SUBMITTED TO – Mr. ROHIT CHANDEL

BY- PERCY (B120020188)

SOURABH RANA (B120020282)

NAVNEET ARORA (B120020323)

Petrol engines come under the category of

Internal Combustion engines.

Petrol engines can be four stroke or two stroke.

4-stroke petrol engines work on Otto Cycle.

As the ignition in engine is due to spark,

therefore petrol engine is known as spark

ignition engine.

Four strokes of petrol engines is as follows:

1. Suction stroke

2. Compression stroke

3. Expansion or Working stroke

4. Exhaust stroke

During this stroke (also known as induction stroke) the piston moves from top dead centre (T.D.C) to bottom dead centre.

The inlet valve opens and proportionate fuel –air mixture is sucked in the engine cylinder.

This operation is represented by the line 5-1 (fig 5.32).

The exhaust valve remain closed throughout the stroke.

In this stroke, the piston moves (1-2) towards (T.D.C) and compresses the enclosed fuel air mixture.

The pressure of the mixture rises in the cylinder to a value of about 8 bar.

Just before the end of this stroke the operating –plug initiates a spark which ignites the mixture and combustion takes place at constant volume (line 2-3) (fig 5.32).

Both the inlet and outlet valve remain closed

during this stroke.

When the mixture is ignited by the spark plug the hot gases are produced which drive or throw the piston from T.D.C to B.D.C and thus work is obtained.

The flywheel mounted on the engine shaft stores energy during this stroke and supplies it during other three strokes.

The expansion of gases is shown by 3-4

(fig 5.32).

Both the valves remain closed during the start

of this stroke but when the piston reaches the

B.D.C the exhaust valve opens.

Removal of gases are accomplished during this stroke.

The piston moves from B.D.C to T.D.C.

The exhaust gases are driven out of the engine cylinder ; this is also known as scavenging.

This operation is represented by the line (1-5) (fig 5.32).

NOTE-the line 5-1 is below the atmospheric line.(this is due to the fact that owing to the restricted area of the inlet passages the entering fuel air mixture cannot cope with the speed of the piston.)

NOTE-the exhaust line 4-5 is slightly above

the atmospheric pressure line.(this is due to

restricted exhaust passages which do not

allow the exhaust gases to leave engine –

cylinder quickly).

The loop which has area 4-5-1 is called

negative loop; it gives the pumping loss due

to the admission of fuel air mixture and

removal of exhaust gases.

The area 1-2-3-4 is the total or gross work

obtained from piston and net work can be

obtained by subtracting the area 4-5-1 from

area 1-2-3-4.

o Fig shows a two stroke engine.

o The cylinder L is connected to a closed crank chamber C.C.

o During the upward stroke of the piston M , the gases in L are compressed and the same time fresh air and fuel(petrol) mixture enter the crank chamber through the valve V.

o When the piston moves downwards , V closes and mixture in C.C is compressed.

o The piston is moving upwards and is compressing an explosive change which has been supplied to L.

o Ignition takes place at the end of this stroke.

o The piston then travels downwards due to the expansion of the gases and near the end of this stroke the piston uncovers the exhaust port (E.P) and burnt gases escape through this port.(refer fig 3)

o The transfer port (T.P) then is

uncovered immediately.

o The compressed charge from

the crank chamber flow into

the cylinder and is deflected

upwards by the hump

provided on the head of the

piston.

o The piston then again start

moving from B.D.C to T.D.C

and the charge gets

compressed when E.P and T.P

is covered by the piston ;

hence cycle is repeated.

S.N

o

ASPECTS FOUR STROKE CYCLE

ENGINE

TWO STROKE CYCLE

ENGINE

1. Completion

of cycle

The cycle is completed in

4 strokes of the piston or

in the two revolution of

crankshaft.

The cycle is completed in 2

stroke of the piston or in

the one revolution of the

crankshaft.

2. Flywheel

required

Because turning

movement is not so

uniform ,therefore

heavier flywheel is

required.

More uniform movement

hence lighter flywheel is

required.

3. Valve and

valve

mechanism

4 stroke engine contains

valve and valve

mechanism.

Instead of valves it has

ports.

4. Volumetric

efficiency

Volumetric efficiency is

more due to more time of

induction.

Volumetric efficiency is less

due to lesser time for

induction.

5. Applications Cars , buses, tractors

etc.

Scooter ,motorcycles etc.

6. Power

produced for

same size of

engine

Because of one power

stroke for two

revolutions ,power

produced for same size

of engine is small.

Because of one power

stroke for one revolution,

power produced for same

size of engine is small.

7. Lubrication

required

Lesser rate of wear and

tear hence lesser

requirement of

lubricants .

Higher rate of wear and

tear and consequently

lesser requirement of

lubricants.

8. Initial cost Because of the heavy

weight and complications

of valve mechanism ,

higher is the initial cost.

Because of light weight and

simplicity due to valve

mechanism absence ,

lower is the initial cost.