*5.4. Exhaust Pipe Pressure*

Figure 8 shows the exhaust pipe pressure results from the exhaust period to the BDC. Comparing the 1D gas flow analysis results at 700 rpm with those of the experiment, it can be seen that the amount of pressure change was small during the exhaust period (130~378 CA◦). The phase difference occurred after the EVC. The reason for the phase difference is that the engine speed is slow and the reflected wave affects it many times [38]. In the case of 700 rpm, the fourth reflected wave was reached during EVC to BDC. At 900 rpm or more, the third or less reflected waves were reached. As the 1D gas flow analysis did not model the exhaust port of bent geometry, it is expected that more errors would have occurred as the influence of the reflected wave increased.

**Figure 8.** Comparison of the experiment and 1D gas flow analysis results of the exhaust pipe pressure during the exhaust period to the BDC.

In the results above 900 rpm, the amount of pressure change was similar or large during the exhaust period. The phase difference occurred after the peak of the pressure wave appeared, which was smaller than that of 700 rpm. The peak of the pressure wave occurred due to the influence of the reflected wave [39]. In all of the results, as the influence of the reflected wave increased, a larger error occurred.

As a result of the calculation by applying the discharge coe fficient, the size of the pressure change was reduced, and the phase was not a ffected. The reason for the phase di fference likely occurred because the e ffect of the reflected wave generated at the open end of the exhaust pipe reached the exhaust port and a ffected the calculation. For the same reason as the intake port, the exhaust port could not be modeled in the same geometry as in the experiment. It is necessary to reduce the error in the 1D gas flow analysis caused by bent pipes not only for the intake system, but also for the exhaust system.

When the discharge coe fficient was applied, the error of the minimum pressure increased by 0.46–6.14%. In addition, the error of the maximum pressure increased by 0.53–5.03% at 900 rpm or higher. Applying the discharge coe fficient was not e ffective in reducing the error of the exhaust pipe pressure.

Summarizing the intake and exhaust pipe pressure results, the error of the intake pipe pressure is improved. However, since it is a result of reducing the di fference in the intake mass flow rate, it is not a fundamental improvement method for the error occurring in the 1D gas flow analysis. Also, the error of the exhaust pipe pressure was not improved even when the discharge coe fficient was applied. It is expected that such errors will be improved only when a method to improve errors occurring in complex shapes, which is a disadvantage of 1D, is used.
