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Review of Diesel Emissions and Control
Abstract:
This review summarizes the latest developments in diesel
emissions regarding regulations, engines, NOx (nitrogen oxides)
control, particulate matter (PM) reductions, and hydrocarbon (HC)
and CO oxidation.
Regulations are advancing with proposals for PN (particle
number) regulations that require diesel particulate filters (DPFs)
for Euro VI in 2013-14, and SULEV (super ultra low emission
vehicle) fleet average light-duty (LD) emissions likely to be
proposed in California for ~2017. CO₂ regulations will also impact
diesel engines and emissions, probably long into the future.
Engine technology is addressing these needs. Heavy-duty (HD)
research engines show 90% lower NOx at the same PM or fuel
consumption levels as a reference 2007 production engine. Work is
starting on HD gasoline engines with promising results. In light
duty (LD), engine downsizing is progressing and deNOx is emerging
as a fuel savings strategy.
Much has recently been reported on optimized selective catalytic
reduction (SCR) systems. The SCR catalyst can be placed before,
after, or on the DPF. Work is progressing on non-urea ammonia
systems, mixed zeolite catalysts, and on fundamental understanding
on issues like ammonia storage, sulfur impacts, and reaction
mechanisms. Developments on HC-based deNOx, like lean NOx traps
(LNTs), result in a better understanding of durability, reduction
in desulfation temperatures, and the use of LNT+SCR systems,
wherein the LNT is calibrated to generate ammonia for use in a
downstream SCR.
PM control is very effective. US2007 HD engines are very clean,
with the DPF systems delivering PM, HC, and CO emissions at levels
lower than 10% of the regulation. DPF regeneration advances are
reported in strategy, modeling PM loading, and catalyst
utilization. The effect of catalyst coatings on PN emissions, and
behavior of captured ash is becoming better understood. NO₂-based
regeneration of soot is very critical for proper functioning of
partial filters. Biodiesel effects on DPF functions are becoming
clearer.
Finally, diesel oxidation catalysts (DOCs) are being developed
for use with premixed combustion engine strategies that function
better at low temperatures, low oxygen levels, and at high HC+CO
levels. The problem of platinum (Pt) migration to SCR catalysts
from DOCs exposed to high temperatures for long times (850°C, 16
hours) is alleviated somewhat by using palladium (Pd) to replace
some of the platinum.
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