There are several reasons why a fuel can fail FBT testing but the most common effects are the following:

• Cold flow additives de-bonding within the fuel
• Waxing of fuel, due to summer grade been affected by environmental impacts
• Metal Carboxylates (Soaps)

The above can be classed as soluble compounds, just like adding sugar to warm water, the sugar now becomes soluble

Insoluble compounds can be in the form of induced contamination as particulate, just like we experience dust build up in our homes from particulates in the atmosphere.

Soluble and Insoluble compounds and effects

Insoluble compounds within fuel will fall partly under Total contamination within the fuel specifications.

A measurement you may be familiar with is the Particle measurement of liquids referenced to as ISO4406, however ISO4406 is independent of any fuel or oil specification but may be used by OEM manufacturers stating the fuels ISO4406 target level of particulate cleanliness prior to engine filtration.

The world fuel Charter recommend that fuel should be supplied at a ISO4406 particulate level of 18/16/13. Measuring fuel particle contamination considers both the size and number of particles per size class contained in the fuel, i.e. the particle size distribution. The ISO 4406 protocol provides a means of expressing the level of contamination by coding the size distribution. Three code numbers, corresponding to the numbers of particles of size greater than 4, 6 and 14 µm per millilitre.

Particulate Contamination

Fuel injection equipment manufacturers continue to develop fuel injection systems to reduce emissions and fuel consumption and to improve performance. Injection pressures have been increasing; currently, they have reached more than 2000 bars. Such levels of injection pressure demand reduced orifice sizes and component clearances, typically from 2 to 5 µm in injectors. Small, hard particles, which may be carried into these engine parts, are potential sources of engine failure. Excessive diesel fuel contamination can cause premature clogging of diesel fuel filters, depending on the level of both hard and organic particles, and premature wear of modern fuel injection system parts. These impacts, depending on the size and the nature of the particles, will lead to: – Reduced part lifetimes; – Part malfunction; – Engine failure; and – Increased exhaust emissions.

Link to the document https://www.acea.auto/publication/worldwide-fuel-charter-2019-gasoline-and-diesel-fuel/ page 94 onwards for Diesel

Insoluble contamination can be removed from the main fuel tanks by fuel polishing, utilizing different size micron rated media, this would be recommended on a fuel report, works summary after both specification and contamination test suits have been conducted.

Testing just ISO4406, water content and Biological activity is only a snapshot of contamination and does not identify out of specification fuel based on the fuel properties.



 The fuel may be out of specification due to soluble compounds suspended within the fuel that will give a high ISO4406 count and the recommendation for fuel polishing would be the wrong course of action, as soluble compounds are not affected by fuel polishing. If we go back to our sugar example once solubilised the sugar is suspended as a liquid within the warm water.