*5.3. Intake Pipe Pressure*

Figure 7 shows the intake pipe pressure results during the intake period. The higher engine speed is the greater negative pressure [30], and the time when the greatest negative pressure occurred was the same as in the experiment and in the 1D gas flow analysis. The phase di fference occurred within 0.5 CA◦.

**Figure 7.** Comparison of the experiment and 1D gas flow analysis results of the intake pipe pressure during the intake period.

After the exhaust valve closes (EVC; 378 CA◦), there is no flow from the cylinder to the exhaust, and the flow enters the cylinder through the intake system. The time it takes for the pressure to rise to atmospheric pressure appeared later as the engine speed increased, which is thought to be due to the large flow rate that entered the cylinder.

By applying a discharge coefficient, errors within the experimental results were reduced. Even when the discharge coefficient was applied, the negative pressure and the pressure change were larger in the 1D gas flow analysis compared to the experiment [31]. The error of the maximum negative pressure decreased by 0.56–1.93% after applying the discharge coefficient and the maximum pressure error decreased by 3.11–7.86%. This is expected to be an error caused by modeling the intake and exhaust ports as straight pipes. When the piston goes down from TDC to BDC, the amount of intake is more in a straight pipe and more gas exchanged in intake and exhaust because there is less loss compared to bent pipes due to an eddy through the pipe [32,33].

In straight pipes, reflected waves are generated at the end of the pipe. However, in bent pipes, reflected waves are generated passing through the bent area [34]. The error in the 1D gas flow analysis is thought to have been caused by not calculating the influence of the reflected wave in bent pipes, as

in the experiment [35]. It is expected that errors can be reduced if the geometry of bent pipes, which cannot be modeled in 1D, such as intake ports, are analyzed in 3D and are coupled [36,37].
