Cetane number: diesel's answer to octane rating
Cetane number measures how readily diesel ignites under compression, the opposite of what octane rating measures for petrol. A higher cetane number generally means smoother running and easier cold starts, not more power or better fuel economy.
In this entry
What cetane number measures
Cetane number measures how readily diesel fuel ignites once it's compressed in an engine cylinder, with no spark needed. A higher cetane number means a shorter delay between injection and ignition, which is what a diesel engine needs to run smoothly.
It's the diesel equivalent of octane rating in the sense that both are fuel-quality numbers printed on standards documents, but the property they measure is essentially the opposite.
Cetane vs octane: opposite properties
This is the point most worth getting right, because the names invite confusion.
Opposite properties, similar-sounding names
A higher octane rating means petrol resists igniting too early. A higher cetane number means diesel ignites quickly and reliably when it's compressed. Both "higher is generally better," but they describe opposite chemical behaviours, so the numbers aren't comparable to each other at all.
How it's measured
Cetane number can be found two ways: by running the fuel in a dedicated test engine, or by estimating it from other measurable properties.
Typical values
Minimum cetane requirements are set by national or regional fuel standards, and premium diesel is often marketed with a higher figure as part of its appeal.
Illustrative scale. The EU's EN 590 standard sets a minimum cetane number of 51; premium diesel is often marketed somewhat higher.
Why it matters in practice
A higher cetane number shows up in a few practical ways for a driver, even if it doesn't change fuel economy on its own.
- Easier, quicker cold starts, since the fuel ignites more reliably in a cold engine.
- Smoother running and less of the knocking diesel clatter, since combustion starts more evenly.
- Often marketed with premium diesel, alongside other additives, as a higher-performance option.
How this shows up on FuelTheGuide
We show diesel and premium diesel as separate grades where a country publishes both, since a numeric cetane figure isn't part of the underlying price data.
We show diesel and premium diesel as separate grades where a country publishes both, but we don't link a numeric cetane figure since it isn't part of the price data.
Questions and answers
Does a higher cetane number mean more fuel economy or power?
Not directly. It mainly affects how readily the fuel ignites, which shows up as easier starting and smoother running, not as a meaningful change in fuel economy or engine power on its own.
Is cetane number the same everywhere in the world?
No. Minimum requirements vary by country. The EU mandates a minimum of 51 for standard diesel under EN 590. The US sets a minimum of 40 under ASTM D975, though most fuel exceeds that. Premium diesel in most markets is formulated to a higher cetane figure than the legal minimum.
Can I raise the cetane number of diesel I already have?
Yes, with cetane improver additives, sometimes sold as diesel treatment products. They can raise the effective cetane number a few points and may help with cold-start behaviour, but they don't change the underlying fuel chemistry as much as using a higher-grade diesel from the pump. The US DOE Alternative Fuels Data Center covers diesel fuel properties and additives.
Does biodiesel affect cetane number?
Yes, usually upward. Most biodiesel feedstocks produce a fuel with a higher cetane number than petroleum diesel, so blending biodiesel into diesel (B5, B7, B20) tends to raise the blended fuel's cetane number slightly.
Sources and further reading
- Diesel fuel quality and cetane number US Department of Energy, Alternative Fuels Data Center, afdc.energy.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