Construction And Working Diagram Of Simple Carburettor Used In Petrol Engine


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 vaporisation
  • Temperature of the incoming air
  • quality of the fuel supply
  • design of combustion chamber and induction system
Simple Carburettor
Simple Carburettor:
  1. Provide air-fuel mixture for all operating conditions
  2. Carburettor depression is pressure differential in the float chamber and venture throat which causes discharge of fuel into the air stream
  3. Flow is controlled by small hole of fuel passage
  4. Pressure at the throat at the fully open throttle condition lies between 4 and 5 cm of Hg and seldom exceeds 8 cm Hg
  5. Drawback of simple carburettor 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 Carburettor:

Additional systems used with simple carburettor can help the engine to operate at all conditions, which are given below,
(i) Main metering system:
  1. Provide constant fuel-air ratio at wide range of speeds and loads.
  2. 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 carburettor 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 carburettor:
  • 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 carburettor:
  • Opening of butterfly allows more air inductance which decreases quantity of fuel drawn
  • Used in aircraft carburettors
(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
Textbook Of IC Engine By Mathur Sharma
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