How to effectively avoid urea crystallization

Urea crystallization failure is easy to occur in cold areas, especially in winter. In most areas in the south, as long as the operation is not illegal, the system will not have problems. This is because the freezing point of urea is -10°C. In areas where the ambient temperature is lower than this temperature, the SCR post-treatment system must provide thawing and warm-keeping measures for the urea aqueous solution, otherwise it will cause urea crystallization.

When the urea temperature sensor detects that the urea temperature is lower than 0°C, the heating solenoid valve on the post-processing system automatically opens to introduce coolant to heat the urea until it stops working at 7°C.

The urea solution is thawed by heating the engine coolant. A heat exchanger is arranged in the urea tank, and the urea in the urea tank is heated and de-iced through the heat transfer of the high-temperature coolant in the engine water jacket, which requires the engine to be warmed up, when the engine coolant temperature reaches 70 ℃, it can enter the heat exchanger to heat the urea.

02. Urea crystals

When the urea gas-liquid mixing channel is blocked, the pressure in the mixing chamber will increase, and the increased pressure will make the gas-liquid mixture (containing urea solution) run up to the compressed air channel, and the urea solution will evaporate moisture in the compressed air channel , resulting in urea crystals, which block the air passage.

03. Urea tube crystallization

There are 3 urea pipes in the urea injection system, which are the liquid supply pipeline (connected from the urea tank to the urea pump assembly), the injection pipeline (from the urea pump assembly to the injector) and the return pipeline (from injector to urea tank).

To deal with the problem of urea tube crystallization, electric heating is usually used for heating. When heating and thawing are required, the post-processing system control unit controls the heating relay to pull in, and the heating material starts to heat up when it is energized. In this way, heating is fast, the layout is simple, and it is not affected by the structure of the whole vehicle.

The electric heating urea tube is composed of urea-resistant nylon tube, heating unit, quick connector, wiring harness and connectors. When the ambient temperature is lower than -5 °C, the control device uses the vehicle power supply (24V) to heat and thaw it, and the pipeline is heated through the heating and thawing strategy control to prevent the urea solution from crystallizing in a low temperature environment. Urea heating pipelines can be divided into the following two types:

1. The heating wire pipeline is wrapped around the outside of the urea tube wall by winding resistance wire, with constant power design, but there is a problem of uneven heat distribution;

   
   

   2. The heating layer pipeline adopts nylon PA12 as the heating material, which is heated evenly, the pipeline has PTC characteristics, the pipeline power can be reduced with the increase of temperature to avoid the pipeline overheating, the product length is not limited, and it can be used in various ways. Various shapes of pipelines are convenient for layout, and the urea pipe joints are provided with heating clips.

04. The nozzle joint is blocked

If urea injection stops after a period of time, urea crystals may accumulate at the joint between the urea pipe and the nozzle or at the joint with the pump, blocking the pipeline. This is mainly because there is a dead angle between the urea line joint and the nozzle or pump connection joint, and a certain amount of urea aqueous solution will accumulate in the dead corner. Under the condition of discontinuous injection, the accumulated urea aqueous solution will evaporate the water and precipitate urea crystals, which will increase the blockage of the channel. .

 05. Blockage caused by small amount of urea injection

If the urea injection amount is small, stopping the injection after a period of time will cause urea crystals to accumulate in the nozzle joints and nozzle holes. The water in the urea aqueous solution evaporates more easily under such conditions, causing the urea liquid to separate out, resulting in urea crystallization. The urea crystals themselves can block the channels, and can also become biuret or cyanuric acid at elevated temperatures, both of which are more difficult to remove.

The urea crystallization process involves complex physical and chemical reactions, the mechanism is complex, and it is very difficult to improve. In places where the temperature is not too low, the urea tank is heated by engine antifreeze, but in cold areas, the urea crystallization problem can be effectively solved by using the urea tube to use electric heating.