Method of controlling ammonia levels in catalytic units
A system of controlling ammonia levels in a catalytic exhaust system comprising: means to provide a target value for ammonia slip/ammonia output from said system or a catalytic unit of said system; first comparison means to compare said target value with a feedback value to provide a command value based on said comparison, and means to control the dosing of a reducing agent such as urea into said exhaust system based on said command value; means to input said command value to a transfer function or model to provide an estimated value of ammonia slip/ammonia output from said catalytic unit/system; means to measure actual ammonia slip/ammonia output from said unit/system; second comparison means to compare said actual value with said estimated value; means to provide said feedback value based on the output from said comparison means.
1. 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).
2. A method as claimed in claim 1 wherein said transfer function is a first order lag function.
3. A method as claimed in claim 2 where the time constant of the first order lag function is
τ
=
1
k
·
m
exh
where m exh , is the measured or estimated exhaust gas flow.
4. A method as claimed in claim 1 wherein said transfer function or said model includes one or more of the following parameters: measured or estimated exhaust gas flow, catalyst temperature and ammonia level within said catalytic system/unit.
5. A method as claimed in claim 1 wherein said feedback value is provided by applying a further transfer function to the difference from the comparison in step v).
6. A method as claimed in claim 5 wherein said further transfer function is a first order lag filter.
7. A method as claimed in claim 1 wherein said reducing agent is urea.
8. A method as claimed in claim 1 , including supplementing said command value for said dosing injector with a further signal for said dosing injector provided from open loop control.
9. A method as claimed in claim 8 wherein said open loop control has one or more input parameters selected from: NOx level output from exhaust and/or an engine; exhaust mass flow; and catalyst temperature.
10. A system of controlling dosing reducing agent in a catalytic exhaust system comprising:
means to provide a target value for ammonia slip/ammonia output from said system or a catalytic unit of said system to an electronic controller;
a dosing injector which doses reducing agent into said exhaust system based on a command value supplied to said dosing injector;
means to input 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;
means to measure actual ammonia slip/ammonia output from said unit/system and communicate the measured actual ammonia slip/ammonia output to said electronic controller;
comparison means to compare said actual value with said estimated value to provide a feedback value;
second comparison means to compare said target value with said feedback value such that this comparison is used; and
means to control dosing of reducing agent into said exhaust system using said dosing injector based on the comparison made by the second comparison means.
11. A system as claimed in claim 10 wherein said transfer function is a slip model of said catalytic system/unit.
12. A system as claimed in claim 10 wherein said transfer function is a first order lag function.
13. A system as claimed in claim 12 where the time constant of the first order lag function is
τ
=
1
k
·
m
exh
where m exh , is the measured or estimated exhaust gas flow.
14. A system as claimed in claim 10 wherein said transfer function includes one or more of the following parameters: measured or estimated exhaust gas flow, catalyst temperature and ammonia level within said catalytic system/unit.
15. A system as claimed in claim 10 having means to provide said feedback value by means to apply a further transfer function to the output from second comparison means.
16. A system as claimed in claim 15 wherein said further transfer function is a first order lag filter.
17. A system as claimed in claim 10 wherein said reducing agent is urea.
18. A system as claimed in claim 10 including means to convert said command signal into a signal for urea injection/dosing means.
19. A system as claimed in claim 10 including means to supplement said signal for urea injection/dosing means with a further signal for urea injection/dosing means provided from open loop control.
20. A system as claimed in claim 19 wherein said open loop control has one or more input parameters selected from: NOx level output from exhaust and/or an engine; exhaust mass flow; and catalyst temperature.
21. A non-transitory computer readable medium or processor embodying a computer program or code adapted to perform the method of claim 1 .