The 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.

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!

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.

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.

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