Cub Cadet T65 Series - Manual (Page 112)

T65 Series Vertical Shaft Engines
108
Mechanical Breakage/ Wear
Sometimes an engine fails because a part breaks.
There are generally three causes of a broken part, outside
of the previously discussed engine failures. They are
abuse, wear, and manufacturing defects.
A very common sign of an abused engine is a bent
crankshaft. Crankshafts bend when they, or something
bolted to them hits something. A prime example of this is
when a mower blade hits a rock. See Figure 11.12.
As the engine runs, there is friction between the
moving 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” relationship to mechanical failures. Which came first? Bent crank-
shafts and imbalanced implements will cause vibration issues. However a vibration issue, such as a overspeed or
loose mounting bolts on the engine, can shake an engine to pieces. The technician must find the source of the vibra-
tion in order to properly diagnosis an engine.
Manufacturing defects are wrongly blamed for 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 gen-
erally show up within the first couple of hours of use.
Detonation/pre-ignition
Detonation is the undesirable condition of the fuel spontaneously igniting 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 combustion chamber
and a second flame front, from the spark plug, will crash into it. The pressure spike caused by this will send shock
waves through the engine. The shock wave can cause a knocking or pinging noise. This is why detonation is some-
times 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 extreme pressures and temperatures that occur in the combustion chamber as a result of detonation will
cause piston damage. In its initial stages, small spots of molten metal will erupt on the piston crown and may stick to
the spark plug. The next stage of damage usually crushes the top ring land, pinching the top piston ring. More
extreme damage happens as the piston crown melts. Metal transfer to the cylinder wall damages the wall and the
piston skirt.
NOTE: Resonance, from the shock waves generated by detonation, can also cause fatigue damage that
resembles a vibration issue.
NOTE: The sudden onset of a violent detonation can blow a hole through the crown of the piston.
A build up of carbon deposits in the combustion chamber will increase the compression ratio. This is a major fac-
tor for the development of detonation. Carbon deposits also hold glowing embers that can trigger pre-ignition.
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 pressure that tries to
push the piston down against the direction of rotation of the crankshaft. The sounds and damage created by this are
the same as detonation.
Figure 11.12
Bent blade