Carburation System

8/12/2019 Carburation System

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Vehicle Engine

TechnologyTheory of Carburetion



Carburetion In SI engines the torque and power are

regulated by controlling the quantity ofcombustible mixture inducted into the

cylinders.



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



Carburetion



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



Carburetion Mixture requirements

Steady state operation Idling – rich mixture

Cruising – lean mixture

High power – rich mixture

Transient operation

Starting – rich mixture Acceleration – rich mixture



Carburetion



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



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



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.



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



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



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



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



Carburetion



Carburetion Air-Fuel Ratio



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



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.



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



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



Modern Carburettor Idling System

Active during starting, idling and low speedoperation

Goes out of action beyond 20% throttle opening



Modern Carburettor Idle system



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



Modern Carburettor Compensation of main metering system

Air bleed system Use of auxiliary air valve or port

Compensating jet

Back suction control



Modern Carburettor

Air Bleed System



Modern Carburettor Auxiliary valve type



Modern Carburettor

Compensating Jet



Modern Carburettor Back suction control



Modern Carburettor Enrichment System or

Economizer Needle type

Piston type

Manifold pressure operated

Modern Carburettor



Modern Carburettor

Needle Type Economizer



Modern Carburettor Piston Type Economizer



Modern Carburettor Manifold Pressure operated economizer



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



Piston Type Accelerator Pump

Modern Carburettor



Diaphragm type accelerator pump



Modern Carburettor Altitude Compensation

Uses an aneroid whichexpands due to

decreased air pressure

Decreases the poweroutput



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



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



Carburettor types Cross draught

Horizontal mixing tube Float chamber place on the sides

Resistance to flow is reduced



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



yp

SU Carburettor



Carburettor Types



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



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



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



Petrol Injection Systems Types

Throttle body injection or TBI Multi-point fuel injection or MPFI

Gasoline direct injection or GDI



Petrol Injection Systems



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



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






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



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


Ti d ti l i j ti



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




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




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




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



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



D Jetronic System

K Jetronic s stems



K Jetronic systems

K stands for Kontinuierlich", meaning

“continuous” Fuel is injected continuously

Also called as Continuous Injection System

K Jetronic Injection system



K-Jetronic Injection system

K Jetronic system



K Jetronic system

K Jetronic system



K Jetronic system

KE Jetronic system



KE Jetronic system

L Jetronic Systems



L Jetronic Systems

L stands for “Luft” meaning ‘air’

It uses air flow sensor to measure the air



Mono Jetronic Systems



Mono Jetronic Systems

Date post:	03-Jun-2018
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Carburation System

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