Fueling control during emission control device purging

A method for controlling an internal combustion engine coupled to an emission control device with an exhaust sensor coupled downstream of the emission control device, the method comprising:

• operating the engine at a lean air-fuel ratio wherein NOx is stored in the emission control device;
• monitoring at least two conditions which include:
• (1) whether an actual fuel amount supplied to the emission control device is greater than an estimate of fuel required to purge the emission control device of stored NOx;
• and
• (2) whether an indication is provided by the sensor;
• operating the engine at a first rich air-fuel ratio to remove said stored NOx, said operation at said first rich air-fuel ratio ending when either of said monitored conditions is detected;
• and
• when operation at said first rich air-fuel ratio is ended based on said actual fuel supplied being greater than said estimate, operating the engine at a second rich air-fuel ratio after operating at said first rich air-fuel ratio at least until an indication is provided by the downstream sensor.