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Vehicle Engine
TechnologyTheory of Carburetion
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Carburetion In SI engines the torque and power are
regulated by controlling the quantity ofcombustible mixture inducted into the
cylinders.
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Carburetion Air-Fuel mixtures
Chemically correct mixture
Enough air for complete combustion
Rich mixture
Less air than the stoichiometric requirement
Lean mixture
More air than the stoichiometric requirement
Air fuel mixture too lean or too rich will not be
able to sustain the flame propagation
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Carburetion Mixture requirements
At full load or wide open throttle
Complete utilization of inducted air to produce best
power
At part load condition
Effective utilization of fuel at any speed
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Carburetion Mixture requirements
Steady state operation Idling – rich mixture
Cruising – lean mixture
High power – rich mixture
Transient operation
Starting – rich mixture Acceleration – rich mixture
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Carburetion Idling range
No load and nearly closed throttle Requires rich mixture
Exhaust gas dilution is more
Cruising range Exhaust gas dilution is insignificant
Power range
Provide best power
Prevent overheating of exhaust valve
Enrichening of mixture reduces the cylinder temperature
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Carburetion Starting
In cold conditions high proportions of fuelcondenses on the cold walls of the manifold
Acceleration Sudden opening of throttle increases the flow of
air more rapidly than the fuel
Sudden opening increases the pressure insidethe manifold, temporarily increasing the
condensation of fuel
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Carburetion Definition
The process of formation of a combustible fuel-airmixture by mixing the proper amount of fuel with
air before admission to engine cylinder.
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Carburetion Process involved
Metering the fuel in proportion to the air flowing
Atomizing or breaking the fuel into fine droplets to
assist evaporation
Distributing the evaporated fuel uniformly to form
homogeneous mixture
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Carburetion Factors affecting carburetion
Engine speed As speed increases the induction time decreases
The velocity of the air has to be increased accordingly
Vaporization characteristics of fuel
Fuel should have good volatility or evaporation
characteristics
Presence of lower hydrocarbons increases volatility
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Carburetion Inlet air temperature
Higher air inlet temperatures increases thevaporization of fuel
Higher temperatures also reduces the volumetric
efficiency and thus the power output Carburettor design
Proper design ensures uniform distribution of mixture
to the cylinders
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Carburetion Induction of mixture
The pressure depression created during intakestroke forces the mixture into the cylinder
The pressure depression depends on
Speed of engine
Size and discharge coefficients of the aperture inside
the manifolds
Throttle valve opening – major controlling factor
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Carburetion Air-Fuel Ratio
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Carburetion If (p1- p2 )is less than gzρf – No fuel flow
As the air flow increases (p1- p2 ) increases and when (p1- p2 ) > gzρf fuel flow begins
At low loads the mixture becomes leaner : the engine requires the
mixture to be enriched at low loads
At intermediate loads, the mixture becomes slightly rich as the air
flow increases.
As the air flow approaches the max wide open throttle value, mixture
becomes progressively rich
Fuel flow rate increases more rapidly than the air flow rate
At high altitudes, the density of air is low – mixture becomes richer
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Carburetion Drawbacks of simple Carburettor
Cannot provide the variation in mixture ratio which theengine requires over the complete load range at any
given speed.
Cannot compensate for transient phenomena in the
intake manifold.
Cannot provide a rich mixture during engine starting
and warm-up.
Cannot adjust to changes in ambient air density due
to changes in altitude.
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Modern Carburettor The Main metering system
Constant lean or stoichiometric mixture over 20to 80% of the air flow range
An Idle system
To meter the fuel flow at idle and light loads toprovide a rich mixture
An enrichment system
The engine can get a rich mixture as WOTconditions is approached and maximum powercan be obtained
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Modern Carburettor An accelerator pump
Additional fuel can be introduced into the engineonly when the throttle is suddenly opened
A choke
To enrich the mixture during cold starting andwarm-up to ensure that a combustible mixture isprovided to each cylinder at the time of ignition
Altitude compensation
To adjust the fuel flow which makes the mixture
rich when air density is lowered
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Modern Carburettor Idling System
Active during starting, idling and low speedoperation
Goes out of action beyond 20% throttle opening
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Modern Carburettor Idle system
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Modern Carburettor Main Metering System
Controls the fuel feed for cruising and full throttleoperations
Functions
To proportion the fuel-air mixture
To decrease the pressure at the discharge nozzle
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Modern Carburettor Compensation of main metering system
Air bleed system Use of auxiliary air valve or port
Compensating jet
Back suction control
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Modern Carburettor
Air Bleed System
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Modern Carburettor Auxiliary valve type
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Modern Carburettor
Compensating Jet
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Modern Carburettor Back suction control
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Modern Carburettor Enrichment System or
Economizer Needle type
Piston type
Manifold pressure operated
Modern Carburettor
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Modern Carburettor
Needle Type Economizer
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Modern Carburettor Piston Type Economizer
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Modern Carburettor Manifold Pressure operated economizer
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Modern Carburettor Accelerating System
Sudden acceleration results in temporary leanmixtures
Accelerating system discharges additional
quantity of fuel into the Carburettor
Types
Piston type
Diaphragm type
Modern Carburettor
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Piston Type Accelerator Pump
Modern Carburettor
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Diaphragm type accelerator pump
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Modern Carburettor Altitude Compensation
Uses an aneroid whichexpands due to
decreased air pressure
Decreases the poweroutput
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Carburettor Types Up draught
Air enters at the bottom and leaves at top Must lift the sprayed fuel droplet
Restricts the size of mixing tube and
throat
If not properly designed it provides only
lean mixtures
Unable to supply mixture at higher
engine speeds
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Carburettor types Down draught
Air enters at the top and leaves at thebottom
Makes use of the gravity force
Mixing tube and throat can be made
large
Can be used in high engine speeds
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Carburettor types Cross draught
Horizontal mixing tube Float chamber place on the sides
Resistance to flow is reduced
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Carburettor types Constant choke carburetor– Constant air and fuel flow areas
Need compensating devices
Depression being varied as per the demand
Solex, Carter and Zenith carburetors
Constant vacuum carburetor or Variable choke carburetor
S.U carburetors (Jeeps)
Air and fuel passage are varied with different engine
speeds
Velocities of the fuel and air remains constant in this
system
Carburettor types
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yp
SU Carburettor
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Carburettor Types
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Petrol Injection Systems Need for Injection systems
Maldistribution of fuel in the
multi-cylinder engines
Need of careful control of
the air-fuel ratio for catalytic
converter To improve the fuel
economy
To avoid freezing in the
carburettor To avoid backfiring of
mixtures
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Petrol Injection Systems Advantages
Uniform Air-fuel mixture distribution Highly accurate Air/fuel ratio control
Superior throttle response
Excellent fuel economy Improved emissions control
High volumetric efficiency
Reduced manifold wetting
Improved warm and cold engine operations
Improved fuel atomization
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Petrol Injection Systems Limitations
High initial cost of equipment Increased care and attention/more servicing problems
Special servicing equipment is necessary to diagnose fuel
injection faults and failures
Special knowledge of mechanical and electrical systems
needed to diagnose and rectify faults
Need of very careful filtration
More electrical/mechanical power needed to drive fuel
pump and/or injection devices
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Petrol Injection Systems Types
Throttle body injection or TBI Multi-point fuel injection or MPFI
Gasoline direct injection or GDI
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Petrol Injection Systems
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Petrol Injection Systems Single Point Injection Systems
Single injector placed over thethrottle body
Placed upstream of the throttle
Low injection pressure – 3 to 6 bar
Low cost
Problems
Fuel tends to condense on the
walls
Accurate distribution of mixtures to
cylinders not possible
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Petrol Injection Systems Multi Point Injection Systems
Individual injectors placed over the intake valve orthe branching pipes of each cylinder
Accommodation of injectors complicates the
manifold design Some heat conducted from the cylinder may
assist evaporation of fuel
More accurate metering and quicker response
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Petrol Injection Systems
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Injection Strategies Continuous injection
Low fuel line pressures of about 2 – 2.5 bars isused
Three fourth of fuel delivered per cycle is stored in
intake manifold One-fourth enters the cylinder directly
Injection duration varies from 10
Injection Strategies
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Continuous injection Low fuel line pressures of about 2 – 2.5 bars is used Three fourth of fuel delivered per cycle is stored in intake
manifold
One-fourth enters the cylinder directly
Injection duration varies from 10° CA (light load) to 300° CA(rated load)
In multi cylinder engines the injection may be divided in to two ormore groups
Injection Strategies
Ti d ti l i j ti
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Timed or sequential injection
Fuel is sprayed at regular intervals with constant fuel dischargepressure
Amount of fuel discharged is controlled by the time period the
injector nozzle valve is open
Fuel is injected only when the relevant inlet valve opens
Reduces the risk of mixture drawn off into an adjacent cylinder
Regulation of air fuel ratio is extremely accurate
I j ti St t i
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Injection Strategies
Air flow metering
Speed Density method Engine speed and Inlet air pressure and temperature
are sensed
Volume Density method The throttle position and the inlet air pressure and
temperature or sensed
Direct Air flow measurement method The amount of air is measured using air flow sensor
I j ti St t i
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Injection Strategies
Direct Air flow measurements
Dependence of volumetric efficiency on speedand exhaust gas back pressure are taken care
Improved idling stability
Automatic adjustments for combustion chamberdeposit build up and change in valve adjustments
P t l I j ti S t
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Petrol Injection Systems
Jetronic is the trade name for petrol injection
systems Designed by Bosch
Some variants
D Jetronic injection systems
K Jetronic injection systems
L Jetronic injection systems
Mono Jetronic injection systems
D J t i S t
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D Jetronic Systems
D stands for “Druck” in German meaning “pressure”
Also called as Manifold Pressure Control system
The inlet manifold vacuum is sensed
Air/ fuel mixture is controlled by controlling fuel injection duration
Used in
Mercedes-Benz: 250E, 280, 300, 350, 450 Porsche: 914
Volkswagen: Type 3 & 4
Volvo: 1800E, 1800ES, 142, 144, 164E BMW: 3.0Si (early types)
Jaguar XJ-S, XJ12
D J t i S t
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D Jetronic System
K Jetronic s stems
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K Jetronic systems
K stands for Kontinuierlich", meaning
“continuous” Fuel is injected continuously
Also called as Continuous Injection System
K Jetronic Injection system
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K-Jetronic Injection system
K Jetronic system
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K Jetronic system
K Jetronic system
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K Jetronic system
KE Jetronic system
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KE Jetronic system
L Jetronic Systems
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L Jetronic Systems
L stands for “Luft” meaning ‘air’
It uses air flow sensor to measure the air
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Mono Jetronic Systems