Data are presented on the gas-dynamical structure of ow in the cylinder when it is lled through poppet valves with heads in the form of a circle and a square. The studies were carried out on a laboratory setup simulating a piston internal combustion engine. The air ow through the gas-dynamical system of the engine was carried out in a stationary mode with an initial speed of 8 to 41 m/s. Thermograms of the ow structure were obtained for two control sections of this system using the thermal imaging method. It has been established that the con guration of the cylinder valve head exerts a signi cant impact on the structure in it and that in a cylinder with a round valve there are pronounced stagnant zones. It has been shown that the use of valves with a square head ensures more uniform lling of the cylinder with air throughout its entire volume compared to a traditional round valve. The data obtained can be useful for re ning mathematical models of the process of cylinder lling and for developing new methods of modernizing valve assemblies of piston machines.