Internal combustion engines

An internal combustion engine comprising:

• at least one pair of opposed, reciprocating pistons forming a combustion chamber therebetween, the at least one pair of opposed pistons comprising an outer piston furthest from the crankshaft and an inner piston;
• and
• a crankshaft driven by the pistons via respective drive linkages;
• wherein the outer piston comprises a skirt extending from its perimeter towards the crankshaft to form a cylinder within which the inner piston reciprocates;
• the skirt of the outer piston comprises one or more inlet ports and one or more exhaust ports and functions as a sleeve valve to enable ingress and exhaust of gases from the combustion chamber as the pistons reciprocate;
• and
• the outer piston is driven to rotate about its central axis as it reciprocates towards and away from the inner piston.