GJ:"
402C
MOO£l
SECTION
AIRPLANE
&
SYSTEMS
DESCRIPTIONS
Fuel
Flow
Variations
With
Changes
In
Manifold
Pressure
The
engine-driven fuel
pump
output
;s
regulated
by
engine speed
and
compressor discharge pressure.
Engine
fuel
flow
is
regulated
by
fuel
pump
output
and
the metering
effects
of
the
throttle
and
mixture
control.
When
the waste gate
is
open, fuel flow will vary
directly
with manifold pres-
sure,
engine speed, mixture
or
throttle
position.
In
this
case, manifold
pressure
is
controlled
by
throttle
position
and
the waste gate
controller,
while fuel flaw varies with
throttle
movement
and
manifold pressure.
When
the waste gate
is
closed
and
manifold pressure changes are
due
to
turbocharger output,
as
discussed previously, fuel flow will follow
mani-
fold pressure
even
though
the
throttle
position
is
unchanged. This
means
that
fuel flow adjustments required
by
the
pilot
are minimized to the
following: (1) small
initial
adjustments
on
takeoff
or
climb-out for the
proper rich climb
setting,
(2) lean-out in
cruise
to the
recommended
lean
cruise
setting,
and
(3) return
to
the
full
rich
position for approach
and
landing.
Manifold
Pressure
Variations
With
Increasing
Or
Decreasing
Fuel
Flow
When
the waste gate
is
open,
movement
of the mixture control
has
little
or
no
effect
on
the manifold pressure
of
the turbocharged engine.
When
the waste gate
is
closed,
any
change
in fuel flow to the engine
will
have
a corresponding
change
in manifold pressure. That
is,
increasing
the fuel flow will increase the manifold pressure
and
decreasing the fuel
flow will decrease the manifold pressure. This
is
because
an
increased
fuel flow to the engine increases the
mass
flow
of
the exhaust. This turns
the turbocharger
faster,
increasing the induction airflow
and
raising
the
manifold pressure.
Momentary
Over
boost
Of
Manifold
Pressure
Under
some
circumstances (such
as
rapid
throttle
movement,
especially
with cold
oil)
it
is
possible
that
the engine
can
be
overboosted
above
the
maximum
allowable 39.0 inches
Hg.
manifold pressure. This
would
most
likely
be
experienced during the takeoff
roll
or during a
change
to
full
throttle
operation in
flight.
Therefore.
it
is
still
necessary
that
the
pilot
observe
and
be
prepared to control the manifold pressure.
Slight
overboosting
is
not considered detrimental to the engine
so~ong
as
it
is
momentary.
Momentary
overboost
of
2 to 3 inches
Hg.
manifola
pressure
can
usually
be
controlled
by
slower
throttle
movement
and
no
corrective
action
is
required
when
momentary
overboost
corrects
itself
and
is
followed
by
normal
engine operation.
However,
if
overboosting of
this
nature
persists,
or
if
the
amount
of
overboost
goes
as high as 4 inches
Hg.
manifold pressure or
more,
the
controller
system should
be
checked for
necessary replacement
or
adjustment of components.
1
November
1979
7-41
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