The process of preparing combustible air fuel mixture in the petrol engine is called carburetion. A device which does this process is called carburetor (Is a mixing device).

1. AUTOMOTIVE FUEL SYSTEM
PART II
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2. Carburetion & Carburetor
 The process of preparing combustible air fuel
mixture in the petrol engine is called carburetion.
 A device which does this process is called
carburetor (Is a mixing device).
 The carburetor atomizes the fuel and mixes it with
air in correct proportions according to the engine
operating conditions.
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3. Atomization
 Atomization is the process of breaking up a liquid
into tiny particles or droplets.
 When a liquid is atomized, the droplets are all
exposed individually to the air.
 For this reason, atomization greatly increases
evaporation by increasing the exposed surface area
of the liquid.
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4. Requirements of Carburetors
The requirements of a good carburetor as follows:
 Easy engine starting, particularly under low
ambient conditions
 Ability to give full power quickly after starting
the engine
 Smooth engine operation at various loads
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5. …cont
 Quick acceleration of the engine
 Developing sufficient power at high engine
speeds
 Good fuel economy
 Ensuring full torque at low speeds
 Simple and compact in construction
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6. Components of carburator
A Simple (or) Elementary Carburetor
 It consists of a float chamber, venturi, discharge jet
or main nozzle, choke valve and throttle valve.
Float Chamber (or) Float Bowl:
 It consists of a float and needle supply valve.
 This system maintains a constant petrol level in the
float chamber.
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7. …cont
Venturi (or) Choke Tube:
 Venturi is a tube of decreasing cross section which
reaches a minimum at the throat (restricted section).
 During suction stroke air is drawn through the venturi.
 The air passing through the venturi increasing in velocity
and the pressure in the venturi throat decreases.
 Main Nozzle (or) Discharge Jet:
 This is a part that directs fuel from the carburetor bowl to
the throat of the venturi.
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8. …cont
Throttle Valve:
 The throttle valve is a round disc located between the
venturi and the engine inlet of the induction system.
The throttle is connected to the accelerator pedal by
a flexible cable.
 Depressing the accelerator pedal opens the throttle
valve and permits an increased amount of air fuel
mixture to reach the engine cylinders.
 This will increase the power output of the engine and
hence the vehicles speed.
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9. …cont
Choke Valve:
 It is a valve which is used for controlling the
amount of air passing to the venturi when starting a
cold engine
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10. …cont
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11. Types of Carburetors (reading assignment)
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 Based on the direction of air flow
 Based on the venturi & depression
 Number of Barrel

12. Carburetor Systems
 The fixed venturi carburetor has six systems and
several devices that provide the correct air-fuel
mixture for different operating conditions. These
include:
1. Float System
2. Idle and Low speed System
3. Main-Metering System
4. Power System
5. Accelerator Pump System
6. Choke System
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13. …cont
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1. Float System
 The float system acts as a fuel reservoir to meet
engine demand.
 The float is pivoted on a pin in the float boss. It rises and
falls with the fuel level in the float bowl. The small metal
tang integrated in the plastic float supports the
float valve, aka float needle.
 As the fuel in the float bowl rises, the float valve is
pushed into the valve seat, until it's high enough to shut
off the fuel flow to the bowl.
 The level in the bowl drops lowering the float which pulls
the float valve from it's seat, and fills again.

14. …cont
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2. Idle and Low Speed System
a. Idle System
 When the throttle valve is closed or only slightly open, only a
small amount of air can pass through the air horn. With low
air speed, there is very little vacuum in the venture. No fuel
will feed from the fuel nozzle. To supply fuel during idle, an
idling system is built into the carburetor. This system has an
opening in the side of the carburetor below the throttle valve.
This hole is called idle port.
 The port is connected by a passage to the float bowl or float
chamber. In addition to the idle port, there is an idle-mixture
screw located behind the idle port to control the amount of
mixture discharge through it.

15. …cont
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b. Low Speed System
 If the throttle valve is open just a little for low speed,
the edge of the throttle valve moves past the idle
port. More air can flow past the throttle valve now,
reducing the vacuum in the intake manifold. So less
fuel flows from the idle port. However, the low
speed port now comes into action. The throttle valve
has moved past and above the low speed port.

16. …cont
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 The vacuum in the intake manifold can act on the
low speed port as well as on the idle port. Both ports
discharge fuel to maintain required amount of air-
fuel mixture for low speed.

17. …cont
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3. Main-Metering System
 If the throttle valve is opened farther, more air will
flow through. This means that there will be less
vacuum in the intake manifold. As a result, the idle
and low speed ports stop discharging fuel.
 However, with more air flow, there is a vacuum in
the venturi. This causes the main nozzle or discharge
jet to discharge fuel. Therefore, fuel gets mixed with
the air passing through.

18. …cont
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4. Power System
 When a driver wants full power, the accelerator
pedal is pushed to the floor (i.e. to the maximum
position). This causes the throttle valve to open wide.
 Another system in the carburetor comes into action
to additional fuel. This system is called power
system. It can be a piston type or metering rod type.

19. …cont
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5, Acceleration Pump System
 There is another operating condition that needs a rich
mixture. This condition occurs when the accelerator
pedal is pushed down suddenly to increase the
speed.
 When the throttle valve is suddenly opened, although the
amount of air drawn into the carburetor immediately
increases, more gasoline is not immediately delivered by
the main nozzle (because gasoline is heavier than air).
For this reason, the air-fuel mixture will become
temporarily lean during acceleration unless this is
prevented by the implementation of an acceleration
circuit.

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 When the accelerator pedal is stepped on, the
gasoline in the pump cylinder is put under pressure
by the pump plunger. This causes the gasoline to
push up outlet steel ball and to be sprayed into
venturi by the pump jet.
 When the accelerator pedal is released, the pump
plunger rises, and the outlet steel ball blocks the
outlet. The inlet steel ball then unblocks the inlet,
allowing gasoline to be drawn up into the pump
cylinder from the float chamber.

21. cont
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6. CHOKE SYSTEM
 When the engine is cold, the fuel tends to condense into
large drops in the manifold, rather than vaporizing.
 By supplying a richer mixture (8:1 to 9:1), there will be
enough vapor to assure complete combustion.
 The carburetor is fitted with a choke system to provide
this richer mixture.
 The choke system provides a very rich mixture to start
the engine and to make the mixture less rich gradually,
as the engine reaches operating temperature. The two
types of choke systems are the manual and automatic

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