FAILURE ANALYSIS
86
Mechanical Breakage/ Wear
Sometimes an engine fails because a part breaks.
There are generally three causes of a broken part, out
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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
crank shaft. Crank shafts bend when they, or some
-
thing bolted to them hits something. A prime example
of this is when a mower blade hits a rock.
See Figure 11.11.
As the engine runs, there is friction between the
moving parts. This friction wears down the parts. Lubri
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cation slows the process, but wear can not be pre-
vented. 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” rela-
tionship to mechanical failures. Which came first? Bent
crankshafts and imbalanced implements will cause
vibration issues. However a vibration issue, 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 a lot
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.
Detonation/preignition
Detonation is the undesirable condition of the fuel
spontaneously combusting the combustion chamber
prior to the spark plug firing. In this state, the flame
front from the detonation will start to travel through the
combustion chamber and a second flame front, from
the spark plug, will crash into it. The pressure differen
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tial 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
failures (melting or breakage), piston skirt damage,
connecting rod breakage and in extreme cases crank
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shaft failures.
A build up of carbon deposits in the combustion
chamber will increase the compression ratio. This is a
major factor for the development of detonation. It insu
-
lates the combustion chamber, allowing it to heat up
above normal operating temperatures.
Preignition is similar to detonation, but on a smaller
scale. Preignition 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
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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.
Figure 11.11
Bent blade
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