Flow visualization and particle tracking velocimetry (PTV) were used to study the in-cylinder flow field produced by a 4-valve engine head during the intake process. Several intake configurations were considered, and their effects on the in-cylinder flow structures were assessed. The engine head was mounted on a special single cylinder water analog for the in-cylinder flow visualization and PTV experiments. The results for this study indicate that the in-cylinder flow field during the induction process is comprised of several large scale vortical motions on different planes within the engine cylinder. These large scale fluid motions are, in most cases, stable and highly repeatable from cycle to cycle. No significant in-cylinder tumbling motion was observed in the engine with both intake valves operating. The only well-defined tumbling flow structure was observed with shrouded intake valve configurations. In this case, the in-cylinder flow was dominated by a strong axial-plane vortex which filled the entire cylinder volume. Using the measured velocity fields, tumble and swirl ratios were evaluated under transient conditions.THE IN-CYLINDER FLUID MOTION in internal combustion engines is one of the most important factors controlling the combustion process. It governs the flame propagation rate in homogeneous charge spark-ignition engines; it controls the fuel-air mixing and burning rates in diesels [1, 2, 3, 4, 5, 6, 7, 8 and 9]. The flow fields prior to combustion in internal combustion engines are generated during the induction process and modified during the compression stroke. Therefore, a good understanding of fluid motion during the induction process is critical to developing engine designs with the most desirable operating and emissions characteristics.One of the most attractive techniques for characterization of the large scale fluid motion during the intake process in internal combustion engines is water flow visualization [10, 11, 12, 13, 14 and 15]. Observation of the intake flow images and in-cylinder velocity measurements provides useful insights into the induction stroke.The aim of the present work is to conduct an extensive in-cylinder flow visualization study of the intake-generated fluid motion produced by a 4-valve engine head with different intake configurations. The engine head was mounted on a single-cylinder transient water analog (engine)  for flow visualization purposes. Aspects of flow evolution during the intake stroke were examined for several different intake configurations. First, the experiment was performed with both intake valves operating. Second, one of the intake valves was shut and the in-cylinder flow motion was visualized. Third, a 180-degree shroud was installed on one of the intake valves and the in-cylinder fluid motion was studied for two different shroud positions. These two shroud positions were selected such that they generated strong tumbling and strong swirl motions, respectively. Finally, 180-degree shrouds were installed on both operating intake valves to generate “pure tumble”.