The Two Stroke
The cylinder liner forms the cylindrical space in which the piston
reciprocates. The reasons for manufacturing the liner separately from the
cylinder block (jacket) in which it is located are as follows;
The liner can be manufactured using a superior material to the
cylinder block. While the cylinder block is made from a grey cast
iron, the liner is manufactured from a cast iron alloyed with
chromium, vanadium and molybdenum. (cast iron contains graphite, a
lubricant. The alloying elements help resist corrosion and improve the
wear resistance at high temperatures.)
The cylinder liner will wear with use, and therefore may have to be
replaced. The cylinder jacket lasts the life of the engine.
At working temperature, the liner is a lot hotter than the jacket.
The liner will expand more and is free to expand diametrically and
lengthwise. If they were cast as one piece, then unacceptable thermal
stresses would be set up, causing fracture of the material.
Less risk of defects. The more complex the casting, the more
difficult to produce a homogenous casting with low residual stresses.
The Liner will get tend to get very hot during engine operation as the
heat energy from the burning fuel is transferred to the cylinder wall. So
that the temperature can be kept within acceptable limits the liner is
Cylinder liners from older lower powered engines had a uniform wall
thickness and the cooling was achieved by circulating cooling water
through a space formed between liner and jacket. The cooling water space
was sealed from the scavenge space using 'O' rings and a telltale passage
between the 'O' rings led to the outside of the cylinder block to show a
To increase the power of the engine for a given number of
cylinders, either the efficiency of the engine must be increased or more
fuel must be burnt per cycle. To burn more fuel, the volume of the
combustion space must be increased, and the mass of air for combustion
must be increased. Because of the resulting higher pressures in the
cylinder from the combustion of this greater mass of fuel, and the larger
diameters, the liner must be made thicker at the top to accommodate the
higher hoop stresses, and prevent cracking of the material.
thickness of the material is increased, then it stands to reason that the
working surface of the liner is going to increase in temperature because
the cooling water is now further away. Increased surface temperature means
that the material strength is reduced, and the oil film burnt away,
resulting in excessive wear and increased thermal stressing.
solution is to bring the cooling water closer to the liner wall, and one
method of doing this without compromising the strength of the liner is to
use tangential bore cooling.
are bored from the underside of the flange formed by the increase in liner
diameter. The holes are bored upwards and at an angle so that they
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 with the
tangentially bored holes.
some large bore, long stroke engines it was found that the undercooling
further down the liner was taking place. Why is this a problem? The
hydrogen in the fuel combines with the oxygen and burns to form water.
Normally this is in the form of steam, but if it is cooled it will
condense on the liner surface and wash away the lube oil film. Fuels also
contain sulphur. This burns in the oxygen and the products combine with
the water to form sulphuric acid. If this condenses on the liner surface, then corrosion can take place. Once the oil film has been
destroyed then wear will take place at an alarming rate. One solution is
to insulate the outside of the liner so that there was a reduction in the
cooling effect. On the latest engines, the liner is only cooled by water at the very
top, relying on the air in the scavenge space to cool the lower part of
The photo shows a
cylinder liner with the upper and mid insulation bands known
is a type of Japanese armour, the word also means literally
" Stomach or Body Warmer". i.e an insulator.
lubrication: Because the cylinder is separate from the
crankcase there is no splash lubrication as on a trunk piston engine. Oil
is supplied through drillings in the liner. Grooves machined in the liner
from the injection points spread the oil circumferentially around the
liner and the piston rings assist in spreading the oil up and down the
length of the liner. The oil is of a high alkalinity which combats the
acid attack from the sulphur in the fuel. The latest engines time the
injection of oil using a computer which has inputs from the crankshaft
position, engine load and engine speed. The correct quantity of oil can be
injected by opening valves from a pressurized system, just as the
piston ring pack is passing the injection point.
mentioned earlier, cylinder liners will wear in service. Correct operation
of the engine (not overloading, maintaining correct operating
temperatures) and using the correct grade and quantity of cylinder oil
will all help to extend the life of a cylinder liner. Wear rates vary, but
as a general rule, for a large bore engine a wear rate of 0.05mm/1000 hours is acceptable. The liner should be replaced as the wear
approaches 0.8 - 1% of liner diameter. The liner is gauged at regular
intervals to ascertain the wear rate.
It has been known for ships to go
for scrap after 20 + years of operation with some of the original liners
in the engine.
Gauging a Liner
As well as corrosive attack, wear is caused by abrasive
particles in the cylinder (from bad filtration/purification of fuel or
from particles in the air), and scuffing (also known as micro seizure or
adhesive wear). Scuffing is due to a breakdown in lubrication which
results in localised welding between points on the rings and liner
surface with subsequent tearing of microscopic particles . This is a very
severe form of wear.