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Influence Of Air Density On The Air-fuel Ratio - Ducati 900SS Workshop Manual

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Table of Contents
CARBURETOR
Influence
of
air density on
the
air-fuel ratio.
Besides
lhese
lhree factors,
there
is one more tha
t
makes
it very
d
ifficult
to
maintain
a consisten
t oir·fuel
ratio: changing
air density.
The
air
density,
or
actual weight
of the
air,
is one
of
the factors mentioned earlier
that
oHects
the compression pressure
of
on
engine.
The
density is dependenl
on
lwo
rhings:
air
lemperolure;
and
air
pressure.
As
the
air temperature increases,
lhe
air molecules
become
spaced farther aport, causing a smaller
amount
of
air
Ia occupy the
some
amount
o
f
space
that
o
larger
volume
did a
t
a lower
temperature.
This is why
obove·normol
a
ir
temperatures
may
require
leaning o
f the
fuel mixture
.
Conversely, as the temperature decreases, the air molecules press closer together
.
causing on
increase
in
the density
of the
air. The mcreose
in
density of the air must
be
compensoled lor by
on increase in
fuel
to
mainta
in the some oir·fuel
ratio.
H
IGHER AIR
TEMP.
LOWER AIR
TEMP.
-
>
LESS
FUEL
REQUIRED
- >
MORE FUEL REQUIRED
The o ther
lector
lho
t
a
ffects a
ir
d
ensity
is a
ir
pressure
A
ir
pressure is determined by both
oltilude
and w
eather
conditions. Air
becomes less dense
as the
altilude
increases because air is compressed by the weight
of
the remaining air
above
it. For
example.
at
sea
level.
air
is
compressed
by
the weight of the entire atmosphere
above
rt. AI an
oltilude
of
10.000
leet,
however, the
air is compressed only by
thai
port of the atmosphere
above
10,000
feet.
Because o
f
changes
in
air
density,
it is
necessary to
modify
the
amount
of fuel supplied
to
the
eng
ine.
HIGHER ALTITUDE
LOWER AlTITUDE
- >
lESS FUEl REQUIRED
- >
M
O
RE
FUEL
REQUIRED
It is imporlont
lo
realize, though, 1ho1 rejetting of corburelors ot hrgher altitudes
only restores
the
air-fuel
rolio
lo
lhe proper
selling,
il does nol replace
lhe loss of
power
lhol accompanies on
increase in oltilude
Weather
conditions also
affect
the
air pressure,
but
not
to
the some extent as
a
change
in
altitude. O nly
the
most
finely tuned engines
requrre rejetting
for changes in weolher
condilions.
REVIEW
1
)All engines require on
a
ir·fuel ratio within o specific range to maintain
combustion.
Too
leon
o
ratio
causes poor combustion (commonly called
"leon
misfire") and poor
performance
.
Too rich o
ratio
not
only
causes
poor
combustion, but
also
resuhs
in excessive
emissions.
2)The
air-fuel
requirements
o
f
the
eng
ine
ore
consta
ntly
changing
w
ith
varia
tions
in
engine
speed,
load s, and
temperatures.
Rrcher mixtures ore
used of low
and
high
speeds
as
well
as
under
heavy
loads.
Leaner mixtures ore
used
at
cruising speeds and under light
loads.
3)The
air
density, or weight, ploys o major pori in determining the
amount
of
fuel required
for
efficient
burning
of the
air-fuel
mixture.
The
lwo
items
which d
etermine
the
a ir
d
ensily
ore
air tempera
ture
and
a
ir
pressure.
HIGHER
TEMP.
LOWER
TEMP.
HIGHER ALTITUDE
lOWER
AlTITUDE
-
>
LESS
FUEl
REQUIRED
- >
MORE
FUEL
REQUIRED
-
>
LESS
FUEL
REQUIRED
- >
MORE
FUEL REQUIRED
From
this
discussion
of
air-fu
el
rolios,
il
should
be
obvious that
precise
metering
is
essential
to
meet oil
the requirements
of
o motorcycle
engine.
The
next
section
intro
duces
lhe
circuits
necessary
to
meet
these
requirements.
The
air-fuel mixture
in
modern
motorcycles
is very
imporlonl
for
overall
performance.
The
mixture
needed
for slorting o cold
engine
is
much
richer
than
the
mixture
needed
for c ruising.
The
mixlure need ed Ia
start o
cold
engine
is obaul
l
0
ports o
f
air
to
1
port
of
fuel, by
w
eight.
A
worm
engine,
al cruising speed,
should
hove
o mixlure
of about
15:
1
to
16:
l. Under
lull-lhrottle operation,
the
mixture should
be
enriched
lo
about
12:
1.
Air-fuel
mixtures
affect
engine
conditions.
Fuel
mixtures
that
ore to
o rich con contribu
te
to
fouled p
lugs,
carbon deposits in
the c
omb
ustion chamber,
poor
mileage,
and
high emissions.
Fuel
mixtures
tha
t
ore
too
leon
con
cause misfiring,
burned
valves,
and overhearing
.
lncorrecl fuel
mixlures could
lead
to engine damage.
g
WARNING: Gasoline
is extremely flammable. To avoid
severe
injury,
do not allow open
flames,
sparks,
cigarettes, or
any other
[.lJ
sources
of
ignition near the
area
in
which you
are
working.
0.12
·
-
. .....

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