Selective Catalyst Reduction System Schematic Diagram
A = EXHAUST GAS; B = DIESEL EXHAUST FLUID (DEF).
| ITEM | DESCRIPTION |
|---|---|
| 1 | Powertrain Control Module (PCM) |
| 2 | Diesel Exhaust Fluid (DEF) tank with DEF tank module |
| 3 | DEF injection pump |
| 4 | DEF line |
| 5 | Post Selective Catalyst Reduction (SCR) Nitrogen Oxide (NOx) sensor |
| 6 | Post SCR particulate matter sensor |
| 7 | SCR catalytic converter |
| 8 | Mixer plate |
| 9 | DEF injector |
| 10 | Pre SCR NOx sensor |
| 11 | Diesel Particulate Filter (DPF) |
| 12 | Catalytic converter |
| 13 | Engine |
The Selective Catalyst Reduction (SCR) catalytic converter reaches the operating temperature at 150°C (302°F). The post Diesel Particulate Filter (DPF) exhaust gas temperature sensor measures the exhaust gas temperature and send the signals to the Powertrain Control Module (PCM).
The Diesel Exhaust Fluid (DEF) injection pump supplies the DEF from the DEF tank to the DEF injector via the heated DEF line. The operating pressure in the DEF line is 6.5 bar The PCM controls the DEF with a Pulse Width Modulation (PWM) signal.
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 post SCR NOx sensor registers the SCR system's efficiency.
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 pre-determined amount of NH3 is available on the SCR catalytic converter at any time.