A 4 stroke petrol engine cylinder head (16) has two inlet pipes (19 and 22) which contain alternate acting inlet valves (14 and 15). Fuel/air induction strokes through valve (14) alternate with air only induction strokes through valve (15) giving an 8 stroke cycle under half load. When gate valve (23) is moved by a control to the dotted position, this seals the air only inlet and opens port (25) to give fuel/air induction on all induction strokes over half load and a 4 stroke cycle. Air only induction strokes or 8 stroke cycles under half load may also be achieved by cutting out some fuel injectors or injection strokes as the load permits.
