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EMISSION CONTROL

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SOURCE OF EMISSION

Emissions from a fuel-driven motor vehicle usually come from four

sources: the fuel tank, the carburetor, the crankcase, and the

exhaust system.

The term emission normally refers to the pollution produced by a

light vehicle during normal use. Emission control systems are

designed to limit the pollution caused by the harmful products of

storing and burning fuel.

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The fuel tank and carburetor allow fuel to evaporate and escape

to the atmosphere. These are called evaporative emissions.

The crankcase and exhaust system emit pollutants directly fromthe engine into the atmosphere. They are caused when

hydrocarbons, lead compounds, and oxygen and nitrogen from

the air, are burned in the combustion chamber.

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COMBUSTION

Approximately 60% of emissions from an uncontrolledvehicle enginecome from the exhaust - a result of combustion of the fuel and the air.

It is a regulated requirement to reduce

these emissions. Some vehicles use

devices or systems that control the

combustion process itself, while others

treat the resulting exhaust gases.

Both Fuel injected and Carbureted engines

meet emission standards by maintaining

accurate mixture control over a full range of

engine conditions. To achieve this, most

fuel systems require an air supply atconstant temperature.

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One simple control system uses a temperature-sensitive valve inside

the air cleaner. It operates a flap that blends the hot air with cool air, so

that the intake and fuel delivery mechanism receives air at 104 degrees

Fahrenheit or about forty degrees Celsius, regardless of outside air

temperature. Maintenance of this temperature assists vaporization ofthe fuel, particularly when the engine is cold.

To reduce this effect the throttle positioned and dashpot slow down the

rate of closure of the throttle plate. This allows more time for air to enter

the manifold, and for the fuel to vaporize, before the throttle is

completely closed.Electronically managed fuel injection systems use sensors and catalytic

converters to control the combustion process and the air-fuel ratio

supplied to the engine at all times.

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COMBUSTION CHAMBER DESIGN

Combustion chamber design can affect the combustion process, and the

level of emissions. Designs that increase gas flow rate and promotevaporizations, distribute fuel more evenly in the chamber.

Combustion chamber design can affect the

combustion process also, and therefore the

level of emissions.

Designs that increase gas flow rate, and

promote vaporization, distribute the fuel more

evenly in the combustion chamber.

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Quenching of the combustion flame can occur in zones in the combustion

chamber where surface temperatures are low. The flame temperature dropsso low in these areas that the flame goes out, or is quenched. Fuel left

unburned in these zones is then exhausted, as hydrocarbon and carbon

monoxide emissions.

If the spark plug is positioned so that the flame front travels evenly through

the combustion chamber, combustion is more complete.

Gas flow rate, and volumetric efficiency, can be improved by using 2 intake

valves in each cylinder. The effective port opening is increased, and the gas

flow rate increases.

Changing valve timing also alters the combustion process. Reducing valve

overlap reduces the scavenging effect. It also reduces hydrocarbon emission.

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EMISSION CONTROL

A positive crankcase ventilation system flushes vapors from thecrankcase into the intake manifold, to join with the inlet air-fuel mixture.

Once there, the vapors are drawn into the engine for burning.

Early vehicles vented the fuel tank

through the filler cap into the

atmosphere. Some of the fuel inthe tank would vaporize. Some

vapors escaped from the filler

cap, some from the carburetor.

Non-vented filler caps are designed to stop the exit of vapors. A

vacuum relief valve can relieve low pressure in the tank when the

temperature drops. This will also stop the tank from collapsing if its

internal pressure falls below atmospheric pressure.

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A vapor line is connected to the vapor space in the tank, or the liquid

vapor separator. It carries fuel vapors from the tank to a storage volume.

This vapor line can incorporate check valves. If the vehicle is tilted too far

from the horizontal, they stop liquid fuel entering the storage volume.

A storage device is used to store the fuel vapors. The fuel tank breathes

through this storage device. Some vehicles use the engine crankcase.

When the temperature of the fuel in the tank increases, fuel vapors are

forced along the vent line, past a liquid check valve, and into the

crankcase.

When the engine starts, the Positive Crankcase Ventilation systemflushes vapors out of the crankcase and into the intake manifold where it

joins with the inlet air-fuel mixture. Once in the inlet manifold, the vapors

are drawn into the engine where combustion can convert them into

carbon dioxide and water vapor.

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CATALYTIC CONVERSION

Maintaining the stoichiometric ratio is necessary for a catalytic converter to

operate efficiently. It receives all the engine's exhaust gases, andchemically converts remaining pollutants to less harmful substances.

Modern petroleum based fueled

vehicles are fitted with three-way

catalytic converters. 3-way

converters convert hydrocarbonsand carbon monoxide to water and

carbon dioxide, as well as convert

the oxides of nitrogen, nitric oxide

and nitrogen dioxide, back into

harmless nitrogen and oxygen

molecules.

The converter uses two different types of catalysts to reduce the

pollutants: a reduction catalyst and an oxidation catalyst.

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When a nitric oxide or nitrogen dioxide molecule comes into contact with the

coating, it strips the nitrogen atom out of the molecule and retains it. This

frees up the one or two oxygen atoms in the molecule which combine in pairs

to form molecules of oxygen.

The nitrogen atoms bond with other nitrogen atoms that are retained in the

catalyst and form molecules of nitrogen. So two molecules of nitric oxide

become one molecule each of nitrogen and oxygen, or two molecules of

nitrogen dioxide become one molecule of nitrogen and two molecules of

oxygen.

The exhaust gases then flow over the oxidation catalyst in the converter. This

has the effect of reducing any unburned hydrocarbons and carbon monoxide

by oxidizing them over the platinum and palladium coating. This aids the

reaction of the carbon monoxide and hydrocarbons with any remaining

oxygen in the exhaust gas.

Each carbon monoxide molecule combines with an oxygen molecule to make

one less harmful carbon dioxide molecule. Because of strict emission

requirements, vehicles with a 3-way catalytic converter have a feedback

system, called looping.

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CLOSED LOOP

A closed loop is part of a feedback system that collects information on how a

system is operating & feeds that information back to affect how the system isworking.

This vehicle has a cruise control unit to

help it maintain a set speed. When it falls

below it, a computer sends a signal that

moves the throttle linkage and increasesthe fuel reaching the engine, and speed.

It has the opposite effect when the

vehicle exceeds the set speed. A cruise

control unit that continually monitors the

system is called a closed loop system.

A closed loop system in an engine exhaust system can monitor the amount

of oxygen in exhaust gases, to maintain a constant air-fuel ratio.

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REGULATED EMISSION

Air pollutants are classified as either primary or secondary contaminants.

Typically the regulated emissions are carbon monoxide, hydrocarbon, nitrogen

oxides and particulate matter.

A primary air contaminant, such as carbon

monoxide gas, or particles of unburned fuel, is

added to the atmosphere as a by-product of

burning gasoline in an internal combustion

engine.

Secondary emissions are emitted as gasesand can combine with other airborne

substances to form particles once in the

atmosphere.

Air contaminants can be divided into gases and

particulates.Particulates, often referred to as Particulate Matter, or PM, are tiny particles

of solid or liquid suspended in the air. They are graded in a size range from

10 nanometers to 100 micrometers in diameter. Particulates of less than 10

micrometers are dangerous to humans because they can be breathed and

reach the lungs. Smaller particles also tend to stay airborne longer than

larger particles, which settle more quickly.

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CRANKCASE EMISSION CONTROL