Transcript Cross Head Engine Cylinder Liner

Cylinder liner
• The cylinder liner forms the
cylindrical space in which the
piston reciprocates. The
reasons for manufacturing the
liner separately from the
cylinder block are as follows:
• The liner can be made of a
superior material to the
cylinder block. While the
cylinder block is made from
grey cast iron, the cylinder liner
is manufactured from cast iron
alloyed with chromium,
vanadium and molybdenum.
Cast iron contains graphite,
which assists lubrication,
whereas the alloying elements
help resists corrosion as well
as improve wear resistance at
higher temperature.
• The cylinder liner may
require replacement as it
wears down. The cylinder
jacket lasts the life of the
engine.
• At working temperature,
the liner is a lot hotter
than the cylinder block.
The liner will expand
more and is allowed to
expand lengthwise and
diametrically. If they were
cast together than
unacceptable thermal
stresses would be set up,
which would cause
fracture.
• Less risk of defects. The
more complex a casting,
more difficult it becomes
to cast a homogeneous
casting with low residual
stresses.
• The liner will tend to get
very hot during engine
operation as the heat
from the burning fuel is
transferred to the cylinder
wall. The liner is cooled
by fresh water so that the
temperature can be kept
within acceptable limits.
• Cylinder liners from older
lower powered engines
had a uniform wall
thickness and cooling
was achieved by
circulating cooling water
through a space formed
between the liner and
jacket. The cooling water
space was sealed from
the scavenge space by
rubber ‘O’ rings. Telltale
passages between the
rings were led outside the
cylinder block to show
leakages.
• To increase the power of
the engine for a given
number of cylinders,
engine rpm must be
increased or more fuel
must be burnt per cycle. .
If more fuel is burnt per
cycle, than the pressure
in the combustion space
will be much higher.
Hence the liner must be
made thicker on top to
withstand higher hoop
stresses and prevent
cracking of material.
• If thickness of the
material is increased,
than it stands to reason
that the surface
temperature of the liner
will increase because the
cooling water is further
away. Increased surface
means that thermal stress
will increase, material
strength will reduce
resulting higher wear
down and burning of lube
oil film, resulting in
excessive wear.
• The solution is to bring the cooling water closer
to the cylinder wall. To achieve this, cylinder
liners are provided with bore coolings. Holes are
drilled from the underside of the flange formed
due to increase in liner diameter. The holes are
bored upwards and at an angle so that approach
the internal surface of the liner at a tangent.
Holes are then bored radially around the top of
the liner so that they join the tangential bored
holes. On some large bore engines long stroke
engines, it was found that further down the liner,
under cooling was taking place.
• Hydrogen of the hydrocarbon fuel forms H2O on
combustion and sulphur (an impurity upto 4.5%)
in the fuel on combustion SO2 and SO3. SO3
combines with H2O and forms sulphuric acid
(H2SO4). The dew point of sulphuric acid is
between 1250 C and 1400 C, depending on the
concentration of sulphuric acid. If liner is under
cooled and its temperature goes below that of
the dew point of the sulphuric acid, the sulphuric
acid will deposit on the liner surface. This excess
acid will cause corrosion and excessive wear of
the liner after the alkaline cylinder lube oil has
been neutralized. The solution to this problem is
to insulate the bottom of the liner and provide
cooling only to the top part of the liner.
Cylinder lubrication
• As the cylinder is separate from the crankcase, there is
no splash lubrication as is the case of trunk type of
engines. Lube oil for lubrication of liner and piston rings
is supplied through drillings in the liner. Grooves
machined in the liner from the injection points spread the
lube oil circumferentially around the liner and piston rings
help in spreading it up and down the length of liner. This
type of lubrication is called once through type of
lubrication. The lube oil is of high alkalinity, which
combats the acid attack from the sulphur of the fuel. The
latest engines time the injection of lube oil using a
computer, which has input from the crankshaft position,
engine speed and the load of the engine.
• The correct quantity of lube oil is injected by
opening from a pressurized system just the
piston ring pack is passing the injection point.
Correct operation of temperature (not
overloading the engine and maintaining correct
operating temperature) and using the correct
grade and quantity of cylinder lube oil will help to
extend the life of the cylinder liner. Wear rates
vary, but as a general rule, for a large bore
diesel engine, a wear rate of 0.05 to 0.1
mm/1000 hrs. is acceptable. The liner should be
replaced as the wear approaches 0.8 to 1% of
the liner diameter. The liner is gauged at regular
interval for ascertaining the wear rate. It has
been known for ships to go for scrap after +20
years of service with original liners.
• Apart from corrosive
attack, wear is caused
due boundary lubrication,
abrasive wear due to
particles in the cylinder
from fuel (due to bad
filtration/purification) or in
the air and scuffing (also
known as micro-seizure
or adhesive wear).
Scuffing is due to
breakdown in lubrication,
which results in localized
welding between points
on the rings and the liner
and subsequent tearing
of microscopic particles.
This is a very severe form
of wear.