How does it work?
The oil
pump,
driven by the engine, aspirates the oil via a sieve (to prevent
aspiration of large contaminants) from the carter
and
pumps it towards all places
that
need lubrication.
Oil filters
can be
designed as 'spin-on filter' or as 'filter-cartridge' in a
housing. A spin-on filter is replaced completely after
its useful
life whereas in the
other
version only the cartridge needs to be replaced in the housing.
Different types of oil filters:
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Full-flow filters |
|
By-pass filters |
|
Full-flow/ By-pass filters (combination) |
Full-flow filters
All the oil aspired by the oil pump passes the oil filter before going to the
places that need lubrication.
Since the oil pump is directly related to the engine rpm, the oil pressure
needs to be limited by an overpressure valve, incorporated in the oil pump
itself.
Usually this overpressure valve limits the pressure in the
filter-lubrication-system to between 3 and 5 kg/cm2.
Should
the pressure rise above this limit, then the risk exists that
gaskets and simmering are damaged or pressed out of their
sittings. Sometimes, when the
pressure
rises too high and the overpressure valve in the oil pump does
not open, either the gasket of the oil filter is blown out
of its sitting and oil
starts
leaking out and/or the oil filter housing starts bulging. The
oil filter then gets unjustly blamed for a problem caused
by the overpressure valve in the
oil pump.
When the engine and the pump are cooled off (when repairing)
the valve seems to be working normally again, eliminating
evidence of the real cause of
the problem.
To prevent a too big pressure difference between the dirty
and the clean side of the filter element, a by-pass valve
is mounted in the filter.
This
valve opens at a pressure difference of 1 to 2 kg/cm2; this
way (unfiltered) oil can still lubricate the engine, even
with a completely plugged
filter
element.
Furthermore an anti-drain valve prevents the filter to drain its oil back to the
carter when the engine is stopped. In some cases an anti-siphon pipe is present
to avoid oil siphoning out of the filter when the engine is stopped. Pressure
drop and filter-life (dust loading capacity) are designed according the
requirements of the OEM. Therefore care must be taken to have a filter that
meets these specs rather than any filter that 'will fit’, usually sold at
a lower purchase cost than a high quality filter, correctly designed for the
specific application.
By-pass aspirated/ combination filters
The
oil
aspired from the carter is pumped straight to the areas that
need lubrication. In a line parallel to the main line,
the
by-pass filter is located.
About
10% of the main oil flow is by-passed to a parallel line to
be filtered. The by-pass ratio is calibrated by a fixed
restriction
in the by-pass filter.
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Although the main oil flow goes unfiltered to the engine, the efficiency of the
by-pass filter is very high.
In
contrast to air filters which are 'single pass', an oil filter
is 'multi-pass' (the same oil is repeatedly passing the same
filter); when a
particle
passes through the mainstream, next time it will be caught
in the by-pass. As such, it takes longer for a by-pass system
to catch all particles.
Full-flow
versus by-pass provide therefore the same level of protection
for the engine. A combination of full-flow and by-pass filters
is usually applied in
the
heavy-duty markets (diesel engines in truck-, industrial-,
and earthmoving applications). About 90% of the oil is filtered
through the full-flow filter
whereas
the remaining 10% returns to the carter via the by-pass filter.
The result of this filter combination is a very good conditioning
of the oil,
whereby
the life of oil and filter is increased drastically.
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