Diesel Particulate Filters
Exhaust gas from diesel engines contains solids known as diesel particulate matter,
which is hazardous to health. This particulate matter is largely comprised of carbon,
but also contains hydrocarbons and a certain amount of metallic particles as a result
of engine wear and the burning of lubricating oil.
The role of a diesel particulate
filter is to separate these from the exhaust gas stream. However, the amount of
particulate matter produced by even a modern diesel engine is quite significant,
so the issue of filter blockage soon arises, and preventing such blockage is a paramount
consideration to an engineer in this field. One obvious option is to simply throw
the filter away when it is dirty, and use of such disposable filters is a viable
option for exhausts which only need to be filtered intermittently. If an exhaust
needs to be filtered on a long-term basis, then a means of periodically cleaning
the filter (preferably without removing it from the engine) must be found.
Fortunately
diesel particulate matter can be made to burn given the right conditions, producing
mainly harmless carbon dioxide and water vapour as a result, so nearly all strategies
for cleaning diesel particle filters rely on burning (or oxidation) for their effectiveness.
The term ‘regeneration’ is used to refer to a process for cleaning a diesel particulate
filter. Apart from the disposable variety, there are two main categories: ‘wall-flow’
filters which achieve filtration efficiencies of 95% plus, and ‘partial’ filters
which can facilitate up to about 50% reduction.
