The US EPA emission standards for 2010 on-highway and 2014
non-road diesel engines are extremely stringent, both in terms of
oxides of nitrogen (NOX) and particulate matter (PM). Diesel
engines typically operate lean and use at least 40-50 percent more
air than what is needed for stoichiometric combustion of the fuel.
As a result, significant excess oxygen (O₂) is present in diesel
exhaust gas which prevents the application of the mature three-way
catalyst (TWC) technology for NOX control used in gasoline
engines.
The objective of this work was to investigate whether or not the
catalyzed DPF had a TWC-type of effect on NOX emissions and if
so, why and to what extent when used on a diesel engine operating
at reduced A/F ratio conditions. The approach was a two-step test
plan: first to reduce the operating A/F ratio of the engine, close
to stoichiometric, with an acceptable engine-out smoke increase;
second, to investigate the possibility of using the catalyzed DPF
(cDPF) as a PM/NOX control device at low A/F ratio operation.
Low A/F ratio operation, close to stoichiometric, was achieved
and tuned for low soot emissions, as indicated by the AVL filter
smoke number (FSN) at five steady-state operating conditions
(modes), without the aftertreatment system installed. Two
aftertreatment system configurations were tested: diesel oxidation
catalyst (DOC) and catalyzed DPF (DOC + cDPF) and cDPF alone. Both
aftertreatment system configurations were found to have a TWC
effect when the excess exhaust O₂ was reduced sufficiently and was
completely consumed by the DOC and/or cDPF. This effect was
observed at all five modes. The measured NOX concentration
reduction ranged from 88 to 99%. The tailpipe brake specific NOX
emissions were well below 0.20 g/hp-hr at the five steady-state
operating conditions.