Method and system for controlling a nitrogen oxide (nox) conversion efficiency monitor

A method for use aboard a vehicle having an internal combustion engine, an exhaust system, a selective catalytic reduction (SCR) device that catalytically converts nitrogen oxide (NOx) gasses in an exhaust stream from the engine into water and nitrogen, and a controller, the method comprising:

• using the controller to calculate a NOx conversion efficiency rate of the SCR device using an upstream NOx level from a first NOx sensor positioned upstream of the SCR device and a downstream NOx level from a second NOx sensor positioned downstream of the SCR device;
• selectively determining, at the end of a key cycle and when an accumulated amount of upstream NOx is less than a calibrated upstream NOx level, whether the NOx conversion efficiency rate is presently passing or presently failing;
• and
• recording the accumulated amount of upstream NOx in memory for use in calculating the NOx conversion efficiency rate during a subsequent key cycle only when the NOx conversion efficiency rate is presently passing at the end of the key cycle.

A control system for a vehicle having a selective catalytic reduction (SCR) device that catalytically converts nitrogen oxide (NOx) gasses in an engine exhaust stream into water and nitrogen, the control system comprising:

• an upstream NOx sensor positioned in proximity to an exhaust port of the engine, and configured to measure an upstream NOx level;
• a downstream NOx sensor positioned in proximity to an outlet side of the SCR device, and configured to measure a downstream NOx level;
• and
• a controller configured to calculate a NOx conversion efficiency rate of the SCR device as a function of the upstream NOx level and the downstream NOx level;
• wherein the controller is configured to detect the end of a key cycle and when an accumulated amount of upstream NOx is less than a calibrated upstream NOx level as an enabling condition, and, in response to the enabling condition:
• determines whether the NOx conversion efficiency rate is presently passing or failing relative to a calibrated threshold efficiency;
• and
• records the accumulated amount of upstream NOx in memory for use in calculating the NOx conversion efficiency rate during a subsequent key cycle only when the NOx conversion efficiency rate is presently passing at the end of the key cycle.