Method for improving the combustion of a diesel combustion engine comprising at least one cylinder with a respective piston and a respective fire ring, which is inserted in a stepped housing defined in a cylinder liner so as to surround a respective combustion chamber, the method comprising the step of: - shaping an outer surface of the fire ring, intended to contact said stepped housing, so as to comprise at least one annular protruding portion shaped as a tip or a ridge so as to reduce a thermal exchange between the fire ring and the cylinder liner; characterized by further comprising: - machining said outer surface at an upper edge of the fire ring in such a manner that said upper edge consists of a cutting edge intended to contact a cylinder head of the combustion engine and to define a sealing effect between the fire ring and the cylinder head. said at least one annular protruding portion being shaped as an annular rim coaxial with a symmetry axis of said cylinder; wherein said outer surface is shaped so as to comprise two annular protruding portions defining respectively a first annular rim and a second annular rim (R1, R2), wherein the first annular rim is close to a lower edge of said fire ring and the second annular rim is close to said upper edge of said fire ring; and wherein, between said first and second annular rims, an intermediate portion of the fire ring is machined so as to define a circular segment, according to a longitudinal sectional view traced so as to include the symmetry axis of the fire ring, said symmetry axis of the fire ring coinciding with the symmetry axis of the cylinder. - shaping an outer surface of the fire ring, intended to contact said stepped housing, so as to comprise at least one annular protruding portion shaped as a tip or a ridge so as to reduce a thermal exchange between the fire ring and the cylinder liner; characterized by further comprising: - machining said outer surface at an upper edge of the fire ring in such a manner that said upper edge consists of a cutting edge intended to contact a cylinder head of the combustion engine and to define a sealing effect between the fire ring and the cylinder head. - machining said outer surface at an upper edge of the fire ring in such a manner that said upper edge consists of a cutting edge intended to contact a cylinder head of the combustion engine and to define a sealing effect between the fire ring and the cylinder head. said at least one annular protruding portion being shaped as an annular rim coaxial with a symmetry axis of said cylinder; wherein said outer surface is shaped so as to comprise two annular protruding portions defining respectively a first annular rim and a second annular rim (R1, R2), wherein the first annular rim is close to a lower edge of said fire ring and the second annular rim is close to said upper edge of said fire ring; and wherein, between said first and second annular rims, an intermediate portion of the fire ring is machined so as to define a circular segment, according to a longitudinal sectional view traced so as to include the symmetry axis of the fire ring, said symmetry axis of the fire ring coinciding with the symmetry axis of the cylinder.