Method for improving the combustion of a diesel combustion engine comprising at least one cylinder (C) with a respective piston (P) and a respective fire ring (FR), which is inserted in a stepped housing (FRH) defined in a cylinder liner (CL) so as to surround a respective combustion chamber (CC), the method comprising the step of: - shaping an outer surface (OS) of the fire ring (FR), intended to contact said stepped housing (FRH), 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 (FR) in such a manner that said upper edge consists of a cutting edge intended to contact a cylinder head (H) of the combustion engine and to define a sealing effect between the fire ring (FR) and the cylinder head (H). said at least one annular protruding portion being shaped as an annular rim coaxial with a symmetry axis (X) 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 (X) of the fire ring, said symmetry axis of the fire ring coinciding with the symmetry axis of the cylinder. - shaping an outer surface (OS) of the fire ring (FR), intended to contact said stepped housing (FRH), 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 (FR) in such a manner that said upper edge consists of a cutting edge intended to contact a cylinder head (H) of the combustion engine and to define a sealing effect between the fire ring (FR) and the cylinder head (H). - machining said outer surface at an upper edge of the fire ring (FR) in such a manner that said upper edge consists of a cutting edge intended to contact a cylinder head (H) of the combustion engine and to define a sealing effect between the fire ring (FR) and the cylinder head (H). said at least one annular protruding portion being shaped as an annular rim coaxial with a symmetry axis (X) 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 (X) of the fire ring, said symmetry axis of the fire ring coinciding with the symmetry axis of the cylinder. <EOS>