The driver of a motor vehicle powered by a diesel engine operates the engine via an accelerator pedal. In a motor vehicle whose engine comprises an electronic control system, the accelerator pedal operates a sensor, sometimes called an accelerator position sensor (APS) that provides an APS signal to the control system indicating the extent to which the driver is depressing the pedal. The control system acts on that signal, along with other signals, to develop appropriate signals for controlling various aspects of engine operation to cause the engine to propel the vehicle in the manner intended by the driver's operation of the accelerator pedal, i.e. accelerate, cruise, or decelerate the vehicle, while striving for efficient use of fuel and minimization of tailpipe emissions. Airflow into the engine and fueling of the engine are two aspects of engine operation that can be controlled.
One configuration for a diesel engine intake system comprises a throttle valve, an EGR (exhaust gas recirculation) valve, and the compressor portion of a turbocharger. One or more of those components (typically all of them) is under the control of the engine control system to control mass airflow into the engine. The fuel system of such an engine comprises electric-actuated fuel injectors under control of the engine control.
In general, a diesel engine runs cooler, slower, and leaner than a spark-ignition engine. At times however, it becomes appropriate for the engine to run rich. The air-fuel ratio is of course controlled by relatively proportioning air and fuel. The combustible mixture may be richened by decreasing the proportion of air, increasing the proportion of fuel, or by a combination of both.
While running lean, the engine generates NOx. The use of a NOx adsorber in the engine exhaust system reduces the amount of NOx that otherwise would enter the atmosphere. The control system of an engine whose exhaust system has such a NOx adsorber monitors the condition of the NOx adsorber and initiates its regeneration when regeneration is needed and the engine is operating in a manner that will allow the regeneration.
When the NOx adsorber is to be regenerated, engine operation transitions from running lean to running rich in order to condition the exhaust for purging the NOx adsorber of adsorbed NOx by generating the excess CO that is needed for the regeneration process. In that instance the transition from running lean to running rich is initiated by the control system itself, rather than the driver. Regeneration occurs from time to time as the engine operates.
Changing the air-fuel ratio in any of the manners mentioned above can have an influence on engine torque production. Consequently, it would be desirable for the regeneration process to be transparent to the driver so that the driver would not sense unexpected change or fluctuations in engine torque due to initiation of a process that he himself did not initiate.
The amount and the timing of engine fueling are two aspects of fueling that are controlled by the 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.