Sealing ring interposed between the block and the cylinder head of an internal combustion engine, having a composite structure

1. A sealing ring set between an engine block of an internal-combustion engine and a cylinder head fixed to said engine block, said ring being received in a seat formed around a cylinder within at least one of the mutually facing surfaces of the engine block and of the cylinder head,

wherein said sealing ring comprises:

an outer hollow annular body having a closed cross-section, said outer hollow annular body made of an elastically deformable metal alloy, said outer hollow annular body comprising ends of a sectional element joined together, wherein said elastically deformable metal alloy has a superelastic behaviour with respect to a deformation of bending necessary for joining ends of said sectional element, and wherein said elastically deformable metal alloy being elastically deformable in response to a squeezing of the outer annular body between the engine block and the cylinder head of the engine, within the seat that receives said ring, to cause a deformation of said cross section of said outer hollow annular body; and

an internal hollow annular body having a closed cross-section, said internal hollow annular body set within said outer hollow annular body and made of a shape-memory metal alloy which, following a passage of its temperature beyond a transition temperature, passes from a martensitic phase to an austenitic phase,

wherein, in an undeformed condition, said ring has an axial dimension larger than an axial dimension of said seat defined between the engine block and the cylinder head, so that mounting the cylinder head on the engine block causes a compression in an axial direction of the cross-section of said sealing ring, which generates an elastic reaction principally due to the superelastic material of the outer hollow annular body,

and wherein shape-memory metal alloy of the internal hollow annular body is chosen so as to have a transition temperature higher than the ambient temperature, so that when the temperature of said internal hollow annular body rises above said transition temperature due to engine operation, the cross-section of the internal hollow annular body expands in the axial direction due to the shape memory effect of said shape memory alloy to cause a further reacting force in addition to the elastic reaction of said outer hollow annular body, which ensures an adequate contact pressure between the sealing ring and the cooperating surfaces of the cylinder head and the engine block even in the presence of vibrations and stresses to which the engine is subject during its operation.

2. The sealing ring according to claim 1 , wherein the external and internal hollow annular bodies have a hollow cross section with a circular profile.