Page 109 - Synthetic Fuels Handbook

FUELS FROM PETROLEUM AND HEAVY OIL 95

Despite the diversity of the processes within a modern petroleum refinery, no single
stream meets all the requirements of gasoline. Thus, the final step in gasoline manufacture
is blending the various streams into a finished product (Fig. 3.18). It is not uncommon for
the finished gasoline to be made up of six or more streams and several factors make this
flexibility critical: (a) the requirements of the gasoline specification (ASTM D 4814) and
the regulatory requirements and (b) performance specifications that are subject to local
climatic conditions and regulations.
Aviation gasoline is form of motor gasoline that has been especially prepared for use
for aviation piston engines and is composed of paraffins and isoparaffins (50–60 percent),
moderate amounts of naphthenes (20–30 percent), small amounts of aromatics (10 percent),
and usually no olefins, whereas motor gasoline may contain up to 30 percent olefins and up
to 40 percent aromatics. It has an octane number suited to the engine, a freezing point of
−60°C (−76°F), and a distillation range usually within the limits of 30 to 180°C (86–356°F)
compared to −1 to 200°C (30 to 390°F) for automobile gasoline.
The narrower boiling range of aviation gasoline ensures better distribution of the vapor-
ized fuel through the more complicated induction systems of aircraft engines. Aircraft oper-
ate at altitudes at which the prevailing pressure is less than the pressure at the surface of
the earth (pressure at 17,500 ft is 7.5 psi compared to 14.7 psi at the surface of the earth).
Thus, the vapor pressure of aviation gasoline must be limited to reduce boiling in the tanks,
fuel lines, and carburetors. Thus, the aviation gasoline does not usually contain the gaseous
hydrocarbons (butanes) that give automobile gasoline the higher vapor pressures.
Under conditions of use in aircraft, olefins have a tendency to form gum, cause pre-
ignition, and have relatively poor antiknock characteristics under lean mixture (cruising)
conditions; for these reasons olefins are detrimental to aviation gasoline. Aromatics have
excellent antiknock characteristics under rich mixture (takeoff) conditions, but are much
like the olefins under lean mixture conditions; hence the proportion of aromatics in avia-
tion gasoline is limited. Some naphthenes with suitable boiling temperatures are excellent
aviation gasoline components but are not segregated as such in refinery operations. They
are usually natural components of the straight-run naphtha (aviation base stocks) used in
blending aviation gasoline. The lower boiling paraffins (pentane and hexane), and both
the high-boiling and low-boiling isoparaffins (isopentane to isooctane) are excellent avia-
tion gasoline components. These hydrocarbons have high heat contents per pound and are
chemically stable, and the isoparaffins have high octane numbers under both lean and rich
mixture conditions.
Gasoline performance and hence quality of an automobile gasoline is determined by its
resistance to knock, for example, detonation or ping during service. The antiknock quality
of the fuel limits the power and economy that an engine using that fuel can produce: the
higher the antiknock quality of the fuel, the more the power and efficiency of the engine.
Thus, the performance ability of gasoline is measured by the octane number.
Octane numbers are obtained by the two test procedures, those obtained by the
first method are called motor octane numbers (indicative of high-speed performance)
(ASTM D-2700 and ASTM D-2723). Those obtained by the second method are called
research octane numbers (indicative of normal road performance) (ASTM D-2699 and
ASTM D-2722). Octane numbers quoted are usually, unless stated otherwise, research
octane numbers.
In the test methods used to determine the antiknock properties of gasoline, comparisons are
made with blends of two pure hydrocarbons, n-heptane and isooctane (2,2,4-trimethylpentane).
Isooctane has an octane number of 100 and is high in its resistance to knocking; n-heptane is
quite low (with an octane number of 0) in its resistance to knocking.
Extensive studies of the octane numbers of individual hydrocarbons have brought to
light some general rules. For example, normal paraffins have the least desirable knocking
characteristics, and these become progressively worse as the molecular weight increases.