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Characteristics and properties of diesel

Characteristics and properties of diesel

Diesel oil is a mixture of liquid hydrocarbons, obtained by fractional distillation of crude oil and is used as fuel for diesel engines, for heating or for the production of electricity.

It is often inappropriately called naphtha, which is distinguished by a better refining process and for different uses.

Diesel fuel finds its first applications in the mechanical field between 1893 and 1897, when in the MAN workshops (Maschinenfabrik Augsburg Nuremberg) in Augusta, Rudolf Diesel carried out the first investigations that led to the invention of the Diesel engine. Its name derives from having been used, in the past, to obtain gas from mineral oil, by vaporization and pyrolysis.

Diesel oil is a mixture containing aliphatic hydrocarbons (also cyclic) with 13 to 18 carbon atoms and paraffins.

Diesel oil, which has a higher flash point temperature than gasoline, is less flammable and therefore inherently safer than gasoline. The specific characteristics are related to the final use of the fuel.

How is diesel produced?

The classical method of diesel production is carried out by fractional distillation of crude oil with an average outlet temperature of the fractionation tower of approximately 350 °C.

Gasoil can also be produced through a cracking process in which hydrocarbons with a higher molecular weight are fragmented in the presence of a catalyst or through a process of transforming methane into long-chain alkanes, taking the name GTL (Gas to Liquid ).

Similar biofuel, similar in characteristics and use, derived from renewable sources is called biodiesel.

In current practice, blends of gas oils produced with the various techniques are often used, keeping petrodiesel as the base, in order to optimize production and minimize the intrinsic limits of each technology: for example, the strong dependence on the quality of crude oil for distillates, the cost Cracking process and storage difficulties of biodiesel.

What is petrodiesel?

Diesel obtained from non-renewable sources is simply called gas oil or, in English, petrodiesel. The quality is strongly linked to that of the original crude and the distillation method. The main production method is the fractional distillation of crude oil in which the separation of diesel is obtained in the fractionation tower at a temperature of 350 ° C.

Another method of diesel production is cracking in which higher molecular weight hydrocarbons are broken down in the presence of a catalyst.

What is synthetic diesel?

Diesel oil can be artificially synthesized by different processes (synthetic diesel).

Gas oil can be produced from methane gas through a cobalt catalyst process. This process takes the name of GTL (Gas to Liquid) and allows you to have a diesel without sulfur and that at first (with the same catalyst system) can reduce the formation of nanoparticles and nitrogen oxides by half. In this field, the Volkswagen group, in collaboration with Royal Dutch Shell, is a pioneer because it considers it a valuable alternative fuel that does not eliminate food production or forests. Shell uses GTL as a component of V-Power diesel fuel blended with petroleum oil.

Diesel production is also possible from various types of biomass, such as wood, sunflower seeds, food waste, sewage waste, etc. through drying and gasification processes to obtain synthetic gas, similar to natural gas, composed mainly of methane. After purification by the Fischer-Tropsch process, synthetic gas oil is produced.

What is diesel used for?

Diesel for transportation

Diesel oil is mainly used for the supply of spontaneous combustion internal combustion engines, called diesel cycle engines, used for motor vehicles. In these engines, diesel is injected directly into the combustion chamber, it is brought to high pressure and temperature values ​​by compression and spontaneously combusts in the presence of air.

The requirements that diesel must meet for sale are defined at European level by the reference specification EN 590 issued by CEN and implemented in Italy as UNI EN 590. The characteristics of diesel that have an environmental impact (for example, the content sulfur) have been defined by the European Union in Directive 2009/30/EC.

Diesel for stationary combustion

The use of stationary combustion oil, or to feed a combustion chamber integrated with a heating boiler or a steam generator, imposes less severe characteristics. Compared to transportation diesel, it has no limits on the cetane number, admits a higher sulfur content (0.1% vs. 0.035%), and has a higher boiling point.

Diesel destined for stationary combustion

For gas oil intended for stationary combustion, a maximum temperature at which 95% of the product must be distilled, the so-called T95, is not indicated. This parameter is very important in diesel (set at 360 ° C) because it allows to limit the presence of heavy fractions of diesel that are basically responsible for the increase in particles.

Diesel for boat engines

Mid-size marine diesel engines use a light gas oil that is sometimes called diesel fuel. It is a particular diesel, which perhaps would be more appropriate to refer to as naphtha, with a high boiling point and a high sulfur content.

Currently the international reference standard is ISO 8217, more stringent regulations will probably be issued in the future, especially in the Mediterranean area.

Characteristics of diesel for transportation

The main characteristics observed are:

Physical appearance, what color is the diesel?

The diesel must have a clear appearance. Its color is not precisely defined, since it depends a lot on the production process and the presence of dyes introduced for fiscal purposes.

An opalescent color can indicate abnormality or sophistication such as the presence of unstable compounds or the presence of impurities or bacterial contamination.

What is its composition?

The sulfur content is a very important parameter, in fact it is considered a strong pollutant and over time there has been a continuous reduction of the sulfur content in diesel for environmental reasons.

Currently the limit is set at a maximum of 10 mg/kg. However, the same sulfur is responsible for the natural lubricating power of diesel and, for this reason, additives are introduced into diesel to increase its lubricating power. With the EN 590 standard, the possibility of adding a part of biodiesel to diesel oil is introduced, establishing a maximum of 7%.

Biodiesel or more appropriately FAME (fatty acid methyl esters) is, as the abbreviation says, a mixture of esters obtained by a process of transesterification of fatty acids of vegetable origin with methyl alcohol.

Pollution and impact on the environment

Diesel contaminants are divided according to their origin, liquids, solids or bacteria. The main liquid contaminant is water, which is very detrimental to the entire fuel system (designed to be self-lubricating) because it cannot maintain its lubrication. This causes oxidation, premature wear of the pumps and injectors, as well as not being able to burn. In the reference specification EN 590 it is stated that the maximum water content must be less than 200 mg / kg, that is, less than 0.02%.

Solid contaminants include sand and dust which can cause premature wear of pumps and injectors as well as being capable of preventing proper flow in the fuel system. In the reference specification EN 590, a maximum of 24 mg / kg of this type of contaminant is set.

Bacterial contamination of diesel is of particular importance: in fact, it is a good substrate for the growth of microbes and especially moulds. The allowed addition according to the reference specification EN 590 by a percentage (currently 7%) of biodiesel has caused an increase in the problem since the percentage of biodiesel brings with it both "living substrate" and water which allows a very fast reproduction of bacteria and microorganisms.

The problem manifests itself fundamentally in the generation of black algae and a rubbery consistency that over time fills the pump that extracts the diesel from the tank, which causes a greater effort until complete rupture.


An important requirement for automotive diesel is its cooling properties, represented by the temperature at which the fuel begins to cloud and opposes increased resistance to flow within a given filter.

For this reason, the reference specification EN 590 contemplates the presence during the winter period in some areas of the so-called "Arctic" diesel, fully efficient up to temperatures of -21 ° C.

What is the cetane number of diesel?

The cetane number indicates the delay between injection and the start of combustion. A high cetane number indicates a more efficient burning oil and therefore offers better performance. The minimum cetane number required today in diesel oils is 51.

For reasons of safety and regularity of combustion, it is important that it is free of light or heavy fractions, responsible for the formation of carbon deposits and incomplete combustion. The density is 835 kg / m³, its calorific value is about 40.9 MJ / kg (or 10200 kcal / kg).

Publication Date: October 30, 2018
Last Revision: October 30, 2018