Ppt

1. .
MARINE INTERNAL COMBUSTION ENGINES & TECH –II
V –SEMESTER (2021Entry)
August-2023
PPT - 1
S K Mukherjee, Faculty
Indian Maritime University, Kolkata Campus.

2. Marine Internal Combustion Engines - II
Forces & Stresses in Engines: (introduction)
• All Engines (and, in every cylinder), disturbing forces act
along the cylinder axis due to acceleration and deceleration
of the piston assembly mass.
• At the same time, a rotating out of balance force due to
the crank throw and big end bearing, produces a disturbing
force which at a fixed engine speed, is constant in
magnitude but rotates through 360 degrees as the
crankshaft rotates.
• These continuously varying forces impose loads on the main
bearings and shake the whole structure.

3. Marine Internal Combustion Engines - II
• With more than one cylinder, a series of such forces occur
simultaneously at phase angles dependent on the crank unit
in use, but in planes axially parallel to one another.
• These together give resulting disturbing forces and couples
tending to produce pitching, yawing and sometimes rolling
motion of the whole engine structure.

4. Marine Internal Combustion Engines - II
FORCES & STRESSES in Engine Structures:
The diagram shows
the forces from the
moving components of
the engine:
The piston and X-Head
assembly gives
vertical forces, while
the Big End bearing
mass gives Vertical &
Horizontal forces
which are
continuously changing
in direction.
The lateral thrust at
X-Head axis arises
from angularity of con
rod.

5. Marine Internal Combustion Engines - II
From the given diagrams the following points regarding forces
need be noted:
1. The compressive forces on piston rod never changes direction
(2-St Engines), but varies in magnitude.
2. The forces in the X-Head Guides vary in both:- direction AND
magnitude.
One guide takes up the forces when firing forces arise, and the
other (or opposite side) takes up during compression. Changes
take place during astern movement.
These forces rock the engine sideways to cause fluctuating
stresses in holding down bolts.

6. Marine Internal Combustion Engines - II
3. The force in the con rod, like the piston rod will vary in
magnitude but not in direction.
4. The result from the thrust of the con-rod will generate both-
radial and tangential forces at the bottom end bearing mass.
5. It is the tangential force that turns the crankshaft and develops
power, whilst the radial force either tries to compress the webs
or extend them, as the cycle is completed.
6. There will be a force generated by masses as they rotate about
the shaft axis.
7. The centrifugal force acts against the radial force at one part
of the cycle and with it at another.

7. Marine Internal Combustion Engines - II
8. The centrifugal forces taken in the horizontal plane are
responsible for ‘couples’ that try to turn the engine in the
horizontal plane as each couple is formed.
9. This causes a ‘snake like’ movement to be attempted in the
bedplate. The holding down bolts and side chocks protect against
such movement, as does the built in stiffness of the bed plate
structure.

8. MICE & Tech II
 Types of Moments: The Vibration characteristics developed from
varying moments can be split into FOUR categories;
 1.External unbalanced moments which are classified as 1st Order &
2nd order external moments relevant to engines with less than 5
cylinders.
 2. Guide Force moments
 3. Axial Vibrations of the shaft system.
 4. Torsional Vibrations in the shaft system.

9. MICE & Tech II
Diagram shows Forces & Moments from 2 units of a low speed
Engine.

10. Marine Internal Combustion Engines - II
Torques, Moments in Engine Structures:
Lateral forces due to reactions at the crossheads and main
bearings cause the engine to vibrate in a sideways direction
either in H-Mode or in X-Mode.
Such forces causing movement is not normally detrimental
to the engine itself, but can cause damage to attached parts
such as the holding bolts of the Turbo-chargers, connected
piping, bracings if not provided with flexibility.

11. Marine Internal Combustion Engines - II
X-Mode & H-Mode couples in Engine Structures:
The X-Mode & H-mode
Couples tend to cause
Rolling, pitching and
Yawing motions on the
Engine.
These movements are
Restricted through use
Of chocks,holding down
bolts
And bracings at the top.

12. Marine Internal Combustion Engines - II
Combined Forces
& Moments
arising in a
working 2-Stroke
Engine.

13. Marine Internal Combustion Engines - II
Hydraulic Bracing.
Purpose: to dampen
side to side movement
of upper region of a 2-
St Cross-head engine.

14. Marine Internal Combustion Engines - II
Ships Motions……similarity of Propulsion Engines…..

15. Marine Internal Combustion Engines - II
Reciprocating & rotating masses have a tendency to cause Rolling, pitching & Yawing motions on the engine.
The holding down bolts & bracings restrict such motions

16. Marine Internal Combustion Engines - II
H-Couple would result in a periodic motion of the engine, where the
topmost part of the engine is in a phase opposite to the part in contact
with the engine foundation.
X-Couple would result in the forward end of the engine being in a
phase opposite to that of the aft end of the engine. When the frequency
of these forces are in the range of the natural frequency of the engine
foundation, the foundation resonates, resulting in local vibrations in the
engine room bottom structure.
• Thus, to prevent this motion, lateral stays or top bracings are used to
connect the top structure of the engine to the hull girder. Sometimes,
at earlier stages of structural design, a redesign of the engine
foundation is recommended so as to change the stiffness of the
structure.

17. MICE & Tech II
 Balancing of moving components:
 In the context of ‘out of balance’ forces arising due to rotating and
reciprocating masses ‘Balancing’ becomes necessary. The following
methods are generally used:
1. Counterweights. Mounted on webs opposite to the crank-pin.
2. Balance shafts. Counter rotating shafts to c/s.
3. Vibration dampers. These are axial & torsional ‘detuners’.
Other forms of balancing and reducing vibratory movements are:
Resilient mountings/active vib control systems
(computerized)/propeller design & maintenance/sound insul &
damping material/bracings.

18. MICE & Tech II
BALANCE SHAFT
CRANK SHAFT.
How Balance shaft is arranged alongside a crank shaft to bance out of bance
forces.
Balance weights

19. MICE & Tech II
Bracing for 2-Stroke Engines: Reduces transverse Rolling motion of the engine.
Fitted at top of engine (entablature). Either on one side, or both – Fwd & Aft.

20. Marine Internal Combustion Engines - II
Vibrations in Ships & Engines:
• This is common in ships and is undesirable.
• The two most noticeable effects of such vibration is Structural
Fatigue, and Discomfort to crew & passengers.
• Vibration in ships is categorized into two types:
a) Machinery Vibration
b) Hull Vibration.
All machinery with parts moving at certain frequencies induce
vibrations. Main engines/shafts/gear boxes/propellors/pumps
etc transmit vibrations.

21. Marine Internal Combustion Engines - II
Vibration causes Damages:
If the vibration level or amplitude in engine increases more than normal,
then following will result:
• Cracks in attached piping.
• Effects in turbo charger as it’s a high speed machine.
• Fretting in the engine structure joints (between A frame and entablature).
• Loosen of engine chocks and holding down bolts.
• Damage in the intermediate shaft, its bearing or bearing support
structure.
• Damage to the thrust bearing.
• Damage to the main bearings.

22. Marine Internal Combustion Engines - II
Machinery vibrations can then be categorized into THREE types
depending on the nature of the vibrations.
1. Torsional Vibrations (rotating shafts)
2. Axial (linear, or Longitudinal ) Vibrations
3. Transverse (lateral, or sideways) Vibration.

23. Marine Internal Combustion Engines - II
Sometimes referred as: Torsional
Vibration

24. Marine Internal Combustion Engines - II
• Torsional Vibration:
• The varying gas pressure in the cylinder during the working cycle
and the c/s-con-rod mechanism create a varying torque in the c/s.
• It is these variations that cause the excitation of torsional vibration
of the shaft system.
• Torsional excitation also comes from the propellor through the
interaction with the non-uniform wake field.
• Te solution is to use torsional vibration dampers or ‘detuners’.

25. Marine Internal Combustion Engines
AXIAL Vibration of C/shafts:
i) The repeated bending of the crank pins alternatively, as the
crankshaft rotates results in an ‘in and out’ movement of the webs.
When such microscopic movements occur rapidly, an axial vibration
follows.
ii) The varying pressures acting on the propellor blades cause
fluctuating forces on the shafting. This too results in contributing to the
axial vibration. The Thrust block and pads bear some brunt of these
vibratory forces.
iii) Torsional vibration with accompanying shear stresses also contributes
to axial vibrations in shafts. Solution: Axial Vibration dampers.

26. Marine Internal Combustion Engines - II
Vibrations – Types, Effects & Damping:
• Vibrations in DE are most complex as magnitude and direction of
the forces creating the vibrations vary throughout one
revolution. (Mathematical approach is required)
• The firing forces in a slow running engine create large low
frequency vibrations in contrast to the blades of a turbocharger
which have high frequency but low magnitude of vibration. Both
these types can cause component failure.
• The failure that results from vibration is almost universally
“fatigue failure”.

27. MICE & Tech II
 RADIAL VIBRATION:
 Occurs when there is an oscillation or movement of engine
components in a radial direction. The term ‘radial’ direction is that
which extends outward from the centre of the engine. This type of
vibration is typically perpendicular to the engine rotational axis
around which the c/s rotates.
 Solutions: Balancing/re-alignment/vibration isolation
mounts/resonance avoidance/active vibration control/stiffness &
damping/correct lubrication/real time monitoring/component
inspection & maintenance/quality manufacturing.

28. Marine Internal Combustion Engines - II
• This type of fatigue failure accounts for the greatest proportion
of material failure in engine components.
• Vibrations can be separated into one of two forms:
i) one is the natural vibration which is a function of the
material itself and its resistance to movement.
ii) The other form is ‘forced vibration’. This is the result of the
frequency with which the applied force occurs.
Ex: A 6 cylinder engine rotating at 100 rpm will have a forcing
frequency of 6x (100 x60) = 36000Hz.

29. Marine Internal Combustion Engines - II
• The main problem arises when the natural and forced
frequencies coincide. Resonance is said to occur.
• The forcing frequency acting at the same time and in the same
direction tends to amplify the natural frequency substantially to
such an extant that the strength of the material may no longer
be able to withstand the stressing within the component.
• Ultimately fatigue failure occurs, with cracks passing through
the material until insufficient area is left to carry the load and
complete failure (rupture) takes place.

30. Marine Internal Combustion Engines - II
The so called “CRITICAL SPEED” is that at which the torsional
forces created by the firing impulses, and the reactions from
the shafting system (with propellor) synchronize with the
natural frequency of the shafting to give amplified Torsional
vibrations. These vibrations reverberate through the entire
ship.
Such effects can result in severe damage to the marine
engine’s internal moving parts, cracks in the structure,
loosening of bolts and securing arrangements, and damage to
bearings. Vibration of Marine Engines is mainly due to- Axial
and Torsional Vibration or combination of both.

31. Marine Internal Combustion Engines - II
Fatigue Failures in Engine Components:

32. Marine Internal Combustion Engines - II

33. MICE & Tech.II
 Effects of vibration:
 Component increased wear & damage.
 Reduced efficiency.
 Unwanted noise.
 Electrical issues (dislocation of fittings and loss of contacts)
 Misalignment.
 Safety concerns.
 Increased maintenance costs.
 Structural damage through early fatigue failures.

34. Marine Internal Combustion Engines - II
DAMPING: As the name suggests, dampers are used to dampen or
reduce the frequency of oscillation of the vibrating components of
the machine by absorbing a part of energy evolved during vibration.
 Axial Damper: The Axial damper is fitted on the crankshaft of the
engine to dampen the shaft generated axial vibration i.e.
oscillation of the shaft in forward and aft directions, parallel to the
shaft horizontal line.
 It consists of a damping flange integrated to the crankshaft and
placed near the last main bearing girder, inside a cylindrical
casing. The casing is filled with system oil on both side of flanges
supplied via small orifice. This oil provides the damping effect.

35. Marine Internal Combustion Engines - II
When the crankshaft vibrates axially,
the oil in the sides of damping flange
circulates inside the casing through
a throttling valve provided from one
side of the flange to the other,
which gives a damping effect.
 The casing is provided with high
temperature alarm and pressure
monitoring alarms located on both
sides of damping flanges. They
give alarm if one side oil pressure
drops more than the set value as a
result of low LO supply, sealing
ring failure etc. Transverse
Girder
SHAFT
FLANGE
Seals

36. MICE & Tech - II
Principle of an Axial vibration
Damper:
The axial movement of the
crankshaft is cushioned in
either direction by
compression and throttle
release of pressurized oil.
A piston with seal rings
isolates the above and below
chambers. The squeezed oil is
released through a needle
valve which connects the two
chambers.

37. Marine Internal Combustion Engines - II
Balance Weights: These may be fitted to change the natural frequency of
the shaft as well as to counter some of the rotational out of balance forces
generated by the crank throw.
 In the case of the 4-
stroke medium speed engine , balance weights on crank webs
restricted to half the total reciprocating mass is fitted to crank
webs onlyfor the odd number of cylinder inline engines. This
reduces the resultant primary and secondary couple
balance by about 50 percent.
 In the case of the 2-stroke
slow speed engine moment compensator are fitted to
neutralise the primaryand/or secondary couple imbalance.

38. Marine Internal Combustion Engines - II
4-Stroke Engines
2-Stroke Engines

39. Marine Internal Combustion Engines - II
Torsional Vibration Damping….
‘DETUNERS’: usually in the form of floating
mass in the shaft system are particularly
useful in damping out torsional vibrations.
This is achieved by changing the natural
frequency of the shaft as the floating mass
drives back into the shaft. A detuner is
essentially a torsional vibration damper.

40. Marine Internal Combustion Engines - II
TORSIONAL Vibration dampers.(contd)

41. MICE & Tech II
OVERLOADING: When increased demands are placed on the engine
beyond its designed capacity, various components are subjected to
stress levels which over a prolonged period will cause the
components to fail.
All components are made with a “Factor of Safety”. Numerically :
Factor of Safety = Ultimate tensile stress/Permissable working
stress.
Stresses during overload occur chiefly in the following areas:
1. Crankshaft & Bearings (Main/X-Hd/Big End): subjected to increased
to higher bending and torsional stresses. Rapid bearing wear.

42. MICE & Tech - II
2. Pistons & Con-rods: intense fluctuating stress on crown compounded
with higher temps may cause diaphragm cracks (early fatigue).Piston
ring fails as Lubrication fails to cause blowpast. Cracks at grooves.
3. Cylinder heads & valves: Exposed to higher temps & pressures , they
become thermally stressed to result in warping (distortion) and
cracking. Valves when overheated, distort the seats to cause
leakage, and further overheating and damage to v/v disk, seats.
4. T/C & Intercoolers: The T/C overheats to cause rapid oxidation,
thickening and carbonizing of the turbine end lub oil for the bearing.
This damages the bearing to give misaligned rotor and consequences.
The casing also will distort or crack. The intercoolers will fail to
function within designed capacity.

43. Marine Internal Combustion Engines - II
Exhaust System: The exh. manifold overheats and is a potential fire
hazard. The expansion bellows crack to leak exh gas into engine room.
Uptake fires may result with soot accumulation in Economizer tube
stacks.The pipe flanges, nuts & bolts may fuse into each other.
Cylinder liners: These will suffer rapid wear and scuffing due to possible
lub oil failure. Will further result in reduced compression and
consequential blow-past, and scavenge fires. Cracking may result.
Will cause early failure of liners necessitating expensive replacements.

44. Marine Internal Combustion Engines - II
Overall:
Overloading can lead to accelerated wear and tear, reduced efficiency,
increased fuel consumption, and higher risks of mechanical failures.
To avoid mishaps , it is prudent to run an engine within its designed
parameters and continuously monitor its performance.

45. Marine Internal Combustion Engines & Tech - II
THANK YOU
S K Mukherjee,
Faculty – IMU Kolkata Campus.