Fuel System
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 the tank.
Fuel Tank
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 cleaned.
Fuel Filter
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 fuel tanks.
Fuel Pump
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.
Mechanical Fuel Pump
The mechanical fuel pump differs in that it has a vacuum booster section.
The vacuum section is operated by the fuel pump arm; otherwise,
it has nothing to do with the fuel system. During the suction (or first)
stroke, the rotation of the eccentric on the camshaft puts the pump operating
arm into motion, pulling the lever and diaphragm down against the pressure
of the diaphragm spring and producing suction (vacuum) in the pump chamber.
The suction will hold the outlet valve closed and pull the inlet valve
open, causing fuel to flow through the filter screen and down through the
inlet valve of the pump chamber.
During the return stroke, the diaphragm is forced up by the diaphragm
spring, the inlet valve closes and the outlet valve opens to allow fuel
to flow through the outlet to the carburetor. The operating
lever is hinged to the pump arm, so that it can move down but cannot be
raised by the pump arm. The pump arm spring forces the arm to follow the
cam without moving the lever. The lever can only be moved upward by the
diaphragm spring. This process causes fuel to be delivered to the carburetor
only when the fuel pressure in the outlet is less than the pressure maintained
by the diaphragm spring. This happens when the passage of fuel from the
pump into the carburetor float chamber is open and the float needle is
not seated.
Electrical Fuel Pump
Electric fuel pumps have been used for many years on trucks,
buses and heavy equipment, and they have also been used as replacements
for mechanically operated fuel pumps on automobiles, but only recently
have they become part of a car's original equipment. The replacement types
usually use a diaphragm arrangement like the mechanical pumps, except that
it is actuated by an electrical solenoid.
The electrically driven turbine type of pump, first used on the Buick
Riviera, was a great departure from the usual fuel pump design.
It uses a small turbine wheel driven by a constant speed electric motor.
The entire unit is located in the fuel tank and submerged in the fuel itself.
This pump operates continuously when the engine is running. It keeps up
a constant pressure which is capable of supplying the maximum fuel demands
of the engine. When less fuel is required, the pump does not deliver at
full potential, because the turbine is not a positive displacement type
like the mechanical pump. Consequently, the turbine will run without pumping
fuel and so, needs no means of varying fuel delivery rate like its mechanical
counterpart. Since the fuel can flow past the spinning turbine blades,
there is no need for pump inlet and outlet valves nor is there any need
to vary its speed.
A relay for the electric fuel pump is used to complete the circuit to the
fuel pump. This cuts off current to the fuel pump in the event of an accident.
Vacuum Pump
Several fuel pumps have a vacuum booster section that operates the windshield
wipers at an almost constant speed. The fuel section then functions in
the same way as ordinary fuel pumps. One difference is that the rotation
of the camshaft eccentric in the vacuum pump also operates the vacuum
booster section by actuating the pump arm, which pushes a link and the
bellows diaphragm assembly upward, expelling air in the upper chamber
through its exhaust valve out into the intake manifold. On the return stroke
of the pump arm, the diaphragm spring moves the bellows diaphragm down,
producing a suction in the vacuum chamber. The suction opens the intake
valve of the vacuum section and draws air through the inlet pipe from the
windshield wipers.
When the wipers are not operating, the intake manifold suction (vacuum)
holds the diaphragm up against the diaphragm spring pressure so that
the diaphragm does not function with every stroke of the pump arm.
When the vacuum is greater than the suction produced by the pump, the air
flows from the windshield wiper through the inlet valve and vacuum chamber
of the pump and out the exhaust valve outlet to the manifold, leaving the
vacuum section inoperative. With high suction in the intake manifold, the
operation of the wiper will be the same as if the pump were not installed.
When the suction is low, as when the engine is accelerated or operating
at high speed, the suction of the pump is greater than that in the manifold
and the vacuum section operates the wipers at a constant speed. Some pumps
have the vacuum section located in the bottom of the pump instead of in
the top, but the operation is basically the same.
Fuel Gauges
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
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.
Breathers
The breather is the positive crankcase ventilation system directing atmospheric
pressure to the crankcase. The atmospheric pressure then pushes the blowby
gases to a low pressure area. The air that is directed into the crankcase
must first be filtered; if it is not, the dust and sand particles will
destroy the engine parts. When there is too much blowby,
it is routed back through the crankcase breather element. It then enters
the carburetor or throttle body with the incoming fresh air to be burned
in the cylinders. In addition, the breather helps to keep the regular air
filter cleaner for a longer period of time, since blowby contains oil vapor
from the crankcase.
Throttle Valve
All gasoline engines have a throttle valve to control
the volume of intake air. The amount of fuel and air that goes into the
combustion chamber regulates the engine speed and, therefore, engine power.
The throttle valve is linked to the accelerator (gas pedal). The throttle
valve is a butterfly valve that usually consists of a disc mounted on a
spindle. The disc is roughly circular, and it has the same diameter as
the main air passage in the throat or "venturi". In a carburetor, the throttle
valve is usually located at the bottom of the carburetor, between the jet
nozzle and the intake manifold. The throttle spindle is connected to the
accelerator in such a manner that when the pedal is depressed, the valve
opens. When the pedal is released, the valve closes. Fuel injected engines
use throttle valves to regulate engine power, even though the fuel is also
regulated through the injectors.
