In this study, 3D air-flow-field evolution in a single cylinder optical research engine was determined using tomographic particle imaging velocimetry (TPIV) at different engine speeds. Two directional projections of captured flow-field were pre-processed to reconstruct the 3D flow-field by using the MART (multiplicative algebraic reconstruction technique) algorithm. Ensemble average flow pattern was used to investigate the air-flow behavior inside the combustion chamber during the intake and compression strokes of an engine cycle. In-cylinder air-flow characteristics were significantly affected by the engine speed. Experimental results showed that high velocities generated during the first half of the intake stroke dissipated in later stages of the intake stroke. In-cylinder flow visualization indicated that large part of flow energy dissipated during the intake stroke and energy dissipation was the maximum near the end of the intake stroke. Non-homogeneous and highly fluctuating flow of intake stroke became uniform during the compression stroke. Air velocity was significantly higher at higher engine speeds and vorticity also increased with increasing engine speed. Peak vorticity was located near piston in the intake stroke however in compression stroke, it was located closer to the cylinder head. Higher piston speed and improved volumetric efficiency at higher engine speeds led to increased intake jet velocity and turbulence.