The risk of urea crystallization is high, which will cause problems such as unqualified emission, power attenuation and high fuel consumption. Through “1. Heating engine coolant” and “2. Heating urea pipeline”, urea crystallization can be effectively avoided.
The national VI emission standard will be implemented from July 1 this year. The proportion of urea and fuel consumption is as high as 8% ~ 10%, and the crystallization risk is high. For the national VI diesel engine with SCR technology, urea crystallization will cause problems such as vehicle emission substandard, power attenuation and high fuel consumption. Therefore, it is very important for national VI vehicles to effectively avoid urea crystallization.
Urea crystallization failure is easy to occur in cold areas, especially in winter. In most areas of the south, as long as there is no illegal operation, the system will not have problems. This is because the freezing point of urea is – 10 ℃. In areas where the ambient temperature is lower than this temperature, the SCR post-treatment system must provide thawing and thermal insulation measures for urea aqueous solution, otherwise urea crystallization will be caused.
When the urea temperature sensor detects that the urea temperature is lower than 0 ℃, the heating solenoid valve on the aftertreatment system will automatically open and introduce coolant to heat urea until 7 ℃.
The way of heating the engine coolant is adopted for the thawing of urea solution. A heat exchanger is arranged in the urea tank to realize the heating and ice melting of urea in the urea tank through the heat transfer of the high-temperature coolant in the engine water jacket. This requires the engine to warm up and enter the heat exchanger to heat urea when the engine coolant temperature reaches 70 ℃.
When the urea gas-liquid mixing channel is blocked, the pressure in the mixing chamber will rise, and the increased pressure will make the gas-liquid mixture (containing urea solution) rush up to the compressed air channel. The urea solution will evaporate water in the compressed air channel and precipitate urea crystals, which will block the air channel.
There are three urea pipes in the urea injection system, which are liquid supply pipeline (connecting the urea tank to the urea pump assembly), injection pipeline (from the urea pump assembly to the injector) and return pipeline (from the injector to the urea tank).
The crystallization of the supply and return liquid urea pipe shall be heated by electric heating. When heating and thawing is required, the control unit of the post-treatment system controls the heating relay to pull in, and the heating material is powered on to start heating. This method has the advantages of fast heating, simple layout and is not affected by the structure of the whole vehicle.
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:
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;
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.
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.
If the injection amount of urea is small, stopping the injection after a period of time will accumulate urea crystals at the nozzle joint and nozzle hole to block the channel. The water in the aqueous urea solution is more likely to evaporate under this condition, resulting in the precipitation of urea liquid, resulting in urea crystallization. The urea crystallization itself will block the channel and turn into biuret or cyanuric acid when the temperature increases, These two substances 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.