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Fuel Filtration

Frequently Asked Fuel Filtration Questions
FUEL FILTRATION Frequently Asked Questions

Q1: Please explain the difference(s) between the
primary and secondary fuel filters in terms of the type Q4: How have new engine designs affected fuel
filtration?
of medium used, micron rating, and so forth.
Differences between primary and secondary filters vary In the past, diesel engines had either mechanical fuel
from system to system, but in general, primary filters injectors or unit injectors. The drive to develop engine that
are used to separate water and larger particles (7-25 µm meet emissions regulations has led to the application of
efficiency). Secondary filters are for final filtration (3-5 µm common rail fuel injection systems. The higher pressures of
efficiency). Primary filters usually will have treated media common rail systems enables more precise control of fuel
to provide water separation performance. This can be either delivery and control of the combustion process. The goal of
cellulose or a multi-layered synthetic media called melt- the new technology is to reduce the particulate matter and
blown coupled with cellulose like Donaldson’s Synteq TM NOx coming out of an engine system, thereby reducing the
media. Secondary filters have untreated, multi-layered burden on after treatment systems.
cellulose or purely synthetic media. These differences
mainly have to do with the water separation requirements The very high pressures in the common rail systems
placed on primary fuel filters. require tighter tolerances, elevating the requirements for
cleanliness and efficiency on new and future fuel systems.
This has created the need for increasingly better fuel
filtration technology. Donaldson offers a range of products
Q2: Have micron (µm) ratings become smaller and for those demanding conditions and is developing solutions
smaller as injection technology has advanced? When for tomorrow’s requirements.
replacing filters, how do you make sure you have the
micron rating that’s appropriate for your generation
of engine and its injection system? Q5: Will common rail systems bring any changes in
terms of fuel filter requirements? If so, can you say
As injection technology has advanced and injection system what will they be?
pressures have increased the filtration requirements have
become more demanding. These systems have required
filtration technology to be more and more efficient. Most fuel injection systems today are already common rail
When replacing your filters be sure you use an OEM or close derivatives. The technology itself does not drive
approved replacement or a direct cross from a reputable specific changes, the injection pressures and desired filter
filter manufacture to ensure you are using a filter that is service intervals are more influential.
appropriate for your engine.
Q6: How important is filtering fuel stored in bulk
Q3: Some truckers used to use a fine primary filter tanks?
to avoid changing the secondary, while the original It’s becoming very important and can reduce future vehicle
equipment concept was to use a coarse primary maintenance downtime. If you’re using a bulk fuel tank,
(on the suction side) and a fine secondary (on the filtering the fuel BEFORE putting in your vehicle is another
pressure side). This took extra changes, but they liked great practice that can reduce contaminant and water from
the idea of avoiding changing the secondary. Is doing the fuel before refilling your vehicle tank. Over times, tanks
this impractical on modern engines? can corrode, water condensation can build up, contaminant
could enter the tank opening during fills.
Primary and secondary filters are usually balanced to
provide the required engine protection and the optimum
filter life. Placing a fine filter in a primary (suction) filter
location is impractical because they can not tolerate as
much pressure drop and will need to be changed very often.
Generally, fine filters do not contain the require water
separation in a primary filter.

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