Parts of the
aftertreatment
system explained
Randy Griffith, Sales & Tech Manager for Emission
& Cooling Solutions provides details on the definitions
for each aspect of the engine aftertreatment
system in commercial vehicle diesel engines:
DIESEL OXIDATION CATALYST (DOC)
“The DOC is the first thing you get to coming off of
the turbo in the emissions system, as far as big components,”
Griffth explains. “The DOC’s sole purpose
is to bring up the temperature in the exhaust system
to create the reaction from soot, which is a bigger
carbon, into ash, which is a smaller carbon.”
The smaller particles can be more easily stored in the
filtration system, or diesel particulate filter (DPF).
DIESEL PARTICULATE FILTER (DPF)
Diesel engine manufacturers have selected DPF
technology as their prime path for reducing particulate
emissions. The DPF accomplishes this
by mechanically filtering the exhaust gases.
Griffith equates the DPF to a trash can, which collects the
residual particles that have been heated by the DOC.
“I always use the analogy, if you take a 55-gallon drum,
and you fill it full of trash, and then you light it, at the
end of that burn you’re going to have a layer of stuff
that can’t be burned,” Griffith explains. “If you do
that a thousand times, you’ve got a whole bunch of
stuff that can’t be burned that fills up your trash can.
You have to pull it out and dump the trash can.
“DPF cleaning is basically the dumping of the
trash can. The taking out of that stuff that can’t
be burned anymore or reduced in size.”
SELECT CATALYTIC REDUCTION
(SCR) SYSTEMS
Implemented in 2010 and newer heavy duty diesel vehicles,
the select catalytic reduction (SCR) system allows commercial
vehicles to reduce NOx emissions. This is done through
an oxidation process, with the use of diesel exhaust fluid.
November/December 2018 | VehicleServicePros.com 27
and a select catalytic reduction (SCR) system
to reduce NOx emissions.
“Diesel-powered commercial equipment
manufactured beginning in 2007 contained the
DOC and DPF only,” explains Gilbert Ramirez,
manager, heavy duty product management at
Denso. “Th e EPA introduced the 2010 greenhouse
gas emissions requirements to eliminate
NOx from the emission stream in 2010, which
led to the inclusion of the SCR to the aft ertreatment
system.”
Denso, a supplier of OEM and aft ermarket vehicle
parts, and AP Emissions Technologies, a global
manufacturer and distributor of vehicle emissions
control systems and products, have partnered to
manufacture and market a new line of OE-quality
DPF and DOC products for heavy duty on-road
and off -road vehicles and equipment.
“What the DOC does is it changes the chemistry
in the exhaust stream and utilizes heat to
burn the soot into ash, and the DPF stores the
ash until it’s time to clean and have it removed,”
says John Lightner, Cummins technical sales
support manager. “Th e SCR system, through a
chemical reaction, by the introduction of the
diesel exhaust fl uid (DEF), changes the chemistry
and eliminates the NOx and greenhouse
gases, so that you have a little water and nitrogen
coming out of the tailpipe. You’re not having
the carbon dioxide coming out that gives you the
greenhouse gas.”
Cummins designs, distributes and services
diesel and natural gas engines and related technologies,
including fuel system, controls, air
handling, fi ltration, emission solutions and electrical
power generation systems.
Th e Technology & Maintenance Council’s
(TMC) Recommended Practice (RP) 355 on
Maintenance and Inspection Guidelines for
OEM-Installed Exhaust Particulate Filters for
Diesel-Powered Vehicles provides details on the
numerous original equipment diesel particulate
fi lter (DPF) technologies for heavy duty commercial
vehicle diesel engines. In addition, RP 355
off ers information about how the DPF works,
and details on how to maintain the system and
diagnose issues.
According to RP 355, the aft ertreatment system
can help reduce particulate matter through a
number of methods, including:
Improved air management (e.g. optimized
turbocharger)
Improved combustion system (e.g. combustion
bowl shape and location)
Improved oil control (e.g. piston ring design and
cylinder bore honing)
Improved injection system (e.g. optimized injection
duration, optimized injector hole size)
Rated speed optimization (e.g. rated engine speed)
Find the root cause of
aftertreatment issues
While traditionally many have pointed to the DPF
as the cause of aft ertreatment issues, fl eets should
note problems with the DPF are typically symptomatic
of an issue further upstream in the system.
“If your engine is not running correctly, then
it’s going to emit more emissions and can plug up
your fi lter and create more maintenance,” says
Cummins’ Lightner.
Th ere are two areas to address when it comes
to servicing the aft ertreatment system: proper
diagnosis, and routine maintenance. Emission
& Cooling Solutions’ Griffi th advises setting a
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