A method for fuelling an internal combustion engine, and a fuel system for delivering a variety of fuel types to the engine, configured to accommodate liquid fuels such as gasoline, ethanol or a blend thereof, and gaseous fuels such as CNG, LNG or LPG. The engine is configured to operate on any of the designated liquid fuels, and can switch between the liquid and gaseous fuels. The fuel system includes a common delivery arrangement, for selectively delivering fuel into the combustion chamber of each cylinder of the engine, comprising a fluid delivery injector and a liquid metering device configured for operation in concert. The same common fluid delivery injector is used for delivery of gaseous fuel only, delivery of liquid fuel only (by way of an air assist delivery process), or delivery of a fuel mixture comprising the gaseous fuel and the liquid fuel.