Heavy-duty diesel engine scheduling aftertreatment system
About diesel engine post-schedule standards
China's Stage 6 emission regulations (National VI) are currently one of the most stringent emission regulations in the world
Diesel engines have uneven mixing of fuel and air in the cylinder
DOC is a diesel oxidation catalyst (DOC)
1. Its principle is to use precious metal catalysts such as platinum, palladium and rhodium coated on the channel wall
2. Oxidize HC, CO, NOx in exhaust gas and soluble organic components in particulate matter
SOF) into harmless gases
3. High temperature is conducive to improving the conversion efficiency of the catalyst
DPF is a diesel particulate filter (DPF)
Its principle is to filter the particulate matter in the exhaust gas through the staggered blocked channels, so that the soot particles in it are deposited. At this time, DPF regeneration is required. The diesel engine aftertreatment system is equipped with a fuel injection system (HCI) for this purpose. By injecting diesel into the exhaust pipe and burning it, heat is released in the front DOC, and the exhaust temperature is increased from 250℃ to 550-650℃, so as to burn the carbon particles blocked in the DPF and restore the basic performance of the DPF.
SCR is Selective Catalytic Reduction (SCR)
The principle is to inject urea aqueous solution into the exhaust pipe, decompose it into ammonia and water through a catalyst (vanadium-based, copper zeolite or iron zeolite), and then further reduce nitrogen oxides (NOx) in the exhaust gas into nitrogen and water. Therefore, the diesel engine aftertreatment system also needs to be equipped with a urea injection system.
ASC is an ammonia slip catalyst (ASC)
Its function is to oxidize excess ammonia into nitrogen and water to prevent ammonia leakage.
Taking the Bosch after-treatment family as an example, a total of 9 sensors, 2 independent controllers and 2 injection systems are required to pass this complex after-treatment system.
The capture rate of particulate matter, the main pollutant in diesel engine exhaust, reaches more than 95%. In the diesel engine National VI after-treatment system, DPF regeneration has a direct impact on the driver. When the vehicle is idling for a long time, driving at a low speed or frequently starting and stopping, the exhaust temperature is too low, which is easy to cause DPF blockage. If the vehicle cannot perform DPF regeneration while driving for a long time, the performance of the engine will be affected by the high exhaust back pressure. Therefore, the National VI engine ECU often reports a torque limit fault. At this time, DPF parking regeneration is required, requiring the driver to park the vehicle in an open place without combustibles. Let the engine and after-treatment system automatically enter parking regeneration, and enter the high speed total speed through additional fuel injection to burn the carbon soot particles deposited in the DPF at high temperature (the whole process takes about 20 to 30 minutes). If the carbon load of the DPF is too high or there is too much ash that cannot be burned, the engine ECU lights up the red fault light. In this case, it can only be taken to the service station for manual disassembly and cleaning. In addition, the high sulfur content of diesel will make many catalysts in the after-treatment system ineffective, so the National VI stage also puts forward higher requirements for diesel quality.
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