Servicing
Wall-flow filters are usually very reliable as long as they were correctly specified
for the application in the first place, however they need regular inspection and
servicing to keep them that way.
Most wall-flow filters these days are fitted with a monitoring system which stores
data on exhaust gas temperature and backpressure. The first task of an inspection
service is to download this stored data to a laptop to see whether the backpressure
is staying within acceptable limits. If the backpressure is getting too high then
it is likely that either the filter is not regenerating frequently enough or that
there is a build up of ash which needs to be removed. In many cases the frequency
of regeneration can be increased by means such as improving the insulation of the
exhaust pipe, increasing the dose of fuel-borne catalyst or persuading the driver
to switch off the engine when it is not required. On the other hand, if the backpressure
is too low then the filter might be cracked or the exhaust pipe leaking. The other
important tasks of an inspection service are to check the exhaust system for cracks
or defective mountings, check the exhaust insulation and top up the fuel borne catalyst
tank if applicable.
An inspection service may highlight the need for ash removal, which is typically
required at intervals of 1000 hours for systems using fuel-borne catalyst and slightly
longer for other systems. This involves removing the filter and cleaning it in a
special machine which uses air pulses to dislodge ash from the filter pores and
collects this ash for safe disposal. Companies offering a filter cleaning service
often provide a ‘clean for dirty’ exchange service to minimise downtime. Some companies
simply blow out the ash using an airline but this is not nearly as effective as
using a proper machine, meaning that the job needs doing more frequently (there
is also a risk of ash being released or inhaled).
(Photo: Thermal regeneration of wall-flow filter)
If a filter fails to regenerate then it will become blocked by diesel particulate
matter and may reach the point where it is incapable of in-situ regeneration. This
is especially likely if the filter relies on a catalytic coating since such a coating
cannot work unless it is exposed to exhaust gas. In these situations it is necessary
to remove the filter and thermally regenerate it in an oven. The usual approach
is first to blow off as much loose DPM as possible in a pulse cleaning machine,
then the filter is placed in an oven and the temperature is gradually raised to
about 400 C. At this temperature, the DPM is ‘dried’ as hydrocarbons are evaporated
off. After a suitable period of time at 400 C the temperature is raised again to
600 C at which point the carbon component of the DPM will be burned. The complete
process, including cooling down at the end, takes about a day so it is common for
the company providing this service to work on an exchange basis.