Urban Buses
Reducing emissions from buses was identified as a priority many years ago, as a
result of their concentration in urban areas, long service lives and the fact that
they often receive public subsidies. Governments in many countries provided grants
and tax incentives to encourage bus operators to retro-fit their older vehicles
with wall-flow diesel particulate filters. Despite a high take-up, many retro-fit
programmes were not wholly successful as the first-generation technology available
at the time was not ideal for vehicles with predominantly low exhaust gas temperatures
resulting from operation in congested city centres. The result was frequent filter
blockages which are still causing problems for maintenance teams today. In addition,
the systems offered by most suppliers had the unfortunate side-effect of increasing
emissions of nitrogen dioxide, which is itself toxic.
Fortunately the technology for reducing emissions from diesel engines has improved
significantly in recent years, as demonstrated by the low emissions and high reliability
of Euro 4 and Euro 5 vehicles. Improved technology is also now available in the
retro-fit market, and this can be applied successfully to Euro 1, 2 and 3 buses.
Partial filters are a recent innovation which can achieve worthwhile emissions reductions
from older buses at low cost, and also have the advantage of being maintenance free.
Wall-flow filters are the choice where the virtual elimination of particulate matter
is required, and if correctly specified and maintained, are now very reliable.
(Diagram: Passive wall-flow filter as commonly installed on buses)