Construction and functioning diagram of a simple carburetor used in gasoline engines

June 27, 2024 Roberto Magalhães

Construction and operation diagram of a simple carburetor used in a gasoline engine

Carburization:

The process of preparing a fuel-air mixture outside the engine cylinder in the SI engine is known as carburization.

Important factors affecting the carburization process are given below;

time available for preparation of the mixture, i.e. atomization, mixing and vaporization
Inlet air temperature
quality of fuel supply
combustion chamber and induction system design

Simple Carburetor:

Provide air-fuel mixture for all operating conditions

Carburetor depression is the pressure differential in the float chamber and engine throat, which causes fuel to discharge into the air stream.

Flow is controlled by a small fuel passage orifice

Throat pressure at wide open throttle condition is between 4 and 5 cm Hg and rarely exceeds 8 cm Hg

The disadvantage of the simple carburettor is that it provides a very rich and very lean mixture due to the vacuum created in the throat being very high and very small, which is undesirable.

Complete carburetor:

Additional systems used with simple carburetor can help the engine to operate in all conditions, which are given below,

(i) Main measurement system:

Provides a constant fuel-to-air ratio over a wide range of speeds and loads.

Based mainly on best economy at full throttle (A/F ratio of about 15.6:1)

The different measurement systems are,

Compensating jet device: – In addition to the main jet, a compensating jet is provided to provide the thinning effect.

Emulsion tube or air bleed device:

Mixture correction is done only by air bleeding
In this arrangement, the main metering jet is installed about 25 mm below the gasoline level, called a submerged jet .

Reverse suction or pressure reduction control method:

In this arrangement, a large vent line connects the carburetor inlet with the top of the float chamber and another small orifice line is connected with the top of the float chambers with the vent throat.
Creates pressure differences depending on engine operating conditions

Auxiliary valve carburetor:

The auxiliary valve spring lifts the valve during increased engine load, which increases vacuum when venting
Allows more additional air intake and the mixture is not too rich

Auxiliary Port Carburetor:

The butterfly opening allows greater air inductance, which reduces the amount of fuel sucked in
Used in aircraft carburetors

(ii) Idle system:

Idle jet is added for idling and low load operation, which requires a rich mixture with A/F ratio of about 12:1
It consists of a small fuel line that runs from the float chamber to a point on the throttle side
Gradually opening the throttle may stop the jet from idling

(iii) Enrichment or energy saving system:

This system provides a richer mixture for a maximum operating power range
Features large orifice metering rod economizer that opens to the main jet as the throttle is opened beyond a certain point
The stem is tapered or stepped

(iv) Acceleration pump system:

Engine acceleration condition or rapid increase in engine speed may open the throttle quickly, which will not be able to provide a rich mixture
Spring-loaded plunger acceleration pump is used for fuel supply

(v) Suffocation:

A rich mixture is required during the cold starting period, at low starting speed and before the engine is warmed up.
Butterfly or choke valve is used between carburetor inlet and engine throat to meet the requirements
Spring-loaded bypass choke is used at higher speeds

Reference
: IC Engine Textbook by Mathur Sharma