Amongst the many advantages diesel fuel can boast over its alternatives, such as far superior fuel
economy and cheap road tax thanks to low negligible carbon emissions, there is one advantage which is
easily overlooked: the safety of diesel. Diesel has historically proven to be an exceptionally safe fuel
for commercial and private use. The high level of safety inherent in diesel is a byproduct of its
chemical construction, and the way diesel burns.
Here's some information regarding diesel safety, and how the low fire risk makes diesel of particular
benefit to you.
How Diesels Work
We'll begin with a really quick, simplified explanation of diesel and how it works in a diesel engine.
By doing this, we can quickly see how diesel is a safer fuel than its nearest rival petrol.
Diesel fuel is derived by a complex process of distillation - although less complex than the
distillation of petrol - from crude oil. Diesel fuel is a combination of hydrogen and carbon molecules;
in fact diesel can range anywhere between 10 and 28 carbon molecules and still work effectively. The
engine in which diesel fuel is consumed works by compression ignition, as opposed to spark ignition as
in petrol engines. In a diesel engine, air is compressed in the cylinder. This compression causes the
air to heat rapidly and the diesel fuel is injected as a fine, high-pressure spray. The diesel ignites
on contact with the hot compressed air, and the energy of this ignition is what powers the engine. For
this type of engine to work efficiently, diesel needs a high flashpoint and a low autoignition
temperature.
Diesel's Flashpoint
The flashpoint of a fuel is measured as the lowest temperature at which it will ignite when exposed to
a flame in normal air conditions. The flashpoint is the degree of a material's flammability, and of how
volatile it is, and it's used to classify flammable liquids. Since the flashpoint can be controlled
during the distillation process, the fuel can be made as flammable or not flammable as is required by
legislation, and the flashpoint does not affect the performance of the engine. The higher the flashpoint,
the safer the fuel, and the more manageable the ignition.
The flashpoint of diesel is generally around 55-65ºC. The flashpoint of a fuel varies inversely with
its volatility; the more volatile the fuel, the lower the flashpoint, and the more dangerous and
unpredictable the fuel becomes. Diesel's flashpoint is quite high in comparison with the other fuels on
the market, and due to this high flashpoint, diesel is an inherently safer fuel than - for example -
petrol. Diesel doesn't burn as easily or readily as petrol - a highly volatile fuel by comparison - which
makes diesel far safer and more efficient too.
It's worth pointing out too that diesel in its liquid state is extremely difficult to ignite; it's the
fumes which ignite. Due to diesel's higher viscosity - its thickness - there is far less evaporation of
fuel. The temperature at which diesel begins to evaporate into flammable gases is much higher than that
of petrol, and therefore the risk of accidental fire is again greatly reduced. Under normal atmospheric
conditions in fact, you can flick a lit match into a pool of diesel and it won't ignite. Try that with
petrol and it's a different story. The truth is that good, treated diesel oil is not much more dangerous
than regular, off-the-shelf vegetable oil.
Autoignition
The autoignition of a fuel is a measure of the temperature at which that fuel will ignite without the
use of a spark or other source of ignition; in other words, the autoignition of a fuel is the lowest
ambient temperature which will cause a fuel to come to flames.
The autoignition can be recorded and measured by its Cetane Number (CN). The science behind the
Cetane Number is a bit long winded, but it should suffice to say the CN is a measure of the quality of
diesel, and the higher the CN number the faster the engine.
In any event, the ambient temperature in which diesel fuel will typically ignite is 210ºC, whereas
petrol will autoignite at 246º. Since diesel relies on air temperature more than any other factor to
facilitate its ignition, the lower the autoignition temperature the better. Therefore, diesel fuel
offers the safety of a high flashpoint and a low autoignition temperature; a feature which cannot be
matched by the far more volatile petrol fuel.
Other Safety Features
The burning of diesel is known to release fewer carcinogens and other pollutants into the atmosphere,
which makes diesel fuel safer for health than petrol. It's less harmful for diesel to make contact with
the skin than it is for petrol. Diesel is easier to distill than petrol, so less people are involved and
there is less to go wrong during the process. Diesel is thicker, and therefore easier to control in
transit and easier to manage if it gets into water.
In summary, diesel's low volatility, high flashpoint, efficient burning process, inherent chemical
composition and human intervention all combine to ensure diesel is the safest it can be, and that the
risk of fire is significantly lower than petrol.