Selective Catalyst Reduction (SCR) System Schematic Diagram
| ITEM | DESCRIPTION |
|---|---|
| A | Exhaust gas |
| B | Diesel Exhaust Fluid (DEF) |
| 1 | Powertrain Control Module (PCM) |
| 2 | Diesel Exhaust Fluid (DEF) tank with DEF tank module |
| 3 | Diesel Exhaust Fluid (DEF) injection pump |
| 4 | Diesel Exhaust Fluid (DEF) line |
| 5 | Post Selective Catalyst Reduction (SCR) Nitrogen Oxide (NOx) sensor |
| 6 | Post Selective Catalyst Reduction (SCR) soot sensor (NAS market vehicles only) |
| 7 | Selective Catalyst Reduction (SCR) catalytic converter |
| 8 | Mixer plate |
| 9 | Diesel Exhaust Fluid (DEF) injector |
| 10 | Pre Selective Catalyst Reduction (SCR) NOx sensor (NAS market vehicles only) |
| 11 | Diesel Particulate Filter (DPF) |
| 12 | Catalytic converter |
| 13 | Engine |
The SCR catalytic converter reaches the operating temperature at 150 °C (302 °F). The temperature of the exhaust gas is measured by the post DPF exhaust gas temperature sensor, which is connected to the PCM.
The DEF injection pump supplies the DEF from the DEF tank at a pressure of 6.5 bar to the DEF injector via the heated DEF pressure line. The DEF injector is controlled with a Pulse Width Modulation (PWM) signal by the PCM.
The injected DEF is carried along by the exhaust gas flow and is evenly distributed in the exhaust gas by the mixer plate. The mixer plate is located in the exhaust pipe upstream of the SCR catalytic converter and downstream of the DEF injector. After the injection, the DEF is converted into ammonia (NH3) and carbon dioxide (CO2) during chemical reactions.
In the SCR catalytic converter, the ammonia (NH3) reacts with the nitrogen oxides (NOx) to produce nitrogen (N2) and water (H2O) vapor. The SCR system's efficiency is registered by the post SCR NOx sensor.
In order to optimize the SCR system's efficiency, the correct amount of ammonia (NH3) is required in the exhaust gas. The ammonia (NH3) is stored on the catalytic converter and used as a function of the NOx feed.
The objective of the control system is to ensure that the predetermined amount of NH3 is available on the SCR catalytic converter at any given time.