Counter Pro
Course - Carburetor/Fuel Pump Training 5
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Why internal combustion engines need carburetors: |
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An automobile or other gasoline-powered engine needs both gasoline and air (oxygen,
actually) in order to run. If the engine doesn't get both fuel and oxygen at the right time
and in the right quantities, the spark plug will have nothing to ignite and the engine will not
run. One of the major problems car designers have had is getting the correct amount of fuel
and oxygen into the engine.
Gasoline in its purely liquid form doesn't burn. Before it can burn, it must be vaporized;
that is, it must be mixed with the atmosphere which contains oxygen.
Gasoline vapor which has too little air or too much air cannot burn. In fact, gasoline vapor
requires rather specific amounts of air and gasoline in order to burn. Scientists and engine
designers measure these quantities in terms of pounds of air and fuel required. They found
it takes about 8 to 18 pounds of air to each pound of gasoline to create and sustain a flame.
Since air is so much lighter than gasoline, it takes enormous quantities of air to mere drops
of fuel to create a burnable ratio of fuel and air. This ratio offers a rather wide variation.
However, at either end of this spectrum, the air and fuel will burn badly or not at all.
With too much air, towards 18 pounds on the spectrum (lean mixture), or too little air,
towards 8 pounds on the spectrum (rich mixture), there tends to be incomplete combustion.
After any burning is complete, unburned gasoline and other by-products will remain. In
the old days, these by-products were called "soot" or "residue." Today, this unwanted
automotive exhaust is called "pollution."
There is, however, a ratio of air and fuel which results in almost "perfect burning." It is
called the "stoichiometric ratio," which is 14.7 pounds of air to one pound of gasoline.
If you could put exactly 14.7 pounds of air for each pound of gasoline into the engine at all
times, you would have complete and perfect burning, maximum gas mileage and minimal
pollutants entering the atmosphere.
Experimental engineers can achieve this perfect 14.7/1 ratio by holding an engine at a
steady speed (to create a condition where a steady volume and weight of air is entering the
engine) and then carefully adjusting the volume of fuel that gets into the entering airstream.
This "steady state perfection" is relatively easy to achieve. Unfortunately, however, the
typical vehicle engine doesn't operate at a single speed; instead, it operates like a moving
target.
The engine speed varies from cranking speed all the way up to 6,000 or so revolutions per
minute. Because of this, the weight of air entering the engine varies at all times. As a
result, there is a need for a varying amount of fuel weight at all times as well.
This variance is further complicated by the quality of the air and fuel mixture entering the
engine. The gasoline cannot enter the airsteam as a big "squirt." Instead, it must be
vaporized, or turned into a very fine mist of droplets which can be mixed almost completely
with the incoming air. This vaporizing enables the spreading of the droplets to be
maximized throughout the air/fuel charge.
This is a very complex and difficult task. Maintaining an exact 14.7/1 ratio under all speed
and operating conditions which a car is asked to perform is nearly an impossible job. The
device that has been doing this job for over eighty years (from the late 1800's to the late
1970's anyway) is the carburetor.
A carburetor mixes a vapor of fuel and air for burning inside an engine. It uses air pressure
to control the amount of mixing which occurs.
Don't mistake the purpose of the fuel pump in this system. The fuel pump (which will be
discussed later in greater detail) is merely a device that moves fuel from the tank to a
reservoir in the carburetor. This reservoir is called a "float bowl." The fuel pump does not
force fuel into the airstream on a carburetor--unless a fault causes it to do so.
To see how a carburetor works, let's look at a very simple carburetor, similar to the type
used on small, constant-speed engines such as lawn mowers.
| A Basic Principle. |
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To understand how a carburetor works, it is first necessary to understand some simple
facts about air and pressure. The actual pressure of any static, unmoving air at ground
level is roughly 15 pounds per square inch. This represents the weight of the column of
air, one inch square, going straight up to the stratosphere. This static weight of air is
everywhere.
When air moves, the pressure decreases. The faster it moves, the more the pressure
decreases. This is why the fast-moving air on the top of an airplane's wing -- a low
pressure area -- allows the higher pressure air underneath it to lift the airplane off the
ground.
This is why "spoilers" are used on the rear ends of some cars. By slowing down the air,
the spoiler increases the air pressure and puts greater weight on the rear wheels.
Static air = static 15 psi pressure, everywhere
Fast moving air = lower pressure
| Venturi action. |
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It is the difference between static air pressure and moving air pressure that creates the
pressure differential required to move air and fuel into the engine at the right mixture levels.
Figure 2 shows our simple carburetor device. The motion of the piston in the cylinder
sucks a column of air into the engine. Because the air is moving, it is at a slightly lower
pressure than static atmospheric pressure.
We cause this pressure to drop even further by putting a device called a "venturi" within the
moving airstream. It squeezes and accelerates the air so it drops more significantly in
pressure. Thus, the faster the piston moves, the faster the air column moves, and the more
the pressure drops.
In our example carburetor and virtually every other carburetor used on engines, there will
be two venturies, as shown.
| Fuel wells.. |
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From a fuel "well" where the fuel is stored, the fuel travels through a fuel delivery tube to
the venturi. Static air pressure acts on the fuel as it sits in the fuel well. Above the well is
an air bleed, which is also exposed to static atmospheric air pressure.
As air rushes through the venturi, the pressure inside the venturi is less than static
atmospheric pressure. Pressure at the fuel well and air bleed is higher. As a result, the
static atmospheric pressure pushes the fuel into the fuel delivery tube, and pushes air
through the air bleed into the moving fuel in the tube. The air "bubbles" the fuel in the
tube, causing it to turn into an "emulsion" of air and fuel. When this emulsion spills into
the venturi, it is swirled and mixed with the fast-moving air to become a vapor.
The use of air bleeds (or "air bleed jets" as they are sometimes called) is absolutely essential
to this process. Without air bleeds, the fuel would be too "squirtlike" and difficult to
vaporize. Plugged air bleeds are a major cause of carburetor performance problems.
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