about fuel supply system for si engines

1. UNIT 2
FUEL SUPPLY SYSTEMS FOR SI AND CI ENGINES AND IGNITION SYSTEMS

2. Content
Fuel supply systems for SI and CI engines: Properties of air-fuel mixtures - Mixture requirements for steady state and
transient operation, Fuel Supply system for gasoline and diesel engine. Basic principle of fuel pump, carburetor and fuel
injector.
Ignition systems: Battery Ignition systems, magneto Ignition system, Electronic Ignition, Automatic Ignition advance
systems.

3. Fuel supply systems for SI and CI engines
There are many types of fuel supply system in petrol engine. The basic fuel supply system in an automobile
petrol engine consists of a fuel tank, fuel Lines, fuel pump, fuel filter, air cleaner, carburetor, intake
manifold.
For the supply of fuel from fuel tank to engine cylinder following fuel feed systems are used:
1. Gravity system
2. Air pressure system
3. Vacuum system (suction and gravity system)
4. Pump feed system
5. Fuel injection system

4. In this system, a steel pipe carries petrol to the fuel pump which Pumps it into the float Chamber of the
carburetor through the pipe. here we use a mechanical or electrical or a diaphragm pump supplies the fuel from
the fuel tank is placed at any suitable location and is vented to the atmosphere.
THE COMPONENTS OF FUEL SUPPLY SYSTEM:
Fuel tank:
The fuel tank holds the fuel for the engine. It is made of steel or aluminum or synthetic rubber compounds and fiber
reinforced plastics which are flame resistant. And these tanks are coated with lead-tin alloy to protect the tank from the
corrosion effect.
Fuel pump:
The fuel pump is used to deliver the fuel from the fuel tank to the carburetor.
There are generally two pumps are used:
1.A.C. Mechanical Pump
2.S.U. Electrical pump
Fuel lines:
These tubes are used for connects the fuel tank with the pump and pump to the carburetor. Generally, these tubes are
made of Copper or Steel.
The two joints of the tubes are made flexible because of the flexible joints help the fuel tank to moves back or front
with the body, and also pump is moves according to the body. This joint prevents the loosening of fuel line by front-back
movement of the body.

5. Air cleaner:
This is very necessary for an engine to get fresh air, otherwise, the polluted air causes several damages to the
engine chamber. Particularly Piston, Piston chamber, Piston ring, and Valves. And if the polluted air enters the
crankcase where we store engine oil that can be damaged the lubrication parts like bearings. Therefore we need
to install an air filter which purifies the air before entering the engine cylinder
Fuel filters:
A fuel filter is necessary for a vehicle to clean the fuel.
For this, a very affected device is used and that is chamois leather, which only allows to passing the petrol into it
and the water is intercepted by this. And also fine particles can not pass through it.
Carburetor:
A perfect air-fuel mixture is necessary for a petrol engine to run. So, therefore, we use a carburetor. It is device
which controlled the air-fuel mixture.
There are two chambers in carburetor one is float chamber which is used to maintain the fuel level with the help
of needle valve and another one is mixing chamber where the mixture of air-fuel takes place.

6. Properties of air-fuel mixtures - Mixture requirements for steady state and transient operation
Rich fuel mixture, on the other hand, is a type of air-fuel
mixture that has less air than the required quantity of air for the
complete combustion of the fuel- may produce more power
and burn cooler
Lean fuel mixture is a type of air-fuel mixture that has more
air than the required quantity of air for the complete
combustion of the fuel.- Lean mixtures are more efficient but
may cause higher temperatures, which can lead to the
formation of nitrogen oxides.

7. PROPERTIES OF AIR-FUEL MIXTURE
▪ Air-fuel ratio (15:1) is the chemically correct air-fuel ratio, and is known as the stoichiometric mixture
▪ The combustion quality of a mixture is indicated by the rate of combustion or the manner of flame propagation in the
combustion chamber
▪ Engine develops maximum power when the air-fuel mixture is slightly rich (about 85% air than the air required for
stoichiometric mixture).
▪ Engine has maximum thermal efficiency when 'the mixture contains about 120% air with respect to the chemically
correct mixture.

8. ENGINE REQUIREMENTS
▪ When the engine runs without load, i.e. at idle speed, the engine requires a richer mixture
(not more fuel consumption)
After 20% load, the engine needs a leaner mixture because the throttle valve is sufficiently opened, volumetric
efficiency is increased, mechanical efficiency is increased, the air-fuel mixture becomes more homogeneous, and
the percentage of residual gases decreases.

9. As the load increases, the throttle valve is opened. This reduces the residual gases· in the combustion
chamber and the engine requires a lesser fuel air ratio
•After 80% load
•The throttle valve is almost fully opened, and the excess fuel in the air-fuel mixture (of greater proportion of
fuel) consumes all the air available.
•Engine temperature is increased as such cooling losses also increase.
•Engine power increases, and the percentage of residual gases reduces.
•Therefore a richer mixture is needed to develop more power.

11. CARBURETOR 1.Throttle Valve
2.Strainer
3.Venturi
4.Metering system
5.Idling system
6.Float Chamber
7.Mixing Chamber
8.Idle and Transfer port
9.Choke Valve
Functions of a carburettor:
The main functions of a carburettor are
1.The main function of carburettors to mix air and gasoline and provides a high combustion mixture.
2.It controls the engine speed.
3.It also regulates the air-fuel ratio.
4.Increase or decrease the amount of mixture according to the engine speed and load changing.
5.To keep certain head of fuel in the float chamber all the time.
6.Vaporize the fuel and mix to air to a homogeneous air-fuel mixture.
7.To supply the correct amount of air-fuel mixture at the correct strength under all conditions of load
and speed of the engine.

12. Types of Carburettor:
There are three types of carburettors according to the direction in which the mixture is supplied.
•Up-draft carburettor
•Horizontal type carburettor
•Down-draft type carburettor

13. The carburettor consists of following circuits.
1. Float Chamber Circuit.
2. Starting Circuit.
3. Idle and Low Speed Circuit.
4. Part and Full Throttle Circuit
5. Acceleration Circuit.

14. There are two types of pumps which are most commonly used
1. Mechanical type fuel transfer pump [A.C. Mechanical pump].
2. Electrical fuel pump [S U. Electrical pump]

15. 1. Mechanical type fuel transfer pump [A.C. Mechanical pump]

16. •Eccentric or Cam
•Diaphragm
•Pull rod
•Strainer
•Inlet Valve
•Pressure Valve
•Outlet valve to Carburettor
The drive for the pump is taken from the camshaft by means of an eccentric or cam. The eccentric operates
the rocker arm which is in conjunction with the diaphragm return spring, which pushes the diaphragm up
and down.
The downward movement of the diaphragm causes vacuum in in the chamber which causes the inlet valve
to open and the fuel then goes through the strainer into the chamber.The next upward movement of the
diaphragm causes the inlet valve to close while the outlet valve opens and the fuel goes out to the
carburettor float chamber.

17. 2. Electrical Fuel Pump:

18. S.U. Electrical Fuel Pump:
In this type also, the diaphragm is used. Alternate vacuum and pressure are produced due to the
movement of the diaphragm which is caused electrically in this case.
•Diaphragm
•Breaker Point
•Power from the Ignition switch
•Solenoid
•Armature
Working of S.U. Electrical Fuel Pump:
Closing the ignition switch energizes the solenoid winding and the magnetic flux is created, which
pulls the armature to which the diaphragm is attached. Thus the diaphragm moves to cause suction in
the pump chamber and the fuel is drawn into the chamber.
But as soon as the armature moves, it interrupts the electric supply by disconnecting the breaker
points, the solenoid is de-energized and the armature falls back causing the diaphragm to move so as
to create the pressure in the pump chamber which opens the outlet valve and the fuel goes out to the
carburetor float chamber.

19. Injection Systems-Multi point and single point fuel injection systems
Fig. (a) Direct injection system Fig. (b) Port injection system Fig. (c) Throttle body injection
Single Point Fuel Injection Systems

20. Single-point or throttle body injection (TBI) Port or multi-point fuel injection (MPFI):

21. Single point injection system is one type of injection system that uses an injector. Even though the engine has 4
cylinders, the number of injectors remains one.
This system was found in the early discovery of injection systems in vehicle engines. At that time, an injector was placed
on the throtle body as a substitute for playing a jet carburetor.
The result is the same as the carburetor system. Gasoline can be sprayed into the throtle body room but this gasoline
comes out of an injector not from carburetor jet.
When compared to the carburetor type, it is clearly better because how much the volume of gasoline has been calculated
before by ECU.
So that it can be more accurate.

22. The MPFI is a system or method of injecting fuel into internal combustion engine through multi ports situated on intake
valve of each cylinder. It delivers an exact quantity of fuel in each cylinder at the right time. There are three types of
MPFI systems – Batched, Simultaneous and Sequential.
In the batched MPFI system fuel is injected to the groups or batches of the cylinders without bringing their intake stroke
together. In the simultaneous system, fuel is inserted to all cylinders at the same time, while the sequential system
injection is timed to overlap with intake stroke of each cylinder.
How fuel injection system works?
MPFI includes a fuel pressure regulator, fuel injectors, cylinders, pressure spring and a control diaphragm. It uses
multiple individual injectors to insert fuel in each cylinder through intake port situated upstream of cylinder’s intake
value. The fuel pressure regulator, connected to the fuel rail by means of an inlet and outlet, directs the flow of the
fuel. While the control diaphragm and pressure spring controls the outlet valve opening and the amount of fuel that
can return. The pressure in the intake manifold significantly changes with the engine speed and load.
Advantages of multi point fuel injection system?
•The multi-point fuel injection technology improves fuel efficiency of the vehicles. MPFI uses individual fuel injector for
each cylinder, thus there is no gas wastage over time. It reduces the fuel consumption and makes the vehicle more
efficient and economical.
•The vehicles with MPFI automobile technology have lower carbon emissions than a few decades old vehicles. It
reduces the emission of the hazardous chemicals or smoke, released when fuel is burned. The more precise fuel
delivery cleans the exhaust and produces less toxic byproducts. Therefore, the engine and the air remain cleaner.
•MPFI system improves the engine performance. It atomizes the air in small tube instead additional air intake, and
enhances the cylinder-to-cylinder fuel distribution that aid to the engine performance.

23. Battery Ignition System
Ignition systems:
1. Battery Ignition systems,
2. Magneto Ignition system,
3. Electronic Ignition,
4. Automatic Ignition advance systems.
The essential components of a
battery ignition system are a
1. Battery,
2. Ignition switch,
3. Ballast resistor,
4. Ignition coil,
5. Breaker points,
6. Condenser,
7. Capacitor
8. Distributor and
9. Spark plugs.

24. Most of the modern spark-ignition engines use battery ignition systems. The essential components of
a battery ignition system are a battery, ignition switch, ballast resistor, ignition coil, breaker points,
condenser, capacitor distributor and spark plugs.
The breaker points, condenser, distributor rotor and spark advance mechanisms are usually housed in
the ignition distributor. The breaker points are actuated by a shaft driven at half-engine speed for a
four-stroke cycle engine. The distributor rotor is directly connected to the same shaft.
The system has a primary circuit of low-voltage current and a secondary circuit for the high-voltage
circuit.
The primary circuit consists of the battery, ammeter, ignition switch, primary coil winding and breaker
points. The primary coil winding usually has approximately 240 turns of relatively heavy copper wire
wound around the soft iron core of the ignition coil.
The secondary circuit contains the secondary coil windings, distributor, spark plug leads and the spark
plug. The secondary windings consist of about 21000 turns of small, well-insulated copper wire.

25. Parts
•Rotating Magnets
•Primary Winding
•Secondary Winding
•Fixed Armature
•Condenser
•Breaker-points or Contact Breaker
•Distributor
•Ignition Switch and
•Spark plug
Magneto Ignition System

26. The working principle of the Magneto ignition system will be the same as that of the coil ignition system.
The primary circuit flux is changed and a high voltage is produced in the secondary circuit With the help of a
cam.
The variation of the breaker current with speed for the coil ignition system and the Magneto ignition
system.
Armature having wound upon it in the primary coil of a few hundred turns of enameled copper wire and the
secondary coil of several thousand turns of fine insulated wire rotates in a permanent magnetic field.
The modern trend is to keep the armature stationary and the magnets rotating
By this action, the primary voltage is generated in accordance with the laws of the electric generator which
is then transformed into a very high secondary voltage exactly on the lines described in the battery ignition
system.
It can be seen that since the cranking speed at the start is low the current generated by the Magneto is
quite small.
The engine speed will increase as we increase the current. Thus, with Magneto there is always a starting
difficulty and sometimes a separate battery is needed for starting.
Magneto is better for high speeds and therefore is widely used for sports and racing cars, aircraft engines,
etc.

27. Electronic Ignition System
Battery
Ignition switch
Electronic control module
Armature
Ignition coil
Distributor
Sparkplug

28. A battery plays an important role in supplying power. The negative terminal is grounded and a positive terminal is
connected to the ignition switch.
Now when we turn on the ignition switch the power supply gets ON and further, the wire is connected to the
electronic ignition module.
Here all the work will be controlled electronically.
From here it sends to the ignition coil. In Ignition coil there is two winding:
1.Primary winding
2.Secondary winding
Both winding are insulated and Primary are thicker than secondary winding. Now, there is one iron rod in between
them that generates a magnetic field.
An armature is connected to the electronic module, the power comes and armature rotates.
Here you can see magnetic pick-up. When the magnetic pick-up and armature touches, the voltage signal generates.
and it generates further until a strong voltage signal comes. Here ON and OF process continues.
This voltage comes to the distributor section and here a rotor is connected. When the rotor rotates and touches the
spark plug line it supplies the voltage and sparks plug ionized or we can say the distributor distributes the voltages.

29. The two mechanisms used are
1. Centrifugal advance mechanism,
2. Vacuum advance mechanism.
Automatic Ignition advance systems Ignition Advance

30. The centrifugal advance mechanism responds to engine speed. It is usually in the bottom of the distributor body under
the contact-breaker baseplate.
Two steel weights are attached to a revolving plate on the distributor shaft by pivots, and held in the closed position by
strong springs.
As the engine speeds up, centrifugal force throws the weights outwards.
They turn on their pivots, twisting the contact-breaker cam forwards so that the points open earlier, and the sparkplug
fires earlier as the speed increases.
The vacuum advance mechanism responds to the vacuum in the engine inlet manifold , which is caused by the suction of
the moving pistons. When the engine is lightly loaded the vacuum increases.
A narrow pipe runs from the manifold to a vacuum chamber on the distributor, inside which there is a
flexible diaphragm .
As the vacuum increases, the diaphragm bends, moving a rod connected to its centre which causes the contact-breaker
baseplate to swivel slightly. This moves the contact-breaker heel relative to the distributor cam and advances the ignition.
When the engine is under load, vacuum decreases, the diaphragm springs back and the ignition is retarded to suit the
changed conditions.