sectional model of valve timer of four stroke engine

2. 
 Fawaz Khalid
 H.Zia-ur-Rahman
 Usman Ali Sher
 Talha Hanif
Member’s Name

3. 
 Introduction
 Valve overlap
 External Combustion Engine
 Parts
 Ported Engine
 Working
Contents

4. 
Valve Timing
In a piston engine, the valve timing is
the precise timing of the opening and
closing of the valves. In an internal
combustion engine these are
usually poppet valves and in a steam
engine they are usually slide
valves or piston valves.
Introduction

5. 
Continue……….
The inlet valve is made to open
10degree to 30degree before the
piston reaches the Top Dead
Center (TDC) during Suction
Stroke and is allowed to close only
after 30degree to 40degree after the
piston reaches and leaves the BDC
in the beginning of compression
stroke

6. 
 The exhaust valve is made to open 30degree to
60degree before the TDC in the exhaust stroke and
allowed to close only after 80 to 10 in0 the beginning
of the suction stroke
Continue……

7. 
 With traditional fixed valve timing, an engine will have a period of "valve
overlap" at the end of the exhaust stroke, when both the intake and exhaust
valves are open. The intake valve is opened before the exhaust gases have
completely left the cylinder, and their considerable velocity assists in
drawing in the fresh charge. Engine designers aim to close the exhaust
valve just as the fresh charge from the intake valve reaches it, to prevent
either loss of fresh charge or unscavenged exhaust gas. In the diagram, the
valve overlap periods are indicated by the overlap of the red and blue arcs.
Key:
 TDC = Top dead centre
 BDC = Bottom dead centre
 IO = Inlet valve opens
 IC = Inlet valve closes
 EO = Exhaust valve opens
 EC = Exhaust valve closes
Valve Overlap

8. 
 Many two-stroke cycle and all wankel
engines do not have a camshaft or
valves, and the port timing can only be
varied by machining the ports, and/or
modifying the piston skirt (two stroke
applications). However,
some supercharged two-stroke diesel
engines (such as the Wilksch aero-
engine) do have a cylinder head and
poppet valves, similar to a four-stroke
cycle engine.
Ported Engine

9. 
 Cam
 Camchaft
 Follower
 Inlet valve
 Outlet valve
 Fly wheel
Parts

10. 
 A cam is a rotating or sliding
piece in a mechanical
linkage used especially in
transforming rotary motion
into linear motion or vice
versa. It is often a part of a
rotating wheel (e.g. an eccentric
wheel) or shaft (e.g. a cylinder
with an irregular shape) that
strikes a lever at one or more
points on its circular path.
 Our cam is made of wood
Cam

11. 
 A camshaft is a shaft to
which a cam is fastened or
of which a cam forms an
integral part.
 Our camshaft is made of
wood.
Cam Shaft

12. 
A inlet valve is a valve typically used to
control the timing and quantity of gas or
vapour flow into an engine. It consists of
a hole, usually round or oval, and a
tapered plug, usually a disk shape on
the end of a shaft also called a valve
stem. The portion of the hole where the
plug meets with it is referred as the 'seat'
or 'valve seat'. The shaft guides the plug
portion by sliding through a valve
guide. In exhaust applications a pressure
differential helps to seal the valve and
in intake valves a pressure differential
helps open it
Inlet and Outlet valve

13. 
 A flywheel is a rotating
mechanical device that is used to
store rotational energy. Flywheels
have a significant moment of
inertia and thus resist changes in
rotational speed
Fly wheel

14. 
 The Camshaft is the "heart" of the gasoline
engine. Your engine will not perform to it’s highest
potential unless the cam is precise to provide
performance at the speeds that you require.
Everything else in your engine may be performing
tip-top, the carburetor and ignition systems for
example.
 You are familiar with the strokes of the four cycle
engine, intake, compression, power and exhaust.
Working

15. 
 Each stroke is one half of a revolution or 180° of
crank movement. All four strokes (4 x 180° = 720° )
represent two complete revolutions of the crank.
 The purpose of the cam is to operate the valves in the
correct sequence in relation to piston movement.
 The cam is connected by a 2-1 ratio sprocket or gears,
so it only turns once to the crank’s two turns (Two
crank degrees equal one cam degree).
Continue….

16. 
 In order for a diesel engine to operate, all of its
components must perform their functions at very
precise intervals in relation to the motion of the
piston. To accomplish this a camshaft is used.
Continue….

17. 
 A camshaft is a long bar with egg-shaped eccentric lobes,
one lobe for each valve and fuel injector. Each lobe has a
follower as shown on Figure 10. As the camshaft is
rotated, the follower is forced up and down as it follows
the profile of the cam lobe. The followers are connected to
the engine's valves and fuel injectors through various
types of linkages called pushrods and rocker arms. The
pushrods and rocker arms transfer the reciprocating
motion generated by the cam shaft lobes to the valves and
injectors, opening and closing them as needed. The valves
are maintained closed by springs.
Continue….

18. 
Continue….

19. 
 As the valve is opened by the camshaft, it compresses the
valve spring. The energy stored in the valve spring is then
used to close the valve as the camshaft lobe rotates out
from under the follower.
 The camshaft is driven by the engine's crank shaft
through a series of gears called idler gears and timing
gears. The gears allow the rotation of the camshaft to
correspond or be in time with, the rotation of the crank
shaft and thereby allows the valve opening, valve closing
to be timed to occur at precise intervals in the piston's
travel.
Continue….

20. 
Any Question ???
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