1.
FUNCTIONS OF THE CARBURETTOR
The main carburettor
functions are:
- To form a proper homogeneous inflammable mixture of fuel and air
- To supply the engine with varying amounts of this mixture
The fuel-air mixture is formed through vapourising and by uniformly spraying
fuel into the airstream or at least by atomising it into very small droplets.
Atomisation takes place in this way: liquid fuel from the atomiser nozzle
meets the flow of air which carries it, broken into very fine droplets, to
the combustion chamber.
We have spoken of a "proper" mixture because the mixture strength, defined
as the amount of air in weight mixed with a fuel unit of weight, must have
a precise value,ie it must be within the limits of inflammability so that
the mixture can be easily ignited by the spark in the combustion chamber.
lnflammmability limits for commercial petrol are: 7:1 (rich limit ie. 7 kgs
of air and 1 kg of petrol), down to 20:1 (lean limit ie. 20 kgs of air and
1 kg of petrol).
To obtain optimum combustion between these inflammability limits, a value
very close to the so-called stoiciometric value is needed ie. about 14.5 -
15.0 kgs of air to 1 kg of petrol.
A stoiciometric mixture ratio is one which ensures complete combustion of
fuel with only the formation of water and carbon dioxide.
The stoiciometric mixture ratio depends on the kind of fuel used, so if the
fuel is changed, this fuel-air ratio will also change (see SECTION
5.1).
The selection of the fuel-air ratio is therefore very important both for engine
performance and for exhaust emission levels.
The throttle valve (usually a flat or piston-type gate valve, also called
a slide) is the main part by which the engine is tuned ie. the engine power
output is varied by controlling the amount of mixture being drawn into the
cylinder.
During bench tests,the engine is usually run in top gear in two characteristic
conditions: full throttle and part throttle.
The full throttle test simulates conditions for a vehicle on a progressive
climb with the throttle wide open.
In the bench test, this condition is reproduced by running the engine with
the throttle fully open; from this maximum horsepower condition, the engine
is braked at various speeds and the specific power and consumption figures
are taken.
The part throttle test simulates the conditions for vehicle on a level road
at varying speeds.
On the test bench, this condition is simulated by running the engine again
from the maximum engine power conditions, but progressively closing the throttle
valve of the carburettor.
At various speeds, specific power and consumption figures are taken again.