DESCRIPTION AND OPERATION
Page 45
PARTIC ULATE MATTER
The compounds remaining in the exhaust after
passing through the EGR system contain
extremely small particles of soot and ash called
particulate matter (PM). When these particles
contact certain metals under high temperature,
they are chemically transformed. The metals that
cause this transformation are called catalysts.
CATALYTIC FILTERS
A catalytic filter (converter) is a porous form (like
a sponge) that has been coated with a catalytic
material. The flow of exhaust gas must pass
through the pores on its way to the atmosphere.
As a result, a high percentage of the PM is
transformed.
33
Figure 33 — Diesel Particulate Filter Diagrams
DIESEL OXIDATION CATALYST
A catalytic filter, the diesel oxidation catalyst
(DOC) reduces emissions in diesel exhaust by an
average of approximately one third over a system
without a DOC. Its honeycomb-like structure
(called substrate) is coated with an active
catalyst. The PM is changed to carbon dioxide
and water. The substrate can be made of a wide
range of materials. Ceramic (cordierite) and
sintered metal are common.
Some PM usually remains unconsumed by the
heat and collects on the filter. Eventually, the
residue must be removed by other means.
Remove the filter and physically remove the
remaining material. There is a special machine to
do this cleaning.
DIESEL PARTICULATE FILTER
Basically a trap, a diesel particulate filter (DPF)
collects the soot and ash to hold until they can be
burned off. A catalyzed DPF also transforms
unburned fuel, engine lubricant and carbon
monoxide into carbon dioxide and water.
Depending on vehicle vocation, the operating
temperature of the exhaust may or may not be
sufficient to consume the trapped soot and ash. If
necessary, a means of elevating the temperature
is included in the system.
FILTER REGENERATION
Cleaning a filter so it can continue to function is
called “regeneration.” High heat with a catalyst
breaks down the chemical properties. Depending
on the vehicle usage, the engine will be equipped
with a passive or an active regeneration system.
As with any filter, eventually the DOC and the
DPF will become clogged. Rising back pressure
caused by plugging the exhaust system
decreases fuel economy and reduces engine
efficiency. It is necessary to clean these filters
from time to time.
The filters can be regenerated to continue in use.
The high temperature needed to complete the
process exists in the exhaust itself. Because
some vehicle vocations do not allow the exhaust
temperature to rise to, or continue at, adequate
levels, a device to control the temperature is
included.
PASSIVE REGENERATION
For vehicles that produce and maintain adequate
exhaust temperature, the soot and ash are
consumed at a rate that prevents clogging the
filter prematurely. Setting a DOC ahead of the
catalyzed DPF in the exhaust stream reduces the
temperature required to regenerate the filters.
However, the process is not complete. Eventually,
the filter must be taken out to remove the
accumulated residue by other means.
1. Diesel Oxidation Catalyst
2. Catalyzed Diesel
Particulate Filter
3. Non-Catalyzed Diesel
Particulate Filter
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