The high speed particle image velocimetry(PIV) was used to measure the in cylinder flow fields of a gasoline direct injection optical engine under motoring conditions. The dynamic mode decomposition(DMD) algorithm was used to extract the multi scale flow structures from early intake stroke to late compression stroke. The decay process of specific kinetic energy of the multi scale flow structures during both strokes was quantified. The results showed that from the early stage of the intake stroke

the in cylinder flow fields were mainly composed of large scale flow structures represented by low order DMD modes and small scale vortex structures represented by high order DMD modes. The variations of specific kinetic energy of DMD modes could clearly reflect the energy cascade and dissipation processes from the large scale flow structures to the small scale vortex structures. Compared with the compression stroke

the in cylinder flow fields during the intake stroke showed a faster energy decay process with more small scale vortex structures. The energy decay of the intake stroke was more sensitive to engine speed.