The most efficient way and the best available technology (BAT)
to radically reduce the critical diesel emission components
particles (PM&NP) and nitric oxides (NOx) are
combined exhaust gas aftertreatment systems (DPF+SCR). SCR
(selective catalytic reduction) is regarded as the most efficient
deNOx-system, diesel particle filters are most efficient
for soot abatement. Today, several suppliers offer combined systems
for retrofitting of HD vehicles.
The presented results are part of the work in the international
network project VERT *) dePN (de-activation, de-contamination,
disposal of particles and NOx), which has the objectives
to establish test procedures and quality standards and to introduce
the SCR-, or combined DPF+SCR-systems in the VERT verification
procedure.
Examples of results for some of the investigated systems are
given and the most important findings are: - the average
NOx conversion rate at transient operation strongly
depends on the operation load profile, on the exhaust gas
temperature and the resulting urea dosing control, - the particle
number filtration efficiency, which is verified at stationary
engine operation, is valid also at the transient operation, -
secondary nanoparticles are produced due to urea injection, they
nevertheless do not impact significantly the overall filtration
efficiency of the system (here: DPF upstream & SCR downstream,
differences of PCFE in the range of 0.1%), - the OEM
NOx-sensors of the investigated systems are appropriate
tools for the in-use control, - the system with catalyzed DPF
(upstream) attains higher overall deNOx-efficiencies due
to NO₂-production in the DPF, - for the investigated systems there
are no critical emissions of unregulated components, NH₃ &
N₂O.