4.11 What are the effects of the specified fuel properties?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
4.11 What are the effects of the specified fuel properties?
Volatility
This affects evaporative emissions and driveability, it is the property that
must change with location and season. Fuel for mid-summer Arizona would be
difficult to use in mid-winter Alaska. The US is divided into zones,
according to altitude and seasonal temperatures, and the fuel volatility is
adjusted accordingly. Incorrect fuel may result in difficult starting in
cold weather, carburetter icing, vapour lock in hot weather, and crankcase
oil dilution. Volatility is controlled by distillation and vapour pressure
specifications. The higher boiling fractions of the gasoline have significant
effects on the emission levels of undesirable hydrocarbons and aldehydes,
and a reduction of 40C in the final boiling point will reduce the levels of
benzene, butadiene, formaldehyde and acetaldehyde by 25%, and will reduce
HC emissions by 20% [44].
Combustion Characteristics
As gasolines contain mainly hydrocarbons, the only significant variable
between different grades is the octane rating of the fuel, as most other
properties are similar. Octane is discussed in detail in Section 6. There
are only slight differences in combustion temperatures ( most are around
2000C in isobaric adiabatic combustion [45]). Note that the actual
temperature in the combustion chamber is also determined by other factors,
such as load and engine design. The addition of oxygenates changes the
pre-flame reaction pathways, and also reduces the energy content of the fuel.
The levels of oxygen in the fuel is regulated according to regional air
quality standards.
Stability
Motor gasolines may be stored up to six months, consequently they must not
form gums which may precipitate. Reactions of the unsaturated HCs may
produce gums ( these reactions can be catalysed by metals such as copper ),
so antioxidants and metal deactivators are added. Existent Gum is used to
measure the gum in the fuel at the time tested, whereas the Oxidation
Stability measures the time it takes for the gasoline to break down at 100C
with 100psi of oxygen. A 240 minute test period has been found to be
sufficient for most storage and distribution systems.
Corrosiveness
Sulfur in the fuel creates corrosion, and when combusted will form corrosive
gases that attack the engine, exhaust and environment. Sulfur also adversely
affects the alkyl lead octane response, and will adversely affect exhaust
catalysts, but monolithic catalysts will recover when the sulfur content of
the fuel is reduced, so sulfur is considered an inhibitor, rather than a
catalyst poison. The copper strip corrosion test and the sulfur content
specification are used to ensure fuel quality. The copper strip test measures
active sulfur, whereas the sulfur content reports the total sulfur present.
Manufacturers many also add additional tests, such as filterability, to
ensure no distribution problems are encountered.
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