PPT on Thermal-II (PPT on Valve Timing Diagram For Four Stroke SI Engine)

1. SIGMA POLYTECHNIC
Saini Tarandeep Singh (126420319613)
Department : Mechanical Engineering (19)
Sem : 5th Sem
Year : 2015-2016
Subject : Thermal Engineering – II
(3351901)

2. Valve Timing Diagram
for four Stroke s.i. Engine

3. Four Stroke S.I. Engine :

4. Main Components of S.I. Engine :
• Cylinder
• Piston
• Piston Ring
• Combustion Chamber
• Inlet Manifold
• Exhaust Manifold
• Inlet and Exhaust Valves
• Spark Plug
• Connection Rod
• Crank Shaft
• Crank Case

5. Spark Ignition (S.I.) :
- Combustion is initiated by spark.
- Air and fuel can be added together.
Compression Ignition (C.I.) :
- Combustion is initiated by spraying the fuel.
- Require fuel Injector to control ignition.
S.I. AND C.I. ENGINES :

6. There are four stroke acting inside the
cylinder :
a.Suction Stroke
b.Compression Stroke
c.Power Stroke OR
Working Stroke OR
Expansion Stroke
d.Exhaust Stroke
WORKING OF FOUR STROKE S.I. ENGINE :

7. In spark ignition engine, the
mixture of air & fuel are usually
entering into the cylinder.
Conventionally, a mechanical
device known as carburetor is
used to mix air and fuel.
AIR-FUEL MIXING :

8. WORKING OF FOUR STROKE S.I. ENGINE :

9. If we have to understand the timing of the valve, we
have to refer the “VALVE TIMING DIAGRAM”.
What is valve timing diagram ?
The Valve Timing Diagram is the diagram which
shows us the angular presentation of opening and
closing of the valves (Inlet & Exhaust Valve) at
different points during the engine is working or
running.
There are two types of Valve Timing Diagram :
i. Theoretical Valve Timing Diagram
ii. Actual Valve Timing Diagram
LETS FOCUS ON THE TIMING OF THE VALVE :

10. THEORETICAL VALVE TIMING DIAGRAM :

11. The valve timing diagram is the diagram which
represents the angular presentation of opening and closing
of valves at different points. For ideal cycle, theoretically,
inlet and exhaust valve must open at dead Centre. Hence,
as shown in figure, inlet valve is opened at TDC. This valve
will close at BDC. As shown in figure, exhaust valve is
opened at BDC when exhaust stroke is starts. This valve
must close when the piston reaches TDC. The above
condition is applicable in cases of theoretical forces, the
actual valve timing diagram is different than theoretical one.
Puppet valve is generally operated by cam-
mechanism. If wear and noise are to be reduced then this
valve must be opened and closed very slowly. Thus,
because of this reason, the cannot be closed instantly.
Since, if this is done than the valve will strike with the valve
seat…..

12. Accordingly, valve must start opening
much earlier progressively before it is
fully open and not instantly. The same is
the case when the valve is being closed
and its time be decided accordingly. This
mechanical effect over and above the
dynamic effect of gas flow is important in
deciding the time setting of the valve
opening and closing.

13. ACTUAL VALVE TIMING DIAGRAM :

14. Figure shows the position of inlet valve,
opening 5 degree to 20 degree before TDC and it
closes 44 degree to 60 degree after BDC. The
reason for this is discussed below :
Generally the valve operate very slowly
when the engine is running fast, then the stroke of
the piston is completed in very short time. During
this, very small quantity of air or charge will enter
the cylinder. This quantity is not sufficient to
produce power. Hence the inlet valve is opened
before TDC and closes after BDC. Hence more air
or charge can enter the cylinder. By closing inlet
valve 40 degree to 60 degree after BDC, there is
no change in compression…
A. INLET VALVE :

15. Because due to motion of air during suction,
compression starts not at BDC but about 40 degree
after BDC. Hence the advantage is taken of the
velocity of air. This is known as Ram effect.
Even through if the Inlet valve is kept open then
air or charge may go out of the inlet valve. The speed
of the engine will be deciding factor of degree of
opening of inlet valve. In slow speed engine, air
velocity during suction is less. Hence, in slow speed
engine, inlet valve is generally closed 10 degree
after BDC. While in the case of high speed engine
the advantage of high speed of the air during suction
is taken into account and larger angle, is kept.

16. The timing of exhaust valve is shown in
figure. As shown in figure, the exhaust valve
opens 30 degree to 40 degree earlier than BDC.
The exhaust valve closes 3 degree to 20 degree
after TDC.
By opening exhaust valve earlier, most of
the expanded exhaust gases leaves during power
stroke. Little less power is produced because of
this. But most of the exhaust gases are removed
before the end to power stroke. Hence, the work
required to be supplied during exhaust stroke is
reduced. Hence there is a net gain. If the exhaust
valve is closed on TDC, then some of the exhaust
gas is compressed in clearance space…
B. EXHAUST VALVE :

17. This will reduce the air or charge taken in
during suction stroke. Based on the
experimental results it is found out that good
results are obtained when the exhaust valve is
closed after 12 degree from TDC.
At some time during cycle both the valves
are open which can be seen from above
discussion. This period is known as “valve
over lap”. The angle of the overlap is generally
kept between 15 degree to 30 degree.

18. THEORETICAL AND ACTUAL P-V DIAGRAM FOR FOUR
STROKE PETROL (S.I.) ENGINE :

19. Petrol engine is usually works on Otto
cycle. In this cycle, the heat is added at
constant volume. Heat is also rejected at
constant volume. Theoretically the
compression and expansion processes
are adiabatic. But in actual practice both
these processes are not adiabatic but as
shown in figure. Hence theoretical and
actual P-V diagrams are different. The
thermal efficiency in actual cycle is much
less than theoretical cycle.