An extraction of cycle-to-cycle variation of in-cylinder flow using temporal / spatial averaging and a proposal of variable sized spatial filtering

Abstract

Cycle-to-cycle variation (CCV) of in-cylinder flow occurs in internal combustion engines. It is necessary to analyze CCV of flow to separate averaged-flow (as low frequency / low wave number) from turbulence (as high frequency / high wave number), because an averaged flow varies from cycle to cycle. Two averaging methods are used for the extraction of mean component from instantaneous flow. One is temporal-averaging method, the other is spatial-averaging method. In the temporal -averaging method, a fluctuation of flow is captured at fixed point in Eulerian, turbulence is regarded as the high frequency component, and it is removed by a low pass filtering. In the spatial-averaging method, the turbulence in spatial arrangement of flow velocity is directly averaged by using vortex scale as a threshold (e.g. Moving-averaging filter and Gaussian-averaging filter). However, the temporal-averaging and the spatial-averaging have completely different characteristics. Therefore, it is necessary to clarify the difference of filtering characteristics in each averaging filter. In this study, comparisons of averaged flow patterns of temporal-average and spatial-average are carried out. Moreover, variable sized spatial filter which is based on Taylor's frozen-turbulence hypothesis is proposed. As a result, variable sized filtering is found close to the filter characteristic of the time average method.

Keywords

• internal combustion engine
• cycle-to-cycle variation
• high-speed piv
• in-cylinder flow
• temporal-averaging
• spatial-averaging
• flow analysis