Control strategy for lean NOx trap regeneration

Method for controlling a direct injection internal combustion engine selectively operative in one of a homogeneous charge combustion mode and a stratified charge combustion mode and having an exhaust gas conduit fluidly connected to a NOx trap generally effective to accumulate NOx emissions during lean operation of the engine and to release said accumulated NOx emissions during rich operation of the engine comprising:

• defining a first engine operating region as the only region in which stratified charge combustion mode is enabled;
• defining an area of low engine speed and engine load, wherein stratified charge combustion mode is highly preferred;
• defining a second operating region, comprising an area greater than the area of low engine speed and engine load and consisting of a reduced portion of the first operating region, the second operating region operative to redefine the only area in which stratified charge combustion mode is enabled;
• monitoring engine operation;
• determining a cumulative mass of NOx stored on the NOx trap device;
• operating the engine in the stratified charge combustion mode only when the engine operation is within the first operating region and the cumulative mass of NOx stored on the NOx trap device is less than a first threshold;
• and
• operating the engine in a stratified charge combustion mode when the engine operation is within the second operating region and the cumulative mass of the NOx stored on the NOx trap device is greater than the first threshold.

Article of manufacture comprising:

• a storage medium having a computer program encoded therein for causing an engine controller to control a direct injection internal combustion engine selectively operative in one of a homogeneous charge combustion mode and a stratified charge combustion mode and having an exhaust gas conduit fluidly connected to a NOx trap device said program including:
• code to define a first engine speed/load operating region;
• code to define a second engine speed/load operating region consisting of a reduced portion of the first operating region;
• code to monitor engine operation;
• code to monitor temperature of the NOx trap;
• code to determine a cumulative mass of NOx stored on the NOx trap device;
• code to operate the engine in the stratified charge combustion mode only when the engine operation is within the first operating region and the cumulative mass of NOx stored on the NOx trap device is less than a first threshold;
• code to operate the engine in the stratified charge combustion mode when the engine operation is within the second operating region and the cumulative mass of NOx stored on the NOx trap device is greater than the first threshold;
• code to control the engine-out air/fuel ratio to regenerate the NOx trap when the temperature exceeds a predetermined temperature threshold;
• and,
• code to control the engine in the homogeneous charge combustion mode and control the engine-out air/fuel ratio to regenerate the NOx trap when the determined cumulative mass of NOx stored on the NOx trap device exceeds a second threshold, said second threshold greater than the first threshold.