Here's a description of the system from an article I wrote in 2007...
The DOC (Diesel Oxidation Converter) & DPF (Diesel Particulate Filter) are the primary components involved with reducing Particulate Matter (PM) and nitrogen oxides (NOx). The DOC is a honey-combed structure that is washcoated with a layer of catalyst materials (Platinum, Palladium and other base metals). These catalysts chemically react with the exhaust gases to convert nitrogen oxides (NOx) into nitrogen dioxide and to oxidize (burn) hydrocarbons. During a regeneration event, a "Catalyst System Efficiency" test will run.
The ECM monitors the efficiency of the DOC by ensuring EGT sensor 1 reaches a temperature high enough to achieve the desired effect. The heated exhaust gases then flow into the DPF, which captures any remaining PM. The DOC is an open-ended flow-through device, while every adjacent cell of the honey-combed porous structure within the DPF is closed, so PM cannot pass through. Soot particles remain trapped in the DPF, and can accumulate over time. A regeneration process is periodically required to burn off the accumulated soot buildup. This requires a heating process. DPF regeneration is initiated based on one of the following four criteria.
- Miles driven since last regeneration.
- Fuel used since last regeneration.
- Engine run time since last regeneration.
- Pressure differential across DPF. A lower pressure differential is better. A high pressure differential could indicate a soot buildup in the DPF.
Two Exhaust Gas Temperature sensors are employed, one ahead of (as shown above) and one behind the DPF (shown on the right), which are used by the vehicle ECM to control or schedule DPF regeneration. Regeneration temperature is critical, both from a soot reduction standpoint as well as DPF life. Temperatures too low, and the soot won't burn - too high (above 800°C or 1472°F), and the DPF substrate could be damaged. Regeneration will be terminated above 1472°F.
A differential pressure sensor is used to monitor pressure difference across the DPF, which indicates the degree of soot loading within the DPF substrate.
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