

wherein the exhaust gas temperature is measured via a temperature sensor disposed in the exhaust passage.

12. The method of claim 8 , further comprising:

providing exhaust gas at a predetermined air-fuel ratio over a second duration via open loop fuel control of the engine.

13. The method of claim 12 , further comprising:

responsive to the plurality of diagnostic entry conditions being met:

determining a second NH 3 amount based on a second NO x sensor output;

wherein the second NO x sensor output is output by a second NO x sensor disposed in the exhaust passage.

14. The method of claim 13 , wherein the second NO x sensor is disposed upstream of the first NO x sensor in the exhaust passage and the hydrocarbon trap is positioned intermediate the second NO x sensor and the first NO x sensor.

15. The method of claim 14 , wherein indicating the degradation of the hydrocarbon trap based on at least the first NH 3 amount includes:

determining a difference between the first NH 3 amount and the second NH 3 amount; and

responsive to the difference being less than a difference threshold, indicating the degradation of the hydrocarbon trap.

16. A method for an engine, comprising:

storing emissions including at least HC, NH 3 , and water during a lower, first temperature of engine operation in a hydrocarbon trap;

releasing the stored emissions once exhaust gas temperature has reached a higher, second desorption temperature in which downstream catalysts and/or a sensor is active; and

responsive to a plurality of diagnostic entry conditions being met, indicating degradation of the hydrocarbon trap based on an NH 3 amount in an exhaust gas.

17. The method of claim 16 , wherein the plurality of diagnostic entry conditions comprises:

an exhaust gas air-fuel ratio being less than an air-fuel ratio threshold; and

the engine having been in operation over an initial duration immediately following a cold start.

18. The method of claim 17 , wherein indicating the degradation of the hydrocarbon trap based on the NH 3 amount in the exhaust gas includes:

determining a diagnostic temperature at which the NH 3 amount is released by the hydrocarbon trap;

determining a voltage difference between a first voltage output by a first heated exhaust gas oxygen (HEGO) sensor when exhaust gas temperature at the first HEGO sensor is at the diagnostic temperature and a second voltage output by a second HEGO sensor when exhaust gas temperature at the second HEGO sensor is at the diagnostic temperature, the first HEGO sensor disposed upstream of the hydrocarbon trap in an exhaust passage coupled to the engine, and the second HEGO sensor disposed downstream of the hydrocarbon trap in the exhaust passage; and

responsive to the voltage difference being lower than a voltage difference threshold, indicating the degradation of the hydrocarbon trap.

19. The method of claim 17 , wherein the plurality of diagnostic entry conditions further comprises:

an exhaust gas temperature being greater than a temperature threshold.

20. The method of claim 19 , wherein the NH 3 amount is determined based on at least one NO x sensor output.