Particle image velocimetry (PIV) is used to investigate the structure and evolution of the mean velocity field in the swirl (r-θ) plane of a motored, optically accessible diesel engine with a typical production combustion chamber geometry under motoring conditions (no fuel injection). Instantaneous velocities were measured were made at three swirl-plane heights (3 mm, 10 mm and 18 mm below the firedeck) and three swirl ratios (2.2, 3.5 and 4.5) over a range of crank angles in the compression and expansion strokes. The data allow for a direct analysis of the structures within the ensemble mean flow field, the in-cylinder swirl ratio, and the radial profile of the tangential velocity. At all three swirl ratios, the ensemble mean velocity field contains a single dominant swirl flow structure that is tilted with respect to the cylinder axis. The axis of this structure precesses about the cylinder axis in a manner that is largely insensitive to swirl ratio. Higher swirl ratios reduce the cycle-to-cycle variability in the orientation of this structure. The tangential velocity profiles are well represented by the Bessel function profile often used to initialize the tangential velocity in CFD codes. A value of 2.2 is recommended for the value of the parameter α which controls the shape of the Bessel function profile. During the compression stroke, the measured in-cylinder swirl ratios were similar to those determined from steady-state flow bench measurements. The swirl ratios decreased significantly after TDC, and continued to decline during the expansion stroke.