4.3.3 SCR System
4.3.3.1 SCR catalyst – Operating principle
“Selective Catalytic Reduction”, or SCR for short) is a chemical reaction in the course of which harmful ni-
trous oxides (NO
X
) in the exhaust gas are converted largely into water (H
2
O) and nitrogen (N
2
).
The entire exhaust mass flow is routed through the catalytic converter system, bringing it into contact with
the catalytically active surface.
The process is expedited by continuously injecting a reducing agent (32.5% solution of ultra pure urea in de-
ionized water) into the exhaust flow. This reducing agent (RA) reacts with the exhaust gas in a fraction of a
second to create ammonia (NH
3
). This ammonia subsequently converts the nitrous oxides in the SCR cata-
lyst.
In addition to the SCR catalyst itself, other component parts of an SCR system are a reducing agent reservoir
(e.g. tank), a reducing agent dosing system and an appropriate system controller.
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