Operational Information
The Medium Speed 4 Stroke Trunk Piston Engine
The Piston
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Pistons for medium speed trunk piston engines which burn residual fuel are composite pistons; i.e the crown
and the skirt are made of different materials.
The crown is a heat resisting steel forging which may be alloyed with chromium, molybdenum and nickel to
maintain strength at high temperatures and resist corrosion. It is dished to form a combustion chamber with
cutouts to allow for the valves opening. The topland (the space between the top ring and the top of the piston)
may be tapered to allow for expansion being greater where the piston is hottest.
The skirt can either be a nodular cast iron or forged or cast silicon aluminium alloy. Aluminium has the
advantage of being light, with low inertia, reducing bearing loading. However because aluminium has a higher
coefficient of expansion than steel, increased clearances must be allowed for during manufacture. This means
that the piston skirt clearance in the liner is greater than that for cast iron when running at low loads. The skirt
transmits the side thrust, caused by the varying angularity of the con rod, to the liner. Too big a clearance will
cause the piston to tilt.
The piston pin for the con rod small end bearing is located in the piston skirt. The piston pin floats in the piston
skirt and is located in place by circlips. Depending on the material used for the skirt (esp. cast aluminium), a
bushing may be used for the pin.
The piston rings may be located in the crown or in both crown and skirt. Normally, the rings are chrome plated
or plasma coated to resist wear. Because the liner is splash lubricated, an oil scraper (oil control) ring is fitted
to the piston skirt.
The piston is oil cooled. This is achieved by various means; The simplest is for a jet of oil to be directed
upwards from a hole in the top of the con rod onto the underside of the crown. A more efficient method is to
use an oil catcher as shown in the picture above. This directs oil into the cooling spaces on the underside of
the crown where the cocktail shaker effect of the reciprocating piston ensures a positive cooling effect. It is
unusual for the oil return temperature to be monitored (unlike the 2 stroke slow speed crosshead engine,
where both temperature and quantity are monitored).
Some engines are fitted with one piece pistons manufactured from either cast iron or silicon alloy aluminium .
These cannot be used with residual fuel, because the higher temperatures causes burning of the piston crown.
Aluminium also suffers from carbon build up above 300º C. Ring grooves in aluminium pistons usually take the
form of a chrome plated cast iron insert.
The Rotating Piston
The Rotating piston is used on the Sulzer ZA40
engine. Instead of a conventional piston pin and
bearing, the top end consists of a two part spherical
The advantages of this system are
At each stroke a different part of the freshly oiled skirt
is in contact with the part of the liner wall absorbing
the side thrust. This reduces wear and the risk of
seizure.
The rings rotate, so the ring gap is not always in the
same position; this reduces local overheating due to
blowby.
Because the loading on the spherical bearing is
symmetrical, and because the piston is symmetrical The operating principle of the ratchet is shown above. The
(no hole for piston pin), the piston can be made with arrow indicates the direction of the con rod swing.
smaller clearances which reduces piston rocking.
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