A vortex-stratified combustion process for hydrogen-fueled reciprocating internal combustion engines is introduced to increase the thermal efficiency by reducing the convective heat transfer losses to the surrounding walls during combustion. The process imposes a highly ordered rotational field upon the charge in a separate, transverse, cylindrically shaped combustion chamber by means of channels that connect with the main chamber enclosed by the engine cylinder and piston. Gaseous hydrogen is injected directly during the compression stroke, while air enters into the combustion chamber tangentially and preferentially along the circumference due to the Coandă effect. The two streams entrain one another and develop into a vigorous vortex by virtue of the chamber and channel geometries. As mixing proceeds, the fuel is confined radially from the combination of finite-time diffusion being outpaced by the replenishment of pure air at the periphery, and the centripetal field formed by the rotating flow acting on the different density gas mixture. Combustion takes place with a flame propagation that initially follows the rotation of the bulk flow but also curls radially inward toward the center. This work investigates the process in a fired, optically accessible 2-stroke hydrogen-fueled direct-injected engine tested at up to 5000 RPM. This paper, the first of two parts, explains the theoretical background, presents the schlieren observations and the results of zero-dimensional cylinder pressure indication and apparent heat release measurements comparing two combustion chamber designs – one that actualizes a homogeneous mixture without specific charge motion directionality and another with the here-introduced vortex-stratified approach.