EN 15751 is the European test method for oxidation stability of FAME and FAME/diesel blends containing at least 2% v/v FAME, run at 110 °C and measuring induction periods up to 48 hours. It is the method EN 590 references for automotive diesel, which requires a minimum induction period of 20 hours for blends containing 2% FAME or more.
The 20-hour blend limit against the 8-hour B100 limit confuses people regularly. The two numbers are not inconsistent — they measure different products against different service expectations, and the blend requirement is genuinely more demanding relative to what the fuel contains.
Why blends need 20 hours when B100 needs 8
Three reasons.
Longer service life. B100 is generally consumed relatively quickly by a specialist user. Automotive diesel sits in a distribution chain — refinery tank, terminal, tanker, retail tank, vehicle tank — and may be months from blending to combustion. Standby generator and agricultural fuel sits far longer.
Less tolerance for consequences. Modern common-rail injection systems operate at extreme pressures with fine clearances. Gums and insolubles that a robust older engine tolerated will foul a modern injector.
Diesel itself is less stable than it used to be. Deep desulfurisation to 10 mg/kg removes naturally occurring sulfur and nitrogen species that had an incidental antioxidant effect. Low-sulfur diesel is a less protective matrix than the diesel that preceded it, so adding an oxidisable component to it is more consequential.
The result: a small percentage of FAME disproportionately determines blend stability, and the standard sets the bar accordingly.
The standards landscape
| Standard | Product | Requirement | Method |
|---|---|---|---|
| EN 14214 | B100 (neat FAME) | min 8 h at 110 °C | EN 14112 or EN 15751 |
| EN 590 | Automotive diesel, FAME ≥2% v/v | min 20 h at 110 °C | EN 15751 |
| EN 590 | Automotive diesel (all) | max 25 g/m³ total insolubles | EN ISO 12205 |
| EN 16568 | FAME/diesel blends ≥2% v/v | alternative method | 120 °C, up to 20 h |
EN 590 currently permits up to 7% v/v FAME (B7). B10 is specified separately where adopted; vehicle warranty coverage above B7 remains limited, which is one reason B10 uptake has been uneven across Europe.
Verify limits against the current published editions before contractual use. EN 15751 was most recently revised in 2025.
What determines blend stability
The FAME quality going in
The dominant factor. A B7 blend cannot be more stable than the arithmetic allows if the FAME component is marginal. FAME meeting EN 14214 at exactly 8 hours — with no antioxidant margin — will drag the blend down. FAME dosed to 12 hours contributes proportionally more.
Practical implication for blenders: specify FAME above the EN 14214 minimum, not at it. Buying to 8.0 hours is buying a blend problem.
Diesel base stock quality
Hydrotreated, deeply desulfurised base stock has low natural antioxidant content. Cracked components — light cycle oil, coker distillate — contain reactive olefins and nitrogen species that reduce stability further. A blend built on a highly cracked base needs more antioxidant protection than one built on straight-run.
Trace metals
Copper is the worst offender, and it is common in fuel systems — brass fittings, bronze components, copper tubing. Iron from tanks and lines contributes. Both catalyse radical initiation at very low concentrations. Where metal contact is unavoidable, metal deactivators chelate the catalytic species and protect the antioxidant package. Metal deactivator and antioxidant work together — neither substitutes for the other.
Water and microbial activity
Water at the tank bottom supports microbial growth, and FAME is a more attractive nutrient source than mineral diesel. Microbial metabolites are acidic and accelerate degradation, while biomass fouls filters directly. Blend stability programmes that ignore water management address only half the problem. See bacteria management.
Storage conditions
Temperature, air exposure and time. Every one of them works against the fuel. A fixed-roof tank in summer with high turnover breathing is a worse environment than a floating-roof tank in winter.
Meeting 20 hours in practice
Start with the FAME specification. Set your purchase spec above 8 hours. This is the cheapest intervention available because it moves cost to the FAME producer, who can treat more efficiently at production than you can post-blend.
Dose the blend, not just the FAME. Antioxidant added to B100 protects the FAME but distributes into the blend at 7% of its original concentration. Post-blend dosing is often more efficient at achieving blend specification. Both approaches are used, and which is better depends on your supply chain.
Establish a dose–response curve for your blend. As with B100, response is non-linear and matrix-specific. Test your actual base stock and FAME combination across a dose range rather than applying a standard treat rate.
Add early. Antioxidants prevent oxidation; they do not reverse it. Once the induction period has fallen, dosing recovers less than dosing the same fuel fresh would have.
Monitor through the chain. Test at blending, at terminal despatch, and periodically in storage. A single blending-point result does not describe what arrives at the retail tank.
Symptoms of blend instability
- Filter blocking and short filter life — see filter blocking tendency improvers
- Rising acid number in storage
- Injector deposits and fouling
- Darkening, haze and sediment
- Elevated total insolubles against EN ISO 12205
Haze is worth distinguishing from oxidation. Water-induced haze is a separate problem addressed by dehazers and demulsifiers, not by antioxidants. Confusing the two leads to dosing the wrong chemistry at the wrong point.
Frequently asked questions
What is EN 15751?
The European test method for oxidation stability of FAME and FAME/diesel blends containing at least 2% v/v FAME, by accelerated oxidation at 110 °C, measuring induction period up to 48 hours.
What is the EN 590 oxidation stability requirement?
Minimum 20 hours induction period by EN 15751 for diesel containing 2% v/v FAME or more, plus a maximum 25 g/m³ total insolubles by EN ISO 12205.
Why is the blend limit 20 hours when B100 only needs 8?
Blends face longer distribution and storage periods, modern injection systems are less tolerant of deposits, and deeply desulfurised base diesel has lost the natural stability contributors that higher-sulfur diesel had.
What FAME content is allowed in EN 590 diesel?
Up to 7% by volume (B7). B10 is specified separately where adopted, with more limited vehicle warranty coverage.
Should antioxidant be added to the FAME or to the finished blend?
Both approaches are used. Dosing FAME at production is efficient for meeting EN 14214; dosing the finished blend can be more efficient for meeting the EN 590 20-hour requirement. The right answer depends on supply chain structure and should be established by testing.
Does EN 15751 apply to HVO blends?
No. HVO is a paraffinic hydrotreated diesel covered by EN 15940, not a methyl ester, and it does not present the same oxidation mechanism.
Where to go next
The 20-hour blend requirement is usually met or missed at the FAME purchase specification, well before any antioxidant is dosed.
Rodanco supplies fuel antioxidants and metal deactivators for FAME and finished blends, with dose–response screening on client samples through our laboratory.
See also biodiesel antioxidants for the B100 side, and our downstream additives range.
Rodanco B.V. is a specialty chemicals supplier based in Alkmaar, the Netherlands, serving refiners, blenders, terminals and biofuel producers across Europe.
Related articles
References
- BS EN 15751:2025 — scope, 110 °C, induction period measurable to 48 h, applicability to blends of at least 2% (V/V) FAME, and the note on cetane improver reducing measured stability. https://www.en-standard.eu/bs-en-15751-2025-automotive-fuels-fatty-acid-methyl-ester-fame-fuel-and-blends-with-diesel-fuel-determination-of-oxidation-stability-by-accelerated-oxidation-method-at-110-c/
- Fuels: EU — Automotive Diesel Fuel. EN 590:2009 introduced the EN 15751 oxidation stability requirement with a 20 h limit. DieselNet. https://dieselnet.com/standards/eu/fuel_automotive.php
- Guidelines for handling and blending FAME, Report No. 9/09 — notes that the EN 590 Rancimat limit applies only to diesel containing 2% v/v FAME or more. Concawe. https://www.concawe.eu/wp-content/uploads/rpt_09-9-2009-05088-01-e.pdf
- Laboratory Oxidation Stability Study on B10 Biodiesel Blends. Concawe. https://www.concawe.eu/wp-content/uploads/rpt_13-10-2013-02818-01-e.pdf
- Evaluation of the oxidation stability of diesel/biodiesel blends — on the 20 h minimum induction period at 110 °C for finished blends. Fuel, ScienceDirect. https://www.sciencedirect.com/science/article/abs/pii/S001623611000147X
About this article
Written by the Rodanco technical team. Rodanco B.V. operates an in-house laboratory in Alkmaar, the Netherlands, running screening, compatibility testing and tailored formulation work, and a field service team conducting on-site trials and dosing optimisation across European and global energy markets.
Standards notice. Values quoted from CEN standards are given for guidance. Standards are periodically revised and national annexes differ. Always verify against the current published edition before contractual or specification use.
Last reviewed: August 2026