SCR catalytic converter, basic function

P14.00-2187-74

Overview

This document contains information on:

 General

 Function requirements

 Function

General

The active SCR catalytic converter is a vehicle catalytic converter for diesel engines for reducing nitrogen oxide emissions (NOx) with the aid of ammonia (NH3). Ammonia can be added directly or generated with the aid of a urea-based reduction agent. The nitrogen oxides contained in the exhaust gas react with ammonia and oxygen (O2) to become nitrogen (N2) and water vapor.

Function requirements

 Reduction agent available

 Catalytic converter at operating temperature (250 °C to 450 °C)

Function

In addition to the oxidation catalytic converter and soot particulate filter, an actively operated SCR catalytic converter with urea injection is used in diesel engines to reduce nitrogen oxide emissions.

The metering valve injects the reduction agent directly upstream of the SCR catalytic converter into the exhaust flow. The combustion engine control unit determines the required amount in a characteristics-map-dependent manner. The SCR catalytic converter converts the reduction agent into ammonia. The constituents ammonia and oxygen convert the nitrogen oxide contained in the exhaust gas into nitrogen and water vapor by means of a chemical reduction process. The conversion rate of the NOx concentration in the exhaust gas is approx. 80%.

To break down waste ammonia, an ammonia slip catalytic converter is attached to the end of the SCR catalytic converter. It consists of a precious-metal coating. It oxidizes the ammonia that was not able to react with the nitrogen oxides into nitric oxide (NO) and water vapor (H2O).

NOx sensors upstream and downstream of the SCR catalytic converter measure the NOx concentration in the exhaust gas and regulate the addition of the reduction agent.