Flash
Point
The
flash point of a fuel is the lowest temperature at which sufficient
vapor is given off to cause temporary burning when a flame
is brought near the surface. This is not the same as the autoignition
temperature, which is the lowest temperature at which the
fuel will spontaneously ignite in a normal atmosphere without
a spark or other source of external ignition. Gasoline is designed for use in a spark-ignition engine because it has a low
flashpoint (-40C=-40F) and a high autoignition temperature
(246C=475F). Thus, when mixed air, gasoline can be readily
ignited by a spark but it will not preignite simply because
the engine is hot. Diesel fuel, on the other hand, is designed for use
in a high-compression engine. It has a comparatively high
flash point (at least 62C=143F) and low autoignition temperature
(210C=410F). In a diesel engine there is no ignition source
(I.e., spark plugs). Simply, the air is compressed until it
is heated above the autoignition temperature of diesel; then
the fuel is injected as a high[-pressure spray, keeping the
air/fuel mixture within the flammable limits of diesel.
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