Pressing a carburetted car's pedal lets in air, and the air fetches the fuel
In an old carburetted car, flooring the accelerator does not squirt in more petrol directly. It opens a valve that lets more air rush through, and faster air has lower pressure, which sucks extra fuel from a reservoir into the stream. The whole device is essentially Bernoulli's principle bolted to an engine.
Air enters through a filter, speeds up through a narrowed throat called a venturi, and there picks up fuel from small tubes fed by the main jet. The fuel waits in a float bowl vented to outside air pressure, kept at a steady level by a floating valve much like the one in a toilet cistern. Because the pressure drop in the airstream and the resulting fuel flow scale together, the mixture stays roughly balanced across engine speeds. Below the throat sits a butterfly throttle linked to the pedal.
Real engines need corrections, so carburettors grew extra circuits. At idle, when air is too slow to lift fuel through the main jet, a separate idle circuit feeds fuel near the nearly closed throttle plate, and an off-idle jet smooths the handover. Because fuel is heavier than air and lags behind when the throttle snaps open, an accelerator pump squirts in a quick extra dose. Under heavy load, a spring-loaded power valve held shut by engine vacuum opens as vacuum falls, enriching the mixture to boost power and reduce knocking.
Cold starts were the classic headache. Chilly fuel vaporises poorly and condenses on the intake walls, so a choke valve partly closes the air inlet, raising vacuum to pull in more fuel. Until the late 1950s drivers worked it with a dashboard knob; automatic chokes then took over, using bimetallic thermostats, electric heaters or exhaust-warmed air. A choke left shut after warm-up wastes fuel and makes the engine run rough. Hot weather caused the opposite trouble, vapour lock, when fuel boiled into bubbles in the bowl.
The word comes from carburet, to charge with hydrocarbons. Since the 1990s fuel injection has largely replaced carburettors in cars and trucks, but they survive in lawnmowers, generators, motorcycles, small outboard motors and many piston-engined aircraft. Diesel engines never used them, since they inject fuel late in the compression stroke instead.
Source: Carburetor
