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
Diesel is a liquid fuel refined from crude oil, burned in engines that ignite it by compression alone rather than with a spark plug. It's the dominant fuel for trucks, buses and heavy machinery worldwide, and a common choice for passenger cars in much of Europe.
It packs more energy into each litre than petrol, one reason diesel vehicles often travel further on the same volume of fuel, though the two aren't directly interchangeable between engines built for one or the other.
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
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.
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