Process for controlling the charging pressure in an exhaust gas turbocharger with an adjustable turbine geometry

#This invention relates to a process for controlling the charging pressure in an internal-combustion engine exhaust gas turbocharger having an adjustable turbine geometry, to a predetermined operating-point-dependent desired value.

#German Patent Document DE 40 25 901 C1, discloses a process for preventing the occurrence of uncontrolled super elevations of pressure after a positive load change when the charging pressure p2 in the exhaust pipe upstream of the turbine is still rising. Thereby the internal-combustion engine needs no longer exhaust against a super elevated exhaust back pressure, facilitating increased efficiency of the internal-combustion engine. For this purpose, a limit pressure switch is provided in the exhaust pipe to control the exhaust back pressure p3. The limit pressure switch changes its switching condition upon the occurrence of a certain limit pressure; that is, in the case of an exhaust back pressure which exceeds a limit pressure or a threshold value, the limit pressure switch is changed from a closed position into an open position.

#The disadvantage of this process is that, in the case of such a monitoring of the exhaust back pressure p3, only the momentary exhaust back pressure existing upstream of the turbine is detected and compared with a limit or threshold value. This measurement does not reflect the current atmospheric pressure (that is, the elevation above the sea level); also, the current charging pressure p2 cannot be taken into account during monitoring. In addition, the limit pressure switch permits only a larger or smaller differentiation but not continuous evaluation.

#FIG. 1 is a schematic diagram of an advantageous arrangement for implementing the process according to the invention;

#FIG. 2 is a functional block diagram of a method of operation of the electronic control unit according to the invention which has the reference number 20 in FIG. 1; and

#FIG. 3 is functional block diagram of the method of operation of the fuzzy control unit according to the invention illustrated in FIG. 2.

#FIG. 1 illustrates an internal-combustion engine 11 with an exhaust gas turbocharger 12 whose turbine 13 is arranged in the exhaust pipe 14 and whose compressor 15 is arranged in the intake pipe 16. To improve the response of the exhaust gas turbocharger 12 in the case of a positive load change (particularly from low load and rotational speed ranges), its turbine 13 has a turbine guide apparatus 17, which for reasons of clarity, is outlined only schematically in the drawing. By means of this turbine guide, on the one hand, both the approach flow cross section and the approach flow direction of the exhaust gases onto the turbine blades can be changed. The turbine guide apparatus 17 is actuated by a servo motor 18 which, in turn, can be controlled by way of the control line 19 by an electronic control unit 20.

#A signal which corresponds to the current charging pressure p2 is fed to the control unit 20 by a sensor 21, via a measuring line 22. A sensor 23 and a measured-value line 24 supply a signal which corresponds to the current exhaust gas pressure p3 in front of the turbine 13, while a sensor 25 and a measured-value line 26 provide a signal which corresponds to the current rotational speed n of the internal-combustion engine, both of which signals are fed to the electric control unit 20. By way of the sensor 27 in the injection system 29 and the measured-value line 28, the control unit 20 also receives a signal which corresponds to the current injection quantity of the internal-combustion engine 11. The current injection quantity is determined from the position of the accelerator pedal (not shown), which in turn is determined by the driver in the case of a positive load change, as, for example, from idling to a full load.

#In the case of a positive load change of the internal-combustion engine 11, the turbine guide apparatus 17 is first changed into a closed position; that is, into a position which reduces to a minimum the approach flow cross-section for the exhaust gases onto the turbine 13. Because the approach flow cross-section of the turbine 13 is constructed to be very small, the exhaust gas pressure p3 rises very rapidly so that, even before the charging pressure p2 has reached the desired value p2 corresponding to this operating point, the exhaust back pressure p3 is considerably super elevated which is a detriment to the efficiency of the internal-combustion engine 11. As a result, it is necessary that the super elevation of the exhaust back pressure p3 is monitored and controlled so that an optimal torque buildup can be achieved which results in a high efficiency of the internal-combustion engine 11.