The purpose of the fuel system is to provide a mixture
of fuel and air to the engine of the car. The air-fuel mixture must
be in proportion to the speed and load placed on the engine. Major
parts of the system include: fuel tank and cap, emission controls,
fuel line, fuel pump, fuel filter, carburetor, and intake manifold
as well as the fuel gauge, which indicates the amount of fuel in
All modern fuel systems are fed through a pump, so the fuel tank
is usually at the rear of the chassis under the trunk compartment.
Some vehicles have a rear engine with the tank in the forward compartment.
The fuel tank stores the excess fuel until it is needed for operation
of the vehicle. The fuel tank has an inlet pipe and an outlet pipe.
The outlet pipe has a fitting for fuel line connection and may be
located in the top or in the side of the tank. The lower end is
about one-half inch above the bottom of the tank so that collected
sediment will not be flushed out into the carburetor. The bottom
of the tank contains a drain plug so that tank may be drained and
The gas tank of the early cars was placed higher than
the engine. The idea was that the gas would flow down to the engine.
This arrangement caused a problem when the car went uphill -- the
gas flowed away from the engine. Solution: drive up the hill backwards!
Clean fuel is important, because of the many small jets and passages
in the carburetor and openings in a fuel injector. To ensure this
cleanliness, fuel filters are installed in the fuel line. Fuel filters
can be located at any point between the fuel tank and the carburetor.
One may be in the tank itself, in the fuel pump or in the carburetor.
The most common placement is between the fuel tank and a mechanical
fuel pump. In this case, the fuel enters a glass bowl and passes
up through the filter screen and out through an outlet. Any water
or solid material which is trapped by the filter will fall to the
bottom of the glass bowl where it can be easily seen and removed.
Dirt particles usually come from scales of rust in the tank cars,
storage tanks or drums. Water comes from condensed moisture in the
The fuel pump has three functions: to deliver enough fuel to supply
the requirements of an engine under all operating conditions, to
maintain enough pressure in the line between the carburetor and
the pump to keep the fuel from boiling, and to prevent vapor lock.
Excessive pressure can hold the carburetor float needle off its
seat, causing high gasoline level in the float chamber. This will
result in high gasoline consumption. The pump generally delivers
a minimum of ten gallons of gasoline per hour at top engine speeds,
under an operating pressure of from about 2 1/2 to 7 pounds. Highest
pressure occurs at idling speed and the lowest at top speed. Although
fuel pumps all work to produce the same effect, there are various
types that may operate somewhat differently.
Cars are equipped with fuel gauges which are operated along with
the vehicle's electrical system. There are two types: the thermostatic
type and the balancing coil type. The thermostatic type is made
of a standing unit, located in the fuel tank, and the gauge itself
(registering unit), which is located on the instrument panel. The
fuel gauge used in some cars and trucks is of the electrically operated
balanced coil type. These have a dash unit and a tank unit. The
dash unit has two coils, spaced about 90 degrees apart, with an
armature and integral pointer at the intersections of the coil axis.
The dial has a scale in fractions between "Empty" and
"Full". The tank unit has a housing, which encloses a
rheostat, and a sliding brush which contacts the rheostat. The brush
is actuated by the float arm. The movement of the float arm is controlled
by the height of the fuel in the supply tank. The height of the
fuel (called variations in resistance) changes the value of the
dash unit coil so that the pointer indicates the amount of fuel
available. A calibrated friction brake is included in the tank unit
to prevent the wave motions of the fuel from fluctuating the pointer
on the dash unit. Current from the battery passes through the limiting
coil to the common connection between the two coils, which is the
lower terminal on the dash unit. The current is then offered two
paths, one through the operating coil of the dash unit and the other
over the wire to the tank unit. When the tank is low or empty, the
sliding brush cuts out all resistance in the tank unit. Most of
the current will pass through the tank unit circuit because of the
low resistance and only a small portion through the operating coil
to the dash unit. As a result, this coil is not magnetized enough
to move the dash unit pointer, which is then held at the "Empty"
position by the limiting coil.
If the tank is partly full or full, the float rises
on the surface of the fuel and moves the sliding brush over the
rheostat, putting resistance in the tank unit circuit. More current
will then pass through the operating coil to give a magnetic pull
on the pointer, which overcomes some of the pull of the limiting
coil. When the tank is full, the tank unit circuit contains the
maximum resistance to the flow of the current. The operating coil
will then receive its maximum current and exert pull of the pointer
to give a "Full" reading. As the tank empties, the operating
coil loses some of its magnetic pull and the limiting coil will
still have about the same pull so that the pointer is pulled toward
the lower reading. Variations in battery voltage will not cause
an error in the gauge reading because its operation only depends
on the difference in magnetic effect between the two coils.
Fuel lines, which connect all the units of the fuel system, are
usually made of rolled steel or, sometimes, of drawn copper. Steel
tubing, when used for fuel lines, is generally rust proofed by being
copper or zinc plated.
Fuel lines are placed as far away from exhaust
pipes, mufflers, and manifolds as possible, so that excessive heat
will not cause vapor lock. They are attached to the frame, the engine,
and other units in such a way that the effect of vibration is minimal,
and so that they are free of contact with sharp edges which might
cause wear. In areas where there is a lot of movement, as between
the car`s frame and rubber-mounted engine, short lengths of gasoline
resistant flexible tubing are used.
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