WO2021068014A1 - Method and test bench for knock calibration of an internal combustion engine - Google Patents

Abstract

According to the invention, a compensation amplitude (KP_PKkomp) is determined from the maximum amplitudes (KP_PK) of a number of cycles (Ak) recorded at a first operating point, which compensation amplitude is used to compensate maximum amplitudes (KP_PK) to determine a number of absolute maximum amplitudes (KP_PKabs), a knock limit (KG) at the first operating point is determined from the absolute maximum amplitudes (KP_PKabs), the knock limit (KG) determined in the first operating point is used at least at one second operating point of the internal combustion engine (1) to vary engine control parameters of an engine controller of the internal combustion engine (1) so that the knock limit (KG) is observed, and the engine control parameters determined in this way are stored in the engine control unit for controlling the internal combustion engine (1) at the second operating point, in order to be able to perform a knock calibration of an internal combustion engine on a test bench (5) easily and reliably.

Description

Procedure and test stand for knock calibration of an internal combustion engine

The invention relates to a method for knock calibration of an internal combustion engine in which the internal combustion engine is operated on a test stand and a knock signal from at least one cylinder of the internal combustion engine is detected by means of a knock sensor and a maximum amplitude of a pressure oscillation recorded by the knock sensor is determined from the knock signal in a working cycle of the internal combustion engine and a number of maximum amplitudes recorded at a first operating point of the internal combustion engine are evaluated for knock calibration, as well as an associated test stand for carrying out the method.

Knocking is understood to mean an uncontrolled combustion process of the air-fuel mixture after ignition in the cylinder of an internal combustion engine, in particular an Otto engine. This can result in high pressure peaks in the cylinder, which can damage components such as pistons, bearings, cylinder head, valves and spark plugs. Knocking is therefore extremely undesirable in the operation of an internal combustion engine and should be avoided. Typical causes of knock are excessive compression in the cylinder, temperatures that are too high or ignition timing that is too early. Apart from this, knocking is also dependent on the operating point of the internal combustion engine, i.e. essentially on the speed and torque, but also on the compression ratio. A common reaction of the engine management system to knock is to shift the ignition point backwards. If the ignition point is delayed, however, the output of the internal combustion engine is reduced because the combustion energy can be used more poorly and consumption increases. It is therefore important in today's internal combustion engines to know the knock limit in the entire operating range of the internal combustion engine. In modern internal combustion engines, knock control is also often provided, usually in the engine control system, which is intended to ensure that the internal combustion engine is operated close to the knock limit for maximum utilization of the combustion energy while at the same time avoiding knocking.

A significant part of the calibration of an internal combustion engine is therefore the knock calibration, in which, for example, the knock limit is parameterized in the engine management system.

WO 2018/172665 A1 describes a method for knock control of an internal combustion engine, but does not concern knock calibration.

With the knock calibration, the knock limit of the internal combustion engine is to be determined in the entire operating range, essentially by influencing the ignition parameters, in particular the ignition time. The aim is to determine the ignition parameters for each operating point in the operating range, which lead to operation of the internal combustion engine as close as possible to the knock limit without exceeding the knock limit. This is for example in KR 101969908 B1 or CN 109973280 A. Such a knock calibration is usually carried out on an engine test bench on which the internal combustion engine is connected to a load machine (dynamometer) in order to be able to set various operating points. On the engine test bench, cylinder pressure sensors are usually used as knock sensors in order to directly record the cylinder pressure during a work cycle of the internal combustion engine. In the knock calibration, cylinder pressure sensors (one piece per cylinder) are preferably used because the pressure oscillations can be measured and assessed directly at the point of origin in the combustion chamber. Other knock sensors only record the cylinder pressure indirectly, by recording the effects of the pressure fluctuations in the cylinder. For example, acceleration sensors detect the acceleration of a part of the internal combustion engine that is excited by structure-borne noise. Sound sensors record the acoustic propagation of the knocking noise. However, these sensor signals also represent the cylinder pressure.

The measured sensor signal from the knock sensor, e.g. the signal from the cylinder pressure sensor, is evaluated in order to identify knocking events that are expressed in high pressure peaks. An attempt is made to determine the knock limit by varying various settings, in particular the ignition point.

Such a knock calibration is very complex in practice, in particular because it has to be carried out in a large number of operating points, and there can also be other influencing variables that influence the calibration, for example the geometric compression ratio, the intake air temperature, the fuel quality, etc. Before Modern internal combustion engines with a variable compression ratio in particular increase the calibration effort, because the adjustable, geometric compression ratio adds another degree of freedom.

During the knock calibration, the pressure fluctuations in the cylinder caused by uncontrolled combustion are evaluated. These are recorded using knock sensors, for example cylinder pressure sensors, acceleration or sound sensors. For knock calibration, the maximum amplitudes (knock peaks) of these pressure fluctuations occurring in the individual work cycles of the internal combustion engine are usually evaluated, with statistical evaluations of a number of these maximum amplitudes being carried out as a rule.

Knock sensors, such as cylinder pressure sensors, not only pick up the knock signal of interest, but also interfering signals (noise). Interfering signals can either be of an electrical nature and can essentially be eliminated by suitable cabling and design of the electrical measuring chain. The normal combustion in the cylinder also creates gas oscillations, which are also noticeable in pressure fluctuations do. Knocking events also generate pressure oscillations in the cylinder, which are superimposed on the pressure oscillations caused by normal combustion. However, these two pressure oscillations have the same frequency content, ie they occur in the same frequency bands. This means that these two cannot simply be separated by filtering in the recorded knock signal. Such normal pressure oscillations are also recorded as maximum amplitudes of a work cycle and are noticeable as background disturbances. In the case of a statistical evaluation of the maximum amplitudes of a number of work cycles, this can lead to inaccurate and unreliable knock detection. This also makes knock calibration more difficult. For a reliable knock calibration, it is therefore advantageous to exclude such background disturbances from the knock detection.

It is therefore an object of the present invention to provide a method for knock calibration of an internal combustion engine that can be carried out simply and reliably.

According to the invention, this object is achieved by the features of the independent claims. Due to the use of absolute maximum amplitudes, it is not necessary to determine an associated knock limit at every operating point at which the knock calibration is to be carried out. It is sufficient to determine the knock limit at a first operating point and to apply the knock limit thus determined to a second operating point that is different from the first. This means that the internal combustion engine must approach the respective operating point on the test bench, but when the operating point is changed, the knock limit does not need to be set again because, according to the invention, absolute maximum amplitudes are now used and the knock limit can therefore be transferred to other operating points. This simplifies and shortens the knock calibration considerably.

Further advantages of the invention emerge from the dependent claims and from the associated description.

The present invention is explained in more detail below with reference to FIGS. 1 to 8, which show exemplary, schematic and non-limiting advantageous embodiments of the invention. It shows

Fig. 1 pressure curves of a combustion in a cylinder of an internal combustion engine, Fig. 2 a test bench for performing a knock calibration,

3 shows the evaluation of a knock signal recorded with a knock sensor,

4 shows an FFT-transformed pressure curve of a work cycle,

Fig. 5 determined maximum amplitudes of a number of work cycles,

6 shows a histogram of the determined maximum amplitudes, FIG. 7 ascertained absolute maximum amplitudes of a number of working cycles and FIG. 8 a histogram of the ascertained absolute maximum amplitudes.

Fig. 1 shows two typical pressure curves of a combustion in a cylinder of an internal combustion engine (here 4-stroke Otto engine). The cylinder pressure p is shown over part of the work cycle (in degrees crank angle cp) around top dead center (0 ° crank angle cp). After the ignition happens in the area of top dead center and the combustion is decisive for the knock calibration, this is the area of particular interest for knock calibration. Two pressure curves VV1, VV2 are shown for different conditions influencing the cylinder pressure curve, such as different compression ratios, different intake air temperature, speed, load, etc. In modern internal combustion engines, the compression ratio can, for example, be adapted as required.

In the pressure curve VV1, e.g. with a first compression ratio, a steep pressure increase (time derivative of the pressure) can be seen after ignition in the area of top dead center, which causes a combustion oscillation V. A knocking event due to self-ignition of the end gas of the combustion, which manifests itself in a knocking oscillation K, is also shown in the pressure curve VV1. In the pressure curve VV2, e.g. with a second compression ratio (which is greater than the first compression ratio), one recognizes the later ignition and also the lower combustion oscillation V due to the lower pressure increase. The combustion oscillation V and the knock oscillation K have superimposed frequency ranges. These pressure oscillations (knock oscillation K and combustion oscillation V) in the cylinder of the internal combustion engine 1 during an operating cycle Ak are evaluated during the knock calibration. Due to the superimposed frequency ranges, no filter methods can be used because this would always impair the detection of the knocking vibration.

The internal combustion engine 1 is arranged on a test stand 5 for knock calibration

(Fig. 2). The test bench 5 is usually an engine test bench on which the

Internal combustion engine 1 is connected to a loading machine 2 (usually an electric motor). The internal combustion engine 1 and the load machine 2 are on

Test stand 5 controlled and operated by a test stand control unit 3 (computing hardware and software). For example, the internal combustion engine 1 is activated to set a desired torque M and the loading machine 2 to set a desired speed n (but this can also be the other way around). For knock calibration, the internal combustion engine 1 is usually operated at a stationary operating point (speed,

Torque). The internal combustion engine 1 could, however, also be operated on a different test bench for knock calibration, for example on one

Powertrain test bench, in which the internal combustion engine has a powertrain (or a Part of it) drives and the loading machine is connected to the drive train (for example on a side shaft of the drive train), or a roller dynamometer on which an entire vehicle can be arranged and the loading machine 2 is the roller drive.

Other settings for the knock calibration can also be made on the test bench. For example, the ambient climate (temperature, humidity, air pressure, etc.) could be set. Likewise, the fuel or the combustion air, or another medium required for the operation of the internal combustion engine 1, could be conditioned (temperature, pressure, humidity, etc.).

At least one knock sensor 4 is arranged on the internal combustion engine 1, with which a knock signal S, for example a pressure curve as described in FIG. 1, is recorded. For example, a cylinder pressure sensor is provided for each cylinder or an acceleration sensor or sound sensor for one or more cylinders (it being possible to infer the respective cylinder based on the ignition sequence). With the knock signal S, the pressure oscillations in the cylinder, directly or indirectly, are recorded. The knock sensor 4 can be, for example, a cylinder pressure sensor on the cylinder, or a sound pick-up or acceleration pick-up on the engine block, or any other suitable sensor. The knock signal S recorded by the knock sensor 4 is fed to a knock evaluation unit 6 (computing hardware and / or software) and evaluated there. The knock evaluation unit 6 can be integrated in the test stand control unit 3 or can be its own hardware and / or software.

Of course, the knock signal S of an entire work cycle (e.g. 720 ° for a 4-stroke gasoline engine) does not have to be evaluated for knock calibration. Typically, the knock signal S is evaluated from the ignition point or from shortly before the ignition point or from the top dead center up to a predetermined crank angle cp, e.g. 100 ° crank angle. With the evaluated crank angle range, the pressure oscillations of interest due to the combustion should of course be recorded.

A signal processing unit 7 (hardware and / or software) can be provided in the knock evaluation unit 6 (as indicated in FIG. 3), in which the knock signal S of a working cycle supplied by the knock sensor 4 can be processed. For example, a high-pass filter can be implemented in the signal processing unit 7 in order to filter out the low frequency ranges that are not relevant for knocking from the knock signal S. For knock calibration, it is advantageous if only the higher-frequency components, which include the pressure oscillations, remain in the knock signal S. Likewise, the knock sensor 4 could already undertake such a filtering. The knock signal S, which represents the pressure oscillations, is sent to a maximum value recognition unit 8 (hardware and / or software) supplied, in which the maximum amplitude KP_PK of the pressure oscillation of the respective work cycle is determined. This can be done either time-based (usually resolved via crank angle) or frequency-based. An FFT (Fast Fourier Transformation) of the knock signal S can be provided for the frequency-based evaluation.

4 shows an example of an FFT-transformed pressure curve VV of a work cycle, for example recorded with a cylinder pressure sensor, as a knock signal S. The low-frequency components, e.g. less than 5,000 Hz, can be filtered out, for example in the signal processing unit 7, or ignored in the determination of the maximum amplitude KP_PK because they are not related to a knock vibration. The maximum amplitude KP_PK of the pressure oscillation occurs in this example at approximately 8,000 Hz.

In an evaluation unit 9 (hardware and / or software) an evaluation of the determined maximum amplitudes KP_PK takes place for knock calibration. As a rule, a statistical evaluation of the maximum amplitudes KP_PK of the current and a number i of past work cycles takes place. A number i of maximum amplitudes KP_PK are therefore considered together in such a statistical evaluation. However, since knocking does not occur in every work cycle, maximum amplitudes KP_PK of a pressure oscillation generated by normal combustion are also taken into account.

In the evaluation of the maximum amplitudes KP_PK, the frequency of maximum amplitudes KP_PK exceeding a certain limit amplitude within a certain window (e.g. number of work cycles) or the exceeding of a maximum permissible maximum amplitude KP_PK are usually evaluated.

FIG. 5 shows the recorded maximum amplitudes KP_PK of a number i work cycles Ak, ..., Ak-i at a specific operating point of the internal combustion engine (speed, torque, compression ratio, etc.). The work cycles are usually consecutive work cycles, but this need not necessarily be the case. For example, work cycles with misfires, which can be easily determined using the knock signal S, can be excluded from the evaluation. The individual maximum amplitudes KP_PK are here connected with a line for the purpose of better representation, but this is of course not absolutely necessary. It can be seen that the pressure oscillations in each work cycle Ak,..., Ak-i considered causes a maximum amplitude KP_PK, in particular also in a work cycle in which no knocking occurs. However, these maximum amplitudes KP_PK of a work cycle without knocking represent a basic disturbance for the knock calibration. Thus, maximum amplitudes KP_PK that are not based on a knocking event also flow into an evaluation for knocking calibration. These basic disturbances are strongly dependent on the respective operating point of the combustion engine (speed, torque, geometric compression ratio, intake air temperature, etc.). In FIG. 5, the maximum amplitudes KP_PK are shown in dashed lines at another operating point of the internal combustion engine in order to clarify this. One recognizes the higher level of the basic disorder, for example.

With the previous knock calibration, the maximum amplitudes KP_PK are therefore more dependent on operating parameters such as speed, torque or compression ratio, etc. (because these influence the basic disturbances), which makes knock calibration even more difficult and expensive.

In the knock calibration, the frequency of occurrence of certain peaks of the maximum amplitudes KP_PK within a certain window, such as a number of work cycles, and / or compliance with a maximum permissible maximum amplitude KP_PK _max are evaluated. For example, the knock calibration may require that at least 80% of the maximum amplitudes KP_PK that occur in a specific number of previous work cycles must be less than a specific knock limit. Due to the basic disturbances, however, it was previously necessary to determine the knock limit for various operating points, which is very complex. The engine settings, in particular ignition parameters such as the ignition angle, are varied at the respective operating point until the knock limit is observed. Furthermore, it can be required that none or only a certain number of maximum amplitudes KP_PK may be greater than a certain maximum permissible maximum amplitude KP_PK _max .

This can easily be shown in the form of a histogram as in Fig. 6. In FIG. 6, two histogram curves K are shown, each showing the frequency H (in percent) of the occurrence of certain maximum amplitudes KP_PK over a certain number of working cycles Ak, typically the 30 to 50 last working cycles Ak. It can be seen from the histogram curves that the frequency H of the occurrence of certain maximum amplitudes KP_PK decreases with an increasing value of the maximum amplitudes KP_PK. The basic disturbance can also be seen in the histogram curves at 100%. The histogram curve for a different operating point is shown in dashed lines analogous to FIG.

A knock limit KG can now be required so that the frequency H that the maximum amplitudes KP_PK exceed a specific limit amplitude KP_PK _{G is} less than a specific frequency limit HG, for example 20%. In FIG. 6, for example, less than 20% of the maximum amplitudes KP_PK are greater than the limit amplitude KP_PK _G. It should be noted, however, that this limit amplitude KP_PK _G is not known in advance, but has to be determined at each operating point during the knock calibration, for example based on the experience of the knock engineer. Likewise, it can additionally or alternatively be required that the greatest maximum amplitude KP_PK be less than a specified one maximum permissible maximum amplitude KP_PK _max or only a number of maximum amplitudes KP_PK may be greater than a specified maximum permissible maximum amplitude KP_PKm _ax . During the knock calibration, parameters of the engine control, in particular ignition parameters such as the ignition angle, are adjusted so that the frequency _{limit H G} and / or the maximum permissible maximum amplitude KP_PK _{max are} maintained. This also gives the value of the limit amplitude KP_PK _G. The knock limit KG can be determined according to various criteria. _{The limit amplitude KP_PK G} does not necessarily have to be minimized. On the contrary, one will seek to make the limit amplitude KP_PK _G as large as possible, taking into account the knock peaks that occur (maximum amplitudes KP_PK _max ) or the frequency of occurrence of knocking events. Often, slight knocking is even desired, for example in order to “clean” the combustion chamber in the cylinder from deposits, but of course the knocking peaks must not be too large in order to avoid engine damage. The maximum permitted knocking peaks are also based on the mechanical load capacity of the engine and also on acoustic requirements, because knocking emits an annoying ringing noise. It should also be noted that the limit amplitude KP_PK _{G could} also be determined according to another criterion. For knock calibration, it is crucial that engine control parameters, in particular ignition parameters such as the ignition angle, are adjusted in order to set a specific, defined knock limit KG.

The engine control parameters set, in particular ignition parameters such as the ignition angle, are then stored in the engine control system for this operating point, so that the internal combustion engine can be operated depending on the operating parameters that define the operating point (such as speed, torque, compression ratio, etc.) to control. It is evident from FIG. 6 that a different limit amplitude KP_PK _G would result for a different operating point due to the basic disturbance.

The use of a histogram as in FIG. 6 for the knock calibration is advantageous because frequencies H are assessed, but of course not absolutely necessary.

In order to reduce this negative influence of the basic disturbances in the knock calibration, it is provided according to the invention that the basic disturbance is compensated for in the maximum amplitudes KP_PK determined, for example, in the maximum value detection unit 8. For this purpose, a compensation amplitude KP_PK _ko m _{p is} determined, which is subtracted from the maximum amplitudes KP_PK used for the knock calibration in order _{to determine absolute maximum amplitudes KP_PK abs} with which the knock calibration is then carried out. Instead of differences, the compensation can of course also be carried out differently in an equivalent manner. For example, a weighting factor can be determined from the compensation amplitude KP_PK _ko m _{p with which the} Maximum amplitudes KP_PK are weighted in order to determine the absolute maximum amplitudes KP_PK _abs .

The compensation amplitude KP_PK _ko m can be determined in various ways.

For example, the minimum maximum amplitude KP_PK _mi n can be a number z, for example of the last 10 to 40 work cycles, of past work cycles Ak-1, ..., Ak-z used in the number i work cycles Ak, ..., Ak used for the knock calibration -i are determined and this determined minimum maximum amplitude KP_PK _mi n are subtracted from the determined i maximum amplitudes KP_PKk, ..., KP_PKk-i as compensation amplitude KP_PK _ko m _p. The number z can match the number i, but can also be larger or smaller than the number i. The determination of the compensation amplitude KP_PK _ko m is preferably determined anew in each working cycle k. In FIG. 5, a minimum maximum amplitude KP_PK _mi n is shown as an example. The minimum maximum amplitude KP_PK _mi n could, for example, also be determined in the maximum value recognition unit 8. The smallest maximum amplitude KP_PKmin is preferably determined in each current work cycle Ak from the past z work cycles Ak-1,..., Ak-z.

In another embodiment, a mean value KP_PK _mea n, for example an arithmetic or geometric mean value, could be determined over a number m of past minimum maximum amplitudes KP_PK _m mi,..., KP_PK _mi nm (which corresponds to a number m of compensation amplitudes KP_PK _ko mm) be, for example in the

Form the arithmetic mean value KP _PK, and the average value

KP_PKm _ea n as the compensation amplitude KP_PK _ko m can be subtracted from the maximum amplitudes KP_PKk, ..., KP_PKk-i used for the knock calibration.

_{In a further possible embodiment, the number x smallest maximum amplitudes KP_PK mi} ni, ..., KP_PKmin _{x could} be determined in the number z of past work cycles Ak-1, ..., Ak-z, and from this a mean value KP_PK _mea n, for example again an arithmetic or geometric mean value can be determined, which is then subtracted as the compensation amplitude KP_PK _ko m from the maximum amplitudes KP_PKk, ..., KP_PKk-i used for the knock calibration or used for weighting.

Of course, the mean value over a number x of the smallest maximum amplitudes KP_PK _mi ni, ..., KP_PKmin _x and the mean value over a number m of past minimum maximum amplitudes KP_PK _m m could also be combined with one another to form a mean value g KP_PKm _ea n to determine as the compensation amplitude KP_PK _ko mp, which is subtracted from the maximum amplitudes KP_PKk, KP_PKk-i used for the knock calibration or used for weighting.

In a further possible embodiment, a frequency H _comp can also be specified, for example H _comp = 85%, and in the current work cycle Ak from a number z of past work cycles Ak-1,..., Ak-z, preferably z = i, the associated Compensation amplitude KP_PK _ko m can be determined, for example from a histogram as in FIG. 6, which corresponds to _{the frequency H comp.} The compensation amplitude KP_PK _ko m determined in this way is then subtracted from the maximum amplitudes KP_PKk,..., KP_PKk-i used for the knock calibration or used for weighting.

It would also be conceivable to determine the compensation amplitude in various ways described above and then to average it and to use the value averaged in this way or the largest or smallest value as the compensation amplitude KP_PK _ko m and from the maximum amplitudes KP_PKk, ... To subtract KP_PKk-i or to use it for a weighting.

Normally, knocking always occurs stochastically, i.e. randomly in individual work cycles.

That is, the determination of the compensation amplitude KP_PK _ko m from a few previous work cycles almost always ensures that at least some non-knocking work cycles are present in order to determine the basic disturbance from the above-mentioned methods. In order to prevent strong knocking actually occurring in very strong knocking operating ranges of the engine in all working cycles lying in the evaluation window, the compensation amplitude KP_PK _ko m _{p can be} limited to a defined maximum value in order to ensure the compensation. Otherwise the value of the compensation amplitude KP_PK _ko m could correspond to the actual knocking peak and a “pulling up” into increasingly stronger knocking events could be the result. Alternatively, this could also be intercepted independently of the knocking peaks, for example purely via the maximum cylinder pressure.

The determination of the compensation amplitude KP_PK _ko m and the absolute maximum amplitudes KP_PK _abs can take place in the maximum value recognition unit 8, the evaluation unit 9 or another unit (hardware and / or software).

The effect of these compensation methods is the same insofar as the basic disturbance in the determined maximum amplitudes KP_PK used for the knock calibration is removed in order _{to determine the absolute maximum amplitude KP_PK abs} , which is shown in FIG. 7 for the two courses of the maximum amplitudes KP_PK in FIG . The effect becomes particularly clear in a histogram display, as shown in FIG. By compensating the Basic disturbance, the influence of the internal combustion engine's operating point (speed, torque, compression ratio, etc.) is essentially eliminated. It is therefore basically only necessary to determine the knock limit KG at a single operating point, because the histogram curves are similar at different operating points. This knock limit KG can then be compensated preferably in all others

Operating points are used, which significantly reduces the effort for knock calibration. However, it would also be possible to determine the knock limit KG at two to five different operating points and then use these knock limits for other operating points in the vicinity of the operating point. This could also significantly reduce the effort involved in knock calibration.

The knock limit KG determined can advantageously be used for other operating points and engine control parameters of the engine controller in an engine control unit ECU for controlling the internal combustion engine 1, in particular ignition parameters such as the ignition angle, can be varied until the knock limit KG is observed. However, parameters such as the air mass, air ratio or valve control times can also be set. The engine control parameters determined in this way, in particular ignition parameters such as the ignition angle, are then stored at the operating point in the engine control for controlling the internal combustion engine 1 in normal operation. In this way, a basic data set for an engine control unit ECU can be determined in which the ignition angle or another or at least one further parameter, typically in a torque / speed map, is stored. Of course, other dependencies of the parameter, such as the dependency on a variable compression ratio, can also be mapped in the basic database.

Claims

Claims 1. A method for knock calibration of an internal combustion engine (1) in which the internal combustion engine (1) is operated on a test stand (5) and a knock signal (S) of at least one cylinder of the internal combustion engine (1) is detected and transmitted from the Knock signal (S) a maximum amplitude (KP_PK) of a pressure oscillation recorded with the knock sensor (4) is determined in a working cycle (Ak) of the internal combustion engine (1) and a number of maximum amplitudes (KP_PK) recorded in a first operating point of the internal combustion engine (1) _{Knock calibration is evaluated, characterized in that a compensation amplitude (KP_PK komp} ) is determined from the maximum amplitudes (KP_PK) of a number of work cycles (Ak) recorded in the first operating point, by which the maximum amplitudes (KP_PK) are compensated by a number of absolute maximum amplitudes (KP_PK _abs ) to determine that from the absolute maximum amplitudes (KP_PK _abs ) a knock limit (KG) im first operating point is determined that in at least one second operating point of the internal combustion engine (1) the knock limit (KG) determined in the first operating point is used to vary engine control parameters of an engine control of the internal combustion engine (1) so that the knock limit (KG) is maintained and that the engine control parameters determined in the process are stored in the engine control system for controlling the internal combustion engine (1) in the second operating point. 2. The method according to claim 1, characterized in that the compensation determined amplitude (KP_PK _komp) from the maximum amplitudes (KP_PK) is withdrawn or a weighting factor is determined from the determined compensation amplitude (KP_PK _kom), with which the maximum amplitudes (KP_PK) are weighted. 3. The method according to claim 1 or 2, characterized in that a smallest maximum amplitude (KP_PK _mm ) of a number of work cycles (Ak) is determined as a compensation amplitude (KP_PK _kom ). 4. The method according to claim 2 or 3, characterized in that _{a mean value is determined from a number of determined compensation amplitudes (KP_PK kom} ), and the mean value is used as the compensation amplitude for the knock calibration. 5. The method according to claim 1 or 2, characterized in that frequencies (H) of the maximum amplitudes (KP_PK) of a number of work cycles (Ak) are determined and that maximum amplitude (KP_PK) is used _{as the compensation amplitude (KP_PK kom) that corresponds to a predetermined frequency} (H _kom ) corresponds. 6. The method according to any one of claims 1 to 5, characterized in that the compensation amplitude (KP_PKkomp) is limited to a predetermined maximum value. 7. Test stand for knock calibration of an internal combustion engine (1), at least one knock sensor (4) being arranged on the internal combustion engine (1) in order to detect a knock signal (S) of at least one cylinder of the internal combustion engine (1), and one on the test stand (5) Knock evaluation unit (6) is provided, which determines a maximum amplitude (KP_PK) of a pressure oscillation recorded by the knock sensor (S) from the knock signal (S) in a working cycle (Ak) of the internal combustion engine (1) and, for knock calibration, a number in a first operating point of the Internal combustion engine (1) evaluates recorded maximum amplitudes (KP_PK), characterized in that the knock evaluation unit (6) determines a compensation amplitude (KP_PKkomp) from the maximum amplitudes (KP_PK) of a number of working cycles (Ak) recorded in the first operating point and determines the maximum amplitudes (KP_PK) with the determined compensation amplitude (KP_PKkomp) compensated to a number of absolute maximum amplitudes (KP_PK _a bs) z u determine that the knock evaluation unit (6) determines a knock limit (KG) in the first operating point from the absolute maximum amplitudes (KP_PKabs), that the test bench (5) operates the internal combustion engine (1) at a second operating point, and the knock evaluation unit (6) in second operating point engine control parameters of an engine control of the internal combustion engine (1) varies in order to maintain the knock limit (KG) determined in the first operating point and that the knock evaluation unit (6) stores the determined engine control parameters in the engine control for controlling the internal combustion engine (1) in the second operating point.