## 5 Conclusions  
In this study, measurement of the back pressure of an exhaust pipe system and the test of exhaust emissions using a gas analyzer were conducted to analyze the effect of the back pressure change according to the confluence geometry of the exhaust pipe system on exhaust emissions in an idling state.  
The most important back pressure measurement position in the back pressure measurement result is number 1. The reason is that it is a location that is greatly affected by changes in the confluence geometry.  
Therefore, in the case of H Pipe, which exhibited the highest back pressure at measurement location number 1, a maximum of 64.88 ppm was shown in the result of THC exhaust emission. The reason is that the flow area of the exhaust pipe is the largest, but the flow resistance, such as a vortex, formed at the confluence point. This is because the flow resistance formed at the confluence position prevented the exhaust gas from flowing naturally and increased the pressure inside the exhaust pipe as the flow rate decreased due to interference and resistance. Therefore, the highest back pressure was 0.362 kPa at position 2 where the back pressure was measured. In addition, in the case of X Pipe, which showed the lowest back pressure at measurement location 1, the minimum value of 51.68 ppm was reduced by 21.4% in the result of THC exhaust emission. In the case of X Pipe, the flow of exhaust gas was naturally formed at the confluence position, creating a high flow rate, and as a result, the lowest back pressure of 0.144 kPa at measurement position 2 was shown. The Y pipe showed a higher pressure than the X pipe, and the THC emission increased by 13%. In the case of Y pipe, exhaust gas flows naturally at the confluence position, but the pressure inside the exhaust pipe is 0.321 kPa, which is 11.3% lower than that of H pipe due to the reduced flow area and long residence time.  
Therefore, it was found through this study that the change in back pressure at measurement position 1, according to the change in the geometry confluence, affects the discharge of THC. These results were influenced by the valve overlap mentioned in the introduction. It results from a part of the exhaust gas that was burned back flowing into the cylinder  
Figure 8: Scatter plot using the result of back pressure measurement position 1 and exhaust emission THC.  
under the influence of the back pressure formed inside the exhaust pipe. As a result, the THC emission increased from the exhaust pipe system where the relatively high back pressure was measured. These results are because the combustion performance decreased as the exhaust gas exhausted back to the cylinder under the influence of the valve overlap mentioned in the introduction. Therefore, the X Pipe, which showed the lowest back pressure in the experimental results of this study, minimized the exhaust gas THC emission by minimizing the backflow of the burned exhaust gas, and this result can be evaluated as the confluence geometry of the optimized dual exhaust pipe system.  
Conceptualization, I.-s.K.; methodology, S.-m.Y.; validation, I.-s.K. and S.-m.Y.; formal analysis, I.-s.K.; investigation, I.-s.K.; resources, I.-s.K.; writing--original draft preparation, I.-s.K.; writing--review and editing, I.-s.K.; visualization, I.-s.K.; supervision, S.-m.Y.; project administration, I.-s.K.; funding acquisition, I.-s.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors declare no conflict of interest.