Control of a NOX reductant delivery system

A method of controlling an air source and a reductant source to deliver compressed air and reductant respectively to a mixer, for supply through a nozzle into an engine exhaust gas upstream from a catalyst, the method comprising:

• maintaining an approximately constant predetermined differential pressure across the nozzle;
• calculating a desired reductant flow rate in response to engine and catalyst conditions;
• and
• controlling a flow rate of the reductant to produce the desired reductant flow rate calculated;
• wherein the step of controlling the flow rate of the reductant comprises the steps of:
• calculating an internal reductant control signal in response to calculating the desired reductant flow rate;
• transforming the internal reductant control signal into an electrical power in response to calculating the internal reductant control signal;
• and
• outputting the electrical power to the reductant source in response to transforming the internal reductant control signal, to produce the desired reductant flow rate.

A method of controlling an air source and a reductant source that deliver compressed air and a reductant respectively into a mixer, through a nozzle, and into an exhaust gas at a position upstream from a catalyst, the method comprising:

• providing a predetermined nozzle differential pressure set point;
• determining an engine speed, an engine load, a temperature of the catalyst, a gas space velocity flowing through the catalyst, an actual nozzle differential pressure existing across the nozzle, and a load pressure seen by the reductant source;
• calculating a differential pressure error value equaling a difference between the predetermined nozzle differential pressure set point and the actual nozzle differential pressure in response to determining the actual nozzle differential pressure;
• calculating an air control signal based upon and in response to calculating the differential pressure error value;
• outputting the air control signal to the air source in response to calculating the air control signal, to drive the differential pressure error value toward zero;
• calculating a desired reductant flow rate for the reductant based upon and in response to determining the engine speed, the engine load, the temperature of the catalyst the gas space velocity flowing though the catalyst;
• calculating a reductant control signal based upon the desired reductant flow rate and the load pressure seen by the reductant source in response to calculating the desired reductant flow rate and determining the load pressure seen by the reductant source;
• and
• outputting the reductant control signal to the reductant source in response to calculating the reductant control signal, to direct an actual flow rate of the reductant to be approximately equal to the desired reductant flow rate.

A system for controlling an air source and a reductant source to deliver compressed air and reductant respectively to a mixer, for supply through a nozzle into an engine exhaust gas pipe at a location upstream from a catalyst, the system comprising:

• a control unit for calculating a desired reductant flow rate in response to engine and catalyst conditions and for controlling a flow rate of the reductant from said reductant source to said mixer;
• and
• a pressure sensor for sensing at least the pressure of the exhaust gas in said exhaust pipe;
• said control unit responsive to pressure sensed by said pressure sensor for controlling the output of said air source to said mixer to maintain an approximately constant predetermined differential pressure across said nozzle.