The Medium Speed 4 Stroke Trunk Piston
The connecting rod in a medium speed 4 stroke engine is subject to an
inertia whip loading due to the mass of the con rod swinging about the
piston pin. (Because of the lower speed of a 2 stroke engine, the whip
loading is not large enough to influence the design of the con rod)
Added to this, the inertia loads due to the mass of the reciprocating
parts cause a stress reversal from high compressive stress (during power
and compression stroke) to a low tensile stress between
the exhaust and inlet strokes.
of the rod influences its design, and to withstand the loading described
above, connecting rods are often forged from a manganese
molybdenum steel in an I or H section which reduces its mass from one
made of round section steel (and thus reduces the whip loading) while
maintaining strength. This is not always the case, as can be seen from
the pictures shown, and often a round section rod is of sufficient
Because of the large diameter of the crankpin to
increase bearing area and decrease bearing load, the
width of the bottom end of the con rod is greater than the
diameter of the cylinder liner.
So that the piston can be withdrawn from the liner, 3 different
designs are used:
The con rod can be fitted with a marine palm as in the photo
The con rod can be split in two parts as shown opposite.
The bottom end can be split obliquely. Serrations are
used to locate the two halves relative to one another.
The advantage of using a vee engine is that the
overall length of the engine is reduced for a given power
If a normal bottom end arrangement is used then the con rods
must be placed side by side which means the opposite cylinders
are offset. The crankpins must be long enough to accommodate two
bottom ends side by side, and of large enough diameter to resist
bending. The increased length of the crankshaft means a longer
Two alternative arrangements are pictured (right). Both allow
the cylinders to be opposite one another. However, both
arrangements restrict access to the crankcase, and because the
design of the bottom ends are different, more spares have to be
BOTTOM END BOLTS
Because of the stress reversal mentioned above, bottom end bolts have
a limited life. This varies from engine to engine, but is generally
around 12-15000 hours. If a bottom end bolt was to fail in operation,
then the results would be disastrous.
Bottom end bolts should be treated with care when removed from the
engine during overhauls. They should be inspected for any damage to the
surface from which a crack could start. This damage could be due to
corrosion (water in LO) or because of incorrect handling.