Better understanding of flow phenomena inside the combustion chamber of a diesel engine and accurate measurement of flow parameters is necessary for engine optimization i.e. enhancing power output, fuel economy improvement and emissions control. Airflow structures developed inside the engine combustion chamber significantly influence the air-fuel mixing. In this study, in-cylinder air flow characteristics of a motored, four-valve diesel engine were investigated using time-resolved high-speed Tomographic Particle Imaging Velocimetry (PIV). Single cylinder optical engine provides full optical access of combustion chamber through a transparent cylinder and flat transparent piston top. Experiments were performed in different vertical planes at different engine speeds during the intake and compression stroke under motoring condition. For visualization of air flow pattern, graphite particles were used for flow seeding. In the experiments, highly swirling air flow pattern was found, which was dominant in the vicinity of valves. Swirling patterns significantly affected by engine speed and increased at higher engine speed. The average structure of the flow field was analyzed, which showed a clear orientation of the average velocity, which changed during different phases of the cycle. Vorticity analysis showed higher vorticity beneath the intake valve that affects turbulence in air flow. Different components of velocity as Vx, Vy and Vz showed that engine speed and z-location affected all velocity components in which maximum variation takes place in Z-direction.