Partial Filters
In terms of filtration efficiency, partial filters fill a gap between diesel oxidation
catalytic converters and wall-flow filters, with up to about 50% reduction in particulate
matter being possible over a duty cycle. Rather confusingly they are known by several
different names, including ‘flow-through filters’ and ‘particle catalytic converters’,
but from now on we’ll stick with the term ‘partial filters’.
The central idea of a partial filter is that it has channels which are significantly
bigger than the size of particles it is intended to trap, and the trapping effect
only occurs because the gas containing the particles is persuaded to follow a non-straight
path. Like any filter, the storage capacity of a partial filter is finite, but an
important feature is that even when the storage capacity has been used up, there
are still open paths which gas can follow to enable the engine to continue running.
For this reason, it is not necessary to use a complex monitoring system such as
is required for a wall-flow filter, which reduces the purchase and installation
cost significantly. Of course the intention is not for the filter to fill up once
and then stay full. Partial filters are designed to be regenerated by nitrogen dioxide
(NO2) which is produced either by a diesel oxidation catalytic converter located
upstream, or by a catalytic coating applied to the partial filter itself. The production
of NO2 is temperature dependent, and an ideal application for a partial filter will
have exhaust gas temperatures of at least 350 C for at least 30% of the duty cycle.
The presence of the diesel oxidation catalytic converter or the catalytic coating
on the filter means that significant reductions in carbon monoxide and hydrocarbon
emissions are achieved as well, once the exhaust gas temperature has reached the
necessary level. As with a diesel oxidation catalytic converter, partial filters
must only be used with fuels that are very low in sulphur i.e. below 50 ppm.
(Diagram: Chemical reactions in a partial filter)
There are many different approaches to making a partial filter, however the one
which has gained the most acceptance in heavy-duty markets is based on alternating
layers of corrugated metal foil and fleece made from metal fibres. The corrugated
foil has miniature aerofoils pressed into it, and these deflect the exhaust gas
so that it impinges on the fleece layer. If the fleece has storage capacity available
then the exhaust gas will pass through, leaving its solids content behind, whereas
if the fleece is full the exhaust gas will continue along the channel to the next
aerofoil. If the fleece becomes full at every point, then the filtration efficiency
will be reduced to zero but at least the filter will not become blocked. The illustration
below shows this kind of filter, preceded by a diesel oxidation catalytic converter
to facilitate the regeneration.
