Selective Catalytic Reduction (SCR) Overview
Selective catalytic reduction is a system for reducing nitrogen oxides (NOx) in the exhaust gas. For this purpose, a reducing agent (urea/water solution) is injected into exhaust gas downstream of the diesel particulate filter.
The nitrogen oxide reduction reaction then takes place in the SCR catalytic converter. The urea-water solution is carried in two reservoirs in the vehicle. The quantity is measured out such that it is sufficient for one oil change interval.
The following is a simplified representation of the system.
The reason for using two reservoirs is that the urea-water solution freezes at a temperature of -11°C (12.2°F). For this reason, the smaller "active" reservoir is heated but the larger passive reservoir is not. In this way, the entire volume of the urea-water solution need not be heated, thus saving energy. The amount in the active tank is sufficient , however, to cover large distances.
The small, heated reservoir is referred to as the active reservoir. A pump conveys the urea-water solution from this reservoir to the metering module. This line is also heated.
The larger, unheated reservoir is the passive reservoir. A transfer pump regularly conveys the urea-water solution from the passive reservoir to the active reservoir.