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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.