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.

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.

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 3-11, 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.

Monolithic type uses a one-piece ceramic structure in a honeycomb style form (Figure 3-11, 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.

Figure 3-11— Catalytic converter.

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.


The muffler reduces the acoustic pressure of exhaust gases and discharges them to the atmosphere with a minimum of noise (Figure 3-12). 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 3-12 — 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.

Air Injection System

An air injection system forces fresh air into the exhaust ports of the engine to reduce HC and CO emissions (Figure 3-13). 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 3-13 — 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.

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 3-14) and serves to avoid the air-fuel mixture by doing the following:

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.

Figure 3-14 — Positive crankcase ventilation system.

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 will seal 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.

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 3-15). This system is found on older vehicles.

Figure 3-15 — 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.

Fuel Evaporation Control System

The fuel evaporization control system prevents vapors from the fuel tank and carburetor from entering the atmosphere (Figure 3-16). 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 3-16 — 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.

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.

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.

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.

3.8.4 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


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.