Method for monitoring a regulated emission concentration in the exhaust gas of an internal combustion engine

A method for determining NOx concentration in an exhaust stream of an engine, comprising:

• estimating an exhaust gas recirculation (EGR) amount based on a turbine rotational speed;
• estimating NOx concentration based on the estimated EGR amount and an upstream exhaust air-fuel ratio, and further based on current operating conditions;
• and
• adjusting an engine operating parameter responsive to the estimated NOx concentration.

A system comprising:

• an engine;
• a turbine arranged in an exhaust stream downstream of the engine;
• an exhaust gas recirculation (EGR) system upstream of the turbine to direct at least a portion of the exhaust stream back to the engine;
• and
• a controller having instructions executable to:
• estimate an amount of EGR directed to the engine based on a rotational speed of the turbine;
• and
• adjust an operating parameter responsive to an emissions concentration, the emissions concentration based on the amount of EGR and an upstream exhaust air-fuel ratio, and further based on current operating conditions.