An internal combustion engine comprising at least one cylinder, each cylinder comprising: a piston; a cylinder head comprising an inlet duct incorporating an inlet valve to allow flow through said inlet duct in only one direction; and an exhaust duct; a single chamber valve through which both inlet and exhaust fluids pass from their respective ducts (3,4) into and out of the cylinder; and a combustion chamber in the cylinder that allows the chamber valve to be significantly open when the piston is at top dead centre enabling a transfer of fluid, fluid flow pressure and pressure waves between the cylinder, and the inlet and exhaust ducts; characterised by: a chamber being a hollow interior volume that extends from the chamber valve seat in a direction away from the combustion chamber, the chamber permanently connecting the inlet and exhaust ducts when the chamber valve is seated in the chamber valve seat and when the chamber valve is unseated in the chamber valve seat; and by the exhaust duct being tuned, in operation, to: accelerate the inlet fluid when the piston is near the top of its stroke; create negative pressure in the cylinder to aid cylinder filling with inlet fluid through at least part of the induction stroke by harnessing the momentum of the exhaust fluid in the exhaust duct; and produce reflected positive pressure waves in the exhaust fluid to force fluid drawn from the inlet duct into the exhaust duct back into the cylinder. a piston; a cylinder head comprising an inlet duct incorporating an inlet valve to allow flow through said inlet duct in only one direction; and an exhaust duct; a single chamber valve through which both inlet and exhaust fluids pass from their respective ducts (3,4) into and out of the cylinder; and a combustion chamber in the cylinder that allows the chamber valve to be significantly open when the piston is at top dead centre enabling a transfer of fluid, fluid flow pressure and pressure waves between the cylinder, and the inlet and exhaust ducts; characterised by: a chamber being a hollow interior volume that extends from the chamber valve seat in a direction away from the combustion chamber, the chamber permanently connecting the inlet and exhaust ducts when the chamber valve is seated in the chamber valve seat and when the chamber valve is unseated in the chamber valve seat; and by the exhaust duct being tuned, in operation, to: accelerate the inlet fluid when the piston is near the top of its stroke; create negative pressure in the cylinder to aid cylinder filling with inlet fluid through at least part of the induction stroke by harnessing the momentum of the exhaust fluid in the exhaust duct; and produce reflected positive pressure waves in the exhaust fluid to force fluid drawn from the inlet duct into the exhaust duct back into the cylinder. accelerate the inlet fluid when the piston is near the top of its stroke; create negative pressure in the cylinder to aid cylinder filling with inlet fluid through at least part of the induction stroke by harnessing the momentum of the exhaust fluid in the exhaust duct; and produce reflected positive pressure waves in the exhaust fluid to force fluid drawn from the inlet duct into the exhaust duct back into the cylinder.