NOx purge air/fuel ratio selection

A method for improving a conversion efficiency of an exhaust gas aftertreatment device coupled downstream of a lean-burn internal combustion engine during a NOx purge, the method comprising:

• providing a base rich air-fuel ratio of an exhaust gas mixture entering the device;
• adjusting said base air-fuel ratio so as to increase a first exhaust gas mixture component conversion efficiency when the device is in a first operating temperature range, wherein said first exhaust gas mixture component is hydrocarbon;
• and
• adjusting said base air-fuel ratio so as to increase a second exhaust gas mixture component conversion efficiency when the device is in a second operating temperature range.

A method for improving a conversion efficiency of an exhaust gas aftertreatment device coupled downstream of an internal combustion engine during a NOx purge, the method comprising:

• determining whether a rich purge cycle should be performed to release stored NOx;
• and during said purge:
• calculating an air-fuel ratio of a rich exhaust gas mixture entering the device as a first function of a device temperature when said temperature is below a predetermined threshold;
• and
• calculating said rich air-fuel ratio as a second function of said device temperature, when said temperature is above said predetermined threshold,
• wherein said calculated rich air-fuel ratio is richer when said temperature is below said threshold than when said temperature is above said threshold.

A method for improving a conversion efficiency of an exhaust gas aftertreatment device coupled downstream of an internal combustion engine during a NOx purge, the method comprising:

• determining whether a rich purge cycle should be performed to release stored NOx;
• and during said purge:
• providing a base rich air-fuel ratio for an exhaust gas mixture entering the device;
• calculating a first adjustment value for said base air-fuel ratio as a first function of a device temperature when said temperature is within a first operating range;
• and
• calculating a second adjustment value for said base air-fuel ratio when said temperature is within a second operating range, where said second temperature operating range is higher than said first temperature operating range, wherein said first adjustment value produces a first rich air-fuel ratio that is richer than a second rich air-fuel ratio produced by said second adjustment.