Reductant delivery performance diagnostics system

1. A method for performing diagnostics of reductant delivery performance of a NOx conversion system, the method comprising the steps of:

providing an outlet pressure sensor at a reductant outlet of a reductant delivery system;

providing a virtual pressure sensor in an exhaust stream at a location where the reductant is injected into the exhaust stream;

calculating a downstream reductant flow rate using at least one of the outlet pressure sensor and the virtual pressure sensor;

calculating an upstream reductant flow rate within the reductant delivery system using at least one of a temperature sensor, a pressure sensor and a control valve;

comparing the calculated downstream reductant flow rate with the calculated upstream reductant flow rate;

and conducting diagnostics based on the results from the comparison of the calculated downstream reductant flow rate with the calculated upstream reductant flow rate.

2. The method of claim 1 , wherein said conducting diagnostics step further includes issuing a fault code corresponding to a malfunction of at least one sensor of the NOx conversion system.

3. The method of claim 1 , wherein, when the calculated downstream reductant flow rate differs from the calculated upstream reductant flow rate by at least a predetermined threshold, the conducting diagnostics step further includes determining whether the calculated downstream reductant flow rate is inaccurate.

4. The method of claim 3 , wherein, when the calculated downstream reductant flow rate is determined to be inaccurate, the conducting diagnostics step further comprises conducting downstream diagnostics to detect a malfunction in at least one of the outlet pressure sensor and the virtual pressure sensor.

5. The method of claim 4 , wherein the step of conducting downstream diagnostics comprises conducting intrusive diagnostics.

6. The method of claim 5 , wherein the intrusive diagnostics includes monitoring the relationship between the pressure measured by the outlet pressure sensor and the virtual pressure sensor at two or more different reductant flow rates.

7. The method of claim 3 , wherein, the calculated downstream reductant flow rate is not determined to be inaccurate, and further comprising the step of conducting reductant delivery system diagnostics.

8. The method of claim 7 wherein the step of conducting reductant delivery system diagnostics detects a fault of at least one of a reductant gas pressure sensor and a reductant temperature sensor.

9. The method of claim 1 wherein the reductant is ammonia.

10. The method of claim 9 , wherein the ammonia is ammonia gas inside the reductant delivery system.

11. The method of claim 1 , wherein reductant flow inside the reductant delivery system is super-sonic.

12. The method of claim 11 , wherein the reductant flow outside the reductant delivery system is sub-sonic.

13. A system for diagnosing reductant delivery performance of a NOx conversion system comprising:

a reductant delivery system configured for generating a reductant flow, the reductant delivery system comprising at least one reductant gas pressure sensor, at least one reductant gas temperature sensor, at least one solid ammonia cartridge, at least one control valve and at least one delivery system outlet;

a reductant delivery line configured for delivering reductant from the reductant delivery system to an exhaust stream of an engine, the reductant delivery line comprising an injection mixer;

an outlet pressure sensor installed at the outlet of the reductant delivery system;

a virtual pressure sensor installed in the exhaust stream at the injection mixer; and

a control mechanism configured for calculating the flow rate of reductant within the reductant delivery system and wherein;

the reductant flow in the reductant delivery system comprises gaseous ammonia flowing at a super-sonic flow rate and the reductant flow in the reductant delivery line comprises ammonia flowing at a sub-sonic flow rate.