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 70% tightening of
fleet average light-duty (LD) criteria emissions likely to be
proposed in California for ~2016-22. CO₂ regulations in both the
heavy- and light-duty sectors will also tighten and impact diesel
engines and emissions, probably long into the future.
Engine technology is addressing these needs. Light-duty diesel
engines are making incremental gains with combustion enhancements
that allow downsizing for CO₂ savings. Heavy-duty (HD) engine show
trade-offs between hardware recipes, exhaust deNOx control, and
fuel consumption.
Much has recently been reported on optimized selective catalytic
reduction (SCR) systems, with many reports on improving low
temperature performance with proper reductant management and new
catalyst formulations. Developments on HC-based deNOx are focused
on lean NOx traps (LNTs) and the LNT+SCR systems, wherein the LNT
is calibrated to generate ammonia for use in a downstream SCR.
PM control with DPFs (diesel particulate filters) are very
effective. DPF regeneration advances are reported in strategy,
catalyst utilization, and substrate design. Biodiesel effects on
DPF functions are becoming clearer.
Finally, diesel oxidation catalysts (DOCs) are investigated in
the context of impacting NO₂ coming from a downstream catalyzed DPF
for use in an SCR system. Hydrocarbon removal from the DOC is quite
important in this regard.