Tumble flow pattern, during the intake and compression strokes, is investigated on a four-valve, four stroke transparent single-cylinder research engine. Two-dimensional in-cylinder velocity distributions are measured with cross-correlation Particle Image Velocimetry (PIV) on planes both parallel and perpendicular to the piston. The velocity fields are obtained at several crank angles. The single-cylinder engine has a transparent quartz liner and an extended piston with a quartz top.PIV investigation in the different planes allows the tumble flow pattern to be deduced from the two-dimensional ensemble-averaged flow pattern. The tumble motion is similar to a 2D roll motion with a rotation axis deformed on the two extremities towards the piston head. With such a pattern, the two-dimensional velocity fields in horizontal sections exhibit two vortices in the upper part of the cylinder. Furthermore, it is demonstrated that the tumble axis presents a precession motion during the intake and compression strokes.Comparison between cycle-resolved bulk flow and ensemble-averaged flow has shown wide differences, particularly near the rotation center. These are due to the cycle to cycle variations which artificially increase the ensemble velocity fluctuations in the axis region of the roll motion. In addition, the spatial filtering velocity field exhibits large scale structures.