Government Engineering College Bhavnagar Assignment on

Elements Of Mechanical Engineering

Prepared by:

Name Enrollment no.

Bhavika Chavda140210107008 Sagar Harsora 140210107018 Kuldip vasghsya 140210107029 Bhavya Sheth 140210107056 Keval vora 140210107062

Topic: Four Stroke Engine

A four-stroke engine (also known as four-

cycle) is an internal combustion engine in

which the piston completes four separate

strokes which constitute a single

thermodynamic cycle

TYPES OF STROKES

FOUR STROKE ENGINE

FOUR STROKE CYCLE ENGINES

• In a four-stroke engine the cycle of the operation

of engine is completed by four strokes of the

piston inside the cylinder.

• During these four strokes fuel is once injected and

burnt inside the engine and two revolutions of the

crankshaft are obtained.

• In a four-stroke spark ignition (SI) engine the

burning of the fuel occurs by the spark generated

from the spark plug. 

Crankshaft- a round shaft with lobes, that rotates to open and close the fuel and exhaust valves.

BASIC COMPO-NENTS OF FOUR-STROKE ENGINES

Intake Valve

Valve Cover

Intake Port

Head

Coolant

Engine Block

Oil PanOil Sump

Crankshaft

Camshaft

Exhaust Valve

Spark Plug

Exhaust Port

Piston

Connecting Rod

Rod Bearings

TYPES OF STROKES USED IN FOUR STROKES

ENGINE

1. Suction stroke

2. Compression stroke

3. Expansion stroke or Power Stroke

4. Exhaust stroke

TYPES OF FOUR STROKES ENGINE

1. Spark ignition engines ( Petrol Engine )

S.I

2. Compression engine (Diesel Engine ) C.I

WORKING OF FOUR STROKE S.I ENGINE

• Mostly working fuel of these engine is Petrol or Gas

• Compression ratio used in these engines varies from 5

to 10

1) SUCTION (INTAKE)STROKE

• At the beginning of this stroke the piston is at the top

dead center or near the cylinder head and is about to

move down.

• At this instance the inlet valve fitted in the cylinder head

is opened and the exhaust valve remains closed due to

the pressure difference.

• As the piston moves down the suction pressure is

created inside the cylinder, drawing an air-fuel mixture

into the cylinder.

• When the piston reaches the bottom most position or

bottom dead center, the suction stroke ends and the inlet

valve is closed. 

2) COMPRESSION STROKE:

• During this stroke the piston starts moving from bottom dead

center to top dead center.

• As the piston moves up, the air-fuel mixture gets compressed

into the clearance volume of the cylinder.

• At the end of the stroke the spark is generated by the spark

plug, which causes the burning of the fuel and the release of

large amounts of thermal energy.

• Due to this heat, high pressures are generated. 

3) EXPANSION OR POWER STROKE:

• he large amount of pressure generated at the end of the compression

stroke pushes the piston towards the bottom dead center.

• It is during this stroke that the actual power is produced by the

engine, hence this stroke is called the power stroke and since the

expansion of gases occurs during this process, it is also called the

expansion stroke.

• During this stroke, both the inlet and exhaust valves remain closed. 

4) EXHAUST STROKE

• Towards the end of the expansion stroke the inlet valve remains

closed while the exhaust valve opens due to the internal and

external pressure difference.

• The piston starts moving in an upward direction and all the

residual gases that are left after the expansion stroke are swept

outside the cylinder and escape through the exhaust chamber.

• At the end of the exhaust stroke, the piston reaches top dead

center position and then starts moving in the downward direction

to suck the air-fuel mixture and complete the suction stroke. 

WORKING OF FOUR STROKE C.I ENGINE

• The cycle of operations of the four-stroke compression ignition (CI)

engine are completed in four strokes of the piston inside the

cylinder.

• During these four strokes two revolutions of the crankshaft are

produced.

• In compression ignition (CI) engines, burning of fuel occurs due to

compression of the fuel to very high pressures. At very high

pressures the fuel, i.e. diesel, starts burning automatically without

the need of any external flame.

1) SUCTION STROKE

• At the start of the suction stroke the piston is located at top dead

center position. As it moves down,

• the inlet valve located in the cylinder head opens, while the

exhaust valve remains closed.

• From the inlet valve, air is drawn into the cylinder which

continues until the piston reaches bottom dead center or the

bottom most position inside the cylinder.

• At this point the suction stroke completes and the suction or inlet

valve closes. 

2) COMPRESSION STROKE

• During the compression stroke the piston starts moving in

upward and compresses the air in the clearance volume.

• While in the case of spark ignition (SI) engines, the

compression ratio is about 6-10, the CI engine this ratio is

about 16-20.

• This clearly indicates that the compression pressure

exerted in the CI engines is much more than in SI

engines. 

3) EXPANSION STROKE

• Towards the end of the compression stroke, the fuel is injected

into the clearance volume.

• Due to excessively high pressures, the fuel starts burning

instantly, creating large amounts of thermal energy, which

further raises the pressure.

• Because of this pressure the piston starts moving down.

• The fuel injection rate is such that the pressure inside the

cylinder is maintained constant even though the piston moves

down.

• The expansion stroke ends when the piston reaches the

bottom position. During this stroke the inlet and exhaust

valves remain closed. 

4) EXHAUST STROKE

•  After the expansion stroke a number of residual gases are left in the cylinder and

need to be cleared from of the cylinder.

• During the exhaust stroke the exhaust valve opens due to the difference between

the atmospheric pressure and the pressure of exhaust gases inside the cylinder.

• As the piston moves from the bottom to the top position the exhaust gases are

swept out of the cylinder.

• When the piston reaches the topmost position all the exhaust gases are released.

• As the piston starts moving down, the inlet valves open and fresh air is drawn into

the cylinder.