In an engine the clearance between
the piston ring and liner bore is extremely small,
therefore it is the small light airborne dust particles
which are the biggest threat when a leak occurs in an
induction system. Under
ideal conditions the working surfaces of a component are
kept apart by a thin film of oil. This oil film prevents
direct contact between the surfaces, reducing the amount
of friction and the rate of wear. The oil film will also
absorb shock loads and help distribute the load over the
whole surface. The introduction of even a small amount of
dust into this environment will seriously disrupt this.
Once the dust particle ahs entered the oil film it forms
a direct link between the two surfaces, nullifying the
effect of the oil film.
The first and immediate
effect is a "scratching" of the surface as the
particle is dragged and rolled across the surfaces. The
second and potentially more serious problem is that once
the dust particle is introduced in between the two
surfaces, it changes the loading of the surface from an
even distribution to a point load concentrated on the
particle with a tremendous increase in pressure at this
point.
The increase in pressure
causes a deflection of the surface, which will eventually
result in metal fatigue and the surface breaking up.
The solution is to keep
the dust out. To do this, design engineers use air
cleaners, breathers and seals at any point that dust may
enter .
Air cleaners filter
air as it is drawn through a system, e.g. in engines,
compressors, etc. They have to deal with large volumes of
air, as an engine needs up to 2000 litres of air to burn
1 litre of fuel efficiently. The most common type of air
cleaners have three main components: a centrifugal
pre-cleaner, a large pleated paper element and a smaller
paper or cloth "safety element:.
The pre-cleaner consists
of radial fins and a collection bowl. The fins cause
incoming air to swirl rapidly around the inside of the
housing. The heavier dust particles are thrown outward by
centrifugal force and are channeled into the dust
collection bowl. Some bowls have an automatic discharge
valve, other bowls have to be removed from time to time
to clean out the accumulated debris before it overflows
into the filter housing.
The main filter element
traps the remaining dust particles as the air is drawn
through the paper pleats.
The smaller safety element
is fitted after the main element in case the main element
is damaged in any way. Its purpose is to trap any dust
introduced when the main filter is changed. It does not
filter as effectively, however, nor can it hold the same
volume of dust.
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