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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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Mixture formation in SI engine-DISC
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Mixture formation in SI engine-DISC
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Mixture formation in SI engine
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Mixture formation in SI engine
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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-MM
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Mixture formation in SI engine-MM
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Mixture formation in SI engine-MM
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Mixture formation in SI engine
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Mixture formation in SI engine
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Mixture formation in SI engine
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Mixture formation in SI engine
