## 2 Simulation Model
### Simulation Object and Initial Conditions  
The engine used in the study was 1 CH12/13. This is a single-cylinder diesel engine with a stroke of 130 mm, cylinder diameter of 120 mm and compression ratio of 15.4. In addition, the engine displacement is 1.47 L with 4 valves (2 intake valves and 2 exhaust valves). The main parameters of the diesel engine are shown in Table 2.  
The engine's original combustion chamber was measured directly when removing the engine. This engine uses an omega combustion chamber. The dimensions were determined and re-drawn as shown on the longitudinal section in Figure 1a.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Sub-Models** & **Name of Sub-Models** \\ \hline Combustion model & ECFM-3Z \\ Spray model & Wave \\ Turbulent dispersion model & Enable \\ Spray wall interaction model & Walljet1 \\ Droplet Evaporation model & Dukowicz \\ Turbulence model & k-zeta-f \\ Ignition model & Auto-ignition \\ NO formation & Extended Zeldovich \\ Soot formation & Kinetic Model \\ \hline \hline \end{tabular}
\end{table}
Table 1: Sub-models of 3D model in AVL Fire[38].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Bore, (mm) & 120 \\ Stroke, (mm) & 130 \\ Connecting rod length, (mm) & 224 \\ Number of cylinders (-) & 1 \\ Displacement, (L) & 1.47 \\ Compression ratio (-) & 15.4 \\ Number of intake valves (-) & 2 \\ Intake valves open, deg. BTDC & 10 \\ Intake valves close, deg. ABDC & 15 \\ Number of exhaust valves & 2 \\ Exhaust valves open, deg. BBDC & 50 \\ Exhaust valves close, deg. ATDC & 6 \\ Number of nozzle holes (-) & 7 \\ Nozzle hole diameter, (mm) & 0.1 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The 1 CH 12/13 engine Specifications[38].  
In this study, the piston shape was kept as omega. We investigated the change in the depth of the combustion chamber and the diameter of the combustion chamber under the condition that the engine's compression ratio remained unchanged.  
Under that requirement, 5 survey modes were selected with corresponding piston sizes, as shown in Table 3. the parameters \(T\), \(T_{m}\), \(D_{b}\), and \(L\) are as shown in Figure 1b.  
Simulation conditions include temperature, pressure of inlet air, cylinder and piston temperatures, etc., which are entered into the model based on the actual measured values of the engine in test modes. These values respond to engine conditions when operating stably. The engine speed was selected to be 1400 rpm, which corresponds to the mode that promotes the maximum torque of the low-speed diesel engine group. Within the scope of the study, the change in inlet air pressure was also investigated, and it increased from 0.15 to 0.45 MPa. Table 4 presents the boundary condition values of the 3D simulation model in AVL Fire.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Engine speed, (rpm) & 1400 \\ Boost pressure, (MPa) & 0.15 to 0.45 \\ Intake air temperature, (K) & 307 \\ Cylinder head temperature, (K) & 550 \\ Cylinder wall temperature, (K) & 475 \\ Piston top temperature, (K) & 575 \\ Fuel injection temperature, (K) & 330 \\ Fuel injection pressure, (MPa) & 300 \\ Fuel mass per cycle, (mg) & 60 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Simulation conditions in the 3D model.  
Figure 1: Piston shape of the 1 CH 12/13 engine: (a) longitudinal section of the piston; (b) basic dimensions of the piston.  
Fuel injection characteristics were determined experimentally on a non-engine test bed. The fuel injection pressure varied from 50 to 300 MPa in steps of 50 MPa. Additionally, the fuel mass per cycle was 60 mg, the injection signal duration was 0.7 ms, and the injection pressure was 300 MPa. The fuel injection characteristics of the nozzle and the mode chosen for simulation are displayed in Figure 2.