⛽ Oxidation stability of FAME and of diesel fuel with FAME — the accelerated oxidation method (Rancimat)
In short
A stream of purified air (10 ± 1 l/h) through the sample at 110 °C; the volatile acids pass into water with a conductometric electrode — a sharp rise of the conductivity marks the induction period (up to 48 h). The test is performed according to PN-EN 15751:2014-05; Range: An induction period up to 48 h; FAME and blends with ≥ 2 % (V/V) of FAME; not for pure petroleum diesel fuels. The procedure comprises 6 steps; it is used for: Quality control of FAME as a blending component and as a fuel in its own right (PN-EN 14214: ≥ 8 h), Control of diesel fuel with FAME ≥ 2 % (V/V) according to PN-EN 590 (≥ 20 h), Assessment of the effectiveness of antioxidants and of additive packages (the Oil and Gas Institute's studies of B10 fuels).
At a glance
- Standard: PN-EN 15751:2014-05
- Category: Petrochemistry
- Procedure steps: 6
- Temperature: 110 °C in the usual application; the heating block controllable to (150 ± 0.1) °C
- Flow of air: (10 ± 1.0) l/h, air purified through a 0.3 nm molecular sieve
- Range: An induction period up to 48 h; FAME and blends with ≥ 2 % (V/V) of FAME; not for pure petroleum diesel fuels
Overview
The standard specifies a method of testing the oxidation stability of fuels for diesel engines by measuring the induction period of the fuel up to 48 h; it applies to fatty acid methyl esters (FAME) intended as a biofuel in their own right or as a blending component of diesel fuel and to blends of FAME with diesel fuel containing at least 2 % (V/V) of FAME. The method is based on EN 14112, developed for pure FAME; modifications made it possible to cover blends, but a new validation showed that the method is not suitable for pure petroleum diesel fuels. The present description is based on the 2014 edition (PN-EN 15751:2014-05), which the Polish Committee for Standardization has withdrawn and replaced by PN-EN 15751:2025-10 — the changes are to be checked before implementation.
The 2014 edition limited the scope to an induction period of 48 h (corresponding to the range of the precision), pointed out a possible change of the induction period in the presence of cetane number improvers (limited tests with EHN — 2-ethylhexyl nitrate — showed a fall within the limits of reproducibility) and gave the result of a check on paraffinic diesel fuels from Fischer-Tropsch synthesis and from hydrotreatment (no other interaction; their stability usually exceeds the range of the method). The induction period is the time from the start of the measurement to the moment when the formation of the products of oxidation begins to rise sharply; the oxidation stability is the induction period in hours.
The requirements: according to PN-EN 590 the oxidation stability of diesel fuel with FAME should be at least 20 h, and that of FAME according to PN-EN 14214 at least 8 h. Diesel fuel with FAME degrades during storage in contact with oxygen (oxidation and autoxidation) and undergoes thermo-oxidative decomposition at higher temperatures; the Oil and Gas Institute uses the Rancimat method beside PetroOXY (PN-EN 16091) for the assessment of B10 fuels with additive packages.
Method principle
A stream of purified air is passed through the sample heated to the target temperature (110 °C in the usual application). The volatile compounds formed in the process of oxidation pass with the air into a vessel with demineralised or distilled water fitted with a conductometric electrode connected to a measuring and recording device; the end of the induction period is marked by a sharp rise of the conductivity caused by the dissociation of the volatile carboxylic acids formed in the oxidation and absorbed in the water. The apparatus (commercially available for example as the Rancimat, model 743 or higher, or the OSI Instrument) comprises an air filter with a molecular sieve, a diaphragm pump with an adjustable flow of (10 ± 1.0) l/h, borosilicate reaction vessels with a closure and gas inlet and outlet tubes (projecting at least 130 mm above the heating block, so that the volatile constituents of the fuel condense; a constriction to break the foam), closed measuring cells of about 150 ml, electrodes for 0–300 µS/cm, an amplifier with a recorder and a cast aluminium heating block controllable to (150 ± 0.1) °C (alternatively an oil bath).
Applications
- Quality control of FAME as a blending component and as a fuel in its own right (PN-EN 14214: ≥ 8 h)
- Control of diesel fuel with FAME ≥ 2 % (V/V) according to PN-EN 590 (≥ 20 h)
- Assessment of the effectiveness of antioxidants and of additive packages (the Oil and Gas Institute's studies of B10 fuels)
- Studies of the stability of fuels during storage (beside PetroOXY and the peroxide value)
Key parameters
| Parameter | Value |
|---|---|
| Temperature | 110 °C in the usual application; the heating block controllable to (150 ± 0.1) °C |
| Flow of air | (10 ± 1.0) l/h, air purified through a 0.3 nm molecular sieve |
| Range | An induction period up to 48 h; FAME and blends with ≥ 2 % (V/V) of FAME; not for pure petroleum diesel fuels |
| Measurement | The conductivity of the water in a measuring cell of about 150 ml, electrodes for 0–300 µS/cm |
| Reaction vessel | Borosilicate glass, ≥ 130 mm above the block; for example 250 mm long (Rancimat 743), 300 mm (OSI) |
| Requirements | Diesel fuel with FAME: ≥ 20 h (PN-EN 590); FAME: ≥ 8 h (PN-EN 14214) |
Standard
- Standard number
- PN-EN 15751:2014-05
- Title (PL)
- Paliwa do pojazdów samochodowych — Estry metylowe kwasów tłuszczowych (FAME) jako samoistne paliwo i ich mieszaniny z olejem napędowym — Oznaczanie stabilności oksydacyjnej metodą przyspieszonego utleniania
- Title (EN)
- Automotive fuels — Fatty acid methyl ester (FAME) fuel and blends with diesel fuel — Determination of oxidation stability by accelerated oxidation method
Step-by-step procedure
-
Sampling
Take the sample according to EN ISO 3170 or EN ISO 3171, or according to the national regulations on the sampling of fuels.
-
Preparation of the apparatus
Wash the reaction vessels with the methanol/toluene/acetone mixture, with the alkaline solution and with 2-propanol; dry the molecular sieve (150 °C) and fill the air filter; check the temperature of the block with the calibrated thermometer.
-
Measuring out and assembly
Measure a test portion of the sample into the reaction vessel (pipettes or measuring cylinders), fill the measuring cell with distilled or demineralised water, fit the electrode and connect the gas tubes with PTFE or silicone tubing.
-
Measurement
Place the vessel in the heating block at the target temperature of 110 °C, switch on the flow of air of (10 ± 1.0) l/h and the recording of the conductivity.
-
Determination of the induction period
Read from the conductivity curve the time from the start of the measurement to the sharp rise of the conductivity (the end of the induction period); give the result in hours as the oxidation stability.
-
Assessment
Compare it with the requirements: diesel fuel with FAME ≥ 20 h (PN-EN 590), FAME ≥ 8 h (PN-EN 14214); induction periods above 48 h lie outside the range of the precision of the method.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Apparatus for the determination of oxidation stability | For example the Metrohm Rancimat 743 or higher, the Omnion OSI Instrument (examples given by the standard, without recommendation) | — |
| Air filter with a molecular sieve | A tube with filter paper at its ends, filled with a 0.3 nm molecular sieve with a moisture indicator, dried at 150 °C | — |
| Diaphragm pump | An adjustable flow of (10 ± 1.0) l/h | — |
| Reaction vessels and measuring cells | Borosilicate vessels with a closure, gas tubes and an anti-foam constriction; cells of about 150 ml with a tube reaching the bottom and vent holes | — |
| Conductometric electrodes and recorder | Range 0–300 µS/cm; an amplifier and a recorder for the signal of each electrode | — |
| Heating block or oil bath, thermometer | An aluminium block controllable to (150 ± 0.1) °C with a thyristor and a contact thermometer reading to 0.1 °C or a Pt100; a thermometer calibrated from 0 to 150 °C in 0.1 °C steps; an oven for (150 ± 3) °C; PTFE or silicone tubing | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Distilled or demineralised water | — | For the measuring cells (the absorption of the volatile acids) |
| A mixture of methanol/toluene/acetone 1:1:1 (V/V/V) | — | For washing the glassware |
| Alkaline solution for washing laboratory glassware | — | Cleaning of the reaction vessels |
| 2-propanol | 67-63-0 | For rinsing the glassware |
| 0.3 nm molecular sieve with a moisture indicator | — | Dried at 150 °C and cooled in a desiccator before use |
Health and safety (OHS)
- A heating block at 110–150 °C and hot glass vessels — heat-resistant gloves
- The washing solvents (methanol, toluene, acetone, 2-propanol) are flammable and toxic — work in a fume hood
- Volatile products of oxidation (carboxylic acids) — lead the outlet of the measuring cells into a fume hood
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN 15751:2014-05 — “Automotive fuels — Fatty acid methyl ester (FAME) fuel and blends with diesel fuel — Determination of oxidation stability by accelerated oxidation method”.
How does this method work?
A stream of purified air is passed through the sample heated to the target temperature (110 °C in the usual application). The volatile compounds formed in the process of oxidation pass with the air into a vessel with demineralised or distilled water fitted with a conductometric electrode connected to a measuring and recording device; the end of the induction period is marked by a sharp rise of the conductivity caused by the dissociation of the volatile carboxylic acids formed in the oxidation and absorbed in the water.
What is the measuring range and accuracy?
Temperature: 110 °C in the usual application; the heating block controllable to (150 ± 0.1) °C; Flow of air: (10 ± 1.0) l/h, air purified through a 0.3 nm molecular sieve; Range: An induction period up to 48 h; FAME and blends with ≥ 2 % (V/V) of FAME; not for pure petroleum diesel fuels; Measurement: The conductivity of the water in a measuring cell of about 150 ml, electrodes for 0–300 µS/cm.
How long does the test take?
The procedure comprises 6 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Apparatus for the determination of oxidation stability, Air filter with a molecular sieve, Diaphragm pump, Reaction vessels and measuring cells, Conductometric electrodes and recorder, Heating block or oil bath, thermometer.
Where is this test used?
Quality control of FAME as a blending component and as a fuel in its own right (PN-EN 14214: ≥ 8 h); Control of diesel fuel with FAME ≥ 2 % (V/V) according to PN-EN 590 (≥ 20 h); Assessment of the effectiveness of antioxidants and of additive packages (the Oil and Gas Institute's studies of B10 fuels); Studies of the stability of fuels during storage (beside PetroOXY and the peroxide value).
What safety precautions apply?
A heating block at 110–150 °C and hot glass vessels — heat-resistant gloves; The washing solvents (methanol, toluene, acetone, 2-propanol) are flammable and toxic — work in a fume hood; Volatile products of oxidation (carboxylic acids) — lead the outlet of the measuring cells into a fume hood.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN 15751.