The engine power drop, flameout or difficult start-up and excessive exhaust gas caused by the blockage of SCR catalyst may interfere with our fault judgment. The serious blockage of SCR catalyst will make the exhaust pipe red and even cause the vehicle spontaneous combustion.
Today, let’s learn how to judge the blockage of SCR catalyst and distinguish the degree of blockage.
Detection method of SCR catalyst blockage:
Feel the temperature difference between the front and rear of SCR catalyst to judge whether it is blocked. A large temperature difference indicates that it is blocked;
Reach out to the exhaust pipe to see if you can feel the air flow. If you can’t feel it, it means it’s blocked;
Failure to reach the maximum speed during test run, poor acceleration, increased fuel consumption and reduced power, which is also the blockage of SCR catalyst Possible performance of;
Abnormal noise
1. When you step on the accelerator to accelerate, you will hear the noise under the chassis, such as fried beans in an iron pot. It is the sound of the cracked and fallen catalytic carrier hitting and shaking the exhaust pipe wall continuously under the high-temperature and high-pressure air flow;
2. When the catalytic carrier inside the SCR catalyst is broken, the vehicle can hear abnormal and obvious abnormal noise during operation;
3. When parking and idling, you can hear a slight gravel rolling and bouncing noise. At this time, when you step on the accelerator and release it immediately, you will hear the “jingle jingle” impact sound. The abnormal noise is emitted from under the chassis, not from the engine room.
4. When you release the accelerator, you will immediately hear the noise of gravel hitting the steel barrel wall. It is the sound of the exhaust flow pressure suddenly weakening, the broken body rebounding and hitting the steel barrel wall at the front end of the exhaust pipe;
5. When the accelerator is released during driving, the feeling of frustration is obvious, which is caused by the sudden weakening of the exhaust pressure after the accelerator is released, which makes the compressed debris rebound and block the front exhaust port, thus affecting the engine work and forming a sudden restraining resistance.
6. After parking and flameout, the heat under the chassis is abnormal. When you stop after driving for a long time and people are beside the car, you will vaguely feel the steaming hot gas from the bottom of the car, which is the hot gas emitted by the steel barrel in the front section of the roasted exhaust pipe.
7. The exhaust emission test fails to meet the standard. The causes are complex, and the blockage of SCR catalyst is one of the important reasons. SCR catalyst is an effective device to reduce the emission of harmful substances from vehicles. In order to meet the increasingly stringent emission regulations, the catalytic converter should not only have high conversion efficiency and long service life, but also have good ignition characteristics (catalytic temperature), flow characteristics (uniform flow) and temperature characteristics (uniform temperature distribution and high temperature resistance).
The flow process in the catalytic converter is complex, including eddy current, heat transfer, mass transfer and chemical reaction. In the area with too high temperature in the SCR catalyst, the catalyst is easy to deteriorate, shorten the service life, and the catalyst in the area with low temperature is not fully utilized, resulting in the reduction of overall conversion efficiency, the uneven temperature distribution, the increase of the temperature gradient in the neck of the carrier, and the thermal stress and thermal deformation and damage of the carrier.
Therefore, in the face of the increasingly high-tech SCR catalyst, it is recommended that car owners should shut down in time for repair in case of abnormal working conditions, such as overheating, surge, tempering or repeated stall, because these conditions can lead to permanent damage to the SCR catalyst!
Six misunderstandings of SCR catalyst
1. Cleaning the SCR catalyst is to wash the carbon deposit SCR catalyst is located in the exhaust part of diesel vehicle. There are many particles in diesel vehicle exhaust and carbon deposition needs to be washed. However, the purpose of cleaning SCR catalyst is to remove the compounds attached to the surface of catalytic carrier, relieve the poisoning of catalyst, restore the activity of catalyst and reduce the purification ability!
The fuel with high sulfur content is easy to form chemisorption complex after combustion, resulting in the blockage of SCR catalyst; Diesel oil containing lead, manganese antiknock agent, poor oil quality and many colloids is easy to form metal deposits, which will block the sintering of SCR catalyst; Only sulfur and phosphorus poisoning of the catalyst is reversible, while others, such as crushing, water poisoning, high temperature deactivation, etc., are irreversible and belong to permanent failure. Only a new catalyst can purify the tail gas.
2. The appearance is intact, and the catalyst is good. Some riders believe that the appearance of the catalyst is intact, so the catalyst will not fail. The catalytic converter is different from the washbasin. As long as there is no hole in the washbasin, it can be said to be good and still play a role. The function of the catalyst is to purify the tail gas, so the standard to judge whether the catalyst is good or bad is the purification ability of the tail gas, not the integrity of the appearance. When the catalytic converter’s ability to reduce exhaust pollutants is less than 50%, it can be judged as invalid.
3. Soak and wash with acid-base detergent The reason why SCR catalyst can purify tail gas is that the surface of catalyst carrier is coated with catalysts, such as platinum, palladium, and rare earth compounds. Many maintenance shops remove the catalyst, soak it with acid-base cleaning agent and then flush it. The result of this cleaning method is that the mud and sand are completely removed, the impurities are washed away, and the precious metals and other components are washed away, losing the purification ability. The oxalic acid transmitted through the network can be cleaned, but the catalytic function of precious metals will deteriorate.
If the catalyst is blocked and cleaned with acid-base cleaning agent, the catalyst can be dredged and cleaned. Moreover, many riders feel that the power is enhanced after cleaning. It is only because the exhaust is smoother and the back pressure is lower. The problem is that the purification ability is weakened. Unless the blockage is particularly serious, use a small amount of acid-base cleaning agent. If it is not serious, wash it with high-pressure water gun. Dry it completely before use. Remember that there should be no liquid water residue. The best choice of cleaning agent is professional weak acid and weak alkali.
4. SCR catalyst requires other catalysts to assist The precious metal produced by the SCR catalyst plays a catalytic role and does not need other auxiliary catalytic substances. The use or addition of other auxiliary catalysts may lead to chemical reaction with the precious metal in the catalytic carrier, resulting in the failure of the catalyst.
5. The blockage of catalyst is caused by cleaning This view is very common. But what I don’t know is that when the professional SCR catalyst cleaning agent acts, it generates gas, which is impossible to cause blockage.
6. After the catalyst is blocked, the catalyst carrier is directly poked out
Once the catalytic converter is blocked and the exhaust is not smooth, the power performance of the vehicle will decline. If it is not handled in time, the blockage will become more and more serious Although the catalytic converter makes a lot of noise in the shop, it can’t deal with the problem directly. Although it can’t deal with a lot of power. The correct approach is to replace the SCR catalyst or catalytic carrier with a new one
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