The flow into and out of the piston top-land crevice of a spark-ignition engine has been studied, using a square-cross-section single-cylinder engine with two parallel quartz glass walls which permit optical access to the entire cylinder volume. Schlieren short-time exposure photographs and high speed movies were used to define the essential features of this flow.The top-land crevice and the regions behind and between the rings consist of volumes connected through the ring gaps. A system model of volumes and orifices was therefore developed and used to predict the flow into and out of the crevice regions between the piston, piston rings and cylinder wall.It was shown that two types of flow out of the top-land crevice entrance occur during the expansion stroke: (i) a low velocity flow which expands out of the crevice entrance, around the piston circumference, shortly after peak cylinder pressure occurs; (ii) a jet-type flow through the top piston-ring gap, later in the expansion process, which starts when the pressure above the ring gap falls below the pressure beneath the gap. Both these flows were observed when a production engine piston crown was inserted into the square-cross-section piston crown of the transparent engine facility in visualization experiments.The flow model was coupled with a ring motion model and applied to a real engine geometry. The model predicts the amount of unburned fuel trapped in the crevice regions, and calculates the portion of this trapped unburned fuel that is lost to the crankcase and the portion that returns to the combustion chamber. The effects of changes in ring gap area, volume of the crevice regions, wall temperature, load and piston speed on the amount of unburned fuel returning to the combustion chamber are examined, and are related to exhaust hydrocarbon emission trends. It is shown that these crevice gases constitute a major source of unburned HC emissions, as well as a significant loss in power and efficiency.