An exhaust system of a diesel engine that comprises a NOx adsorber is capable of adsorbing significant amounts of NOx in exhaust gas passing through the exhaust system from the engine, thereby reducing the amount of NOx that otherwise would enter the atmosphere.  From time to time, such a device must be regenerated in order to purge it of adsorbed NOx so that it can continue to be effective in adsorbing NOx as the engine continues to run.  A known technique for regenerating a NOx adsorber comprises creating an excess of CO for reaction with adsorbed NOx to reduce the NOx to molecular nitrogen (N2) while the CO oxidizes to CO2 during the process. 
One known method for creating excess CO comprises injecting fuel in proper amount into the exhaust leaving engine combustion chambers.  Because that fuel does not contribute to the thermal energy of combustion that is converted by thermodynamic processes in the combustion chambers acting on the engine's kinematic mechanism to create engine torque, it has essentially no influence on engine torque. 
For one or more reasons, post-injection of fuel that does contribute to the thermal energy of combustion that produces engine torque may be considered a more desirable alternative, although both methods require the injection of extra fuel to purge the NOx adsorber.  However, the post-injection alternative has consequences on engine torque output that can lead to undesirable torque fluctuations that contribute to engine and vehicle harshness as the engine continues to run during NOx adsorber regeneration. 
A known electronic engine control system comprises a processor-based engine controller that processes data from various sources to develop control data for controlling certain functions of the engine.  The amount and the timing of engine fueling are two functions that are controlled by an engine control system.  A typical diesel engine that comprises fuel injectors for injecting fuel into the engine cylinders under control of an engine control system controls both the duration and the timing of each fuel injection to set both the amount and the timing of engine fueling.  During an engine cycle, it is also capable of pre-injection of fuel (pilot-injection) in advance of a main injection and post-injection after the main injection, although the use of either typically depends on how the engine is being operated.