Recently, proper orthogonal decomposition (POD) technique has gained wide attractiveness as a powerful analysis tool in engine flows. The velocity vector fields resulted from high speed time-resolved particle image velocimetry (PIV) of the flow field inside an engine cylinder under steady-state conditions were investigated by proper orthogonal decomposition with the objective to compare the in-cylinder flow structures evolution and variation at different pressure differences 300, 450 and 600 mmH2O. The result demonstrated that mode 1 flow pattern was a good estimate of the ensemble-averaged tumble pattern. Moreover, there was a good correlation between mode 1 coefficients and the applied pressure difference. © 2018 Author(s).