Suppression of Sulfide Emission During Lean Nox Trap Desulfation

Lean NOx traps are being extensively examined (Ref. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12) because they can be efficiently used to reduce the NOx emissions from port fuel injected and direct fuel injected spark ignited gasoline engines. A lean NOx trap (LNT) stores NOx during lean A/F engine operation. However, its storage capacity is limited and the LNT must be regenerated periodically by subjecting the LNT to momentary rich A/F operation for several seconds. The regeneration process releases the NOx that is chemically bonded to the washcoat and subsequently reduces it to N₂ and O₂. Fuel that contains a non-zero amount of sulfur will contaminate an LNT by significantly reducing its NOx storage capacity. Therefore, except for the case of a zero level of sulfur in the fuel, the LNT must be desulfated on a periodic basis. The desulfation process requires that the temperature of the LNT be raised to a temperature of about 650°C for several minutes. Earlier work (Ref. 10) has shown that the stored sulfur is emitted as SO₂ and H₂S during desulfation. The emission of H₂S is undesirable because of its highly odiferous nature. Two methods of hydrogen sulfide suppression were investigated. The first method involved adding a gettering material, such as nickel, to the lean NOx trap washcoat. The second method involved providing a low frequency AFR modulation whose purpose is to “pump” air (oxygen) into the LNT periodically during desulfation. Both methods have been found to attenuate markedly the H₂S peak concentration and to shift the mass emitted ratio of the two sulfur-containing constituents from predominantly H₂S to SO₂. Test results and control system design are presented for both steady state and transient operation for a 1.8L I4 engine in a 1477 kg (3250 lb) test vehicle. The transient tests were carried out on the U.S. FTP75 drive cycle and on the European NEDC drive cycle.