An internal combustion engine comprising at least one cylinder and a pair of opposed, reciprocating pistons within the cylinder forming a combustion chamber therebetween. The engine has at least one combustion igniter associated with the cylinder, a portion of the combustion igniter being exposed within the combustion chamber formed between the opposed pistons.

An optical measurement device is disclosed and includes a body configured to be mounted to at least one piston cylinder, and a compression ratio determination unit operatively connected to the body to receive optical measurement data and compute a clearance between at least two opposed pistons disposed within the at least one piston cylinder.

An optical measurement device is disclosed and includes a body configured to be mounted to at least one piston cylinder, and a compression ratio determination unit operatively connected to the body to receive optical measurement data and compute a clearance between at least two opposed pistons disposed within the at least one piston cylinder.

An internal combustion engine comprising at least one pair of opposed, reciprocating pistons forming a combustion chamber therebetween a crankshaft driven by the pistons via respective drive linkages. The outer piston furthest from the crankshaft comprises a skirt extending from its perimeter towards the crankshaft to form a cylinder within which the other, inner piston reciprocates.

An internal combustion engine comprising at least one pair of opposed, reciprocating pistons forming a combustion chamber therebetween a crankshaft driven by the pistons via respective drive linkages. The outer piston furthest from the crankshaft comprises a skirt extending from its perimeter towards the crankshaft to form a cylinder within which the other, inner piston reciprocates.