Diesel: the compression-ignition fuel
Diesel is a compression-ignition fuel refined from crude oil, used mainly in trucks, buses and many passenger cars. It holds more energy per litre than petrol and changes composition with the seasons to resist gelling in cold weather.
In this entry
What diesel is — definition, composition, and uses
What is diesel? Diesel — also called diesel fuel or diesel diesel fuel in everyday speech — is a liquid fuel refined from crude oil, burned in compression-ignition engines that ignite it by pressure alone rather than with a spark plug. It is the dominant fuel type diesel for trucks, buses, heavy machinery, and many passenger cars, particularly in Europe.
What is diesel made of? Diesel is a blend of hydrocarbon compounds in the C10–C19 range, heavier than petrol's C4–C12 fraction, with added lubricity additives, cold-flow improvers, and (in most regulated markets) a percentage of biodiesel. Energy in diesel per litre is approximately 35–37 MJ/litre, roughly 10–15% higher than petrol, which is why diesel vehicles typically travel further on the same tank.
What is diesel fuel used for? The uses are broad: road haulage, passenger cars, agricultural machinery, marine engines, rail locomotives, and stationary generators. Diesel types vary by market: standard road diesel (ULSD in the US, EN 590 in the EU), premium diesel (higher cetane, additive-enhanced), off-road/red diesel (dyed, reduced duty), and marine gas oil. All share the same compression-ignition chemistry — the differences are in additives, sulphur limits, and tax treatment.
Diesel compared with petrol
Diesel and petrol are both refined from crude oil, but the engines that burn them, and the fuels themselves, work quite differently.
Diesel and petrol, side by side
What it's called around the world
Ordinary road diesel goes by a few different names depending on where you are, though the underlying fuel is broadly similar.
Older, higher-sulphur diesel has been phased out across most of the world's road fuel markets over the past two decades, mainly for air-quality reasons. The "ultra-low-sulphur" label on US Diesel #2 reflects that shift, and equivalent limits apply under standards like the EU's EN 590.
Why diesel changes with the seasons
Diesel behaves differently in cold weather than petrol does, in a way that shapes how it's actually sold.
In very cold weather, paraffin wax naturally present in diesel can start to crystallise, thickening the fuel and eventually clogging filters. This is what people mean by diesel "gelling," and it's the reason winter-blend diesel and cold-flow additives exist.
How this shows up on FuelTheGuide
We show diesel and premium diesel as separate grades where a country publishes both, using the local name for the grade but grouping it under the same comparison category across countries.
We use each country's own local name where it differs, such as Diesel #2 in the United States, but group both under the same comparison category.
Questions and answers
What is diesel and what is it made of?
What is diesel? Diesel is a compression-ignition liquid fuel refined from crude oil, made of heavier hydrocarbon compounds (C10–C19) than petrol, with lubricity additives, cold-flow improvers, and biodiesel blended in. Standard road diesel in the EU must meet EN 590; in the US it is called Ultra-Low-Sulphur Diesel (ULSD). What is diesel fuel used for? Road haulage, passenger cars, agriculture, marine, and power generation. Energy in diesel is approximately 35–37 MJ/litre — about 10–15% more than petrol per litre.
Does diesel have an octane rating?
No. The octane rating of diesel is not a real measure — diesel engines ignite by compression, not spark, so octane resistance is irrelevant. The correct quality metric for diesel is the cetane number (or cetane rating), which measures how quickly and reliably diesel ignites under compression. The EU EN 590 standard sets a minimum cetane number of 51; US ASTM D975 sets a minimum of 40. See the cetane number glossary entry for the full explanation.
Why is diesel sometimes more expensive than petrol, and sometimes cheaper?
It varies by country and by season. Some markets tax diesel more lightly than petrol to support commercial transport, which makes it cheaper; others do the reverse. Seasonal winter-blend costs can also push diesel prices up temporarily in colder regions.
Can I put diesel in a petrol car by mistake, or the other way round?
Putting diesel in a petrol car is less common because diesel pump nozzles are usually larger and don't fit most petrol filler necks. Putting petrol in a diesel car is easier to do by mistake and more damaging, since petrol lacks the lubricity that diesel fuel systems depend on. Both require draining the tank before starting the engine.
What is HVO or renewable diesel?
Hydrotreated Vegetable Oil (HVO) is a fuel made from vegetable oils or animal fats using a different process from biodiesel. Unlike biodiesel (FAME), HVO is chemically very similar to petroleum diesel and can be used as a drop-in replacement in most diesel engines without modification. It is sold as a premium product in some markets. The IEA tracks biofuel developments including HVO as part of its transport energy analysis.
Do all diesel cars need AdBlue or a similar additive?
No. AdBlue (a urea solution) is used in Selective Catalytic Reduction (SCR) systems fitted to Euro 6 and later diesel engines to reduce nitrogen oxide emissions. Older diesel cars, and some newer ones without SCR systems, do not require it.
Diesel fuel grades and ultra low sulfur diesel
Diesel fuel grades differ by cetane number, additive package and sulphur content. The most significant regulatory shift in the past two decades has been the global move to ultra low sulfur diesel (ULSD). In the United States, the U.S. Energy Information Administration confirms that all on-road diesel sold since 2006 must contain no more than 15 parts per million (ppm) of sulphur — down from 500 ppm under the previous standard. In the EU, EN 590 similarly caps sulphur at 10 ppm. The ultra-low sulphur requirement enables modern exhaust after-treatment systems such as diesel particulate filters (DPF) and selective catalytic reduction (SCR) to function without being poisoned by sulphur compounds.
Beyond the standard grade, many markets sell a premium tier with a higher cetane number (typically 55+), additional detergent additives, and improved cold-flow properties. The UK government's fuels guidance outlines how diesel sold in the United Kingdom meets the BS EN 590 specification, while the OPEC Annual Statistical Bulletin tracks global diesel demand by region — useful context for understanding why diesel fuel grades and pricing vary so widely between countries.
More questions
What is ultra low sulfur diesel (ULSD)?
Ultra low sulfur diesel (ULSD) is diesel fuel with a sulphur content of 15 ppm or less in the US (ASTM D975) and 10 ppm or less in the EU (EN 590). The low sulphur content is essential for the proper functioning of modern emission-control systems including diesel particulate filters and SCR catalysts. Virtually all on-road diesel sold in regulated markets today is ULSD — the term simply distinguishes it from older, higher-sulphur formulations that are now banned for road use in most countries.
Sources and further reading
- Diesel fuel explained US Energy Information Administration, eia.gov
- EN 590 automotive diesel fuel standard European Committee for Standardisation, cen.eu
- Fuel prices tool and grade lists OpenVan.camp, openvan.camp/compare/fuels/fuel-prices