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.
Air Cleaners
Air cleaners are made to separate dust and other particles in the incoming air before it enters the carburetor. Thousands of cubic feet of air are drawn from within the car hood and passed through the engine cylinders, so it is important that the air is clean.
When driving on dirt or other dusty roads, dust particles are drawn through the radiator and find their way into the engine if it is not filtered and cleaned. Dust and other foreign materials in the engine will cause excessive wear and operating problems.
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Carburetor
The purpose of the carburetor is to supply and meter the mixture of fuel vapor and air in relation to the load and speed of the engine. Because of engine temperature, speed, and load, perfect carburetion is very hard to obtain.
The carburetor supplies a small amount of a very rich fuel mixture when the engine is cold and running at idle. With the throttle plate closed and air from the air cleaner limited by the closed choke plate, engine suction is amplified at the idle-circuit nozzle. This vacuum draws a thick spray of gasoline through the nozzle from the full float bowl, whose fuel line is closed by the float-supported needle valve. More fuel is provided when the gas pedal is depressed for acceleration. The pedal linkage opens the throttle plate and the choke plate to send air rushing through the barrel. The linkage also depresses the accelerator pump, providing added gasoline through the accelerator-circuit nozzle. As air passes through the narrow center of the barrel, called the "venturi", it produces suction that draws spray from the cruising-circuit nozzle. The float-bowl level drops and causes the float to tip and the needle valve to open the fuel line.
To cause a liquid to flow, there must be a high pressure area (which in this case is atmospheric pressure) and a low pressure area. Low pressure is less than atmospheric pressure. The average person refers to a low pressure area as a vacuum. Since the atmospheric pressure is already present, a low pressure area can be created by air or liquid flowing through a venturi. The downward motion of the piston also creates a low pressure area, so air and gasoline are drawn through the carburetor and into the engine by suction created as the piston moves down, creating a partial vacuum in the cylinder. Differences between low pressure within the cylinder and atmospheric pressure outside of the carburetor causes air and fuel to flow into the cylinder from the carburetor.
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Engine Fuel
Engine fuel is mainly made up of hydrogen and carbon, mixed so that it will burn with oxygen present, and will free its heat energy into mechanical energy. Liquid fuels are ideal for internal combustion engines, because they can be economically produced, have a high heat value per pound, an ideal rate of burning, and can be easily handled and stored. The most common engine fuels are gasoline, kerosene and Diesel fuel oil.
Gasoline has many advantages and is used to a greater extent than any other fuel in internal combustion engines having spark ignition. It has a better burning rate than other fuels, and, because it vaporizes easily, it gives quick starting in cold weather, smooth acceleration and maximum power.
Diesel fuel oil ranks next to gasoline in quantity used. It can be produced as cheaply as gasoline, but its use is limited to Diesel type engines. The use of kerosene as a fuel is usually limited to farm tractors, marine and stationary engines, all which operate at a fairly constant speed. Its traits are such that it cannot be properly mixed with air and controlled in variable speed engines.
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Fuel Additives
Tetraethyl lead was used in some gasolines to reduce or prevent knocking. However, in 1975, it became illegal to use leaded gasoline except in cars built prior to this time. Methyl Tertiary Butyl Ether (MTBE) is used in unleaded fuel to increase the octane. Gasoline exposed to heat and air oxidizes and leaves a gummy film. Detergents are now added to gasoline to prevent this. The detergents keep the carburetor passages and fuel injectors free from deposits, which could cause hard starting and problems in driving. Deposits also restrict the flow of fuel and cause a rough idle, hesitation of acceleration, surging, stalling, and lack of power.
Alcohol is frequently used as an additive to commercial gasoline, because it will absorb any condensed moisture which may collect in the fuel system. Water will not pass through the filters in the fuel line, so, when any water collects, it will prevent the free passage of fuel. It also tends to attack and corrode the zinc die castings of which many carburetors and fuel pumps are made. This corrosion will not only destroy parts, but also clog the system and prevent the flow of fuel. By using alcohol in gasoline, any water present will be absorbed and pass through the fuel filter and carburetor jets into the combustion chamber. Alcohol additives are often purchased and added separately into the gas tank to prevent gas-line freeze and vapor lock.
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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.
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Fuel Injection
The carburetor, despite all it advances: air bleeds, correction jets, acceleration pumps, emulsion tubes, choke mechanisms, etc., is still a compromise. The limitations of carburetor design is helping to push the industry toward fuel injection.
Direct fuel injection means that the fuel is sprayed directly into the combustion chamber. The fuel injected nozzle is located in the combustion chamber.
Throttle Body injection systems locate the injector(s) within the air intake cavity, or "throttle body". Multi-point systems use one injector per cylinder, and usually locate the injectors at the mouth of the intake port.
The fuel injector is an electromechanical device that sprays and atomizes the fuel. The fuel injector is nothing more than a solenoid through which gasoline is metered. When electric current is applied to the injector coil, a magnetic field is created, which causes the armature to move upward. This action pulls a spring-loaded ball or "pintle valve" off its seat. Then, fuel under pressure can flow out of the injector nozzle. The shape of the pintle valve causes the fuel to be sprayed in a cone-shaped pattern. When the injector is de-energized, the spring pushes the ball onto its seat, stopping the flow of fuel.
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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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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.
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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!
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Intake Manifold
An intake manifold is a system of passages which conduct the fuel mixture from the carburetor to the intake valves of the engine. Manifold design has much to do with the efficient operation of an engine. For smooth and even operation, the fuel charge taken into each cylinder should be of the same strength and quality.
Distribution of the fuel should, therefore, be as even as possible. This depends greatly upon the design of the intake manifold. Dry fuel vapor is an ideal form of fuel charge, but present-day fuel prevents this unless the mixture is subjected to high temperature. If the fuel charge is heated too highly, the power of the engine is reduced because the heat expands the fuel charge. Therefore, it is better to have some of the fuel deposited on the walls of the cylinders and manifold vents. Manifolds in modern engines are designed so that the amount of fuel condensing on the intake manifold walls is reduced to a minimum.
In a V-8 engine, the intake manifold is mounted between the cylinder heads. The L-head engine's manifold is bolted to the side of the block, and the I-head manifold is bolted to the cylinder head.
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Manifold Heat Control
Most engines have automatically operated heat controls which use the exhaust gases of the engine to heat the incoming fuel-air charge during starting and warm-up. This improves vaporization and mixture distribution. When the engine is cold, all of the exhaust gas is deflected to and around the intake manifold "hot spot". As the engine warms up, the thermostatic spring is heated and loses tension. This allows the counterweight to change the position of the heat control valve gradually so that, at higher driving speeds with a thoroughly warmed engine, the exhaust gases are passed directly to the exhaust pipe and muffler.
In the ram induction system, there is a heat control chamber in each manifold to operate the automatic choke and to heat the fuel mixture after warm-up. A heat control valve in each exhaust manifold will by-pass the exhaust gas through an elbow to the intake manifold heat control chamber. Heat outlet pipes then carry the gas down to the "Y" connector under the heat control valve.
Heat control is regulated by a coiled thermostatic spring mounted on the exhaust manifold. A counterweight is mounted on the other end of the heat control valve shaft and this counterweight, in conjunction with the thermostatic spring, operates to close and open the heat control valve.
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Octane Rating
A gasoline's ability to resist detonation is called its "octane" or anti-knock rating. Gasoline from asphaltic base crude oil produces less knock than one from paraffinic base crude. Cracked gas has less tendency to knock than straight run gas. All marketed gasolines are a blend of straight run and cracked gasolines, so unless their blending is controlled, the anti-knock qualities will vary.
A mixture of iso-octane, which has a very high anti-knock rating, and heptane, which makes a pronounced knock, is used as a reference fuel to establish an anti-knock standard. The anti-knock value or octane number is represented by the percentage of volume of iso-octane that must be mixed with normal heptane in order to duplicate the knocking of the gasoline which is being tested. These ratings range from 50 in third grade gasolines to 110 in aviational fuels. The rating of 100 means a fuel having an anti-knock value equal to that of iso-octane. If the octane rating of a gasoline is naturally low, the fuel will detonate as it burns and power will be applied to the pistons in hammer-like blows. The ideal power is that which pushes steadily on the pistons, rather than hammer against them. The octane rating of a gasoline can be raised by treating it with a chemical which is not a fuel. The best chemical known is tetra-ethyl lead compound, which is added to the gasoline.
Tetra-ethyl lead is a liquid which mixes thoroughly with gasoline and vaporizes completely. Ethylene dibromide prevents the tetra-ethyl lead from forming lead oxide deposits on spark plugs and on valve seats and stems. Red dye is added to identify an ethyl treated gasoline and to warn against its being used as anything but an engine fuel. In 1975, it became illegal to use a leaded gasoline except in cars built prior to this time. With the addition of the catalytic converter, it is undesirable to burn leaded fuel, because leaded fuel will clog the converter and increase the back-pressure of the exhaust.
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Supercharger
A supercharger is a compressor. Hence, a supercharged engine has a higher overall compression than a nonsupercharrged engine having the same combustion chamber volume and piston displacement and will burn more fuel. Unfortunately, the increase in power is not proportional to the increase in fuel consumption. There are two general models of superchargers, the Rootes type and the centrifugal type. The Rootes "blower" has two rotors, while the centrifugal uses an impeller rotating at high speed inside a housing.
Superchargers can be placed between the throttle body of the carburetor or fuel injection system and the manifold; or at the air inlet before the throttle body. Racing cars usually have it located between the throttle body and the manifold. This design has the advantage that the fuel can be supplied through the throttle body without modification to any part of the system. If the supercharger is placed in front of the throttle body, fuel must be supplied under sufficient pressure to overcome the added air pressure created by the supercharger. The advantage of a supercharger over a turbocharger is that there is no lag time of boost; the moment the accelerator pedal is depressed, the boost is increased.
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Turbocharger
A turbocharger, or supercharger, can boost engine power up to 40%%. The idea is to force the delivery of more air-fuel mixture to the cylinders and get more power from the engine. A turbocharger is a supercharger that operates on exhaust gas from the engine.
Although turbochargers and superchargers perform the same function, the turbocharger is driven by exhaust gases, while the supercharger is driven by belts and gears. The turbocharger has a turbine and a compressor, and requires less power to be driven than a supercharger. The pressure of the hot exhaust gases cause the turbine to spin. Since the turbine is mounted on the same shaft as the compressor, the compressor is forced to spin at the same time, drawing 50%% more air into the cylinders than is drawn in without the turbocharger. This creates more power when the air-fuel mixture explodes.
A turbocharged engine's compression ratio must be lowered by using a lower compression piston, since an excessive amount of pressure will wear on the piston, connecting rods, and crankshaft, and destroy the engine. All of these parts then, as well as the transmission, must be strengthened on a turbocharged engine or it will be torn apart by the increased horsepower.
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