The top of the piston has a spherical recess (4) at the crossing point of two v-shaped or spherical grooves (3) aligned with the inlet gas flow paths from the inlet slots (2) and forming a combustion chamber. The recess is under the spark plug and the burnt gasses are ducted out via an outlet slot (1). The cylinder head (7) has a flat profile over the cylinder, or a raised profile. The recess and grooves duct the flushing gas into the cylinder and effectively remove most of the burnt gasses prior to the next firing cycle. The grooves slope down into the recess and are angled to a middle line through the cylinder.
