## 3 Results and Discussion  
### Flow Process Analysis  
In this study, the flow process was simulated to obtain the change of pressure in the cylinder during the flow process. The pressure change is not obvious during the whole intake process, with slight fluctuations. At the beginning of the intake process, because the intake valve is not fully closed, the pressure in the cylinder is close to that of the exhaust port. In the initial stage of intake due to the overlap period of the valve, the pressure in the cylinder fluctuates. Due to the suction of the piston, negative pressure is generated in the cylinder and the pressure is reduced. After the exhaust valve is closed (\(-315\)\({}^{\circ}\)CA), the pressure gradually rises again. In the late stage of intake, the pressure continues to rise after BDC (bottom dead center, \(-180\)\({}^{\circ}\)CA) to intake valve closes (\(-121\)\({}^{\circ}\)CA) owing to the delayed closing of the intake valve. The lift curves of the intake and exhaust valve are shown in Figure 3.  
Visualizing the flow field of the intake process, as shown in Figure 4, the cut plane is parallel to the center line of the cylinder, and it passes through the center of the valve. The velocity of the air flow value in the calculation result is added to the instantaneous velocity field under different crank angles. After the intake valve is opened, the air in the intake manifold begins to enter the cylinder. In the initial stage of intake, through the analysis of the instantaneous velocity field in the cylinder, it can be observed that a little gas flows back into the intake manifold through the intake valve, which causes the in-cylinder pressure to fluctuate. As the intake valve opening, the air entering the cylinder forms the tumble flow. In the early stage of the intake stroke, there are distinct turbulent motions in two opposite directions around the intake side and exhaust side, respectively. However, the tumble flow  
Figure 3: Lift curves of the intake valve and exhaust valve.  
Figure 2: L-type ignition energy.  
near the intake side disappears gradually in the compression stroke since the motion of the piston and the valve.  
Figure 5 shows the contrast results of tumble ratio during the intake and compression strokes simulated by the G-RNG (generalized renormalization group) \(k-\varepsilon\) model and the RNG \(k-\varepsilon\) model, respectively. From Figure 5, it can be seen that the tumble ratio in the combustion chamber by the G-RNG \(k-\varepsilon\) model had the same trend to that of the RNG \(k-\varepsilon\) model similarly reported in literature [31]. As shown in Figure 4, when the intake valves opened, the pressure in the combustion chamber was higher than that in the intake ports, which led to a back flow. As the piston moved down, fresh air began to rush into the combustion chamber and the sign of the tumble ratio varied. After IVO, the tumble motion became more organized due to the piston movement during the compression stroke, leading to a slight increase in the tumble motion, and the tumble of the gas gradually decreased at the end of the compression stroke.  
Figure 4: Instantaneous velocity field of the flow process.