Lean-burning engines, or engines that run on an air/fuel mixture with a stoichiometrically greater amount of air than fuel, can offer improved fuel economy relative to engines configured to run on stoichiometric air/fuel mixtures.
However, lean-burning engines also may pose various disadvantages. For example, burning a lean air/fuel mixture may decrease the reduction of nitrogen oxides (collectively referred to as “NOx”) in a conventional three-way catalytic converter.
Various mechanisms have been developed to reduce NOx emissions in lean-burning engines. One mechanism is a NOx trap. The NOx trap is a catalytic device typically positioned downstream of a catalytic converter in an emissions system, and is configured to retain NOx when the engine is running a lean air/fuel mixture and then release and reduce the NOx when the engine runs a more rich air/fuel mixture.
A typical NOx trap includes one or more precious metals, and an alkali or alkaline metal oxide to which nitrogen oxides adsorb as nitrates when the engine is running a lean air/fuel mixture. The engine can then be configured to periodically run a richer air/fuel mixture. The nitrates decompose under rich conditions, releasing the NOx. This reacts with the carbon monoxide, hydrogen gas and various hydrocarbons in the exhaust over the precious metal to form N2, thereby decreasing the NOx emissions and regenerating the trap.
The use of a NOx trap can substantially reduce NOx emissions from a lean-burning engine. However, SO2 produced by the combustion of sulfur in fuel can form sulfates, which can poison the NOx storage sites and lower the NOx storage capacity of the trap.
The NOx storage capacity of the trap may be recovered by operating the trap for several minutes at a high temperature (for example, around 700° C.) under rich conditions. However, this process can result in the formation and emission of hydrogen sulfide, which has an unpleasant odor. The emission of hydrogen sulfide may be suppressed by alternating between lean and rich conditions while holding the NOx trap at desulfation conditions. However, this may slow desulfation significantly.
German Published Patent Application No. DE 198 49 082 A1 teaches a multistage desulfation process. In the first stage, a NOx trap is exposed to slightly rich conditions (air/fuel ratio=0.98) and a relatively low desulfation temperature for a first period of time. In the second stage, the air/fuel ratio is modulated about the initial value. As the second stage progresses, the amplitude of the modulation is increased, the temperature is increased, and the frequency and midpoint of the modulation are decreased. This method may decrease the time required for desulfation relative to fixed amplitude/frequency modulation schemes. However, this method may still cause the production of excess hydrogen sulfide, and/or take more time than necessary to complete desulfation, as it does not take into account an amount of hydrogen sulfide in a trap at any instant during the desulfation process.
The inventors herein have recognized that the formation and emission of hydrogen sulfide during desulfation may be more efficiently addressed by utilizing a method of desulfating a catalytic NOx storage and conversion device that includes determining an amount of sulfur stored in the catalytic NOx storage and conversion device; determining an interval for exposing the catalytic NOx storage and conversion device to a rich exhaust stream based upon the determined amount of sulfur stored, wherein the interval is longer for lower amounts of sulfur stored and shorter for higher amounts of sulfur stored; and exposing the catalytic NOx storage and conversion device to the rich exhaust stream for the determined interval.