What is the EN590 specification for diesel fuel?

This article will answer the following questions?

  • How do I know if my fuel is of a good quality?

  • Why do I need to test my diesel fuel?

  • What does BS EN590 mean when I see it written on fuel pumps?

  • How can a laboratory confirm if fuel is of a good quality or become contaminated or adulterated?

  • What effect does biodiesel have on my vehicle?

  • Why misfueling is a serious problem and can potentially impact a warranty claim?

The EN590 Specification is the legal specification for road diesel in the UK. You will see it written on the fuel pumps when you re-fuel and is a requirement by all the UK fuel suppliers to meet this specification when supplying fuel to consumers.

The specification involves a batch of laboratory tests that must be performed by an independent laboratory prior to a batch of fuel going on sale. Each test is designed to confirm different aspects of the fuel composition to ensure the fuel you buy meets the highest quality standards you expect. The testing can also be performed post sale as part of a vehicle or bulk storage tank check to confirm the fuel has not become contaminated after leaving the fuel producer.

Some of the tests include:

  • Appearance (visual) – this is a visual check of the sample to confirm it is clear and bright as diesel should look and be free from any visible sediment or free water. If the fuel appears discoloured, cloudy this can indicate possible contamination has taken place and will usually be supported by failed lab test results for other parameters too.
  • Cetane Index – this is a similar reading to the more widely known octane rating in petrol (gasoline) engines. The higher the cetane Index the lower the ignition delay. The value should be a minimum of 46 to meet the specification and to avoid running issues. The most common cause of failure is contaminated or adulterated fuel. You may have also come across aftermarket cetane improver additives which can be added to improve this rating for certain applications. However, it is always advisable to check with your engine manufacturer before using any aftermarket products as the engines are typically designed based on EN590 fuel rather than a modified version.
  • Cold filter plugging point (CFPP) – It may be surprising to hear that diesel is not the same everywhere and during the different seasons the blends are changed to suit the seasonal temperatures. The reason for this, is diesel is not one single chemical but a whole range of different hydrocarbon chain lengths and the heavier paraffinic chains form waxes during cold temperatures. These waxes can block filters and stop a vehicle from starting during cold weather. Hence, during winter months additional cold flow additives are added to disrupt wax chain formation and in some cases slightly lighter blends may be preferred depending on the regional geography across the world. In the UK in terms of fuel blending, Summer is classed from the 16th March to 15th November, whilst Winter is classed as 16th November to 15th March. The laboratory testing of CFPP aims to replicate the situation where a fuel filter would become blocked by cold formed waxes, but problems of partial blocking may already be occuring before this temperature is reached and a cloud point can help indicate this point I.e. the point where the fuel begins to cloud. The maximum temperature for a CFPP in the UK is -5’C during for summer blends and -15’C for winter blends i.e. well beneath a typical UK weather. Although not part of an EN590 Specification, cloud point is used in many other fuel specifications and as a general guide only, as it is not a mandatory requirement for fuel you would expect a summer cloud point of less than +3’C and a Winter Cloud Point of less than -5’C. Typically operating conditions of fuel tanks that are 10’C above the cloud point you can be reasonably confident of no cold wax issues, but a common problem that can occur is after prolonged cold temperatures even upon warming the wax does not instantly melt and the temperature sometimes has to be significantly raised to break the wax crystals from the fuel. That is why for bulk storage, tanks are now often heated to avoid such issues.
  • Density @ 15’C is quite a standard physical property you can measure of a fluid. The causes of low and high densities are usually contamination with low density e.g. petrol/ gasoline or high density e.g. biodiesel products. The minimum and maximum values for the EN590 diesel are between 820 and 845 kg/m3.
  • Distillation properties – diesel is a range of hydrocarbon molecules to make a liquid that will easily be burnt in a diesel engine. Too light a blend can result in problems such as pre-ignition where the fuel ignites before top dead centre, whilst too heavy a blend can result in higher running temperature of the engine and general running issues. Hence there are some set parameters set out to ensure the fuel suppliers are supplying fuel that is suitable for an engine designed to run on EN590 fuel as all UK diesel engine road vehicles (DERV) are. These include: recovery at 250’C max of 65%, which means at 250’C the max amount of fuel that should have boiled off is 65%. This threshold prevents too volatile fuels being classed as diesel. On the other hand, recovery at 350’C Min of 85% and Min 95% recovery at 360’C ensures the fuel will actually combust because 85% of the fuel has boiled off by 350’C and by 360’C 95% has too. It is uncommon to see any of these parameters fail laboratory testing unless the fuel has been contaminated with another fuel type that has lower or higher boiling range than diesel.
  • Fatty Acid Methyl Ester (FAME) / Biodiesel – you may have heard of biodiesel, which is a renewable form of fuel, which can now be up to 7% of all diesels under UK legislation to reduce dependency on fossil fuels. FAME is made from heavy fats and oils including the most common which is Rapeseed oil (RME). However FAME also covers products derived from used cooking oils, animal fats (Tallow), Soya oil, Palm oil and sunflower oil. The fats by themselves are far too heavy to be useful fuels, and so they are split into smaller methyl ester molecules in a process called trans-esterification. Thie involves mixing 1 part fat to 3 parts methanol with a catalyst such as potassium hydroxide to produce 3 parts FAME, 1 part glycerol. FAME, even though it is lighter than the original fats, is not very similar to the petrochemical diesel as it has a much higher flash point (>100’C) and density (860-900 kg/m3) than diesel. Hence it is often diluted with petrochemical diesel to bring it’s combustion properties back to a diesel like product. It is sold as it’s percentage amount of FAME in diesel e.g. B7 is 7% FAME to 93% diesel and B20 is 20% biofuel content. The mixes can go to B50 and even B100, although B7 is the max allowable under the EN590 as the fuel becomes less diesel in behaviour the more FAME that is added. Although the FAME content of fuel is consistently rising to meet environental regulations and the EN590 will likely have to change the 7% max threshold when the legislation demands higher biofuel content, it is wise to ensure the vehicle can handle higher FAME contents before using levels above the EN590 such as B20 or B50. This is because biofuel is still a relatively new technology when compared to diesel itself and so engine manufacturers have to play catch-up to make the vehicles able to handle this new type of fuel. Equally known issues such as tendency of FAME to absorb water can mean using too high a FAME content can potentially cause engine combustion issues. It is also worth noting this is a UK regulation, so if you fill-up your vehicle abroad you may find the diesel there has higher FAME contents.
  • Flashpoint (closed) – it is a common misconception that diesel is a flammable liquid and is often because both Petrol and Diesel are used in road vehicles they are assumed to be as flammable as Petrol/gasoline. In fact, diesel is not particularly flammable at all and that is why spark ignition type systems are not used and instead the combustion works on compression instead. It may also confound most to discover that liquids do not actually burn, only the vapours. This has an impact on transportation and storage of fuels where the vapours cannot escape to the air. These vapours collect in the headspace above the fuel over time and in the presence of a flame or spark can ignite. However for sufficient vapours to collect the liquid needs to be hot enough to produce them. For a liquid such as petrol/gasoline anything above -46’C is suffient for the fuel to flash, meaning pretty much everywhere on earth it’s going to be a fire risk. Diesel by contrast must be greater than +55’C to pass EN590 so at room temperature you should not be able to ignite the vapours, which is why it is much less of a fire risk than petrol. Now that FAME is added to diesel the flashpoints tend to be even higher with typical flash points being 60 to 75’C, and even higher with higher FAME contents. It is not only safety that flashpoint is tested, but a low flashpoint fuel can actually lead to damage to injectors and the engine itself as it burns and ignites at the wrong time within the engine. The reason for a low flashpoint is usually down to contamination with a low flash point product such as Petrol or kerosene. There is no maximum limit for flashpoint, however unusually high flashpoints of >90’C are likely because of high biofuel contents, or adulteration of the fuel with lubricating oils or other high flash point products. Although not a fail in flashpoint as there is no maximum, in these cases other test parameters such as the distillation properties or FAME content will likely be highlighted as out of specification.
  • Sulphur content – sulphur has been actively decreased in fuels over the years with road vehicles having to meet the ever tightening emission regulations first, shortly followed by off-road machinery and finally by marine diesels. Sulphur is both beneficial and harmful to the engine so it’s removal has also meant other additives need to be added to replace the beneficial parts of sulphur. So why is sulphur bad? The most obvious reason is sulphur when burnt in your engine forms sulphurous oxides that when mixed with water make sulphuric acid. Sulphuric acid is both harmful to engines by its high acidity on metal components, and when it leaves the vehicle exhaust where it causes acid rain. Sulphur however is a great antiwear additive, so it’s removal meant to protect the fuel pump from wear FAME is now used to replace this functionality of improving lubricity of the fuel. For road diesel the maximum sulphur content is 10mg/kg. Reasons for high sulphur are usually down to contamination with non road diesel, which contains high sulphur or lubricating oil, which too contains high a sulphur content.
  • Total Contamination (insolubles/sediment) – this measures the non-dissolved content of the fuel, which is usually dirt. Although it will pass at point of refinery testing this is the most common cause of failure on fuel samples from vehicle tanks. This is because the diesel specifications requirements are actually very clean. Hence, a few small specs of dirt can cause contamination to go above the max of 24mg/kg threshold. When interpreting the data it is worth considering the actual value rather than just as a pass or fail. So 26 and 126mg/kg results are very different. For instance a sample of fuel from a vehicle with no problems, using good quality fuel and having been well maintained, depending on the working environments of the vehicle could have a marginal fail of e.g. 26mg/kg, which is a technical fail and depending on your warranty agreement could mean it is invalidated if it is black and white in terms of pass and fail. However, most fuel systems filtration capabilities can handle these marginal fails and it is not until serious contamination is present problems occur, beyond what is expected as normal day to day contamination ingress into the system. It is also worth noting the sampling location when assessing this information as a bottom of a tank sample will contain a higher concentration of sediment as it settles over time.
  • Viscosity @ 40’C – this is the thickness of the fuel. Since the fuel is also the lubricant to moving parts within the fuel system it needs to have sufficient viscosity to reduce wear, but also not be too thick to be difficult to pump. The allowable range of viscosities is from 2 to 4.5mm2/s. It’s is very unusual for this parameter to fail and usually requires considerable contamination from a lower viscosity product such as petrol or a higher viscosity product such as lubricating oil to show a noticeable change in viscosity of the fuel
  • Water content – the process of combustion and water clearly don’t go well together. This is why there is a max limit of 200mg/kg (0.02%). The usual causes for this being high are fuel storage tank breathing (the repeated introduction of fresh air in the system containing water vapour, which condenses on cold tank surfaces). Sampling location is also key for this test as water will collect at the bottom of tanks so drawing from the bottom of a tank for a sample or when fueling the vehicle will increase the likelihood of water ingress. It is best practice on bulk storage to use dessicated breathers to dry air entering the tank and regularly drain off water from the bottom of any bulk storage. With high FAME fuels it is also worth noting these will have a tendency to absorb water and so extra vigilance should be taken in reducing water ingress. If water ingress does occur then paper filters will likely need changing too as they swell and warp in the presence of water meaning large debris particles can pass through the filter. Equally, the wet filter is a perfect breeding ground for microbes (fuel bugs) that block filters and fuel system pathways.
Min and Max figures represent the current EN590 standard at time of writing 2018.