By the strictest definition, the term "diesel fuel" refers only to fuel oils which are intended for compression ignition engines. Such engines were invented in Germany in the nineteenth century by Rudolf Diesel and make use of the heat generated as the fuel-air mixture is compressed to 500 or 550 pounds per square inch as the sole ignition source. In theory, many different hydrocarbons and other highly flammable compounds could be used as fuel in a diesel engine, but in actuality nearly all diesel engines are fueled with a mixture of petroleum hydrocarbons which boil between 330 and 700 degrees Fahrenheit.
Diesel engines are not only highly efficient power plants; they are also very versatile in the fuels they can use. Rudolf Diesel first conceived of the engine that now bears his name as running on powdered coal. A ruinous engine explosion taught him to value liquid fuels. He subsequently hit on the idea of using vegetable oil. The engine that he demonstrated at the World Exhibition in Paris in 1900 ran on oil extracted from peanuts.
In common usage, the term "Diesel Fuel" is applied to several kerosine grades of fuel oil. But strictly speaking, the term "Diesel Fuel" refers not to the composition of the fuel, but rather to the fact that it can be used as a fuel in a Diesel cycle engine. In a diesel engine, fuel is injected as a mist into a chamber at a pressure hundreds of times higher than in a gasoline-powered engine. As a piston compresses the charge, the fuel temperature rises and the mixture burns, pushing the piston down to turn the crank shaft. This occurs in each cylinder thousands of times each minute. An important characteristic of diesel fuel is its ability to ignite on its own, as it must in a diesel engine. This ability is quantified by the fuel's cetane number, where a higher cetane number means that the fuel ignites more quickly.
Diesel engines are more expensive to build than their gasoline-fueled counterparts. Their virtue lies in their sparing use of a relatively inexpensive fuel, and in their long life and low maintenance costs. Early diesel engines were mainly large, low rpm machines, but modern diesel engineering has seen the development of light, high speed engines. The major uses of diesel engines are in: commercial trucks, propulsion of ships and boats, railroad locomotives, industrial engines, and private automobiles. Diesel oils are among the products considered "fuel oils" Home heating oil is similar to diesel oil but has a separate CAS number (68476-30-2). Just as Diesel oil 1 is similar in chemical composition to Fuel oil 1, with the exception of additives, so is Diesel oil 2 similar in chemical composition to Fuel oil 2, with the exception of additives. Diesel fuels, and typical home heating oil and high aromatic content home heating oil, are all forms of no. 2 fuel oil. Diesel oils, as well as typical heating oil, fall under the broader category of Fuel Oil Number 2 (CAS 68476-30-2).
Fuel oils are refined from crude petroleum and may be categorized as either a distillate fuel or a residual fuel depending on the method of production. In terms of refining crude oil, diesel fuels are middle distillates. The middle distillates include kerosene, aviation fuels, diesel fuels, and fuel oil #1 and 2. These fuels contain paraffins (alkenes), cycloparaffins (cycloalkanes), aromatics, and olefins from approximately C9 to C20. Aromatic compounds of concern included alkylbenzenes, toluene, naphthalenes, and PAHs.
Fuel oils no. 1 and no. 2 are distillate fuels which consist of distilled process streams. Residual fuel oils such as fuel oil no. 4 are residues remaining after distillation or cracking, or blends of such residues with distillates. Diesel fuels are approximately similar to fuel oils used for heating (fuel oils no. 1, no. 2, and no. 4). All fuel oils consist of complex mixtures of aliphatic and aromatic hydrocarbons. The aliphatic alkanes (paraffins) and cycloalkanes (naphthenes) are hydrogen saturated and compose approximately 80-90% of the fuel oils. Aromatics (e.g., benzene) and olefins (e.g., styrene and indene) compose 10-20% and l%, respectively, of the fuel oils. Fuel oil no. 1 (straightrun kerosene) is a light distillate which consists primarily of hydrocarbons in the C9-C16 range; fuel oil no. 2 is a heavier, usually blended, distillate with hydrocarbons in the C11-C20 range. Straight-run distillates may also be used to produce fuel oil no. 1 and diesel fuel oil no. 1. Diesel fuel no. 1 and no. 2 are similar in chemical composition to fuel oil no. 1 and fuel oil no. 2, respectively, with the exception of the additives. Diesel fuels predominantly contain a mixture of C10 through C19 hydrocarbons, which include approximately 64% aliphatic hydrocarbons, l-2% olefinic hydrocarbons, and 35% aromatic hydrocarbons.
Fuel no. 4 (marine diesel fuel) is less volatile than diesel fuel no. 2 and may contain up to 15% residual process streams, in addition to more than 5% polycyclic aromatic hydrocarbons. Residual fuel oils are generally more complex in composition and impurities than distillate fuel oils; therefore, a specific composition cannot be determined.
The term "diesel" is actually a catch-all for various diesel mixtures. The three most common forms of diesel are:
Grade 1-D: Straight-run fractions including kerosenes to intermediate distillates from mixedbase crudes. Used for mobile service such as trucks, railroads and submarines.
Grade 2-D: Similar to Grade 1-D but with lower volatility. Used for industrial and heavy mobile service.
Grade 4-D: Residual fuel oils blended with more viscous distillates. Used for larger stationary installations (ASTM D 975).
To some people, the term "diesel fuel" means any petroleum distillate which boils between 300 and 700 degrees Fahrenheit, and so includes another group of products which are more properly called heating oils. Heating oils are intended for use in domestic or commercial space-heating furnaces, or as fuel for small steam or hot water boilers. In practice, some small petroleum refiners produce a single product which is marketed as both No. 2 diesel oil and No. 2 heating oil. The specifications for the two products overlap sufficiently that a dual-purpose product is possible. Larger refiners, however, make separate products with separate properties.
Diesel fuels are classed as middle distillates and are more dense than gasoline, thus providing more energy per unit volume than gasoline. The product definition for diesel oil in the U.S. Chemical Substances Inventory under the Toxic Substances Control Act is: " A complex combination of hydrocarbons produced by the distillation of crude oil. It consists of hydrocarbons having carbon numbers predominantly in the range of C9-C20 and boiling in the range of approximately 163-357 degrees C ."
It has been stated that because of the way diesel is produced, most lighter weight hydrocarbons (C1 to C8) and most of the volatile aromatic compounds such as benzene, toluene, xylene, and ethylbenzene (BTEX), and most PAHs are removed from the diesel during the distillation process. However, the proceeding statement is only true in a relative sense, since PAHs and BTEX are still important hazardous components of diesel spills.
Short-term hazards of the some of the lighter, more volatile and water soluble compounds (such as toluene, ethylbenzene, and xylenes) in diesels include potential acute toxicity to aquatic life in the water column (especially in relatively confined areas) as well as potential inhalation hazards. Diesel fuels have moderate volatility and moderate solubility. Diesel products possess moderate to high acute toxicity to biota with product-specific toxicity related to the type and concentration of aromatic compounds. Diesel spills could result in potential acute toxicity to some forms of aquatic life. Oil coating of birds, sea otters, or other aquatic life which come in direct contact with the spilled oil is another potential short-term hazard. In the short term, spilled oil will tend to float on the surface; water uses threatened by spills include: recreation; fisheries; industrial, potable supply; and irrigation.
Long-term potential hazards of the some of the lighter, more volatile and water soluble compounds (such as toluene and xylenes) in diesel fuels include contamination of groundwater. Long-term water uses threatened by spills include potable (ground) water supply. Chronic effects associated with middle distillates are mainly due to exposure to aromatic compounds.
Some chemical characteristics and natural impurities in diesel fuel can affect exhaust emissions from diesel engines, can damage or impede the operation of emission control devices, and can increase secondary pollutant formation in the atmosphere. Low-sulfur diesel fuel began replacing conventional diesel starting in 2006. The new fuel contains 97% less sulfur than conventional diesel-sulfur will be reduced from 500 parts per million (ppm) to 15 ppm. Low-sulfur diesel fuel is cleaner-burning, producing less particulate emissions in both older and new engines. It will also allow the use of improved exhaust treatment devices to reduce emissions of particulates and smog-forming nitrogen oxides (NOx). These devices can be "poisoned" by the sulfur in conventional diesel fuel.
In the early 20th century, diesel engines were adapted to burn petroleum distillate, which was cheap and plentiful. Gradually, however, the cost of petroleum distillate rose, and by the end of the 1970s there was renewed interest in biodiesel. Biodiesel is a form of diesel fuel that can be manufactured from vegetable oils, animal fats, or recycled restaurant greases. It is safe, biodegradable, and produces less air pollutants than regular diesel. Biodiesel can be used in its pure form (B100) or blended with petroleum diesel. Lower-level biodiesel blends-up to 20% biodiesel-can be used safely in most diesel engines. Higher blends may also be used in engines built since 1992-94 with little or no modification. Biodiesel has several advantages over petroleum diesel fuel, but there are also some challenges to using it, including slightly lower fuel economy and power (10% lower for B100, 2% for B20).
A long ignition delay (low cetane) in a diesel engine will result in rapid pressure rise that can cause undesirable audible knock, high stresses and severe engine vibration. Also, difficult starting in cold weather, misfiring and excessive white smoke often result from too low cetane.
Supercetane, derived from plant and tree oils by high-pressure hydrotreating, is a fuel additive used for increasing diesel's cetane value. The cetane value measures the fuel's ability to self-ignite, like the octane value for gasoline. Supercetane derived from plant oils has a cetane value of 90 to 100, and from tree oils a value of 70 to 75. The cetane value of #2 diesel is about 45, while biodiesel has a value of 50 to 55. The minimum cetane value for a fuel used in a U.S. diesel engine is 40. Currently, most petroleum diesel fuel contains synthetic additives to increase its cetane value above 40. Supercetane is cost-competitive with synthetic cetane enhancers and does not have the diminishing marginal effectiveness of synthetics. Supercetane appears to be more effective than synthetic additives on low cetane engine base fuels. The only problem with supercetane is that it has a high pour point (or freezing point); this can be alleviated when the additive is diluted in conventional fuel.
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