METHOD 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

[0001] Description Title Method and apparatus for testing a fuel injector

[0002] State of the art

[0003] 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 an injection valve that is hydraulically 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 have led to a greater number of these common-rail systems being used.

[0004] The need for partial injections per injection process or per operating cycle of the internal combustion engine means that the fuel quantity of each injection is constantly decreasing and the variance of the injection quantity between multiple injection processes or operating cycles is being more tightly tolerated. This also places new demands on methods and equipment for injector testing.

[0005] Disclosure of the invention

[0006] One object of the invention is to provide a cost-effective and robust solution for injector testing with increased accuracy.

[0007] This problem is solved by methods and a device according to the invention. 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 the evaluation of different features in the pressure profile. Since the pressure profile changes 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.

[0008] 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.

[0009] The correlation between injection times and control durations can be determined, for example, by calculating the Pearson correlation coefficient. The agreement of the absolute values ​​of the injection times and the control durations is irrelevant.

[0010] The method selected in this way shows the best linear relationship, but errors at the zero point and / or slope may still be present. To determine the relationship between the injection times and the activation durations exactly, a regression function is calculated from the pairs of values ​​for the activation durations and the determined injection times, e.g., using the "least squares method". The injection time can be determined from the pressure curve using the slope and the axis segment. Such linearization is particularly feasible when only a relatively small area around the respective operating point of the injector is considered.

[0011] 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.

[0012] The invention also relates to a method for determining the injection quantity of individual injection events of a fuel injector, which can be 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; actuating 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.

[0013] 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.

[0014] The method is applicable to any injector type and across the entire operating range of the respective injector, covering the full flow rate 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.

[0015] Using a pressure sensor that is often already present in the supply line reduces acquisition and maintenance costs. The process It 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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 curve. This allows the start and end of the injection process to be determined particularly effectively, reliably and easily.

[0021] 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.

[0022] 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.

[0023] 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. Exemplary embodiments of the invention are explained in more detail below with reference to the accompanying figures:

[0024] Figure 1 schematically shows a device according to the invention for testing an injector.

[0025] Figure 2 shows a schematic flowchart of a test method according to the invention.

[0026] Fig. 3a shows an example of the control of an injector during the injection quantity correlation.

[0027] Fig. 3b shows the injection quantity as a function of the control duration.

[0028] Figures 4a and 4b show the determined corresponding injection times for different control durations, whereby two different methods were used to determine the injection times.

[0029] Figures 5a and 5b show the optimal correlation values ​​determined for different operating points as a function of the control duration.

[0030] Figures 6a and 6b show, by way of example, the control of an injector at the operating point and the resulting pressure profile in the supply line.

[0031] Figures 7a to 7d show the measured pressure profile over time (Fig. 7a and 7d) and in the frequency domain (Fig. 7b and 7c).

[0032] Fig. 8 shows an enlarged section of a processed pressure profile over a certain period.

[0033] Fig. 9 shows a number of injections and the corresponding injection quantities. 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 enables the continuous flow rate or the sum of the injection quantities of several injections to be recorded. As shown in Fig. 1, the volumetric measuring unit 16 can be located on the low-pressure side, i.e., in the outlet of the 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. Figure 2 shows an exemplary schematic flow diagram of a method according to the invention.

[0034] 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 resulting pressure profiles in supply line 4 are measured and, if necessary, stored. In the subsequent evaluation (step 200), the pressure profiles are analyzed. This can either involve using previously stored pressure profiles or evaluating the measured pressure profiles immediately without intermediate storage. In particular, the pressure profiles with various

[0035] Methods determine the respective injection times (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 highest correlation value, is selected for the evaluation of the next measurement (step 230). For the selected method, a relationship between the injection time and the 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

[0036] To determine injection time and injection quantity. In order for a proportional relationship between injection quantity and injection time to be calculated, the average control durations must differ from the corresponding injection quantities.

[0037] 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 due to measurement variations. Alternatively, the injection time can be considered. The relationship between injection quantity and injection time is established at two points around the operating point, and a regression function is fitted between these two points. To calculate the injection quantity from the injection time, interpolation is performed between these points. Linearization is particularly effective when only a small range around the respective operating point is considered.

[0038] 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. The data measured at the operating point are evaluated (step 400). In particular, the injection times of the individual injection events are determined from the recorded pressure profiles using the method determined during injection quantity correlation (step 410), and the individual injection quantities are determined from the injection times (step 420). This can be done either by using previously stored pressure profiles or by using the measured data.

[0039] Pressure curves can be evaluated directly without intermediate storage. The individual injection quantities are statistically evaluated (step 500) to determine the quality of injector 2.

[0040] Fig. 3a shows an example of the control of an injector 2 during the injection quantity correlation. In the diagram shown in Fig. 3a, the control duration T is Ans t (y-axis) plotted against time t for different control phases.

[0041] Injector 2 is initially activated at the operating point with the control duration T. B p is activated (phase A). Then the activation duration T Ans t for a duration ΤΊ below the operating point T B p is reduced and, after a stabilization phase, the pressure profile in the supply line 4 is measured and recorded (phase B). Simultaneously, a flow rate V-ι is measured for a number of injection processes with a control duration Ti.

[0042] Later the control duration T Ans t gradually (step-like) up to an upper control duration T2 above the operating point T B p of the injector is increased (phase C).

[0043] For the upper control duration T2, which is above the operating point T B After a stabilization phase, the flow rate V2 is measured again for a number of injections with the control duration T2 (phase D).

[0044] For each control duration T Ans 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.

[0045] From the flow rates V-ι , V2 measured for the control durations ΤΊ and T2, the relationship between the injection time and the injection quantity is determined using a regression line (linear approximation).

[0046] Fig. 3b shows the measured flow rate Q (y-axis) as a function of the control duration T. A (x-axis) for three different control durations, especially at the operating point (P2), below and above the operating point (P1 , P3). Figure 3b shows that the flow rate as a function of the injection time in the area under consideration can be very well approximated by a straight line.

[0047] Figures 4a and 4b show different control durations T A (x-axis) the corresponding injection times T determined from the pressure profiles in the supply line E (y-axis), where each of the two figures shows a different method for determining the injection time T. E has been used.

[0048] Figures 4a and 4b clearly show that the injection times T determined using the first method (Fig. 4a) E a significantly better correlation with the control durations T Aexhibit, than the injection times T determined using the second method (Fig. 4b). E For evaluating the measurement data at the operating point, the first method (Fig. 4a) is therefore preferable in this case. Figures 5a and 5b show the optimal correlation values ​​K (y-axis) determined for different operating points as a function of the control duration T. A (x-axis).

[0049] The data in Figure 5a were recorded at an injection pressure of 1000 bar, as occurs, for example, during partial load operation of the engine, and the data in Figure 5b were recorded at an injection pressure of 400 bar, as occurs, for example, during idle operation.

[0050] Figures 5a and 5b show only the correlation values ​​K determined using the optimal method for the respective operating point. The different methods are indicated by different symbols for the measurement points.

[0051] The data shown in Figures 5a and 5b demonstrate 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.

[0052] The results further show that in this example the correlation is better overall at a lower injection pressure (e.g. in idle operation) and with changing control duration T. A is subject to lesser fluctuations (Fig. 5b) than with a higher injection pressure, such as occurs in the partial load range (Fig. 5a).

[0053] Figures 6a and 6b show, by way of 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).

[0054] From features of this pressure profile, such as maxima, minima and / or inflection points, the pressure required for a given control duration T can be determined. A proper injection time T E This can be calculated using various methods, each weighting the individual characteristics differently. The previously described procedure selects the most suitable method for the respective operating point.

[0055] One method can also involve transforming the measured pressure profile into the frequency domain and processing it further there.

[0056] Fig. 7a shows an example of such a pressure profile over space or time, and Fig. 7b shows the signal transformed into the frequency domain, e.g., 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.

[0057] In the frequency spectrum shown in Fig. 7c, low-frequency components (< 1000 Hz) are filtered out, so that the higher-frequency components (> 1000 Hz) are much more clearly visible and evaluable.

[0058] Fig. 7d shows the processed signal transformed back into the relevant time period. For comparison, the electrical control signal is also shown as a dashed line.

[0059] Fig. 8 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. Figure 9 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 specified upper limit OG or lower limit UG. The variance of the varying injection quantities can also be determined and compared with a specified target value.

Claims

Patent claims 1. Method (200) for selecting a method for determining the injection time of individual injection events of a fuel injector (2) which can be supplied with pressurized fuel by a supply line (4), comprising the steps: - Controlling the fuel injector (2) with different, known control durations around a predetermined operating point of the fuel injector (2); - Recording (100) the pressure profile over time in the supply line (4) for a number of injection processes; - Evaluate (21 1 , 212, 213) the recorded time-dependent pressure profiles using at least two different methods to determine the respective injection time; - Determining the correlation (221 , 222, 223) between the specified injection times and the respective control duration; - Selecting (230) the method with the highest correlation.

2. Method for determining the injection quantity of individual injection events of a fuel injector (2) which can be supplied with pressurized fuel via a supply line (4), comprising the steps: - Selecting (200) the most suitable method for determining the injection time for the respective operating point by a method according to claim 1; - Activating (300) the fuel injector (2) at at least one predetermined operating point and measuring the pressure profile occurring in a supply line (4); - Determining (410) the injection time of each individual injection operation from the measured pressure profile using the selected method; - Determining (420) the injection quantity of each individual injection operation from the injection time.

3. Method for testing a fuel injector (2) comprising the steps: - Determining the respective injection quantity of a number of individual injection events of a fuel injector (2) at at least one operating point using the method according to claim 2; - statistical evaluation of the injection quantities determined in this way.

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

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

6. Method according to one of the preceding claims, wherein the actuation of the fuel injector (2) includes actuation of the fuel injector (2) with actuation durations that increase or decrease in a step-like manner.

7. Method according to any of the preceding claims, wherein each of the injection processes includes several partial injection processes.

8. Method according to one of the preceding claims, wherein at least one method (21 1 , 212, 213) for evaluating the pressure profile over time includes the transformation of the recorded pressure profile into the frequency domain.

9. Method according to one of the preceding claims, wherein at least one method (21 1 , 212 , 213) for evaluating the pressure profile over time includes the determination of maxima, minima and / or inflection points of the pressure profile.

10. Device for testing a fuel injector (2) with - 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 profile over time in the supply line (4); - at least one volume measuring unit (16) which is set up to detect the flow through the injector (2); - at least one control device (8) designed to control the fuel injector (2); - at least one evaluation unit (14) which is functionally connected and configured with the volume measuring unit (16), the sensor (10) and the control unit (8), to perform at least one of the methods according to one of the preceding claims during operation.