Exhaust system implementing feedforward and feedback control

A method of operating a combustion engine, comprising:

• directing a flow of exhaust from the combustion engine through a reduction catalyst;
• collecting particulate matter from the exhaust flow upstream of the reduction catalyst;
• determining an amount of particulate matter build up upstream of the reduction catalyst;
• determining an amount of reductant that should be injected into the exhaust flow upstream of the reduction catalyst based at least partially on the amount of particulate matter build up;
• and
• injecting the determined amount of reductant.

A power system, comprising:

• an engine configured to combust fuel and generate a flow of exhaust;
• an exhaust passageway leading from the engine to the atmosphere;
• an SCR catalyst disposed within the exhaust passageway;
• an injection device located to inject urea into the exhaust passageway upstream of the SCR catalyst;
• a particulate filter located upstream of the injection device;
• a soot sensor configured to determine an amount of soot loading in the particulate filter;
• and
• a controller in communication with the engine, the soot sensor, and the injection device, the controller being configured to:
• determine an amount of urea that should be injected based at least partially on the soot loading in the particulate filter.