Alternative Fuels 469
2. The fuel can be completely vaporized. The nal temperature of the
vapor will then have to be 522 K, as shown by the saturated-vapor
curve at an enthalpy of 1050 kJ/kg.
3. The fuel can be partially vaporized at a temperature between 522
and 600 K, depending on the pressure. For example, at a pressure of
0.7 MPa, partial vaporization would yield a liquid-fuel-vapor mix-
ture at a temperature of 567 K, with about half the total fuel in the
form of vapor.
10.5.3 Spontaneous-ignition Temperature
An important factor affecting aircraft re and explosion hazards is the ten-
dency of the fuel toward spontaneous ignition. When an aircraft crashes, a
common cause of re is the spontaneous ignition of fuel coming into con-
tact with hot surfaces, especially near engines. The problem is especially
important for high-speed aircraft because of kinetic heating of the aircraft
structure. Above altitudes of 11 km, at about Mach 2.7, the leading edge of
an aircraft attains a temperature that just exceeds the spontaneous-ignition
temperature (SIT) of the fuel.
The SIT depends greatly on molecular structure, since the oxidation
reactions leading to spontaneous ignition can occur only when the chemical
bonds within the fuel are broken by thermal agitation. Consequently, the
SIT is lower for fuels with higher relative density, whose molecules are less
compact and more vulnerable to cracking [1]. Straight-chain parafns are
more easily ignited than either branched-chain parafns or cycloparafns
containing the same number of carbon atoms. Very highly branched com-
pounds and aromatics are highly resistant to spontaneous ignition [29].
The SIT is usually measured by injecting a small sample of the fuel into
a heated crucible or ash and noting the delay until the onset of ignition.
The test is repeated at lower temperatures (with associated longer delays),
until the minimum ignition temperature is found [1]. Some typical results
are shown in Figure 10.9.
Generally, for a hydrocarbon fuel, the SIT increases with decreasing pres-
sure. Tests carried out on a JP-4 fuel [30] showed that decreasing the pressure
to one-third its original value increased the SIT by a factor of 2.
A number of workers have investigated the effect of additives on the SIT of
aviation fuels. Thomas [31] examined a number of additives, the most effec-
tive being tetraethyl lead. In a concentration of 3 mL per imperial gallon, this
additive increased the minimum pressure needed to produce spontaneous
ignition from 27.6 to 40.7 kPa. Tetraethyl lead is added as part of a mixture
that also contains ethylene dibromide and dye. Ethylene dibromide acts as a
scavenger for lead oxide and reacts to produce a mixture of lead bromide and
lead oxybromides. These compounds are volatile and are exhausted from
the engine; in this manner, deposits of lead oxide are prevented from collect-
ing on valves and spark plugs. Of the number of additives investigated by