How To Solve The Problem Of Urea Crystallization
The national six emission standards will be implemented from July 1 this year. The ratio of urea to fuel consumption is as high as 8% to 10%, and the risk of crystallization is high. For National VI diesel engines with SCR technology, urea crystallization can cause problems such as substandard vehicle emissions, power decay and high fuel consumption. Therefore, it is very important to effectively avoid urea crystallization for National VI vehicles.
1、 Basic principles of urea crystallization and thawing
Urea crystallization failures are prone to occur in cold regions, especially in winter. In most areas in the south, the system will not have problems as long as there are no irregularities. This is because the freezing point of urea is -10°C. In areas where the ambient temperature is below this, the SCR aftertreatment system must provide thawing and insulation of the aqueous urea solution, otherwise urea crystallization will result.
When the urea temperature sensor detects that the urea temperature is below 0°C, the heating solenoid valve on the aftertreatment system will automatically open and introduce coolant to heat the urea until 7°C.
The thawing of the urea solution is carried out by heating the engine coolant. A heat exchanger is arranged in the urea tank to heat and thaw the urea in the urea tank by heat transfer from the high temperature coolant in the engine water jacket. This requires the engine to heat up and enter the heat exchanger to heat the urea when the engine coolant temperature reaches 70°C.
2、 Urea crystallization
When the urea gas-liquid mixing channel is blocked, the pressure in the mixing chamber rises, and the increased pressure causes the gas-liquid mixture (containing the urea solution) to rush up the compressed air channel. The urea solution causes the water in the compressed air passages to evaporate and precipitate urea crystals, thus blocking the air passages.
3、 Urea tube crystallization
There are three urea lines in the urea injection system, the supply line (connecting the urea tank to the urea pump assembly), the injection line (from the urea pump assembly to the injector) and the return line (from the injector to the urea tank).
The crystallization of the supply and return urea tubes should be electrically heated. When heating is needed for thawing, the control unit of the aftertreatment system controls the heating relay to pull in, energizes the heating material and starts heating. This method has the advantages of fast heating speed, simple arrangement and not affected by the whole vehicle structure.
The electric heating urea pipe is composed of urea resistant nylon pipe, heating unit, quick plug connector, harness and connector. When the ambient temperature is lower than – 5 ℃, the control device uses the on-board power supply (24V) to heat and thaw it, and controls the heating of the pipeline through the heating and thawing strategy to prevent the crystallization of urea solution in low temperature environment. The urea heating pipeline can be divided into the following two types:
1. Heating wire pipeline
The heating wire pipe is slightly wrapped outside the urea pipe wall by means of resistance wire winding, with constant power design, but there is a problem of uneven heat distribution;
2. Heating layer pipeline
Nylon PA12 is used as the heating material for the heating layer pipeline. The heating is balanced. The pipeline has PTC characteristics. The pipeline power can be reduced with the increase of temperature, so as to avoid pipeline overheating. The product length is not limited. It can make pipelines of various shapes for convenient layout. The urea pipe joint is equipped with heating buckle.
4、 Nozzle joint blocked
After urea injection for a period of time, stopping injection may accumulate urea crystals at the joint between urea pipe and nozzle or at the joint with pump, blocking the pipe. This is mainly because there is a dead corner at the joint between the urea line joint and the nozzle or pump, in which a certain amount of urea aqueous solution will accumulate. Under the condition of discontinuous injection, the accumulated urea aqueous solution will evaporate water and precipitate urea crystals, which will then plug the channel.
5、 Blockage caused by small urea injection
If the amount of urea injection is low, after stopping the injection for a period of time, urea crystals will accumulate at the nozzle joint and nozzle hole, blocking the passage. In this case, the water in the aqueous urea solution is more likely to evaporate, leading to precipitation of the urea solution and causing urea crystals. The urea crystals themselves can block the passages and turn into bio-urea or cyanuric acid when the temperature rises, both of which are more difficult to remove.
Urea crystallization process involves complex physical and chemical reactions, the mechanism is complex, and it is very difficult to improve. The urea tank is heated with engine antifreeze. There is no problem where the temperature is not too low. The urea pipe adopts electric heating to effectively solve the problem of urea crystallization in cold areas.