08-10-2012, 05:36 PM
FUEL INJECTION IN S.I. ENGINE (CARBURETOR)
Carburettor.ppt (Size: 2.61 MB / Downloads: 82)
Carburetion
In S.I. engine a combustible air-fuel , mixture is prepared outside the engine cylinder.
The process of preparing A-F mixture outside the engine cylinder, is known as “Carburetion”.
The process of carburetion is achieved through Induction System & the device in which the fuel atomizes &mixes with air ,is called as “Carburetor.”
The pipe which carries the prepared A-F mixture to combustion chamber is called is the “Intake manifold.”
Induction System
During the suction stroke, vacuum is created in the cylinder causes the air to flow through the carburettor & fuel to be sprayed from the fuel jets.
Because of the volatility of the fuel , most of the fuel vaporizes and forms a combustible air-fuel mixture. However some of the larger droplets may reach in cylinder and must be vaporized & mixes with air during the compression stroke before ignition by the spark plug.
Factors Affects The Carburetion
Atomization , mixing & vaporization are the processes which require a finite time to occur. In high speed engine the time available for mixture formation is very small.
For example , for an engine running at 3000 rpm ,the induction process should be completed in less than 0.02 seconds. To complete these process during such a small period requires such a great ingenuity in designing the carburetion system.& hence efficient mixing , vaporization and distribution is difficult to achieve.
Mixture Requirement
1:Idling Range: (From No Load to about 20% of Rated Power):The no load running mode of the engine is called is “idling”. During idling, the air supply is restricted by the nearly closed throttle & the suction pressure is very low. This condition of low pressure gives rise to backflow of exhaust gases(since exhaust pressure is higher than the intake pressure) ,this increases the amount of residual gases. During suction process, the residual gases expand ,thereby reducing the fresh mixture inhaled. Increase dilution causes the combustion to be erratic & even impossible. Thus poor Thermal efficiency & higher exhaust emission. So Thus richening of mixture increases the probability of contact between fuel & air particles & thus improves combustion.
Working of Carburettor
A simple carburettor consists of a Float Chamber, Nozzle with metering orifice, a venturi & throttle valve.
During suction stroke ,air is drawn through the venturi. Venturi is a tube of decreasing cross section which reaches the maximum at throat (Venturi is also known as choke tube and is so shape that it gives minimum resistance to air flow).
The air passing through the venturi increases in velocity at the expense of its pressure.
From the float chamber , the fuel is fed to a discharge jet ,the tip of which is located in throat of the venturi.
Now because the pressure in the float chamber is atmospheric & that at the discharge jet below atmospheric, a pressure differential called carburettor depression, exists between them. This causes the discharge of fuel in to the air stream and the rate of flow is controlled or metered by the size of smallest section in the fuel passage. This is provided by the discharge jet and the size of this jet is chosen empirically to give the required engine performance.