Exhaust aftertreatment system

A method determining a flow rate of a reductant in an aftertreatment system for an exhaust stream of an engine, comprising:

• receiving a first input from one or more first gas sensors positioned between the engine and a selective catalyst reduction system to determine a first concentration of ammonia (NH3) and a first concentration of NOx in the exhaust stream;
• receiving a second input from one or more second gas sensors positioned between the selective catalyst reduction system and a secondary catalyst to determine a second concentration of ammonia (NH3) and a second concentration of NOx in the exhaust stream;
• receiving a third input from one or more third gas sensors positioned downstream of the secondary catalyst to determine a third concentration of ammonia (NH3) and a third concentration of NOx in the exhaust stream;
• estimating a first change in the exhaust steam between the first gas sensor and the second gas sensor;
• estimating a second change in the exhaust stream between the second gas sensor and the third gas sensor;
• and
• varying a flow rate of a reductant to the selective catalyst reduction system based upon the first change and the second change.

An aftertreatment system for treating an exhaust stream of a lean burn engine, comprising:

• a selective catalyst reduction system positioned downstream of the lean burn engine;
• the selective catalyst reduction system comprising an injector for a stream of a reductant;
• a secondary catalyst positioned downstream of the selective catalyst reduction system;
• a first gas sensor positioned between the engine and the selective catalyst reduction system;
• a second gas sensor positioned between the selective catalyst reduction system and the secondary catalyst;
• a third gas sensor positioned downstream of the secondary catalyst;
• and
• a controller in communication with the first gas sensor, the second gas sensor, the third gas sensor, and the injector, wherein the controller receives sensor input from at least the second gas sensor and the third gas sensor to determine concentrations of ammonia (NH3) in the exhaust stream.