## 2 Materials and Methods
### Input Parameters Selection  
The research object of this paper is a 6 EX340 EF low-speed two-stroke diesel engine, and some of its parameters are shown in Table 1 below. Since the research object is a low-speed two-stroke diesel engine, the cost of the bench test is high, and only limited experimental data can be obtained. Table 2 below shows the experimental data under four typical working conditions obtained from the bench test. According to the working principles of marine diesel engines and the complex physical-chemical reaction processes of fluids such as combustible gas mixtures in three-dimensional space, computational fluid dynamics software can be used to create algebraic equations to obtain numerical solutions at discrete time/space points that can accurately calculate the required performance parameters. The experimental data were used to calibrate the 3D simulation model of the marine diesel engine, and then this model was used to construct orthogonal experimental data to obtain more experimental data. The process of obtaining data is shown in Figure 2 below: \begin{table}
\begin{tabular}{c c} \hline
**Item** & **Data** \\ \hline Cylinder diameter & 340 mm \\ Single cylinder power & 750 kW \\ Engine speed & 169 rpm/m \\ Firing order & 1-6–2–4–3–5 \\ Stroke & 1600 mm \\ Fuel injection & Common rail \\ Exhaust valve driving mode & Electrical servo drive oil \\ Scavenging form & Direct current \\ Other features & Turbocharged and intercooled \\ \hline \end{tabular}
\end{table} 
Table 1: Low-speed machine parameters. \begin{table}
\begin{tabular}{c c c c c} \hline
**Item** & \multicolumn{3}{c}{**Data**} \\ \hline Load (\%) & 24.8 & 49.5 & 75.7 & 100.4 \\ NOx (ppm) & 1271.3 & 919.44 & 726.46 & 723.57 \\ Speed (r/min) & 106.8 & 133.9 & 153.6 & 169 \\ Power (kW) & 1367 & 2732 & 4178 & 5543 \\ Scavenging temperature (K) & 44.8 & 42.7 & 45.8 & 53.7 \\ Scavenging pressure (MPa) & 1.477 & 2.527 & 3.617 & 4.547 \\ Oil consumption (g/kW-h) & 175.34 & 170.98 & 169.61 & 178.37 \\ Oil temperature (K) & 30 & 29 & 28 & 27 \\ Common rail pressure (Gbar) & 645 & 700 & 760 & 850 \\ Inject timing (CA) & 0.8 & 0.8 & 1.8 & 1 \\ Inject duration (CA) & 6.6 & 10.5 & 12.5 & 16.16 \\ Exhaust valve open (CA) & 256.8 & 257 & 260 & 261 \\ Exhaust valve close (CA) & 139 & 138 & 139 & 136 \\ \hline \end{tabular}
\end{table} 
Table 2: 6 EX340 EF typical load experiment data. According to the data, the NOx content of exhaust emissions produced by the marine low-speed diesel engine burning heavy oil (HFO) is 8.7% according to the mass ratio (NOx/HPO) [37]. The NOx is divided into the thermal type (83.92%), fuel type (11.08%), and fast type (5%) [38]. For the research on the formation mechanism of NOx, the thermal NOx formation explanation of the Zeldovich mechanism is widely recognized. This mechanism clarifies that NOx is produced by the chemical reaction of \(N_{2}\) and \(O_{2}\) in the combustion mixture. Since NOx is a product of combustion, the relevant parameters of the scavenging process, compression, fuel injection, combustion process, and structural characteristics of the marine diesel engine will affect the combustion and then affect the formation of NOx. From the formation mechanism of the thermal NOx chemical reaction, it is known that the distribution of temperature and pressure in the cylinder, the concentration and distribution of oxygen, and the flame propagation rate control the formation of NOx. The impact relationship of structural parameters, initial state parameters, working process parameters, and typical performance characteristic parameters involved in these three conditions in NOx generation are outlined in Figure 3.  
Figure 3: Influencing factors in the formation of NOx emissions. Figure 2: Dataset source sketch. Many factors affect the formation of NOx. Selecting appropriate input parameters can reduce the computational complexity of the NOx simulation model and achive higher accuracy.