Abstract: An internal combustion engine (10) has at least one combustion chamber (15) defined by a piston (13) accommodated in a cylinder (11). The method of operating the engine (10) comprises opening an exhaust means (30) as the combustion chamber (15) expands to permit fluid to discharge from the combustion chamber (15), opening an inlet means (20) as the combustion chamber continues to expand to admit the intake air into the combustion chamber (15); closing the exhaust means (30) as the combustion chamber still continues to expand to interrupt discharge of fluid from the combustion chamber (15), and closing the inlet means (20) to interrupt admission of the intake air into the combustion chamber (15). With this operating sequence, scavenging of the combustion chamber (15) is incomplete and so there is a relatively large residual fluid within the combustion chamber.