ENGINE BLOCK, PISTONS, & CRANKSHAFT
by Kyle McFadden
The engine block is the foundation of the engine. It is constructed
of cast iron or aluminum. The engine block is a platform in which the
components necessary for the operation of the engine are mounted. The
engine block has passages cast into it that distribute oil for lubrication
and coolant to prevent overheating.
The crankshaft is mounted in the lower section of the engine block.
This section is called the crankcase. The crankshaft is bolted in place
with small sections called main bearing caps. The portion of the crankshaft
that is contained by the main bearing caps is called journals. The main
bearing caps contain the bearings that allow the crankshaft to rotate
freely. The bearings are divided into two semi-circular halves, with
one half mounted in the engine block and the other in the main bearing
cap. The bearings contain a soft metal alloy bonded to a steel outer
shell. The soft surface faces the crankshaft journal. The crankshaft
rides on a film of oil between the crankshaft main journal and the bearing
surface. The crankshaft has weights built into its design that along
with the flywheel, help to balance the rotating motion of the engine.
It also provide inertia to keep the engine rotating at slow speed.
block also contains the cylinders that contain the pistons. Pistons
are generally constucted of cast or forged aluminum. This material expands
at about twice the rate of the cast iron engine block, as the engine
warms up to operating temperature. Specific clearance is machined into
the cylinder to allow for expansion of the piston. The top of the piston
is designed to withstand the tremendous heat and force present when
the air/fuel mixture is ignited in the combustion chamber. There are
grooves or slots under the top of the piston. These grooves contain
the rings which seals the piston in the cylinders. Rings are constructed
of metal such as iron or chrome-alloy. They are almost completely circular
with a small gap in the ring. The gap is designed to prevent overlap
of the ring as the engine reaches operating temperature. Most automobile
and light truck engines use a piston with three rings. The top two rings
are used to prevent leakage of the compessed air fuel mixture past the
piston. The bottom ring is used to control oil loss by preventing oil
from reaching the combustion chamber.
The connecting rod is attached at one end of the piston and to the crankshaft
at the other. It transmits the up and down motion of the piston in the
cylinder bore to the crankshaft. It is generally constructed of forged
or cast iron and is rigidly designed to withstand the force generated
by the downward thrust of the piston. As each cylinder fires the crankshaft
is subjected to torsional force that causes it to wind and unwind as
if it were a spring. While the actual distance that the crankshaft winds
and unwinds can only be measured with a micrometer, the action of the
torsional forces on the crankshaft is enough to produce vibrations that
could be felt by the driver. To control this a dampner is used to absorb
vibration. This is usually a weighted two part metal disc. It consist
of an outer ring attached to an inner ring using a thin layer of rubber.
The vibration dampner is placed on the front part of the crankshaft
that protrudes from the engine.
has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad