Diesel Emissions Equipment

DOC, DPF, EGR, & SCR...What do They Mean, How do They Work?



DOC - Diesel Oxidation Catalyst

The DOC is the catalytic converter of diesel engines. They use the elements palladium and/or platinum as catalysts to reduce particulate matter and convert harmful emissions. Specifically, the diesel oxidation catalyst converts hydrocarbons and carbon monoxide into carbon dioxide and water. Most DOCs operate at approx. 90% efficiency.


DPF - Diesel Particulate Filter + Active Regeneration

A diesel particulate filter (DPF) is used to nearly eliminate particulate emissions (diesel soot). They are 80 - 100 percent effective in reducing particulate emissions. DPF equipped trucks should emit no visible smoke, even under high load conditions. Active regeneration is used to keep the filters clean, as a truck with a clogged DPF will experience significant reductions in performance and fuel economy. When sensors in the exhaust system relay to the ECM that the filter has accumulated a pre-determined amount of particulate matter, the truck enters active regeneration mode. During active regeneration, fuel is introduced into the exhaust stream and burned to increase the heat of the DPF. The filter needs to reach a temperature of approx. 600 degrees C (just over 1100 degrees F) for the particulate matter to burn. Since soot is the result of partially combusted fuel, burning it out of the filter is an effective way of cleaning it. There are several methods of delivering fuel into the exhaust stream, including injecting fuel into the combustion chamber during the exhaust stroke or utilizing a dedicated fuel injector in the exhaust system. During active regeneration, the engine usually idles higher and the engine consumes a significantly higher amount of fuel. If active regeneration occurs too frequently, fuel economy will suffer greatly. If active regeneration does not occur frequently enough, the filter may become clogged and driveability/fuel economy will suffer.


EGR - Exhaust Gas Recirculation

Exhaust gas recirculation systems introduce exhaust gases into the intake of the engine. By introducing exhaust gases into the engine, the amount of oxygen is reduced and combustion temperatures are lowered, both of which help to reduce the emissions of nitrous oxides (NOx). While EGR systems are effective in reducing NOx emissions, fuel economy, performance, and in some cases engine longevity are negatively affected. EGR flow is regulated so that the flow of exhaust into the engine depends on engine load.


SCR - Selective Catalytic Reduction + Urea Injection

Selective Catalytic Reduction (SCR) is a process in which diesel exhaust fluid (DEF), a mixture of urea and water, is mixed into the exhaust stream before entering a catalyst where a chemical reaction occurs. The heat of the exhaust converts urea into carbon dioxide and ammonia. Inside the catalyst, ammonia and nitrous oxides (NOx) are converted into nitrogen gas and water.



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