Abstract:
A method for determining the rotary speed of a compressor, e.g., a turbocharger of an internal combustion engine, includes detecting the pressure in a region that is downstream from the compressor and generating a corresponding pressure signal. The rotary speed of the compressor is obtained from periodic fluctuations of at least one component of the pressure signal.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method for determining the rotary speed of a compressor, e.g., a turbocharger of an internal combustion engine, as well as to a computer program and/or a control device for controlling an internal combustion engine. 
   BACKGROUND INFORMATION 
   In internal combustion engines, e.g., gasoline or Diesel piston engines, to increase the performance, the air charge in a combustion chamber of the internal combustion engine is increased by the use of a compressor, such as an exhaust gas turbocharger. The pressure with which the air is pressed into the combustion chamber of the internal combustion engine is also designated as boost pressure, and is generally measured in the vicinity of the combustion chamber by a pressure sensor. The pressure signal is supplied to a closed control loop which controls the exhaust gas turbocharger and thereby sets a desired boost pressure. 
   Exhaust-gas turbochargers have a characteristic time constant, and thus they react comparatively sluggishly to changed control signals, which makes the regulation of the boost pressure more difficult. Therefore, it is advantageous if a direct state variable of the exhaust gas turbocharger that is to be regulated is recorded, e.g., the rotary speed of the compressor of the turbocharger, which is particularly suitable for this purpose. 
   It is an object of the present invention to provide a method which makes possible a cost-effective and reliable recording of the rotary speed of a compressor. 
   SUMMARY OF THE INVENTION 
   In an example method according to the present invention, the pressure sensor that is utilized for the determination of the boost pressure is also used for determining the rotary speed of the compressor. This is based on the recognition that usual compressors do not convey the air continuously, but in a “gushing manner” with respect to a certain location downstream from the compressor. This is caused by the fact that, for example, in an axial compressor, each time that a vane of the compressor wheel passes a certain position, the speed, and thereby also the pressure, of the conveyed air changes. This leads to periodic pressure fluctuations, at least at certain locations downstream from the compressor, whose periodicity is related to the rotary speed of the compressor. This relationship is utilized, according to the present invention, to obtain the rotary speed of the compressor. 
   As a result, a non-contact method for ascertaining the rotary speed of the compressor is made available, which works on a very robust, basic physical principle and is therefore highly reliable. In addition, in accordance with the method of the present invention, the efficiency of the intake systems of the internal combustion engine and the exhaust gas turbocharger is not reduced, since no additional sensor system is required in comparison to the usual numbers of sensor systems deployed in internal combustion engines. Also, because of the non-contact measurement, if there is any wear, it is slight. Finally, pressure sensors are comparatively simple and inexpensive types of sensor whose signals are able to be simply processed. 
   Directly downstream from the compressor, the periodic fluctuations in the pressure, which are important to the method according to the present invention, and thus also the recorded pressure signals, are particularly concise, which simplifies the evaluation and thus also the determination of the rotary speed. The costs of assembly are reduced even more if the pressure sensor is integrated into a control component of the compressor, e.g., a pop-off valve. Such a pop-off valve is used as a bypass of the compressor, which is opened in response to the closing of a throttle valve of the internal combustion engine, in order to enable as fast a pressure reduction as possible. 
   For the separation of the periodic fluctuations from the pressure signal, high-pass filtering can be used, which is simple to implement in software technology. From the separated periodic fluctuations, which are also designated as “alternating components” of the pressure signal, the frequency is able to be ascertained in a simple manner, e.g., by a Fourier transform. By dividing the frequency by the number of vanes of the compressor, or rather, of the compressor wheel, one directly obtains the rotary speed of the compressor. 
   From the signal of the pressure sensor, not only can the rotary speed of the compressor be obtained, but the boost pressure can also be ascertained, which is an important operating variable for the control of an internal combustion engine. The corresponding pressure value is simply obtained by an averaging of the pressure signal, for instance by low-pass filtering. 
   However, since the pressure sensor is situated advantageously in the vicinity of the compressor, and since there are various other components between the compressor and the combustion chambers, e.g., a charge-air cooler and a throttle valve, in such a case, the average value of the pressure signal does not correspond to the charge air that is of interest for the control of the internal combustion engine. However, the desired value of the charge air can be obtained in a simple way by correcting the average value of the pressure signal appropriately. 
   The correction factors used for this are ascertained in preliminary tests, for instance, on a test stand, for the specific type of internal combustion engine. The accuracy of the method is able to be improved in the process if at least one correction factor is used that is a function of a current operating variable of the internal combustion engine, e.g., of an air mass throughput or an air volume throughput. 
   Because of the position of the pressure sensor in the immediate vicinity of the compressor, its pressure signal can also be used for the functional monitoring of an air filter. For this purpose, the difference between the ascertained pressure and the pressure of an environmental pressure sensor is ascertained. If the pressure reduction exceeds a certain measure, the air filter should be replaced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic representation of an internal combustion engine having an exhaust gas turbocharger and a pressure sensor according to the present invention. 
       FIG. 2  shows a schematic flowchart of an example method for evaluating the signals made available by the pressure sensor shown in  FIG. 1 . 
       FIG. 3  shows a schematic representation of another example embodiment of an internal combustion engine having an exhaust gas turbocharger and a pressure sensor according to the present invention. 
   

   DETAILED DESCRIPTION 
   In  FIG. 1 , an internal combustion engine in its entirety is designated by reference numeral  10 . Although internal combustion engine  10  shown in  FIG. 1  is designed as a gasoline internal combustion engine having intake manifold injection, however, important basic contents of the following description apply in exactly the same way to Diesel internal combustion engines, as well as to internal combustion engines having direct fuel injection. 
   The internal combustion engine  10  includes a plurality of cylinders, of which at present only one is shown, which includes a combustion chamber  12 . Combustion air reaches the latter through an intake valve  14  via an intake duct  16 . Into this fuel is injected, immediately upstream of intake valve  14 , by an injector  18 , which is connected to a fuel system  20 . Upstream of the latter, there is a throttle valve  21  in intake duct  16 . 
   A fuel-air mixture present in combustion chamber  12  is ignited by a spark plug  22 , which is connected to an ignition system  24 . Hot combustion exhaust gases are carried off from combustion chamber  12  through an exhaust valve  26  and an exhaust pipe  28 . In the exhaust pipe there is a turbine  30 , which is able to be bypassed via a bypass valve  32 . 
   A compressor  34  is situated in intake duct  16 , which is mechanically connected to turbine  30 . Turbine  30  and compressor  34  together form an exhaust gas turbocharger  36 . For the compression of air, compressor  34  has a plurality of compressor vanes or compressor blades, which are not shown in  FIG. 1 , however. The intake air heated by the compression is cooled by a charge-air cooler  38 , which is situated in intake duct  16 , between compressor  34  and throttle valve  21 . 
   The operation of internal combustion engine  10  is controlled and regulated by a control and regulating device  40 . In particular, throttle valve  21 , injector  18 , ignition system  24  and bypass valve  32  are controlled by control and regulating device  40 . The latter receives signals from various sensors, such as from an HFM sensor  42  which records the air mass flowing through intake duct  16  upstream of compressor  34 , and from a pressure sensor  44 , which records the current pressure in intake duct  16  immediately downstream from compressor  34 . 
   The combustion air supplied to combustion chamber  12  is compressed by compressor  34 , which makes possible a greater performance of internal combustion engine  10 . The pressure of the air charge pressed into combustion chamber  12  (the “boost pressure”) is made available by pressure sensor  44  in a manner that will be shown below, and is adjusted in a closed control loop by control and regulating device  40 . To do this, the performance of turbine  30  (and thereby the performance of compressor  34 ), is varied by opening bypass valve  32  more or less. 
   In order to achieve regulation of the boost pressure that is as rapid and precise as possible, the boost pressure is regulated not only based on the boost pressure made available by pressure sensor  44 , but also based on the current rotary speed of compressor  34 . Boost pressure p L  and rotary speed n ATL  are ascertained starting from a signal U p  that is made available by pressure sensor  44 , with the aid of a method which will now be explained with reference to  FIG. 2 . 
   First of all, output signal U p  of pressure sensor  44  is submitted in  46  to an A/D conversion. Then, in  48 , periodic fluctuations (“alternating components”) U n  of signal U p  are separated. These periodic fluctuations U n  are brought about by the pressure waves of compressor  34 , which are caused by the individual compressor vanes or compressor blades of compressor  34 . In order for the periodic fluctuations of compressor  44  to be able to be recorded, it is necessary to situate pressure sensor  44  comparatively close to compressor  34 , as shown in  FIG. 1 . Besides that, pressure sensor  44  has to have appropriate dynamics. 
   The periodic fluctuations separated by the high-pass filter in  48  are now submitted in  50  to a Fourier transformation, by which frequency F of the periodic fluctuations is ascertained. This frequency F is the product of rotary speed n ATL  and the number n S  of the compressor blades or compressor vanes. Therefore, in  52 , ascertained frequency F is divided by the number n S  of the compressor blades, which finally leads to the rotary speed n ATL  of compressor  34 . 
   As was mentioned above, signal U p  of pressure sensor  44  is also used to ascertain boost pressure P L  which prevails immediately upstream of intake valve  14  and in combustion chamber  12  itself. For this purpose, signal U p  is submitted to a low-pass filtering in  54 , which leads to an average value U p—m  of pressure signal U p . This average value U p—m  is equivalent to the pressure between compressor  34  and boost pressure cooler  38 . In order to obtain from this the pressure immediately upstream of intake valve  14 , the value U p—m  is submitted to a correction in  56 , by applying to it, in a multiplicative or additive way, at least one correcting factor, here designated as K. 
   Correcting factor K is determined during the design of the parameters of control and regulating device  40 , for instance, on an engine test stand, by measuring the pressure before and after boost pressure cooler  38  at different operating states of internal combustion engine  10 . Correcting factor K may, in turn, be a function of operating variables of internal combustion engine  10 , for instance, of air mass throughput dm/dt, which is recorded by HFM sensor  42 . 
     FIG. 3  depicts an alternative example embodiment of an internal combustion engine  10 . In this context, it should be noted that such elements and regions which have equivalent functions to elements and regions in  FIG. 1  are not explained again in detail. 
   In internal combustion engine  10  shown in  FIG. 3 , pressure sensor  44  is not situated directly in intake duct  16 , downstream from compressor  34 , but is integrated, together with a pop-off valve  58 , in a unit  60 . Pop-off valve  58  opens when throttle valve  21  is closed, in order to make possible a rapid reduction in pressure in intake duct  16 . 
   In  FIG. 3 , upstream of HFM sensor  42  in intake duct  16 , an air filter  62  is also situated, and upstream of it, in turn, an environmental pressure sensor  64  is present. As may be seen in  FIG. 2 , its signal U u , together with averaged signal U p—m , which is obtained using pressure sensor  44 , is fed to a comparison block  66 . If it is determined that the difference between these two signals, or rather the pressure values determined from them, exceeds a boundary value, a measure is carried out in  68 . This measure may be, for instance, an entry into a fault memory, by which it is signaled, during a maintenance procedure, that air filter  62  has been used up or clogged, and has to be replaced.