Diesel is made by refining crude oil through a separation process called distillation. Not all diesel fuels come from petroleum, though. The process to make alternative fuels like biodiesel is slightly different.

Named after German engineer Rudolf Diesel, diesel fuel is one of the liquid fuels commonly used to power diesel engines in semi-trucks, trains, buses, and even passenger vehicles—but the process to make it can be complicated to understand.

If you’ve ever wondered how diesel is made and how it differs from gasoline, you’re in the right place.

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How is diesel fuel made from crude oil?

Diesel fuel is refined from crude oil, a darkly colored and highly viscous liquid that lies beneath the earth’s surface hidden in bedrock. Viscosity is a substance’s thickness—which means that crude oil is very thick.

Crude oil is the base ingredient for many types of fuel and petroleum products, including gasoline, jet fuel, and even asphalt.

Diesel undergoes a process called distillation, which separates the oil’s components, and then is purified to make it usable by our trains, planes, and automobiles.

(A side note—crude oil is used for both gas oil and diesel oil, but each undergoes a different oil refining process.)

Diesel is often desirable for large vehicles like semi-trucks because it has a higher energy density, meaning each gallon of diesel contains more energy than a gallon of gas.

Below, we take a closer look at the refining process for two types of diesel fuel: petroleum diesel and biodiesel.

Petroleum diesel

After crude oil is extracted, it’s transported to a petroleum refinery. There, the crude oil is transferred into a large distillation tank for refinement, which involves three steps:

Separation: The first step in the refinement process is the most complicated and involves exposing the crude oil to extremely high temperatures. Under pressure and heat, the crude oil’s components—or “fractions”—separate into liquids and gasses. Heavier fractions (like lubricants) sink to the bottom, and lighter ones (like propane gas and diesel) stay closer to the top. The entire process of separating crude oil into its component parts is called fractional distillation.

Conversion: Once the crude oil has been distilled, it’s further processed and reformed via exposure to catalysts. In this step, the long hydrocarbon chains in petroleum are broken up and rearranged into shorter ones.

Purification: Finally, the fuel products are cleaned of impurities, such as sulfur content, to make them suitable to pass through a pipeline and be used as fuel oil.

Key Takeaway Petroleum products become diesel through a process called fractional distillation.

MORE:

How to choose the best gasoline or fuel for your car

Biodiesel

Biodiesel is not made out of petroleum—instead, it comes from biomass, which is a word for various organic products like vegetable oils (soybeans, corn, etc.), animal fat, and slaughter waste. Even algae can be made into biodiesel!

In the U.S., the most common base for biodiesel is soybean oil. The oil is combined with fats and alcohol, then exposed to a catalyst (just like petroleum-based diesel). This chemical process is (brace yourself) known as transesterification.

Generally, biodiesel is blended in small amounts with traditional diesel fuel to enhance its performance.

Some perks of biodiesel include:

Eco-friendliness. Unlike crude oil, which is a fossil fuel, biodiesel is a renewable energy source. It also emits fewer pollutants when burned, and the carbon emissions biodiesel releases are offset by the carbon dioxide intake of the soybean plants used to make it.

Safety. Biodiesel is also generally safer to burn and has a lower point of combustion (catching on fire) compared to petroleum-based diesel.

Key Takeaway Biodiesel is a type of “renewable diesel” made from plant or animal material. It is generally less harmful to the environment than petroleum diesel.

MORE: How to pass emissions testing

Uses of diesel fuel

Diesel fuel can be used to power many different machines, including:

Cars, buses, and trucks

Airplanes (most jet fuel is diesel)

Trains

Military vehicles

Generators and furnaces (as an alternative to natural gas)

Farm and construction equipment

Submarines

Biofuels—including biodiesel—are mostly used as transportation fuels for cars, trucks, and buses. In some cases, biofuels can also be used to power generators or heaters.

MORE: How to choose an eco-friendly car

Find hassle-free car insurance

If you drive a vehicle with a diesel-powered engine, you’re no stranger to the benefits of diesel fuel, including better fuel economy and longer-lasting mileage—but diesel prices are often more expensive than regular gasoline.

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FAQs

How is diesel produced from crude oil?

Diesel is produced from crude oil in a process known as fractional distillation, which separates the components of the crude oil by weight and chemical composition.

Once separated, diesel is typically treated and purified before being released for consumption.

Is diesel a byproduct of making gasoline?

No. While both diesel and gasoline come from crude oil, they result from entirely different refining processes.

What are the benefits of diesel?

Among diesel fuel’s many benefits are a lower evaporating point, better mileage, and high energy efficiency.

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