The Medium Speed 4 Stroke Trunk Piston
The cylinder liner is cast separately from the main cylinder frame for
the same reasons as given for the 2 stroke engine which are:
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 nodular 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
Modern liners employ bore cooling at the top of the
liner where the pressure stress is high and therefore the liner
wall thickness has to be increased. This brings the cooling water
close to the liner surface to keep the liner wall temperature
within acceptable limits so that there is not a breakdown in
lubrication or excessive thermal stressing. Although the liner is
splash lubricated from the revolving crankshaft, cylinder
lubricators may be provided on the larger engines.
On the example shown opposite, the lubricator drillings are bored
from the bottom of the liner circumferentially around the liner
wall. Another set of holes are drilled to meet up with these
vertically bored holes at the point where the oil is required at
the liner surface.
Other engines may utilise axial drillings as in a two stroke
Sulzer ZA40 Liner (vee engine; The straight
engine is similar)
MAN-B&W L58/64 Liner
Where the cooling water space is formed between the
engine frame and the jacket, there is a danger that water could
leak down and contaminate the crankcase if the sealing O rings
were to fail. As a warning, "tell tale" holes are led
from between the O rings to the outside of the engine.
modern engines tend not to use this space for cooling water.
Instead a separate water jacket is mounted above the cylinder
frame. This stops any risk of leakage of water from the cooling
space into the crankcase (or oil into the cooling water space),
and provides the cooling at the hottest part of the cylinder
Note that the liner opposite is fitted with a fireband. This is
sometimes known as an antipolishing ring. It is slightly smaller
in diameter than the liner, and its purpose is to remove the
carbon which builds up on the piston above the top ring. If this
carbon is allowed to build up it will eventually rub against the
liner wall, polishing it and destroying its oil retention
The liner must be gauged regularly to establish the wear rate and
check that it is within manufacturers tolerances. The wear rate for a
medium speed liner should be below 0.015mm/1000hrs. Excessive wear is
caused by lack of lubrication, impurities in fuel air or Lubricating
oil, bad combustion and acid attack.