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The measurement and analysis of the flow in the single expansion chamber exhaust pipe
Exhaust noise constitutes the major part of the total noise emitted from motorcycle engines, especially for two stroke engines with small displacement volume. The temporal variations of the velocity and pressure inside the exhaust pipe play an important role in determining the total sound pressure level and the spectral characteristics of exhaust noise. Usually one dimensional unsteady gasdynamic model was adopted to calculate the exhaust flow in the past. This approach worked well for exhaust pipes of simple geometry. However, high frequency components of the noise generated from exhaust flow could be underestimated for complex exhaust pipes with the conventional one dimensional approach. The objective of this research is to investigate the flow characteristics in the exhaust pipe of a small two stroke motorcycle engine. A multi dimensional flow model coupled with an engine cycle model was used instead of the conventional one dimensional model to calculate the periodic flow in exhaust pipes.
Both analysis and measurements of the temporal variations of the velocity and pressure variations inside the exhaust pipe were carried out in this study. The engine cycle simulation software BOOST was adopted to model the two stroke motorcycle engine. The intake system was analyzed with one dimensional model while the crankcase and the cylinder were analyzed with zero dimensional model. A CFD package FIRE was applied to model the three dimensional periodic flow in the exhaust pipe. The three dimensional exhaust pipe model and the one/zero dimensional engine model were then combined together and the associated numerical programs were executed concurrently to obtain the periodic variations of the pressure and flow in the exhaust pipe. As in the part of measurement, a commercial moped engine was used for testing in this study. An exhaust pipe with single expansion chamber was attached to the engine exhaust port. The engine was driven with an electric motor at constant speeds. The temporal as well as the spatial variations of the flow inside the exhaust pipe were measured with a hot wire anemometer and the pressure variations were measured with pressure transducers located at several locations along the axial length of the exhaust pipe.
Results of calculation of the three dimensional exhaust pipe model showed that as the flow in the exhaust pipe reached a stable periodic state, two circulating zones occurred in the expansion chamber. These two circulating zones grew and decayed sequentially and then merged together to become a large circulation at the end of an engine cycle. The process of growth and decay repeated in each engine cycle. Calculation results of the three dimensional exhaust pipe model were quite different from those of the one dimensional exhaust pipe model obtained previously. The complex flow pattern occurring inside the expansion chamber has not been observed in the results of conventional one dimensional exhaust pipe model. However, the pressure variations in the exhaust pipe obtained from the conventional one dimensional model are close to those obtained in the present study. The spatial pressure variations in the expansion chamber are within 0.1 kPa at all times during the flow period, and the pressure distribution along the length of the pipe is close to a plane wave. As in the straight pipe connecting the engine exhaust port and the muffler, the calculated velocity distribution in the present study was close to that obtained from previous one dimensional calculation.
Comparing the measured data with the calculated results showed that the location and the moving speed of the circulating zones as well as the major frequency components of the velocity variations in the expansion can be predicted correctly. However, the calculated velocity amplitudes and phase angles did not agree very well with the measured data. As comparisons of the instantaneous flow at the exit of the exhaust pipe, results of the three dimensional model are closer to the measured data than those obtained from the conventional one dimensional model concerning the velocity amplitudes and phase angles. However, discrepancies of the average flow in the whole cycle still exist between the calculated results and the measured data for both the cases of one dimensional model and three dimensional model.
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