Diesel is a fuel made from crude oil, composed mainly of hydrocarbon chains that are longer and heavier than those in gasoline. It powers everything from pickup trucks and semis to locomotives, ships, and backup generators. What makes diesel distinct is not just its chemistry but how it ignites: instead of needing a spark plug, diesel fuel combusts under pressure alone, a process that extracts more energy per gallon than gasoline engines can.
How Diesel Is Made
Diesel fuel comes from fractional distillation, the core process in an oil refinery where crude oil is heated and separated into different products based on their boiling points. Diesel separates out at temperatures between 200°C and 350°C (roughly 392°F to 662°F), which is hotter than the range for gasoline but cooler than the range for heavy lubricating oils. The result is a dense, oily liquid with more energy packed into each gallon than lighter fuels.
Chemically, diesel is 65 to 85% saturated hydrocarbons, with 5 to 30% aromatic compounds and a small fraction of other molecules. The carbon chains typically range from 8 to 25 carbon atoms long, with the most abundant ones falling in the C13 to C17 range. By comparison, gasoline molecules are shorter, usually C4 to C12. Those longer chains are what give diesel its higher energy density and its thicker, oilier feel.
How Diesel Engines Work
A gasoline engine mixes fuel and air, compresses the mixture, then fires a spark plug to ignite it. A diesel engine skips the spark entirely. During the intake stroke, only air enters the cylinder. That air is then compressed at a much higher ratio than in a gasoline engine, which heats it to extremely high temperatures. Near the top of the compression stroke, a high-pressure injector sprays diesel fuel directly into the cylinder, where the superheated air causes it to ignite on contact.
This compression ignition approach is inherently more efficient. A diesel engine can be up to 40% more thermally efficient than a gasoline engine producing the same power. Even after accounting for the heavier weight of diesel engines and the difference in fuel energy density, diesel still delivers roughly 20% better overall efficiency. That’s why diesel dominates in applications where fuel economy and pulling power matter most: long-haul trucking, farming equipment, and marine transport.
Cetane Number: Diesel’s Quality Rating
Gasoline quality is measured by octane rating. For diesel, the equivalent is the cetane number, which indicates how quickly the fuel ignites after being injected into the cylinder. A higher cetane number means a shorter delay between injection and combustion, which translates to smoother, quieter engine operation and lower emissions. Standard commercial diesel typically has a cetane number around 51.
Different types of hydrocarbon molecules within diesel have wildly different cetane ratings. Straight-chain molecules can score very high, while compounds with multiple fused aromatic rings score below 20. Refiners blend these fractions to hit the target cetane number for the market they’re supplying.
No. 1 vs. No. 2 Diesel
At the pump, you’ll most commonly see No. 2 diesel, which is the standard grade for everyday driving and most commercial use. It has more energy per gallon, so you get better mileage and more power. The trade-off is that No. 2 diesel is more prone to gelling in cold weather.
No. 1 diesel, sometimes called “winter diesel,” is a lighter, more refined version. It flows better at low temperatures but carries less energy, so fuel economy drops. In cold climates, stations often sell a seasonal blend of the two, or you can find No. 1 sold separately during winter months. The choice comes down to temperature: if you’re driving in freezing conditions, the lighter grade keeps your fuel system from clogging.
Cold Weather and Fuel Gelling
Diesel’s longer hydrocarbon chains are an advantage for energy density, but they become a liability in cold weather. As temperatures drop, wax crystals begin forming in the fuel. The temperature at which this first happens is called the cloud point, and for standard No. 2 diesel it typically falls between -18°F and 20°F (-28°C to -7°C), though lower-quality fuel can start clouding as warm as 40°F (4.4°C).
If the temperature keeps falling past the cloud point, those crystals grow and clump together until the fuel thickens into a gel that can’t flow through fuel lines or filters. The temperature at which fuel stops flowing entirely is the pour point. Drivers in cold climates manage this with winter-blend fuel, fuel additives that lower the gelling temperature, or engine block heaters that keep everything warm overnight.
Sulfur Content and Modern Standards
Diesel fuel naturally contains sulfur from the crude oil it comes from. When burned, that sulfur produces emissions that contribute to acid rain and respiratory problems. For decades, diesel had a reputation as a dirty fuel partly because of its high sulfur content. That changed significantly with regulations requiring ultra-low sulfur diesel (ULSD), which caps sulfur at just 15 parts per million. Virtually all on-road diesel sold in the United States today meets this standard.
The shift to ULSD also enabled modern emissions-control technology. Newer diesel vehicles use a system called selective catalytic reduction, which requires a separate fluid (often sold under the brand name AdBlue or simply called diesel exhaust fluid, or DEF). This fluid is a solution of roughly 32.5% urea in deionized water. It gets injected into the exhaust stream, where heat breaks it down into ammonia. That ammonia reacts with nitrogen oxide pollutants inside a catalytic converter, converting them into harmless nitrogen gas and water. If you drive a diesel vehicle made after about 2010, you’ll need to refill a small DEF tank periodically, usually every few thousand miles.
Biodiesel and Renewable Diesel
Two alternative fuels are often grouped under the diesel umbrella, but they’re chemically distinct from each other and from petroleum diesel.
- Biodiesel is made by chemically processing vegetable oils or animal fats into fatty acid methyl esters. Unlike petroleum diesel, biodiesel contains oxygen in its molecular structure, which gives it about 7% less energy per gallon. It’s typically blended with petroleum diesel rather than used on its own. A label like “B20” means the blend is 20% biodiesel.
- Renewable diesel starts from similar feedstocks (plant oils, animal fats) but uses a different refining process that strips out the oxygen entirely. The end product is a pure hydrocarbon, chemically identical to petroleum diesel. It can be used as a direct, drop-in replacement with no blending required and no loss in energy content.
Both reduce lifecycle carbon emissions compared to petroleum diesel, but renewable diesel’s chemical similarity to conventional fuel makes it compatible with existing engines and fuel infrastructure without modification.