Diesel

Up until a few years ago, gas oil was a product of the primary distillation of crude oil. Therefore, its quality essentially depended on the characteristics of the crude oil itself and on the distillation method. In more recent years, more use has been found for components from conversion plants, such as stocks from cracking or from hydrocracking, and, therefore, the influence of the quality of the crude oil, while always important, has been reduced.
On average, the gas oil has a distillation interval comprised between 160°C and 380°C.
Gas oil is mainly used for the supply of internal combustion engines with spontaneous ignition or else for the combustion in thermal systems for civil heating.

When used in diesel engines, the diesel fuel is injected into the combustion chamber where it ignites when it comes into contact with the air brought to high temperatures by compression. Therefore, diesel fuel must have good combustion characteristics, so as to limit the delay between the start of injection and the start of ignition. The "cetane number" is the indicator chosen for the quality of ignition.
The European reference specification which harmonizes the national specifications for automotive diesel fuel in all of Western Europe is EN 590, which is in force in Italy as UNI EN 590.
The characteristics of diesel fuel which have an environmental impact (for example: the sulphur content) where defined by the European Union in the 2003/17/CE directive.

The applicative needs and, therefore, specific to the use of gas oil in stationary combustion, are less severe compared to those for automobiles.
Gas oil for heating, in fact, differs from the automotive product in three aspects:

• It has a higher sulphur content (0,2% vs. 0,035%)
• It is "higher-boiling" (T95 not indicated vs. max 360°C)
• The cetane number is not indicated in the specification (automotive min. 51)

For gas oil for heating, each country has its own specification. In Italy, UNI-CTI 6759 is in force.
A particular type of diesel fuel with a high final boiling point and a high sulphur content is used in marine fuels for medium-sized engines.
The international contractual reference for this diesel fuel is established in the ISO 8217 specification.