A simple carburetor can only provide the correct air-fuel ratio at one throttle position. It is incapable of meeting the engine's demands under varying operating conditions like load and speed. To address this, modern carburetors add additional systems like an idling system, auxiliary port system, power enrichment system, and accelerating pump. The purpose of a carburetor is to atomize fuel, mix it with air, and deliver the air-fuel mixture to the engine under different operating conditions.

1. 140050119028 Kanani Manthan
Faculty,
Subhasis Sarkar

2.  Simple Carburators
 Automobile Carburators

3.  A simple carburetor as described suffers from the fact that it
provides the required air-fuel ratio only at one throttle
position.
 At all other throttle positions, the mixture is either leaner or
richer depending on whether the throttle is opened less or
more.

4.  A simple carburetor is capable to supply a correct air-fuel
mixture to the engine only at a particular load and speed.
 In order to meet the engine demand at various operating
conditions, the following additional systems are added to
the simple carburetor.
 idling system
 auxiliary port system
 power enrichment by economizer system
 accelerating pump system
 choke
Complete Carburetor

5. Idling System

6.  During starting or idling, engine runs without
load.
 The throttle valve remains in closed position.
 Engine produces power only to overcome
friction between the parts.
 A rich mixture is to be fed to the engine to
sustain combustion.
IDEAL SYSTEM

7. (1) A single jet carburetor can not provide a very rich mixture as required at
the time of starting the engine
(2) It can not provide very rich mixture required for sudden acceleration of
the engine.
(3) For gradually increasing pressure differences over the jet (at higher speed
of the engine), the weight of the petrol discharged from a single jet
increases at a greater rate than does the air supply. Hence, a single jet
carburetor gives a progressively richer mixture as the air speed increases
when set to give a correct mixture at low air speeds.
(4) It can not reduce the quantity of air flow during starting as required in
cold weather conditions.
(5) The automation control of air and fuel according to the required
conditions is not possible.
LIMITATIONS OF SIMPLE
CARBURETTOR

8. In the SI engine a combustible fuel-air mixture
is prepared outside the engine cylinder. The
process of preparing this mixture is called
carburetion.
The carburetor, a device which atomises the
fuel and mixes it with air. The pipe that carries
the prepared mixture to the engine cylinders is
called the intake manifold.
AUTOMOBILE CARBURATORS

9. AUTOMOBILE CARBURATORS
A carburetor’s
primary purpose is
to produce a
mixture of air and
fuel for different
engine operating
conditions.

10. Store
Deliver
Filter
Mix air and fuel
Control air and fuel mixture
PURPOSE OF CARBURETION

11. CARBURATION AIR FUEL
REQUIREMENT
Ratio of air to
fuel.
Example 14.7
lbs of air to 1 lb
of fuel
Fuel ratios
Rich / lean

13. Cold
Low speed
Mid speed
Acceleration
High speed
CARBURETOR OPERATING
CONDITIONS