PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 1/TASK 2
d. A Basic Carburetor. The ideal state for the fuel to be in when it
reaches the cylinder is to be vaporized completely. Good intake manifold
design will help to vaporize the fuel, but the carburetor must properly
atomize the fuel beforehand. Atomization of the fuel occurs as it is drawn
into the venturi. As the fuel comes out of the discharge nozzle, it is
broken into tiny droplets which enter the airflow. To ensure that there is
a high degree of atomization, a tiny hole called an air bleed is used to
allow air to mix with the fuel in the discharge tube. The fuel is then
further atomized as it enters the venturi. To ensure proper fuel flow, a
secondary venturi or a venturi booster may be used. It will further
decrease the pressure at the discharge nozzle.
e. AirFuel Ratio. The proportions of an airfuel mixture are
expressed in terms of the airfuel ratio. It is the relationship by weight
of the mixture. An example of how this is expressed would be:
AirFuel Ratio = 12:1.
In this airfuel mixture, the air would be 12 times as heavy as the fuel.
The operational range of airfuel ratios in the average gasoline engine are
from approximately 9:1 to approximately 17:1. Airfuel ratios on the lower
end (less air) are considered to be rich mixtures; the airfuel ratios at
the higher end (more air) are considered to be lean mixtures. A gasoline
engine, propelling a vehicle at a steady speed, operates on an airfuel
ratio of approximately 15:1. Considering that gasoline weighs approximately
640 times as much as air, it can be seen that a gasoline engine consumes a
tremendous amount of air. If, in fact, the airfuel ratio was considered by
volume rather than weight, it would be seen that a gasoline engine operating
on an airfuel ratio of 15:1 consumes approximately 9600 gallons of air for
every gallon of gasoline.
Construction of the Basic Carburetor
a. Throttle Valve (figure 11 on the following page). The throttle
valve is used to regulate the speed and power output of the engine. It is
controlled by the accelerator pedal, and usually consists of a flat, round
plate that tilts with the throttle shaft. As the accelerator pedal is fully