carburetor for pdf

Internal Combustion EnginesBy

Dr. Amr Ibrahim

© Dr. Amr Ibrahim, 2010, all rights reserved. The commercial use of these slides without permission is strictly prohibited.

I will teach you the following topics:• fuel systems in gasoline (petrol) engines

o carburetors (carburettors)

o fuel injection systems

• ignition systems in spark-ignition engines

• chemical kinetics

• flames

Combustion requirements:

• fuel

• oxygen (or air)

• the air to fuel ratio must lie

in the flammability range

• heat source (such as spark

or hot air)

What are the main functions of fuel systems?

• transfer the fuel from the tank to the mixing point where it ismixed with air

• meter the fuel quantity to form an appropriate air-fuel ratiothat can satisfy engine operating conditions

• break the liquid fuel into fine droplets (atomization) tovaporize the fuel efficiently and help good mixing with air

Hydrocarbon fuel + air → combus on products

Combustion products include CO2, H2O, O2, CO,

N2,...etc

• The minimum quantity of air which is used to

burn the fuel completely (i.e. without producing

O2 in the combustion products) is called

theoretical or stoichiometric air.

•If the actual mass of air is more than the

stoichiometric mass of air, then, the fuel-air

mixture is called lean mixture (lean

combustion).

•If the actual mass of air is less than the

stoichiometric mass of air, then, the fuel-air

mixture is called rich mixture.

For gasoline: (A/F)th ≈ 14.7 kgair/kgfuel

For methane: (A/F)th ≈ 17.1 kgair/kgfuel

If (A/F)act > (A/F)th → lean mixture

If (A/F)act = (A/F)th → stoichiometric mixture

If (A/F)act < (A/F)th → rich mixture

• Relative air-fuel ratio= excess air factor= λ

th

act

FAFA

)/()/(

• Fuel air equivalence ratio = φ = 1/λ

Air- fuel mixture formation in gasoline engines:• For conventional engines: air and fuel are initially mixed outside the engine cylinders by using:• Carburetors (carburettors) or• Fuel injection systems

•For some modern engines (Direct Injection “DI”

engines): gasoline is injected directly inside

engine cylinders.

The function of accelerator-pedal in gasoline engines:

• The accelerator pedal ismechanically connected to athrottle valve.

• The throttle valve controls the amount of the air received by engine cylinders. Watch video

• The throttle valve is located at the entrance of theintake manifold

Fuel metering principle:

• The change of air-fuel ratio (A/F) in aconventional gasoline engine is limited comparedto diesel engines. This means generally:

When air flow increases, fuel flow increasesWhen air flow decreases, fuel flow decreases

•The fuel system provides the engine with thecorrect amount of fuel suitable for the amount ofair flow and engine operating conditions.

Engine operating modes:

Idling (no load), throttle valve is closed Part load, throttle valve is partially open

Full load, throttle valve is fully open

Carburetors

© Dr. Amr Ibrahim, 2010, all rights reserved. The commercial use of these slides without permission is strictly prohibited.

Carburetor with an air filter attached

Carburetor with no air filter

Fuel supply system:

• Fuel tank

• Fuel filter

• Pump

• Carburetor

• Fuel lines

• Charcoal canister

Fuel filter

Charcoal canister

Fuel pumps:

Mechanical fuel pump(diaphragm type)

Electric fuel pump

Mechanical fuel pump (diaphragm type):Watch video

Carburetor main parts:• Air horn, which contains a choke valve •Main body, which contains the float system and a venturi.• Throttle body, which contains the throttle valve

The fuel is delivered to the float chamber via a fuel pump.

The float system consists of:• Float chamber• Float• Needle valveThe float system maintains the fuel level constant

Main metering system (main nozzle):As the thro le valve opens wider, air flow ↑, venturi throat vacuum ↑, fuel flow ↑

VA ρmair

const2gV

γp 2

Throttle valve

fm ,p As

Idling (no load) operation:

•The throttle valve is closedduring engine idling• Therefore, the vacuum inthe intake manifold (belowthe throttle valve) is high• the vacuum in the intakemanifold decreases as thethrottle valve opens

• The vacuum in the intake manifold draws the air-fuelmixture via the idle port• The amount of air-fuel mixture is controlled by anidle-mixture screw

Low speed operation:

Air-fuel mixture is drawn from both the idle and low speed ports

The photo shows both the idle and low speed ports in addition to the idle mixture screw

• As the throttle valve opens further , the vacuum near the throttle valve decreases, and the flow from the idle and low speed ports stop• The fuel is now supplied from the main nozzle

Full load (full power) operation:Rich fuel-air mixture is needed in order to get the maximum power

Extra fuel is supplied by the power system

Power valve is closed Power valve is open

Power valve is closed Power valve is open

The power system

Engine acceleration:• While the accelerator pedal isbeing depressed, the throttlevalve opens suddenly and moreair rushes to the engine

• To prevent that, extra fuel must be injected during acceleration for propbaly1 second

• The increase of fuel flow may take milliseconds.Also, the sudden increase in manifold pressuretemporarily increases the condensation of fuel out ofthe mixture. That might lead to an excessive leanmixture that could stop the engine

• The fuel enrichment is provided by the accelerator pump

Pump plunger is down Pump plunger is up

The accelerator pump

Engine cold start:

• With the carburetor andengine cold, only part of thefuel evaporates• Some fuel condenses onthe cold intake manifoldwalls•Extra fuel is needed so enough will vaporize to produce a combustible mixture

•To start a cold engine, the carburetormust deliver a very rich mixture

•The choke valve produces the required fuel enrichment during engine cold start

The choke valve opens during engine normal operation

The choke valve is closed during engine cold start

• Air flow decreases• The vacuum at theventuri throat increasesleading to fuel dischargefrom the main nozzle• Rich fuel-air mixture isproduced

Closing the choke valve during engine cold start leads to:

• The choke valve must be fully open during normal engine operation• The choke valve is usually controlled automatically

A thermostatic spring (bi-metal element) controls the choke valve movement

• Other chokes may use the heat from enginecoolant or exhaust manifold

• Some automatic chokes use electric heating to operate the thermostat

Multiple-barrel carburetors:Two- barrel (and sometimes four-barrel) carburetors can be used with multi-cylinder engines

Two-barrel carburetors can be designed as follows:• each barrel supplies the air fuel mixture for half ofthe engine cylinders (2 carburetors in 1)• staged carburetor: the primary barrel supplies allcylinders until the primary throttle opens more than45 or 60 degrees, then, the secondary throttle opens.

The use of staged carburetors can improve themedium to high speed engine operation

Fuel-air ratio requirements for gasoline engines:Rich mixture is needed for:• Engine cold start and warm-up• Idling (no load)• Full load (full power)• AccelerationLean mixture is needed for:Part-load operationStoichiometric mixture is needed for:Idling and part-load operation for engines fitted with a three-way catalyst

HC = unburned hydrocarbonsNOx= nitrogen oxides

1.Nitrogen oxides (NOx) arereduced to N2 and O2

2. CO is oxidized to CO2

3.Unburned hydrocarbon (HC)is oxidized to H2O and CO2

Three-way catalyst is used to reduce engineemissions:

The three-way catalyst operates efficiently when the air- fuel mixture is adjusted at almost stoichiometriccondition.