The document discusses the operation of 4-stroke and 2-stroke engines. It describes the four strokes of a 4-stroke engine: intake, compression, power, and exhaust. The actual valve timings for 4-stroke diesel and petrol engines are provided. For 2-stroke engines, it explains how the power and compression strokes occur simultaneously, with the intake of fresh air assisted by exhaust gases pushing out through ports. Theoretical and actual valve timings are presented for 2-stroke petrol and diesel engines.

1. Branch : Mechanical Engineering
Div: 6C-1
Name: Saahil R. Kshatriya
Enrolment Number: 150120119164

2. Theoretical Valve Timing Diagram
• In this type of diagram suction & exhaust valve
opens and close at TDC and BDC.
Actual Valve Timing Diagram
• In this type of diagram suction & exhaust valve
opens and close after and before TDC and BDC.

3. 4-stroke Diesel & Petrol engine
There is mainly four stroke in 4-stroke engine
1. Suction Stroke
2. Compression Stroke
3. Power Stroke
4. Exhaust Stroke

4. STROKE 1 : SUCTION
TDC
BDC
180
DEGREE
Inlet valve
• SUCTION STROKE
During this stroke, inlet valve opens and exhaust valve
closed, the pressure in the cylinder will be atmosphere .
As the piston moves from the TDC To BDC , the volume in
the cylinder increase , while simultaneously pressure
decreases.
This create
atmosphere
a pressure difference between the
and inside of the cylinder.Due to this
pressure difference the petrol and air mixture will enter
into the cylinder in S.I.Engine & in C.I.Engine air will enter
into the cylinder.
The crankshaft has now made half rotation i.e. 180
degree of crank angle.
At the end of this stroke, the cylinder will filled
completely with charge and inlet valve is closed.

5. • COMPRESSION STROKE
During this stroke both the inlet valve and exhaust valve
are closed, the piston moves from BDC to TDC .
As this stroke being performed ,the charge contained in
the cylinder will be compressed ,
temperature of charge increases.
so pressure and
The process of
compression is shown in fig.
Near the end of this stroke , In S.I.Engine the petrol and air
mixture is ignited by the electric spark given out by the
spark plug & In C.I.Engine the diesel is spray through
nozzle.
The combustion of petrol/diesel releases the hot gases
which will increases the pressure at the constant volume.
TDC
BDC

6. • POWER STROKE
During this stroke both the inlet valve and exhaust valve are
closed, the piston moves from TDC to BDC.
The high pressure and high temperature burnt gases force the
piston to perform this stroke, called power stroke. This stroke is
also known as expansion or working stroke.
The engine produces mechanical work or power during this
stroke.
As the piston moves from TDC to BDC , the pressure of hot gases
gradually decreases and volume increases.
Near the end of this stroke, the exhaust valve opens which will
release the burnt gases to the atmosphere.
This will suddenly bring the cylinder pressure to the
atmospheric pressure.
STROKE 3 : POWER OR
EXPANSION

7. • EXHAUST STROKE
During this stroke, the exhaust valve opens and the inlet
valve is closed .
The piston moves from BDC to TDC and during this motion
piston pushes the exhaust gases (combustion product) out
of the cylinder at constant pressure.
Again the inlet valve open and the new cycle starts.
TDC
BDC

8. Theoretical & Actual Valve Timing Diagram For
4-Stroke Diesel Engine

9. Actual Valve Timing For 4-Stroke Diesel
Engine
I.V.O. → 25° Before T.D.C.
I.V.C. → 30° After B.D.C.
F.V.O. → 5° Before T.D.C.
F.V.C. → 25° After T.D.C.
E.V.O. → 45° Before B.D.C.
E.V.C. → 15° After T.D.C.
Valve Overlap → 30°

10. Theoretical & Actual Valve Timing Diagram For
4-Stroke Petrol Engine

11. Actual Valve Timing For 4-Stroke Petrol
Engine
I.V.O. → 20° Before T.D.C.
I.V.C. → 35° After B.D.C.
F.V.O.→ 35° Before T.D.C.
E.V.O. → 35° Before B.D.C.
E.V.C. → 10° After T.D.C.
Valve Overlap → 30°

12. 2-Stroke Petrol & Diesel Engine
Their is mainly two stroke in 2-stroke engine:
1. Expansion Stroke
2. Compression Stroke

13. At the beginning of the first stroke piston is at the TDC as shown in fig. A. Piston moves from TDC
to BDC.
The electric spark ignites the compressed charge in petrol engine & Diesel spray in diesel engine
when compression stroke is about complete . The combustion of the charge will release the hot
gases which increase the temperature and pressure in cylinder. The high pressure combustion
engine to force piston downward .
The piston perform power stroke till it uncovers the exhaust port As shown in fig. B. The
combustion gases which are at the pressure slightly higher than atmosphere pressure escape
through exhaust port.
The piston uncovers the transfer port the fresh charge flow from the crankcase into cylinder
through transfer port as shown in fig. B.
Which enters the cylinder pushes the burnt gases , so more amount of exhaust gases come out
through exhaust port as shown in fig. B.
This swiping out of exhaust gases by incoming fresh charge is called scavenging.
This will continue till the piston covers both the transfer and exhaust port during next upward
stroke.
First stroke

14. Fig. A
Beginning of the first stroke
Fig. B
Piston uncovers transfer port
During first stroke
First stroke

15. Second Stroke
In this stroke piston moves from BDC to TDC.
When it covers the transfer port in fig. C. , the supply of charge is stopped and
then when it moves further up it covers the exhaust port completely in fig. D
stop the scavenging.
Further upward motion of the piston will compressed the charge in the cylinder.
After the piston reaches TDC the first stroke repeats again.

16. Second Stroke
Fig. C.
Transfer port covered
Fig. D.
Compression commenced

17. Theoretical & Actual Valve Timing
Diagram For
2-Stroke Petrol Engine

18. Actual Valve Timing For 2-Stroke Petrol
Engine
• → 70° After B.D.C.
• → 60° Before B.D.C.
• → 60° After B.D.C.
• → 50° Before T.D.C.
• →50° After T.D.C.
E.P.O. → 70° Before B.D.C.
E.P.C.
T.P.O.
T.P.C.
I.P.O.
I.P.C.

19. Theoretical & Actual Valve Timing Diagram For
2-Stroke Diesel Engine

20. Actual Valve Timing For 2-Stroke Diesel
Engine
E.P.O. → 80° Before B.D.C.
E.P.C. → 80° After B.D.C.
T.P.O. → 60° Before B.D.C.
T.P.C. → 60° After B.D.C.