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A square piston engine and a side-view collimating (SVC) cylinder engine are developed for side-view schlieren visualization of in-cylinder processes. The square piston engine has a square cylinder with two flat quartz windows permitting optical access to the entire cylinder volume. Compression sealing is provided by three polyimide ring assemblies which maintain the windows clean during the actual observation. In contrast with the square piston engine, the SVC cylinder engine permits to take schlieren photographs in practical engine geometries. This is made possible by designing the SVC cylinder so that it does not disperse parallel light rays. The physical principle and optical property of this cylinder are described. High speed schlieren movies were taken with these two transparent cylinder engines. Typical pictures, such as those of flame propagation, unburned gas motion, and formation of fuel and air mixture, are presented.