Flash point

The flash point of a flammable liquid is the lowest temperature at which it can form an ignitable mixture with air. At this temperature the vapor may cease to burn when the source of ignition is removed. A slightly higher temperature, the fire point, is defined at which the vapor continues to burn after being ignited. Neither of these parameters is related to the temperatures of the ignition source or of the burning liquid, which are much higher. The flash point is often used as one descriptive characteristic of liquid fuel, but it is also used to describe liquids that are not used intentionally as fuels.

Mechanism

Every flammable liquid has a vapor pressure, which is a function of that liquid's temperature. As the temperature increases, the vapor pressure increases. As the vapor pressure increases, the concentration of evaporated flammable liquid in the air increases. Hence it is the temperature which determines the concentration of evaporated flammable liquid in the air under equilibrium conditions. Different flammable liquids require different concentrations of the fuel to be present in the air to sustain combustion. The flash point is that minimum temperature at which there is enough evaporated fuel in the air to start combustion.

Measuring flash points

A commonly used device for measuring the flash point of liquid fuels is the Pensky-Martens closed cup. That apparatus consists of a small cup containing the liquid to be observed, which is gradually heated toward its flash point while being stirred continuously in order to distribute the heat uniformly. At regular intervals an open flame is directed into the cup; at the flash point, it will ignite the contents of the cup.

Examples of fuel flashpoints

Petrol (gasoline) is designed for use in an engine which is driven by a spark. The fuel should be premixed with air within its flammable limits and heated above its flash point, then ignited by the spark plug. The fuel should not preignite in the hot engine. Therefore, gasoline is required to have a low flash point and a high autoignition temperature.

Diesel is designed for use in a high-compression engine. Air is compressed until it has been heated above the autoignition temperature of diesel; then the fuel is injected as a high-pressure spray, keeping the fuel-air mix within the flammable limits of diesel. There is no ignition source. Therefore, diesel is required to have a high flash point and a low autoignition temperature.

Petrol:
- Flash point: >-45 °C
- Autoignition temperature: 246 °C
Diesel:
- Flash point: >62 °C
- Autoignition temperature: 210 °C
Jet Fuel:
- Flash Point: >50 °C
- Autoignition Temperature: 210 °C

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Mechanism

Measuring flash points

Examples of fuel flashpoints

See also