You know from my previous posts that,in a petrol engine ,a carburettor is used to mix air and fuel in the required ratio and this mixture is drawn into the engine cylinder by suction stroke.
But in Diesel Engine,which is working on constant pressure cycle,the fuel has to be supplied when the air inside the cylinder is compressed to a very high pressure.Therefore the fuel must be injected at a high pressure than that developed in the cylinder at the end of compression stroke.
Only air is compressed to a pressure of about 35 kilogram per centimeter square and the temperature after this compression is reach near to 600 degree centigrade.
The fuel sprayed into this compressed air must be uniformly distributed so that enough Oxygen in the air is available to every particle of fuel for complete combustion.The distribution of the fuel is also assisted by turbulence of air which can be brought about by properly designed inlet ports,the top of the piston and the combustion chamber.Due to which there is considerable relative motion between the fuel and the air,which leads to a proper mixing.
The time for which the fuel is injected into the engine cylinder must be very small,since high injection velocities give a good atomization of fuel and the fuel has to be burnt at top dead centre position of the piston.
In compression ignition engine (diesel engine),a pressure as high as 100 to 300 kilogram per centimeter square are required to be developed for fuel injection systems.
The main advantage of diesel engine is,the maintenance cost of fuel injection equipments are less than that those of the carburettor and spark plug of petrol engine.
Requirements Of Fuel Injection System:-
The fuel injection system in diesel engine must satisfy the below requirements,
01) At beginning as well as the end of injection of fuel should take place sharply.
02) The injection of fuel must occur at the correct moment,correct rate and in correct quantity as required by the various load conditions of engines.
03) The fuel must be injected in a fine atomised condition and must be distributed uniformly in the combustion chamber of engine.
Methods Of Fuel Injection:-
01)Air Blast Injection:-
In this,air is compressed to a very high pressure by means of compressor and the blast of air the injected carrying fuel along with it into the cylinder.This method is obsolete now.
In this,the fuel under pressure is directly injected into the combustion chamber in atomised state.It requires pump to deliver the fuel at very high pressure and used in all big and small diesel engines.This system is also subdivided in,
a) Individual Pump System:-
In this system,each cylinder has its own individual high pressure pump and metering unit.The cost of this system is very high.
b) Common Rail System:-
In this system,the fuel is pumped by multi-cylinder pump into a common rail,the pressure in which is controlled by a relief valve.A metered quantity of fuel is supplied to each cylinder from the rail.It requires higher accuracy during manufacturing.
I hope,this will help you to understand about the fuel supply of your diesel engine.
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