2 Carburettor

7/24/2019 2 Carburettor

http://slidepdf.com/reader/full/2-carburettor 1/42

S.I. Engine

Mixture Preparation

Carburetion



Mixture Requirements

Engine induction and fuel system must prepare a fuel-airmixture that satisfies the requirements of the engine overits entire operating regime.

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

1. required power output

2. with lowest fuel consumption3. consistent with smooth and reliable operation

In Practice

1. The constraints of emission control may dictate a different

air/ fuel ratio2. The recycling of a fraction of the exhaust gases(EGR) intothe intake system

Relative proportions of fuel and air that give the aboverequirements depend on engine speed and load.



Mixture requirements for various conditions

• Steady running condition– Continuous operation at a givenspeed and power output with normal engine temperatures

• Transient operation– starting, warm-up and acceleration

• The evaporation of the fuel may be incomplete, thequantity of liquid fuel in the inlet manifold may be

increasing or decreasing• The distribution of fuel to various cylinders may be

different



Steady state condition

• Idling condition: A/ F 7:1 ( Mixture must be enriched)

• To prevent the dilution of fresh charge

• The richening of mixture increases the probability of contact

between fuel and air particles• Cruising range: A/ F ratio 16:1

• Exhaust gas dilution problem is insignificant

• Power range: A/F 12:1

•

To prevent the overheating of exhaust valves & the area nearit



Transient conditionStarting & warm up requirements: A/ F 5:1

•The ratio of evaporated fuel in the air may be too lean toignite (much of fuel in liquid form)

• As the engine warm up the amount of evaporated fuelincreases and hence the mixture ratio progressively madeleaner to avoid too rich evaporated fuel – air ratio

• Too high or too low volatility, both create difficulties inoperation

Acceleration: mixture must be enriched

• Liquid fuel lags behind

• Temporary lean mixture causing the engine to misfire and

a temporary reduction in power output



Factors affecting Carburetion

• Engine speed. In a 4-stroke engine running at 3000

rev/ min, the intake will take about 10 ms during whichthe fuel has to evaporate, mix with air and be inductedinto the engine.

• Vaporization characteristics of the fuel. Will require a volatile fuel for quick evaporation and mixing with air.

• The temperature of the in coming air . Must be highenough to be able to evaporate the fuel and yet not toohigh as to reduce mass of fresh charge.

• Design of the carburetor . This will help in properintroduction of fuel into the air stream and provide proper

distribution of the mixture to the various cylinders.



Air-fuel ratio neglecting compressibility of air



Air – fuel ratio provided by a simple carburetor

• If (p1-p

2)is less than gz

f– No fuel flow

• As the air flow increases (p1- p2) increases and when (p1-

p2) > gz f 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



Deficiencies of Elementary Carburetor

• Thus, the elementary carburetor cannot provide the

variation in mixture ratio which the enginerequires over

the complete load range at any given speed.

• The elementary carburetorcannot compensate for transient

phenomena in the intake manifold.

• It alsocannot provide a rich mixtureduring engine starting

and warm-up.

• Itcannot adjust to changes in ambient air density due to

changes in altitude.



Modern Carburetor Design

• The Main metering system: Constant lean or

stoichiometric mixture over 20 to 80% of the air flow range• An Idle system: To meter the fuel flow at idle and light

loads to provide a rich mixture

• An enrichment system: The engine can get a rich mixture

as WOT conditions is approached and maximum powercan be obtained

• An accelerator pump: Additional fuel can be introducedinto the engine only when the throttle is suddenly opened

• A choke: To enrich the mixture during cold starting and

warm-up to ensure that a combustible mixture is providedto each cylinder at the time of ignition

• Altitude compensation: To adjust the fuel flow whichmakes the mixture rich when air density is lowered



Main metering system• Air bleed jet-

•

Use of compensating jet – increasing flow of air throughfuel passage

• Use of emulsion tube – air bleeding alone

• Back suction control or pressure reduction in float chamber

• Use of an auxiliary air valve or port – admits additional air



Compensating Jet



Acceleration Pump System



Power Enrichment System



Some manufacturers make the choke operate automatically by

means of a thermostat such that when the engine is cold the choke

is closed by a bimetallic element

Fig. Idling systemFig. Choke system



Boost Venturi

• The discharge edge of the boost venturi is located at the throat of

the main venturi• To reduce the pressure loss across the total venturi system, without

increasing the height of the carburetor

• High depression in the region of fuel nozzle

• Fuel is introduced at boost venturi throat

• Fuel is better atomized in the smaller boost venturi with its higherair velocity

• Velocity of air is as high as 200 m/ s



Multiple – Barrel Carburettor

• Each of multiple barrel contains a fuel jet, a venturi tube , an

idling system, a choke and a throttle• Common float chamber and accelerating pump

• Cars with six or more cylinders – each venturi supplies the air-

fuel mixture to half of the cylinder

•

Charge of air-fuel mixture distributed in a uniform manner • In some designs, half of the multi barrel carburetor operates as a

unit during light load and cruising speeds. (Primary unit)

• The other half of the carburetor acts a supplementary unit during

top speed and full throttle operation. (Secondary unit)

• The throttle of secondary unit remains closed at lower engine

speeds.

• No idle and choke systems in secondary unit

• The secondary barrel – larger cross sectional area



Dual Barrel carburettor

Carter 4 barrel

Chevrolet 4 barrel



Altitude compensation



Carburettor Types

• Downdraught -High engine speed

• Updraught– It must lift the sprayed fuel droplet by airfriction

• Designed for small mixing tubes and throat

• Cross draught – Low resistance to flow



Types of carburetors

• 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 enginespeeds



Constant vacuum carburetor



Anti – Dieseling system

• Dieseling : A SI engine sometimes continuous to run for a

very small period after the ignition is switched off.• Engine idling speed set to high

• Engine overheating

• Too high spark plug heat range

•Increase in compression ratio due to carbon deposits

• Anti-dieseling system: Has a solenoid valve operated idlingcircuit



Carburetor Icing

• At 5ºC & High Humidity

• At throttle valve, idle port & venturi• Add additives like alcohol

• Preheat the intake air



Carburettor size• Carburetor size must be matched to flow for displacement

and rpms that engine is used. Changing either will requirean adjustment to the carburetor to match.

• Size of venturi throat – 30 to 35 mm

• Jet diameter – 1/ 16 of venturi

•

Pr drop – 50mm of HgCarburettor Throttle Bore Diameter

•D = K x SQRT( C x N )

•D is throttle bore diameter, in millimeters

•K is a constant ( approx. 0.65 to 0.9, derive fromexisting carburetor bore)

•C is cylinder displacement, in liters

•N is RPM at peak power



FEEDBACK CONTOL CARBURETOR

2 Carburettor

Documents

Transcript of 2 Carburettor