LANCE
IISERVICE
MANUAL
Overboost
Overshoot
Bootstrapping
Critical Altitude
An overboost
condition
means
that
manifold
pressure
is
exceeding
the
limits at
which the
engine
was tested
and FAA
certified
and
can
be determental
to
the life
and performance
of
the engine.
Overboost
can be caused
by malfunctioning
controllers
or improperly
operating wastegate
in the
automatic
system
or by pilot
error in a manual
controlled
system.
Overshoot is
a condition of the automatic
controls not having
the ability to respond
quickly enough to check the
inertia of
the turbocharger
speed
increase with
rapid engine
throttle
advance.
Overshoot
differs from
overboost
in that the
high
manifold pressure
lasts only
for a few
seconds. This
condition
can usually
be overcome
by smooth
throttle
advance.
This
is a term used
in conjunction
with turbo
machinery.
If
you were to
take all the air coming
from a turbocharger
compressor and duct it directly back into the turbine of that
turbocharger,
it
would be called
a bootstrap
system and
if no
losses were encounted,
it would thereoretically
run
continuously.
It would also be very
unstable because if for
some reason
the turbo speed would change,
the compressor
would pump more air
to drive the turbine faster,
etc. A
turbocharged
engine above critical
altitude (wastegate closed)
is similar
to the example mentioned
above, except now there
is an engine placed between the compressor discharge and
turbine inlet. Slight
system changes caused
the exhaust gas to
change
slightly, which causes
the turbine speed to
change
slightly, which
causes the compressor
air to the engine to
change slightly, which in turn again affects the exhaust gas,
etc.
A turbocharged
engine's
wastegate
will be in
a partially open
position at sea
level. As the aircraft is
flown to high altitude
(lower ambient pressures) the wastegate closes gradually to
maintain the preselected
manifold pressure. At
the point
where the
wastegate
reaches its
full closed
position, the
preselected manifold pressure will start to drop and this is
considered
critical
altitude.
NOTES:
1. Refer
to Lycoming
Service Bulletin No.
369 for recommended
engine inspections
after any
Overspeed or Overboost
conditions.
Added:
9/2/78
POWER PLANT
- TURBO
2C22
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