Due to the independent operation of the aftertreatment system
from the engine, the aim of the OBD system (On Board Diagnostic) is
to guarantee the powertrain emissions stay within the emissions
standards during all the vehicle useful life. In the case of
heavy-duty diesel vehicles, that uses the SCR system (Selective
Catalytic Reduction) as the aftertreatment technology to meet the
stringent emissions levels, the use of a NOx sensor in the exhaust
pipe will be indispensable for a reliable monitoring of the
emissions.
Analyzing the European directives that are legislating the OBD
system, the NOx sensor must be monitored by the emissions control
system, guaranteeing faults detection in the aftertreatment system
that could increase the emissions. According to the annex 4 of the
directive 2005/78/EC item 3.3.2.1 (requirements for the OBD system
Stage 2), it is needed to monitor the catalyst efficiency as an
isolated component of the system.
In case of efficiency reduction of the aftertreatment system,
due to the low quality of the SCR reagent liquid, physical damage
or catalyst aged, the current softwares are ready to detect the
faults, but providing a generic and inaccurate diagnostic.
The SCR catalyst aging, due to the high level of sulfur in the
diesel, to the lubricant oil burning or to the high exhaust gas
temperature, will be verified in real applications when the SCR
system is implemented. When it is aged, the main feature of the
catalyst (the ammonia storage capacity) is reduced, which can cause
ammonia emissions (slip) in the ambient. The treatment of this
particular feature can also be found in sophisticated software
strategies, but applying a theoric factor to the catalyst aging
based on the system operating conditions to be applied as a
correction factor in the closed-loop dosing control strategy.
This work consists in the development of a software algorithm to
be integrated to the OBD system that allows diagnosing, through a
dynamic functional test, a NOx conversion efficiency reduction in
the SCR catalyst as an isolated component of the complete system.
With this diagnostic, applied within the strategies for catalyst
aging detection, the dosing controller can correct the reagent
liquid injection, preventing ammonia emissions in the ambient, and
getting a faster and easier fault resolution.