Diesel Engine Fundamentals
These rings function as the seal between the piston and the cylinder wall and also act to
reduce friction by minimizing the contact area between the piston and the cylinder wall.
The rings are usually made of cast iron and coated with chrome or molybdenum. Most
diesel engine pistons have several rings, usually 2 to 5, with each ring performing a
distinct function. The top ring(s) acts primarily as the pressure seal. The intermediate
ring(s) acts as a wiper ring to remove and control the amount of oil film on the cylinder
walls. The bottom ring(s) is an oiler ring and ensures that a supply of lubricating oil is
evenly deposited on the cylinder walls.
The connecting rod connects the piston to the crankshaft. See Figure 2 and Figure 3 for
the location of the connecting rods in an engine. The rods are made from drop-forged,
heat-treated steel to provide the required strength. Each end of the rod is bored, with the
smaller top bore connecting to the piston pin (wrist pin) in the piston as shown in
Figure 6. The large bore end of the rod is split in half and bolted to allow the rod to be
attached to the crankshaft. Some diesel engine connecting rods are drilled down the
center to allow oil to travel up from the crankshaft and into the piston pin and piston for
A variation found in V-type engines that affects the connecting rods is to position the
cylinders in the left and right banks directly opposite each other instead of staggered
(most common configuration). This arrangement requires that the connecting rods of two
opposing cylinders share the same main journal bearing on the crankshaft. To allow this
configuration, one of the connecting rods must be split or forked around the other.
The crankshaft transforms the linear motion of the pistons into a rotational motion that
is transmited to the load. Crankshafts are made of forged steel. The forged crankshaft
is machined to produce the crankshaft bearing and connecting rod bearing surfaces. The
rod bearings are eccentric, or offset, from the center of the crankshaft as illustrated in
Figure 7. This offset converts the reciprocating (up and down) motion of the piston into
the rotary motion of the crankshaft. The amount of offset determines the stroke (distance
the piston travels) of the engine (discussed later).
The crankshaft does not ride directly on the cast iron block crankshaft supports, but rides
on special bearing material as shown in Figure 7. The connecting rods also have
bearings inserted between the crankshaft and the connecting rods. The bearing material
is a soft alloy of metals that provides a replaceable wear surface and prevents galling
between two similar metals (i.e., crankshaft and connecting rod). Each bearing is split
into halves to allow assembly of the engine. The crankshaft is drilled with oil passages
that allow the engine to feed oil to each of the crankshaft bearings and connection rod
bearings and up into the connecting rod itself.
The crankshaft has large weights, called counter weights, that balance the weight of the
connecting rods. These weights ensure an even (balance) force during the rotation of
the moving parts.