Purpose of liners Liners are provided in order to increase the service life of the engine ie wear resistant surface for bore It simplifies the production of cast iron engines Material used Liners are made of cast iron and special alloys of iron containing silicon manganese nickel and chr ID: 694164
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Slide1
Cylinder Liners
Liners are cylindrical components that fit inside the cylinder bore.Slide2
Purpose of liners:
Liners are provided in order to increase the service life of the engine i.e., wear resistant surface for boreIt simplifies the production of cast iron engines
Material used:
Liners are made of cast iron and special alloys of iron containing silicon, manganese, nickel and chromium.
There are two types of liners
Dry liner
and
Wet linerSlide3
Dry liner
It is in the form of a barrel having flange at the top which fits in the grooves of block. It is not directly in contact with water. Hence it is called dry liner. It is machined from both the sides.
Wet liner
It is in the form of barrel shape provided with flange at top which fits into the grooves of the cylinder block.
Grooves are provided to cylinder or liner. Rubber packings are inserted into the bottom grooves.
These are having direct contact with water. Hence called as wet liners.
Machining from both the sides are not required because it does not bear against the cylinder block.Slide4Slide5
Comparison of dry liners with wet liners
Sl. No.Description
Dry Liners
Wet Liners
1.
Contribution
of rigidity of cylinder block
More rigidity
Less rigidity
2.
Introduction of thermal barrier at the adjoining surfaces
More
Less
3.
Cooling
Inferior
Better
4.
Renewal after wear
Comparatively difficult
Easy
5.
Chances of coolant leakage
Less
More Slide6
PISTONSlide7Slide8Slide9
Functions of a Piston
To transmit the force of explosion to the crankshaftTo form a seal for high pressure gasses in the combustion chamber do not escape into the crankcase
To serve as a guide and a bearing for small end of the connecting rodSlide10
Piston must satisfy the following conditions
Silent in operation
Rigidity for high pressure
The design should be such that seizure does not occur
Resistant to corrosion due to some products of combustion e.g. sulphur dioxide
It should have shortest possible length so as to decrease overall engine size
Lighter in weight so that inertia forces created by its reciprocating motion are minimized
High thermal conductivity for efficient heat transfer so that higher compression ratios may be obtained without the occurrence of detonation Slide11Slide12
Top of the piston is called
head or crown Some pistons head is provided valve relief.
Pistons used in some high powered engines may have raised domes, which increases compression ratio as well as controls the Combustion.
In some engines pistons may be specially dished to form the desired shape of the combustion chamber. Slide13
Piston ‘Skirt’
The part of the piston below the rings is called ‘Skirt’.Its function is to form a guide suitable for absorbing the side thrust produced on account of the inclination of the connecting rod.
It must be of sufficient length to resist tilting of the piston under load.
The combustion pressure from the piston crown is transmitted to the connecting rod through the ‘webs’ inside the piston.
The webs also form heat path from the piston crown to the gudgeon pin bosses and the skirt. The ‘Bosses’ form a bearing surface for the rocking motion of the connecting rod. Slide14
Piston Materials
Common materials used are cast iron, cast aluminium, forged aluminium, cast steel, forged steel and alloys of aluminium.
Cast iron is the ideal material for rubbing surfaces of piston.
Cast iron and steel have the high strength required for good wearing qualities at required temperature and low thermal expansions.
But heavier pistons than aluminium and low thermal conductivity
Aluminium alloy pistons have the advantage of low weight and high thermal conductivity which makes it to run cool.
But it is less strong and hence thicker sections have to be used. Slide15
Since it is softer fine particles of lubricating oil get embedded on it. Piston life gets shortened due to abrasion
Expansion is about 2.5 times that of the cast iron Strength also reduces as temperature rises
Due to unequal coefficient of expansion from the cylinder if large cold clearance is kept the ‘piston slap’ would occur.Slide16
To prevent heat from the piston skirt following methods are adoptedBy providing horizontal slot
By providing inclined slot in the oil ring groove
By making a heat dam
By vertical slot
By T-slot
By tapered piston
Using special alloys
Wire wound pistons
Autothermic piston
Bi-metal piston
Aeconoguide pistonSlide17Slide18Slide19
Wire wound pistons:A band of steel wire under initial tension is put between the
piston pin and the oil control ring, thus restricting the expansion of the skirt .
Autothermic piston:
This type of piston contains low thermal expansion steel insert at the piston pin bosses. These inserts are so moulded that their
ends are anchored in the piston skirt as shown
.Slide20
Bi-metal piston:In this type of piston skirt is formed by steel and the aluminium alloy cast inside it forms piston head and piston pin bosses.
As the coefficient of thermal expansion for steel is quite small, the piston will not expand much and hence smaller cold clearances can be maintained.Aeconoguide piston:
This is a method to reduce skirt friction. The skirt contact area is reduced by about 75% compared to conventional pistons.
And consist of raised pads which are specially shaped to assist the hydrodynamic lubrication.Slide21
Piston head designs Slide22
Piston Failures
Scuffing
Scoring
Burnt Piston
Worn out Circlip groove
Damaged piston pin boss
Damaged Ring landSlide23
Scuffing :
This occurs due to excessive heat, the piston expands and becomes tight in the cylinder. As a result lubricant is squeezed out from the cylinder walls and metal to metal contact takes place. Scoring:
Piston is scored as a result of carbon build-up
.
Accumulation of carbon and other deposits on the piston skirt. Particles of carbon breaking away
from the exhaust ports, lodging between the piston skirt and cylinder results in scoring the piston.
Damage to Ring land:
This occurs mainly because excessive ring groove clearance and
attempting to remove the piston without first removing the cylinder ridge.
Damage to piston pin boss and circlip groove:
This occurs rocking motion of the connecting rod due to bent connecting rod or tapered crankpins or loosely installed circlip
Piston FailuresSlide24
PISTON RINGS
These are circular rings and made of special steel alloys which retain elastic properties even at high temp.
These are housed in the circumferential grooves of piston outer surface
Main
function of the piston ring is to impart the necessary radial pressure to maintain the seal between piston and the cylinder bore. Slide25
It prevents gas leakage into the crank case. It has contacts with cylinder walls evenly and tightly fit into the grooves.
It is made by fine grained cast iron. Modern rings are made by steel.Types
of piston rings:
Compression rings or Pressure rings
Oil control rings or Oil scraper rings
The compression rings are located at the top portion of the piston.
Two or three compression rings are fitted in order to increase the compression ratio and one or two oil rings to remove the excess oil from the cylinder walls.Slide26
Functions of compression rings
To seal between the piston and the cylinder liner.To transfer the
heat
To
absorb piston
fluctuations due to side thrust.
Any
number of piston rings.
The heat transfer is better from rings to the
liner.
It is better to use a number of narrow rings than a few wide shallow rings.Slide27
Types of rings and groovesSlide28
Functions of oil control rings
To scrape the lubricating oil from the surface of the liner. During the upward stroke these should allow sufficient oil to go upwards for the proper lubrication of the liner.Slide29
Types of piston ring Ends
Butt cutAngle
cut
Square step cut
joint or Lap
Round step cut jointSlide30
CONNECTING ROD
Manufactured by drop forgingShould have adequate strength, stiffness with minimum weightMaterial
Shape of connecting rod:
Rectangular
Circular
Tubular
I-section
H-sectionSlide31
Connecting Rod
There are 2 types of small end
and big end bearings
Split
at right angle to its length
Split a
t
an angle Slide32
Connecting
Rod AssemblySlide33
Cross Section of Connecting
RodSlide34
Crank ShaftSlide35Slide36
The Front end of the crankshaft carries three devices- gear or sprocket that drives the camshaft, vibration damper to control torsional vibration and the fan belt pulley.
Rear end of the crankshaft carries fly wheel.Crank shaft have drilled oil passages through which oil can flow from the main bearings to the connecting rod bearingSlide37
ValvesSlide38
Valve is a device to admit the Air-Fuel mixture and expel gases from cylinder.
Inlet valve is made by
Nickel Chromium alloy steel
and Exhaust valve is made by
Silichrome steel
.
The face of the valve should be ground to make an angle
45
0
to 30
0
in order to match the angle of valve seat in the head or block.
Valves used in modern vehicles are treated as mushroom valves.
There are two types of valve mechanisms namely
“Straight mushroom valve mechanism” and
“Overhead mushroom valve mechanism”Slide39
Overhead Valve MechanismSlide40
As crank shaft rotates, cam shaft rotates will also rotates there by rotating cams mounted on it. Cams lift the push rod by valve lifter.
Push rod actuates the rocker arm and opens the valve. Valves are mounted in the cylinder head and valve clearance is in between rocker arm and valve stem.Slide41
Straight Mushroom Valve MechanismSlide42
When the crank shaft rotates cam shaft rotates, valve lifter slides up and down due to rotation of cams.
When the valve lifter moves upwards valve stem moves upward which operates the valves. Valve mechanism is provided in the engine block. Here valve clearance is between valve stem and valve lifterSlide43
Valve Timing diagram
EVO
EVC
IVO
IVC
Valve OverlapSlide44
To see how valve-timing works in a 4-stroke engine cycle, let’s show piston motion as a circle. In this simple cycle, each stroke is shown as a semi-circle.
The intake valve opens at top dead center, and closes at bottom dead center. The blue line shows that period and it matches the intake stroke.
The exhaust valve opens at bottom dead center, then closes at top dead center before the new air-fuel mixture enters the cylinder.
In practice, the intake valve usually opens earlier than top dead center, and stays open a little past bottom dead center.
The exhaust valve opens a little before bottom dead center, and stays open a little past top dead center.
When the valves actually open and close, can be measured by angles. To make these angles easier to read, let’s use a
spiral
instead of a circle.Slide45
This intake valve opens 12° before the piston reaches top dead center.
And it closes 40° after bottom dead center.The exhaust valve opens 47° before bottom dead center - and stays open - until 21° past top dead center. This gives exhaust gases more time to leave.By the time the piston is at 47° before bottom dead center on the power stroke, combustion pressures have dropped considerably and little power is lost by letting the exhaust gases have more time to exit.
When an intake valve opens before top dead center and the exhaust valve opens before bottom dead center, it is called
lead.
When an intake valve closes after bottom dead center, and the exhaust valve closes after top dead center, it is called lag.
On the exhaust stroke, the intake and exhaust valve are open at the same time for a few degrees around top dead center. This is called valve
overlap.
On this engine, it is 33°.
Different engines use different timings. Manufacturer specifications contain the exact information. Slide46
Port Timing Diagram
(TDC)
(BDC)
IPO
IPC
EPO
EPCSlide47
Exhaust Port opens before piston reaches BDC and after sometime inlet valve opens. During next stroke of the piston, first inlet valve closes then exhaust valve closes. After exhaust port closes compression of fuel-air mixture takes placeSlide48