Diesel fuel is made from crude oil, refined through a process called fractional distillation that separates petroleum into different products based on their boiling points. The diesel fraction consists of hundreds of different hydrocarbon molecules, typically containing 10 to 22 carbon atoms per molecule. These hydrocarbons break down into three main categories: 50 to 60% alkanes (straight or branched chains of carbon and hydrogen), 20 to 30% cycloalkanes (carbon chains arranged in rings), and 7 to 30% aromatics (a different type of ring structure).
How Diesel Is Separated From Crude Oil
Crude oil is a mixture of thousands of hydrocarbon molecules of varying sizes. To turn it into usable fuels, refineries heat crude oil in a distillation column, a tall tower where different products rise to different levels based on their boiling points. Lighter molecules with fewer carbon atoms (like those in gasoline) boil off first and rise to the top. Heavier molecules stay lower.
Diesel collects in the middle range of the column, where temperatures sit between roughly 175°C and 275°C (about 350°F to 530°F). This is the same general zone where kerosene, jet fuel, and heating oil are drawn off, though each gets further separated and processed. The raw diesel fraction then goes through additional refining steps to remove impurities, especially sulfur, before it’s ready for sale.
What’s Actually in the Fuel
If you could zoom in on a drop of diesel, you’d find a complex soup of hydrocarbons. The alkanes that make up the majority are simple chains of carbon atoms bonded to hydrogen. These burn cleanly and ignite easily under compression, which is exactly what a diesel engine needs. Longer carbon chains store more energy per gallon than shorter ones, which is why diesel delivers better fuel economy than gasoline (whose molecules are smaller, typically 4 to 12 carbons).
Cycloalkanes, the second largest group, have their carbon atoms arranged in ring shapes rather than straight lines. They contribute to diesel’s energy density and stability. Aromatics, the smallest fraction, are ring-shaped molecules with a different bonding structure that makes them harder to burn completely. Higher aromatic content generally means more soot and particulate emissions, so refiners try to keep this percentage low.
Sulfur Removal and Regulations
Raw diesel straight from the distillation column contains sulfur compounds that, when burned, produce sulfur dioxide, a pollutant that causes acid rain and damages catalytic exhaust systems. Beginning in 2006, the EPA phased in regulations requiring diesel sold in the United States to contain no more than 15 parts per million of sulfur. This ultra-low sulfur diesel (ULSD) is now the standard at every pump in the country.
Removing sulfur happens through a process called hydrotreatment, where the diesel fraction is exposed to hydrogen gas at high temperatures and pressures. The hydrogen reacts with sulfur compounds and pulls the sulfur out, leaving behind cleaner-burning fuel. This single step is one of the most important in modern diesel production, since it’s what allows the particulate filters and catalytic converters on newer diesel vehicles to function properly.
Additives Mixed Into Finished Diesel
The base fuel that comes out of the refinery isn’t quite what ends up in your tank. Fuel producers blend in several types of additives before distribution. Cetane improvers help diesel ignite more readily under compression, reducing engine knock and improving cold starts. Detergent additives keep fuel injectors clean, preventing the carbon buildup that degrades engine performance over time. Lubricity additives compensate for the natural lubricating compounds that get stripped out during sulfur removal, protecting fuel pumps and injectors from wear. All diesel fuel additives sold in the U.S. must be registered with the EPA.
In cold climates, diesel gets special attention. The long-chain hydrocarbons in diesel start forming wax crystals as temperatures drop, which can clog fuel filters and prevent engines from running. Cold flow improvers work by modifying how those wax crystals grow, changing their shape from flat plates (which stack together and block filters) into smaller needle-like structures that pass through more easily. Some winter diesel blends also mix in kerosene, a lighter fuel that resists gelling at lower temperatures.
Biodiesel and Renewable Diesel
Not all diesel comes from crude oil. Biodiesel is produced from vegetable oils, animal fats, used cooking oils, or yellow grease through a chemical process called transesterification. This reaction breaks the fat molecules apart and recombines them with an alcohol (usually methanol) to create a fuel that can be blended with petroleum diesel or, in some cases, used on its own. The process also produces glycerin as a byproduct. Biodiesel blends are labeled by percentage: B20 means 20% biodiesel mixed with 80% petroleum diesel.
Renewable diesel is a newer alternative that starts from similar raw materials (plant oils, animal fats, waste greases) but uses a completely different production method. Instead of transesterification, renewable diesel is made through hydrotreating, the same hydrogen-based process used to remove sulfur from petroleum diesel. The result is a fuel that is chemically identical to petroleum diesel, meaning it meets the same specifications and can be used as a direct replacement without any blending limits. Biodiesel, by contrast, is a different type of molecule (a mono-alkyl ester) with its own fuel standard and blending restrictions.
Why Diesel Differs From Gasoline
Diesel and gasoline both come from crude oil, but they’re drawn from different sections of the distillation column and have fundamentally different molecular profiles. Gasoline is made of smaller, lighter hydrocarbons (typically 4 to 12 carbon atoms) that evaporate easily and ignite with a spark. Diesel’s larger molecules (10 to 22 carbons) don’t evaporate as readily, which is why diesel fuel feels oily to the touch while gasoline evaporates almost instantly from your skin.
This size difference has practical consequences. Diesel packs about 10 to 15% more energy per gallon than gasoline, which is a big reason diesel engines get better mileage. But those larger molecules also need higher temperatures and pressures to ignite, which is why diesel engines use compression ignition rather than spark plugs. The engine squeezes air in the cylinder until it’s hot enough to ignite the fuel on contact, a design that extracts more work from each drop of fuel but requires heavier, more robust engine components.