The design and operating characteristics of a single-cylinder transparent spark-ignition engine for Schlieren flow visualization are described. The engine is built on a CFR engine crankcase using the CFR piston and cylinder as a crosshead for the square cross-section piston and cylinder assembly. The square cross-section assembly has two parallel steel walls and two parallel quartz glass walls to permit optical access to the entire cylinder volume over the complete engine operating cycle. The CFR head and valve mechanism completes the assembly. It is shown that the engine operates satisfactorily with propane fuel under typical engine operating conditions.Schlieren short time-exposure photographs and high speed movies were taken to define details of the flow and density fields through the engine cycle. Photographs which illustrate key features of these fields are presented and described. Important features identified are: Jet flow through the intake valve; large scale rotating motion in the cylinder during intake and compression; details of ignition, flame kernel development, flame structure, flame propagation; the extent of cycle-by-cycle variations; flow out of the crevices behind and beneath the piston rings during expansion; flow out of the spark plug crevice, and boundary layer entrainment during blowdown; the growth of the vortex in the piston cylinder-wall corner during the exhaust stroke; thermal boundary layer thicknesses.