The noise radiated from the tail pipe plays an important role in the total noise emitted for small two stroke engines. In this paper, an engine cycle and exhaust system model for a commercial 50 c.c., crankcase-scavenged type, single cylinder two stroke engine was developed to determine the noise emitted from the exhaust pipe.
In modeling the engine cycle, a zero dimensional thermodynamic model was used to represent the compression and the expansion process in cylinder, and a two zone quasi dimensional model was used to represent the combustion process. As for the scavenging process, the perfect mixing model was used. In modeling the gas dynamics in the intake and the exhaust system, an unsteady one dimensional model was used, and the characteristic line method was employed to solve the partial differential equations.
Seven different configurations of exhaust pipes were studied in this paper, including three straight pipes, two diverging-converging pipes, one pipe with an expansion chamber, and a real exhaust pipe. The test engine was run in both motoring and firing conditions. Pressure variations in the exhaust pipe and the total sound level and the one third octave frequency distributions outside the exhaust pipe were recorded.
For the simple geometry pipes at motoring conditions, the calculated results of pressure variations inside exhaust pipe and the noise level outside exhaust pipe agree quite well with the measured data. However, for the complex geometry pipe at firing conditions, the calculated results are consistent with the experimental data at low engine speeds only. At high engine speeds, the model used in this paper can not predict the high frequency components of the pipe pressure, and the total sound levels predicted are underestimated.