Table of Contents

Carburetor – Diagram , working , parts ,types

Carburetion:
The process of preparing a combustible fuel-air mixture outside engine cylinder in SI engine is known as carburetion.

Important factors which affect the process of carburetion are given below;
-time available for the mixture preparation i.e. atomisation, mixing and the vaporization
-Temperature of the incoming air
-quality of the fuel supply
-design of combustion chamber and induction system

Simple Carburetor:

-provide air-fuel mixture for all operating conditions
-Carburetor depression is pressure differential in the float chamber and venture throat which causes discharge of fuel into the air stream
-flow is controlled by small hole of fuel passage
-pressure at the throat at the fully open throttle condition lies between 4 and 5 cm of Hg and seldom exceeds 8 cm Hg
-petrol engine is quantity governed
-Drawback of simple carburetor is that it provides too rich and too lean mixture due to vacuum created at the throat is too high and too small which is undesirable

Complete Carburetor :

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

(i) Main metering system:
-provide constant fuel-air ratio at wide range of speeds and loads.
-mainly based upon the best economy at full throttle (A/F ratio about 15.6:1)

The different metering systems are,

Compensating jet device:-

Addition to the main jet, a compensating jet is provided to provide the leanness effect

Emulsion tube or air bleeding device:
-mixture correction is done by air bleeding alone
-in this arrangement main metering jet is fitted about 25 mm below the petrol level which is called as submerged jet

Back suction control or pressure reduction method:
-in this arrangement large vent line connects the carburetor entrance with the top of the float chamber and another small orifice line is connected with the top of the float chambers with venture throat
-it creates pressure differences according to engine operating conditions

Auxiliary valve carburetor:
-Valve spring of auxiliary valve lift the valve during increase of engine load which increases the vacuum at venture
-Allows more admittance more additional air and the mixture is not over rich

Auxiliary port carburetor:
-opening of butterfly allows more air inductance which decreases quantity of fuel drawn
-used in aircraft carburetors

(ii) Idling system:
-Idling jet is added for the idling and low load operation which requires rich mixture sof about A/F ratio 12:1
-consists of small fuel line from the float chamber to a point of throttle side
-gradual opening of throttle may stop the idling jet

(iii) Power enrichment or economizer system:
-this system provides the richer mixture for maximum power range of operation
-It has meter rod economizer of large orifice opening to the main jet as the throttle is opened beyond a certain point
-rod is tapered or stepped

(iv) Acceleration pump system:
-Engine acceleration condition or rapid increase in engine speed may open the throttle rapidly which will not able to provide rich mixture
-acceleration pump of spring loaded plunger is used for fuel supply.

(v) Choke:
-Rich mixture is required during cold starting period, at low crancking speed and before the engine warmed up condition
-butterfly type valve or choke is used between the entrance to the carburettor and venture throat to meet the requirement
-spring loaded by-pass choke is used in higher speeds

Carburettor types:

(i) Open choke:Zenith, solex and carter
Constant vacuum type: S.U. carburettor
(ii) updraught type
Horizontal or downdraught: mixture is assisted by gravity in its passage to the engine induction

(a) Solex carburettor:
-provide ease of starting, good performance, and reliability
-used in Fiat and standard cars and Willey jeep
-Bi-starter is used for cold starting
-well of emulsion system is used for idling and slow running condition
-diaphragm type acceleration pump is used for increasing speed case

(b) Carter carburettor:
-downdraft type carburettor used in jeep
-has triple venture diffusing type choke in which smallest lies above the level float chamber, other two below the petrol level, one below other.
-multiple venturies result in better formation of the mixture at very low speeds causing steady and smooth operation at very low and high engine speed
-mechanical metering method is used
-choke valve is provided in the air circuit for cold starting
-plunger type acceleration pump is used

(c) S.U. carburettor:
-constant air-fuel ratio is maintained due to vacuum depression
-has only one jet
-no separate idling jet or acceleration pump
-constant high velocity air across the jet may avoid the use of idling jet
-jet lever arrangement provides the rich mixture in cold starting
-used in many British cars and Hindustan ambassador car

Drawbacks of modern carburettor:
-improper mixture proportion in multi-cylinder engine
-loss of volumetric efficiency due to obstruction of flow of mixture from choke tubes, jets, throttle valve etc.
-wear of carburettor parts
-Freezing at low temperature
-surging when carburettor is tilted or during acrobatics in aircraft
-backfiring in fuel pipe line