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 Number | Particles per ml. More Than | Particles per ml. Less or Equal |
| 22 | 20,000 | 40,000 |
| 21 | 10,000 | 20,000 |
| 20 | 5,000 | 10,000 |
| 19 | 2,500 | 5,000 |
| 18 | 1,300 | 2,500 |
| 17 | 640 | 1,300 |
| 16 | 320 | 640 |
| 15 | 160 | 320 |
| 14 | 80 | 160 |
| 13 | 40 | 80 |
| 12 | 20 | 40 |
| 11 | 10 | 20 |
| 10 | 5 | 10 |
| 9 | 2.5 | 5 |
| 8 | 1.3 | 2.5 |
| 7 | .64 | 1.3 |
| 6 | .32 | .64 |
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.
| Diesel Catagory | Cleanliness Level |
| 1 | 10 mg/kg |
| 2 | ISO 18/16/13 |
| 3 | ISO 18/16/13 |
| 4 | ISO 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.
| Injection | Pressure | Viscosity | Clearances | Cleanliness |
| EUI | 16,000 psi | 1-4 cSt | 5-8μm | 18/16/13 |
| HPCR | 36,000 psi | 1-4 cSt | 1-4μm | 18/16/13 |
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.
| Company | Specified ISO Cleanliness Level |
| World Wide Fuel Charter | 18/16/13 |
| Engine OEM’s | 18/16/13 |
| Fuel Injector OEM | 12/9/6 |
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.
| Fuel Cleanliness | Volume of Contamination |
| ISO 23/21/18 | 26.4 lb/year |
| ISO 18/16/13 | 0.10 lb/year |
| ISO 12/9/6 | 0.8 lb/year |
Reference: Cleanliness Standards
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