Carburettor, a device attached to an internal combustion engine which uses a volatile fuel such as petrol. Before the fuel can be effectively burned in the engine cylinder it must be vaporised and mixed with the air supply in definite proportions: usually about 15 parts of air to 1 part of fuel by weight in the case of petrol. The carburettor must maintain this mixture strength over the full range of speed and load variation, and in addition a richer mixture must be supplied momentarily for sudden acceleration and for ease of starting.
The earlier carburettors consisted of cotton wicks saturated with fuel across which heated air was drawn. Modern carburettors have a restricted throat or choke through which the air is drawn, the depression created drawing a fine spray of petrol through a small orifice or jet. The great majority of carburettors are of the 'constant choke' type, in which the jet area is fixed while the depression varies. The only surviving 'constant vacuum' carburettor is the SU carburettor, in which the effective jet area is varied while the depression remains constant. At large depressions, there is a tendency for the mixture to be too rich and this is compensated for in the constant choke carburettor by an air bleeder arrangement which maintains a reasonably constant mixture strength. At the low depression resulting from idling of the engine, an additional idling jet maintains the correct mixture, while for starting a strangled flap, or 'choke', can be operated in order to increase the depression. In the carburettor shown in the figure, an accelerator pump is incorporated in order to provide the additional amount of fuel when the throttle is suddenly opened.
Additional refinements have been incorporated into recent designs to reduce the polluting properties of the engine exhaust gases. An alternative solution uses a fuel injection pump to meter fuel to each cylinder according to load and speed.