An operative internal combustion engine is transparent for demonstration of internal combustion engine principles. A transparent cylinder houses a noncontacting piston, a combustion chamber being sealed by means such as protruding annular teflon rings seated in an annular groove of the piston or a flexible flap formed integrally with the piston which is pressed firmly to the internal surface of the cylinder wall by pressure developed within the combustion chamber during operation of the engine. Other portions of the engine are transparent to enable their contained parts to be observed during operation. The engine also has an enlarged flywheel to permit operation of the engine at low, observable speeds. The engine burns low carbon fuels such as alcohol to keep the cylinder walls clean.The engine is designed with various sizes of piston caps to change the compression ratio. It utilizes a timing lever which varies the ignition timing from −20° to +20°. Study of flame propagation, combustion, and emissions are enhanced by the glass cylinder which is air cooled for optical reasons.This oil-less engine is coupled to an electric dynamometer for measurements of torque, engine rpm, cylinder pressure, manifold pressure, and cylinder head temperature. The engine is patented (U.S. Pat. No. 3,698,370 and Re-issue No. 30,253).