What is a carburettor? PH Explains

Need to mix some fuel and air without any electrical assistance? Then the classic carburettor could be just the ticket.

For an engine to function properly it must be supplied with a regulated mixture of fuel and air. Prior to the advent of affordable and accurate electronic fuel injection systems, manufacturers would typically rely on a device called a carburettor to fulfil this requirement.

The popularity of carburettors was due in part to their straightforward nature, with most consisting effectively of just five key parts - the main body, a float chamber, a fuel discharge nozzle, a 'venturi' and a throttle system.

When you crank a carburettor-equipped engine, the pressure drop created when a piston travels down the cylinder on the intake stroke causes air to be drawn into the carburettor's 'barrel' - an opening that runs through its body. As this air travels into the carburettor's barrel it must pass through the venturi, which contains the discharge nozzle.

For the air to get through the venturi, which is smaller than the primary barrel or 'bore' of the carburettor, it has to speed up. This causes its pressure to fall, to below that of atmospheric levels. Fuel, at atmospheric pressure, is consequently drawn out of the discharge nozzle; this then mixes with the air in the venturi, before being fed into the engine.

The throttle, typically a butterfly valve behind the venturi, controls the engine speed by regulating the flow of air into the engine. With the throttle almost closed, little air - and thus little fuel - is drawn into the engine, and vice versa.

Maintaining the correct fuel pressure at the discharge nozzle is achieved by the use of a float chamber, which functions like the cistern on a toilet. It contains a float, typically brass or plastic, which rises as the chamber fills. When it reaches a level that will result in the correct pressure as the discharge nozzle, it cuts off the fuel supply - and opens it again when the level falls. If the pressure were too high, fuel could be forced into the venturi, upsetting the mixture.

More complicated carburettors will feature multiple barrels, discharge nozzles and other fuel supply ports, regulated by fuel-metering 'circuits' - which are passageways that supply fuel from the float chamber to the discharge points. Typically, they account for different engine speed ranges and can be adjusted, often by the use of screw-in fittings called 'jets'. Some carburettors also have 'accelerator pumps', which deliver a shot of fuel when demand suddenly increases, maintaining smooth operation.

Carburettors are also often equipped with a choke, which usually takes the form of a flap or a butterfly. When in use it restricts the flow of air through the carburettor, increasing the vacuum on the engine side of the venturi and resulting in more fuel being drawn out. This is used to help get the engine started when cold, when myriad factors - such as the temperature being so low as to stop most of the fuel vaporising - are working against it.

A properly maintained carburettor can prove a relatively accurate and straightforward method of delivering fuel - and you'll still find them in applications where simplicity and reliability are key, ranging from garden equipment to light aircraft. Otherwise, in most modern applications, they have been superseded by more accurate, self-adjusting electronic fuel injection systems.

Lewis Kingston

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