# Patent

## Title
Method, recording support and device to calibrate fuel injection

## Abstract

A method to calibrate the fuel injection in at least one combustion chamber of a Diesel engine includes:

    
    
        a) recording the combustion noise power or amplitude in the combustion chamber over a piston position range [γ1i;γ2i],
        b) at the same time, recording the piston position during the same piston position range [γ1i;γ2i],
        c) determining from the preceding recordings for which piston position Kmin-i the measured combustion noise power passes through a minimum Pmin-i when the piston moves from position γ1i to position γ2i,
        d) adjusting the fuel injection according to the determined piston position Kmin-i.

## Background
BACKGROUND AND SUMMARY
The present invention relates to a method, a recording support and a device to calibrate fuel injection.
The Applicant knows a method to calibrate the fuel injection in at least one combustion chamber of a Diesel engine. Each combustion chamber has a piston that moves along a piston stroke between a top dead center and a bottom dead center.
For example, it is known from DE 196 12 179 to measure the combustion noise using knock sensors and to compare the measured combustion noise to a predetermined threshold SW.
By “combustion noise” we mean the noise produced by the fuel when it is injected in the combustion chamber and the noise produced by the explosion of the injected fuel in the combustion chamber. This combustion noise is not a mechanical noise produced by impacts or chocks on mechanical parts of the Diesel engine. The combustion noise is the vibration of the engine structure caused by the fuel injection and the fuel explosion.
For memory, it is reminded that the time at which the fuel explosion takes place in a combustion chamber of a Diesel engine is not known accurately because such explosion is not triggered by a spark like in other combustion engines.
In DE 196 12 179, the time Tc at which the measured combustion noise exceeds threshold SW is considered as being the time when the fuel explosion begins in the combustion chamber. This information on time Tc allows to adjust the fuel injection in this particular combustion chamber.
However, this method is not very reliable. In fact, time Tc varies from one engine cycle to the other even if every Diesel engine parameters are maintained constants from one cycle to the other. The engine cycle is defined as being the time range or the piston position range over which only one fuel explosion takes place in each of the combustion chambers of the engine.
Time Tc is not a very accurate representation of the actual time at which the fuel explosion begins. Thus, any fuel injection adjustment based on this time Tc is not very reliable.
Accordingly, it is desirable to provide a more reliable method to calibrate the fuel injection in at least one combustion chamber of a Diesel engine.
The invention provides, according to an aspect thereof, a fuel injection calibration method comprising for at least one combustion chamber:
a) recording the combustion noise power or amplitude in the combustion chamber over a piston position range [γ1i;γ2i] where γ1i, is a piston position where the fuel is injected into the combustion chamber and γ2i is a piston position where the explosion of the injected fuel has already begun,
b) at the same time, recording the piston position during the same piston position range [γ1i;γ2i],
c) determining from the preceding recordings for which piston position Kmin-i relative to the top dead center of the combustion chamber, the measured combustion noise power passes through a minimum Pmin-i when the piston moves from position γ1i to position γ2i,
d) adjusting (94) the fuel injection according to the determined piston position Kmin-i.
As a result, the piston position Kmin-i, is more accurately correlated with the piston position where the fuel explosion takes place than the position where the combustion noise power exceeds a predetermined threshold. Thus, using piston position Kmin-i to adjust the fuel injection increases the reliability of the method.
The embodiments of the above method may comprise one or several of the following features:

    
    
        the piston position Kmin-i is determined for each combustion chamber and adjusting the fuel injection comprises adjusting the fuel injection timing in each combustion chamber so that the piston positions Kmin-l in each combustion chamber are closer to each other,
        the method further comprises:
        
            computing an average position Kmin from the positions Kmin-i determined for each combustion chamber, and
            adjusting the fuel injection timing in each combustion chamber so that the piston positions Kmin-i in each combustion chamber are equal to this average position Kmin,
        
        
        the combustion noise power or amplitude is only recorded for frequencies ranging between 7.5 kHz and 8.5 kHz,
        for at least one combustion chamber:
        
            the fuel injection adjustment is performed for different engine speed and engine torque and corresponding fuel injection correction factors are recorded in a memory, and
            for a given engine speed or an engine torque, the correction factors to be applied to adjust the fuel injection are recovered from said previously recorded correction factors according to the present engine speed and the present engine torque without proceeding to a new determination of the piston position Kmin-i, and
        
        
        the piston position range [γ1i;γ2i] is shorter than the piston position range extending from the bottom dead center to the top dead center.
    
    


The above embodiments of the method present the following advantages:

    
    
        adjusting the fuel injection timing so that positions Kmin-i are equal or nearly equal in every combustion chamber ensures a smoother rotation of the crankshaft and reduces engine vibrations,
        adjusting the fuel injection timing so that positions Kmin-i in every combustion chamber are equal to the average position Kmin further reduces the engine vibrations,
        only using noise frequencies ranging between 7.5 kHz and 8.5 kHz increases the reliability of the method because this frequency range is less disrupted by mechanical noises,
        recording the fuel injection correction factors for specific engine speeds and torques simplifies the fuel injection adjustment.
    
    


The invention also relates to an information recording support comprising instructions to execute the above method when the instructions are executed by an electronic calculator.
The invention also relates to a device to calibrate the fuel injection in at least one combustion chamber of a Diesel engine, the combustion chamber having a piston that moves along a piston stroke between a top dead center and a bottom dead center, the device comprising:

    
    
        at least one knock sensor fixedly registered to the Diesel engine to measure the combustion noise power or amplitude in the combustion chamber,
        at least one piston position sensor able to sense the piston position along the piston stroke,
        an electronic calculator able to:
        
            record the measured combustion noise power or amplitude in the combustion chamber over a piston position range [γ1i;γ2i], where γ2i is a piston position where the fuel is injected into the combustion chamber and γ2i is a piston position where the explosion of the injected fuel has already begun,
            at the same time, record the piston position over the same piston position range,
            determine from the preceding recordings for which piston position Kmin-i relative to the top dead center of this combustion chamber the measured combustion noise power passes through a minimum Pmin-i when the piston moves from position γ1i to position γ2i, and
            adjust the fuel injection according to the determined piston position.
        
        
    
    


These and other aspects of the invention will be apparent from the following description, drawings and claims.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a truck including a device to calibrate the fuel injection of a Diesel engine.
FIG. 2 is a timing chart of the pressure within a combustion chamber of the engine of the truck of FIG. 1.
FIG. 3 is a timing chart of an injector drive pulse used in the engine of the truck of FIG. 1.
FIG. 4 is a timing chart showing a combustion noise measurement window.
FIG. 5 is a timing chart showing the signal outputted by a knock sensor of the truck of FIG. 1.
FIG. 6 is a flowchart of a method to calibrate the fuel injection of the truck of FIG. 1.
FIG. 7 is a timing chart of a measured combustion noise power according to the piston position in a combustion chamber of the truck of FIG. 1.



In these drawings, the same reference numbers are used to designate the same elements.
In the following description, well-known functions or constructions by a person of ordinary skill in the art are not described in details.
DETAILED DESCRIPTION
FIG. 1 shows a truck 2 equipped with a Diesel engine 4. For example, Diesel engine 4 has six cylinders, each cylinder defining a combustion chamber, respectively, 6-11. Each combustion chamber 6-11 has a respective piston 14-19 that runs along a piston stroke. Each piston stroke extends between a top dead center and a bottom dead center. When the piston is in the top dead center position, the volume of the combustion chamber is minimal. In contrast, when the piston is in the bottom dead center position, the volume of the combustion chamber is maximal.
Each piston 14-19 is mechanically linked to a crankshaft 22 that rotates at engine angular speed ω. More precisely, each piston starts to exert a force to rotate crankshaft 22 at an angle γci where the fuel explosion begins in the corresponding combustion chamber.
In the following specification, index i or letter i refers to the number of a combustion chamber. For example, herein, numbers 1 to 6 are assigned to combustion chambers 6-11, respectively.
In engine 4, the positions γci in each combustion chamber are spaced apart by 120° so that one engine cycle corresponds to a rotation of 720° of crankshaft 22.
Crankshaft 22 exerts a torque that is transmitted through transmission mechanisms to the traction wheels of truck 2. The transmission mechanisms have not been represented in FIG. 1 for simplicity.
At least one fuel injector is registered with each combustion chamber of engine 4. In FIG. 1, for simplicity, only fuel injectors 24-29 registered to respective combustion chambers 6-11, are shown. Each fuel injector propels the fuel inside the combustion chamber.
Each fuel injector is fluidly connected to a fuel injection unit 34 from which each fuel injector draws fuel.
Truck 2 is also equipped with a device to calibrate the fuel injection start time in each combustion chamber. This device comprises:

    
    
        two knock sensors 36 and 38 fixedly mechanically registered to engine 4,
        one angular speed and position sensor 40 that measures both the angular position and the angular speed ω of crankshaft 22,
        a torque estimator 42 that estimates torque Ω exerted by crankshaft 22 from a model of engine 4 and from other measured information,
        a knock processing chip 44 that processes the signal outputted by sensors 36 and 38 and that outputs the combustion noise power of one combustion chamber according to time,
        an electronic calculator 46 connected to chip 44 and to sensor 40 and estimator 42 to adjust the fuel injection start time and to command each fuel injectors 24-29, and
        a memory 48 connected to calculator 46.
    
    


Sensors 36 and 38 are mechanically fixed to engine 4 without any degree of freedom. More precisely, sensor 36 is fixed to engine 4 as close as possible of combustion chambers 6-8 Reversely, sensor 38 is fixed to engine 4 as close as possible of combustion chambers 9-11. For example, knock sensors 36 and 38 are accelerometers sensors and usually have transducers built from piezoelectric materials.
The acceleration measured by sensors 36 and 38 are transmitted to chip 44 through connection lines 50 and 52.
Chip 44 is able to process the signals outputted by sensors 36 and 38 to output to calculator 46 the presently measured combustion noise power. The signal processing performed by chip 44 will be described in more details in view of FIG. 6.
For example, calculator 46 is a programmable electronic calculator able to execute program instructions to perform the method of FIG. 6. To this end, calculator 46 is connected to memory 48 and memory 48 stores the program instructions necessary to execute the method of FIG. 6.
For example, calculator 46 is known as an EMS ECU (Engine Management System Electronic Control Unit).
Calculator 46 includes an injection scheduler 54 that determines the injection start time in each combustion chamber according to a predetermined angular piston position αi and according to the measured angular speed ω. Angular position αi represents the position of the piston in combustion chamber i at which the fuel injection must start. Position αi is expressed in degrees and is relative to the top dead center position of this piston in combustion chamber i.
It is pointed out that in an engine, a piston angular position can be easily converted into a time. This results from the fact that it is always possible to find the time corresponding to an angular position using the angular speed ω and vice versa. In this description, piston angular positions are mainly used but time and time range could have been used instead of angular position and angular range.
Memory 48 stores also one correction angle map 56 per combustion chamber. Each map 56 records the value of a correction factor for different engine speeds ω and engine torques Ω Therein, the correction factor is a correction angle βi. Correction angle βi is an angle intended to be added to a nominal pre-set angle αi0 at which fuel injection should start in combustion chamber i in order to obtain angle αi, used by scheduler 54. Angles αi0 are nominal angle values that are determined independently of the signal measured by sensors 36 and 38. For example, angles αi0 are recorded in memory 48 at the time of manufacturing truck 2. Typically, angles αi0 are spaced apart from each other by angular range equal to 120° in this embodiment.
FIG. 2 shows the evolution of the pressure within combustion chamber i according to the piston position γi. Piston position γi is expressed in degrees and is relative to the top dead center in combustion chamber i.
As it can be shown from this figure, the pressure grows to a maximum pressure and then decreases. During the growing phase, the fuel explosion takes place.
FIG. 3 shows the injector drive pulse of the same combustion chamber i according to position γi. The rising front of the drive pulse corresponds to an angle γ1i. At position γ1i, fuel starts to be injected in combustion chamber i. The pulse lasts during a time interval Δi that corresponds to a specific angular range. During interval Δi, unit 34 continuously injects fuel in combustion chamber i. For example, time range Δi is determined according to an engine torque set point or an engine speed set point. In this embodiment, time range Δi is determined independently from the combustion noise.
FIG. 4 shows a piston position range Δyi that starts from position γ1i and that ends for a piston position γ2i. During range Δyi either one of sensors 36 and 38 is used to continuously measure the combustion noise in combustion chamber i. Range Δyi is chosen long enough so that the knock measurement windows lasts from the beginning of the fuel injection to the beginning of the explosion of the fuel injected in the combustion chamber. However, range Δyi is much shorter than the piston stroke. For example, range Δyi is smaller than 60° and preferably smaller than 40°. In fact, range Δyi should be chosen as small as possible because it reduces the calculation load and improves the accuracy of position Kmin-i.
The function of range Δyi will be understood in view of the description of FIG. 6.
FIG. 5 shows the combustion noise measured by one of sensor 36 or 38 according to position γi. As shown, it is very difficult to determine from such a raw signal the piston position at which begins the fuel explosion without further signal processing.
The operation of the calibration device will now be described with reference to FIG. 6.
The calibration method starts by a calibration phase 70 during which correction angles βi are determined for each combustion chamber i so as to obtain a more uniform and smoother crankshaft rotation.
At the beginning of phase 70, in step 72, once the engine is running at a constant speed ω and at a constant torque Ω, calculator 46 identifies the combustion chamber in which the next fuel injection will take place. Herein, we consider that speed ω and torque Ω are constant if they do not vary by more than 10%. For example, in step 72, the identification of such a combustion chamber is carried out from the angular position of crankshaft 22 measured by sensor 40 and from the knowledge of the nominal angles αi0.
Subsequently, in step 74, calculator 46 selects the knock sensor which is the closest from the identified combustion chamber and adjusts the gain of this knock sensor according to the distance that separates the selected knock sensor from the identified combustion chamber. Thereafter, we assume that knock sensor 36 is selected.
Upon starting of the fuel injection in the identified combustion chamber, i.e. at position γ1i, in step 76, the selected sensor measures the combustion noise and, in step 78, chip 44 and calculator 46 are used to record the power of the measured combustion noise in the frequency range [fmin;fmax]. In this embodiment, frequencies fmin and fmax are equal to 7.5 kHz and 8.5 kHz, respectively. This frequency range [fmin,fmax] corresponds to a frequency range where the power of the combustion noise is predominant in comparison to other noises like mechanical noises.
More precisely, the power of the combustion noise in the frequency range [fmin;fmax] is obtained as follows.
The signal outputted by the knock sensor is first filtered by an anti-aliasing filter. Then, the signal is converted to a digital signal by an analog-to-digital converter. Subsequently, the digital signal is amplified according to the gain determined as a function of the distance between the knock sensor and the selected combustion chamber. The signal is then transmitted through a bandpass filter which rejects most of the frequencies which are out of frequency range [fmin;fmax]. Subsequently, the bandpass filtered signal is sent through a rectifier that outputs the absolute value of the bandpass filtered signal. This absolute value is sent to an integrator that integrates the signal over a predetermined integrator period in order to output the power of the combustion noise in the frequency range [fmin;fmax] to calculator 46. For example, this predetermined integrator period is chosen at least twenty times smaller than the time period corresponding to piston position range [γ1i;γ2i]. This predetermined integrator period corresponds to the frequency at which chip 46 delivers to calculator 46 new measured combustion noise power values.
In parallel to steps 76 and 78, in step 80, at the same time, sensor 40 measures the angular position of crankshaft 22 and, in step 82, calculator 46 records the position γi of the piston in the identified combustion chamber relative to its top dead center. The position γi of the piston is deduced from the measured angular position of crankshaft 22.
Steps 76 to 82 are repeated as long as position γ2i is not reached.
FIG. 7 shows an example of the evolution of the combustion noise power recorded in range Δyi, for cylinder i according to position γi. Typically, at the beginning of range Δyi, the combustion noise power is high. In this period of time, the noise is produced by the fuel injection. Then, the combustion noise power passes through a minimum Pmin-i, at a position Kmin-i. Then, the combustion noise power sharply increases to reach a maximum. The sharp increase of the combustion noise power is due to the fuel explosion.
Subsequently, in step 84, calculator 46 determines position Kmin-i, from the recorded data over range Δyi. For example, in an operation 86, calculator 46 identifies a rapid increase in the curve shown in FIG. 7. Then, in an operation 88, calculator 46 finds the best parabolic curve that fits the curve of FIG. 7 just before the rapid increases. Then, this best parabolic curve is derivated according to position γi to find an accurate value for Kmin-i. Operation 88 reduces the dependency on integrator period and increases the robustness.
Steps 72 to 84 are reiterated for each combustion chamber i of engine 4.
Subsequently, in a step 90, calculator 46 proceeds to the balancing of the positions when each piston applies a force to crankshaft 22 so as to obtain a smoother and more regular rotation of crankshaft 22.
For example, at the beginning of step 90, in an operation 92, calculator 46 computes an average position Kmin-i of the positions Kmin-i. For example, average position Kmin is calculated according to the following relationship:




  
    
      
        
          
            K
            _
          
          
            m
            ⁢
            
              
            
            ⁢
            i
            ⁢
            
              
            
            ⁢
            n
          
        
        =
        
          
            1
            N
          
          ⁢
          
            
              ∑
              
                i
                =
                1
              
              N
            
            ⁢
            
              K
              
                
                  m
                  ⁢
                  
                    
                  
                  ⁢
                  i
                  ⁢
                  
                    
                  
                  ⁢
                  n
                
                -
                i
              
            
          
        
      
    
    
      
        (
        1
        )
      
    
  




where N is equal to the number of combustion chambers.

Subsequently, in operation 94, calculator 46 adjusts the individual injection starting times so that each position Kmin-i becomes equal to Kmin. To this end, for example, in sub-operation 96, calculator 46 calculates the individual correction angle βi for each combustion chamber i according to the difference between Kmin-i and Kmin. For example, each correction angle βi is calculated according to the following relationship:

βi= Kmin−Kmin-i  (2)

Thereafter, in a sub-operation 98, calculator 46 applies the correction angle βi to each nominal pre-set angle αi0. More precisely, the fuel injection starting position αi is computed with the following relationship:

αi=αi0+βi  (3)

At the end of sub-operation 98, injection scheduler 57 controls fuel injection unit 34 so that the fuel injection in combustion chamber i starts at piston position αi.
Then, in step 100, steps 72 to 84 are reiterated once to obtain new values for each position Kmin-i.
Subsequently, in step 102, new values for each position kmin-i are compared to average position Kmin. More precisely, in step 102, the following condition is assessed for each combustion chamber i:

|Kmin-i− Kmin|≦ε  (4)

where ε is a pre-set constant.

For example, ε is smaller than 0.2° or even smaller than 0.1°.
If the above condition (4) is false for at least one combustion chamber, the method returns to step 90 to adjust once again the fuel injection starting time for each combustion chamber. On the contrary, if condition (4) is true, in step 104 the calibration phase ends and each correction angle βi is recorded in the corresponding map 56 associated with the present engine speed ω and the present engine torque Ω estimation. Thus, from time to time, maps 56 are built and stored. As a result, each map 56 stores the value of respective correction angles βi for different engine speeds and engine torques.
Later on, in step 106, if calculator 46 needs to proceed to a new adjustment of angle αi because the engine speed or the engine torque has changed, then, if possible, the correction angles βi corresponding to this new engine speed or new engine torque are recovered from maps 56 and then used to determine the appropriate positions αi at which should start the fuel injection. Then, steps 100 to 102 are reiterated to verify that the recorded correction angles βi are still correct and, otherwise, to return to step 90 to determine new correction angles βi. Thus, the above method, continuously or, at least from time to time, monitors and updates the correction angles βi when engine 4 is running.
Many other embodiments are possible. For example, the Diesel engine may have from four to twelve cylinders or even more.
Only one knock sensor may be used. In another embodiment, more than two knock sensors can be used. For example, one knock sensor per combustion chamber can be used.
The fuel injection unit can be whatever existing fuel injection unit. For example, fuel injection unit can be a common rail-injector or a unit pump injection.
Positions αi, may be adjusted so that the positions Kmin-i are all equal to another predetermined value than average position Kmin. For example, this predetermined value can be the median value of the different positions Kmin-i.
The position at which should start the fuel injection can also be defined relative to the bottom dead center. This does not change anything to the preceding explanation because the angular range between the top dead center and the bottom dead center is constant. Thus, when a position is defined relative to the bottom dead center, it is also defined relative to the top dead center.
Chip 44 may output the average amplitude of the combustion noise rather than the power. However, it should be noticed that the average amplitude is directly correlated with the frequency power of the noise.
In another embodiment, during the time interval Δi, the fuel injection does not take place continuously but rather by burst or pulse.
Piston position γ1i does not have to correspond exactly to the position where the fuel starts to be injected in the combustion chamber. For example, position γ1i may be offset by a few degrees to correspond to a piston position where fuel injection is already running.
The frequency range [fmin,fmax] should be selected according to the engine structure to correspond to a frequency range where the combustion noise power is higher than the other noise. Preferably, frequency fmin is always chosen higher than 1 kHz and frequency fmax is always chosen inferior to 20 kHz. The frequency range [fmin; fmax] width varies from 0.5 kHz to 10 kHz.
In another embodiment, other relations than relation (2) can be used. For example, relation (2) can be replaced by the following relation:

β1=a( Kmin−Kmin-1)+bβic  (5)

where:

    
    
        βic is the former value of the correction angle βi recorded in map 56, for example, and
        coefficients a and b are constants chosen to avoid fast transition from the former value βic of the correction angle to the new value of the correction angle, where a+b=1.
    
    


Positions Kmin-i can also be used to adjust other parameters of the fuel injection than starting angular position αi. For example, positions Kmin-i, can be used to adjust:

    
    
        the quantity of fuel injected in the combustion chamber per cycle,
        the fuel injection ending position or time, or
        the value of range Δi.
    
    












LIST OF REFERENCES


















 2
truck



 4
engine



 6-11
combustion chamber



14-19
pistons



22
crankshaft



24-29
injectors



34
injection control unit



36, 38
knock sensor



40
speed and angular position sensor



42
torque estimator



44
signal processing chip



46
calculator



48
memory



50, 52
communication lines



54
fuel injection scheduler



56
correction angle maps

## Claims

1. A method to calibrate the fuel injection in at least one combustion chamber of a Diesel engine, the combustion chamber having a piston that moves along a piston stroke between a top dead center and a bottom dead center, the method comprising for at least one combustion chamber:
a) recording the combustion noise power or amplitude in the combustion chamber over a piston position range [γ1i; γ2i], where γ1i is a piston position where the fuel is injected into the combustion chamber and γ2i is a piston position where the explosion of the injected fuel has already begun,
b) at the same time, recording the piston position during the same piston position range [γ1i; γ2i],
c) determining from the preceding recordings for which piston position Kmin-i relative to the top dead center of the combustion chamber, the measured combustion noise power passes through a minimum Pmin-i when the piston moves from position γ1i to position γ2i,
d) adjusting the fuel injection according to the determined piston position Kmin-i.

2. The method according to claim 1, wherein the piston position Kmin-i is determined for each combustion chamber and wherein adjusting the fuel injection comprises adjusting the fuel injection timing in each combustion chamber so that piston positions Kmin-i in each combustion chamber are closer to each other.

3. The method according to claim 2, wherein the method further comprises:
computing an average position Kmin from the positions Kmin-i determined for each combustion chamber, and
adjusting the fuel injection timing in each combustion chamber so that the piston positions Kmin-i in each combustion chamber are equal to this average position Kmin.

4. The method according to claim 1, wherein the combustion noise power or amplitude is only recorded for frequencies ranging between 7,5 kHz and 8,5 kHz.

5. The method according to claim 1, wherein for at least one combustion chamber:
the fuel injection adjustment is performed for different engine speed and engine torque and corresponding fuel injection correction factors are recorded in a memory, and
for a given engine speed or an engine torque, the correction factors to be applied to adjust the fuel injection are recovered from said previously recorded correction factors according to the present engine speed and the present engine torque without proceeding to a new determination of the piston position Kmin-i.

6. The method according to claim 1, wherein the piston position range [γ1i; γ2i], is shorter than the piston position range extending from the bottom dead center to the top dead center.

7. Information recording support comprising instructions to execute a method according to claim 1, when the instructions are executed by an electronic calculator.

8. A device to calibrate the fuel injection in at least one combustion chamber of a Diesel engine, the combustion chamber having a piston that moves along a piston stroke between a top dead center and a bottom dead center, the device comprising:
at least one knock sensor fixedly registered to the Diesel engine to measure the combustion noise power or amplitude in the combustion chamber,
at least one piston position sensor able to sense a piston position along the piston stroke,
an electronic calculator able to:
record the measured combustion noise power or amplitude in the combustion chamber over a piston position range [γ1i; γ2i], where γ1i is a piston position where the fuel is injected into the combustion chamber and γ2i is a piston position where the explosion of the injected fuel has already begun,
at the same time, record the piston position over the same piston position range,
determine from the preceding recordings for which piston position Kmin-i relative to the top dead center of this combustion chamber the measured combustion noise power passes through a minimum Pmin-i when the piston moves from position γ1i to position γ2i, and
adjust the fuel injection according to the determined piston position Kmin-i.