Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
1999-09-17
2001-07-31
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S213700, C423S239100, C502S514000
Reexamination Certificate
active
06267937
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the heating of a gas storage trap and in particular of a trap, mounted in the exhaust system of an internal combustion engine, for storing oxides of nitrogen.
BACKGROUND OF THE INVENTION
When an internal combustion engine is operating in a lean burn mode, there is always an excess of oxygen present in the exhaust system. As a result, oxides of nitrogen (NOx) present in the exhaust gases while the engine is operating in lean burn mode cannot be reduced by a three-way catalytic converter. To avoid such NOx gases from being discharged into the ambient atmosphere, it has been proposed to store them temporarily in a trap and to release them into the exhaust stream at a later stage when the engine is operated with a rich or at least stoichiometric mixture.
NOx traps can be formed as modified three-way catalytic converters in which the matrix has additional NOx storage sites consisting of base metal oxides. In the presence of surplus oxygen and NOx gases, the base metal oxides are converted to nitrates thereby trapping the NOx gases. In a reducing atmosphere on the other hand, the metal nitrates decompose releasing the NOx back into the exhaust stream but at this time the functioning three-way catalyst will reduce the NOx gases to nitrogen that can safely be discharged into the atmosphere.
A problem encountered with such traps is that the base metal oxides will also react with oxides of sulphur to produce sulphates which are more stable compounds than nitrates and do not normally decompose when the engine is again run in a rich or stoichiometric mode. As a result, unless further steps are taken, the trap is poisoned by sulphur which, at least in Europe, is usually present in the fuel.
It has been found that the damage done to a trap by sulphur in the fuel is reversible in the presence of a reducing atmosphere if the trap is heated to above 650° C.
OBJECT OF THE INVENTION
The present invention seeks to provide a method of heating a lean NOx trap to a sufficiently high temperature to enable it to be desuplhated.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a method of desulphating a lean NOx trap (LNT) in an exhaust system of a spark ignition internal combustion engine having a three-way catalyst and the lean NOx trap connected in series along the exhaust system, the NOx trap containing both NOx and oxygen storage components, the method comprising modulating the air to fuel ratio (AFR) calibration of the engine cyclically, first with a lean calibration for a sufficient time to result in the presence of excess air in the exhaust gases to provide oxygen to be stored within the lean NOx trap, then with a rich calibration for a sufficient time to result in the presence of excess fuel in the exhaust gases initially to react completely with any oxygen stored in the three-way catalyst itself and subsequently for the remainder of the excess fuel to break through the three-way catalyst to react with the oxygen stored in the NOx trap so as to heat the NOx trap to a temperature in excess of 650° C.
Three-way catalysts currently in use all have oxygen storage capacity to cope with rich AFR excursions. Purely for the purpose of heating the NOx trap, the three-way catalyst is not required to store oxygen. However, as oxygen storage capacity is invariably present in the three-way catalyst, its presence must be allowed for in the fuel computation as it is essential for the excess fuel (i.e. unburnt hydrocarbons and carbon monoxide) to break through the catalytic converter and find its way to the storage trap if the latter is to be heated.
The concept of alternating between lean and rich calibrations, that is to say modulation of the AFR, is known for example from EP-A-0 627 548, and is used to maintain a mean stoichiometric mixture, as is required by a three-way catalyst. The invention differs from such engine operation in that it requires a much greater degree of AFR modulation and for a more prolonged modulation cycle period because heating the NOx trap to a temperature at which desulphating can occur requires many AFR modulation cycles during each of which oxygen must first be stored in the NOx trap and subsequently fuel break through must occur past the catalytic converter and reach the NOx trap to react with the stored oxygen.
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Drouillard Jerome R.
Ford Global Technologies Inc.
Griffin Steven P.
Vanoy Timothy C
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