Combustion chamber of an internal combustion engine

An internal combustion engine comprising a cylinder block having a cylinder bore therein;

• a cylinder head mounted on said cylinder block and having an inner wall;
• a first raised portion having on its lower end a flat bottom face and being formed on the periphery of the inner wall of said cylinder head so as to project downwards;
• a piston reciprocally movable in said cylinder bore and having a top face which has a flat peripheral portion approachable to said flat bottom face so as to create a first squish area therebetween at the end of the compression stroke for spouting out a first squish flow along the top face of said piston;
• a combustion chamber defined by the top face of said piston and the inner wall of said cylinder head;
• an intake valve movably mounted on said cylinder head for leading a combustible mixture into said combustion chamber;
• an exhaust valve movably mounted on said cylinder head for discharging exhaust gas into the atmosphere;
• a second raised portion formed on the top face of said piston at a position opposite to said first raised portion with respect to an axis of said piston and having a rear face and a front face exposed to said combustion chamber, said rear face being approachable to the inner wall of said cylinder head so as to create a second squish area therebetween at the end of the compression stroke for spouting out a second squish flow which moves forward in the upper interior of said combustion chamber in the direction opposite to the spouting direction of said first squish flow, said first and second squish flows cooperating with each other to create a strong swirl motion rotating about a horizontal axis in said combustion chamber, and;
• a spark plug having a spark gap located in said combustion chamber, wherein the front face of said second raised portion comprises a gently inclined front face portion located beneath said exhaust valve, and a steeply inclined front face portion located beneath said intake valve, said gently inclined front face portion and said steeply inclined front face portion being connected by a step portion having a smoothly curved surface, said gently inclined portion having a greater surface area than said steeply inclined portion, said inclined portions extending in substantially the same direction, said inclined portions each having a planar face and being disposed such that a line drawn horizontally across the planar face of the gently inclined portion is substantially parallel to a line drawn horizontally across the planar face of the steeply inclined portion.