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  • 8/14/2019 26 Tarun Singh



  • 8/14/2019 26 Tarun Singh



    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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    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 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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    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


    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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    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


    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


    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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    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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    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


    Secondary emissions are emitted as gasesand can combine with other airborne

    substances to form particles once in the


    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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