and air in the proportions and quantity the
needs at any time.
It does this by spraying the fuel into the moving air stream through a
, so that the fuel vaporises and makes an explosive mixture.
The faster an
runs, the more air it sucks in. The air passes through a narrowed neck inside the
), which speeds up its flow at that point.
As air flows faster its
drops, so there is a slight
inside the venturi. The fuel jet opens into the venturi, and the partial vacuum sucks fuel through the jet into the air stream.
The speed of the engine is controlled by a throttle, a movable circular flap linked to the
pedal which partly blocks the venturi to admit variable amounts of air.
There has to be some way of governing the fuel flow through the jet so that the mixture is right so that the fuel is in the right proportion to the air. The simplest way of doing this is with a variable jet.
The variable-jet carburettor
The fuel jet is partly blocked by a tapered needle, which can be raised progressively to unblock it.
The needle is fixed to a
, which is free to slide up and down in a chamber above the jet. The top of the chamber is linked to the inlet
through a narrow passage.
When the engine is idling, low manifold depression and a light spring causes the piston to sit at the bottom of the chamber, and the needle almost completely blocks the jet. Little fuel flows.
As the throttle opens, air flow to the engine increases. The engine speeds up and sucks in yet more air.
This suction creates a partial vacuum in the inlet manifold, and therefore also in the top of the chamber, which is connected to it.
The vacuum is more powerful than the slight vacuum in the venturi under the piston, so it draws the piston up, unblocking the jet and letting more fuel flow.
A sudden burst of acceleration causes a sudden rush of mixture into the inlet manifold, so that the vacuum there lessens for an instant.
That would allow the piston to fall, closing the jet and weakening the mixture; but the problem is avoided by having an oil-filled
attached to the piston, which prevents it from moving quickly. Therefore, the mixture does not suddenly become too weak for smooth
Starting from cold
An engine needs an extra-rich mixture more petrol, less air for starting from cold.
On some variable-jet carburettors, this is provided for by the jet being lowered a short way, so that it is less blocked by the needle and dispenses more fuel than usual.
On others, neat fuel is atomised by the rotation of a disc of progressively larger-sized holes.
resembles the simpler variable-jet type (See
How variable-jet carburettors work
) in having a
- a constricted neck - through which air flows on its way to the
The purpose of the
system is to generate a very high
age from the car's 12 volt
, and to send this to each sparkplug in turn, igniting the fuel-air mixture in the