The Chemistry of Diesel Fuel


    • Diesel fuel is a liquid fuel that is made from the byproduct of petroleum. Diesel fuel was originally supposed to be coal dust, but in 1895, Rudolf Diesel discovered the use of petroleum byproducts for liquid fuel in diesel engines. A commonly known example of diesel engines would be: school buses, construction machines, and public buses. It is also used in trucks, trains, boats, military vehicles, and even generators. The military likes using diesel because it is less flammable and is less prone to stalling, unlike gasoline engines. Diesels are also more capable of producing higher torque numbers than gasoline engines.
    • I chose to study the chemistry of diesel fuel because diesel has a big impact on America. Almost every construction, military, and key transportation vehicles are powered by diesel fuel, in fact about 94% of freight relies on diesel. They are more fuel efficient and have unbeatable reliability. I also have a strong interest in diesels as well.
    • Diesel fuel is something that is actually involved in my life everyday. Because it is something that I have a deep interest in, and I also personally own a diesel, so I am driving a vehicle powered by diesel fuel everyday. I also love doing research and learning more about diesel and being able to work on my truck.

Composition of ...

    • Crude Oil
      • Hydrocarbons(hydrogen and oxygen)
        • Paraffins (roughly 75%)
          • Carbon atoms that link, forming chain like molecules.
            • N-paraffins
            • Isoparaffins
            • Ranging from C10H22 to C20H42
        • Aromatics (roughly 25%)
          • Similar to Naphthenes, a ring like structure is created, but they are joined by aromatic (double) bonds, instead of single bonds.
          • C10H8 - C20H34
    • Sulfur
    • Nitrogen

Main Chemicals, Compounds, Components

    • Paraffins
      • There are two kinds of paraffins: N (normal) paraffins and Isoparaffins. N-paraffins have carbon atoms that will form chain-like molecules. Isoparaffins are similar to N-paraffins except they have offshoots or branches of carbons from the chain. It was first commercially produced in 1867, paraffin wax is known for its colorless, or white appearance when solid; however when it is a liquid it turns mostly translucent. It’s melting point ranges from 120 to 150° F. However, there isn’t just one type of paraffin wax, common types of paraffins in diesel fuel are: decane, n-pentadecane, methyltetradecane, eicosane, and methylnonadecane. Because paraffins have a straight molecular structure, paraffins are the main reason why diesel fuel will eventually begin to turn into a solid state, a process known as gelling, a big problem for diesel engines. At 32 ° F the paraffin in the fuel will begin to cloud the fuel, and at 15 to 10° F it will begin to “gel” and make a diesel engine unable to run. Kerosene can be added to the fuel beforehand, reducing the fuel’s viscosity and making it much less prone to gelling.
    • Aromatics
      • Aromatics create a ring like structure with some of their carbon atoms. Aromatic hydrocarbon rings are created with 6 carbon atoms. They alternate single and double bonds throughout their ring structure. Another type of aromatic hydrocarbon is a polycyclic aromatic. Polycyclic aromatics are simply put as aromatic compounds with two or more aromatic rings. The simplest of all the aromatic compounds is benzene, with a chemical composition of C6H6. The name comes from the fact that the compounds of aromatics have a very strong aroma. Aromatic compounds are used to thin out oil or grease based compounds, which explains why when kerosene and other compounds can be added to diesel fuel to change the diesel fuel’s viscosity, making it less prone to gel up. Common compounds found in diesel fuel are: naphthalene, tetralin, anthracene, and tetradecylbenzene.

Chemistry's Role

None of the components in diesel fuel are individually made. Diesel fuel could be seen as something that naturally occurs as it is extracted from the earth (oil), but the final product, diesel fuel, is a man made product. The crude oil that is extracted from the earth is put in a distillation tower, the oil is then heated up to over 400°C. This begins a process, known as separation, that separates different components that have different boiling temperatures. As you go up the distillation tower the boiling temperature is cooler and the refining process is more tedious. Diesel fuel is created at the lower end of the tower, then comes kerosene, gasoline, butane, and propane. These compounds are then collected by a distillation plate, which takes that compound away and stores it in a storage tank. Chemistry plays a big role in this process, as scientists need to know the chemical composition of each mixture (ex. how many hydrogen and carbon atoms are in each compound) that is being separated at each boiling point. They need to know where to put the distillation plates in the tower to collect each compound effectively and efficiently.

Background Research

Diesel’s most obvious differences are its physical properties. Diesel fuel is sometimes called “diesel oil”, because of how oily it is, it smells different, it’s heavier and more much more oily, it evaporates much slower than gasoline, and has a higher melting point, ranging from 200 - 380°C. Chemically, diesel fuel contains more carbon atoms than gasoline does. Gasoline usually being C9H20 and Diesel usually being C12H23.

    • How is it Made?
      • Crude oil is placed into a distillation tower and the liquid is then heated to over 400°C.
      • Once the liquid begins to heat up, different chains of hydrogen and carbon atoms (hydrocarbons) begin to separate.
      • Diesel fuel begins to emerge between temperatures of 200°C and 380°C. Which is collected on distillation plates and siphoned off to a diesel holding tank.
    • Why Diesel?
      • Diesel fuel is cheaper to make, because of it’s less meticulous refining process when making it.
        • However, diesel fuel is currently more expensive due to the demand.
      • Diesel fuel also yields a higher energy density than gasoline.
        • 14% more energy than gasoline by volume.
      • Diesel engines, on average, are 20 to 30% more efficient than gasoline engines.


About the Author

Russell Schiller is a junior in highschool who has a deep passion for diesels. He also has an interest in chemistry and taking things apart and learning about things inside and out. He wants to keep on truckin in the world of diesel, but wants to attend Montana Tech for a bachelors in materials or metallurgical engineering.