(2-07) Analysis of Fractal Characteristics of tumbling flow in a Spark Ignition Engine Using Wavelet Transform((SI-3)S. I. Engine Combustion 3-Flow and Combustion)

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It is well known that in a four-valve spark ignition engine with pentroof combustion chamber, the tumbling flow is broken down into small eddies in the latter stage of the compression stroke under the condition of higher compression ratio and as the turbulence intensity increases. However, many characteristics of the turbulence at the time when the tumbling flow is broken down are not yet understood. Generally, the fractal dimension of the turbulence is effective for showing the eddy structure. However, when the turbulence is analyzed with this ordinary method, only a statistically averaged value is obtained, and it is difficult to evaluate the change of the eddy structure when the tumbling flow breaks down. In this study, we propose a new method to calculate the fractal dimension by using a wavelet transform which is a time-frequency analysis method and try to evaluate the change of the eddy structure in the turbulence when the tumbling flow is broken down. The procedure of our method is 1) Carry out the wavelet transform of the turbulence and obtain the information of the turbulence about both time and frequency. 2) Calculate the fractal dimension at each time from the wavelet transform of the turbulence. 3) Evaluate the change of the eddy structure in the turbulence by using the fractal dimension obtained. The Figure shows the result in the case of compression ratio ε=5.5, where Fig. (a) is the wavelet transform of the turbulence and Fig. (b) is the result of the fractal analysis. As shown in Fig. (b), the fractal dimension decreases a little near 320 degrees crank angles because the tumbling flow is broken down and the turbulence velocity becomes large as shown in Fig. (a). Then, the fractal dimension increases and becomes D_ω=1.591 near TDC because the power of the small eddies become larger. That is, the eddies that were generated by the tumbe flow broken down have larger scale and transmit the energy which they have to the small eddies in the compression stroke. After the TDC, the gas flow is also generated by the piston motion and the fractal dimension increases as shown in Fig. (b). Therefore, based on the above discussion, the fractal dimension D_ω proposed is effective to evaluate the change in structure of the eddies quantitatively.
一般社団法人日本機械学会の論文
2001-07-01

(2-07) Analysis of Fractal Characteristics of tumbling flow in a Spark Ignition Engine Using Wavelet Transform((SI-3)S. I. Engine Combustion 3-Flow and Combustion)

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