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Quelques explications
concernant l'essence avion - AVGAS
L'essence Avgas est une essence pour aéronefs
à moteurs à pistons ou alternatifs. Comme toutes les essences, elle est très
volatile et extrêmement inflammable aux températures de service normales. Les
marches à suivre et le matériel prévus pour une manipulation sécuritaire de ce
produit doivent donc faire l'objet de la plus grande attention.
Les grades d'essence Avgas sont principalement définis par leur indice d'octane.
Il existe deux indices pour les essences aviation (l'un pour le mélange pauvre
et l'autre pour le mélange riche), ce qui donne lieu à un système de numérotage
multiple. Ex. : Avgas 100/130 (où l'indice du mélange pauvre est 100 et celui du
mélange riche 130).
Il y avait, dans le passé, plusieurs grades différents d'essence aviation
d'usage général. Ex. : 80/87, 91/96, 100/130, 108/135 et 115/145. Avec le recul
de la demande, ces grades ont été ramenés à un grade de base, Avgas 100/130.
Afin d'éviter toute confusion et de réduire au minimum les erreurs de
manipulation de l'essence aviation, on désigne couramment le grade uniquement
par l'indice du mélange pauvre. L'essence Avgas 100/130 devient alors de
l'essence Avgas 100 - couleur verte -
Puis, un grade supplémentaire a été mis sur le marché afin de permettre
l'utilisation d'un carburant dans les moteurs conçus à l'origine pour des grades
à plus faible teneur en plomb. Il s'agit du grade
Avgas 100LL - couleur bleue - , LL correspondant, en anglais, à une faible
teneur en plomb.
Avgas is gasoline fuel for reciprocating piston engine aircraft and is
not to be confused with jet fuel. As with all gasolines, avgas is very volatile
and is extremely flammable at normal operating temperatures. Procedures and
equipment for safe handling of this product must therefore be of the highest
order.
Avgas grades are defined primarily by their octane rating. Two ratings are
applied to aviation gasolines (the lean mixture rating and the rich mixture
rating) which results in a multiple numbering system e.g. Avgas 100/130 (in this
case the lean mixture performance rating is 100 and the rich mixture rating is
130).
In the past, there were many different grades of aviation gasoline in general
use e.g. 80/87, 91/96, 100/130,108/135 and 115/145. However, with decreasing
demand these have been rationalised down to one principle grade, Avgas 100/130.
(To avoid confusion and to attempt to eliminate errors in handling aviation
gasoline, it is common practice to designate the grade by just the lean mixture
performance, i.e. Avgas 100/130 becomes Avgas 100). More recently, an additional
grade was introduced to allow one fuel to be used in engines originally designed
for grades with lower lead contents: this grade is called Avgas 100LL, the LL
standing for 'low lead'.
All equipment and facilities handling avgas are color coded and display
prominently the API markings denoting the actual grade carried. Currently the
two major grades in use internationally are Avgas 100LL and Avgas 100. To ease
identification the fuels are dyed; for example Avgas 100LL is colored blue,
while Avgas 100 is colored green.
Avgas fuelling nozzles for overwing dispensing are painted red. To help prevent
the possibility of jet fuelbeing supplied to a piston engine aircraft, the
nozzle of an Avgas fueller is limited to a maximum diameter of (internationally)
40 mm (49 mm in U.S.A) and the aperture on an aircraft Avgas tank to a maximum
of 60 mm diameter. Nozzles for Jet A-1 are larger than 60 mm and thus cannot be
placed into an aircraft's Avgas tank.
| which is the
everyday gasoline
used in cars.
Avgas is only used in aircraft that use internal combustion engines:
jet aircraft
and turboprops
use kerosene jet
fuel.
Avgas has a lower volatility than mogas and doesn't evaporate as
quickly, which is important for high-altitude use. The particular
mixtures in use today are the same as when they were first developed
in the 1950s
and 1960s,
and therefore the high-octane ratings are achieved by the addition
of
tetra-ethyl lead, a fairly toxic substance that was phased out
for car use in most countries in the
1980s. The
main
petroleum component used in blending Avgas is
alkylate,
which is essentially a mixture of various isooctanes, and
some refineries also use some
reformate.
Avgas is currently available in several grades with differing lead
concentrations.
- Avgas 80/87 has the lowest lead content at 0.5
gram lead per US
gallon, and is
only used in very low compression ratio engines.
- Avgas 100/130 is a higher octane grade aviation gasoline, containing
about 4 gram of
lead per US gallon,
maximum 1.12 gram/litre. 100LL "low
lead" was designed to replace avgas 100/130.
- Avgas 100LL contains about 2
gram of lead per
US gallon, or
maximum 0.56 gram/litre, and is the
most commonly available and used aviation gasoline.
In the past other grades were also available, particularly for military
use, such as avgas 115/145. Note that the octanes of avgas cannot be
directly compared to those of mogas, as a different test engine and method
is used to determine the octane. The first (lower) number is the lean
mixture rating, the second (higher) number is the rich rating.
To aid pilots in identifying the fuel in their aircraft, dyes are added
to the fuel. 80/87 is red, 100/130 is green, and 100LL is blue, while jet
fuel, JET A1, is clear or straw, being undyed.
Many
general aviation
aircraft
engines were designed to run on 87 octane, the standard for automobiles
today. Direct conversions to run on automotive fuel, are fairly common.
However the alloys used in aviation engine construction are rather outdated,
and engine wear in the valves is a serious problem on mogas conversions.
Additionally, the wider range of allowable vapor pressures found in mogas
pose some risk to aviation users if fuel system design considerations are
not taken into account. Mogas can vaporize in fuel lines causing a vapor
lock (a bubble in the line) starving the engine of fuel. This does not
constitute an insurmountable obstacle, but merely requires examination of
the fuel system to ensure adequate shielding from high temperatures and the
existence of sufficient pressure and flow in the fuel lines.
The only large consumers of avgas these days are in
North America,
Australia,
Brazil, and
Africa (mainly
South Africa).
There are very few remaining supply sources outside the
United States.
In Europe,
avgas prices are so high that the entire general aviation industry is being
wiped out. There are a number of efforts to convert the industry to
diesel instead,
which is common, inexpensive and has a number of advantages for aviation
use. It remains to be seen whether there is enough of a market left for this
to occur. |
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