Investigation of Three-Way Catalyst Effect of a Catalyzed Diesel Particulate Filter on a Diesel Engine Operating at Reduced A/F Ratio, 03-R8031

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Principal Investigators
Stefan Simescu
Gary D. Neely
Vlad L.C. Ulmet
Magdi K. Khair

Inclusive Dates:  02/16/09 – 06/01/09

Background - The U.S. Environmental Protection Agency (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 to 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 a catalyzed diesel particulate filter (cDPF) had a TWC-type effect on NOX emissions and if so why and to what extent when used on a diesel engine operating at reduced air-fuel (A/F) ratio conditions.

Approach - The approach was a two-step test plan: first reduce the operating A/F ratio of the engine, close to stoichiometric, with an acceptable engine-out smoke increase; second, investigate the possibility of using the catalyzed DPF (cDPF) as a PM/NOX control device at low A/F ratio operation. The investigation was performed at five steady-state operating conditions.

Accomplishments - Low A/F ratio operation, close to stoichiometric, was achieved and tuned for low soot emissions at five steady-state operating conditions without the aftertreatment system installed. Following this initial step, two aftertreatment system configurations were tested: diesel oxidation catalyst (DOC) + cDPF and cDPF alone. Both aftertreatment system configurations were found to have a TWC-type 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. For the combined DOC + cDPF configuration, both the oxidation catalyst and the catalyzed DPF exhibited this effect. The following were inferred from the experimental results and subsequent analysis:

  1. The TWC-type effect was observed at slightly rich of stoichiometric operating conditions (A/F ratio of 14), where all excess exhaust O2 had been consumed by the DOC and/or cDPF
  2. The NOX consisted essentially of only NO at these conditions
  3. The measured NOX concentration reduction ranged from 88 to 99 percent
  4. The tailpipe brake specific NOX emissions were well below 0.20 g/hp-hr
  5. The reduction in NOX did not result in production of harmful N2O emissions
  6. The reduction in NOX was associated with NH3 formation in the DOC and/or cDPF at all operating modes, for both aftertreatment configurations
  7. The main reactions responsible for the observed NOX reduction are typical TWC reactions and are believed to be:


 

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