• petrol consists of these constituents of crude petroleum which have boiling points between the
temperatures of, roughly, 30 and 200 °C.
• For an average sample of petrol the calorific value is about 44 MJ/kg.
• the air/fuel ratio for complete combustion - called the chemically correct mixture - is about 15 in the case of
petrol.
• the heat absorbed during vaporization have a beneficial cooling effect on the inside of the combustion
chamber. best results will be obtained if the fuel can be introduced into the cylinder in liquid form provided
it can all be vaporized before combustion.
• vapor is given off from the surface of a liquid, the rate of evaporation can be increased by breaking up the
liquid into a fine spray of minute droplets.
• Atomization: Generally, the time is too short to vaporize the fuel completely before combustion. To break up
the fuel mechanically into small particles is called atomization, either by subjecting fuel to a turbulent air
flow, or by pumping the fuel into the air stream through small holes in an injector. This action not only aids
the production of a fast-burning mixture but also helps to mix the air and fuel equally, i.e., it aims to produce
a homogeneous mixture
• Carburation systems bring together the petrol and air
through a device called carburetor.
• This device mixes the air and petrol in suitable
proportions, it also atomizes the petrol to give good
combustion.
• Nowadays higher performance engines use a fuel
injection system instead of a carburetor.
• A good injection system returns a higher performance
from the engine, i.e., the engine will: develop more power
to give a higher speed and to accelerate at a greater rate,
be more economical in its use of fuel.
• Figure
shows, in
simple
form, two
systems of
carburation
in use
today.
Simple Carburettor
• A carburetor consists essentially of two parts: a float
chamber for regulating the entry of fuel to the carburetor
according to the rate at which the fuel is used; the mixing
chamber for atomizing the fuel and mixing it with the
necessary amount of air. These two parts are not necessarily
separate from one another.
The float chamber
• Figure illustrates the way the level in the chamber is kept
constant.
• The top of the chamber is closed by a lid in which there is a
small hole or air vent to maintain atmospheric pressure on
the liquid in the chamber.
• The lid also has a connection for the pipe from the fuel pump.
• Inside the chamber is a float of plastic. Needle attached to
its top blocks the hole of the inlet pipe when maximum fuel
level is achieved.
• When the chamber is
empty the float lies on
the bottom of the
chamber and the fuel
inlet is open.
• Operation of the fuel
pump delivers fuel to
the chamber
• When max level is
achieved, fuel entry is
cut off until some fuel is
drawn away through the
jet.
• The height of fuel in the
chamber is arranged to
be slightly below the top
of the discharge nozzle.
The mixing chamber
• the region is centered around a venturi.
• a venturi increases the air speed and forms a depression.
• the mixing chamber is situated in the induction pipe that
supplies the engine with air.
• Fitted at the throat of the venturi, also known as the
choke tube, is an outlet from a pipe or drilling that
conveys petrol from the float chamber.
• The combination of the depression created by the venturi
and the petrol supply from the float chamber allows the
petrol to mix with the air.
• The depression intensifies as the quantity of air passing
through the induction pipe increases, the amount of
petrol supplied to mix with this air also increases.
• Figure: A simple carburetor
• the throttle valve regulates the
engine power by controlling the
air flow to the engine.
• When the fuel supply is turned
on, fuel level rises in both the
float chamber and also the fuel
discharge nozzle. When the
needle goes into the needle
seating, the entry of fuel is
cutoff that time the level is just
below the top of the discharge
nozzle.
• If the engine is rotated, air is
drawn in through the air
intake and choke tube,
producing a pressure drop or
depression inside the choke
tube.
• This depression draws fuel
from the float chamber via
the passage and jet into the
mixing chamber.
• The rush of air through the choke tube will,
atomize the fuel as it issues from the
discharge nozzle.
• The mixture of air and atomized fuel will be
drawn into the engine at a rate which
depends upon engine speed and the
extent of opening of the throttle valve.
• The driver controls the throttle by the
accelerator pedal.
• As the rate of air flow increases, because
of either an increase in engine speed or a
wider throttle opening or both, the
depression in the choke tube also
increases, thus drawing more fuel from the
jet.
• The size of the
choke tube is
selected so that
the air velocity
through it is
sufficient to
atomize the fuel at
the lowest speed
at which the engine
is required to run.
• The desired
mixture can be
obtained by using a
size of jet which
allows the correct
amount of fuel to
flow and join the
air stream
