Chapter 11: Failure Analysis
107
Mechanical Breakage/ Wear
Sometimes an engine fails because a part breaks.
There are generally three causes of a broken part, out-
side of the previously discussed engine failures. They
are abuse, wear, and manufacturing defects.
A very common way to abuse an engine is a bent
crankshaft. Crankshafts bend when they, or something
bolted to them hits an object. A prime example of this is
when a mower blade hits a rock. See Figure 11.13.
As the engine runs, there is friction between the mov-
ing parts. This friction wears down the parts. Lubrica-
tion slows the process, but wear can not be prevented.
Over time the parts wear to the point they break or fail
in some way. Car tires are a good example of wear. A
tire will only last for so many miles before all the rubber
is worn off and the tire goes flat. Bushings are another
example, they are designed to wear so that the wear of
other parts will be minimized.
Vibration issues have a “chicken and the egg” relation-
ship to mechanical failures. Which came first? Bent
crankshafts and imbalanced implements will cause
vibration issues. However a vibration issues, such as a
over speed or loose mounting bolts on the engine, can
shake an engine to pieces. The technician must find
the source of the vibration in order to properly diagno-
sis an engine.
Manufacturing defects are wrongly blamed for alot of
failed parts. A manufacturing defect is when a part is
made wrong. It could be a porous casting, parts
assembled wrong, the wrong parts used or so on. A
manufacturing defect will generally show up within the
first couple of hours of use.
Figure 11.13
Bent blade
Detonation/pre-ignition
Detonation is the undesirable condition of the fuel
spontaneously combusting in the combustion chamber
prior to the spark plug firing. In this state the flame front
from the detonation will start to travel through the com-
bustion chamber and a second flame front, from the
spark plug, will crash into it. The pressure differential
caused by this will send shock waves through the
engine. The shock wave cause a knocking or pinging
noise. This is why detonation is sometimes called
“knocking”, “spark knocking” or “pinging”. The shock
wave will also try to push the piston down against the
direction of rotation of the crankshaft.
The shock wave from detonation can cause piston fail-
ures (melting or breakage), piston skirt damage, con-
necting rod breakage and in extreme cases crankshaft
failures.
A build up of carbon deposits in the combustion cham-
ber will increase the compression ratio. This is a major
factor for the development of detonation. It insulates
the combustion chamber, allowing it heat up above nor-
mal operating temperatures.
Pre-ignition is similar to detonation, but on a smaller
scale. Pre-ignition is cause by a localized hot spot or a
hot deposit in the cylinder. As the fuel/air mixture is
drawn into the cylinder it is ignited. This creates pres-
sure that tries to push the piston down against the
direction of rotation of the crankshaft. The sounds and
damage created by this is the same as detonation.
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