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In petrol engines, the air and fuel is mixed outside the engine, and supplied to the engine.
The fuels such as petrol, benzol and alcohol used in S.I.(Spark Ignition) engines, vaporizes easily if injected in the flow of air.
Therefore, the engine suction is sufficient to draw the mixture easily into the cylinder.
The fuels which are used in C.I.(Compression Ignition) engines do not vaporize easily. Therefore, a separate injection system is used consists of Fuel Injection Pump (FIP) and Fuel Injectors.
Fuel System Components
S.I. Engine
1.Fuel Tank
2.Fuel Lines
3.Fuel Pump
4.Fuel Filter
5.Carburettor
6.Air Filter
7.Inlet Manifold
C.I. Engine
1.Fuel Tank
2.Fuel Lines
3.Fuel Pump
4.Fuel Filter
5.Fuel Injection Pump
6.Fuel injector
Types of Fuel System
1.Gravity feed System
In the gravity-feed system, the fuel tank is mounted at the highest position from the carburettor.
This system uses the force of gravity to transfer the fuel from the tanks into the carburettor float chamber.
The system is very simple and cheap, but the rigidity of placing the fuel tank over the carburetor is a disadvantage.
This system is useful for the smaller propeller driven aircrafts. The fuel tanks are kept in the wings of the aircrafts
2. Pressure fuel System
In the pressure system, a hermetically (airtight) sealed fuel tank is used.
Pressure is created in the tank by means of Helium gas or a separate air pump.
For starting, the pump is primed by hand. Under the pressure produced, the fuel will flow to the float chamber of the carburettor.
There are chances of pressure leak, but the advantage is that, the fuel tank can be placed at any suitable location.
This system is used in Rockets and space-crafts.
3. Pump System:
In this system, a steel pipe carries petrol to the fuel pump, which supplies it into the float chamber of the carburetor, through a flexible pipe.
If the fuel pump is mechanical, it has to be driven from the engine camshaft.
So the mechanical pumps are attached to the engine itself.
However, electrically operated fuel pump can be placed anywhere, (away from the hot engine), to reduce the chances of forming vapour lock.
Most present-day cars use this system, and the fuel pump is placed inside the fuel tank. The reason is to avoid vapour lock, that will disrupt the smooth fuel supply.
4. Fuel Injection System:
An ideal fuel injection system can precisely provide, exactly the right amount of fuel under all engine operating conditions.
The petrol injection system has been used successfully on modern vehicles (MPFI engines).
In this system, the fuel is atomized, by means of a nozzle and then delivered into an air stream.
Separate fuel injection system is used for each cylinder, which controls the level of fuel mixture, under different load and speed conditions.
Precise air-fuel-ratio is very essential for:
1.Easy engine operation even at low engine temperatures (cold start),
2.good adaptation to a wide range of altitudes and ambient temperatures,
3.good fuel efficiency, for a better mileage,
4.Lowest level of exhaust emissions.
MPFI: Multi-Point Fuel Injection
Components of Fuel System
1.Air Filter (Cleaner)
An air filter is a device made of fibrous or porous materials, which removes particles such as dust, pollen, and water droplets from the air.
Air filters are part of Fuel systems, to send clean air into Carburettors, to prepare petrol-air mixture for combustion.
Air filters are also used to provide clean air, in Combustion Ignition Engines.
2.Inlet Manifold
An inlet manifold or intake manifold is the part of an engine, that supplies the fuel-air mixture to the cylinders.
The main function of the intake manifold is, to evenly distribute the fuel mixture, to each intake port in the cylinder heads.
Even distribution is important to optimize, the efficiency and performance of the engine.
It can also serve as a mount for the carburetor, fuel injectors of the engine.
3.Carburetor
Carburetors have been modernized, and improved over the years. However, the basic function of mixing fuel and air has not changed.
A carburetor’s job is to supply, combustible air/petrol mixture in a proper ratio, that will power the engine.
Carburetors regulate the flow of air through their main bore (Venturi).
This flowing air extracts the fuel, and the mixture enters the engine, via the intake valve.
The Venturi Effect is critical to the successful operation of carburetor.
Carburetors consist of a bowl, center bore, passages, jets, vents, a slide, a Choke, idle speed adjustment, and air/fuel ratio adjustment and an accelerator pump
Venturi Effect
Twisting the throttle on a motorcycle handlebar does not control how much fuel enters the engine.
The throttle controls just how much air is allowed, to flow through the carburetor. The flow of air is that controls, how much fuel is sent into the engine.
The Venturi Principle/Effect states that, when air flows through a restriction of a tube, the air speed increases, and air pressure decreases.
This drop in air pressure draws out fuel, from the carburetor bowl, through jets and passages, and sends into the cylinder head.
How a carburettor works
1.Air flows into the top of the carburetor, passing through a filter.
2.When the engine is first started, the choke can be set, so that it almost blocks the pipe, to reduce the amount of air coming in. This will increase the fuel content, of the mixture entering the cylinders.
3.In the center of the tube, the air is forced through a narrow neck called a venturi. This increases the speed and causes its pressure to drop.
4.The drop in air pressure creates suction on the float chamber, drawing-in the fuel through the jet.
5.When the throttle valve is open, more air and fuel flows to the cylinders, so the engine produces more power and the vehicle goes faster.
a.Fuel is supplied from a mini-fuel tank called the float chamber.
b.As the fuel level falls, a float in the chamber falls and opens a valve at the top.
c.When the valve opens, more fuel flows in to replenish the chamber from the main fuel tank.
Fuel Injection System for CI engines
CI(Compressed ignition) engines use diesel as fuel, and fuel injection system to run the engine.
The purpose of fuel system in a diesel engine is, to supply fuel to the cylinder by atomising into fine droplets, and then distribute it in the combustion chamber.
Early CI engines used air pressure to deliver the fuel into the cylinder.
But nowadays, fuel is usually drawn from the fuel tank by a Fuel Injection pump, and forced through a filter to the injection pump.
The injection pump sends fuel under pressure, through pipes to the injector nozzles, located in each cylinder head.
***End of Session 6 of Unit-1***
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