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A detailed knowledge of the scavenging process becomes necessary during the development process, when the performance and in particular the emission output of two-stroke engines needs to be qualified and evaluated. This paper presents an experimental approach to describe the composition of the flow at the exhaust port by means of flow visualization and to quantify the relative changes of the scavenging losses imposed by design changes. The experimental set-up has been described in previous SAE papers and has been expanded by a transparent exhaust port, which gives optical access to the flow through the exhaust port. The gases in the cylinder are represented by differently colored water-based fluids. Typically, the burnt gas is clear and the fresh charge is colored to visualize its progress during the scavenging cycle and its distribution inside the cylinder.

A method is presented which allows the investigation of the flow inside the cylinder and the transfer channels of small two-stroke cylinders under the condition of an expanded time scale by using the similarity laws. A test rig has been designed to incorporate the application of visualization techniques as well as quantitative measurement systems like LDV or PIV. The results obtained by this method are needed to understand the complex nature of the three-dimensional flow inside the cylinder and can be used to verify CFD calculations. Moreover, modifications can be applied to the model in an easy and fast manner, which enables the designer to examine various versions during the early stage of the development process.

Experimental methods are needed to understand unsteady three-dimensional flows. They also become an important tool where geometrical details or the complexity of the applied model would require an over-proportional numerical effort to investigate a scavenging flow. The experimental method presented in this paper applies visualization techniques to analyze the scavenging flow of small two-stroke cylinders. The principle at work is based on similarity laws, which transfer the dimensions of an actual engine into an enlarged model. The modular set-up allows fast modifications of the port geometry and detailed visualization of the flow coming out of individual transfer ports of multiple port cylinders by using different dyes. Results are presented as movies, which give a very graphic impression of the unsteady three-dimensional flow inside the cylinder.