SCR selective catalytic reduction principle SCR selective catalytic reduction principle
1. The SCR system converts nitrogen oxides (NOx) from engine exhaust to nitrogen and water.
2. This reaction requires the injection of diesel exhaust fluid into the exhaust.
3. The vehicle Adblue is injected into the exhaust upstream of the catalyst by a precise charging device. The number of injected Adblue is controlled by the engine ECM
4. Ammonia reacts with NOx in a catalytic converter to produce harmless nitrogen (N2) and water (H2O)
Diesel engine nitrogen oxide reducing agent (Adblue for vehicles)
Car Adblue is a high purity urea aqueous solution with a urea concentration of 32.5% by weight.
1. Non-toxic and non-polluting
2. Non-flammable
3. Clear liquid
4. May have a slight ammonia smell
5. Start freezing at -11 °C
6. The leaked car urea is easily converted into white DEF crystals due to evaporation of water.
Car Adblue storage recommendations
Shelf storage, the following state is the ideal environment for long-term preservation of Adblue for vehicles
a. Storage temperature is between -5oC and 25oC
b. Store in a sealed container to avoid contamination
c. Avoid direct sunlight
d. If the above requirements are met, it can be kept for at least 18 months
e. Every 5oC increase in ambient temperature, reduce life expectancy by 6 months
f. For example, the shelf life is 12 months at 30oC and 6 months at 35oC.
Storage period on the vehicle
a. Not recommended for storage on vehicles for more than 6 months
b. If the vehicle must be stored for a long time, it is necessary to regularly check the concentration of urea used in the vehicle.
Car processing with Adblue
If the car Adblue overflows, use a suitable container to collect it, or use a suitable item to dry it, and then dispose of it in accordance with local regulations.
Do not allow drainage into the drainage system
Do not discharge into surface water such as rivers, lakes, and oceans.
Very small amount of car Adblue can be rinsed with plenty of water
If the spilled diesel exhaust treatment is air dried or wiped with a cloth only, a white residue will be left. Can lead to incorrect diagnosis of diesel exhaust fluid injection system leakage
Car Adblue notes
It is not allowed to use agricultural urea and water to prepare Adblue for vehicles. Agricultural urea does not meet the requirements of the specification and may damage the aftertreatment system.
It is not allowed to add water or other non-compliant liquid to the car Adblue tank, otherwise it will cause the aftertreatment system to malfunction.
It is not allowed to add any chemicals and additives to the car Adblue (for example, trying to use additives to reduce the freezing point of the car Adblue), otherwise it will cause the aftertreatment system to malfunction.
SCR post-processing (UA2) system working process
In the normal working situation, there are 4 working stages:
Initialize
The engine ignition switch is turned on but the engine is not started, the system is initialized and self-tested.
2. Pre-injection phase
The pre-injection phase must meet either of the following two conditions:
Engine running for more than 120 seconds
Catalyst inlet exhaust temperature exceeds 150oC.
Meet the requirements of the SCR heating system at the same time:
Tank temperature exceeds -7oC
Spray unit temperature exceeds -7oC
3. Injection stage
After the pre-fill is successful, the air shut-off valve opens and the diaphragm pump motor enters the standby state, at which point it enters the ready-to-spray phase.
The metering injection phase is directly controlled by the ECM, and the metered injection depends on the amount of nitrogen oxides produced and the corresponding exhaust temperature.
The injection unit does not inject DEF into the exhaust system until the exhaust gas temperature at the inlet of the catalyst reaches 220oC, but the air always flows into the exhaust system through the injection unit and the nozzle.
Spray DEF condition
a. The catalyst inlet temperature exceeds 220 °C, and the average temperature of the two inlet and outlet temperature sensors exceeds 210 °C.
b. There are some current SCR system fault codes
c. Vehicle Adblue tank level is higher than 0%
d. The pressure of the vehicle Adblue/air mixture is higher than 300KPa, and the pressure of the vehicle Adblue is higher than 480KPa.
e. According to the Cummins NOx algorithm, the ECM calculates a NOx production amount greater than a predetermined threshold.
4. Emptying stage
When the ignition switch is turned off, the system automatically enters the emptying phase.
After the emptying phase lasts for 30 seconds, the system stops working.
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