Diagnostic method for internal combustion engine

Abstract

To provide an internal combustion engine diagnostic method considering a state where a fatigue caused by vibration and a fatigue caused by temperature changes acts on the internal combustion engine simultaneously.SOLUTION: An internal combustion engine diagnostic method includes: a first step to extract a target region where a damage due to a high-cycle stress caused by stress is equal to or higher than a given value; a second step to a target region where a damage due to low-cycle stress caused by temperature changes is equal to or higher than a given value; and confirming a target region extracted in either or both of the first and second steps. Further, adding the confirmed damage, in the target region, due to high-cycle stress and damage due to low-cycle damage, and, if the value produced by the addition is larger than a given value, a determination is made that there is a possibility of damage in the target region.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for diagnosing an internal combustion engine such as an exhaust pipe of an automobile.

Background Art

[0002] A method for predicting damage to an exhaust pipe caused by engine vibration has been developed. For example, Patent Document 1 discloses a method of calculating the total time when the engine speed is less than the idling speed, and determining that there is a possibility that the exhaust pipe of the vehicle is damaged when the total time becomes equal to or more than a predetermined value.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The inventor has found the following problems regarding the method for diagnosing an internal combustion engine. The exhaust pipe is also fatigued by a change in temperature in addition to vibration. Until now, in order to determine the possibility of damage at a plurality of locations of the exhaust pipe of the vehicle in advance, fatigue due to vibration and fatigue due to temperature change have been analyzed separately, and it has been determined whether or not each result is equal to or more than an allowable value. Therefore, it has not been possible to determine the possibility of damage due to the superposition of fatigue due to vibration and fatigue due to temperature change as an actual phenomenon.

[0005] The present invention has been made in view of such circumstances, and an object thereof is to provide a method for diagnosing an internal combustion engine in consideration of a state in which fatigue due to vibration and fatigue due to temperature change act simultaneously.

Means for Solving the Problems

[0006] The method for diagnosing an internal combustion engine according to the present invention is A first step of extracting a target part where the damage of high-cycle stress caused by stress on the target part is equal to or greater than a predetermined value; A second step of extracting a target part where the damage of low-cycle stress caused by temperature change on the target part is equal to or greater than a predetermined value; A confirmation step of confirming the extracted target part in either or both of the first step and the second step; The damage caused by high-cycle stress and the damage caused by low-cycle stress of the part confirmed in the confirmation step are added together. If the added value is greater than a predetermined value, it is determined that there is a possibility of breakage at the target part.

[0007] By using the diagnosis method, it is possible to diagnose a state in which fatigue due to vibration and fatigue due to temperature change act simultaneously.

Advantages of the Invention

[0008] According to the present invention, it is possible to provide a diagnosis method for an internal combustion engine that can determine the possibility of breakage due to the superposition of fatigue due to vibration and fatigue due to temperature change.

Brief Description of the Drawings

[0009]

Figure 1

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Figure 8

Mode for Carrying Out the Invention

[0010] Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings. However, the present invention is not limited to the following embodiments. Also, for clarity of explanation, the following description and drawings are simplified as appropriate.

[0011] (First Embodiment) <Fatigue Failure> First, the mechanism of fatigue failure of materials will be described. For a material that breaks after N times of repeated loading of stress (stress amplitude), let D be the damage when the stress amplitude is loaded n times. According to the linear cumulative damage rule, when the total of the damage D becomes 1 or more, fatigue failure of the material occurs.

[0012] In components such as exhaust manifolds and converters, low cycle fatigue (LCF) caused by thermal cycling and high cycle fatigue (HCF) caused by vibration act. LCF is confirmed using Abaqus thermal stress analysis based on multi-point temperature measurement in the ekmani thermal cycling durability test. Here, the ekmani thermal cycling is hereinafter referred to as "ek cold". On the other hand, HCF is confirmed using CeroSim static analysis based on ekmani Rr flange vibration and ekmani stay strain. At this time, in the conventional method, LCF and HCF are confirmed independently. That is, the conventional method does not consider the case where LCF and HCF overlap. Therefore, even if the damage D of each is diagnosed as less than 1, if LCF and HCF overlap at the same part, it will lead to failure.

[0013] FIG. 1 is a flowchart showing a diagnostic method of a diagnostic apparatus for an internal combustion engine according to a first embodiment. Here, DLCF represents damage caused by LCF. Similarly, DHCF represents damage caused by HCF. First, HCF is checked, and a target part where DHCF is 0.5 or more is extracted (step ST1). At this time, the predetermined value of DHCF does not necessarily have to be 0.5. Next, LCF is checked, and a target part where DLCF is 0.5 or more is extracted (step ST2). At this time, the order of the first step ST1 and the second step ST2 may be reversed. Also, the predetermined value of DLCF does not necessarily have to be 0.5. Thereafter, in either or both of step ST1 and step ST2, the extracted target part is checked (step ST3). Regarding the confirmed target part, the sum of DLCF and DHCF at the same part is calculated, and it is checked whether the total damage is 1 or less. (Step ST4). If the total damage of DLCF and DHCF at the same part is less than 1 (step ST4 YES), there is no possibility of breakage, so the process ends. On the other hand, if the total damage of DLCF and DHCF at the same part is 1 or more, there is a possibility of breakage at the target part, so countermeasures are considered (step ST4 NO). After the countermeasures are implemented, the diagnosis of steps ST1 to ST4 described above is repeated for the part where the countermeasures are implemented. Here, the predetermined value of the total damage does not necessarily have to be 1.

[0014] In the diagnostic method according to the first embodiment, the extraction condition for a site where there is a concern about the overlap of LCF and HCF is set as a threshold value of 0.5 or more for both DLCF and DHCF. FIG. 2 is a diagram showing the overlapping effect of DHCF on DLCF under various stress amplitude conditions. According to the Miner's rule, the sum of DLCF and DHCF is 1, suggesting that failure will occur. Since the Miner's rule is an ideal in material failure, in actual phenomena (0.6%, 0.4%, 0.25% stress amplitudes in FIG. 2), the plots are not on a straight line, and each plot reaches failure before the sum of DLCF and DHCF reaches 1. For a site where DLCF is less than 0.4, even if HCF overlaps, the life does not decrease significantly. However, when HCF overlaps with a site where DLCF is 0.4 or more, it can be seen that it deviates greatly from the line of the Miner's rule and the life is decreased. From this, in the diagnostic method according to the first embodiment, the extraction condition for a site where there is a concern about the overlap of LCF and HCF is set such that DLCF and DHCF are each 0.5 or more.

[0015] <Fatigue confirmation method> First, a method for extracting the part where the LCF acts and the DLCF becomes 0.5 or more will be described. FIG. 3 shows the temperature distribution of multi-point temperature measurement under the EC cooling condition. The horizontal axis represents temperature, and the vertical axis represents strain. The strain on the vertical axis, a positive value indicates the strain due to tensile deformation. On the other hand, a negative value indicates the strain due to compressive deformation. First, a method for extracting the part where the LCF acts will be described. Prepare the temperature distribution of multi-point temperature measurement under the EC cooling condition shown in FIG. 3. Using FIG. 3, perform Abaqus thermal stress analysis. Through the Abaqus thermal stress analysis, information on temperature and strain at each part can be obtained. The analysis information on temperature and strain obtained by the Abaqus thermal stress analysis is plotted in FIG. 3. Here, the area surrounded by the EC cooling determination line A and the EC cooling determination line B (the hatched area in FIG. 2) indicates that the DLCF is 0.5 or more with respect to the EC cooling determination line. The EC cooling determination line A is the EC main cooling thermal determination line drawn based on the passing results of the endurance test with 2000 cycles of EC cooling. The EC cooling determination line A is determined by, for example, the material temperature and material. The EC cooling determination line B has a value that is half of the strain at the EC cooling determination line A. That is, the part plotted in the area surrounded by the EC cooling determination line A and the EC cooling determination line B corresponds to the part where the DLCF is 0.5 or more. For example, in FIG. 3, the plot a1 is outside the area surrounded by the EC cooling determination line A and the EC cooling determination line B, so it is a part where the DLCF has a value smaller than 0.5. On the other hand, a2 is inside the area surrounded by the EC cooling determination line A and the EC cooling determination line B, so it is a part where the DLCF is 0.5 or more.

[0016] The ΔT-N diagram arranges the temperature difference ΔT between the maximum temperature and the minimum temperature on the vertical axis and the number of repetitions N on the horizontal axis, and is used for life determination against thermal cycling. Figure 4 is a correlation diagram of the damage by the ECC cold determination line and the damage by the ΔT-N diagram. In the straight line C, the correlation between the damage by the ECC cold determination line and the damage by the ΔT-N diagram is 1:1. On the other hand, in the straight line D, the correlation between the damage by the ECC cold determination line and the damage by the ΔT-N diagram is 2.5:1. That is, due to the difference in the drawing method of the ECC cold determination line, the correlation between the damage by the ECC cold determination line and the damage by the ΔT-N diagram is different. Here, since the ECC cold determination line is a determination on the safe side, the ECC cold determination line is used to extract the part where DLCF is 0.5 or more.

[0017] Next, a method for extracting the part where HCF acts and DHCF is 0.5 or more will be described. Abaqus static analysis is performed using the Ekmani Rr flange vibration and the Ekmanist strain. At this time, CeroSim static analysis may be used for the Abaqus static analysis. The top 10 points of the analyzed HCF part are extracted, and the temperature of the extracted part is confirmed using the temperature distribution of the Abaqus thermal stress analysis. Subsequently, a determination line is drawn on the modified GoodMan diagram with reference to the literature values. At this time, it is preferable to draw the determination line with reference to the literature value as close as possible to the temperature of the extracted part. Note that the determination line is preferably drawn with a scale of at least 50°C at intervals. Also, the stress amplitude and the mean stress of the extracted part are plotted on the modified GoodMan diagram. An auxiliary line parallel to the determination line is drawn through the plot of the extracted part and expressed as a linear function (y = ax + b). The intercept b in the linear function is the simulated stress amplitude for calculating the damage rate. By dividing the intercept b by the fatigue strength Sf, DHCF can be calculated. Finally, for the 10 points extracted, the part where DHCF is 0.5 or more is selected. At this time, if DHCF of all the 10 points extracted is 0.5 or more, another 10 points are extracted and the above-mentioned method is repeated to extract all the parts where DHCF is 0.5 or more.

[0018] Referring to Fig. 5, a calculation example of DHCF in the modified Goodman diagram is shown. A calculation example is shown when the stress amplitude, mean stress, and material temperature at the extraction site are 30 MPa, 20 MPa, and 794 °C, respectively. At this time, let the plot at the extraction site in Fig. 4 be P. Since the temperature of plot P is 794 °C, the determination lines at 800 °C, 750 °C, and 700 °C are drawn on the modified Goodman diagram. For the determination line, the material strength at 800 °C, which is close to the temperature at the extraction site, is used. At this time, the tensile strength is 45 MPa and the fatigue strength is 73 MPa (Sf1). Therefore, the determination line is represented by y = -1.622x + 73. Calculating an equation parallel to the determination line passing through plot P at the extraction site gives y = -1.622x + 62.4. That is, the intercept b at the extraction site is 62.4 MPa. Since the DHCF for the extraction site with respect to the determination line is the ratio of the fatigue strength Sf1 to the intercept b, it can be calculated as 0.85 MPa by dividing 62.4 MPa (intercept b) by 73 MPa (Sf1).

[0019] Here, in the modified Goodman diagram, the horizontal axis is the mean stress and the vertical axis is the stress amplitude. On the modified Goodman diagram, a determination line indicating the material strength is drawn based on the literature values at a temperature close to the extraction site. The intersection of the determination line and the horizontal axis represents the tensile strength, and the intersection of the determination line and the vertical axis represents the fatigue strength. If it is below the determination line, it indicates infinite life. On the other hand, in the S-N diagram, the horizontal axis is the number of repetitions and the vertical axis is the stress amplitude, and the life with respect to the stress amplitude is calculated. Fig. 6 is a correlation diagram between the damage calculated using the modified Goodman diagram and the damage calculated using the S-N diagram. In straight line E in Fig. 6, the correlation between the damage calculated using the modified Goodman diagram and the damage calculated using the S-N diagram is 1:1. It can be seen that the judgments of both methods are equivalent when the damage is near 1. However, when the damage is greater than 1, it can be seen that the damage calculation using the modified Goodman diagram makes a safer judgment compared to the S-N diagram. That is, the damage calculation method using the modified Goodman diagram and the damage calculation method using the S-N diagram are equivalent damage calculation methods when the damage is near 1.

[0020] Fig. 7 shows the correlation diagram between the Abaqus analysis values and the CeroSim analysis values. The straight line F in Fig. 7 indicates that the Abaqus analysis values and the CeroSim analysis values have a 1:1 correlation. Slightly more plots are concentrated on the Abaqus side than the straight line F. That is, the Abaqus analysis values tend to be slightly on the safe side. When extracting the site where HCF acts according to this embodiment, either Abaqus static analysis or CeroSim static analysis can be used.

[0021] Table 1 shows the calculation results of DLCF and DHCF in the exhaust manifold using the method according to the first embodiment. At this time, Table 1 arranges the values of DLCF based on the extraction sites where DHCF is 0.5 or more. P1 to P10 represent the extraction sites. Fig. 8 shows the sites where DHCF in the exhaust manifold is 0.5 or more and the temperature at those sites. Fig. 8 represents the high and low temperatures using a color scale.

[0022] Focusing on DHCF, the DHCF at the extraction sites P2, P5, P6, P7, and P8 is greater than 1. That is, it suggests that there is a possibility of reaching failure only by HCF. In order to suppress destruction, it is considered necessary to reduce DHCF to 1 or less. On the other hand, focusing on DLCF, the extraction sites P2, P3, P5, P6, P7, P9, and P10 are the sites where DLCF is 0.5 or more. Therefore, even if DHCF is reduced to suppress destruction, due to the superposition with DHCF, the life may be extremely reduced and lead to destruction. Among them, the DHCF at the extraction sites P3, P9, and P10 is less than 1 but 0.5 or more. Therefore, it may lead to destruction due to the superposition with DLCF. Focusing on the total value of the extraction sites P3, P9, and P10, since the total value of DLCF and DHCF exceeds 1, there is a possibility of reaching destruction due to the superposition of LCF and HCF. The DLCF at the extraction site P10 is greater than 1. That is, it suggests that there is a possibility of reaching failure only by LCF. In order to suppress destruction, it is considered necessary to reduce DLCF to 1 or less.

[0023] The DLCF of the extraction part P10 exceeds 1, and there is a possibility of damage only due to LCF. However, no crack occurred in the exhaust manifold. From this, it can be seen that the determination line of the exhaust gas cooling diagnoses on the safe side.

[0024] In Table 1, it can be confirmed that the material temperature of the part extracted with high damage is about 50 to 100 °C higher than the measurement result of the bellmouth vehicle in the temperature distribution of the exhaust gas cooling. That is, since it is higher than the temperature measurement result of the bellmouth vehicle, it is considered that a large damage is obtained due to the decrease in material strength.

Table 1

[0025] Table 2 shows the calculation results of DLCF and DHCF when the material temperature is calculated based on the temperature measurement result of the bellmouth vehicle. Compared with the results in Table 1, since the temperature of each part decreases, the value of DHCF generally decreases. However, for the extraction parts P1, P3, P4, and P9, the value of DHCF increases. This is because the material temperature of the exhaust gas cooling is lower than that of the bellmouth vehicle, so the yield stress in the bellmouth vehicle increases compared to the yield stress of the exhaust gas cooling, and the stress amplitude at the extraction parts P1, P3, P4, and P9 becomes below the yield stress and can be deformed in the elastic region, resulting in an increase in the mean stress. In the bellmouth vehicle, although the value of DHCF decreases, the DHCF of the extraction parts P2, P7, and P8 still exceeds 1. From Figure 8, the extraction parts P2, P7, and P8 are all edge parts. Since the analytical value of the stress amplitude takes into account safety factors such as material lower limit correction, strength reduction correction of the heat affected zone, and edge part strain correction, it is considered that the diagnosis is on the safe side.

Table 2

[0026] From the above, it is possible to identify the parts where LCF and HCF may overlap and be damaged by the method according to this embodiment. That is, it is possible to consider the state in which fatigue due to temperature change and fatigue due to vibration act simultaneously. Regarding the necessity of the final damage countermeasure, diagnosis based on the confirmation results of the actual temperature and actual stress of the extracted parts is required.

[0027] Note that the present invention is not limited to the above embodiment, and can be appropriately changed without departing from the gist.

Explanation of reference signs

[0028] a1 plot a2 plot P plot P1 to P10 extraction parts

Claims

Claim 1 A high-cycle stress caused by stress on a target part, for a material that breaks after N repetitions of stress loading, a first step of extracting a target part where the damage when the stress amplitude is loaded n times is equal to or greater than a first predetermined value; A low-cycle stress caused by a temperature change in the target part, for a material that breaks after N repetitions of stress loading, a second step of extracting a target part where the damage when the stress amplitude is loaded n times is equal to or greater than a second predetermined value; A confirmation step of confirming the extracted target part in either or both of the first step and the second step; Adding the damage due to high-cycle stress and the damage due to low-cycle stress of the part confirmed in the confirmation step, and if the added value is greater than a third predetermined value, determining that there is a possibility of breakage at the target part; A diagnostic method for an internal combustion engine.

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