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Diesel fuel
Diesel fuel
Diesel fuel
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A tank of diesel fuel on a truck

Diesel fuel, also called diesel oil, fuel oil (historically), or simply diesel, is any liquid fuel specifically designed for use in a diesel engine, a type of internal combustion engine in which fuel ignition takes place without a spark as a result of compression of the inlet air and then injection of fuel. Therefore, diesel fuel needs good compression ignition characteristics.

The most common type of diesel fuel is a specific fractional distillate of petroleum fuel oil, but alternatives that are not derived from petroleum, such as biodiesel, biomass to liquid (BTL) or gas to liquid (GTL) diesel are increasingly being developed and adopted. To distinguish these types, petroleum-derived diesel is sometimes called petrodiesel in some academic circles.[1] Diesel is a high-volume product of oil refineries.[2]

In many countries, diesel fuel is standardized. For example, in the European Union, the standard for diesel fuel is EN 590. Ultra-low-sulfur diesel (ULSD) is a diesel fuel with substantially lowered sulfur contents. As of 2016, almost all of the petroleum-based diesel fuel available in the United Kingdom, mainland Europe, and North America is of a ULSD type. Before diesel fuel had been standardized, the majority of diesel engines typically ran on cheap fuel oils. These fuel oils are still used in watercraft diesel engines. Despite being specifically designed for diesel engines, diesel fuel can also be used as fuel for several non-diesel engines, for example the Akroyd engine, the Stirling engine, or boilers for steam engines. Diesel is often used in heavy trucks. However, diesel exhaust, especially from older engines, can cause health damage.[3][4]

Names

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Diesel fuel has many colloquial names; most commonly, it is simply referred to as diesel. In the United Kingdom, diesel fuel for road use is commonly called diesel or sometimes white diesel if required to differentiate it from a reduced-tax agricultural-only product containing an identifying coloured dye known as red diesel. The official term for white diesel is DERV, standing for diesel-engine road vehicle.[5] In Australia, diesel fuel is also known as distillate[6] (not to be confused with "distillate" in an older sense referring to a different motor fuel), and in Indonesia and most of the Middle East, it is known as Solar, a trademarked name from the country's national petroleum company Pertamina. The term gas oil (French: gazole) is sometimes also used to refer to diesel fuel.

History

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Origins

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Diesel fuel originated from experiments conducted by German scientist and inventor Rudolf Diesel for his compression-ignition engine, which he invented around 1892. Originally, Diesel did not consider using any specific type of fuel. Instead, he claimed that the operating principle of his rational heat motor would work with any kind of fuel in any state of matter.[7] The first diesel engine prototype and the first functional Diesel engine were only designed for liquid fuels.[8]

At first, Diesel tested crude oil from Pechelbronn, but soon replaced it with petrol and kerosene, because crude oil proved to be too viscous,[9] with the main testing fuel for the Diesel engine being kerosene (paraffin).[10] Diesel experimented with types of lamp oil from various sources, as well as types of petrol and ligroin, which all worked well as Diesel engine fuels. Later, Diesel tested coal tar creosote,[11] paraffin oil, crude oil, gasoline and fuel oil, which eventually worked as well.[12] In Scotland and France, shale oil was used as fuel for the first 1898 production Diesel engines because other fuels were too expensive.[13] In 1900, the French Otto society built a Diesel engine for use with crude oil, which was exhibited at the 1900 Paris Exposition[14] and the 1911 World's Fair in Paris.[15] The engine actually ran on peanut oil instead of crude oil, and no modifications were necessary for peanut oil operation.[14]

During his first Diesel engine tests, Diesel also used illuminating gas as fuel, and managed to build functional designs, both with and without pilot injection.[16] According to Diesel, neither was a coal-dust–producing industry existent, nor was fine, high-quality coal-dust commercially available in the late 1890s. This is the reason why the Diesel engine was never designed or planned as a coal-dust engine.[17] Only in December 1899, did Diesel test a coal-dust prototype, which used external mixture formation and liquid fuel pilot injection.[18] This engine proved to be functional, but suffered from piston ring failure after a few minutes due to coal dust deposition.[19]

Since the 20th century

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Before diesel fuel was standardised, diesel engines typically ran on cheap fuel oils. In the United States, these were distilled from petroleum, whereas in Europe, coal-tar creosote oil was used. Some diesel engines were fuelled with mixtures of fuels, such as petrol, kerosene, rapeseed oil, or lubricating oil, which were cheaper because, at the time, they were not being taxed.[20] The introduction of motor-vehicle diesel engines, such as the Mercedes-Benz OM 138, in the 1930s meant that higher-quality fuels with proper ignition characteristics were needed. At first no improvements were made to motor-vehicle diesel fuel quality. After World War II, the first modern high-quality diesel fuels were standardised. These standards were, for instance, the DIN 51601, VTL 9140–001, and NATO F 54 standards.[21] In 1993, the DIN 51601 was rendered obsolete by the new EN 590 standard, which has been used in the European Union ever since. In sea-going watercraft, where diesel propulsion had gained prevalence by the late 1970s due to increasing fuel costs caused by the 1970s energy crisis, cheap heavy fuel oils are still used instead of conventional motor-vehicle diesel fuel. These heavy fuel oils (often called Bunker C) can be used in diesel-powered and steam-powered vessels.[22]

Types

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Diesel fuel is produced from various sources, the most common being petroleum. Other sources include biomass, animal fat, biogas, natural gas, and coal liquefaction.

Petroleum diesel

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A modern diesel dispenser

Petroleum diesel is the most common type of diesel fuel. It is produced by the fractional distillation of crude oil between 200 and 350 °C (392 and 662 °F) at atmospheric pressure, resulting in a mixture of carbon chains that typically contain between 9 and 25 carbon atoms per molecule.[23] This fraction is subjected to hydrodesulfurization.

Usually such "straight-run" diesel is insufficient in supply and quality, so other sources of diesel fuels are blended in. One major source of additional diesel fuel is obtained by cracking heavier fractions, using visbreaking and coking. This technology converts less useful fractions but the product contains olefins (alkenes) which require hydrogenation to give the saturated hydrocarbons as desired. Another refinery stream that contributes to diesel fuel is hydrocracking. Finally, kerosene is added to modify its viscosity.[24]

Synthetic diesel

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Main article: Synthetic fuel

Synthetic diesel can be produced from many carbonaceous precursors but natural gas is most important. Raw materials are converted to synthesis gas which by the Fischer–Tropsch process is converted to a synthetic diesel.[25] Synthetic diesel produced in this way generally is mainly paraffins with low sulfur and aromatics content. This material is blended often into the above mentions petroleum derived diesel.[24]

Biodiesel

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Main article: Biodiesel
Biodiesel made from soybean oil

Biodiesel is obtained from vegetable oil or animal fats (biolipids) which are mainly transesterified with methanol to form fatty acid methyl esters (FAME). It can be produced from many types of oils, the most common being rapeseed oil (rapeseed methyl ester, RME) in Europe and soybean oil (soy methyl ester, SME) in the US. Methanol can also be replaced with ethanol for the transesterification process, which results in the production of ethyl esters. The transesterification processes use catalysts, such as sodium or potassium hydroxide, to convert vegetable oil and methanol into biodiesel and the undesirable byproducts glycerine and water, which will need to be removed from the fuel along with methanol traces. Biodiesel can be used pure (B100) in engines where the manufacturer approves such use, but it is more often used as a mix with diesel, BXX where XX is the biodiesel content in percent.[26][27]

FAME used as fuel is specified in DIN EN 14214[28] and ASTM D6751 standards.[29]

Storage and additives

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Large diesel fuel tanks in Sörnäinen, Helsinki, Finland

In the US, diesel is recommended to be stored in a yellow container to differentiate it from kerosene, which is typically kept in blue containers, and gasoline (petrol), which is typically kept in red containers.[30] In the UK, diesel is normally stored in a black container to differentiate it from unleaded or leaded petrol, which are stored in green and red containers, respectively.[31]

Ethylene-vinyl acetate (EVA) is added to diesel as a "cold flow improver". 50–500 ppm of EVA inhibits crystallization of waxes, which can block fuel filters. Antifoaming agents (silicones), antioxidants (e.g butylated hydroxytoluene), and metal deactivating agents are other additives. Their use is dictated by the particular composition of and storage plans for diesel fuels. Each is added at the 5–50 ppm level.[24]

Standards

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The diesel engine is a multifuel engine and can run on a huge variety of fuels. However, development of high-performance, high-speed diesel engines for cars and lorries in the 1930s meant that a proper fuel specifically designed for such engines was needed: diesel fuel. In order to ensure consistent quality, diesel fuel is standardised; the first standards were introduced after World War II.[21] Typically, a standard defines certain properties of the fuel, such as cetane number, density, flash point, sulphur content, or biodiesel content. Diesel fuel standards include:

Diesel fuel

Biodiesel fuel

Measurements and pricing

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Cetane number

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Main article: Cetane number

The principal measure of diesel fuel quality is its cetane number. A cetane number is a measure of the delay of ignition of a diesel fuel.[32] A higher cetane number indicates that the fuel ignites more readily when sprayed into hot compressed air.[32] European (EN 590 standard) road diesel has a minimum cetane number of 51. Fuels with higher cetane numbers, normally "premium" diesel fuels with additional cleaning agents and some synthetic content, are available in some markets.

Fuel value and price

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Further information: Gasoline and diesel usage and pricing

About 86.1% of diesel fuel mass is carbon, and when burned, it offers a net heating value of 43.1 MJ/kg as opposed to 43.2 MJ/kg for gasoline. Due to the higher density, diesel fuel offers a higher volumetric energy density: the density of EN 590 diesel fuel is defined as 0.820 to 0.845 kg/L (6.84 to 7.05 lb/US gal) at 15 °C (59 °F), about 9.0–13.9% more than EN 228 gasoline (petrol)'s 0.720–0.775 kg/L (6.01–6.47 lb/US gal) at 15 °C, which should be put into consideration when comparing volumetric fuel prices. The CO2 emissions from diesel are 73.25 g/MJ, just slightly lower than for gasoline at 73.38 g/MJ.[33]

Diesel fuel is generally simpler to refine from petroleum than gasoline. Additional refining is required to remove sulfur, which contributes to a sometimes higher cost. In many parts of the United States and throughout the United Kingdom and Australia,[34] diesel fuel may be priced higher than petrol per gallon or liter.[35][36] Reasons for higher-priced diesel include the shutdown of some refineries in the Gulf of Mexico, diversion of mass refining capacity to gasoline production, and a recent transfer to ultra-low-sulfur diesel (ULSD), which causes infrastructural complications.[37] In Sweden, a diesel fuel designated as MK-1 (class 1 environmental diesel) is also being sold. This is a ULSD that has a lower aromatics content, with a limit of 5%.[38] This fuel is slightly more expensive to produce than regular ULSD. In Germany, the fuel tax on diesel fuel is about 28% lower than the petrol fuel tax.

Taxation

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Diesel fuel is similar to heating oil, which is used in central heating. In Europe, the United States, and Canada, taxes on diesel fuel are higher than on heating oil due to the fuel tax, and in those areas, heating oil is marked with fuel dyes and trace chemicals to prevent and detect tax fraud. "Untaxed" diesel (sometimes called "off-road diesel" or "red diesel" due to its red dye) is available in some countries for use primarily in agricultural applications, such as fuel for tractors, recreational and utility vehicles or other noncommercial vehicles that do not use public roads. This fuel may have sulfur levels that exceed the limits for road use in some countries (e.g. US).

This untaxed diesel is dyed red for identification,[39] and the use of this untaxed diesel fuel for a typically taxed purpose (such as driving use) can be fined (e.g. US$10,000 in the US). In the United Kingdom, Belgium and the Netherlands, it is known as red diesel (or gas oil), and is also used in agricultural vehicles, home heating tanks, refrigeration units on vans/trucks which contain perishable items such as food and medicine, and for marine craft. Diesel fuel, or marked gas oil is dyed green in the Republic of Ireland and Norway. The term "diesel-engined road vehicle" (DERV) is used in the UK as a synonym for unmarked road diesel fuel. In India, taxes on diesel fuel are lower than on petrol, as the majority of the transportation for grain and other essential commodities across the country runs on diesel.

Taxes on biodiesel in the US vary between states. Some states (Texas, for example) have no tax on biodiesel and a reduced tax on biodiesel blends equivalent to the amount of biodiesel in the blend, so that B20 fuel is taxed 20% less than pure petrodiesel.[40] Other states, such as North Carolina, tax biodiesel (in any blended configuration) the same as petrodiesel, although they have introduced new incentives to producers and users of all biofuels.[41]

Uses

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Diesel fuel is mostly used in high-speed diesel engines, especially motor-vehicle (e.g. car, lorry) diesel engines, but not all diesel engines run on diesel fuel. For example, large two-stroke watercraft engines typically use heavy fuel oils instead of diesel fuel,[22] and certain types of diesel engines, such as MAN M-System engines, are designed to run on petrol with knock resistances of up to 86 RON.[42] On the other hand, gas turbine and some other types of internal combustion engines, and external combustion engines, can also be designed to take diesel fuel.

The viscosity requirement of diesel fuel is usually specified at 40 °C.[32] A disadvantage of diesel fuel in cold climates is that its viscosity increases as the temperature decreases, changing it into a gel (see Compression Ignition – Gelling) that cannot flow in fuel systems. Special low-temperature diesel contains additives to keep it liquid at lower temperatures.

On-road vehicles

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Trucks and buses, which were often otto-powered in the 1920s through 1950s, are now almost exclusively diesel-powered. Due to its ignition characteristics, diesel fuel is thus widely used in these vehicles. Since diesel fuel is not well-suited for otto engines, passenger cars, which often use otto or otto-derived engines, typically run on petrol instead of diesel fuel. However, especially in Europe and India, many passenger cars have, due to better engine efficiency,[43] diesel engines, and thus run on regular diesel fuel.

Railroad

[edit]
See also: Dieselisation and Diesel locomotive

Diesel displaced coal and fuel oil for steam-powered vehicles in the latter half of the 20th century, and is now used almost exclusively for the combustion engines of self-powered rail vehicles (locomotives and railcars).[44][45]

Aircraft

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Packard DR-980 9-cylinder diesel aircraft engine, used in the first diesel-engine airplane

In general, diesel engines are not well-suited for planes and helicopters. This is because of the diesel engine's comparatively low power-to-mass ratio, meaning that diesel engines are typically rather heavy, which is a disadvantage in aircraft. Therefore, there is little need for using diesel fuel in aircraft, and diesel fuel is not commercially used as aviation fuel. Instead, petrol (Avgas), and jet fuel (e. g. Jet A-1) are used. However, especially in the 1920s and 1930s, numerous series-production aircraft diesel engines that ran on fuel oils were made, because they had several advantages: their fuel consumption was low, they were reliable, not prone to catching fire, and required minimal maintenance. The introduction of petrol direct injection in the 1930s outweighed these advantages, and aircraft diesel engines quickly fell out of use.[46] With improvements in power-to-mass ratios of diesel engines, several on-road diesel engines have been converted to and certified for aircraft use since the early 21st century. These engines typically run on Jet A-1 aircraft fuel (but can also run on diesel fuel). Jet A-1 has ignition characteristics similar to diesel fuel, and is thus suited for certain (but not all) diesel engines.[47]

Military vehicles

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Until World War II, several military vehicles, especially those that required high engine performance (armored fighting vehicles, for example the M26 Pershing or Panther tanks), used conventional otto engines and ran on petrol. Ever since World War II, several military vehicles with diesel engines have been made, capable of running on diesel fuel. This is because diesel engines are more fuel efficient, and diesel fuel is less prone to catching fire.[48] Some of these diesel-powered vehicles (such as the Leopard 1 or MAN 630) still ran on petrol, and some military vehicles were still made with otto engines (e. g. Ural-375 or Unimog 404), incapable of running on diesel fuel.

Tractors and heavy equipment

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Today's tractors and heavy equipment are mostly diesel-powered. Among tractors, only the smaller classes may also offer gasoline-fuelled engines. The dieselization of tractors and heavy equipment began in Germany before World War II but was unusual in the United States until after that war. During the 1950s and 1960s, it progressed in the US as well. Diesel fuel is commonly used in oil and gas extracting equipment, although some locales use electric or natural gas powered equipment.

Tractors and heavy equipment were often multifuel in the 1920s through 1940s, running either spark-ignition and low-compression engines, akroyd engines, or diesel engines. Thus many farm tractors of the era could burn gasoline, alcohol, kerosene, and any light grade of fuel oil such as heating oil, or tractor vaporising oil, according to whichever was most affordable in a region at any given time. On US farms during this era, the name "distillate" often referred to any of the aforementioned light fuel oils. Spark ignition engines did not start as well on distillate, so typically a small auxiliary gasoline tank was used for cold starting, and the fuel valves were adjusted several minutes later, after warm-up, to transition to distillate. Engine accessories such as vaporizers and radiator shrouds were also used, both with the aim of capturing heat, because when such an engine was run on distillate, it ran better when both it and the air it inhaled were warmer rather than at ambient temperature. Dieselization with dedicated diesel engines (high-compression with mechanical fuel injection and compression ignition) replaced such systems and made more efficient use of the diesel fuel being burned.

Other uses

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Poor quality diesel fuel has been used as an extraction agent for liquid–liquid extraction of palladium from nitric acid mixtures.[49] Such use has been proposed as a means of separating the fission product palladium from PUREX raffinate which comes from used nuclear fuel.[49] In this system of solvent extraction, the hydrocarbons of the diesel act as the diluent while the dialkyl sulfides act as the extractant.[49] This extraction operates by a solvation mechanism.[49] So far, neither a pilot plant nor full scale plant has been constructed to recover palladium, rhodium or ruthenium from nuclear wastes created by the use of nuclear fuel.[50]

Diesel fuel is often used as the main ingredient in oil-base mud drilling fluid.[51] The advantage of using diesel is its low cost and its ability to drill a wide variety of difficult strata, including shale, salt and gypsum formations.[51] Diesel-oil mud is typically mixed with up to 40% brine water.[52] Due to health, safety and environmental concerns, Diesel-oil mud is often replaced with vegetable, mineral, or synthetic food-grade oil-base drilling fluids, although diesel-oil mud is still in widespread use in certain regions.[53]

During development of rocket engines in Germany during World War II J-2 Diesel fuel was used as the fuel component in several engines including the BMW 109-718.[54] J-2 diesel fuel was also used as a fuel for gas turbine engines.[54]

Chemical analysis

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Chemical composition

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Diesel does not mix with water. This picture also showcases the phenomenon of Thin-film interference.

In the United States, petroleum-derived diesel is composed of about 75% saturated hydrocarbons (primarily paraffins including n, iso, and cycloparaffins), and 25% aromatic hydrocarbons (including naphthalenes and alkylbenzenes).[55] The average chemical formula for common diesel fuel is C12H23, ranging approximately from C10H20 to C15H28.[56]

Chemical properties

[edit]
Further information: Diesel exhaust and Gel point (petroleum)

Most diesel fuels freeze at common winter temperatures, while the temperatures greatly vary.[57] Petrodiesel typically freezes around temperatures of −8.1 °C (17.4 °F), whereas biodiesel freezes between temperatures of 2 to 15 °C (36 to 59 °F).[57] The viscosity of diesel noticeably increases as the temperature decreases, changing it into a gel at temperatures of −19 to −15 °C (−2 to 5 °F), that cannot flow in fuel systems. Conventional diesel fuels vaporise at temperatures between 149 °C and 371 °C.[32]

Conventional diesel flash points vary between 52 and 96 °C, which makes it safer than petrol and unsuitable for spark-ignition engines.[58] Unlike petrol, the flash point of a diesel fuel has no relation to its performance in an engine nor to its auto ignition qualities.[32]

Carbon dioxide formation

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As a good approximation the chemical formula of diesel is C
nH
2n. Diesel is a mixture of different molecules. As carbon has a molar mass of 12 g/mol and hydrogen has a molar mass of about 1 g/mol, so the fraction by weight of carbon in EN 590 diesel fuel is roughly 12/14.

The reaction of diesel combustion is given by:

2 C
nH
2n + 3n O
2 ⇌ 2n CO
2 + 2n H
2O

Carbon dioxide has a molar mass of 44 g/mol as it consists of 2 atoms of oxygen (16 g/mol) and 1 atom of carbon (12 g/mol). So 12 g of carbon yields 44 g of carbon dioxide.

Diesel has a density of 838 g per liter.

Putting everything together the mass of carbon dioxide that is produced by burning 1 liter of diesel fuel can be calculated as:

The figure obtained with this estimation is close to the values found in the literature.

For gasoline, with a density of 0.75 kg/L and a ratio of carbon to hydrogen atoms of about 6 to 14, the estimated value of carbon emission if 1 liter of gasoline is burnt gives:[59]

Hazards

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See also: Diesel exhaust

Environment hazards of sulfur

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In the past, diesel fuel contained higher quantities of sulfur. European emission standards and preferential taxation have forced oil refineries to dramatically reduce the level of sulfur in diesel fuels. In the European Union, the sulfur content has dramatically reduced during the last 20 years. Automotive diesel fuel is covered in the European Union by standard EN 590. In the 1990s specifications allowed a content of 2000 ppm max of sulfur, reduced to a limit of 350 ppm by the beginning of the 21st century with the introduction of Euro 3 specifications. The limit was lowered with the introduction of Euro 4 by 2006 to 50 ppm (ULSD, Ultra Low Sulfur Diesel). The standard for diesel fuel in force in Europe as of 2009 is the Euro 5, with a maximum content of 10 ppm.[60]

Emission standard At latest Sulfur content Cetane number
N/a 1 January 1994 max. 2000 ppm min. 49
Euro 2 1 January 1996 max. 500 ppm min. 49
Euro 3 1 January 2001 max. 350 ppm min. 51
Euro 4 1 January 2006 max. 50 ppm min. 51
Euro 5 1 January 2009 max. 10 ppm min. 51

In the United States, more stringent emission standards have been adopted with the transition to ULSD starting in 2006, and becoming mandatory on June 1, 2010 (see also diesel exhaust).

Algae, microbes, and water contamination

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Main article: Microbial contamination of diesel fuel

There has been much discussion and misunderstanding of algae in diesel fuel. Algae need light to live and grow. As there is no sunlight in a closed fuel tank, no algae can survive, but some microbes can survive and feed on the diesel fuel.[61]

These microbes form a colony that lives at the interface of fuel and water. They grow quite fast in warmer temperatures. They can even grow in cold weather when fuel tank heaters are installed. Parts of the colony can break off and clog the fuel lines and fuel filters.[62]

Water in fuel can damage a fuel injection pump. Some diesel fuel filters also trap water. Water contamination in diesel fuel can lead to freezing while in the fuel tank. The freezing water that saturates the fuel will sometimes clog the fuel injector pump.[63] Once the water inside the fuel tank has started to freeze, gelling is more likely to occur. When the fuel is gelled it is not effective until the temperature is raised and the fuel returns to a liquid state.

Road hazard

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Diesel is less flammable than gasoline/petrol. However, because it evaporates slowly, any spills on a roadway can pose a slip hazard to vehicles.[64] After the light fractions have evaporated, a greasy slick is left on the road which reduces tire grip and traction, and can cause vehicles to skid. The loss of traction is similar to that encountered on black ice, resulting in especially dangerous situations for two-wheeled vehicles, such as motorcycles and bicycles, in roundabouts.

See also

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References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Diesel fuel

View on Grokipedia
from Grokipedia
Diesel fuel is a middle-distillate refined from crude oil through , primarily comprising saturated and unsaturated hydrocarbons with carbon chain lengths of 10 to 22 atoms, boiling in the range of 180 to 360 °C. It serves as the primary fuel for compression-ignition engines, which rely on high compression ratios to achieve autoignition, yielding thermal efficiencies of 35 to 45 percent—substantially higher than the 20 to 30 percent typical of spark-ignition gasoline engines due to the fuel's higher and . Developed in the late by to enable efficient operation on diverse fuels including vegetable oils, petroleum-based diesel became dominant for its availability and performance in heavy-duty applications. Key properties include a density of approximately 0.82 to 0.86 kg/, volumetric energy content of 35 to 38 MJ/ (about 15 percent greater than 's 32 to 34 MJ/), and lower volatility compared to lighter fuels, which enhances storage stability but requires precise injection for control. Widely used in transportation sectors such as trucking, rail, marine shipping, and transit—as well as in stationary power , , and —diesel accounts for roughly one-third of global consumption, powering equipment where and fuel economy outweigh . While its efficiency reduces emissions per unit of work relative to , diesel engines produce elevated nitrogen oxides and particulate matter, spurring advancements in ultra-low-sulfur formulations and exhaust aftertreatment systems to mitigate environmental impacts without sacrificing operational advantages.

History

Invention by Rudolf Diesel

, a German mechanical engineer born in 1858, developed the concept for a highly efficient in the late 1880s while employed by the Linde refrigeration company in . Motivated by the inefficiencies of steam engines (typically around 10% ) and existing gas engines, Diesel sought to realize a closer to the Carnot ideal through compression ignition, where air is compressed to elevate its temperature sufficiently for fuel auto-ignition without a spark, followed by at constant pressure. This approach theoretically permitted efficiencies up to 75% under ideal conditions, far exceeding contemporaries, by leveraging higher compression ratios viable with cheaper, denser fuels less prone to pre-ignition than volatile . Diesel filed his first for the compression-ignition engine in 1892, receiving German Patent DRP No. 67207 on February 23, , for the "method of operating and construction of an ." Corresponding U.S. patents followed, including No. 542,846 in 1895 and No. 608,845 in 1898, detailing the engine's core principles of high compression (up to 25:1 or more), air-blast , and delayed to optimize expansion work. The design specified a four-stroke cycle with a large single , such as the prototype's 150 mm bore and 400 mm stroke, built by Maschinenfabrik Augsburg-Nürnberg (MAN) and first fired on August 10, , though early tests encountered mechanical issues like seizures from excessive pressures. Refinements culminated in a successful demonstration on February 17, 1897, using a 250 mm bore, 400 mm stroke engine producing 14.7 kW (20 hp) at 172 rpm, achieving a practical thermal efficiency of 26.2%—over twice that of steam engines—and a specific fuel consumption of 317 g/kWh. This engine employed air injection to atomize and introduce fuel into the hot compressed air (around 550–700°C), enabling operation on heavier, less refined liquid fuels unsuitable for spark-ignition engines. Diesel initially considered diverse feedstocks, including vegetable seed oils like peanut oil (demonstrated at the 1900 Paris Exposition) and even coal dust or tar, to capitalize on abundant, low-cost resources for industrial applications. However, the engine's tolerance for high-viscosity, high-cetane fuels lacking volatility—contrasting gasoline's requirements—paved the way for "diesel fuel" as a standardized petroleum fraction, typically the heavier distillate boiling between 200–350°C from crude oil refining, which offered superior energy density (around 35–36 MJ/L) and lubricity while minimizing refining costs compared to lighter kerosenes. This fuel-engine synergy, rather than a predefined fuel invention, defined the diesel system's emergence, with commercialization by firms like Sulzer in 1898 shifting toward petroleum derivatives for reliability and scalability.

Early commercialization and adoption

The first commercial diesel engines, designed to operate on heavier petroleum distillates known as diesel fuel, emerged shortly after Rudolf Diesel's successful prototype test on February 17, 1897, at in , where the engine achieved 26.2% at 172 rpm. Diesel licensed his patents to manufacturers including Sulzer Brothers in , which produced the first operational in June 1898, a four-stroke model developed in collaboration with Diesel himself. In March 1898, the inaugural commercial installation occurred at the plant of Vereinigte Zündholzfabriken A.G., a German matchstick , featuring two 30 PS (22 kW) engines built under , marking the shift from experimental prototypes to industrial power generation using diesel oil—a medium distillate fraction from refining, distinct from lighter or . These early engines, typically large and slow-speed (under 300 rpm), were favored for stationary applications in factories and breweries due to their higher efficiency—up to 75% better fuel economy than engines—and ability to burn lower-grade fuels that were previously underutilized heavy oils. By 1900, over 200 licenses had been granted worldwide, primarily in , spurring adjustments to isolate consistent diesel fractions for reliable ignition under compression. Marine adoption accelerated commercialization, with Diesel licensing his technology to (the Nobel family's Russian oil company) in 1898 for propulsion in oil tankers, leveraging the engine's and for long-haul shipping. The first seagoing diesel-powered vessel, a Danish freighter, entered service in , followed by widespread retrofits in fleets by the 1910s, as diesel propulsion reduced coal dependency and enabled self-bunkering with carried . Early diesel fuel specifications emphasized and cetane-like ignition quality, derived empirically from engine trials rather than standardized testing, with distillates boiling between 200–350°C proving optimal for compression-ignition without issues plaguing lighter fuels. Adoption in submarines began around 1913 with U.S. installations, prioritizing diesel's and reduced risk over . By the early 1920s, stationary and marine diesels dominated heavy industry in , though global spread was limited by high initial costs and manufacturing scale-up challenges until high-speed variants enabled and use.

20th-century refinements and global spread

High-speed diesel engines, introduced in the 1920s for commercial vehicles and the 1930s for passenger cars, necessitated refinements in fuel quality beyond the heavier, unrefined distillates used in Rudolf Diesel's original low-speed designs. These advancements included lighter distillate fractions with improved ignition characteristics, as measured by the scale developed in the late 1920s to quantify fuel's compression-ignition delay. Refineries adapted thermal cracking processes, initially pioneered around 1910 for automotive fuels, to produce diesel fractions with higher typically exceeding 40 for reliable high-speed operation. Additive development accelerated in to address instability arising from evolving refinery techniques, such as cracked stocks that promoted oxidation and gum formation. By mid-century, common additives included detergents for injector cleanliness, anti-foam agents to reduce handling issues, and early cetane improvers like alkyl nitrates to boost ignition in variable-quality feeds. Post- standardization efforts established specifications for , , and content, enabling consistent performance across engines; for instance, military specifications during the war emphasized limits to minimize in and ship applications. reduction gained momentum later in the century, with European mandates beginning phased cuts in the to below 0.2% by weight, driven by emissions concerns rather than initial performance needs. Diesel fuel's global spread accelerated with demands, where it powered submarines, merchant fleets, and some armored vehicles, contributing to diesel engines handling approximately 25% of sea trade by 1939. Post-war reconstruction fueled adoption in locomotives and trucking; by the 1950s, diesel-electric trains supplanted steam globally, while heavy-duty road transport in and leveraged diesel's superior torque and efficiency over . In passenger vehicles, led with introducing the 260D in 1936, but mass uptake occurred post-1950 amid fuel taxes favoring diesel, contrasting the U.S. where dominance persisted due to abundant supplies and consumer preference for smoother operation. By century's end, diesel accounted for over 90% of heavy-duty vehicle fuel worldwide, underpinning industrial growth in developing regions through stationary generators and agricultural machinery.

Recent developments since 2000

In the early 2000s, regulatory efforts worldwide focused on drastically reducing content in diesel fuel to enable advanced engine aftertreatment systems for lower emissions. In the United States, the Environmental Protection Agency (EPA) initiated a phase-in of ultra-low diesel (ULSD), limiting sulfur to 15 parts per million (ppm) for highway diesel by June 2006 and extending to non-road, , and marine applications by 2010–2012. This change addressed lubricity loss from desulfurization, often mitigated by additives or blending, as low-sulfur fuels alone increased wear on fuel pumps and injectors. Similar reductions occurred in Europe under Euro 4 standards from 2005, requiring sulfur below 50 ppm initially, then 10 ppm by Euro 5 in 2009, supporting particulate filters and catalysts that reduced particulate matter and nitrogen oxides by up to 98% in heavy-duty engines since the baseline. Parallel to sulfur controls, biodiesel—fatty acid methyl esters derived from vegetable oils or animal fats—gained traction as a renewable blendstock. U.S. production, negligible before 2000, surged post-2005 Energy Policy Act incentives and the Renewable Fuel Standard (RFS), reaching 1.8 billion gallons by 2020, often blended at B5–B20 levels to enhance lubricity and meet RFS biomass-based diesel mandates, which set volumes like 2.43 billion gallons for 2021. Standards evolved accordingly; ASTM D6751 for biodiesel was updated in 2023 to include low-metal ULSD-compatible grades limiting sodium, potassium, calcium, and magnesium to 4 mg/kg, addressing contamination risks in high blends. Renewable diesel, chemically identical to diesel, emerged as a higher-quality alternative, avoiding biodiesel's cold-flow and stability issues. U.S. production grew from 40 million gallons in 2010/11 to 2.3 billion gallons in 2022/23, overtaking amid state low-carbon standards and corporate decarbonization goals, with total biodiesel-plus-renewable consumption projected to exceed 5 billion gallons in 2024. These fuels, sourced from waste oils and soy, integrate drop-in into existing but face scalability limits from feedstock competition with production. Post-2010 innovations included synthetic fuels via Fischer-Tropsch processes for near-zero sulfur and aromatics, though adoption remained niche due to high costs; regulatory pressures like California's 2018 low-carbon fuel standard further boosted renewable variants over fossil diesel. These shifts prioritized compatibility with emissions hardware, reducing tailpipe pollutants but raising refining energy demands and dependency on biomass supply chains.

Chemical Composition and Properties

Molecular makeup and fractions

Diesel fuel comprises a complex mixture of hydrocarbons derived from the middle distillate fraction of crude oil, specifically the gas oil cut obtained during atmospheric distillation with boiling points ranging from approximately 150°C to 380°C. This straight-run fraction is further processed through hydrotreating or cracking to remove impurities like sulfur and adjust properties for engine performance. The resulting fuel contains predominantly saturated hydrocarbons, with minor unsaturated components, and its composition reflects the original crude's characteristics—paraffinic crudes yielding higher straight-chain content, while aromatic or naphthenic crudes contribute more cyclic and ring structures. The primary molecular classes include paraffins (straight- and branched-chain alkanes), naphthenes (cycloalkanes), and aromatics (such as alkylbenzenes and polycyclic aromatic hydrocarbons), with carbon chain lengths typically spanning C9 to C20. Aliphatic hydrocarbons, encompassing paraffins and naphthenes, constitute about 64% of the mixture, while aromatics account for roughly 35%, and olefins (unsaturated hydrocarbons) make up 1–2%. These proportions can vary; for instance, hydrotreated diesels exhibit reduced aromatic content to enhance and minimize formation during combustion. In terms of fractions, diesel emerges between lighter (boiling up to ~200°C) and heavier fuel oils or lubricants (above 350°C) in the distillation column, capturing molecules suited for compression-ignition engines due to their higher molecular weight and compared to (C4–C12). Refining adjustments, such as catalytic dewaxing, further tailor the paraffin-to-naphthene ratio to optimize cold-flow properties and , ensuring the fuel remains pumpable in diverse climates.

Physical characteristics

Diesel fuel exists as a at ambient temperatures and pressures, comprising a of hydrocarbons distilled from with a characteristic pungent, kerosene-like . Its appearance is typically clear to pale yellow, though color can vary from light straw to darker hues depending on the source crude oil and refining processes, with high-gravity crudes yielding lighter, thinner fuels and low-gravity crudes producing thicker, darker variants. Density, often expressed as specific at 15.6°C (60°F), ranges from 0.82 to 0.88 for ASTM Grade No. 2-D diesel, equivalent to an of 30 to 42 degrees; the weight per liter of diesel fuel equals its density (mass per unit volume, typically in kg/L), so a change in density causes a proportional change in weight per liter—higher density increases weight per liter, lower density decreases it—with typical values of 0.82–0.85 kg/L at 15°C (e.g., 0.832 kg/L commonly cited), varying with temperature (higher when colder) and composition. This property influences fuel volume, pump delivery, and efficiency but is not directly mandated in ASTM D975, instead derived from related tests like and . Kinematic at 40°C, critical for injector lubrication and fuel atomization, must lie between 1.9 and 4.1 mm²/s (cSt) for No. 2-D to ensure proper flow without excessive wear or poor spray characteristics. Dynamic viscosity, derived from kinematic viscosity multiplied by density (typically 0.82–0.86 g/cm³), ranges from 1.6 to 3.5 cP at 40°C (typical 2.0–3.0 cP) per ASTM D975. Viscosity increases with decreasing temperature, reaching 3–5 cP at 20–25°C and exceeding 10 cP below 0°C; biodiesel blends slightly increase it relative to pure petroleum diesel. The , the lowest temperature at which vapors ignite when exposed to an open flame, is specified at a minimum of 52°C for No. 2-D diesel under ASTM D93, providing a margin over gasoline's lower threshold and reducing during storage and handling. characteristics, measured via ASTM D86, define the boiling range: for No. 2-D, the temperature at which 90% by volume is recovered falls between 282°C and 338°C, reflecting the mid-range volatility suited to compression-ignition engines with initial boiling points around 150–200°C and endpoints up to 380°C.
PropertyASTM D975 Specification (No. 2-D)Test MethodNotes
Kinematic Viscosity (at 40°C)1.9–4.1 mm²/sD445Ensures lubricity and atomization
Flash Point≥52°CD93Minimum for safety
Distillation (90% vol. recovery)282–338°CD86Defines volatility range
Density (max. at 15°C)≤876 kg/m³D1298Indirect via other properties
Other physical traits include a typically above -15°C for winter-grade variants to prevent gelling in cold climates, and around 25–28 mN/m, which affects droplet formation in sprays, though not standardized in D975.

Key performance metrics

Diesel fuel's content is a primary performance metric, enabling high in compression-ignition engines due to its relatively high lower heating value of approximately 42.5 to 43.5 MJ/kg and volumetric of 35 to 36 MJ/L, which exceeds that of by about 10-15% on a basis. This advantage stems from diesel's higher (typically 0.82 to 0.86 kg/L at 15°C) compared to , allowing greater per unit despite similar gravimetric values. The net heating value correlates directly with fuel economy, with variations arising from hydrocarbon composition—paraffinic fractions yielding higher values than aromatics. Viscosity governs fuel atomization, injection timing, and internal in high-pressure systems, with ASTM D975 specifying 1.9 to 4.1 mm²/s (cSt) at 40°C for No. 2 diesel to balance flow and prevent excessive wear or poor spray patterns. Insufficient risks leakage and incomplete , while excess hinders pump operation and increases energy losses.
PropertyTypical Value/RangeStandard Reference (ASTM D975)Performance Impact
Density (at 15°C)0.82–0.88 g/cm³Indirect (via 30–42)Influences volumetric energy delivery and combustion completeness; higher density boosts power output but may elevate emissions.
Flash Point≥52°CMinimum 52°CDetermines handling safety and storage requirements; values below this increase fire hazard during transport or refueling.
Pour Point-15°C to -35°C (varies by grade)No fixed minimum; guided by regional needsMeasures low-temperature flowability; exceeding the pour point causes gelling and filter blockage, impairing cold-start reliability.
Lubricity (HFRR wear scar)≤520 μmMaximum 520 μm (D6079 test)Protects fuel pumps and injectors from wear, especially in ultra-low diesel lacking natural ; deficiencies accelerate component failure and reduce lifespan.
These metrics collectively ensure reliable ignition, efficient energy release, and minimal operational disruptions, though they must align with design and environmental regulations for optimal performance.

Production Methods

Refining from

Diesel fuel is obtained from crude primarily through followed by hydrotreating and other upgrading processes in refineries. Crude , a complex of hydrocarbons, is first heated in an atmospheric distillation unit to temperatures around 350–400°C under , vaporizing the components which then rise in a fractionation column where they condense at different heights based on points. The diesel , known as straight-run gas , is collected from the middle distillate cut with a boiling range typically between 200–370°C, corresponding to hydrocarbons with 12–25 carbon atoms. This straight-run distillate often requires additional refining to meet fuel specifications, particularly hydrotreating to remove , , and oxygen compounds that can cause deposits, , or emissions issues. Hydrotreating involves reacting the distillate with gas over a cobalt-molybdenum or nickel-molybdenum catalyst at temperatures of 300–400°C and pressures of 30–130 bar, converting sulfur compounds to (H₂S), which is then stripped out. This process also saturates olefins and aromatics, improving stability and while reducing density. In modern refineries, deep achieves ultra-low sulfur diesel (ULSD) with sulfur content below 10–15 ppm, as required by regulations like the U.S. EPA's 2006 highway diesel standard and Euro V/VI norms. To increase diesel yields beyond straight-run distillation (which varies by crude type, yielding 15–30% diesel from light to heavy crudes), refineries employ conversion processes such as hydrocracking, where heavier gas oils are cracked under pressure over catalysts to produce additional middle distillates. Final blending may incorporate additives or lighter fractions for and cold-flow properties, but the core petroleum-derived diesel remains dominated by paraffinic, naphthenic, and aromatic hydrocarbons from these refining steps. configurations optimized for diesel, such as those in post-2000, prioritize these processes to balance gasoline-diesel ratios amid varying crude slates and market demands.

Synthetic production processes

Synthetic diesel fuel is produced through processes that convert synthesis gas (syngas), a mixture of (CO) and (H₂), into liquid hydrocarbons via catalytic reactions, primarily the -Tropsch synthesis (FTS). Developed in the 1920s by chemists Franz and Hans Tropsch, FTS polymerizes syngas over metal catalysts such as iron or at temperatures of 200–350°C and pressures of 1–5 MPa to yield straight-chain paraffins, olefins, and oxygenates, which are then hydrocracked and isomerized to produce diesel-range hydrocarbons (C₁₀–C₂₀). The resulting synthetic diesel exhibits a high (typically 70–85), negligible and aromatics content (<1%), and improved cold-flow properties compared to petroleum-derived diesel, enabling cleaner combustion with reduced particulate matter and NOx emissions. The primary feedstocks for syngas generation determine the variant: coal-to-liquids (CTL), gas-to-liquids (GTL), or biomass-to-liquids (BtL). In CTL, coal is gasified at high temperatures (above 700°C) with steam and oxygen to produce syngas, followed by FTS; this indirect route has been commercialized since the 1950s by in South Africa, where plants produced over 150,000 barrels per day of synthetic fuels by the 1980s, and continues in China, with a planned 4 million tonnes per year facility announced in 2024 emphasizing diesel output. CTL yields high-quality diesel but requires substantial water (up to 1–2 barrels per barrel of product) and generates high CO₂ emissions (2–3 times that of petroleum refining without carbon capture). GTL processes reform natural gas via steam methane reforming or partial oxidation to syngas, then apply FTS; Shell's Pearl GTL facility in Qatar, operational since 2012, converts 1.6 billion cubic feet of gas per day into 140,000 barrels of liquids, including premium diesel with sulfur levels below 1 ppm. This method produces diesel with excellent lubricity and stability, suitable for blending, though economic viability depends on gas prices below $3–5 per million Btu. BtL follows a similar gasification-to-FTS pathway but uses lignocellulosic biomass, pretreated via pyrolysis or torrefaction before gasification at 800–1,000°C to yield syngas, which is cleaned of tars and impurities prior to synthesis. Pilot plants, such as those tested in Europe since the 2000s, demonstrate diesel yields of 70–80% from dry biomass input, with lifecycle greenhouse gas reductions of 80–90% versus fossil diesel when accounting for biomass regrowth, though commercialization remains limited by high costs ($100–150 per barrel equivalent) and feedstock logistics. Across all variants, upgrading steps like hydrotreating ensure compliance with standards such as EN 590, emphasizing FTS's role in producing drop-in diesel compatible with existing infrastructure.

Bio-based and renewable variants


Bio-based diesel fuels encompass biodiesel and renewable diesel, both derived from renewable feedstocks such as vegetable oils, animal fats, and waste lipids through distinct chemical processes. Biodiesel consists of fatty acid methyl esters (FAME) produced primarily via transesterification, where triglycerides in feedstocks react with methanol in the presence of a catalyst, typically sodium hydroxide or potassium hydroxide, to yield biodiesel and glycerol as a byproduct. Common U.S. feedstocks include soybean oil, which accounts for the majority of production, alongside animal fats from meat processing and recycled restaurant greases. This process operates under mild conditions, around 60°C and atmospheric pressure, but requires purification steps like washing to remove contaminants and achieve fuel-grade quality meeting standards such as ASTM D6751. Renewable diesel, also known as hydrotreated vegetable oil (HVO) or green diesel, is a paraffinic hydrocarbon fuel synthesized by hydrotreating renewable feedstocks under high pressure and temperature with hydrogen, involving hydrodeoxygenation to remove oxygen, hydrocracking to break long-chain hydrocarbons, and isomerization for improved cold-flow properties. Feedstocks mirror those of biodiesel but extend to non-edible oils and waste materials, enabling broader sustainability without competing with food production. Unlike biodiesel, renewable diesel lacks oxygen, resulting in chemical properties nearly identical to petroleum diesel, including higher energy density (around 36-38 MJ/L versus 35-37 MJ/L for FAME biodiesel) and compatibility with existing pipelines and engines without modifications. Production facilities often co-process with petroleum, leveraging refinery infrastructure, with global capacity exceeding 1.4 billion gallons annually as of recent estimates. Other bio-based variants include biomass-to-liquid (BtL) diesel from gasification of lignocellulosic biomass followed by Fischer-Tropsch synthesis, which converts syngas into long-chain hydrocarbons, though commercial scale remains limited due to high costs and complexity. These renewable options reduce lifecycle greenhouse gas emissions by 50-90% compared to fossil diesel, depending on feedstock and process efficiency, but face challenges like feedstock availability and land-use impacts. Adoption has grown, with U.S. biodiesel production reaching over 2.1 billion gallons in 2022, while renewable diesel capacity expands rapidly to meet mandates like California's Low Carbon Fuel Standard.

Standards and Quality Specifications

Cetane number and ignition quality

The cetane number (CN) serves as a primary metric for assessing the ignition quality of diesel fuel, quantifying the duration of the ignition delay—the interval between fuel injection into the combustion chamber and the initiation of combustion under compression-ignition conditions. A higher CN indicates a shorter ignition delay, facilitating more rapid autoignition and promoting efficient combustion, while lower values prolong the delay, potentially leading to incomplete fuel-air mixing and suboptimal performance. This property is particularly critical in diesel engines, where fuel must self-ignite without a spark, and variations in CN directly influence combustion timing and stability. Measurement of CN follows ASTM D613, a standardized procedure utilizing a single-cylinder Cooperative Fuel Research (CFR) engine operated at fixed conditions: 900 rpm, 45.5 compression ratio, and intake air temperature of 52°C (126°F). The test compares the fuel's ignition delay—typically measured from the start of injection to 10% heat release—to reference blends of n-cetane (CN=100, representing ideal paraffinic ignition) and heptamethylnonane (CN=15, adjusted to represent poor ignition characteristics previously assigned to alpha-methylnaphthalene at CN=0). The CN scale spans 0 to 100, though practical diesel fuels fall between 30 and 85; engine calibration with primary reference fuels ensures reproducibility within ±1.5 units. Alternative methods, such as derived cetane number (DCN) via ignition quality tester (IQT) under ASTM D6890, offer faster, non-engine-based assessments correlating closely with ASTM D613 results but with distinct operational ranges. Commercial diesel fuels exhibit CN values mandated by regional standards to ensure reliable ignition; in the United States, ASTM D975 specifies a minimum of 40 for No. 2 diesel, with typical market values ranging from 42 to 45, though premium fuels or additives can exceed 50. European EN 590 standards require at least 51, reflecting demands for advanced engine technologies sensitive to ignition timing. Fuels with CN below 40 often result from high aromatic content, which resists autoignition due to stable molecular structures, whereas paraffinic hydrocarbons inherently yield higher CN through easier radical formation during compression heating. Ignition quality profoundly affects diesel engine operation: elevated CN shortens delay periods to under 10-15 milliseconds, enabling precise control of injection timing, reduced combustion noise, and minimized unburned hydrocarbons or particulate matter emissions via improved mixing. Conversely, low CN extends delays, causing pressure spikes, knocking, white smoke during cold starts, and torque fluctuations, exacerbating wear in high-speed engines and increasing NOx or CO outputs under mismatched conditions. Empirical studies confirm that raising CN by 5-10 units via additives like 2-ethylhexyl nitrate can enhance cold-start reliability by 20-30% in sub-zero temperatures and boost fuel economy by 1-2% through optimized combustion phasing, though benefits diminish in modern common-rail systems with electronic controls. Thus, CN optimization balances ignition promptness with overall fuel chemistry to sustain engine durability and efficiency.

Sulfur limits and ultra-low sulfur diesel

Sulfur in diesel fuel originates from organic compounds in crude oil and, upon combustion, forms sulfur oxides (SOx) that contribute to acid rain, respiratory issues, and sulfate particulate matter (PM), exacerbating air pollution. High sulfur levels also poison catalytic converters and diesel particulate filters (DPFs), hindering their effectiveness in reducing nitrogen oxides (NOx), hydrocarbons (HC), and PM emissions from engines. Regulatory limits on sulfur content have thus been imposed to enable advanced exhaust aftertreatment technologies and directly curb SOx and PM outputs, with empirical data showing sulfur reductions correlating to measurable declines in these pollutants. Ultra-low sulfur diesel (ULSD) refers to diesel fuel refined to contain no more than 15 parts per million (ppm) of sulfur by weight, a standard mandated in the United States for on-road applications since June 1, 2006, following a phase-in from prior limits of 500 ppm (effective 1993). This threshold extended to non-road, locomotive, and marine diesel by 2012, achieving a 99.7% drop in sulfur emissions from pre-1990s levels. In the European Union, sulfur caps progressed from 50 ppm under Euro IV standards (January 2005) to 10 ppm by 2009, aligning with stricter emission norms that require low-sulfur fuel to prevent catalyst deactivation. Globally, as of January 2024, over 115 countries enforce 50 ppm or lower limits for on-road diesel, though regions like parts of Africa and Asia retain higher thresholds up to 500 ppm or more, limiting deployment of modern emission controls. Achieving ULSD requires deep hydrodesulfurization during refining, which removes sulfur via hydrogen reaction under high pressure and temperature but also strips natural lubricity compounds, increasing wear on fuel pumps and injectors unless mitigated by additives. Studies confirm minimal impact on engine-out NOx, HC, or CO from sulfur variation alone, but higher sulfur (>50 ppm) elevates PM via sulfate formation and impairs DPF regeneration efficiency in Euro VI or equivalent engines. While ULSD production raises refining costs and slightly reduces fuel density (by about 1%), the net environmental gains include substantial SOx cuts and compatibility with selective catalytic reduction (SCR) systems, though long-term storage stability decreases due to heightened oxidation susceptibility.
Region/StandardSulfur Limit (ppm)Implementation DateKey Reference
US On-Road (EPA)≤15June 1, 2006
Non-Road≤152010-2012
(Euro V/VI)≤102009 onward
Global (Many Countries)≤50Phased to 2025

International and regional standards

Diesel fuel specifications lack a single international standard for automotive applications, with the (ISO) providing supporting frameworks through ISO/TC 28/SC 4 for fuel classifications and test methods, such as ISO 5165 for determination, rather than prescriptive requirements. Regional standards predominate, tailored to local engine technologies, climatic variations, and emission mandates, though global trends favor ultra-low sulfur levels (typically 10-15 ppm) to enable advanced aftertreatment systems like diesel particulate filters. These standards define critical properties including , , , and biodiesel blend limits to ensure fuel-engine compatibility and combustion efficiency. In the United States, the standard D975 (revised 2021) governs diesel fuel oils, specifying seven grades differentiated by sulfur content (S15 for ≤15 ppm highway diesel, S500 for off-road) and application, with a minimum of 40, kinematic of 1.9-4.1 mm²/s at 40°C, and allowance for up to 5% (B5) or higher blends under separate provisions. European specifications follow (2022 edition, developed by CEN), mandating ≤10 mg/kg , a minimum of 51, of 2.0-4.5 mm²/s, and up to 7% (FAME) content for automotive diesel, aligning with EU directives on emissions and winter grades for cold flow properties. Japan's JIS K 2204 (2023) outlines five diesel grades primarily varying by cold flow properties (e.g., Special No. 1 to No. 3), with sulfur limited to ≤10 ppm, minimum of 45-50 depending on grade, and 2.5-4.0 mm²/s, supporting domestic vehicle fleets and limited integration up to 5%. In , GB 19147 (China VI, effective nationwide by 2020) requires ≤10 mg/kg sulfur for automotive diesel, ≥49, and 2.0-4.0 mm²/s, while GB 252 covers general-purpose fuels; these standards reflect phased implementation to curb urban .
StandardRegionSulfur Max (ppm)Cetane Min (Number/Index)Viscosity (mm²/s at 40°C)Biodiesel Max
ASTM D975 (S15)1540 (number)1.9-4.15% (up to 20% optional)
1051 (number)2.0-4.57% FAME
JIS K 22041045-50 (index)2.5-4.05%
GB 191471049 (index)2.0-4.0Varies by region
Variations persist in parameters like polycyclic aromatic hydrocarbons and oxidation stability, reflecting differing priorities, but ongoing ISO harmonization efforts aim to reduce trade barriers for paraffinic and renewable diesel variants. For marine applications, ISO 8217 (2024) provides a more unified global benchmark, limiting sulfur to 0.50% m/m outside emission control areas and accommodating up to 100% biodiesel blends.

Additives and Storage Practices

Types of additives and their functions

Additives are chemical compounds blended into diesel fuel at treat rates typically under 1% by weight to address inherent limitations in base fuel properties, such as ignition quality, , and stability, without substantially changing characteristics like or . These enhancements support compliance with standards like ASTM D975 and optimize performance in high-pressure injection systems prevalent since the early 2000s. Cetane improvers, or ignition enhancers, elevate the —often from a base of 40-45 to 50 or higher—by decomposing to release free radicals that accelerate autoignition, thereby reducing ignition delay, engine knock, white smoke on startup, and particulate emissions while improving cold starts and fuel economy. Alkyl nitrates, such as 2-ethylhexyl at 0.05-0.3% concentrations, are widely used for this purpose, with benefits most evident in fuels from heavier crude fractions. Detergents, including polymeric amines and polyether amines, maintain cleanliness in fuel injectors, pumps, and chambers by solubilizing and dispersing carbonaceous deposits, preventing injector tip fouling that can increase emissions by up to 10% and reduce power output in common-rail systems operating above 1,500 bar since the . They are routinely added at refineries or terminals to counteract deposit precursors from incomplete or blends. Lubricity improvers, such as esters of fatty acids or amides, restore boundary lubrication lost in ultra-low diesel (below 15 ppm since U.S. implementation in 2006), forming adsorbed films that reduce and wear on high-speed pumps and injectors by 20-50% in high-frequency injection cycles. This is critical as compounds, naturally present in higher- fuels, previously provided . Cold flow improvers, comprising copolymers, alter crystallization in diesel (which clouds at 0-10°C and gels at -10 to -20°C), producing smaller, non-agglomerating crystals that pass through filters, extending operable temperatures by 5-10°C and enabling higher yields from paraffinic crudes. Optimal effectiveness requires addition at least 10°C above the to ensure homogeneous dispersion. Corrosion inhibitors, often amine-based or carboxylic salts, form monomolecular protective layers on and non-ferrous metals in storage tanks, pipelines, and engines, mitigating from or acidic oxidation products, with efficacy demonstrated in preventing pitting in systems exposed to 200 ppm . Antioxidants, such as hindered or amines at 10-50 ppm, interrupt radical reactions during storage, limiting formation and subsequent gum buildup that raises by up to 20% over 6-12 months, thereby preserving fuel injectability and efficiency. Biocides, including quaternary ammonium compounds or isothiazolinones, eradicate , fungi, and proliferating at fuel- interfaces (with growth rates doubling every 20 hours at 30°C), averting slime-induced filter clogs and microbially influenced that can occlude systems within weeks in contaminated storage. Other specialized additives include de-icers, which lower the freezing point of water emulsions to prevent ice blockages in cold climates, and anti-foam agents, such as polymers, which suppress formation during tank filling to improve handling efficiency by reducing spill risks and entrapment of air. Multifunctional packages combining several types are common in commercial formulations to streamline application across the from to end-user.

Storage stability and contamination prevention

Diesel fuel storage stability refers to the fuel's resistance to chemical and physical degradation over time, primarily through oxidation processes that form peroxides, gums, sediments, and acids, alongside microbial growth from water contamination and exposure to temperature extremes. The ASTM D5304 standard test method assesses this stability for middle distillate fuels like No. 2 diesel by subjecting samples to elevated temperature and oxygen pressure to accelerate aging and measure insoluble matter formation. Ultra-low diesel (ULSD), common since regulatory mandates in the mid-2000s, exhibits shorter stability due to reduced natural antioxidants from removal, with untreated ULSD typically maintaining usability for 6-12 months under ambient conditions around 20°C with minimal air exposure, though durations shorten further at higher temperatures above 70°F, with contamination, or extreme cold. Stability can extend to 2 years or more through the addition of antioxidants and metal deactivators, which interrupt radical formation and chelate pro-oxidant ions, respectively, combined with practices such as storing in cool, dark, sealed tanks, regular filtration, stock rotation, and fuel stabilizers. Key degrading factors include , metal catalysis from tank , and exposure to atmospheric oxygen, which initiate autoxidation chains producing varnishes and particulates. Regular fuel testing per ASTM guidelines, including visual inspections for clarity and particulate counts, ensures early detection of instability. Contamination prevention focuses on excluding , particulates, and microbes, as accumulation at tank bottoms fosters "diesel bug"—hydrocarbon-utilizing and fungi that degrade and clog filters. Diesel's hydrophilic nature draws condensation, exacerbated by temperature fluctuations; thus, tanks should feature vapor recovery vents, breathers, and sloped bottoms for drainage, with weekly checks recommended for long-term storage. Biocides, such as compounds dosed at manufacturer-specified rates (e.g., 1:400 for heavy contamination), kill microbial colonies but require compatibility testing to avoid issues. Particulate ingress from filling operations is mitigated by 10-micron at tank inlets and periodic tank cleaning to remove sediments, while cross-contamination with incompatible fuels like is avoided through dedicated . Opaque, above-ground or tanks stored in cool, shaded areas minimize UV and thermal degradation, with headspace reduction via full tanks or blanketing limiting oxygen ingress. Compliance with standards like ASTM D975's storage guidance, including annual quality assessments, underpins effective prevention strategies across industrial applications.

Applications and Uses

Transportation sectors

Diesel fuel serves as the source for heavy-duty road vehicles, locomotives in non-electrified rail networks, and marine vessels, leveraging its superior of approximately 35.8 MJ/L compared to gasoline's 32.2 MJ/L, which enables efficient operation under high demands. In the global transportation sector, diesel accounts for a substantial share of freight movement, with consumption projected to reach around 252.81 billion USD in 2025, driven largely by these applications. In road transportation, diesel powers the majority of commercial trucks and buses, which handle the bulk of short- to medium-haul freight. In the United States, trucks transported roughly 72.7% of the nation's freight by weight in 2024, with diesel comprising about 81% of use in trucks and buses, reflecting the prevalence of compression-ignition engines optimized for load-hauling . Globally, diesel's role in freight persists despite trends in light vehicles, as battery limitations constrain heavy-duty applications, maintaining diesel's dominance in sectors requiring long ranges and capacities exceeding 20 tons per . Rail freight relies heavily on diesel-electric locomotives, particularly in regions without extensive . Diesel traction accounted for 67% of the global market in 2024, powering systems that move approximately 40% of U.S. freight ton-miles where rail is the mode of choice for bulk commodities like and intermodal containers. In the U.S., diesel locomotives held an 81% share of rail freight traction in 2024, underscoring their reliability in vast networks spanning over 140,000 miles of track. Marine transportation employs (MDO) and similar variants for in cargo ships, tankers, and ferries, where low-speed, high-power engines benefit from diesel's and properties. Diesel and distillate fuels support about 90% of global seaborne trade volume, equivalent to over 11 billion tons annually as of 2023, with intermediate and MDO comprising key grades under IMO regulations limiting to 0.5% since January 2020. Diesel's adoption in marine sectors has increased with technologies and low-sulfur mandates, reducing reliance on heavier residuals while sustaining consumption at around 300 million metric tons yearly for distillates.

Industrial and stationary power

Diesel fuel powers stationary internal combustion engines in industrial applications, including , pumping, and compression systems, due to its high and compatibility with robust engine designs. These engines, often exceeding 600 horsepower, serve primary roles in sectors like oil and gas exploration, where reliability under continuous operation is critical. In power generation, sets (gensets) provide standby backup for facilities such as hospitals, data centers, and manufacturing plants during grid outages, with capacities ranging from 6 to 5,720 ekW. They also function as sources in remote or off-grid locations, including operations and sites, where diesel's long enables extended runtime without frequent refueling. Industrial diesel engines achieve thermal efficiencies of 35% to 45%, outperforming counterparts through higher compression ratios and complete fuel . Stationary diesel systems excel in cogeneration setups, combining electricity production with recovery for process heating in factories, enhancing overall utilization. For the same engine size, diesel gensets deliver approximately twice the kilowatts compared to equivalents, reducing consumption per unit of output. Modern advancements have pushed peak thermal efficiencies beyond 50%, as demonstrated by engines reaching 53.09% in controlled tests. These attributes make diesel preferable for high-load, intermittent operations like peak shaving in commercial buildings.

Military and specialized uses


Diesel fuel powers a wide array of military vehicles, including tanks, trucks, and generators, due to the high torque and fuel efficiency of diesel engines in heavy-duty applications. The U.S. military predominantly employs JP-8, a kerosene-based jet fuel with military additives, in diesel engines under the "single fuel forward" policy to simplify logistics across aircraft, vehicles, and equipment. This approach incurs a torque penalty of up to 5-10% and reduced fuel economy compared to conventional diesel due to JP-8's lower cetane number (around 42 versus 40-55 for diesel) and density, yet it enables unified supply chains in expeditionary operations. In naval applications, diesel fuel drives diesel-electric submarines in numerous navies, providing propulsion when surfaced or to recharge batteries, as these vessels rely on electric motors underwater. Non-nuclear diesel submarines, such as those in allies' fleets, store diesel in dedicated tanks, often compensated with ballast to maintain trim as fuel is consumed. These platforms offer cost advantages over nuclear submarines for littoral missions, with modern variants extending underwater endurance. Specialized military uses include diesel engines in select and unmanned systems, where high-efficiency compression-ignition designs like the historical DR-980 have been tested for USAF applications, offering better economy than gasoline counterparts. Contemporary efforts focus on heavy- diesel variants for UAVs, compatible with to support multi-domain operations. Diesel also fuels stationary generators for base power and systems, prized for reliability in remote or austere environments. In extreme conditions, military diesel formulations incorporate additives for cold-weather flow or anti-corrosion, ensuring operability in arctic or desert theaters.

Performance and Efficiency

Energy density and thermal efficiency

Diesel fuel exhibits a volumetric of approximately 38.3 MJ/, surpassing that of at 33.5 MJ/, due to its higher of around 0.83–0.85 kg/ compared to 's 0.72–0.78 kg/. This 15% greater content per unit volume enables longer ranges in diesel-powered vehicles under equivalent tank capacities. Gravimetrically, diesel provides 42–45 MJ/kg, marginally lower than 's 44–46 MJ/kg, as diesel's molecular structure incorporates more carbon and hydrogen bonds yielding sustained release. Thermal efficiency in diesel engines, defined as the ratio of mechanical work output to fuel's input, typically ranges from 30% to 40% in automotive applications, exceeding engines' 20–30% due to higher compression ratios (14:1 to 25:1) enabling more complete expansion of gases. Large-scale diesel engines, such as those in marine or power generation, achieve 43–50% efficiency through optimized , turbocharging, and reduced losses. Experimental advancements, including precise control of phasing, have demonstrated peaks of 53.09% in controlled prototypes as of 2024. Factors like operation and absence of throttling losses contribute causally to this superiority, converting a greater fraction of diesel's inherent into usable power while minimizing exhaust rejection.

Comparisons to gasoline and alternatives

Diesel fuel possesses a higher volumetric than , delivering approximately 36.9 MJ/L compared to 's 33.7 MJ/L, which equates to roughly 15% more per liter and enables greater range in comparable tank sizes. This advantage stems from diesel's denser composition, primarily longer-chain paraffins and aromatics, allowing diesel engines to extract more work per unit of . In terms of , diesel engines achieve 35–45% conversion of to mechanical work through higher compression ratios (typically 14:1 to 25:1) and compression-ignition cycles, outperforming spark-ignition engines at 30–40% efficiency under similar conditions. Overall, this results in diesel powertrains yielding 15–40% better economy on a per-mile basis, though actual figures vary with , load, and .
PropertyDiesel FuelGasoline
Energy Density (MJ/L)36.933.7
Thermal Efficiency (%)35–4530–40
Typical MPG AdvantageBaseline15–40% lower
Compared to biofuel alternatives like , diesel maintains parity in engine compatibility and efficiency when blended (e.g., B5–B20), but pure biodiesel (B100) exhibits 8–10% lower due to its oxygenated structure, potentially reducing power output and mileage without engine recalibration. Renewable diesel, a variant, closely matches petrodiesel's energy content and (45–55), preserving efficiency while offering without infrastructure changes. Gaseous alternatives such as (CNG) suffer from lower volumetric (about 9–10 MJ/L at standard pressures) and require spark-ignition Otto-cycle engines, which cap efficiency at 25–35%—inferior to diesel's compression-ignition process—leading to 20–30% higher fuel consumption for equivalent work in heavy-duty applications. (LNG) improves density but still demands specialized storage and yields lower lifecycle efficiency in compression-ignition adaptations compared to straight diesel. These alternatives often trade diesel's density and longevity for reduced emissions in lighter-duty scenarios, but diesel prevails in high-load efficiency due to its inherent stability and fuel properties.

Operational advantages in heavy-duty contexts

Diesel engines fueled by diesel exhibit superior characteristics compared to counterparts, delivering peak at lower engine speeds, which facilitates efficient and sustained power under heavy loads in applications like long-haul trucking and construction machinery. This low-RPM advantage stems from the higher compression ratios in diesel cycles, typically 14:1 to 25:1, enabling robust performance for towing capacities exceeding 20,000 pounds without excessive revving. In heavy-duty operations, diesel's higher —approximately 36.9 MJ/L versus 33.7 MJ/L for —translates to extended operational ranges and reduced refueling frequency, critical for fleet vehicles and off-road equipment operating in remote areas. Brake thermal efficiencies in heavy-duty diesel engines can reach 39.1% over cycles, outperforming engines by converting a greater proportion of into mechanical work, thereby lowering per-mile or per-hour consumption by up to 30% in comparable heavy vehicles. For instance, in medium- and heavy-duty truck evaluations, diesel powertrains demonstrate lower use under high-demand conditions, with variants consuming 13.5% more in direct comparisons. The robustness of diesel-fueled systems supports prolonged service intervals and higher load factors in demanding environments, such as haul trucks and industrial generators, where engines routinely achieve 500,000 to 1,000,000 miles of operation before major overhaul, attributed to the fuel's and the engine's compression-ignition design that minimizes wear under sustained high-torque output. This durability reduces and costs in sectors reliant on continuous heavy-duty , contrasting with engines' shorter lifespan under similar stresses.

Economic Dimensions

Pricing and market dynamics

The retail price of diesel fuel is predominantly influenced by the cost of crude oil, which constitutes the largest share of production expenses, as diesel is derived from crude . Refining costs also play a role, with diesel production involving processes that can be more energy-intensive than in certain configurations, contributing to price premiums in regions with high distillate demand. Additional factors include transportation logistics, local market competition, and regional variations in storage and distribution overheads, which can cause price disparities even within the same country. In the United States, diesel fuel typically commands a higher retail price than due to a federal excise tax of 24.4 cents per on diesel compared to 18.4 cents per on , alongside sustained demand from heavy-duty trucking and industrial sectors that outpaces 's consumer vehicle focus. Globally, diesel prices exhibit wide variation driven by national policies; for instance, subsidies in oil-producing nations like keep prices low, while high taxes in elevate them, with 2024 averages ranging from under 0.50 USD/liter in the to over 1.50 USD/liter in parts of . International distillate demand, particularly from Asia's shipping and manufacturing growth, exerts upward pressure on U.S. export prices, linking domestic markets to global supply chains. Market dynamics for diesel are characterized by sensitivity to geopolitical events and seasonal patterns, such as winter demand that competes with diesel stocks, often leading to price spikes in colder months. Supply disruptions, including outages or sanctions on major producers like , have historically amplified volatility; for example, post-2022 Ukraine conflict, global diesel premiums surged due to reduced Russian exports. In 2024 and 2025, U.S. retail diesel prices averaged around 3.70 USD per , reflecting ample global inventories and increased non-OPEC production that offset demand growth from transportation recovery. The U.S. forecasts a modest decline to approximately 3.50 USD per in 2025, attributed to rising inventories and stable consumption despite economic expansion in diesel-intensive sectors like freight. Long-term, trends in light vehicles may ease competition but sustain diesel's role in heavy-duty applications, potentially stabilizing prices amid blending mandates that add marginal costs.

Taxation, subsidies, and trade

Diesel fuel is subject to taxes in most jurisdictions, primarily to fund transportation infrastructure and generate revenue, with rates often differentiated from to account for diesel's predominant use in freight and heavy , which impose greater wear. , the federal on diesel stands at 24.4 cents per as of 2025, higher than the 18.4 cents per on , reflecting the rationale that trucks cause more pavement per mile traveled. State-level taxes add variability, with California's combined state and sales taxes pushing total diesel levies above 80 cents per in some cases. , the average duty on diesel was €0.445 per liter in 2024, below the €0.548 per liter for , as member states apply lower rates to support commercial transport; the EU minimum is €0.330 per liter, adhered to only by , while countries like impose rates exceeding €0.70 per liter. Subsidies for diesel fuel persist in various forms globally, particularly in developing economies to mitigate costs for agriculture, transport, and industry, though reforms have accelerated amid fiscal pressures and energy price volatility. Explicit consumer subsidies on diesel, part of broader fossil fuel support, totaled around $620 billion worldwide in 2023 according to the International Energy Agency, down from peaks during the 2022 energy crisis but still significant in nations like Indonesia and Nigeria where underpricing keeps retail prices below market levels. In India, diesel subsidies were partially phased out by 2014 but reemerged implicitly through price controls; as of mid-2024, partial subsidies remained for select users following a 50% price hike in June. The International Monetary Fund estimates total fossil fuel subsidies, including diesel, reached $7 trillion in 2022 when factoring in unpriced externalities like environmental damage, though direct budgetary transfers and tax exemptions dominate explicit diesel support in policy debates. Reduced-tax "red diesel" for off-road agricultural and heating uses exemplifies targeted subsidies in the UK and elsewhere, exempting it from full road fuel duties to lower farming input costs. International trade in diesel fuel, a key refined , involves substantial volumes driven by refining imbalances, with net exporters including the , , and Middle Eastern producers supplying importers in , , and . Global diesel exports grew in developing economies by over tenfold in 2024 compared to advanced ones, fueled by trade upturns and refinery expansions, though exact volumes fluctuate with crude prices and demand. Geopolitical events reshaped flows; Europe's diesel imports from plummeted from 29.9 million metric tons in 2022 to 2.9 million in 2024 following the EU embargo, redirecting supplies from the US Gulf Coast and , which emerged as a top exporter with firms like shipping billions in value. U.S. diesel exports to , a major regional market, declined 13% in value to $41 billion in 2024 amid lower prices, highlighting trade sensitivity to regional dynamics. Tariffs and sanctions further influence patterns, as seen in post-2022 restrictions elevating Asian and American sourcing for .

Environmental and Emissions Profile

Combustion products and lifecycle CO2

The primary products of diesel fuel combustion are carbon dioxide (CO₂) and water vapor (H₂O), resulting from the oxidation of its hydrocarbon constituents, typically approximated as CₙH₁.₈ₙ or similar formulas. In diesel engines, which operate under lean-burn conditions with excess air, the exhaust also contains nitrogen (N₂) from intake air and residual oxygen, alongside trace amounts of nitrogen oxides (NOₓ) formed at high temperatures, carbon monoxide (CO) from incomplete combustion, unburned hydrocarbons, and particulate matter consisting of soot and associated organics. Sulfur oxides (SOₓ) are minimal in ultra-low sulfur diesel (ULSD), limited to below 15 ppm sulfur content in many jurisdictions. The CO₂ yield from complete combustion dominates greenhouse gas outputs, calculated stoichiometrically from diesel's density (approximately 0.84 kg/L) and carbon content (about 86% by mass), producing roughly 2.68 kg CO₂ per liter; empirical factors confirm around 2.7 kg CO₂ per liter consumed. Lifecycle CO₂ emissions for diesel fuel encompass well-to-tank (WTT) stages—including crude oil extraction (0.5-1.0 kg CO₂ eq per barrel equivalent), (energy-intensive hydrotreating and adding 0.2-0.4 kg CO₂ eq per liter), and distribution—plus tank-to-wheel (TTW) . WTT contributions typically range 10-20 g CO₂ eq per MJ of fuel energy, representing 15-25% of total emissions depending on crude source and efficiency; for Middle Eastern conventional crudes, this is lower than for heavier oils requiring more processing. Combining with TTW values, total well-to-wheel (WTW) emissions for conventional diesel average 85-95 g CO₂ eq per MJ. Given diesel's lower heating value of 35-36 MJ/L, this translates to 3.0-3.4 kg CO₂ eq per liter overall. These figures derive from models like the U.S. EPA's Renewable Standard baselines (around 94 g CO₂ eq/MJ for petroleum diesel) and EU Joint Research Centre well-to-wheels analyses, which account for regional variations but prioritize empirical assay data over speculative land-use impacts absent in fossil pathways. Unlike biofuels, conventional diesel's lifecycle lacks biogenic carbon credits, making its WTW profile straightforwardly tied to fossil carbon release, with dominating due to the fuel's high carbon intensity relative to content.

Particulate matter and NOx characteristics

Diesel engines, operating on compression ignition, produce particulate matter (PM) primarily through incomplete , yielding agglomerates of (elemental carbon cores) coated with adsorbed organic compounds, sulfates, and metals from fuel and lubricants. This PM exhibits a multimodal size distribution, with significant fractions in the fine (PM2.5) and ultrafine (<0.1 μm) ranges, enabling deep lung penetration and higher particle number concentrations compared to engines. Pre-control technology diesel vehicles emit PM at rates approximately 20 times higher than equivalent counterparts, driven by the heterogeneous air-fuel mixing and characteristics inherent to diesel . Nitrogen oxides (NOx), comprising mainly nitric oxide (NO, ~90-95%) and nitrogen dioxide (NO2), form in diesel engines via the Zeldovich mechanism under high-temperature, oxygen-rich conditions exceeding 1,800 K during the latter combustion phases. The lean-burn operation and elevated compression ratios (typically 14:1 to 25:1) in diesels elevate peak flame temperatures, resulting in NOx yields 2-5 times higher per unit energy output than in stoichiometric gasoline engines, though total inventory shares vary by fleet composition. NO2 fractions remain low (5-10%) absent aftertreatment, but increase under certain loads due to oxidation in the exhaust. Empirical inventories attribute a disproportionate share of mobile-source NOx and PM to diesels, reflecting their prevalence in heavy-duty applications despite lower overall vehicle numbers.

Mitigation via technology and fuels

Diesel particulate filters (DPFs) capture and particulate matter (PM) from exhaust gases through wall-flow filtration, achieving reduction efficiencies exceeding 90% in modern systems. These filters require periodic regeneration, typically via active or passive oxidation of trapped at temperatures above 550°C, to prevent backpressure buildup that could impair engine performance. (SCR) systems inject aqueous solution (, DEF) upstream of a vanadium- or zeolite-based catalyst, converting to and with efficiencies of 80-95% under optimal conditions. (EGR) dilutes intake air with cooled exhaust to lower combustion temperatures, reducing formation by 20-50%, often integrated with SCR and DPF for synergistic effects in heavy-duty engines meeting Euro VI or EPA 2010 standards. Diesel oxidation catalysts (DOCs) upstream of DPFs oxidize hydrocarbons (HC) and (CO) to CO2 and H2O, with conversion rates over 90% at temperatures above 200°C, while also aiding passive DPF regeneration. Combined systems, such as SCR-coated DPFs, integrate reduction and PM filtration into single units, minimizing space and improving cold-start performance, though they demand precise control to balance soot loading and urea dosing. These aftertreatment technologies rely on ultra-low diesel (ULSD, ≤15 ppm ), which prevents and PM formation, enabling PM reductions up to 90% and supporting ancillary cuts when paired with advanced controls. Biodiesel blends, derived from vegetable oils or animal fats, reduce PM emissions by 10-30% and CO by 10-20% relative to petroleum diesel due to higher oxygen content promoting complete combustion, as observed in B20 blends across various engines. However, these blends typically elevate NOx emissions by 2-10%, attributable to altered injection dynamics, higher flame temperatures, or oxygen effects, with pure B100 showing average increases of 10% in controlled tests. Hydrotreated vegetable oil (HVO), a renewable diesel paraffinic fuel, mitigates this by reducing NOx compared to fossil diesel while lowering PM without the stability issues of traditional biodiesel. Fuel additives, such as cerium-based compounds, enhance DPF regeneration and cut PM by 20-50% in older engines, though their efficacy diminishes in ULSD-equipped modern systems.

Safety, Health, and Hazard Management

Flammability and handling risks

Diesel fuel has a ranging from 52°C to 96°C (126°F to 205°F), with #2 diesel specified by ASTM D975 requiring a minimum of 52°C (125°F), classifying it as a combustible liquid under definitions where s exceed 37.8°C (100°F). In comparison, exhibits a of approximately -43°C (-45°F), rendering it highly flammable at ambient temperatures. This property results in diesel vapors forming ignitable mixtures only when heated, reducing spontaneous ignition risks; experiments demonstrate that a lit dropped into a diesel pool extinguishes without , unlike . Despite lower volatility, diesel's of 210°C (410°F) allows ignition under compression or exposure to sustained heat, and once burning, it produces intense, persistent flames difficult to extinguish due to its lower sustaining slower but hotter . Atomized diesel , as generated in fuel systems or high-pressure spills, ignites more readily than bulk liquid, posing elevated fire risks in mechanical failures or accidents. Handling entails static electricity hazards during pumping or transfer, where charge buildup can spark vapors above the flash point, mitigated by mandatory grounding of equipment and conductive hoses per NOAA guidelines. Spills create slip risks and propagation potential if vapors accumulate in enclosed areas and contact ignition sources, with run-off exacerbating environmental spread during . When working on diesel fuel systems, precautions include performing tasks in well-ventilated areas to disperse potential vapors, avoiding ignition sources such as sparks or smoking, using absorbent materials like rags for spill cleanup to prevent slips and fire spread, and keeping a Class B fire extinguisher nearby, given diesel's flammability risks especially with mists or heated fuel. Regulatory frameworks address these via OSHA 29 CFR 1926.152, limiting indoor storage to 25 gallons outside approved cabinets and requiring separation from ignition sources, alongside NFPA 30 protocols for diking to contain spills and bulk tanks positioned at least 50 feet from structures. Approved containers and ventilation prevent vapor buildup, while prohibiting or open flames near operations further minimizes incidents.

Human health effects from exposure

Exposure to diesel fuel liquid can cause acute dermal , including redness, pain, and , particularly with prolonged or repeated contact; large exposures may lead to chemical burns or blisters. of diesel fuel vapors at low levels typically produces of the eyes, nose, throat, and , along with symptoms such as , , lightheadedness, and increased during short-term exposure. Ingestion of diesel fuel risks gastrointestinal , , vomiting, and severe complications from aspiration into the lungs, potentially causing , central nervous system depression, or long-term neurological effects like agitation and . Chronic inhalation of diesel fuel vapors has been associated with kidney damage and impaired blood clotting in occupational settings, though human data remain limited compared to animal studies showing lethality at high concentrations. The predominant health risks from diesel fuel arise from exposure to its combustion products in exhaust, which contains fine particulate matter, nitrogen oxides, polycyclic aromatic hydrocarbons, and other toxins. Acute exhaust exposure exacerbates these irritant effects and can trigger asthma attacks or worsen cardiovascular conditions in vulnerable populations. Long-term exposure to diesel exhaust is causally linked to increased incidence of , with the International Agency for Research on Cancer classifying it as carcinogenic to humans () based on sufficient evidence from epidemiological studies of workers and others showing elevated risk, particularly from particulate-bound carcinogens. also contributes to non-cancer respiratory diseases, including and , as well as cardiovascular events like heart attacks, through mechanisms involving , , and ; these effects are supported by controlled human exposure studies and cohort analyses demonstrating dose-response relationships. Vulnerable groups, such as children and those with pre-existing conditions, face heightened risks of premature mortality and emergency hospitalizations from cumulative exposure.

Microbial growth and water contamination

Water contamination in diesel fuel storage systems primarily arises from atmospheric condensation inside partially filled tanks, particularly during temperature fluctuations, or from ingress via faulty seals, vents, or delivery equipment. This free or emulsified settles at the fuel-tank interface due to its higher , creating an aqueous phase conducive to microbial proliferation. Diesel fuel itself provides hydrocarbons as a carbon source, while the supplies necessary hydration, enabling aerobic and anaerobic (e.g., spp.), fungi (e.g., Hormoconis resinae), and yeasts to metabolize and reproduce, often at optimal temperatures between 10°C and 40°C. Microbial growth manifests as biomass accumulation—slime, sludge, or biofilms—at the fuel-water boundary, tank walls, and filters, leading to fuel degradation through and acid production. These byproducts cause in storage tanks, piping, and engine components like injectors; filter plugging that restricts flow and induces ; and incomplete resulting in power loss or engine stalls. In biodiesel blends (e.g., B20), heightened hygroscopicity exacerbates water retention, accelerating contamination rates compared to pure petroleum diesel. Undetected proliferation can degrade fuel quality over months, with biomass densities exceeding 10^6 cells/mL triggering operational failures in generators, fleets, or marine systems. Detection involves visual inspection for tank-bottom , fuel , or foul odors; laboratory culturing or (ATP) assays quantify viable microbes, with thresholds above 10^4 CFU/mL indicating risk. Prevention prioritizes exclusion: regular draining to remove settled phases, installation of -separating filters during transfer, and maintaining near full capacity to limit headspace . Biocides, such as quaternary ammonium compounds dosed at 200-400 ppm, target planktonic and sessile microbes but require compatibility testing to avoid destabilization; post-treatment removes dead to prevent secondary clogging. Periodic cleaning and fuel polishing circuits further mitigate recurrence, especially in long-term storage exceeding 6 months.

Controversies and Regulatory Debates

Emissions testing scandals and fraud

In September 2015, the United States Environmental Protection Agency issued a notice of violation to Volkswagen Group, revealing that the company had installed software-based "defeat devices" in approximately 482,000 diesel vehicles sold in the U.S. from model years 2009 to 2015, enabling the engines to detect when they were undergoing laboratory emissions testing. These devices recognized test conditions—such as specific patterns of steering wheel movement, acceleration, and steady speeds—and temporarily activated full emissions controls, including selective catalytic reduction (SCR) systems for nitrogen oxides (NOx), to meet regulatory limits like the U.S. Tier 2 Bin 5 standard of 0.043 grams per mile NOx. In real-world driving, the software disabled or reduced these controls, resulting in NOx emissions up to 40 times higher than permitted, with affected 2.0-liter engines emitting around 1.5-1.6 grams per mile. The scandal, dubbed "Dieselgate," extended globally to about 11 million vehicles across Volkswagen, Audi, and Porsche brands, prompting the resignation of CEO Martin Winterkorn and criminal charges against executives, including Winterkorn for fraud and conspiracy in the U.S. and Germany. The defeat devices exploited discrepancies between controlled test cycles (e.g., the EPA's Federal Test Procedure or Europe's New European Driving Cycle) and actual on-road operation, where variables like temperature, speed, and load caused emissions control systems to underperform without software intervention. For instance, Volkswagen's programming would switch off SCR urea injection above certain speeds or after prolonged operation to prioritize fuel efficiency and performance, a tactic later identified in post-scandal testing as "cycle-beating" or thermal window manipulation, where systems deactivate below 15-20°C or during non-test acceleration. Bosch, the supplier of the engine control units (ECUs) like the EDC17 used in these vehicles, faced accusations of complicity in developing the software, leading to settlements and investigations. Volkswagen's actions violated the Clean Air Act and constituted wire fraud, as confirmed by guilty pleas from engineers like Oliver Schmidt, who admitted to concealing the cheating from regulators. Consequences included a U.S. settlement of up to $14.7 billion in June 2016 for buybacks, owner compensation, , and civil penalties, with total global costs exceeding $30 billion by 2022, including fixes for affected engines via hardware retrofits or software updates that reduced power output by up to 30%. The eroded trust in diesel processes, highlighting how lab-test unrealism—failing to capture transient real-world dynamics—created incentives for , though the deliberate remained unlawful. Beyond Volkswagen, similar fraud emerged at other manufacturers. (FCA, now ) equipped Ram 1500 and models with 3.0-liter engines (2014-2020) using defeat devices that manipulated and dosing during tests, leading to a $2 billion settlement in 2023 with the EPA, DOJ, and for excess emissions. faced U.S. DOJ charges in 2020 for software in diesels (2009-2016) that selectively activated emissions controls, resulting in a $1.2 billion and recall of over 250,000 vehicles. In , ongoing lawsuits as of October 2025 accuse , Mercedes, and others of illegal defeat strategies, such as speed- or temperature-based deactivation of AdBlue systems, contributing to widespread exceedances documented in independent real-driving emissions tests. These cases underscore systemic use of software tampering across the industry to bridge the gap between stringent lab standards and practical diesel physics, where formation rises with high temperatures and loads unavoidable in efficient engine operation.

Policy-driven phase-outs and bans

In the , regulations adopted by the in 2022 require new passenger cars and vans to achieve zero tailpipe CO2 emissions starting in 2035, prohibiting sales of new diesel-powered vehicles except those using carbon-neutral e-fuels, which remain technologically constrained and costly to produce at scale. This policy, part of broader net-zero ambitions, targets diesel's contribution to and PM emissions, which form and , though post-2010 engines with diesel particulate filters and systems reduce these pollutants by over 90% compared to pre-2000 models. As of October 2025, and have called for an expedited review of the timeline, citing supply chain vulnerabilities and gaps. The legislated a ban on new petrol and diesel and sales from 2030, advancing to full zero-emission mandates for all new vehicles by 2035, with exemptions for certain hybrids until the later date. This followed a announcement shifting from a 2040 target, driven by air quality directives linking diesel particulates to respiratory illnesses, yet diesel's thermodynamic —yielding 20-50% better than —results in 15-25% lower CO2 emissions per kilometer on a tank-to-wheel basis. In the United States, 's 2022 Advanced Clean Cars II regulation phases out new light-duty diesel and vehicle sales by 2035, adopted by 17 other states representing about 40% of the national vehicle market. For heavy-duty trucks, where diesel dominates due to superior and range, California proposed ending new fossil-fuel sales in 2036 but withdrew the rule in January 2025 amid lawsuits over $15-20 billion in estimated compliance costs for fleets and insufficient zero-emission alternatives for long-haul operations. These phase-outs prioritize despite diesel's lower lifecycle CO2 footprint in applications like trucking, where battery weight reduces by up to 30%. Economic assessments of similar low-emission zones reveal net social costs from forgone mobility and higher logistics expenses, often exceeding localized air quality gains after accounting for modern emission controls. Industry analyses project EU-wide vehicle production disruptions and job losses exceeding 100,000 if mandates ignore diesel's role in decarbonizing freight, where electric trucks currently achieve only 60-70% of diesel's range without refueling infrastructure. Proponents cite modeled health savings from reduced PM2.5 exposure, but empirical data from U.S. diesel retrofits show technology-driven emission drops of 75-80% without outright bans, questioning the necessity of fuel-specific prohibitions.

Empirical trade-offs versus alarmist narratives

Diesel engines demonstrate superior compared to engines, with practical efficiencies of 35-45% versus 30-40%, attributable to higher compression ratios (typically 14:1 to 25:1) and the greater of diesel (approximately 35-38 MJ/L versus 32-35 MJ/L for ). This efficiency translates to 20-50% improved fuel economy in equivalent vehicles, enabling diesel-powered automobiles to achieve up to 24% lower utilization-phase CO2 emissions per kilometer than gasoline counterparts in life-cycle analyses that account for well-to-wheel pathways. In heavy-duty applications, such as trucking, these gains compound, reducing total lifecycle by prioritizing energy conversion over raw fuel properties, despite diesel's marginally higher carbon content per unit mass. A primary empirical trade-off arises from diesel's compression-ignition process, which generates elevated and PM emissions relative to spark-ignition engines under equivalent loads, due to higher combustion temperatures and soot formation in heterogeneous mixtures. can exceed levels by factors of 5-10 without aftertreatment, while PM (including ) contributes to local air quality challenges. However, advanced mitigation technologies—such as (EGR), (SCR) with injection, and diesel particulate filters (DPF)—have achieved reductions exceeding 95% and near-total PM capture in compliant Euro 6 and EPA 2010+ engines, shifting the toward feasible integration without sacrificing efficiency. Real-world data from instrumented fleets confirm these systems maintain 90-98% effectiveness over 500,000+ km, countering claims of inevitable degradation. Alarmist narratives, often rooted in pre-2010 emission scandal data like Volkswagen's defeat devices, portray diesel as categorically hazardous, linking excess to 38,000 annual premature deaths globally based on non-compliant , while extrapolating risks to compliant fleets despite verifiable mitigation. Such assessments, from advocacy groups emphasizing health endpoints over full-system trade-offs, frequently omit diesel's role in minimizing demand—critical for decarbonization pathways reliant on finite biofuels or synthetic fuels—and amplify localized PM/ concerns over net CO2 savings, which equate to millions of tons avoided annually in efficient sectors. Empirical lifecycle modeling reveals these omissions distort , as diesel's 15-40% efficiency edge sustains lower throughput even amid aftertreatment costs (1-3% fuel penalty). Regulatory responses, including urban bans, thus prioritize unnuanced pollutant proxies over causal chains linking fuel choice to total emissions, where diesel optimizes for scale in freight (70%+ of road CO2 from heavies) without comparable feasibility.

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