Strict rules on fuel cleanliness mean more work for filters

Jan 4, 2016 by Capt. Jeff Werner

It is a common belief that the diesel engine fuel filter, installed by the engine manufacturer, will provide the required level of contamination protection to achieve reliable operation. This is a fair and reasonable assumption because the intended purpose of that filter is to “achieve a fuel cleanliness that will enable the fuel injection components to function reliably and within their designed operating parameters throughout the life of the component.”

However, engine fuel filters alone cannot achieve the stringent cleanliness level recommended by high pressure common rail (HPCR) fuel injector manufacturers.

Most major engine manufacturers have determined that fuel must meet or exceed a cleanliness level between 1,300 to 2,500 contaminant particles of four microns in size per milliliter of diesel. To put this into perspective, the average red blood cell is eight microns in diameter, so these organic and inorganic particles are very small indeed.

But the manufacturers of HPCR fuel injectors rely on a higher cleanliness level between 20 to 40 particles four microns in size per milliliter.

The typical passive diesel fuel filtration system installed on a HPCR engine uses both a primary filter and a secondary filter. The primary filter is usually plumbed between the fuel tank and the engine, while the secondary filter is frequently mounted on the engine and is fed by the engine’s low pressure pump. Manufacturers will modify this design depending on the requirements of their engine and their research into what they believe works best.

The primary filter removes larger particles that can damage the low pressure pump and also separates free water from the fuel. The secondary filter removes the smaller particles, about four microns in size, which can damage the downstream engine components such as the high pressure fuel pump and the fuel injectors.

Some new filter designs use a single-stage system that incorporates the water removal capability of the primary filter with the high efficiency particle removal of the secondary filter.

The distribution of contaminant particle sizes within diesel fuel is vastly different from other hydrocarbons. Lubricating oils, engine oils and hydraulic oils typically have a wide distribution of particle sizes within a given sample.

A typical sample of diesel fuel, however, comprises about 80-97 percent of particles at or below four microns in size. With such a high percentage of particles in the fuel at the same size that causes the greatest damage in a HPCR injection system, filter companies have changed the filtration size on engine fuel filters to below four microns.

This transition to a lower micron rated filter has led to a reduction in the life of the filter element because the physical size of the filter canister has not changed. In effect, the same amount of fuel is being pushed through a smaller “screen size”, which causes the filters to work overtime, and they can get plugged up more quickly.

Engine fuel filters are typically changed on a time-based planned maintenance strategy that is measured in hours of engine use. Engine manufacturers list the fuel filter change interval in their operation or service manuals. Today, most HPCR engine manufacturers schedule engine fuel filter changes at 500 hours. However, as the filter micron ratings have been reduced to target particles below four microns, some engine manufacturers advise their customers to reduce the fuel filter change interval to 250 hours.

This reduction in the preventive maintenance interval helps ensure that the engine fuel filter will not get completely blocked. This is, however, an expensive fix and would not be required if the fuel in the tank was clean enough to not overload the engine fuel filter in the first place.

Filters are not intelligent devices; they remove contaminants in the fuel as they pass through the filter media. Reducing the level of contaminants before the fuel reaches the engine filter will increase its service life and dramatically reduce the amount of dollars spent on filters every year.

Having a diesel dialysis system on board to clean the fuel, and used regularly as part of the fuel’s planned maintenance program, will easily assure low counts of particles smaller than four microns. This reduces the risk of damage to and the costly repair of the components of the high pressure common rail injector system. Reliable fuel does matter.

Capt. Jeff Werner has been in yachting for almost 25 years. Contact him through



About Capt. Jeff Werner

Capt. Jeff Werner has been in yachting for more than 20 years on private and charter yachts, both sail and power. He is an instructor for RYA, MCA, USCG and US Sailing courses and owns Diesel Doctor (

View all posts by Capt. Jeff Werner →

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