Transcript PISTON - AIMES, Srinivas Integrated Campus,Mukka

Cylinder Liners
• Liners are cylindrical components that fit inside the
cylinder bore.
Purpose of liners:
• Liners are provided in order to increase the service
life of the engine i.e., 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 chromium.
There are two types of liners Dry liner and Wet liner
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.
Comparison of dry liners with wet
liners
Sl.
No.
1.
2.
3.
4.
5.
Description
Dry Liners
Contribution of rigidity of cylinder
More
block
rigidity
Introduction of thermal barrier at
More
the adjoining surfaces
Cooling
Inferior
Renewal after wear
Comparati
vely
difficult
Chances of coolant leakage
Less
Wet
Liners
Less
rigidity
Less
Better
Easy
More
PISTON
Functions of a Piston
1. To transmit the force of explosion to the crankshaft
2. To form a seal for high pressure gasses in the
combustion chamber do not escape into the
crankcase
3. To serve as a guide and a bearing for small end of
the connecting rod
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
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.
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.
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.
• 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.
 To prevent heat from the piston skirt following
methods are adopted
1. By providing horizontal slot
2. By providing inclined slot in the oil ring groove
3. By making a heat dam
4. By vertical slot
5. By T-slot
6. By tapered piston
7. Using special alloys
8. Wire wound pistons
9. Autothermic piston
10. Bi-metal piston
11. Aeconoguide piston
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.
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.
Piston head designs
Piston Failures
Scoring
Burnt Piston
Scuffing
Damaged piston pin boss
Damaged Ring land
Worn out Circlip groove
Piston Failures
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 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.
 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.
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.
Types of rings and grooves
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.
Types of piston ring Ends
•
•
•
•
Butt cut
Angle cut
Square step cut joint or Lap
Round step cut joint
CONNECTING ROD
• Manufactured by drop forging
• Should have adequate strength, stiffness with
minimum weight
• Material
Shape of connecting rod:
 Rectangular
 Circular
 Tubular
 I-section
 H-section
Connecting Rod
There are 2 types of small end and big end bearings
• Split at right angle to its length
• Split at an angle
Connecting Rod Assembly
Cross Section of Connecting Rod
Crank Shaft
• 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 bearing
Valves
•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 450 to 300 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”
Overhead Valve Mechanism
• 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.
Straight Mushroom Valve Mechanism
• 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 lifter
Valve Timing diagram
Valve Overlap
EVC
IVO
IVC
EVO
• 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.
• 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.
Port Timing Diagram
(TDC)
EPO
EPC
IPC
IPO
(BDC)
 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 place