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 4 stage process:

1. A close coupled diesel oxidation catalyst (DOC) stage
2. A close coupled selective catalyst reduction (SCR)/diesel particulate filter (DPF) stage
3. A rear selective catalyst reduction (SCR) stage
4. A rear diesel oxidation catalyst (DOC) stage

In the first stage, the close coupled DOC removes exhaust HC and CO through an oxidation process. In the next stage, 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. The close coupled SCR and DPF are integrated into one unit. In the DPF, 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. Exhaust emissions are further reduced in the rear SCR and rear DOC.