## 1 Introduction
The urea crystallization phenomenon in SCR systems is caused by a combination of factors, which can be summarized into two main categories: first, operating condition factors, such as changes in exhaust temperature, exhaust flow, and urea injection parameters; second, structural factors of the SCR system, such as mixer design and injector design. A number of related studies have confirmed that exhaust temperature is a key factor affecting the formation of urea crystals in SCR systems. The study conducted by Navistar Corporation in the United States on urea crystallization showed that urea crystals are easily formed under low temperature and low mass flow rate conditions, with the amount of sediment generated being approximately 25\(\sim\)65% of the urea injection amount. When the SCR temperature reaches 350 \({}^{\circ}\)C, urea crystals are almost not formed [16]. [PERSON] et al. used a chemical reaction dynamics model to study the formation process and location of urea crystals and found that when the exhaust temperature is below the Leidenfrost temperature, a liquid film forms on the surfaces of the mixer and elbow; otherwise, after droplet collision, they leave the wall surface and tend to form a small amount of cyanuric acid in the center area of the SCR carrier inlet end surface [17]. Under actual engine operating conditions, due to the changing operating conditions, the formation characteristics of crystallization are different from those under steady-state conditions. Transient exhaust temperature, ambient temperature, and urea injection amount have different effects on the formation of crystallization [18]. [PERSON] studied the effect of temperature changes on the total amount of crystallization by increasing the temperature of crystals generated under stable low-temperature conditions (150 \({}^{\circ}\)C-250 \({}^{\circ}\)C).The results showed that at the same injection amount, intermittent and continuous injection at low temperatures (150 \({}^{\circ}\)C) had a small impact on the total amount of crystallization, but intermittent temperature rise could eliminate the crystals generated during the period of no injection, thereby reducing the total amount of crystallization [19]. Overall, crystallization is greatly affected by exhaust temperature. For both steady-state and transient operating conditions, increasing temperature will improve the crystallization. The spray parameters also have a significant impact on crystallization. The spray state can be described by spray cone angle, penetration distance, and spray particle size and distribution. The spray particle size and penetration distance are directly affected by the spray pressure. With the increase of spray pressure, the penetration distance will become larger while the spray particle size decreases. Spray particle size is generally characterized by Sauter Mean Diameter (SMD) [20]. Smaller particle size has a larger specific surface area, which is conducive to UWS evaporation and decomposition. At the same time, smaller particles are more susceptible to the influence of airflow, and the mixing effect with the exhaust is better, which improves the uniformity of ammonia distribution in the exhaust [21; 22]. The formation of urea crystals can have many adverse effects: crystallization reduces the flow cross-sectional area of the carrier and increases the backpressure of the after-treatment system, which is detrimental to the fuel economy of the engine; crystallization means that some UWS is not decomposed to produce ammonia, resulting in insufficient supply of ammonia to the SCR system, which leads to a decrease in NOx conversion efficiency. At the same time, partial decomposition of the crystallization under high-temperature conditions may lead to an excessive supply of ammonia and excessive escape of ammonia; as the crystallization problem continues to worsen, the after-treatment system may fail due to clogging, further affecting the NOx conversion efficiency. This article studies the urea crystallization risk assessment model for the SCR system.