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Typical example (1) Selective Catalytic Reduction (SCR) system (2) Exhaust outlet connection (3) Diesel Exhaust Fluid (DEF) injector (4) Diesel Particulate Filter (DPF) (5) Diesel Oxidation Catalyst (DOC) (6) Exhaust intake connection | |
The Clean Emissions Module (CEM) for the engine consists of the following components.
- Diesel Oxidation Catalyst (DOC)
- Diesel Particulate Filter (DPF)
- Selective Catalytic Reduction (SCR) system
The Diesel Oxidation Catalyst (DOC) oxidizes the carbon monoxide and the hydrocarbons that are not burnt in the exhaust gas into carbon dioxide and water. The DOC also acts on the oxides of nitrogen to increase the nitrogen dioxide fraction for improved efficiency of the SCR system. The DOC is a through flow device that will continue to operate during all normal engine operating conditions.
The wall flow Diesel Particulate Filter (DPF) collects all solid particulate matter in the exhaust gas.
A flexible exhaust pipe connects the engine to the Clean Emissions Module (CEM). Refer to Disassembly and Assembly for the correct procedure to install the flexible exhaust pipe.
The solid particulate matter that is collected by the DPF consists of soot (carbon) from incomplete combustion of the fuel and inorganic ash from the combustion of any oil in the cylinder.
The rate of accumulation of ash is slow under normal engine operating conditions. The filter is designed to contain all the ash that is produced for the useful emissions life of the engine.
The engine aftertreatment system is designed to oxidize the soot in the DPF at the same rate as the soot is produced by the engine. The oxidization of the soot will occur when the engine is operating under normal conditions. The soot in the DPF is constantly monitored. If the engine is operated in a way that produces more soot than the oxidized soot, the engine management system will automatically activate the exhaust back pressure valve to raise the exhaust temperature. The raising of the exhaust temperature will ensure that more soot is oxidized than the soot that is produced by the engine. The oxidization of more soot returns the DPF to a reduced level of soot. The exhaust back pressure valve is then deactivated when the soot level has been reduced.
The engine ECM must know how much soot is in the DPF. Measurement of soot is accomplished through the following means:
- Radio frequency measurement across the DPF
The information gathered from the radio frequency measurement is then converted into a percentage of soot output that is viewed through the electronic service tool. The soot level may be displayed as a graphical bar, or as an actual percentage.
The Electronic Control Module (ECM) uses the soot measurement information to determine if the engine operating conditions need to be adjusted in order to oxidize the soot at an increased rate.
After the DPF, the exhaust gases are injected with a Diesel Exhaust Fluid (DEF) by a Diesel Exhaust Fluid (DEF) injector. The exhaust gases and the DEF are mixed in a mixing chamber. The mixture decomposes to form ammonia and carbon dioxide. The mixture passes to the main Selective Catalytic Reduction (SCR) reaction chamber.
Exhaust gases and an atomized mist of ammonia and carbon dioxide enter the SCR reaction chamber. Together with the SCR catalyst inside the chamber, the mixture undergoes a chemical reaction that produces nitrogen gas and water vapor.
There is an oxidation catalyst after the SCR catalyst. The oxidation catalyst reacts with the excess ammonia to produce nitrogen.
Refer to Systems Operation, Testing and Adjusting, "DEF Dosing Control System" for more information on the DEF system.