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Diesel Fuel Cleanliness Standards

The awareness of fuel cleanliness in diesel engines is not new – in fact, it was first documented as early as 1931 with the introduction of the Caterpillar Diesel Sixty Tractor. The fuel injection pressure developed by the engine was only 1,250 psi, yet the caption from an article of the time tells a powerful story and it should be reflected on in respect to today’s engines.


In today’s world, defining how clean or dirty fuel is is critically important, and as such, fuel cleanliness levels are now measured and reported according to the ISO Cleanliness Code 4406:1999. The International Organization for Standardization (ISO) created the cleanliness code ISO4406:1999 to quantify particulate contamination levels per millilitre of fluid at three sizes: 4μ, 6μ, 14μ.

Scale NumberParticles per ml. More ThanParticles per ml. Less or Equal

Fuel Cleanliness Vs. Engine Technology

Fuel cleanliness levels using the ISO4406:1999 method were officially documented as a global standard only as recently as 1998 with the development of the Worldwide Fuels Charter (WWFC). Since its inception, the charter has established a minimum cleanliness level for each of the diesel fuels under various available categories around the world.

WWFC Diesel Category Fuel Cleanliness Standards

Diesel CategoryCleanliness Level
110 mg/kg
2ISO 18/16/13
3ISO 18/16/13
4ISO 18/16/13

Most mainstream engine OEM’s now subscribe to these standards. Interestingly, however, and somewhat troubling to note, is that fuel cleanliness levels being specified by engine OEM’s and the WWFC have not changed since their inception in 1998, despite the enormous advances in fuel injection technology. This relationship is best represented in the previous table that identifies the advances in fuel injection systems and clearly highlights how OEM’s and the WWFC have not responded to reduce fuel cleanliness in accordance with advancements in technology.

Diesel Fuel Injection – Advancing Technologies & Cleanliness Levels

EUI16,000 psi1-4 cSt5-8μm18/16/13
HPCR36,000 psi1-4 cSt1-4μm18/16/13

This table identifies that over time, fuel injector critical clearances have halved and fuel pressures have doubled, yet the level of fuel cleanliness being specified has not altered in accordance with such advancements. In fact, the same cleanliness levels specified in 2000 are still being used today despite these magnificent technological advancements.

Leading fuel injector manufacturers around the world have clearly identified and communicated that they require UCD fuels with fuel cleanliness levels as low as ISO12/9/6 to maintain ultimate performance and reliability. It is here where we see an enormous mismatch in what the fuel injection OEM desires as a fuel cleanliness level, to what the engine OEM’s and the WWFC are advising the industry. The following table identifies the discrepancies in fuel cleanliness levels.

Diesel Cleanliness Levels

CompanySpecified ISO Cleanliness Level
World Wide Fuel Charter18/16/13
Engine OEM’s18/16/13
Fuel Injector OEM12/9/6

Owners and operators of diesel engines should take note that the cleanliness levels required by the fuel injector OEM’s are 64x cleaner than what the industry is being advised as an acceptable standard of fuel for engines. This is a critical point when considering contamination control solutions for diesel fuel systems.

Fuel Cleanliness – Deciphering the Code

When referring to the ISO4406:1999 cleanliness code, it can sometimes be difficult to comprehend how much contamination the codes actually represent in a real world operating system. To help clarify this, we can use a real world scenario that is more meaningful. The example used to demonstrate this is as follows:

Model: Cummins DQKB Diesel Generator
Engine: QSK60-G3
Duty: Continuous
Rating: 1450kW (1813kVA)
Cycle: 60Hz
Load: 100% Peek Load
Run Time: 5000 hrs/year
Fuel: 74.8 US GPH (283 L/hr)

The following table provides an indication of how much contamination is being presented to the engine fuel system each year under this example.

Volume of Contamination

Fuel CleanlinessVolume of Contamination
ISO 23/21/1826.4 lb/year
ISO 18/16/130.8 lb/year
ISO 12/9/60.10 lb/year