a four - stroke piston engine in which air is drawn into a cylinder while rotary motion about the cylinder axis is imparted to the air . fuel may be blown / injected into the air during its rotation in the cylinder while the fuel supply is located on the axis of the cylinder and supplies at least one jet of fuel initially directed radially outwardly and which jet of fuel in cooperation with the rotating air forms at least one helical stream of mixture within the cylinder , preferably during the compression stroke , which is transformed into a coherent mixture zone enclosed by a ring of air upon completion of compression . the piston may have a recess so that when the piston approaches top dead center an enriched zone is formed in the lower region of the compression space which can be ignited by electrodes disposed in said lower region of the compression space . the stratified charge in the cylinder of the stratified - charge engine comprises a mixture zone of comparatively small volume and a mixture zone of comparatively large volume which may have different fuel concentrations plus a thermally insulating zone of air .