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Diesel is an energy-dense secondary fuel (or energy currency) used to power many heat engines, including cars, trucks, and diesel generators. It can be a petroleum derivative, or it can be made from biomass. Diesel itself is a mixture of hydrocarbons, ranging from C10H20 to C15H28. The average composition of diesel is C12H23,[1] but it should be restated that these are not actually molecules, just an average composition.

Automotive use

Diesel engines are a kind of internal combustion heat engine.

Diesel is most commonly used a fuel for transportation, specifically. Both diesel and gasoline have about the same energy density; but because diesel has a higher mass density than gasoline, the same volume of diesel has more energy than gasoline. Diesel also allows for engines to run much higher compression ratios (i.e. the ratio of the highest volume to the lowest volume in the compression chamber). Both, diesel's higher mass density and higher compression ratios, allows diesel engines to be more energy efficient than their gasoline burning counterparts. Diesel engines are also different from gasoline in that they don't use spark plugs, instead achieving ignition through compression of the fuel (and a glow-plug when the engine is cold). Diesel engines work by having a piston pressurize air, to heat it up (remember the ideal gas law: at a constant volume, increasing pressure increases the temperature). Fuel injectors then atomize the fuel, turning it into a gas. The heat from the air inside the chamber raises the temperature of the diesel gas until it ignites, which does work against a piston in the chamber.

Types of diesel


Petrodiesel is the name for diesel that is derived from petroleum. This diesel usually must be refined to remove sulfur from it. Currently, petrodiesel is more common than biodiesel as a fuel, although its use in medium or heavy trucks and buses is growing.[2] Petrodiesel is produced through a process called fractional distillation, where crude oil is boiled, and its components separated. Because diesel has a higher boiling point than gasoline, the gasoline separates from the crude oil sooner. This process allows refineries to separate substances like diesel, kerosene, and gasoline from each other.[3]


See Biofuel

Biodiesel is diesel that is made from biomass, such as algae. Biodiesel has lower net emissions than petrodiesel because the carbon it emits was taken from the modern atmosphere when the biomass grows, whereas the carbon released by petrodiesel has been stored in the earth for millions of years.

Pure biodiesel, known as B100 (100% biodiesel), is rarely used as a transportation fuel.[4] Rather, biodiesel is blended with petrodiesel. B20 (20% biodiesel, 80% petrodiesel) is the optimum blend since it burns cleaner than petrodiesel, leaving less combustion products behind, and has better flow properties at low temperatures than pure biodiesel, which would clump up in the cold.[5]

For Further Reading


  1. http://www.newton.dep.anl.gov/askasci/chem07/chem07490.htm
  2. H. Al-Mashhadani and S. Fernando, "Properties, performance, and applications of biofuel blends: a review", AIMS Energy, vol. 5, no. 4, pp. 735-767, 2017. Available: 10.3934/energy.2017.4.735.
  3. http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/rocks/fuelsrev3.shtml
  4. "Alternative Fuels Data Center: Biodiesel Blends", Afdc.energy.gov, 2020. [Online]. Available: https://afdc.energy.gov/fuels/biodiesel_blends.html. [Accessed: 03- Feb- 2020].
  5. H. Al-Mashhadani and S. Fernando, "Properties, performance, and applications of biofuel blends: a review", AIMS Energy, vol. 5, no. 4, pp. 735-767, 2017. Available: 10.3934/energy.2017.4.735.

Authors and Editors

Ethan Boechler, Jordan Hanania, James Jenden, Kailyn Stenhouse, Luisa Vargas Suarez, Jason Donev
Last updated: September 27, 2021
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