Understanding the ISO 4406 fuel cleanliness standard

Curt Dunafin

Today’s High Pressure Common Rail (HPCR) diesel engines are designed to reduce emissions and fuel consumption while improving horsepower and performance. To do this, component orifice sizes and clearances have become much tighter, 2-5 microns (µm), and the fuel is pressurized at over 30,000 psi. These high pressures and tight tolerances mean that smaller particles, which may be carried into engine parts, are potential sources of engine failure. Excessive particle contamination can cause premature clogging of diesel fuel filters and wear of modern fuel system components. This wear leads to increases in emissions, malfunctions and failures. For this reason, we are seeing equipment manufacturers referring to the ISO 4406 Fuel Cleanliness ratings in manuals, warranties and fuel testing. They would like to see fuel that is dispensed into the equipment tank that meets an 18/16/13 rating.

What is an ISO 4406 Fuel Cleanliness rating?

The ISO 4406 Fuel Cleanliness standard establishes a framework for quantifying and reporting the particulate contamination levels in liquid fuels. It utilizes a coding system based on the number and size of particles present in a fuel sample. The standard defines cleanliness levels using a three-number code, such as ISO 18/16/13, where each number represents the maximum particle count in >4 µm, >6 µm and >14 µm size ranges. More specifically, the 18/16/13 rating refers to a maximum count of 2,500 particles >4 µm, 640 particles >6 µm and 80 particles >14 µm per ml of fuel.

Where do particles come from?

To begin with, the air in the tank contains particles that make their way into fuel, such as pollen, dust, smoke, equipment exhaust and bacteria. Secondly, moisture is found in the air and will build up through condensation during storage. Moisture leads to microbial growth, corrosion and degradation of the fuel. Even if the fuel looks clear, particles below 40 µm are not visible with the naked eye without a magnifying lens while particles smaller than 10 µm are visible with a microscope.

How do I improve my fuel rating?

Following a proactive fuel quality management program of diligent water inspections and remediation along with selecting proper filtration such as a 10 µm (or smaller) microglass media filter is recommended. This aids in keeping the particle count of the fuel dispensed into the equipment below the 18/16/13 rating.

For help in designing a fuel quality management program and selecting the proper filtration, please contact your local FS energy specialist.

Curt Dunafin serves as GROWMARK’s energy services manager.

Curt Dunafin