In order to comply with ever more stringent limitations on pollutant emissions, a plurality of actions are taken in present-day engines in order to reduce, in particular, emissions of particulates and of nitrogen oxides.
One essential action is exhaust gas recirculation (EGR), which represents a present-day arrangement for preventing nitrogen oxide emissions. Exhaust gas recirculation lowers the oxygen content in the cylinder, and a decrease in the temperature in the combustion chamber consequently occurs. A rise in particulates with increasing exhaust gas recirculation is problematic. The principal reason for higher particulate emissions is that the oxygen also necessary for carbon oxidation is limited. The reduction in oxygen content caused by exhaust gas recirculation thus always has the effect of decreasing nitrogen oxide emissions, and elevating particulate emissions. This creates a conflict of objectives between particulate and nitrogen-oxide emissions, especially in diesel engines.
As a result of existing regulatory stipulations for the exhaust test cycle, the requirements for passenger cars regarding reduction of pollutant emissions in dynamic operation have been undemanding. In the commercial vehicle sector, dynamic operation has been entirely excluded by a steady-state test.
The development of exhaust gas recirculation control systems has therefore been directed essentially toward reducing emissions under steady-state conditions. The EGR control structure has therefore been based on values for engine speed and engine load that were identified under steady-state conditions and stored in characteristics diagrams. Control of air masses or EGR rates is known from known EGR control systems. Future regulatory requirements for commercial-vehicle and passenger-vehicle engines provide for a substantially larger dynamic component. In the future, certification will focus on emissions occurring in actual driving operation, e.g. in dynamic operation (“real driving emissions” or RDE), and on fuel consumption.
Taking dynamic processes into account requires, in particular, taking into account abrupt load changes and rapid load increases, such as those that often occur in real driving operation and in future test cycles. In a diesel engine, abrupt load changes and rapid load increases result, because of the inertia of the air system, in a delay in boost pressure buildup. The reasons for this inertia include the moment of inertia of the turbocharger and the dead volume between the compressor and the engine intake valves. The injection system that implements the driver's load demand has a considerably shorter reaction time than the engine's air system.
Because cylinder filling in a diesel engine is determined substantially by the slow-reacting boost pressure, the target values of the air system—based on injection volume and engine speed—are mismatched with the dynamic state of the engine. The steady-state target value of an air mass control system thus results, in the context of a slow-reacting boost pressure buildup and thus decreased cylinder filling, in a sharp reduction in the EGR rate, thereby producing dynamic nitrogen oxide spikes. With an EGR rate control system, the result is a smaller air mass and thus elevated particulate emissions. In systems or operating states dominated by high-pressure EGR, the resulting additional negative effect on boost pressure buildup potentially limits the maximum possible injection volume, with a consequent delay in reaching the desired target torque.
Patent document DE 100 10 978 A1 discusses a method for regulating the boost pressure of an internal combustion engine having an exhaust gas-driven turbocharger whose turbine, disposed in the exhaust duct of the internal combustion engine, has a modifiable geometry, regulation of the boost pressure being accomplished by way of an adjustment of the turbine geometry. In order to ensure boost regulation such that in the context of a load change, the boost pressure tracks as quickly as possible the change in the desired target boost pressure value, this method and this apparatus provide for determining a control variable for the turbine geometry from the system as a function of the exhaust backpressure existing in the exhaust duct in front of the turbine. This is because the exhaust backpressure reacts considerably more quickly than the boost pressure to a change in the behavior of the controlled system, e.g. to a load alternation, change in engine speed, change in exhaust gas recirculation, or to malfunctions, e.g. in the actuation system.
Patent document DE 41 07 693 A1 discusses a system for regulating and controlling a turbocharger, in which a predefined target value of the boost pressure is compared with an actual value. As a function of the comparison, a control system generates an actuating signal for applying control to an actuating mechanism. The target value depends at least on an engine speed signal and on a load signal. An arrangement is provided which elevates the boost pressure upon a specific change in the load signal.
The methods from the existing art for identifying target values for the control function are not sufficient to react, in the control loops for exhaust gas recirculation, turbocharging, and fuel injection, to the state of affairs described above in transient driving situations. In methods known from the existing art, transient driving situations of this kind are implemented, for example, by monitoring accelerator pedal gradients or injection volume gradients. The problem here is that these variables are not relevant in terms of preventing the generation of pollutant emissions.
Strictly in principle, it is also possible to use electric motors (e-machines) or electrical auxiliary compressors (e-boosters) to accelerate the buildup of boost pressure or torque. Here as well, a transient driving situation must be optimally detected.