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Gasoline Fuel Systems

As a licensed mechanic, you will be working with different types of fuel systems for the internal combustion engine. It is important to know how these components function to provide fuel to the engine and how to service those systems. You will need to be able to identify the properties of gasoline and the components of a fuel system. The information provided in this course will help you identify the different systems and understand how they operate.

When you have completed this course, you will be able to:

  1. Understand the different types of gasoline fuel systems, including how the components function to provide fuel to the engine in proper quantities and how to service the gasoline fuel systems.
  2. Identify the properties of gasoline.
  3. Identify the components of a fuel system.
  4. Identify and understand the different gasoline fuel injection systems.
  5. Identify components of the exhaust and emission control systems.
  6. Understand the operation of the exhaust and emission control systems.



1.0.0 Gasoline Fuel Systems

2.0.0 Gasoline Fuel Injection Systems

3.0.0 Exhaust and Emission Control Systems

Review Questions


The function of the fuel system is to supply a combustible mixture of air and fuel to the engine. Major elements of the gasoline fuel supply system include the following: fuel tank and cap, fuel system emissions controls, fuel lines, fuel pump, fuel filter, carburetor or fuel injection system, air cleaner, and exhaust system. Before learning about these components of a gasoline fuel system, you should understand the composition and properties of gasoline.


Gasoline is a highly volatile, flammable liquid hydrocarbon mixture used as a fuel for internal combustion engines. A comparatively economical fuel, gasoline is the primary fuel for automobiles worldwide. Chemicals called additives such as lead, detergents, and anti-oxidants, are mixed into gasoline to improve its operating characteristics.

Antiknock additives are used to slow down the ignition and burning of gasoline. This action helps to prevent engine ping or knock. Leaded gasoline has lead antiknock additives. The lead allows a higher engine compression ratio to be used without the fuel igniting prematurely. Leaded gasoline is designed to be used in older vehicles that have little or no emission controls.

The fuel used today is unleaded gasoline. Unleaded gasoline, also called no-lead or lead-free, does NOT contain lead antiknock additives. Congress has passed laws requiring that all vehicles meet strict emission levels. As a result, manufacturers began using catalytic converters and unleaded fuel.

Properties of Gasoline

For a gasoline fuel system to function properly, the fuel must have the right qualities to burn evenly no matter what the demands of the engine are. To help you recognize the qualities required of gasoline used for fuel, let’s examine the three properties of gasoline and their effects on the operation of the engine.


The ease with which gasoline vaporizes is called volatility. A high volatility gasoline vaporizes very quickly. A low volatility gasoline vaporizes slowly. A good gasoline should have the right volatility for the climate in which the gasoline is used.

If the gasoline is too volatile, it will vaporize in the fuel system. The result will be a condition called vapor lock. Vapor lock is the formation of vapor in the fuel lines in a quantity sufficient to prevent the flow of gasoline through the system. Vapor lock causes the vehicle to stall from lack of fuel. In the summer and in hot climates, fuels with low volatility lessen the tendency toward vapor lock.

Antiknock Quality

In modern high compression gasoline engines, the air-fuel mixture tends to ignite spontaneously or to explode instead of burning rather slowly and uniformly. The result is a knock, a ping, or a detonation. For this reason, gasoline refiners have various ways to make gasoline that does not detonate easily.

Octane Rating

A gasoline that detonates easily is called low octane gasoline. A gasoline that resists detonation is called high octane gasoline.

The octane rating of a gasoline is a measurement of the ability of the fuel to resist knock or ping. A high octane rating indicates the fuel will NOT knock or ping easily. It should be used in a high compression or turbocharged engine. A low octane gasoline is suitable for a low compression engine.

Octane numbers give the antiknock value of gasoline. A higher octane number (91) will resist ping better than a gasoline with a low octane number (83). Each manufacturer recommends an octane number for its engine.

Air-Fuel Ratio

For proper combustion and engine performance, the right amounts of air and fuel must be mixed together. If too much fuel or too little fuel is used, engine power, fuel economy, and efficiency are reduced.

 For a gasoline engine, the perfect air-fuel ratio is 14.7:1 (14.7 parts air to 1 part fuel by weight). Under constant engine conditions, this ratio can help assure that all fuel is burned during combustion. The fuel system must change the air-fuel ratio with the changes in engine-operating conditions.

Lean Air-Fuel Mixture

A lean air-fuel mixture contains a large amount of air. For example, 20:1 would be a very lean mixture. A slightly lean mixture is desirable for high gas mileage and low exhaust emissions. Extra air in the cylinder ensures that all the fuel will be burned; however, too lean of a mixture can cause poor engine performance (lack of power, missing) and even engine damage.

Rich Air-Fuel Mixture

A rich air-fuel mixture contains a little more fuel mixed with the air. For gasoline, 8:1(8 parts air to 1 part fuel) is a very rich mixture. A slightly rich mixture tends to increase power; however, it also increases fuel consumption and exhaust emissions. An overly rich mixture will reduce engine power, foul spark plugs, and cause incomplete burning (black smoke at engine exhaust).

Gasoline Combustion

For gasoline or any other fuel to burn properly, it must be mixed with the right amount of air. The mixture must then be compressed and ignited. The resulting combustion produces heat, expansion of the gases, and pressure.

1.4.1 Normal Combustion

Normal gasoline combustion occurs when the spark plug ignites the fuel and burning progresses smoothly through the fuel mixture. Maximum cylinder pressure should be produced after a few degrees of crank rotation after the piston passes TDC on the power stroke.

Normal combustion takes only about 3/1,000 of a second. This is much slower than an explosion. Dynamite explodes in about 1/50,000 of a second. Under some undesirable conditions, however, gasoline can be made to bum quickly, making part of the combustion like an explosion.

1.4.2 Abnormal Combustion

Abnormal combustion occurs when the flame does NOT spread evenly and smoothly through the combustion chamber. The lean air-fuel mixture, high operating temperatures, low octane, and unleaded fuels used today make abnormal combustion a major problem that creates unfavorable conditions, such as the following:

1.5.0 Gasoline Fuel System Components

A gasoline fuel system draws fuel from the tank and forces it into the fuel-metering device (carburetor, gasoline injectors), using either a mechanical (engine-driven) or electric fuel pump. The basic parts of a fuel supply system include the following:

1.5.1 Fuel Tank

An automotive fuel tank must safely hold an adequate supply of fuel for prolonged engine operation. The location of the fuel tank should be in an area that is protected from flying debris, shielded from collision damage, and not subjected to bottoming (Figure 1). A fuel tank can be located just about anywhere in the vehicle that meets these requirements.

Figure 1— Common fuel tank locations.

Figure 2 shows the general construction of a fuel tank used on automotive equipment. Fuel tanks are usually made of thin sheet metal or plastic. The main body of a metal tank is made by soldering or welding two formed pieces of sheet metal together. Other parts (filer neck, fuel tank cap, and baffles) are added to the form to complete the fuel tank assembly. A lead-tin alloy is normally plated to the sheet metal to prevent the tank from rusting.

Figure 2— Fuel tank construction

The fuel tank filler neck is an extension on the tank for filling the tank with fuel. The filler cap fits on the end of the filler neck. The neck extends from the tank through the body of the vehicle. A flexible hose is normally used as part of the filler neck to allow for tank vibration without breakage. In vehicles requiring unleaded fuel, a fuel neck restrictor is used inside the filler neck. This prevents the accidental use of leaded gasoline in an engine designed for unleaded. The restrictor is too small to accept the larger leaded fuel type pump nozzle.



If the restrictor is removed and leaded fuel is used in a vehicle designed for unleaded fuel, the catalytic converter will be damaged. This action is a violation of federal law; therefore, NEVER remove the filler neck restrictor.

Modern fuel tank caps are sealed to prevent escape of fuel and fuel vapors (emissions) from the tank. The cap has pressure and vacuum valves that open only under abnormal conditions of high pressure or vacuum.

Fuel tank baffles are placed inside the tank to prevent the fuel from sloshing or splashing around in the tank. The baffles are metal plates that restrict fuel movement when the vehicle accelerates, decelerates, or turns corners. Fuel tanks give little or no trouble, and generally require no servicing other than an occasional draining and cleaning.



If a fuel tank is punctured or develops leaks, it should NOT be welded or repaired with or near an open flame until all traces of fuel and fuel vapors have been completely removed from the tank. Before attempting to make any repairs to a fuel tank, consult with the shop supervisor for specific instructions on all safety precautions to be observed.

1.5.2 Fuel Gauges

The fuel gauge indicates the fuel level in the fuel tank. It is a magnetic indicating system that can be found on either an analog or digital instrument panel.

The fuel sending unit is combined with the fuel pump assembly and consists of a variable resistor controlled by the level of an attached float mechanism in the tank. When the fuel is low, resistance in the sender is low; therefore movement of lift of the gauge is low. When the resistance is high, such as with a full tank, the indicator is high, showing the gauge higher up the scale on the instrument panel.

1.5.3 Fuel Filters

The fuel injection system is highly sensitive to foreign particles. Fuel filters prevent water, dirt, and rust particles from entering the system. Contaminated fuel can cause incomplete combustion, smoky exhaust, engine knocking, and difficulties starting the engine. Most heavy equipment has a fuel pressure gauge that indicates when the filters are dirty.

The fuel filter operates by passing fuel through a porous element that removes particles large enough to cause problems in the system (Figure 3). Particles are often measured in microns. A micron is one millionth of a meter. Some filters serve as sediment bowls. These types of filters separate water and larger particles from the fuel. After separation, the water and particles settle to the bottom of the bowl, where they can be removed through a drain plug.

Figure 3— Fuel filter operation.

Filter elements can be made of ceramic, treated paper, sintered bronze or metal screen (Figure 4). Some filter elements are made of laminated disks that are spaced 0.0003 inches apart. Foreign matter is blocked out as the fuel passes between the disks.

Figure 4— Fuel filter elements.

1.5.4 Fuel Pump

A fuel pump is the device that draws the fuel from the tank to the engine’s injection system. All late model vehicles use an electric fuel pump.

The fuel pump can be located either inside the tank or in the fuel system after the tank. There are four types of fuel pumps: the diaphragm, plunger, bellows, and impeller or rotary pump. The in-tank electric pump is usually a rotary pump. The others are usually of the demand style, meaning that when the ignition is turned on, the fuel pump starts and when the pressure in the system is correct, it shuts off. When more fuel is required, the pump starts again.

Most vehicles have an in-tank fuel pump. Some other vehicles also have a secondary pump along the fuel line. Fuel pumps are mounted in the tank to help keep the pump cool.

Nearly all electric fuel pump circuits have some sort of rollover protection. Typically this protection includes the installation of an inertia switch that shuts off the fuel pump if the vehicle rolls over or is in an accident.

1.5.5 Fuel Lines and Hoses

Fuel lines and hoses carry fuel from the tank to the filter and fuel injection assembly. They can be made from either metal tubing or flexible nylon or synthetic rubber hoses. The latter must be able to resist gasoline. The hoses must be non-permeable so gas and gas vapors cannot evaporate through the hose.

Fuel supply lines from the tank to the injectors usually follow the frame of the vehicle along the underchassis. Generally, rigid lines are used from the tank to the fuel pump or filter. To absorb vibration, these lines can be joined with short lengths of flexible hose. Many tanks also have vent hoses to allow air in the tank to escape when the tank is being filled. Vent hoses are usually routed alongside the filler neck.

Faulty fuel lines and hoses are a common source of fuel leaks. Fuel hoses can become hard and brittle after being exposed to the engine heat and the elements. Engine oil can soften and swell them. Always inspect hoses closely and replace any in poor condition. Metal fuel lines rarely cause problems; however, they should be replaced if they become smashed, kinked, rusted, or leaky. Remember these rules when working with fuel lines and hoses:



Most fuel injection systems have very high fuel pressure. Follow recommended procedures for bleeding or releasing pressure before disconnecting a fuel line or fitting. This action will prevent fuel spray from possibly causing injury or a fire.

1.5.6 Fuel Pressure Regulator

The fuel pressure regulator controls the amount of pressure entering the injector valves. When sufficient pressure is attained, the regulator returns excess fuel to the tank. This maintains a preset amount of fuel pressure for injector valve operation.

1.5.7 Fuel Injectors

Fuel injection systems can have one or more fuel supply devices called fuel injectors (Figure 5). Fuel injectors are controlled by an ECM. The computer system sends an electrical current to activate the solenoid inside the injector. When the solenoid is activated, the injector nozzle opens and squirts atomized fuel in a cone-shaped pattern. The computer system controls the fuel-air ratio by varying the length of time that the injector nozzle remains open.

 Figure 5— Fuel injector.

In gasoline engines injectors squirt fuel into the intake manifold. In diesel engines, fuel is delivered directly into the combustion chamber. Spring pressure closes the injector nozzle when the solenoid is deactivated.

1.6.0 Air Cleaner

Air cleaners are used to prevent foreign matter, such as sand, dust, and lint, from entering the intake system (Figure 6). Contaminated air in the intake system can cause engine wear, poor combustion, and engine breakdown. In addition to supplying clean air, air cleaners reduce vibration sounds and other noises caused by air entering the intake system.

Figure 6 — Air cleaners.

Test Your Knowledge

1. Which of the following is NOT a property of gasoline?

A. Volatility
B. Antiknock quality
C. Cetane number
D. Octane rating

2. Which of the following air-fuel ratios is considered to be perfect for a gasoline engine?

A. 20:1
B. 14.7:1
C. 17.4:1
D. 18.8:1

3. The fuel pressure regulator controls the amount of pressure entering the ______.

A. intake
B. valves
C. injectors
D. pump


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A modern gasoline injection system uses pressure from an electric fuel pump to spray fuel into the engine intake manifold. Like a carburetor, it must provide the engine with the correct air-fuel mixture for specific operating conditions. Unlike a carburetor, however, PRESSURE, not engine vacuum, is used to feed fuel into the engine. This makes the gasoline injection system very efficient. A gasoline injection system has several possible advantages over a carburetor type of fuel system. Some advantages are as follows:

There are many types of gasoline injection systems. Before studying the most common ones, you should have a basic knowledge of the different classifications. Systems are classified as either single- or multi-point injection and as either indirect or direct injection.

The point or location of fuel injection is one way to classify a gasoline injection system. A single-point injection system, also call throttle body injection (TBI), has the injector nozzles in a throttle body assembly on top of the engine. Fuel is sprayed into the top center of the intake manifold.

A multi-point injection system, also called port injection, has an injector in the port (airfuel passage) going to each cylinder. Gasoline is sprayed into each intake port and toward each intake valve. Thereby, the term multi-point (more than one location) fuel injection is used.

An indirect injection system sprays fuel into the engine intake manifold. Most gasoline injection systems are of this type. Direct injection forces fuel into the engine combustion chambers.

There are several basic configurations of gasoline fuel injection we will discuss the timed, and throttle body.

2.1.0 Timed Fuel Injection System

Timed fuel injection systems for gasoline engines inject a measured amount of fuel in timed bursts that are synchronized to the intake strokes of the engine. Timed injection is the most precise form of fuel injection but is also the most complex. There are two basic forms of timed fuel injection: mechanical and electronic.

The basic operation of a mechanical-timed injection system (Figure 7) is as follows:

Figure 7 — Mechanical-timed injection system.

A high-pressure electric pump draws fuel from the fuel tank and delivers it to the metering unit. A pressure relief valve is installed between the fuel pump and the metering unit to regulate fuel line pressure by bleeding off excess fuel back to the tank.

The metering unit is a pump that is driven by the engine camshaft. It is always in the same rotational relationship with the camshaft so it can be timed to feed the fuel to the injectors just at the right moment.

Each injector contains a spring-loaded valve that is opened by fuel pressure, injecting fuel into the intake at a point just before the intake valve opens.

The throttle valve regulates engine speed and power output by regulating manifold vacuum, which, in turn, regulates the amount of fuel supplied to the injectors by the metering pump.

The more common type of timed fuel injection is the electronic-timed fuel injection, also known as electronic fuel injection, or EFI (Figure 8).

Figure 8 — Electronic fuel injection.

An electronic fuel injection system can be divided into four subsystems:

The fuel delivery system of an EFI system includes an electric fuel pump, a fuel filter, a pressure regulator, the injector valves, and the connecting lines and hoses.

The electric fuel pump draws fuel out of the tank and forces it into the pressure regulator.

The fuel pressure regulator controls the amount of pressure entering the injector valves. When sufficient pressure is attained, the regulator returns excess fuel to the tank. This maintains a preset amount of fuel pressure for injector valve operation.

The fuel injector for an EFI system is a coil or solenoid-operated fuel valve. When not energized, spring pressure keeps the injector closed, keeping fuel from entering the engine. When current flows through the injector coil or solenoid, the magnetic field attracts the injector armature. The injector opens, squirting fuel into the intake manifold under pressure.

The air induction system for the EFI typically consists of a throttle valve, sensors, an air filter, and connecting ducts.

The throttle valve regulates how much air flows into the engine. In turn, it controls engine power output. Like the carburetor throttle valve, it is connected to the gas pedal. When the pedal is depressed, the throttle valve swings open to allow more air to rush into the engine.

The EFI sensor system monitors engine operating conditions and reports this information to the computer. A sensor is an electrical device that changes circuit resistance or voltage with a change in a condition (temperature, pressure, position of parts, etc.). For example, the resistance of a temperature sensor may decrease as temperature increases. The computer can use the increased current flow through the sensor to calculate any needed change in the injector valve opening. Typical sensors for an EFI system include the following:

Since some of these sensors were discussed in the section on computerized carburetor systems, we will concentrate only on the sensors that are particular to the EFI system. These sensors are as follows:

The throttle position sensor is a variable resistor connected to the throttle plate shaft.

When the throttle swings open for more power or closes for less power, the sensor changes resistance and signals the computer. The computer can then enrich or lean the mixture as needed.

The air flow sensor is used in many EFI systems to measure the amount of outside air entering the engine. It is usually an air flap or door that operates a variable resistor. Increased air flow opens the air flap more to change the position of the resistor. Information is sent to the computer indicating air inlet volume.

The inlet air temperature sensor measures the temperature of the air entering the engine. Cold air is denser, requiring a little more fuel. Warm air is not as dense as cold, requiring a little less fuel. The sensor helps the computer compensate for changes in outside air temperature and maintain an almost perfect air-fuel mixture ratio.

The crankshaft position sensor is used to detect engine speed. It allows the computer to change injector openings with changes in engine rpm.

The signal from the engine sensors can be either a digital or an analog type output. Digital signals are on/off signals. An example is the crankshaft position sensor that shows engine rpm. Voltage output or resistance goes from maximum to minimum, like a switch. An analog signal changes in strength to let the computer know about a change in condition. Sensor internal resistance may smoothly increase or decrease with temperature, pressure, or part position. The sensor acts as a variable resistor.

Basic operation of an electronic-timed injection system is as follows:

2.2.0 Throttle Body Injection System

The throttle body injection (TBI) system uses one or two injector valves mounted in a throttle body assembly (Figure 9). The injectors spray fuel into the top of the throttle body air horn The TBI fuel spray mixes with the air flowing through the air horn. The mixture is then pulled into the engine by intake manifold vacuum. The throttle body injection assembly typically consists of the following: throttle body housing, fuel injectors, fuel pressure regulator, Idle air control sensor, throttle position sensor, and throttle plates.

Figure 9 — Throttle body injection.

The throttle body housing, like a carburetor body, bolts to the pad on the intake manifold. It houses the metal castings that hold the injectors, the fuel pressure regulator, and the throttle plates. The throttle plates are located in the lower section of the body. A linkage or cable connects the throttle plates with the accelerator pedal. An inlet fuel line and an outlet return line connect to the fittings on the body.

The throttle body injector consists of an electric solenoid coil, armature or plunger, ball or needle valve and seat, and injector spring. Wires from the computer connect to terminals on the injectors. When the computer energizes the injectors, a magnetic field is produced in the injector coil. The magnetic field pulls the plunger and valve up to open the injector. Fuel can then squirt through the injector nozzle and into the engine.

The throttle body pressure regulator consists of a fuel valve, a diaphragm, and a spring. When fuel pressure is low, the spring holds the fuel valve closed, causing pressure to build as fuel flows into the regulator from the fuel pump. When a preset pressure is reached, pressure acts on the diaphragm. The diaphragm compresses the spring and opens the fuel valve. Fuel can then flow back to the fuel tank, limiting the maximum fuel pressure at the injectors.

Although throttle body injection does not provide the precise fuel distribution of the direct port injection, it is much cheaper to produce and provides a much higher degree of precision fuel metering than a carburetor.

2.3.0 Multi-Port Injection

Multi-port injection systems use a computer, engine sensors, and one solenoid injector for each cylinder. This is the most common fuel injection system in late model vehicles. The multi-port injection system operates similar to that of a throttle body injection system, except that the fuel is injected at each intake port instead of at the top of the intake manifold.

A fuel rail feeds fuel to the injectors. It connects the main fuel line to the inlet of each injector. The injector is pressure fit into a port in the intake manifold. Each injector is aimed to spray fuel toward the engine’s intake valve. Each injector is connected to the computer and is electronically fired just before the intake valve is to open.

2.4.0 Direct Injection

Direct injection has been around for many years on a diesel engine. With gasoline direct injection (GDI), the gasoline is injected directly into the combustion chamber (Figure 10). To make this possible, specially designed injectors deliver fuel into the high pressures and temperatures in the cylinders. To prevent heat from igniting the fuel in the injector, the injectors are designed to completely seal after the fuel is sprayed. The injectors must also be able to spray the fuel at a higher pressure than what is in the cylinder.

Figure 10 — Gasoline direct injection.

GDI allows for very lean operation during cruising. When the engine is operating under heavy loads, the system provides near perfect air-fuel mixture. The ability to run at such lean mixtures allows for increased fuel economy, nearly 30%. Spraying the fuel directly into the cylinder also increases volumetric efficiency. GDI decreases an engine’s tendency to knock, allowing for a higher compression ratio without the need for a higher octane gasoline.

With GDI, fuel can be injected at any time, not just during intake. The injectors can pulse twice during the transition from the compression stroke to combustion. The two pulses promote complete combustion when the PCM senses operating conditions may prevent a complete burning of fuel.

The fuel is injected into the cylinder under enormous pressure, typically between 400 and 1,500 psi. The injector delivers a relatively small, precisely shaped burst of fuel around the spark plug just before ignition. This means only the area around the spark plug has air and fuel to begin combustion; the rest of the chamber is filled with air.

The fuel pump that delivers this high pressure is driven by the engine. The pump is fed by an in-tank electric fuel pump. A PCM controls the timing of injection and ignition for each cylinder.

Test Your Knowledge

4. A gasoline injection system has all these advantages over a carburetor, except which one?

A. Improved atomization
B. Better fuel distribution
C. Lower horse power
D. Lower emissions

5. What gasoline fuel injection system is the most precise and also the most complex?

A. Hydraulic-timed
B. Throttle body fuel
C. Timed fuel
D. Continuous fuel


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Over the past several years, exhaust and emission control has greatly increased because of stringent anti-pollution laws and EPA guidelines. This has made the exhaust and emission control systems of vehicles invaluable and a vital part of today’s vehicles.

The waste products of combustion are carried away from the engine to the rear of the vehicle by the exhaust system where they are expelled to the atmosphere. The exhaust system also serves to dampen engine noise. The parts of a typical exhaust system include the following: exhaust manifold, header pipe, catalytic converter, intermediate pipe, muffler, tailpipe, hangers, heat shields, and muffler clamps.

The control of exhaust emissions is a difficult job. The ideal situation would be to have the fuel combine completely with the oxygen from the intake air. The carbon would then combine with the oxygen to form carbon dioxide (CO2), the hydrogen would combine to form water (H2O), and the nitrogen present in the intake would stand alone. The only other product present in the exhaust would be the oxygen from the intake air that was not used in the burning of the fuel. In a real life situation, however, this is not what happens. The fuel never combines completely with the oxygen, and undesirable exhaust emissions are created as a result.

The most dangerous of the emissions is carbon monoxide (CO), which is a poisonous gas that is colorless and odorless. CO is formed as a result of insufficient oxygen in the combustion mixture and combustion chamber temperatures that are too low.

Other exhaust emissions that are considered major pollutants are as follows:

Hydrocarbons (HC) are unburned fuel. They are particulate (solid) in form, and, like carbon monoxide, are manufactured by insufficient oxygen in the combustion mixture and combustion chamber temperatures that are too low. Hydrocarbons are harmful to all living things. In any urban area where vehicular traffic is heavy, hydrocarbons in heavy concentrations react with the sunlight to produce a brown fog known as photochemical smog.

Oxides of nitrogen (NOx) are formed when nitrogen and oxygen in the intake air combine when subjected to high temperatures of combustion. Oxides of nitrogen are harmful to all living things.

The temperatures of the combustion chamber would have to be raised to a point that would melt pistons and valves to eliminate carbon monoxide and carbon dioxide emissions. Furthermore, oxides of nitrogen emissions go up with any increase in the combustion chamber temperature. Knowing these facts, you can see why emission control devices are necessary.

3.1.0 Exhaust Manifold

The exhaust manifold connects all the engine cylinders to the exhaust system. It is usually made of cast iron. If the exhaust manifold is properly formed, it can create a scavenging action that will cause all of the cylinders to help each other get rid of exhaust gases. Back pressure (the force that the pistons must exert to push out the exhaust gases) can be reduced by making the manifold with smooth walls and without any sharp bends. All these factors are taken into consideration when the exhaust manifold is designed, and the best possible manifold is manufactured to fit into the confines of the engine compartment.

3.2.0 Catalytic Converters

It is impossible to keep carbon monoxide and hydrocarbon emissions at acceptable levels by controlling them in the cylinder without shortening engine life considerably. The most practical method of controlling these emissions is outside the engine using a catalytic converter. The catalytic converter is similar in appearance to the muffler and is positioned in the exhaust system between the engine and muffler. As the engine exhaust passes through the converter, carbon monoxide and hydrocarbons are oxided (combined with oxygen), changing them into carbon dioxide and water.

The catalytic converter contains a material (usually platinum or palladium) that acts as a catalyst. The catalyst is something that causes a reaction between two substances without actually getting involved. In the case of the catalytic converter, oxygen is joined chemically with carbon monoxide and hydrocarbons in the presence of its catalyst. Because platinum and palladium are both very precious metals and the catalyst must have a tremendous amount of surface area in order to work properly, it has been found that the following internal structures work best for catalytic converters:

Pellet type is filled with aluminum oxide pellets that have a very thin coating of catalytic material (Figure 11, View A). Aluminum oxide has a rough outer surface, giving each pellet a tremendous amount of surface area. The converter contains baffles to ensure maximum exposure of the exhaust to the pellets.

Figure 11— Catalytic converter.

Monolithic type uses a one-piece ceramic structure in a honeycomb style form (Figure 11, View B). The structure is coated thinly with a catalytic material. The honeycomb shape has a tremendous surface area to ensure maximum exposure of exhaust gases to the catalyst.

 An adequate amount of oxygen must be present in the exhaust system for the catalytic converter to operate; therefore, a supporting system, such as an air injection system, usually is placed on catalytic converter-equipped engines to dilute the exhaust stream with fresh air.

3.3.0 Muffler

The muffler reduces the acoustic pressure of exhaust gases and discharges them to the atmosphere with a minimum of noise (Figure 12). The muffler usually is located at about midpoint in the vehicle with the exhaust pipe between it and the exhaust manifold, and the tailpipe leading from the muffler to the rear of the vehicle.

Figure 12 — Muffler.

The inlet and outlet of the muffler usually are slightly larger than their connecting pipes so that they may hook up by slipping over them. The muffler is then secured to the exhaust pipe and tailpipe by clamps.

A typical muffler has several concentric chambers with openings between them. The gas enters the inner chamber and expands as it works its way through a series of holes in the other chambers and finally to the atmosphere. They must be designed also to quiet exhaust noise while creating minimum back pressure. High back pressure could cause loss of engine power and economy as well as overheating.

Exhaust system components usually are made of steel. They are coated with aluminum or zinc to retard corrosion. Stainless steel also is used in exhaust systems in limited quantities due to its high cost. A stainless steel exhaust system will last indefinitely.

3.4.0 Air Injection System

An air injection system forces fresh air into the exhaust ports of the engine to reduce HC and CO emissions (Figure 13). The exhaust gases leaving an engine can contain unburned and partially burned fuel. Oxygen from the air injection system causes this fuel to continue to burn. The major parts of the system are the air pump, the diverter valve, the air distribution manifold, and the air check valve.

Figure 13 — Air injection system.

The air pump is belt-driven and forces air at low pressure into the system. A hose is connected to the output of the diverter valve.

The diverter valve keeps air from entering the exhaust system during deceleration. This prevents backfiring in the exhaust system. Also, the diverter valve limits maximum system air pressure when needed, releasing excessive pressure through a silencer or a muffler.

The air distribution manifold directs a stream of fresh air toward each engine exhaust valve. Fittings on the air distribution manifold screw into a threaded hole in the exhaust manifold or cylinder head.

The air check valve is usually located in the line between the diverter valve and the air distribution manifold. It keeps exhaust gases from entering the air injection system.

Basic operation of the air injection system is as follows:

When the engine is running, the spinning vanes of the air pump force air into the diverter valve. If the engine is not decelerating, the air is forced through the diverter valve, the check valve, the air injection manifold, and into the engine. The fresh air blows on the exhaust valves.

During periods of deceleration, the diverter valve blocks air flow into the engine exhaust manifold. This prevents a possible backfire that could damage the exhaust system of the vehicle. When needed, the diverter valve will release excess pressure in the system.

3.5.0 Positive Crankcase Ventilation (PCV) System

The positive crankcase ventilation system uses manifold vacuum to purge the crankcase blow-by fumes. The fumes are then aspirated back into the engine where they are reburned.

A hose is tapped into the crankcase at a point that is well above the engine oil level. The other end of the hose is tapped into the intake manifold.

An inlet breather is installed on the crankcase in a location that is well above the level of the engine oil. The inlet breather also is located strategically to ensure complete purging of the crankcase fresh air. The areas of the crankcase where the vacuum hose and inlet breather are tapped have baffles to keep motor oil from leaving the crankcase.

A flow control valve is installed in the line that connects the crankcase to the manifold. It is called a positive crankcase ventilation (PCV) valve (Figure 14) and serves to avoid the air-fuel mixture by doing the following:

Figure 14 — Positive crankcase ventilation system.

Any periods of large throttle opening will be accompanied by heavy engine loads. Crankcase blow-by will be at its maximum during heavy engine loads. The PCV valve will react to the small amount of manifold vacuum that also is present during heavy engine loading by opening fully through the force of its control valve spring. In this way, the system provides maximum effectiveness during maximum blow-by periods.

Any period of small throttle opening will be accompanied by small engine loads, high manifold vacuum, and a minimum amount of crankcase blow-by. During these periods, the high manifold vacuum will pull the PCV valve to its position of minimum opening. This is important to prevent an excessively lean air-fuel mixture.

In the event of engine backfire (flame traveling back through the intake manifold), the reverse pressure will push the rear shoulder of the control valve against the valve body. This seals the crankcase from the backfire which could otherwise cause an explosion.

 A PCV system keeps the inside of the engine clean and reduces air pollution. Older engines used an open PCV system. This system is no longer in use. The closed system uses a sealed oil filler cap, a sealed dip stick, ventilation hoses, and either a PCV valve or flow restrictor. The gases are drawn into the engine and burned. The system stores the gases when the engine is not being run.

3.6.0 Exhaust Gas Recirculation (EGR) System

The exhaust gas recirculation system allows burned gases to enter the engine intake manifold to help reduce oxides of nitrogen (NOx) emissions. When exhaust gases are added to the air-fuel mixture, they decrease peak combustion temperatures (maximum temperature produced when the air-fuel mixture burns). For this reason, an exhaust gas recirculation system lowers the amount of NOx in the engine exhaust. EGR systems can be controlled by engine vacuum or by the engine control module.

Vacuum controlled EGR systems use engine vacuum to operate the EGR valve (Figure 15). This system is found on older vehicles.

Figure 15 — Vacuum controlled EGR valve.

The basic vacuum EGR system consists of a vacuum-operated EGR valve and a vacuum line from the throttle body or carburetor. The EGR valve is bolted to the engine intake manifold to control the air-fuel ratio and reduce exhaust emissions. Exhaust gases are routed through the cylinder head and intake manifold to the EGR valve.

The EGR valve consists of a vacuum diaphragm, spring, plunger, exhaust gas valve, and diaphragm housing. It is designed to control exhaust flow into the intake when the throttle is opened and the increased vacuum pulls the diaphragm open on the EGR valve, in turn opening the exhaust outlet to allow exhaust gas into the intake manifold.

An electronic-vacuum EGR valve uses both engine vacuum and electronic control for better exhaust gas metering. An EGR position sensor is located on top of the EGR valve. This sensor sends data to the ECM and allows the computer to determine how far to open the EGR valve.

Electronic EGR systems use vehicle sensors, the ECM, and a solenoid-operated EGR valve. This is the most common type of EGR system used on late model engines.

The ECM uses data from the EGR position sensor, engine coolant temperature sensor, mass airflow sensor, throttle position sensor, crankshaft position sensor, and various other sensors to control the air fuel ratio and reduce exhaust emission. The data collected will determine the duty cycle for the EGR valve to allow certain amounts of gases to be recirculated for maximum efficiency.

3.7.0 Fuel Evaporation Control System

The fuel evaporization control system prevents vapors from the fuel tank and carburetor from entering the atmosphere (Figure 16). Older, pre-emission vehicles used vented fuel tank caps. Carburetor bowls were also vented to the atmosphere. This caused a considerable amount of emissions. Modern vehicles commonly use fuel evaporization control systems to prevent this source of pollution. The major components of the fuel evaporization control systems are the sealed fuel tank cap, fuel air dome, liquid-vapor separator, rollover valve, fuel tank vent line, charcoal canister, carburetor vent line, and the purge line.

Figure 16 — Fuel evaporation system.

The sealed fuel tank cap is used to keep fuel vapors from entering the atmosphere through the tank filler neck. It may contain pressure and vacuum valves that open in extreme cases of pressure or vacuum. When the fuel expands (from warming), tank pressure forces fuel vapors out a vent line or line at the top of the fuel tank, not out of the cap.

The fuel air dome is a hump designed into the top of the fuel tank to allow for fuel expansion. The dome normally provides about 10 percent air space to allow for fuel heating and volume increase.

The liquid-vapor separator is frequently used to keep liquid fuel from entering the evaporation control system. It is simply a metal tank located above the main fuel tank. Liquid fuel condenses on the walls of the separator and then flows back into the fuel tank.

The roll-over valve is sometimes used in the vent line from the fuel tank. It keeps liquid fuel from entering the vent line after an accident where the vehicle rolled upside down. The valve contains a metal ball or plunger valve that blocks the vent line when the valve is turned over.

The fuel tank vent line carries fuel vapors up to a charcoal canister in the engine compartment.

The charcoal canister stores fuel vapors when the engine is not running. The metal or plastic canister is filled with activated charcoal granules capable of absorbing fuel vapors.

The purge line is used for removing or cleaning the stored vapors out of the charcoal canister. It connects the canister and the engine intake manifold.

Basic operation of a fuel evaporization control system is as follows:

When the engine is running, intake manifold vacuum acts on the purge line, causing fresh air to flow through the filter at the bottom of the canister. The incoming fresh air picks up the stored fuel vapors and carries them through the purge line. The vapors enter the intake manifold and are pulled into the combustion chambers for burning.

When the engine is shut off, engine heat produces excess vapors. These vapors flow through the vent line and into the charcoal canister for storage. The vapors that form in the tank flow through the liquid vapor separator into the tank vent line to the charcoal canister. The charcoal canister absorbs these fuel vapors and holds them until the engine is started again.

3.8.0 Oxygen Sensor

The oxygen sensor monitors the exhaust gases for oxygen content. The amount of oxygen in the exhaust gases is a good indicator of the engine’s operational state. The oxygen sensor’s voltage output varies with any changes in the exhaust’s oxygen content. For example, an increase in oxygen, which would indicate a lean mixture, will make the sensor output voltage decrease. A decrease in oxygen which occurs during rich mixture conditions causes the sensor output voltage to increase.

In this way, the oxygen sensor supplies data to the computer. The computer can then alter the opening and closing of the injectors to maintain a correct air-fuel ratio for maximum efficiency.

3.8.1 Pre Oxygen Sensor

A pre oxygen (O2) sensor is the O2 sensor located in front of the catalytic converter. The signal from the pre O2 sensor indicates whether the engine’s air-fuel mixture is too lean or too rich.

3.8.2 Post Oxygen Sensor

The post oxygen sensor is located further down the exhaust system after the catalytic converter. It checks the oxygen content of the exhaust gases to determine if the converter is working properly. If the oxygen content after the converter is the same as before, it will send a signal to the trouble light on the dash board to let the operator know there is a problem with the catalytic converter.

3.8.3 Heated Oxygen Sensor

The heated oxygen sensor (HO2) uses an electrical heating element to warm the sensor to normal operating temperature. This element will stabilize the temperature and operation of the sensor. The heating element allows the computer system to utilize the sensor’s input sooner since the sensor operates at a higher temperature.

Test Your Knowledge

6. Of the following chemical compounds, which one is the most dangerous emission?

A. Carbon dioxide
B. Carbon monoxide
C. Hydrocarbons
D. Oxides of nitrogen

7. The exhaust gas recirculation system allows burned gases to enter the engine intake manifold to help reduce what gas?

A. Carbon dioxide
B. Carbon monoxide
C. Hydrocarbons
D. Oxides of nitrogen


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Your knowledge of the gasoline fuel system will enable you to evaluate certain engine problems with confidence. The ability to diagnose a gasoline fuel system will help the environment because your ability to determine that the problem is in the exhaust system will alleviate some of the pollutants being dispersed into the atmosphere. Technicians and engineers have developed automobile parts and various systems to help extend fuel economy, gain horsepower, and lower emissions.


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Review Questions

1. What types of additives are used in gasoline to slow down ignition?

A. Antiping
B. Antiknock
C. Anticombustion
D. Antioxidants

2. Which property is NOT a property of gasoline?

A. Volatility
B. Antiknock quality
C. Cetane number
D. Octane rating

3. What term refers to the measurement of the ability of a fuel to resist knock or ping?

A. Air-fuel ratio
B. Cetane number
C. Volatility
D. Octane rating

4. An air-fuel mixture that is too lean will cause which condition?

A. Increased power
B. Increased fuel consumption
C. Poor engine performance
D. Decreased exhaust emissions

5. Which is NOT a condition of abnormal combustion?

A. Detonation
B. Dieseling
C. Pre-ignition
D. Spark ping

6. Which factor can cause dieseling in a gasoline engine?

A. Low octane fuel
B. Low heat range spark plugs
C. Incorrect timing
D. Hot exhaust valve

7. What device is used in the filler neck of a gasoline fuel tank to prevent the accidental use of leaded fuel?

A. Fuel valve
B. Restrictor
C. Vacuum valve
D. Fuel nozzle

8. What is the function of the baffles in a fuel tank?

A. To reinforce the bottom of the fuel tank
B. To reinforce the sides of the fuel tank
C. To prevent the fuel from sloshing and splashing
D. To prevent the escape of fuel and fuel vapors from the tank

9. Fuel filters are NOT made of which material?

A. Sintered brass
B. Ceramic
C. Treated paper
D. Metal screen

10. What is the function of the fuel pump?

A. To measure the amount of fuel that enters the carburetor or fuel injectors
B. To deliver the fuel from the tank to the engine under pressure
C. To pump fuel from the carburetor to the intake manifold
D. To pump fuel from the carburetor through the fuel filter into the manifold

11. Fuel line tubing is normally made of what material?

A. Single-wall steel
B. Double-wall steel
C. Single-wall copper
D. Double-wall copper

12. Which attribute is NOT an advantage of a gasoline injection system over a carburetor type system?

A. Improved atomization
B. Better fuel distribution
C. Richer fuel mixture
D. Lower emissions

13. In a gasoline indirect injection system, fuel is sprayed into what part?

A. Precombustion chamber
B. Cylinder
C. Combustion chamber
D. Intake manifold

14. Of the gasoline fuel injection systems, what system is the most precise and also the most complex?

A. Hydraulic-timed injection
B. Throttle body fuel injection
C. Timed fuel injection
D. Continuous fuel injection

15. In a mechanical-timed injection system, the throttle valve regulates engine speed and power output by regulating the _______.

A. intake pressure
B. manifold vacuum
C. exhaust pressure
D. metering pump vacuum

16. Which is NOT a subsystem of an electronic-timed fuel injection system?

A. Fuel delivery system
B. Air induction system
C. Computer control system
D. Fuel metering system

17. In an electronic fuel injection system, what sensor measures the amount of outside air entering the engine?

A. Air flow
B. Inlet air temperature
C. Manifold pressure
D. Oxygen

18. In an electronic fuel injection system, where does the fuel pressure regulator divert the excess fuel?

A. Back to the fuel tank
B. To the inlet side of the fuel filter
C. To the inlet side of the fuel pump
D. Back to the inlet side of the fuel line

19. What component of a throttle body injection system contains the fuel pressure regulator?

A. Throttle air horn
B. Throttle body housing
C. Throttle positioner
D. Throttle fuel mixture valve

20. In a multi port fuel injection system, what component supplies fuel to the injectors?

A. Fuel tube
B. Fuel rail
C. Siphon pipe
D. Pressure regulator

21. Of the following chemical compounds, which is the most dangerous emission?

A. Carbon dioxide
B. Carbon monoxide
C. Hydrocarbons
D. Oxides of nitrogen

22. Exhaust manifolds are made from what type of material?

A. Aluminum
B. Steel
C. Cast iron
D. Iron alloy

23. What device is used to reduce the acoustic pressure of exhaust gases and discharge the gases into the atmosphere?

A. Resonator
B. Catalytic converter
C. Muffler
D. Exhaust manifold

24. The catalytic converter changes carbon monoxide and hydrocarbons into carbon dioxide and _____.

A. hydrogen
B. oxygen
C. methane
D. water

25. What two materials that act as a catalyst can be found inside a catalytic converter?

A. Silver and bronze
B. Bronze and platinum
C. Silver and palladium
D. Platinum and palladium

26. In an air injection system, what device is used to prevent air from entering the exhaust system during deceleration?

A. Air distribution manifold
B. Air check valve
C. Air pump
D. Diverter valve

27. What device keeps exhaust gases from entering the air injection system?

A. Air check valve
B. Diverter valve
C. Air distribution manifold
D. Air pump

28. The open type positive crankcase ventilation system has a sealed breather that is connected to the air filter by a hose.

A. True
B. False

29. To control the formation of oxides of nitrogen, the exhaust gas recirculation system recirculates a portion of the exhaust gases back through the _______.

A. intake manifold
B. exhaust manifold
C. muffler
D. catalytic converter

30. At idle, engine vacuum is blocked off so it cannot act on the EGR valve. This is accomplished by a closed _______.

A. diverter valve
B. vacuum diaphragm
C. throttle plate
D. heat control valve

31. What valve prevents fuel from entering the fuel tank vent line in the event of a accident in which the vehicle turns over?

A. Purge
B. Fuel tank
C. Roll-over
D. Spillage

32. The charcoal canister does NOT store fuel vapors when the engine is running.

A. True
B. False

33. What component connects the charcoal canister to the engine intake manifold and is used to clean out stored fuel vapors from the charcoal canister?

A. Purge line
B. Carburetor vent line
C. Fuel tank vent line
D. Liquid-vapor separator


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