A three-component laser Doppler anemometry (LDA) system has been employed to investigate the structure of the ow inside the cylinder of a motored internal combustion engine. This model engine was reasonably representative of a typical, single cylinder, spark ignition engine although it did not permit ÿring. It was equipped with overhead valve gear and optical access was provided in the top and side walls of the cylinder. A principal objective was to study the in uence of the inlet port design on the ow within the cylinder during the induction and compression strokes of the engine. Here, it can be noted that results obtained in an unÿred engine are believed to be representative of the ow behaviour before combustion occurs in a ÿred engine (see P.O. Witze, Measurements of the spatial distribution and engine speed dependence of turbulent air motion in an i.c. engine, SAE Paper No. 770220, 1977; Witze, Sandia Laboratory Energy Report, SAND 79-8685, Sandia Laboratories, USA, 1979). Experimental data presented for an inclined inlet port conÿguration reveal the complex three-dimensional nature of the ow inside the model engine cylinder. Not surprisingly, the results also show that the inclined inlet port created ow conditions more favourable to mixing in the cylinder. Speciÿcally, the inclined inlet ow was found to generate a region with a relatively high shear and strong recirculation zones in the cylinder. Inclining the inlet port also produced a more nearly homogeneous ow structure at top dead centre during the compression stroke. The paper identiÿes the special di culties encountered in making the LDA measurements. The experimental ÿndings are examined and the problems that arise in presenting time-varying three-dimensional data of this type are discussed. Finally, the future potential of this experimental approach is explored.