A method of modifying an internal combustion engine, the internal combustion engine comprising: a cylinder; a piston housed within the cylinder and being movable between a top dead centre position, TDC, and a bottom dead centre position, BDC, a combustion chamber of the cylinder having a volume defined by a head of the piston when the piston is in the top dead centre position, TDC; and apparatus for controlling the opening and closing of an inlet-outlet portal of the cylinder, wherein the inlet-outlet portal is configured when open, to permit air or an air-fuel mixture to enter or enter and exit the cylinder and, when closed, to prevent the air or air-fuel mixture from entering or exiting the cylinder; the method comprising the steps of: modifying the opening and closing of the inlet-outlet portal such that the volume of the air or air-fuel mixture located inside the cylinder when the inlet-outlet portal closes is less than the volume of the cylinder defined when the piston is at the bottom dead centre, BDC, position inside the cylinder when the inlet-outlet portal is closed such that the expansion ratio of the engine is increased; and, fixedly attaching an incompressible member (21, 23) inside the combustion chamber to reduce the volume of the combustion chamber by an amount proportional to the volume of air-fuel mixture when the inlet-outlet portal closes. a cylinder; a piston housed within the cylinder and being movable between a top dead centre position, TDC, and a bottom dead centre position, BDC, a combustion chamber of the cylinder having a volume defined by a head of the piston when the piston is in the top dead centre position, TDC; and apparatus for controlling the opening and closing of an inlet-outlet portal of the cylinder, wherein the inlet-outlet portal is configured when open, to permit air or an air-fuel mixture to enter or enter and exit the cylinder and, when closed, to prevent the air or air-fuel mixture from entering or exiting the cylinder; the method comprising the steps of: modifying the opening and closing of the inlet-outlet portal such that the volume of the air or air-fuel mixture located inside the cylinder when the inlet-outlet portal closes is less than the volume of the cylinder defined when the piston is at the bottom dead centre, BDC, position inside the cylinder when the inlet-outlet portal is closed such that the expansion ratio of the engine is increased; and, fixedly attaching an incompressible member (21, 23) inside the combustion chamber to reduce the volume of the combustion chamber by an amount proportional to the volume of air-fuel mixture when the inlet-outlet portal closes. modifying the opening and closing of the inlet-outlet portal such that the volume of the air or air-fuel mixture located inside the cylinder when the inlet-outlet portal closes is less than the volume of the cylinder defined when the piston is at the bottom dead centre, BDC, position inside the cylinder when the inlet-outlet portal is closed such that the expansion ratio of the engine is increased; and, fixedly attaching an incompressible member (21, 23) inside the combustion chamber to reduce the volume of the combustion chamber by an amount proportional to the volume of air-fuel mixture when the inlet-outlet portal closes.