Ignition of Fuels

Definition

The flash point of a volatile liquid is the lowest temperature at which it can vaporise to form an ignitable mixture in air.

Transportation regulations distinguish fuels as either flammable or combustible depending upon their flashpoint. Flammable fuels have a flashpoint below38°C. and combustible fuels have a flashpoint above 38°C. As an example, diesel fuel flashpoints range from about 50°C. to around 100°C making it relatively safe to transport and handle. Gasoline, by comparison, has a flashpoint of minus 43°C. Sparks and static electricity can easily ignite gasoline fumes.

Aviation fuels fall mainly into two categories, aviation gasoline - commonly abbreviated to “avgas” - and the variants of paraffin (kerosene) used by all gas turbine engines and loosely described as "jet fuel"

AVGAS

AVGAS is broadly similar to automotive gasoline but subject to much more rigorous quality control. The most common form of AVGAS has an octane rating of 100 - octane rating being a measure of the resistance of a fuel to pre-ignition (the chance of combustion occurring before the spark). 

This flashpoint is the same as that of automotive gasoline and there is a significant danger of combustion if it is not handled carefully. Although there are many more aircraft using AVGAS than use jet fuel, they are almost exclusively light aircraft.

Jet Fuels

The most common jet fuels in use are named Jet A (U.S.) and Jet A-1 (international). They are kerosene grade fuels with a flashpoint of 38°C. Commercially available Jet B has a lower flashpoint (minus 18°C.) but it also has a much lower freezing point making it very suitable for use in extremely cold environments.

Fuels such as JP5 and JP7 have higher flashpoints and were developed to provide additional safety margins in specific military applications. All jet fuels are subject to rigorous testing for impurities and any fuel that fails such testing is diverted to ground applications. 

All aviation fuels contain specified small quantities of various additives such as corrosion inhibitors, static dissipaters, and anti-freezing substances.

Diesel

A fairly recent development is light aircraft fitted with diesel engines, although this is still very much an exception.

Some fuel examples

Fuel	Flash point	Autoignition temperature	Freezing point
Ethanol (70%)	16.6°C	363°C
Petrol (Gasoline)	−43°C	246°C
Diesel	> 62°C	210°C
Jet fuel	> 38°C	210°C
Jet A	> 38°C	210°C	< −40 °C
Jet A-1	> 38°C	210 °C	< −47 °C
JP5	> 60°C		< −46°C
JP7	> 60°C
Jet B	−18°C
Kerosene	> 38°C – 72°C	220°C
Biodiesel	> 130 °C