Diesel fuel

A modern diesel dispenser

Petroleum diesel, also called petrodiesel,[7] or fossil diesel is the most common type of diesel fuel. It is produced from the fractional distillation of crude oil between 200 °C (392 °F) and 350 °C (662 °F) at atmospheric pressure, resulting in a mixture of carbon chains that typically contain between 9 and 25 carbon atoms per molecule.[8]

Synthetic diesel can be produced from any carbonaceous material, including biomass, biogas, natural gas, coal and many others. The raw material is gasified into synthesis gas, which after purification is converted by the Fischer–Tropsch process to a synthetic diesel.[9]

The process is typically referred to as biomass-to-liquid (BTL), gas-to-liquid (GTL) or coal-to-liquid (CTL), depending on the raw material used.

Paraffinic synthetic diesel generally has a near-zero content of sulfur and very low aromatics content, reducing unregulated emissions of toxic hydrocarbons, nitrous oxides and particulate matter (PM).[10]

Biodiesel made from soybean oil

Biodiesel is obtained from vegetable oil or animal fats (biolipids) which are mainly Fatty acid methyl esters (FAME), and transesterified with methanol. 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.[11][12]

FAME used as fuel is specified in DIN EN 14214[13] and ASTM D6751 standards.[14]

Fuel equipment manufacturers (FIE) have raised several concerns regarding biodiesel, identifying FAME as being the cause of the following problems: corrosion of fuel injection components, low-pressure fuel system blockage, increased dilution and polymerization of engine sump oil, pump seizures due to high fuel viscosity at low temperature, increased injection pressure, elastomeric seal failures and fuel injector spray blockage.[15] Pure biodiesel has an energy content about 5–10% lower than petroleum diesel.[16] The loss in power when using pure biodiesel is 5–7%.[12]

Unsaturated fatty acids are the source for the lower oxidation stability; they react with oxygen and form peroxides and result in degradation byproducts, which can cause sludge and lacquer in the fuel system.[17]

As biodiesel contains low levels of sulfur, the emissions of sulfur oxides and sulfates, major components of acid rain, are low. Use of biodiesel also results in reductions of unburned hydrocarbons, carbon monoxide (CO), and particulate matter. CO emissions using biodiesel are substantially reduced, on the order of 50% compared to most petrodiesel fuels. The exhaust emissions of particulate matter from biodiesel have been found to be 30% lower than overall particulate matter emissions from petrodiesel. The exhaust emissions of total hydrocarbons (a contributing factor in the localized formation of smog and ozone) are up to 93% lower for biodiesel than diesel fuel.

Biodiesel also may reduce health risks associated with petroleum diesel. Biodiesel emissions showed decreased levels of polycyclic aromatic hydrocarbon (PAH) and nitrited PAH compounds, which have been identified as potential cancer-causing compounds. In recent testing, PAH compounds were reduced by 75–85%, except for benz(a)anthracene, which was reduced by roughly 50%. Targeted nPAH compounds were also reduced dramatically with biodiesel fuel, with 2-nitrofluorene and 1-nitropyrene reduced by 90%, and the rest of the nPAH compounds reduced to only trace levels.[18]

This category of diesel fuels involves converting the triglycerides in vegetable oil and animal fats into alkanes by refining and hydrogenation, such as H-Bio. The produced fuel has many properties that are similar to synthetic diesel, and are free from the many disadvantages of FAME.

Dimethyl ether, DME, is a synthetic, gaseous diesel fuel that results in clean combustion with very little soot and reduced NOx emissions.[11]

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.[21] A higher cetane number indicates that the fuel ignites more readily when sprayed into hot compressed air.[21] 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.

As of 2010, the density of petroleum diesel is about 0.832 kg/L (6.943 lb/US gal), about 11.6% more than ethanol-free petrol (gasoline), which has a density of about 0.745 kg/L (6.217 lb/US gal). About 86.1% of the 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. However, due to the higher density, diesel offers a higher volumetric energy density at 35.86 MJ/L (128,700 BTU/US gal) vs. 32.18 MJ/L (115,500 BTU/US gal) for gasoline, some 11% higher, which should be considered when comparing the fuel efficiency by volume. The CO₂ emissions from diesel are 73.25 g/MJ, just slightly lower than for gasoline at 73.38 g/MJ.[22] Diesel is generally simpler to refine from petroleum than gasoline, and contains hydrocarbons having a boiling point in the range of 180–360 °C (360–680°F). The price of diesel traditionally rises during colder months as demand for heating oil rises, which is refined in much the same way. Because of recent changes in fuel quality regulations, 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,[23] diesel may be priced higher than petrol.[24] 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.[25] In Sweden, a diesel fuel designated as MK-1 (class 1 environmental diesel) is also being sold; this is a ULSD that also has a lower aromatics content, with a limit of 5%.[26] This fuel is slightly more expensive to produce than regular ULSD.

Diesel fuel is very 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,[27] and using this untaxed diesel fuel for a typically taxed purpose (such as driving use), the user 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.[28] 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.[29]

Fairbanks-Morse opposed piston diesel engines on a submarine

Diesel fuel is widely used in most types of transportation. Trucks and buses, which were often gasoline-powered in the 1920s through 1950s, are now almost exclusively diesel-powered. The gasoline-powered passenger automobile is the major exception; diesel cars are less numerous worldwide.

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).[34][35]

Packard DR-980 9-cylinder diesel aircraft engine

The first diesel-powered flight of a fixed-wing aircraft took place on the evening of 18 September 1928, at the Packard Proving Grounds near Utica, Michigan. With Captain Lionel M. Woolson and Walter Lees at the controls the first "official" test flight was taken the next morning, flying a Stinson SM-1B (X7654), powered by a Packard DR-980 9-cylinder diesel radial engine, designed by Woolson. Charles Lindbergh flew the same aircraft and in 1929, it was flown 621 miles (999 km) nonstop from Detroit to Langley Field, near Norfolk, Virginia. In 1931, Walter Lees and Fredrick Brossy set the nonstop flight record flying a Bellanca powered by a Packard diesel for 84 hours and 32 minutes. X7654 is now owned by Greg Herrick and is at the Golden Wings Flying Museum near Minneapolis, Minnesota.

Diesel engines for airships were developed in both Germany and the United Kingdom by Daimler-Benz and Beardmore produced the Daimler-Benz DB 602 and Beardmore Typhoon respectively. The LZ 129 Hindenburg rigid airship was powered by four Daimler-Benz DB 602 16-cylinder diesel engines, each with 1,200 hp (890 kW) available in bursts and 850 horsepower (630 kW) available for cruising.[36] The Beardmore Typhoon powered the ill-fated R101 airship, built for the Empire airship programme in 1931.

With a production run of at least 900 engines, the most-produced aviation diesel engine in history was probably the Junkers Jumo 205. Similar developments from the Junkers Motorenwerke and licence-built versions of the Jumo 204 and Jumo 205, boosted German diesel aero-engine production to at least 1000 examples, the vast majority of which were liquid-cooled, opposed-piston, two-stroke engines.

In the Soviet Union significant progress towards practical diesel aero-engines was made by the TsIAM (Tsentral'nyy Institut Aviatsionnovo Motorostroyeniya – central institute of aviation motors) and particularly by A.D. Charomskiy, who nursed the Charomskiy ACh-30 into production and limited operational use.[37]

Armored fighting vehicles use diesel because of its lower flammability risks and the engines' higher provision of torque and lower likelihood of stalling.[38][39]

Diesel-powered cars generally have a better fuel economy than equivalent gasoline engines and produce less greenhouse gas emission.[40] Their greater economy is due to the higher energy per-litre content of diesel fuel and the intrinsic efficiency of the diesel engine.[41] While petrodiesel's higher density results in higher greenhouse gas emissions per litre compared to gasoline,[42] the 20–40% better fuel economy achieved by modern diesel-engined automobiles offsets the higher per-litre emissions of greenhouse gases, and a diesel-powered vehicle emits 10–20 percent less greenhouse gas than comparable gasoline vehicles.[43][44][45] Biodiesel-powered diesel engines offer substantially improved emission reductions compared to petrodiesel or gasoline-powered engines, while retaining most of the fuel economy advantages over conventional gasoline-powered automobiles.[43] However, the increased compression ratios mean there are increased emissions of oxides of nitrogen (NO_x) from diesel engines.[46] This is compounded by biological nitrogen in biodiesel to make NO_x emissions the main drawback of diesel versus gasoline engines.[46]

Today's tractors and heavy equipment are mostly diesel-powered. Among tractors, only the smaller classes may also offer gasoline 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 is commonly used in oil and gas extracting equipment, though some places use electric or natural gas powered equipment to help reduce environmental effects such as pollution from exhaust gases and spills.

Tractors and heavy equipment were often multifuel in the 1920s through 1940s, running spark-ignition and low-compression engines. Thus many farm tractors of the era could burn gasoline, alcohol, kerosene, and any light grade of fuel oil such as diesel fuel, heating oil, or tractor vaporising oil, according to whichever was most affordable in any region at any given time. On U.S. farms during this era, the name "distillate" often referred to any of the aforementioned light fuel oils. The 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 switch 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.

Poor quality diesel fuel has been used as an extraction agent for liquid–liquid extraction of palladium from nitric acid mixtures.[47] Such use has been proposed as a means of separating the fission product palladium from PUREX raffinate which comes from used nuclear fuel.[47] In this system of solvent extraction, the hydrocarbons of the diesel act as the diluent while the dialkyl sulfides act as the extractant.[47] This extraction operates by a solvation mechanism.[47] 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.[48]

Diesel fuel is also often used as the main ingredient in oil-base mud drilling fluid.[49] 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.[49] Diesel-oil mud is typically mixed with up to 40% brine water.[50] 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.[51]

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.[52] J-2 diesel fuel was also used as a fuel for gas turbine engines.[52]

Diesel does not mix with water.

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).[53] The average chemical formula for common diesel fuel is C₁₂H₂₄, ranging approximately from C₁₀H₂₀ to C₁₅H₂₈.[54]

Most diesel fuels freeze at common winter temperatures, while the temperatures greatly vary.[55] Petrodiesel typically freezes around temperatures of −8.1 °C (17.5 °F), whereas biodiesel freezes between temperatures of 2° to 15 °C (35° to 60 °F).[55] The viscosity of diesel noticeably increases as the temperature decreases, changing it into a gel at temperatures of −19 °C (−2.2 °F) to −15 °C (5 °F), that cannot flow in fuel systems. Conventional diesel fuels vaporise at temperatures between 149 °C and 371 °C.[21]

Conventional diesel flash points vary between 52 and 96 °C, which makes it safer than petrol and unsuitable for spark-ignition engines.[56] Unlike petrol, the flash point of a diesel fuel has no relation to its performance in an engine nor to its auto ignition qualities.[21]

Diesel engines, like other lean-burn (excess of oxygen in proportion to the fuel) forms of combustion, recombine the atmospheric oxygen (O₂) and nitrogen (N₂) into mono-nitrogen oxides NO and NO
₂, collectively known as NO_x, due to the high temperature and pressure. While naturally present in the atmosphere, their excess can contribute to smog and acid rain, as well as influence human health after reacting with ammonia, moisture, and other compounds. Modern diesel engines (Euro 6 & US EPA standards) use exhaust gas recirculation into the intakes to reduce excess oxygen present in combustion, and urea injection to turn NO_x into N₂ and water.

Small particles (PM 2.5) can penetrate deeply into lung tissue and damage it, causing premature death in extreme cases.[57] Inhalation of such particles may cause or worsen respiratory diseases, such as emphysema or bronchitis, or may also aggravate existing heart disease. Unlike petrol engines, modern diesel engines are fitted with diesel particulate filters that help to reduce PM 10 and PM 2.5 emissions.[58] However, those filters tend to get clogged easily with soot and are often removed by car owners.[59]

High levels of sulfur in diesel are harmful for the environment because they prevent the use of catalytic diesel particulate filters to control diesel particulate emissions, as well as more advanced technologies, such as nitrogen oxide (NO_x) adsorbers (still under development), to reduce emissions. Moreover, sulfur in the fuel is oxidized during combustion, producing sulfur dioxide and sulfur trioxide, that in presence of water rapidly convert to sulfuric acid, one of the chemical processes that results in acid rain. However, the process for lowering sulfur also reduces the lubricity of the fuel, meaning that additives must be put into the fuel to help lubricate engines. Biodiesel and biodiesel/petrodiesel blends, with their higher lubricity levels, are increasingly being utilized as an alternative. The U.S. annual consumption of diesel fuel in 2006 was about 190 billion litres (42 billion imperial gallons or 50 billion US gallons).[60]

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 currently in force in Europe for diesel fuel is the Euro 5, with a maximum content of 10 ppm.

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). U.S. diesel fuel typically also has a lower cetane number (a measure of ignition quality) than European diesel, resulting in worse cold weather performance and some increase in emissions.[61]

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.[62]

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.[63]

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.[64] 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.

Diesel is less flammable than gasoline / petrol. However, because it evaporates slowly, any spills on a roadway can pose a slip hazard to vehicles.[65] 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.