This slide explains about basic carburetor design.

1. CARBURETOR

2. INTRODUCTION :
 A carburetor is a device that mixes air and fuel for internal combustion engines in
the proper ratio for combustion. It is sometimes colloquially shortened to carb in
the UK and North America or carby in Australia. To carburate or carburet (and
thus carburation or carburetion, respectively) means to mix the air and fuel or to
equip (an engine) with a carburetor for that purpose.
 Carburetors have largely been supplanted in the automotive and, to a lesser
extent, aviation industries by fuel injection. They are still common on small
engines for lawn mowers, rototillers and other equipment.

3. PRINCIPLES :
 The carburetor works on Bernoulli's principle. the faster air moves, the lower its
static pressure, and the higher its dynamic pressure. The throttle (accelerator)
linkage does not directly control the flow of liquid fuel. Instead, it actuates
carburetor mechanisms which meter the flow of air being pushed into the
engine. The speed of this flow, and therefore its pressure, determines the amount
of fuel drawn into the airstream.
 When carburetors are used in aircraft with piston engines, special designs and
features are needed to prevent fuel starvation during inverted flight. Later
engines used an early form of fuel injection known as a pressure carburetor.

4.  Air flows into the top of the carburetor from the car's air
intake.
 When the engine is first started, the choke (blue) can be set
so it almost blocks the top of the pipe to reduce the amount
of air coming inside.
 In the centre of the tube, the air is forced through a narrow
kink called a venturi.
 The drop-in air pressure creates suction on the fuel pipe
(right), drawing in fuel (orange).
 The throttle (green) is a valve that swivels to open or close
the pipe.
 The mixture of air and fuel flows down into the cylinders.
 Fuel (orange) is supplied from a mini-fuel tank called the
float-feed chamber.
 As the fuel level falls, a float in the chamber falls and opens a
valve at the top.

5. OPERATION :
 Fixed - Venturi :
In which the varying air velocity in the Venturi alters the fuel flow; this
architecture is employed in most carburetors found on cars.
 Variable Venturi :
In which the fuel jet opening is varied by the slide (which simultaneously
alters air flow). In "constant depression" carburetors, this is done by a
vacuum operated piston connected to a tapered needle which slides
inside the fuel jet.

6. BASICS :
Cross-sectional schematic of a downdraft carburetor
A carburetor basically consists of an open pipe
through which the air passes into the inlet
manifold of the engine. The pipe is in the form of
a Venturi. It narrows in section and then widens
again, causing the airflow to increase in speed in
the narrowest part. It increases speed of fuel
supply to engine.

7.  Off – Idle Circuit :
As the throttle is opened up slightly from the fully closed position, the throttle
plate uncovers additional fuel delivery holes behind the throttle plate where there
is a low-pressure area created by the throttle plate/Valve blocking air flow; these
allow more fuel to flow as well as compensating for the reduced vacuum that
occurs when the throttle is opened, thus smoothing the transition to metering
flow through the regular open throttle circuit.
 Main Open – Throttle Circuit :
As the throttle is progressively opened, the manifold vacuum is lessened since
there is less restriction on the airflow, reducing the flow through the idle and off-
idle circuits. This is where the Venturi shape of the carburetor throat comes into
play, due to Bernoulli's principle (i.e., as the velocity increases, pressure falls).
 Power Valve : For
open throttle operation a richer mixture will produce more power, prevent pre-
ignition detonation, and keep the engine cooler. This is usually addressed with a
spring-loaded "power valve", which is held shut by engine vacuum.

8.  Accelerator Pump :
Liquid gasoline, being denser than air, is slower than air to react to a force applied to
When the throttle is rapidly opened, airflow through the carburetor increases
immediately, faster than the fuel flow rate can increase. This transient oversupply of
causes a lean mixture, which makes the engine misfire - an effect opposite to that
which was demanded by opening the throttle. This is remedied by the use of a small
piston or diaphragm pump which, when actuated by the throttle linkage, forces a
amount of gasoline through a jet into the carburetor throat. This extra shot of fuel
counteracts the transient lean condition on throttle tip-in.
 Choke :
To provide the extra fuel, a choke is typically used; this is a device that restricts the
of air at the entrance to the carburetor, before the venturi. With this restriction in
extra vacuum is developed in the carburetor barrel, which pulls extra fuel through the
main metering system to supplement the fuel being pulled from the idle and off-idle
circuits.
 Other Elements :
The interactions between each circuit may also be affected by various mechanical or
pressure connections and also by temperature sensitive and electrical components.
These are introduced for reasons such as response, fuel efficiency or automobile
emissions control.

9.  Fuel Supply :
To ensure a ready mixture, the carburetor has a "float chamber" (or "bowl") that
contains a quantity of fuel at near-atmospheric pressure, ready for use. This reservoir
constantly replenished with fuel supplied by a fuel pump. The correct fuel level in the
bowl is maintained by means of a float controlling an inlet valve, in a manner very
similar to that employed in a cistern (e.g. a toilet tank). As fuel is used up, the float
drops, opening the inlet valve and admitting fuel. As the fuel level rises, the float rises
and closes the inlet valve.
 Diaphragm Chamber :
If the engine must be operated in any orientation (for example a chain saw or a
airplane), a float chamber is not suitable. Instead, a diaphragm chamber is used. A
flexible diaphragm forms one side of the fuel chamber and is arranged so that as fuel
drawn out into the engine, the diaphragm is forced inward by ambient air pressure.
 Multiple Carburetor Barrel :
While basic carburetors have only one Venturi, many carburetors have more than one
Venturi, or "barrel". Two barrel and four barrel configurations are commonly used to
accommodate the higher air flow rate with large engine displacement. Multi-barrel
carburetors can have non-identical primary and secondary barrel(s) of different sizes
and calibrated to deliver different air/fuel mixtures; they can be actuated by the
or by engine vacuum in "progressive" fashion, so that the secondary barrels do not
begin to open until the primaries are almost completely open.

10. THANK YOU