Mixture formation for spark ignition

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    MIXTURE FORMATION IN

    SPARK IGNITION ENGINES

    Chapter 5

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    Mixture formation in SI engine

    Engineinduction andfuel systemmust preparea fuel-air mixture thatsatisfiesthe requirements of the engineover its entire operating regime

    Theconstraints of emissionsmay dictate a differentair fuel ratioandalso requirerecycling some exhaust gas. (EGR)

    Relative proportions of fuelandairthat give the above requirementsdepend onengine speedandload.

    Optimum air-fuel ratio for SI engine is that which gives

    1. Required power output

    2. Lowest fuel consumption

    3. Consistent with smooth and reliable operation

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    Air induction systems

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    SI engine fuel supply system

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    Functional requirement of fuel supply system

    The functional objectives for fuel injection systems can vary, beside the central task ofsupplyingfuel to the combustion process

    There are severalcompetingobjectives such as:

    Power output

    Fuel efficiency

    Emissions performance

    Ability to accommodate alternative fuels

    Reliability

    Driveability and smooth operation

    Initial cost Maintenance cost

    Diagnostic capability

    Range of environmental operation

    Engine tuning

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    Mixture formation in SI engine

    There arethree mixture formation techniquesexist forgasoline engines

    I. Carburetor

    II. Port fuel injection - inject the fuel intothe intake manifold

    Depending the position of the injector

    a. Single point injection

    b. Multipoint injection

    III. Direct injection - inject the fuel directlyinto the cylinder

    The primary difference betweencarburetorsandfuel injectionis that fuel injection

    atomizes the fuel by forcibly pumping it through a small nozzle under high

    pressure

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    Mixture formation in SI engine

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    Mixture formation in SI engine - Carburetor

    The process of formation of acombustible fuel-air mixturebymixing the

    proper amount of fuel with airbeforeadmission to engine cylinderis called

    carburetionand the device which does this job is called acarburetor.

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    Mixture formation in SI engine - Carburetor

    The process ofcarburetionis influenced by

    The velocity of incoming air

    Thevelocity of the air streamatthe pointwherethe fuel is injectedhas to be

    increased. This is achieved by introducing a venturi section in the path of the air.

    The vaporization characteristics of the fuel

    Will require a volatile fuel for quick evaporation and mixing with air

    The temperature of the incoming air and

    Higher atmospheric air temperature increases the vaporization of fuel and

    produces a more homogeneous mixture.

    The design of the carburetor

    Proper design of carburetor elements alone ensures the supply of desired

    composition of the mixture under different operating conditions of the engine.

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    Mixture formation in SI engine - PFI

    Thepoint or location of fuel injection is one way to classify a gasoline

    injection system.

    A single-point injection system, also called 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 .

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    Mixture formation in SI engine - PFI

    Amulti-point injection system, also called port injection, has an injector in the

    port (air-fuel passage) going to each cylinder.

    Gasoline is sprayed into each intake port and toward each intake valve.

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    Mixture formation in SI engine - PFI

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    Mixture formation in SI engine - PFI

    Electronic control unit (ECU) is a generic term for anyembedded system that controls one or more of theelectricalsystems or subsystems in a motor vehicle.

    An engine control unit (ECU) is a type of electronic controlunit that determines the amount of fuel, ignition timing andother parameters an internal combustion engine needs to keeprunning.

    It does this by reading values from multidimensional mapswhich contain values calculated by sensor devices monitoringthe engine.

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    Mixture formation in SI engine-PFI

    Most port injection todayoccurs withthe intake valve closed

    Evaporation and mixture formation outside the cylinder

    preventslarge dropletsfromdirectly entering the combustionchamber

    large drops enter the cylinder, may be deposited on the walls,

    due to the low pressure and density of the gas, and increaseHC emissions.

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    Mixture formation in SI engine - PFI

    Theclassical gasoline injectortoday is thesolenoid injector

    The majority of injectors used these days are low-and medium-pressure injectorswith injection pressures in the range of0.5 to 1.5MPa

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    Mixture formation in SI engine - PFI

    The injectors can survive the excessive temperature and pressure of

    combustion by using the fuel that passes through it as a coolant

    The electronic fuel injector is normally closed, and opens to inject

    pressurized fuel as long as electricity is applied to the injector's

    solenoid coil.

    When the injector is turned on, it opens, spraying atomized fuel at

    the combustion chamber .

    Depending on engine operating condition ,injection quantity will

    vary .

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    Mixture formation in SI engine - PFI

    Injectorsforport fuel injectiondiffer mainly in thetype of nozzleused togenerate the desired spray shape

    The pintle-type injector produces a cone-shaped spray, the single-holeinjectorapencil-shaped spray, andthe multi hole plate injectorproducesmultiple spraysaccording to the number of holes

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    Mixture formation in SI engine - DFI

    Compared tothe conventional diesel injection near top dead center, the

    direct injection of gasolinemay occur already during theinduction strokein

    thecase of homogeneous mixture formation(full load), orvery late during

    the compression strokein the case ofstratified charge operation(part load)

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    Mixture formation in SI engine -DFI

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    Mixture formation in SI engine -DFI

    Among other techniqueslike cylinder cutoffor theapplication of variable

    valve trainsthedirect injection of gasolineis the measure with thehighest

    potentialtoreduce fuel consumptionand thus alsoCO2-emissions

    Compared to a similar PFI engine, about 1525% reduction of fuelconsumption atpart loadare theoretically possible

    Depending on the operating point of the engine, the direct injection of

    gasoline offers different advantagescompared to thePFI technique.

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    Mixture formation in SI engine -DFI

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    Mixture formation in SI engine

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    Mixture formation in SI engine -DFI

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    Mixture formation in SI engine-DFI

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    Mixture formation in SI engine-DFI

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    Mixture formation in SI engine-DFI

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    Mixture formation in SI engine-DFI

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    Mixture formation in SI engine-DFI

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    Mixture formation in SI engine-DIF

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    Mixture formation in SI engine-DFI

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    Mixture formation in SI engine-DISC

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    Mixture formation in SI engine-DISC

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