Full expansion internal combustion engine with co-annular pistons

A cylinder for a four-stroke internal combustion (IC) engine having co-annular dual pistons, the cylinder including:

• i) a main cylinder including a main cylinder wall and a cylinder head, and having at least one exhaust port, an exhaust valve disposed in the exhaust port, and at least one inlet air port, and an inlet air valve disposed in the inlet air port;
• ii) an outer piston having an annular crown, an inner cylinder wall defining an inner cylinder, and an annular outer sidewall extending from the periphery of the crown, the outer piston configured for reciprocating movement within the main cylinder;
• iii) an inner piston having a crown, and an annular inner sidewall extending from the periphery of the crown, the inner piston configured for reciprocating movement within the inner cylinder;
• and
• iv) a securement configured for securing the outer piston with the main cylinder through an intake stroke and a compression stroke, and for disengagement of the outer piston with the main cylinder through a combustion and power stroke and an exhaust stroke.