Process and device for testing a fuel injector

Abstract

The invention relates to a process for selecting a method for determining the injection time of individual injection processes of a fuel injector (2), which can be supplied with pressurized fuel via a feed line (4), comprising the following steps: actuating the fuel injector (2) using various known actuation durations around a predefined operating point of the fuel injector (2); detecting (100) the pressure course over time in the feed line (4) for a number of injection processes; evaluating (200) the detected pressure courses over time using at least two different methods (211, 212, 213) for determining the injection time from the associated pressure course; determining the correlation (221, 222, 223) between the determined injection times and the associated actuation duration; selecting (230) the method with the highest correlation.

Description

State of the art

[0001] Fuel injection systems, which operate at very high injection pressures, are increasingly used to supply fuel to internal combustion engines. In these systems, fuel is pumped by a high-pressure pump into a high-pressure accumulator, from which it is injected into the combustion chambers of the engine via injectors. Diesel engines, in particular, use injectors with a hydraulically operated valve that is opened and closed by a servo valve to control the timing of the injection process into the combustion chamber. The servo valve is actuated by a magnetic or piezoelectric actuator. Increasingly stringent emissions regulations worldwide and the continuous improvement in engine efficiency mean that these common-rail systems require a greater number of partial injections per injection cycle.The amount of fuel required per combustion engine operating cycle is decreasing, the amount of fuel in each injection is becoming smaller, and the tolerance for the variance in the injection quantity between multiple injections or operating cycles is becoming tighter. This also places new demands on methods and equipment for injector testing.

[0002] Documents EP 1854987 A2, WO 2004 / 065775 A1, WO 2008 / 092779 A1, DE 10 2008 033754 A1 and DE 197 40 608 A1 disclose various methods for calculating the injection time of individual injection events of a fuel injector. Furthermore, document DE 198 51 285 C1 discloses a test device for a fuel injection valve. Disclosure of the invention

[0003] One object of the invention is to provide a cost-effective and robust solution for injector testing with increased accuracy.

[0004] This problem is solved by the inventive method according to claim 1 and the inventive device according to claim 10.

[0005] The invention is based on the fundamental idea that opening and closing the injector generates pressure waves in the injector's supply line, and that the injection time (i.e., the duration the injector is open) can be determined by measuring and evaluating the pressure profile in the supply line. Various methods exist for this purpose, based on evaluating different features of the pressure profile. Since the pressure profile varies for each injector type and operating point (temperature, pressure, injection time, etc.) due to various influencing factors, there is currently no universally applicable method that delivers the best possible result for every injector type at every operating point. The invention therefore comprises a method for selecting the most suitable method for each specific application from a number of different methods.

[0006] An inventive method for selecting a method for determining the injection time of individual injection events of a fuel injector, which can be supplied with pressurized fuel via a supply line, comprises the steps of actuating the fuel injector with various known actuation durations in the vicinity of a predetermined operating point of the fuel injector; recording the pressure profile over time in the supply line for a number of injection events for each actuation duration; evaluating the recorded pressure profiles over time with at least two different methods for determining the injection time for each injection event; determining the correlation between the determined injection times and the respective associated actuation duration; and selecting the method with the highest correlation.

[0007] The correlation between injection times and control durations is determined by calculating the Pearson correlation coefficient. The agreement of the absolute values ​​of the injection times and the control durations is irrelevant.

[0008] The method selected in this way shows the best linear relationship, but errors related to the zero point and / or slope may still be present. To determine the relationship between the injection times and the actuation durations exactly, a regression function is constructed from the pairs of actuation durations and the determined injection times, for example, using the method of least squares. In the case of a regression line, the injection time can be determined from the pressure curve using the slope and the y-intercept. Such linearization is particularly feasible when only a relatively small range around the respective operating point of the injector is considered.

[0009] It is also possible to set a threshold for the correlation value such that injection times are only determined when the threshold is exceeded, ensuring that the method exhibits a sufficiently strong linear relationship between injection time and control duration. Alternatively, injection times can also be determined when the threshold is not met and output with a corresponding warning.

[0010] The invention also relates to a method for determining the injection quantity of individual injection events of a fuel injector, which is supplied with pressurized fuel via a supply line, comprising the steps of: selecting the most suitable method for determining the injection time for the respective operating point using the method described above; activating the fuel injector at at least one predetermined operating point and simultaneously measuring the pressure profile occurring in a supply line; determining the injection time of each individual injection event from the measured pressure profile using the selected method; and determining the injection quantity of each individual injection event from the previously determined injection time.

[0011] The inventive method allows the injection time of a single injection process of a fuel injector to be reliably determined with high accuracy, even for short injection times.

[0012] The method is applicable to any injector type and across the entire operating range of the respective injector, covering the full flow rate range of various injectors (passenger cars, trucks, piezo actuators, solenoid valves). The measuring technology itself is limited only by the pressure sensor. This may need to be adjusted or replaced.

[0013] By using a pressure sensor that is often already present in the supply line, acquisition and maintenance costs are reduced. The method is insensitive to the injector's installation position and easy to use, as neither complex mechanics nor the creation of back pressure are required. The method allows for easy retrofitting of existing systems with continuous flow measurement and is suitable for workshop use due to its robustness and resistance to dirt.

[0014] The invention also relates to a method for testing a fuel injector comprising the steps of: determining the respective injection quantity of a number of individual injection events of a fuel injector at at least one operating point using the method described above, and statistically evaluating the injection quantities thus determined. Such a test method enables a particularly accurate and effective testing of modern high-performance injectors that are operated at high injection pressures of several thousand bar and short injection times.

[0015] In one embodiment, the method for testing a fuel injector also includes the evaluation of a measure of dispersion, such as the standard deviation or the variance, of the determined injection quantities. This allows the quality of the test to be improved even further.

[0016] In one embodiment, each injection process includes several partial injection processes. The method is flexible enough to also evaluate injection processes that comprise multiple partial injection processes.

[0017] In one embodiment, the method for evaluating the pressure profile over time includes transforming the recorded pressure profile into the frequency domain. This transformation improves the evaluation; in particular, interfering frequency components can be filtered out before further analysis. In another embodiment, the evaluation method also includes the reverse transformation of the pressure profile from the frequency domain back into the spatial or time domain.

[0018] In one embodiment, the method for evaluating the pressure profile over time includes the determination of maxima, minima, and / or inflection points of the pressure profile. This allows the beginning and end of the injection process to be determined particularly effectively, reliably, and easily.

[0019] In one embodiment, the method includes controlling the fuel injector with control durations above and below the operating point. In particular, the method includes successively controlling the fuel injector with a series of stepwise or incrementally increasing or decreasing control durations. Such a stepwise control allows for a particularly good determination of the correlation between the control duration and the injection time determined from the pressure profile, and enables the most suitable method for evaluating the pressure profile for the respective injector at the operating point under consideration to be selected very effectively.

[0020] The invention also relates to a device for testing a fuel injector. Such a device has at least one receiving device for receiving at least one fuel injector; at least one supply line configured to supply pressurized fluid to the fuel injector; at least one sensor configured to measure the pressure profile over time; a volume measuring unit configured to detect the flow rate through the injector; at least one control device configured to control the fuel injector; and at least one evaluation unit functionally connected to the volume measuring unit, the sensor, and the control device. The evaluation unit is configured to perform at least one of the methods according to the invention.

[0021] The sensor for measuring the pressure profile over time in the supply line can be a pressure sensor located in the supply line or a structure-borne sound sensor attached to the supply line, which measures the sound generated by the pressure fluctuations propagating in the supply line. Such a structure-borne sound sensor can, for example, be designed as a piezoelectric element.

[0022] Exemplary embodiments of the invention are explained in more detail below with reference to the accompanying figures: Figur 1 Figure 1 schematically shows a device according to the invention for testing an injector. Figur 2 shows a schematic flowchart of a test method according to the invention. Fig. 3a This shows, as an example, the control of an injector during injection quantity correlation. Fig. 3b shows the injection quantity as a function of the activation duration. Figuren 4a and 4bThe figures show the corresponding injection times determined for different control durations, whereby two different methods were used to determine the injection times. Figuren 5a and 5b The optimal correlation values ​​determined for various operating points are shown as a function of the control duration. Figuren 6a und 6b This illustrates, for example, the control of an injector at its operating point and the resulting pressure profile in the supply line. Figuren 7a bis 7d show the measured pressure profile over the period ( Fig. 7a and 7d ) and in the frequency domain ( Fig. 7b and 7c ). Fig. 8 shows an enlarged section of an edited print profile over a specific period. Fig. 9 shows a number of injections and the corresponding injection quantities.

[0023] Figur 1 Figure 1 schematically shows a device according to the invention for testing an injector 2. The injector 2 to be tested is arranged in an injector holder 1 and connected via a (high-pressure) supply line 4 to a (high-)pressure accumulator 6, which contains a fluid to be injected, such as (diesel) fuel or a test oil. The injector 2 is electrically controlled by a triggering device 8, e.g., an engine control unit or a test device simulating an engine control unit. A pressure sensor 10 is arranged in the supply line 4 and measures the pressure profile over time in the supply line 4. A trigger sensor 12, which can be designed as a current sensor, detects the start time of the electrical control signal as a trigger. Alternatively, the start time can also be output directly by the triggering device 8. A data acquisition unit 14 records the measurement data, in particular the pressure profile and the trigger signal.A volumetric measuring unit 16 makes it possible to measure the continuous flow rate or the sum of the injection quantities of several injections. The volumetric measuring unit 16 can be, as in the . Fig. 1 The device is shown on the low-pressure side, i.e., in the outlet of injector 2, or in the supply line 4 on the high-pressure side. It can also be directly connected to the measurement data acquisition unit 14.

[0024] Figur 2 A schematic flowchart illustrates, by way of example, the process of a method according to the invention.

[0025] In a first step (step 100), a number of injection events with varying control durations are performed in the vicinity of a measuring operating point (test point), and the pressure profiles occurring in supply line 4 are measured and, if necessary, stored. In the subsequent evaluation (step 200), the pressure profiles are analyzed. Either previously stored pressure profiles can be used, or the measured pressure profiles can be evaluated immediately without intermediate storage. In particular, the corresponding injection times are determined from the pressure profiles using various methods (steps 211, 212, 213), and the correlation of the injection times thus determined with the corresponding control durations is calculated (steps 221, 222, 223). The correlation values ​​determined in this way are compared with each other, and the method with the best correlation, i.e.,The measurement with the highest correlation value is selected for evaluation of the following measurement (step 230).

[0026] For the selected method, a relationship between injection time and injection quantity is established (step 240). For this purpose, a sum of injection quantities measured with the volumetric measuring unit 16 for a number of injection events can be used to determine the relationship between injection time and injection quantity. In order for a proportional relationship between injection quantities and injection time to be calculated, the mean control durations must differ from the corresponding injection quantities.

[0027] If the injection quantities are determined from a continuous flow lasting, for example, 2 to 3 minutes, an average injection quantity is obtained. This eliminates errors caused by measurement variations.

[0028] Alternatively, the injection time can be considered. The relationship between injection quantity and injection time is determined at two points around the operating point, and a regression function is fitted through these two points. The injection quantity is then calculated from the injection time by interpolating between these points. Linearization is particularly effective when only a small range around the respective operating point is considered.

[0029] In step 300, the measurement data, i.e., the pressure fluctuations in the supply line during injector activation at the operating point, are measured and, if necessary, stored. This can occur before or after selecting the most suitable method (injection quantity correlation) in steps 100 and 200. It can also occur before or after step 100, as well as before, after, or during the selection of the most suitable method (injection quantity correlation) in step 200.

[0030] The data measured at the operating point are evaluated (step 400). In particular, the injection times of the individual injection processes are determined from the recorded pressure profiles using the method determined during the injection quantity correlation (step 410), and the individual injection quantities are determined from the injection times (step 420). Either previously stored pressure profiles can be used, or the measured pressure profiles can be evaluated directly without intermediate storage.

[0031] The individual injection quantities are statistically evaluated (step 500) to determine the quality of injector 2.

[0032] Fig. 3a This shows, as an example, the control of injector 2 during injection quantity correlation. In the diagram shown in the Fig. 3a The diagram shown plots the control duration T Anst (y-axis) for different control phases against time t.

[0033] Injector 2 is initially activated at the operating point with a control duration TBP (Phase A). Then, the control duration TAnst is reduced to a duration T1 below the operating point TBP, and after a stabilization phase, the pressure profile in the supply line 4 is measured and recorded (Phase B). Simultaneously, a flow rate V1 is measured for a number of injection events with a control duration T1.

[0034] Later, the control duration T Anst is gradually (step-like) increased to an upper control duration T 2 above the operating point T BP of the injector (phase C).

[0035] For the upper control duration T 2, which lies above the operating point T BP, the flow rate V 2 is measured again after a stabilization phase for a number of injections with the control duration T 2 (phase D).

[0036] For each actuation duration T Anst, the pressure profile over time in the supply line 4 is measured and recorded for a number of injection processes that is statistically sufficient to achieve the required accuracy.

[0037] From the flow rates V1 and V2 measured for the control durations T1 and T2, the relationship between the injection time and the injection quantity is determined using a regression line (linear approximation).

[0038] Fig 3b shows the measured flow rate Q (y-axis) as a function of the control duration TA (x-axis) for three different control durations, in particular at the operating point (P2), below and above the operating point (P1, P3).

[0039] The Figur 3b shows that the flow rate as a function of the injection time in the considered range can be very well approximated by a straight line.

[0040] The Figuren 4a and 4bThe figures show the corresponding injection times TE (y-axis) determined from the pressure profiles in the supply line for different control durations TA (x-axis), whereby a different method for determining the injection time TE has been used in each of the two figures.

[0041] From the Figuren 4a and 4b It is clearly evident that the first method ( Fig. 4a The injection times TE determined with the second method show a significantly better correlation with the control durations TA than those determined with the second method ( Fig. 4b ) determined injection times TE. Therefore, in this case, the first method is the appropriate one for evaluating the measurement data at the operating point ( Fig. 4a ) preferable.

[0042] The Figuren 5a and 5b The optimal correlation values ​​K (y-axis) determined for different operating points are shown as a function of the control duration TA (x-axis).

[0043] The data of Figur 5a were measured at an injection pressure of 1000 bar, as occurs, for example, during partial load operation of the engine, and the data of the Figur 5b The images were recorded at an injection pressure of 400 bar, as occurs, for example, during idle.

[0044] In the Figuren 5a and 5b Only the correlation values ​​K determined using the optimal method for the respective operating point are shown. The different methods are indicated by different symbols for the measurement points.

[0045] The in the Fig. 5a and 5b The data presented show that the optimal method, i.e., the method with the best correlation between the activation duration and the injection time, depends on both the injection pressure and the activation duration. Therefore, the optimal method must be determined anew for each injector and for each operating point.

[0046] The results further show that in this example, the correlation is better overall at a lower injection pressure (e.g., during idle operation) and is subject to less fluctuation when the control duration TA changes ( Fig. 5b ) than with a higher injection pressure, such as occurs in the partial load range ( Fig. 5a ).

[0047] The Figuren 6a und 6b show, as an example, the control of an injector at the operating point ( Fig. 6a ) and the resulting pressure profile p in the supply line 4 ( Fig. 6b ).

[0048] From characteristics of this pressure profile, such as maxima, minima, and / or inflection points, the injection time TE corresponding to a given control duration TA can be calculated. This can be done using various methods, which weight the individual characteristics differently. The procedure described above selects the most suitable method for the respective operating point.

[0049] One method can also involve transforming the measured pressure profile into the frequency domain and processing it further there.

[0050] Fig. 7a This shows an example of such a pressure profile over a specific location or time period and Fig. 7b This shows the signal transformed into the frequency domain, for example using a Fast Fourier Transform (FFT). The signal exhibits strong frequency components in the range around 500 Hz, which complicate the evaluation of the significantly weaker frequency components in the higher frequency range.

[0051] In the Fig. 7c In the frequency spectrum shown, low-frequency components (< 1000 Hz) have been filtered out, so that the higher-frequency components (> 1000 Hz) are much more clearly visible and evaluable.

[0052] Fig. 7d The graph shows the processed signal transformed back into the specified time period. For comparison, the electrical control signal is also shown as a dashed line.

[0053] Fig. 8 The figure shows an enlarged section of the processed signal over a specific period, i.e., the pressure p (y-axis) in line 4 as a function of time t (x-axis). The start (BIP) and end (EIP) of the injection process are determined based on predefined characteristics, in this case, characteristic inflection points. The injection time TIP is calculated as the difference in time between the end (EIP) and the start (BIP) of the injection process.

[0054] Fig. 9 The graph shows a number of injections (x-axis) and the corresponding injection quantities (y-axis), which vary around a mean value (MW) normalized to 1. For evaluation, the mean value (MW) can be compared with a target value specified for the respective operating point, and it can be investigated whether individual injection quantities exceed or fall below a predefined upper limit (OG) or lower limit (UG). The variance of the varying injection quantities can also be determined and compared with a predefined target value.

Claims

1. Method (200) for selecting a procedure for determining the injection time of individual injection operations of a fuel injector (2) in a device for testing the fuel injector (2), wherein the fuel injector (2) is supplied with pressurized fuel through a supply line (4), said method comprising the following steps: - actuating the fuel injector (2) with various known actuation periods close to a predefined operating point of the fuel injector (2) in a device for testing the fuel injector (2); - detecting (100) the pressure curve over time in the supply line (4) for a number of injection operations; - evaluating (211, 212, 213) the detected pressure curves over time using at least two different procedures for determining the respective injection time; - determining the correlation (221, 222, 223) between the determined injection times and the respective known actuation period by calculating the Pearson correlation coefficients; - selecting (230) the procedure that is the most appropriate for the respective predefined operating point as the procedure with the highest correlation, wherein the procedure with the highest correlation can be redefined for each fuel injector (2) and for each operating point.

2. Method for determining the injection quantity of individual injection operations of a fuel injector (2) that is able to be supplied with pressurized fuel via a supply line (4), said method comprising the following steps: - selecting (200) the procedure for determining the injection time that is the most suitable for the respective operating point using a method according to Claim 1; - actuating (300) the fuel injector (2) at at least one predefined operating point and measuring the pressure curve occurring in a supply line (4); - determining (410) the injection time of each individual injection operation from the measured pressure curve using the selected procedure; - determining (420) the injection quantity of each individual injection operation from the injection time.

3. Method for testing a fuel injector (2), said method comprising the following steps: - determining the respective injection quantity of a number of individual injection operations of a fuel injector (2) at at least one operating point using the method according to Claim 2; - statistically evaluating the injection quantities thus determined.

4. Method for testing a fuel injector (2) according to Claim 3, wherein the step of statistical evaluation includes the evaluation of a spread of the determined injection quantities.

5. Method according to any one of the preceding claims, wherein the actuation of the fuel injector (2) includes actuating the fuel injector (2) with actuation periods above and below the operating point.

6. Method according to any one of the preceding claims, wherein the actuation of the fuel injector (2) includes actuating the fuel injector (2) with actuation periods that increase or decrease in steps.

7. Method according to any one of the preceding claims, wherein each of the injection operations includes multiple partial injection operations.

8. Method according to any one of the preceding claims, wherein at least one procedure (211, 212, 213) for evaluating the pressure curve over time includes the transformation of the detected pressure curve into the frequency domain.

9. Method according to any one of the preceding claims, wherein at least one procedure (211, 212, 213) for evaluating the pressure curve over time includes the determination of maxima, minima and / or inflection points of the pressure curve.

10. Device for testing a fuel injector (2) having - at least one receiving device (1) for receiving at least one fuel injector (2); - at least one supply line (4) designed to supply a pressurized fluid to the fuel injector (2) during operation; - at least one sensor (10) designed to measure the pressure curve over time in the supply line (4); - at least one volume measuring unit (16) configured to measure the flow through the injector (2); - at least one actuation apparatus (8) designed to actuate the fuel injector (2); - at least one evaluation unit (14) which is functionally connected to the volume measuring unit (16), the sensor (10) and the actuation apparatus (8) and is designed to carry out at least one of the methods according to any one of the preceding claims during operation.

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