## 2 Real-Time Evaluation Method of a Diesel Vehicle SCR System  
### Analysis of the NOx Conversion Process for a Real-Road Vehicle Test  
In the urea-SCR system for the diesel vehicle, the injector injects the urea aqueous solution into the catalyst converter, where the urea aqueous solution undergoes evaporation, pyrolysis, and hydrolysis to finally produce ammonia (NH\({}_{3}\)) for catalytic reduction [22]. Equations (1)-(3) present the evaporation, pyrolysis, and hydrolysis reactions, respectively. \[\text{CO(NH}_{2})_{2}\cdot\text{7H}_{2}\text{O(aqu)}\rightarrow\text{CO(NH}_{ 2})_{2}\text{ (s)}+\text{7H}_{2}\text{O(g)} \tag{1}\]  
\[\text{CO(NH}_{2})_{2}\text{ (s)}\rightarrow\text{NH}_{3}\text{(g)}+\text{ HNCO(g)} \tag{2}\]  
\[\text{HNCO(g)}+\text{H}_{2}\text{O(g)}\rightarrow\text{NH}_{3}\text{(g)}+ \text{CO}_{2}\text{(g)} \tag{3}\]  
For this SCR catalytic conversion process, the hydrolysis reaction requires the highest temperature, which challenges the emission control in the cold-start stage of a diesel engine. The urea aqueous solution first produces HNCO after evaporation and pyrolysis, and then it is hydrolyzed to NH\({}_{3}\) for catalytic reduction. Generally, the hydrolysis reaction only begins at 150 \({}^{\circ}\)C and continues until it reaches 200 \({}^{\circ}\)C and releases ammonia. Moreover, the adsorption efficiency of NH\({}_{3}\) on the catalyst is low at low temperatures. When the exhaust temperature is lower than 200 \({}^{\circ}\)C, urea cannot be fully pyrolyzed and hydrolyzed, and by-products such as cyanuric acid and urea crystallization are easily generated, so that urea cannot be fully converted into NH\({}_{3}\). According to the Eley-Rideal mechanism, when NH\({}_{3}\) enters the catalyst, it is adsorbed to the active sites on the catalyst surface [23]. There are two active sites on the catalyst surface: one is the Bronsted active site, where NH\({}_{3}\) is adsorbed in the form of NH\({}_{3}^{+}\)\(-\), and the other is the Lewis active site, where NH\({}_{3}\) is adsorbed in the form of NH\({}_{x}\)\(-\) (\(x\) ranges from 1 to 3). When NH\({}_{3}\) enters the catalyst, it first is absorbed on the Bronsted and Lewis active sites until adsorption saturation of NH\({}_{3}\) is reached, and then continues to be absorbed on the inactive sites on the catalyst surface. The process of storing NH\({}_{3}\) on active and inactive sites is called ammonia storage [24]. Generally, the de-NOx mechanism of the SCR system includes both standard and fast SCR reactions, as the following equations show:  
\[4\text{NH}_{3}+4\text{NO}+\text{O}_{2}\text{$\rightarrow$}4\text{N}_{2}+6\text {H}_{2}\text{O} \tag{4}\]  
\[2\text{NH}_{3}+\text{NO}+\text{NO}_{2}\text{$\rightarrow$}2\text{N}_{2}+3 \text{H}_{2}\text{O} \tag{5}\]  
\[8\text{NH}_{3}+6\text{NO}_{2}\text{$\rightarrow$}7\text{N}_{2}+12\text{H}_{2} \text{O} \tag{6}\]  
Equation (4) is the standard SCR reaction, and it is the main reaction between NH\({}_{3}\) and NO in the SCR system [25]. Equation (5) is the fast SCR reaction, and it has a greater chemical reaction rate than the standard reaction because NO2 helps improve the reaction rate [26; 27]. Equation (6) is the slow SCR reaction, and it works when the NO\({}_{2}\) amount exceeds the amount of NO [28]. The reduction reaction of the SCR catalyst needs a specific temperature window for effective NOx conversion, and different catalysts have different temperature windows. For example, the temperature window of a Cu-zeolite catalyst is about 200-500 \({}^{\circ}\)C, and the temperature window of a Fe-zeolite catalyst is about 350-550 \({}^{\circ}\)C. In a specific temperature window, the catalytic conversion efficiency first increases with the increase in temperature and then decreases once it is higher than the temperature of the maximum catalytic conversion efficiency [29] ([PERSON] et al., 2012). For the SCR catalyst, once the temperature is not within the special temperature window, for example, if it is lower than 200 \({}^{\circ}\)C, urea cannot be fully hydrolyzed.