An apparatus for modifying an internal combustion engine, wherein the internal combustion engine is operable to control the volume of air or air and non-combusted fuel mixture inside a combustion chamber and cylinder of said internal combustion engine, the internal combustion engine comprising: (a) an inlet-outlet portal having open and closed states and connected to air or air and non-combusted fuel source(s); (b) a combustion chamber ; (c) a cylinder ; and (d) a piston housed within the cylinder ; characterised in that the apparatus for modifying the internal combustion engine comprises a cam configured to actuate the inlet-outlet portal of the engine to be controlled, when open, to permit air or air and non-combusted fuel mixture to enter or enter and exit the combustion chamber and cylinder and when closed to prevent air or air and non-combusted fuel mixture from entering or exiting the chamber and cylinder , in which the volume of air or air and non-combusted fuel mixture located inside the chamber and cylinder when the inlet-outlet portal closes, is less than the volume of the combustion chamber and cylinder defined when the piston is at the bottom dead centre, BDC, position inside the cylinder when the inlet-outlet portal is closed so that the compression ratio is reduced and is less than an expansion ratio thereof; and the apparatus for modifying the internal combustion engine further comprises a substantially incompressible member configured to be fixedly attached inside the combustion chamber of the engine to provide a reduction in volume of the combustion chamber ; wherein a volume of the incompressible member is selected so that the reduction in volume of the combustion chamber provided by the incompressible member is proportional to the reduction in volume between the volume of air or air and non-combusted fuel inside the cylinder when the piston is at the BDC position and the volume of air or air and non-combusted fuel inside the cylinder when the inlet-outlet portal closes. (a) an inlet-outlet portal having open and closed states and connected to air or air and non-combusted fuel source(s); (b) a combustion chamber ; (c) a cylinder ; and (d) a piston housed within the cylinder ; the apparatus for modifying the internal combustion engine comprises a cam configured to actuate the inlet-outlet portal of the engine to be controlled, when open, to permit air or air and non-combusted fuel mixture to enter or enter and exit the combustion chamber and cylinder and when closed to prevent air or air and non-combusted fuel mixture from entering or exiting the chamber and cylinder , in which the volume of air or air and non-combusted fuel mixture located inside the chamber and cylinder when the inlet-outlet portal closes, is less than the volume of the combustion chamber and cylinder defined when the piston is at the bottom dead centre, BDC, position inside the cylinder when the inlet-outlet portal is closed so that the compression ratio is reduced and is less than an expansion ratio thereof; and the apparatus for modifying the internal combustion engine further comprises a substantially incompressible member configured to be fixedly attached inside the combustion chamber of the engine to provide a reduction in volume of the combustion chamber ; wherein a volume of the incompressible member is selected so that the reduction in volume of the combustion chamber provided by the incompressible member is proportional to the reduction in volume between the volume of air or air and non-combusted fuel inside the cylinder when the piston is at the BDC position and the volume of air or air and non-combusted fuel inside the cylinder when the inlet-outlet portal closes. <EOS>