It is often inappropriately called naphtha, which is distinguished by a process of better refining and for different uses.
Diesel fuel finds its first applications in the mechanical field between 1893 and 1897, when in the workshops MAN (Maschinenfabrik Augsburg Nuremberg) of Augusta, Rudolf Diesel made the first investigations that led to the invention of the diesel engine. Its name derives from having been used, in the past, to obtain mineral oil gas, by vaporization and pyrolysis.
Diesel fuel is a mixture containing aliphatic (also cyclic) hydrocarbons of 13 to 18 carbon atoms and paraffins.
Diesel fuel, which has a flash point at a higher temperature than gasoline, is less flammable and, therefore, intrinsically safer than gasoline. The specific characteristics are related to the final use of the fuel.
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.
Diesel oil can also be produced by a cracking process in which the higher molecular weight hydrocarbons are fragmented in the presence of a catalyst or by a process of transforming methane into long chain alkanes, taking the name of GTL (Gas to Liquid).
The similar biofuel, similar in characteristics and use, derived from renewable sources is called biodiesel.
In current practice, mixtures of gas oils produced with the various techniques are often used, maintaining the petrodiésel as a base, 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 of cracking process and storage difficulties of biodiesel.
Gas oil obtained from non-renewable sources is simply called gas oil or, in English, petrodiésel. The quality is strongly linked to that of the original crude and to the distillation method. The main production method is the fractional distillation of the crude oil in which the separation of the diesel in the fractionation tower is obtained at a temperature of 350 ° C. Another method of diesel production is cracking in which the hydrocarbons of higher molecular weight is fragmented in the presence of a catalyst.
The diesel fuel can be synthesized artificially by different processes (synthetic diesel).
Diesel fuel 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 the beginning (with the same catalyst system) can halve the formation of nanoparticles and nitrogen oxides. 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 the production of food or forests. Shell uses GTL as a component of diesel fuel V-Power mixed with petroleum oil.
Oil production is also possible from various types of biomass, such as wood, sunflower seeds, food waste, waste water 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.
Diesel fuel for transportation
Diesel oil is mainly used for the supply of spontaneous combustion internal combustion engines, called diesel cycle engines, used for motorized vehicles. In these engines, diesel is injected directly into the combustion chamber, is brought to high pressure and temperature values by compression and spontaneous combustion in the presence of air.
The requirements to be met by diesel for sale are defined at European level by the reference specification EN 590 issued by the CEN and implemented in Italy as UNI EN 590. The characteristics of diesel that have an environmental impact (for example, the sulfur content) have been defined by the European Union in Directive 2009/30 / EC.
The main characteristics observed are:
The diesel must have a clear appearance. Its color is not defined with precision, since it depends a lot on the production process and the presence of dyes introduced for fiscal purposes. An opalescent color may indicate anomaly or sophistication such as the presence of unstable compounds or the presence of impurities or bacterial contamination.
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 the 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 the diesel and, for this reason, additives are introduced into the diesel to increase its lubricating power. With the EN 590 standard, the possibility of adding a part of biodiesel to diesel and establishing a maximum of 7% is introduced.
Biodiesel or more appropriately FAME (methyl esters of fatty acids) is, as the abbreviation says, a mixture of esters obtained by a process of transesterification of fatty acids of vegetable origin with methyl alcohol.
Diesel fuel contaminants are divided according to their origin, liquids, solids or bacteria. The main liquid contaminant is water that is very harmful to the whole feeding system (designed to self-lubricate) because it can not maintain its lubrication. Oxidation, premature wear of pumps and injectors, as well as being unable to burn. The reference specification EN 590 states that the maximum water content must be less than 200 mg / kg, that is, less than 0.02%.
As regards solid contaminants, sand and dust are included that can cause premature wear of the pumps and injectors, as well as being able to prevent the correct flow in the feeding system. In the reference specification EN 590 a maximum of 24 mg / kg of this type of pollutants is fixed.
Bacterial contamination of diesel is of particular importance: in fact, it is a good substrate for the growth of microbes and especially of molds. The addition allowed in accordance with the reference specification EN 590 for a percentage (currently 7%) of biodiesel has caused an increase in the problem since the percentage of biodiesel brings with it both "live substrate" and water that allows a very fast reproduction of bacteria and microorganisms.
The problem is manifested mainly in the generation of black algae and gummy consistency that over time fills the pump that extracts the diesel oil from the tank, which causes a greater effort until the complete rupture.
An important requirement for automotive diesel is its cold properties, represented by the temperature at which the fuel begins to cloud and opposes a greater 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 diesel called "Arctic", fully efficient up to temperatures of -21 ° C.
The cetane number indicates the delay between the injection and the start of combustion. A high cetane number indicates a more efficient combustion oil and, therefore, offers better performance. The minimum number of cetane required today in diesel oils is 51.
For reasons of safety and regularity of combustion, it is important that it be 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 around 40.9 MJ / kg (or 10200 kcal / kg).
Diesel for stationary combustion
The use of stationary combustion oil, or to feed an integrated combustion chamber with a heating boiler or a steam generator, imposes less severe characteristics. Compared to diesel for transport, it has no limits in the cetane index, admits a higher sulfur content (0.1% vs. 0.035%), it is a boiling point.
Diesel fuel for stationary combustion
For diesel intended for stationary combustion, no maximum temperature is indicated at which 95% of the product must be distilled, the so-called T95. This parameter is very important in diesel (established at 360 ° C) because it allows to limit the presence of heavy fractions of diesel which are basically responsible for the increase in particles.
Diesel fuel for boat engines
Medium-sized marine diesel engines use a light diesel fuel that is sometimes called diesel fuel. It is a particular diesel, which perhaps would be more appropriate to refer to naphtha, high boiling point and high sulfur content.
Currently the international reference standard is ISO 8217, in the future more strict regulations will probably be issued, especially in the Mediterranean area.