## 4 Results and Discussion
### Analysis of the Measured Exhaust Emissions  
The components of the exhaust gas emitted after combustion inside the cylinders were analyzed using the low-speed idling test method. The items that were compared and analyzed were CO, CO\({}_{2}\), THC, and excess air factor (\(\lambda\)).  
Figure 7A shows the CO results according to the confluence geometry of the exhaust pipe system. Overall, the mean CO result was found to be 0.01% based on three measurements. The CO emission was close to zero due to the influence of the lean AFR values compared above and complete combustion.  
Figure 7B shows the CO\({}_{2}\) results according to the confluence geometry of the exhaust pipe system. Based on three measurements, Y Pipe exhibited a maximum CO\({}_{2}\) result of 14.3% and a mean value of 14.27%. Both X Pipe and H Pipe showed a maximum value of 14.5% and a mean value of 14.5%.  
Therefore, the total mean CO\({}_{2}\) emission was found to be 14.4% under the influence of complete combustion because each exhaust system exhibited leaner AFR compared to the theoretical AFR in the idling state.  
Figure 7C shows the THC results according to the confluence geometry of the exhaust pipe system. THC is representative of harmful gas that is emitted in the idling state. Based on three measurements, H Pipe exhibited relatively high values. The maximum value was 64.88 ppm and the mean value was 63.45 ppm. X Pipe showed relatively low values. The minimum value was 51.68 ppm, which was 25.6% lower compared to the maximum value, and the mean value was 52.25 ppm, which was 21.4% lower compared to that of H Pipe. In the case of Y Pipe, the mean value was 59.05 ppm, which was 7.4% lower compared to that of H Pipe.  
Figure 7D shows the result of comparing the excess air factor (\(\lambda\)) according to the change in the confluence geometry of the exhaust pipe system.  
In the result of the analysis based on three measurements, Y Pipe showed a maximum value of 1.031 and an arithmetic mean value of 1.028. In X Pipe and H Pipe, the arithmetic mean values of three times were 1.019 and 1.015, respectively.  
Therefore, compared to the theoretical air intake, the actual air intake was confirmed to be the leanest in the Y pipe.  
All of the measurement results satisfied ULEV standards in the Unites States for exhaust gas. In the idling state, however, the low exhaust gas temperature caused by lean excess air factor (\(\lambda\)) and ideal CO\({}_{2}\) emission through complete combustion may increase THC emissions by interfering with combustion as opposed to oxygen.  
This is because combustion exhaust gas flows back into the cylinders again due to the pressure difference during the valve overlap and accelerates incomplete combustion together with the residual gas remaining in the cylinders.  
In Figure 6, for back pressure measurement results (A) and (B), the highest pressure is shown in the H pipe, and the lowest pressure is shown in the X pipe.  
Based on this result, when compared with (C), THC in Figure 7, it can be seen that the decrease in back pressure is related to the decrease in THC.  
In order to clearly confirm this relationship, the results of back pressure measurement 1 and THC were used to show the scatter diagram, as shown in Figure 8. The scatter diagram shows that the THC emission increases as the back pressure increases. The result showing the lowest emission in the X Pipe group showing the lowest back pressure can be confirmed.  
Therefore, a relatively large amount of combustion exhaust gas may flow back into the cylinders when a large pressure is formed inside the exhaust pipe.  
The experiment results also confirmed that there was a large difference in THC emissions depending on the back pressure even though only the geometry of the exhaust pipe system was changed under the same conditions.  
Figure 7: Results of measured exhaust emissions (**A**) CO, (**B**) CO\({}_{2}\), (**C**) THC, and (**D**) excess air factor (\(\lambda\)).