Diesel fuel comes primarily from crude oil, refined at high temperatures into a specific range of hydrocarbons. It can also be made from plant oils, animal fats, natural gas, coal, and even biomass. Most of the diesel powering trucks, trains, and ships today is petroleum-based, but renewable and synthetic versions are growing in use.
From Crude Oil to Diesel
The vast majority of diesel starts as crude oil pumped from underground reservoirs. At a refinery, crude oil is heated to over 350°C (about 660°F) in a distillation column, a tall tower that separates the oil into different products based on their boiling points. Lighter molecules rise to the top and become gases or gasoline. Heavier molecules settle lower and become products like jet fuel, heating oil, and diesel.
Diesel occupies a middle-to-heavy band in the distillation column. Its molecules are chains of 12 to 20 carbon atoms, longer than gasoline (which typically has 4 to 12). These longer chains are what give diesel its oily texture, its higher energy content, and its tendency to ignite under compression rather than from a spark. The three main types of hydrocarbons in diesel are paraffins (straight or branched chains), naphthenes (ring-shaped molecules), and aromatics (rings with a different bonding structure).
A standard 42-gallon barrel of crude oil yields about 10 gallons of diesel, roughly 22% of the barrel’s total output. Gasoline gets the larger share at around 19 gallons. The remaining volume becomes jet fuel, heavy fuel oil, asphalt, petrochemical feedstocks, and other products. Nothing in a barrel of oil goes to waste.
Why Diesel Packs More Energy Than Gasoline
Diesel contains about 38,290 kilojoules per liter, compared to 33,526 for gasoline. That’s roughly 14% more energy in the same volume. This energy advantage is a direct result of those longer carbon chains, which store more chemical energy per molecule. It’s the reason diesel engines get better fuel economy per gallon and why diesel dominates heavy transport: long-haul trucks, freight trains, cargo ships, and construction equipment all rely on that extra energy density to move heavy loads efficiently.
Diesel engines also work differently from gasoline engines. Instead of using a spark plug, a diesel engine compresses air in the cylinder until it gets extremely hot, then injects fuel into that superheated air. The fuel ignites on contact. How quickly and smoothly that ignition happens is measured by a rating called the cetane number, which runs on a scale from 0 to 100. Most commercial diesel falls in the range of 40 to 55. Higher cetane numbers mean smoother, more complete combustion and easier cold starts.
What Happens to Diesel in Cold Weather
Those same paraffin molecules that make diesel energy-rich also cause problems in winter. As temperatures drop, the heavier paraffins (chains of 10 to 25 carbon atoms) begin to crystallize into tiny wax particles. The temperature where these crystals first become visible is called the cloud point, because the fuel turns hazy. If temperatures keep falling, the wax crystals grow and clump together, eventually clogging fuel filters and starving the engine.
To prevent this, refiners adjust diesel blends seasonally. Winter diesel contains a higher proportion of lighter, shorter-chain hydrocarbons that resist crystallization. Chemical additives, such as polymers that either break up wax crystals or prevent them from growing large enough to cause blockages, are also mixed in. In extremely cold climates, drivers sometimes blend a small amount of kerosene into their diesel to keep it flowing.
Sulfur Limits and Clean Diesel
Crude oil naturally contains sulfur, and that sulfur carries over into diesel during refining. When burned, sulfur compounds form particulates and sulfur dioxide, both harmful to lungs and the environment. Starting in 2006, the U.S. Environmental Protection Agency began phasing in rules to limit sulfur in diesel to just 15 parts per million, a fuel standard known as ultra-low sulfur diesel (ULSD). By 2010, all highway diesel sold in the U.S. had to meet that limit. By 2014, the requirement extended to diesel used in off-road equipment, locomotives, and marine engines.
This was a dramatic reduction. Older diesel standards allowed sulfur levels hundreds of times higher. The switch to ULSD also enabled modern emissions equipment like diesel particulate filters and catalytic converters, which are poisoned by high-sulfur fuel. Today’s diesel vehicles are far cleaner than their predecessors largely because of this fuel change.
Biodiesel From Plants and Animal Fats
Not all diesel comes from petroleum. Biodiesel is made from renewable fats and oils through a chemical process called transesterification. In this process, a fat or oil (from plants, animals, or used cooking grease) is mixed with an alcohol, typically methanol, in the presence of a catalyst and heated. The reaction breaks apart the fat molecules and rearranges them into new compounds that behave like petroleum diesel in an engine, with glycerol left over as a byproduct.
The feedstocks vary by region. Soybean oil is the primary source in the United States and Brazil. Europe relies heavily on rapeseed oil. Tropical countries use palm oil. Beyond these, a wide range of other sources are viable: waste cooking oil, animal tallow, chicken fat, and non-food crops like jatropha, jojoba, neem, and camelina. Camelina is gaining attention because of its short growing cycle and ability to thrive on marginal farmland, making it less likely to compete with food production.
Biodiesel can be blended with petroleum diesel at various ratios or used on its own. A blend labeled B20, for example, is 20% biodiesel and 80% petroleum diesel. Most modern diesel engines can run B20 without modification.
Synthetic Diesel From Gas, Coal, or Biomass
There’s a third route to diesel that doesn’t require crude oil or crops. Synthetic diesel is made through a process called Fischer-Tropsch synthesis, developed in the 1920s. The starting point is any carbon-rich material: natural gas, coal, or biomass like wood chips and agricultural waste. First, the raw material is converted into a mixture of carbon monoxide and hydrogen called synthesis gas. Then, using a metal catalyst and high heat, the synthesis gas is assembled into hydrocarbon chains, essentially building diesel molecules from scratch.
The industry labels these fuels based on the starting material. GTL (gas-to-liquid) starts with natural gas. CTL (coal-to-liquid) starts with coal. BTL (biomass-to-liquid) starts with organic waste or plant matter. Fischer-Tropsch diesel tends to be very clean, with virtually no sulfur and very few aromatics, making it well-suited for use in modern engines with strict emissions standards. Researchers are also exploring the conversion of waste plastics into synthesis gas, which could then be turned into synthetic diesel as part of a circular waste strategy.
A Fuel With Surprising Origins
Rudolf Diesel, the German engineer who invented the compression-ignition engine in the 1890s, originally envisioned his engine running on a variety of fuels. At the 1900 World Exposition in Paris, a small diesel engine ran on peanut oil at the request of the French government, which wanted locally sourced fuels for its African colonies. Petroleum-based diesel eventually won out because of its abundance and low cost, but the flexibility Diesel built into his engine design is still visible today in the range of fuels it can burn, from petroleum to soybean oil to synthetic hydrocarbons built from natural gas.