POD-based analysis of combustion images in optically accessible engines

This paper reports on 2D images of combustion-related luminosity taken in two optically accessible automobile engines of the most recent generation. The results are discussed to elucidate physical phenomena in the combustion chambers. Then, proper orthogonal decomposition (POD) is applied to the acquired images. The coefficients of the orthogonal modes are then used for the analysis of cycle variability, along with data of dynamic in-cylinder pressure and rate of heat release. The advantage is that statistical analysis can be run on a small number of scalar coefficients rather than on the full data set of pixel luminosity values. Statistics of the POD coefficients provide information on cycle variations of the luminosity field. POD modes are then discriminated by means of normality tests, to separate the mean from the coherent and the incoherent parts of the fluctuation of the luminosity field, in a non-truncated representation of the data. The morphology of the fluctuation components can finally be reconstructed by grouping coherent and incoherent modes. The structure of the incoherent component of the fluctuation is consistent with the underlying turbulent field.