1. Defects in structural design of automotive urea solution catalyst
In early production vehicles, the distance between urea nozzle and SCR catalytic converter is too short. When urea is sprayed out, it reaches the catalyst carrier before gasification, so crystallization is formed; moreover, the welding angle of urea nozzle mounting base and catalytic converter packaging do not meet the requirements.
2. Atomization effect or nozzle failure of vehicle urea solution nozzle
There are two types of post-treatment SCR systems for common vehicle models, i.e. non air assisted SCR system and air assisted SCR system. Generally, the working pressure of urea injection without air assistance is relatively high, so it often occurs that the nozzle valve needle is not closed tightly, which leads to still working in the low discharge temperature stage when it should not work. Urea can not be hydrolyzed at high temperature, resulting in rapid crystallization. The urea nozzle with gas assistance is normally open, and its atomization is assisted by air pressure, such as the failure of the air pressure pipeline of the urea pump or the nozzle cracking at high temperature As a result, the spray atomization is poor and the crystallization is blocked.
3. The urea pump is not completely emptied after the vehicle goes out
Since there is any residual liquid in the vehicle urea solution pump and pipeline after the vehicle stops working, it is necessary to empty the vehicle after turning off the ignition switch to prevent crystallization or freezing.
4. The filter element of vehicle urea solution pump is not replaced regularly
All urea pumps have filters or filter elements, which must be cleaned or replaced regularly. Otherwise, if the time is too long, it will lead to blockage or damage, resulting in the loss of filtration function, which will inevitably cause the system to work out of order. If the dust is brought into the tank when adding urea, it is easy to block the nozzle if the filtration function fails. Generally, the nozzle has 3-4 holes. If one or two holes are blocked, there will be bad atomization and crystallization.
5. Performance failure or damage of other external sensors and actuators of vehicle urea solution
For example, when the exhaust temperature sensor has a performance failure, the actual exhaust temperature may be 100 ℃, while the temperature fed back by the sensor to the computer is 180 ℃, which will cause the system to inject urea in advance. The urea injection can not get hydrolysis gasification in the low-temperature exhaust gas, which will naturally form crystals at the catalyst carrier and form hard urea crystal compounds over time.
6. The catalyst failure is caused by the use of inferior diesel oil and bad working environment
The catalyst of SCR system is to promote the chemical reaction between ammonia decomposed from urea solution and NOx in exhaust gas at high temperature. It has high technical requirements for sulfur content in diesel. Sulfide formed at high temperature has irreversible damage to the catalyst. Once the catalyst fails, the exhaust gas will exceed the standard and limit the engine power output.