System Overview
The diesel exhaust aftertreatment system is designed to reduce the levels of hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOx), and particulate matter remaining in the vehicle's exhaust gases. Reducing these pollutants to acceptable levels is achieved through a 3 stage process:
- A diesel oxidation catalyst (DOC) stage
- A diesel particulate filter (DPF) stage
- A selective catalyst reduction (SCR) stage
In stage 1, the DOC removes exhaust HC and CO through an oxidation process. After stage 1, particulate matter consisting of extremely small particles of carbon remaining after combustion are removed from the exhaust gas by the large surface area of the DPF. In the final or stage 3 process, reductant, also known as diesel exhaust fluid (DEF) or urea, is injected into the exhaust gases prior to entering the SCR stage. Within the SCR, NOx is converted to nitrogen (N2), carbon dioxide (CO2), and water vapor (H20) through a catalytic reduction fueled by the injected reductant.