Maintenance and the mistake using of commercial vehicle SCR exhaust aftertreatment system

Composition of SCR tail gas post-treatment system
SCR system can be divided into three parts: urea injection system for injecting urea, SCR catalyst box assembly for catalytic silencing, sensor and other parts. It mainly includes urea supply unit (urea pump), urea injection unit (urea nozzle), urea tank, urea pipeline and injection control unit (DCU).
Urea pump
The urea pump is responsible for pressurizing the urea solution in the urea tank and sending it to the urea nozzle. At the same time, the excess urea solution is pumped back to the urea tank to maintain the pressure of the system at about 9BAR. After the engine is shut down, the urea pump will pump the urea solution in the system back to the urea tank to avoid system failure caused by residual urea solution. There is a replaceable filter in the urea pump to prevent the micro dust particles (diameter > 30 UM) in the urea solution from entering the injection valve. The filter element and its auxiliary balance elements need to be replaced regularly.

Urea nozzle
The function of the urea nozzle is to inject the urea solution with a certain pressure provided by the urea pump into the tail gas of the exhaust pipe. The urea nozzle has one urea pipe joint and two cooling water joints. The two cooling water joints are the water inlet and return port for the engine coolant to cool the urea nozzle to prevent the high temperature failure of the urea nozzle.

Urea pipe
The urea pipeline is the circulation channel of urea, and there is a heating device to prevent the crystallization of urea solution on the urea pipeline.
SCR box
That is, the catalyst is another core unit of SCR tail gas post-treatment system, which has two functions: tail gas catalytic conversion and noise silencing. SCR box assembly consists of box type and barrel type. Barrel type SCR box assembly has two appearances, one is side inlet and rear end outlet (side inlet and end outlet); One is air inlet at the front end and air outlet at the rear end (end in and end out). The SCR box assembly is fixed on the appropriate position of the vehicle with brackets and straps and connected with the engine exhaust connecting pipe through the air inlet flange. The SCR box assembly is integrated with urea nozzle, exhaust temperature sensor and nitrogen and oxygen sensor. In order to prevent the failure of urea nozzle and nitrogen and oxygen sensor caused by collision during transportation and handling, the urea nozzle and nitrogen and oxygen sensor are equipped with protective frames. The honeycomb carrier is installed in the SCR catalyst. Avoid entering large objects (such as plastic bags, stones, etc.) during disassembly and assembly, otherwise the engine exhaust will be blocked.
Urea Solution
Urea aqueous solution (DEF for short). Vehicle urea uses the international standard urea aqueous solution with a mass fraction of 32.5%, which is composed of 32.5% high-purity urea and 67.5% deionized water