Engine control systems and methods for nitrogen oxide reduction

1. An engine control system, comprising:

a fuel control module that controls fuel injection of an engine based on a predetermined lean air/fuel ratio, wherein the predetermined lean air/fuel ratio is fuel lean relative to a stoichiometric air/fuel ratio for the fuel; and

a cylinder control module that deactivates opening of intake and exhaust valves of M cylinders of the engine to facilitate removal of nitrogen oxide (NOx) from exhaust in response to a determination that:

an amount of ammonia stored by a selective catalytic reduction (SCR) catalyst receiving exhaust output by the engine is less than a first predetermined amount of ammonia;

an engine speed is within a predetermined speed range for facilitating the removal of NOx; and

an engine load is within a predetermined engine load range for facilitating the removal of NOx,

wherein M is an integer greater than 0 and less than a total number of cylinders of the engine,

wherein the fuel control module further:

disables fueling of the M cylinders while opening of the intake and exhaust valves of the M cylinders is deactivated; and

while fueling of the M cylinders is disabled and opening of the intake and exhaust valves of the M cylinders is deactivated, controls fuel injection of other cylinders of the engine based on a predetermined rich air/fuel ratio that is fuel rich relative to the stoichiometric air/fuel ratio for the fuel;

wherein the cylinder control module further re-activates opening of the intake and exhaust valves of the M cylinders in response to a determination that:

the amount of ammonia stored by SCR catalyst is greater than a second predetermined amount of ammonia, wherein the second predetermined amount of ammonia is greater than the first predetermined amount of ammonia; and

the engine load is increasing;

wherein the fuel control module further controls fuel injection of the M cylinders after the re-activation based on the predetermined lean air/fuel ratio in response to the determination that:

the amount of ammonia stored by SCR catalyst is greater than the second predetermined amount of ammonia; and

the engine load is increasing;

wherein the cylinder control module further continues to deactivate the opening of the intake and exhaust valves of the M cylinders of the engine in response to a determination that:

the amount of ammonia stored by SCR catalyst is greater than the second predetermined amount of ammonia; and

the engine load is not increasing; and

wherein, during the continued deactivation, the fuel control module further controls the fuel injection of the other cylinders based on the predetermined lean air/fuel ratio in response to the determination that:

the amount of ammonia stored by SCR catalyst is greater than the second predetermined amount of ammonia; and

the engine load is not increasing.

2. The engine control system of claim 1 wherein M is equal to one-half of the total number of cylinders of the engine.

3. An engine control method, comprising:

controlling fuel injection of an engine based on a predetermined lean air/fuel ratio, wherein the predetermined lean air/fuel ratio is fuel lean relative to a stoichiometric air/fuel ratio for the fuel;

deactivating opening of intake and exhaust valves of M cylinders of the engine to facilitate removal of nitrogen oxide (NOx) from exhaust in response to a determination that:

an amount of ammonia stored by a selective catalytic reduction (SCR) catalyst receiving exhaust output by the engine is less than a first predetermined amount of ammonia;

an engine speed is within a predetermined speed range for facilitating the removal of NOx; and

an engine load is within a predetermined engine load range for facilitating the removal of NOx,

wherein M is an integer greater than 0 and less than a total number of cylinders of the engine;

disabling fueling of the M cylinders while opening of the intake and exhaust valves of the M cylinders is deactivated; and

while fueling of the M cylinders is disabled and opening of the intake and exhaust valves of the M cylinders is deactivated, controlling fuel injection of other cylinders of the engine based on a predetermined rich air/fuel ratio that is fuel rich relative to the stoichiometric air/fuel ratio for the fuel;

re-activating opening of the intake and exhaust valves of the M cylinders in response to a determination that:

the amount of ammonia stored by SCR catalyst is greater than a second predetermined amount of ammonia, wherein the second predetermined amount of ammonia is greater than the first predetermined amount of ammonia; and

the engine load is increasing;

controlling fuel injection of the M cylinders after the re-activation based on the predetermined lean air/fuel ratio in response to the determination that:

the amount of ammonia stored by SCR catalyst is greater than the second predetermined amount of ammonia; and

the engine load is increasing;

continuing to deactivate the opening of the intake and exhaust valves of the M cylinders of the engine in response to a determination that:

the amount of ammonia stored by SCR catalyst is greater than the second predetermined amount of ammonia; and

the engine load is not increasing; and

during the continued deactivation, controlling the fuel injection of the other cylinders based on the predetermined lean air/fuel ratio in response to the determination that:

the amount of ammonia stored by SCR catalyst is greater than the second predetermined amount of ammonia; and

the engine load is not increasing.

4. The engine control method of claim 3 wherein M is equal to one-half of the total number of cylinders of the engine.