5. CATALYSTS FOR DIESEL PARTICULATE FILTERS
5.1. The Particulate Filter
The wall-flow particulate filter is today the
most efficient device for reducing diesel soot emissions,
attaining filtration efficiencies of the order of 90% at nominal operation conditions. Soot
filtering is especially important in the case of
worn-out diesel engines with poorly controlled fuel
combustion. Specific application problems, basically
related to filter durability, have limited the use of
particulate filters, mainly on city buses, some delivery
trucks and fork lift trucks. Intensive research is aimed
at developing diesel filter systems suitable for a wider
application to commercial vehicles or passenger cars.
The particulate filter concept has focused research
and development activities around the world, and a
variety of systems is offered by various manufacturers.
A trap oxidizer system is based on a
durable temperature resistant filter, which removes
particulate matter from the exhaust before it is
emitted to the atmosphere. The accumulated particulate raises filter back pressure, i.e. the pressure
difference across the filter which is necessary to
force the exhaust through it. The typical back pressure level depends on the filter type, and
increases as the filter becomes loaded with particulate. High back pressure is undesirable, since it
increases fuel consumption and reduces available
power. It is necessary to clean the filter periodically by
burning off (oxidizing) the collected particulate; this
process is known as regeneration.