Method of controlling ammonia levels in catalytic units

A method of dosing reducing agent in a catalytic exhaust system comprising:

• i) providing a target value for ammonia slip/ammonia output from said system or a catalytic unit of said system, wherein said target value is provided to an electronic controller;
• ii) dosing reducing agent into said exhaust system with a dosing injector based on a command value supplied to said dosing injector;
• iii) inputting said command value to a transfer function or model, in said electronic controller, to provide an estimated value of ammonia slip/ammonia output from said catalytic unit/system;
• iv) measuring actual ammonia slip/ammonia output from said unit/system using an ammonia sensor and communicating the measured actual ammonia slip/ammonia output to said electronic controller;
• v) comparing said actual value from iv) with said estimated value from step iii), using said electronic controller to provide a feedback value;
• vi) comparing, using said electronic controller, said target value with said feedback value;
• and
• vii) controlling dosing of reducing agent into said exhaust system using said dosing injector based on the comparison of step vi).