Husqvarna 371K Workshop Manual Supplement page 4

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– English
FILTER SYSTEM
Husqvarna Air Injection is a filter system that efficiently cleans the engine intake
air in three separate stages that use different methods of filtering.
The obvious practical benefit of Air Injection is that the service interval is consid-
erably longer than with previous systems.
1. Centrifugal cleaning is the first stage of filtering the intake air on the 371K/
3120K Air injection. Centrifugal cleaning was originally only used on larger ma-
chines in dusty environments, e.g. on contracting machinery (cyclone air cleaning).
The fan blades on the flywheel supply cooling air to the cylinder while also
forming the active component of the centrifugal cleaning system for the engine
intake air. An intake nozzle is located close to the fan blades.
Centrifugal force prevents large particles from following the bent air stream into
the nozzle. Instead they are thrown past the nozzle. Only very small dust particles
are able to follow the diverted air stream towards the engine air intake. Tests show
that up to 80% of all dust is separated out by centrifugal air cleaning.
2. The foam plastic air filter is the next separation stage in cleaning the air. The filter
covers the whole area of the engine cover and thanks to this design has a total filter
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area of 3.5 dm (371K), 3.7 dm
the entire filter area so that the filter is equally effective across its whole area.
The filter is soaked with oil and has three layers with different pore sizes.
The route the air takes through filter is like a labyrinth. Dust particles that come
into contact with the filter material do not bounce off the surface, but are instead
caught by the sticky oil. A dry foam plastic filter is much less effective at filtering
air than one soaked with oil.
An oiled foam plastic filter is by far the most effective filter for stone dust, since
the entire volume of the filter is used as a dust trap, not just the surface. The total
dust-capturing area of the filter is therefore extremely large. The foam filter
absorbs around 95% of the total volume of dust that passes through the centrifu-
gal cleaning stage. The foam filter can be washed clean and must be soaked in
clean oil during servicing.
3. The paper filter takes care of the small amount of dust that escapes through
the foam filter, mostly by chance. Only a very small amount of very fine dust
particles reaches the paper filter. The filter's dense, mesh-like structure of
cellulose fibres traps these particles. The paper filter also acts as a protective
filter during a filter service. The paper filter must be replaced at each service.
Husqvarna air filter oil
Use Husqvarna's pine needle oil, which is made from renewable ingredients and
is completely biodegradable, for oiling foam plastic filters. This oil can be washed
off with soap and water. It collects and traps dust particles very effectively in the
filter, leading to a cleaner engine and less wear. The oil is thixotropic, which gives
it an actively sticky surface, and it does not contain solvents or ingredients that
dry out.
Soak the filter thoroughly and squeeze out.
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(3120K). The filter base plate spreads the air over