# Design and Testing of a Gasoline Engine Muffler Based on the Principle of Split-Stream Rushing
## 2 Materials and Methods
### Acoustic and Aerodynamic Tests  
#### 2.3.1 Gasoline Engine Noise Test and Acoustic Test  
In this paper, the model of the gasoline engine is KTM690, which features light weight, high power, and a high rotational speed. Table 1 shows the technical parameters of the KTM690 engine [19]. According to the requirements of GB/T 33928-2017 [20] \"Measurement Methods for Internal Combustion Engine Exhaust Mufflers\", the exhaust noise characteristics of the target gasoline engine were tested. The exhaust noise testing steps were as follows (similar to the muffler insertion loss test steps):  
1. After removing the muffler of the target engine, the engine was placed in an open area, and it was ensured that there were no reflectors around the site to reduce inaccurate test results due to sound wave reflection. 2. A straight pipe was connected directly to the engine exhaust. The length of the straight pipe was the same as that of the previous muffler. In order to minimize the influence of other engine noises on the exhaust noise, a whole sound insulation panel with an opening in the middle was used. The straight pipe was passed through the opening and extended out of the sound insulation panel, and the opening of the sound insulation panel was sealed for sound and vibration isolation. 3. The engine was placed outdoors, and it was ensured that there was no interference from other noise sources. When the outdoor wind speed was greater than 1 m/s, a wind shield was used as required to reduce the impact on noise testing due to excessive wind speed; when the wind speed exceeded 5 m/s, the measurement was stopped to avoid distorted test data, and there were no reflectors within 10 m around the exhaust pipe. 4. The measurement position was located at a 45\({}^{\circ}\) direction of the exhaust port airflow axis, and the measuring distance was selected as 1 m, with one measuring point taken. When measuring the distance, it was ensured that the measuring distance was greater than three times the diameter of the pipe opening and not less than 0.25 m.
5. After the preliminary arrangement work of the test was completed, the engine was started. After the engine had run for 10 min, a sound level meter was used to measure the exhaust noise of the engine at different speeds, namely idle speed, medium speed, and rated speed. During the test, two data measurements were completed, respectively, and the average value of the two data measurements was taken. In one test, two data records were completed, and the parking time interval between the two data records did not exceed 30 min. The test schematic diagram is shown in Figure 3, and the test equipment is shown in Figure 4. The test equipment used in this test was the B&K 2250 sound level meter and the 4231 type calibrator. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Calibrated power (kW) & 45.8 \\ Quantity (kg) & 158.6 \\ Rated speed (r/min) & 5000 \\ Cylinder diameter (mm) & 102 \\ Stroke (mm) & 84.5 \\ Displacement (L) & 0.69 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Technical parameters of the ktm690 engine. After the engine exhaust noise test was completed, noise spectrum analysis was carried out using the noise characteristic analysis software Noise Partner 1.9 provided by B&K (Hottinger Bruel & Kjaer GmbH., Darmstadt, Germany). Data analysis was performed at an idle speed of 1500 r/min, a medium speed of 3000 r/min, and a rated speed of 5000 r/min, respectively. The results are shown in Figure 5.  
Since the target gasoline engine was a racing engine, the engine was kept at the rated speed for most of the time during the race, so the center of gravity of noise reduction was placed at the rated speed. In the whole frequency band of the rated speed, the peak frequency of the engine exhaust noise was 381 Hz, and the noise reached 110 dB.