EXCESSIVE SPEED
Shutting off the fuel supply to the cylinders of an
engine may be done in various ways. The fuel-control
Engines that are maintained in proper operating
mechanism may be forced to the NO FUEL position;
condition seldom reach speeds above those for which
the fuel line may be blocked by closing a valve; the
they are designed. However, there may be times when
pressure in the fuel injection line may be relieved by
speeds become too high. The operation of an engine at
opening a valve; or the mechanical movement of the
excessive speeds is extremely dangerous. If the engine
injection pump may be prevented. These methods of
speed is high enough, the high inertia and centrifugal
shutting off the fuel supply may be done either
force developed may cause parts to become seriously
manually or automatically.
damaged or even to fly apart. Therefore, it is essential
that you know why an engine may reach a dangerously
OVERSPEED SAFETY DEVICES
high speed, and how it may be brought under control
Automatic operation of fuel and air control
when too much speed occurs.
mechanisms is accomplished by overspeed safety
In some 2-stroke cycle engines, lubricating oil may
devices. As emergency controls, these safety devices
leak into the cylinders as a result of leaky blower seals
operate only in the event the regular speed governor
or broken piping. Even though the fuel is shut off, the
fails to maintain engine speed within the maximum
engine may continue to operate or even run away as a
design limit. Devices that function to bring an over-
result of this combustible material coming from the
speeding engine to a full stop by completely shutting
uncontrolled source. Engines in which lubricating oil
off the fuel or air supply are generally called "overspeed
may accumulate in the cylinders are generally equipped
trips." Devices that function to reduce the excessive
with an automatically operated mechanism that shuts
speed of an engine, but allow the engine to operate at
off the intake air at the inlet passage to the blower. If no
safe speeds, are called "overspeed governors."
air shutoff mechanism is provided and shutting off the
All overspeed governors and trips depend upon a
fuel will not stop an engine that is overspeeding,
spring-loaded centrifugal governor element for their
anything that can be placed over the engine's air intake
operation. In overspeed devices, the spring tension is
to stop airflow, such as a piece of metal or wood, will
great enough to overbalance the centrifugal force of the
stop the engine.
weights until the engine speed rises above the desired
maximum. When an excessive speed is reached, the
centrifugal force overcomes the spring tension and
WARNING
operates the mechanism, which stops or limits the fuel
Do not use your hand! The vacuum is great
or air supply.
enough to cause a large blood blister or it may
When a governor serves as the safety device, the
suck the blood out of your hand. Another very
actual operation of the fuel or air control mechanism by
great danger is that your hand may be pulled into
centrifugal force may be brought about directly, as in a
the blower rotor blades.
mechanical governor, or indirectly, as in a hydraulic
governor. In the case of an overspeed trip, the shutoff
Excessive engine speeds more commonly result
control is operated by a power spring. The spring is
from an improperly functioning regulating governor
placed under tension when the trip is manually set, and
than from any other cause. The usual method of
held in place by a latch. If the maximum speed limit is
accomplishing an emergency shutdown when the
exceeded, a spring-loaded centrifugal weight moves
regulating governor fails to function properly is to shut
out and trips the latch, allowing the power spring to
off the fuel supply to the cylinders. If this fails to slow
operate the shutoff mechanism.
the engine or stop it, the air supply to the engine must be
cut off.
Q4-20.
In a diesel fuel system, a governor is con-
nected between what two components?
1.
The throttle and injectors
CAUTION
2.
The throttle and linkage
3.
The linkage and injectors
Do not risk personal injury to stop an
4.
The injectors and cylinders
overspeeding engine when all normal means have
failed.
4-25
