Method for detecting coking in the intake tract of an internal combustion engine with direct fuel injection

Abstract

Method for detecting coking in the intake tract of an internal combustion engine with direct fuel injection, throttle valve and variable intake valve lift control, comprising the following steps: - Determining a correction value (KW) which is calculated by the intake valve lift control as an offset value with a preset valve lift; - Performing an initial quantity deviation test, by which an initial air-fuel ratio value (w1) is determined from a lambda value (λ) measured during the initial quantity deviation test. real, 1 ) and a desired lambda value (λ soll, 1 ) the fuel combustion in the combustion chambers of the internal combustion engine, whereby in the first quantity deviation test a load control is carried out by means of the variable intake valve lift control; - Performing a second quantity deviation test, by which a second air-fuel ratio value (w2) is determined from a lambda value (λ) measured during the second quantity deviation test. real, 2 ) and a desired lambda value (λ soll, 2 ) the fuel combustion in the combustion chambers of the internal combustion engine, whereby in the second quantity deviation test a load control is carried out by means of the throttle valve; - Determining a comparative result (VE) from the first air ratio value (w4) and the second air ratio value (w2); - Determine whether coking is present in the intake tract of the internal combustion engine by combining the comparison result (VE) and the correction value (KW).

Description

[0001] The invention relates to a method for detecting coking in the intake tract of an internal combustion engine which has direct fuel injection, a throttle valve and variable intake valve lift control, as well as a corresponding engine test device.

[0002] In gasoline direct injection (GDI) engines, carbon buildup can occur in the intake tract, particularly on the intake valve (for example, on the valve stem) or in the intake port at the valve opening. This type of carbon buildup results in the accumulation of carbon-like material in the intake tract. The reason for this tendency in GDI engines is that, since the fuel is injected directly into the combustion chamber, there is no continuous cleaning scavenging of the intake tract, such as the intake valve, with gasoline. Oil residues from the crankcase ventilation, valve stem seals, or turbocharger bearings, combined with soot particles from the cylinder, can be deposited as carbon in the intake tract under the influence of heat. The tendency for carbon buildup tends to increase with the use of low-quality fuels and oils and with driving profiles characterized by low load.

[0003] This carbon buildup deteriorates engine smoothness during cold idling and can lead to increased fuel consumption and problems with warm-up performance. For example, a carbon deposit on the top of the valve stem between the valve head and valve stem can disrupt the tumble flow, causing more pronounced fluctuations in idle speed and, in extreme cases, even misfires. A carbon deposit can also act as a flow resistance, resulting in insufficient cylinder filling and thus a loss of power. Furthermore, a carbon deposit on the intake valve or valve opening can prevent the valve from closing properly, leading to compression loss and sporadic misfires.

[0004] Typically, the degree of carbon buildup in the intake manifold can only be visually assessed in a workshop after disassembling engine parts. If the intake manifold is sufficiently carbonized, it is then cleaned or the affected components are replaced.

[0005] Visually detecting carbon buildup has the disadvantage of requiring significant effort to disassemble engine parts for visual inspection. Furthermore, visual assessment suffers from a lack of objectivity, as carbon buildup is difficult to evaluate through purely visual observation.

[0006] From German patent application DE 102 56 906 B4, a method for controlling an air-fuel mixture of an internal combustion engine is known, wherein an injected fuel mass is corrected for an adaptation of an air-fuel mixture ratio, wherein, in the case of multiple fuel injections with several injection processes, the correction of the injected fuel mass is carried out for each of these injection processes, and the adaptation of the correction value for the fuel mass is carried out at an operating point of the internal combustion engine with substantially constant speed and air supply.

[0007] From the publication DE 10 2011 005 283 B4, a method for detecting faulty components of an electronically controlled fuel injection system of an internal combustion engine is known, wherein standard values ​​determined on a test bench are compared with parameters of the fuel injection system calculated by several calculation models.

[0008] From the publication DE 103 39 251 A1, a method for operating an internal combustion engine is known in which, depending on the prevailing operating conditions, a shift operation or a homogeneous operation is selected and the injected fuel and air quantity is evaluated individually for each cylinder and respective correction data are formed depending on target values.

[0009] From the publication DE 199 45 813 A1, a method for operating an internal combustion engine is known, wherein, upon detection of deposits in the combustion chamber, a knocking combustion is brought about or a cleaning fluid is added to the intake combustion air.

[0010] From the publication DE 10 2006 034 633 A1, a method for cleaning a combustion chamber and an intake tract of an internal combustion engine is known, wherein the combustion chamber and the intake tract are kept free of deposits and coking by igniting a prepared fuel-air mixture in the combustion chamber with the intake valve open in the intake channel.

[0011] From the publication DE 10 2014 105 270 A1, a system and a method for compensating or reducing an accumulation of coking residues on fuel injectors is known, wherein an engine operating parameter is set based on a shift of a fuel injector current profile during an initial current increase of the injector activation in order to compensate for or reduce coking.

[0012] From the publication DE 10 2016 219 067 A1, a method for operating an internal combustion engine is known in which fuel is injected from a high-pressure storage tank into the combustion chamber by means of fuel injectors, wherein a deviation of a quantity characteristic of the injected fuel quantity from a corresponding target value is determined, and wherein, based on the determined deviation, a quantity of oxygen or air to be supplied to the internal combustion engine is adapted to the quantity of fuel injected during the at least one injection.

[0013] German patent application DE 10 2012 213 241 A1 discloses a method for detecting carbon buildup in the intake tract of an internal combustion engine with direct fuel injection without requiring engine disassembly and enabling an objective assessment of the carbon buildup. However, the method described therein can only be applied to internal combustion engines with variable valve timing (VVT) where the air mass flow is controlled by a throttle valve. In this method, the internal combustion engine is operated at idle with an increased idle speed compared to normal idle operation. Furthermore, the opening time of the intake valves is advanced by means of the VVT, thereby increasing the valve overlap (the duration for which the intake valve is already open before the exhaust valve is closed).A characteristic running roughness parameter of the combustion engine is then determined by measurement. Based on this running roughness parameter, the presence of carbon buildup in the intake manifold can then be detected.

[0014] However, this method cannot be used in internal combustion engines where the air mass flow is controlled by variable intake valve lift, as the combustion process is designed differently. While in an internal combustion engine with throttled load control, residual gas control is achieved via the intake, in an internal combustion engine with variable intake valve lift, residual gas control is achieved via the exhaust. An internal combustion engine with variable intake valve lift is a fully variable mechanical valve control system. With this system, load control is regulated not by the throttle valve, but by a valve lift curve. This significantly reduces charge exchange losses during partial load operation of the internal combustion engine, thus achieving fuel consumption improvements.

[0015] The object of the invention is to provide a method for detecting carbon buildup in an internal combustion engine with direct fuel injection and variable intake valve lift control, which does not require disassembly of the engine for detection and provides at least an indication of the presence of carbon buildup. Furthermore, the object is to provide an engine test device with corresponding properties.

[0016] This problem is solved by the features of the independent claims. Advantageous embodiments are specified in the dependent claims.

[0017] A first aspect of the invention relates to a method for detecting coking in the intake tract of an internal combustion engine. The internal combustion engine has one or more cylinder banks, each cylinder bank comprising several cylinders, each with a combustion chamber and at least one injection nozzle. In particular, exactly one injection nozzle is provided in each combustion chamber. A common air mass flow is supplied to the combustion chambers of each cylinder bank. Likewise, a common exhaust gas flow is discharged from the combustion chambers of each cylinder bank. The internal combustion engine has direct fuel injection; in particular, it is a gasoline direct injection engine.

[0018] The procedure runs, for example, on an external engine test device, such as one used in a vehicle repair shop. The engine test device is connected wirelessly or via cable to the vehicle's engine control unit (ECU) using a suitable vehicle interface and can also control the vehicle's operation and retrieve measured values ​​from the ECU. It is also conceivable that the procedure runs on the vehicle's ECU itself, in which case, for example, if a fault is detected, particularly carbon buildup, the system might initiate cleaning of the intake ports and / or intake valves. A cleaning instruction can be issued to the driver or a mechanic in the workshop, and / or a corresponding message can be stored in the vehicle's fault memory.

[0019] The internal combustion engine features a throttle valve in the intake manifold and a variable intake valve lift (VVT). The variable intake valve lift allows for control of the valve lift. This variable lift regulates the amount of air entering each combustion chamber of the engine, eliminating the need for the throttle valve upstream of a cylinder bank during normal operation. The throttle valve is only used in special operating conditions, such as emergency running mode. This throttle-free load control reduces charge exchange losses. A higher airflow velocity leads to better mixing of the air-fuel mixture in the combustion chamber.

[0020] In this process, a correction value is determined, which is then applied by the intake valve lift control as an offset value to a preset valve lift. This correction value accounts for the fact that deposits can form on the intake valves, for example, due to contaminated air (ambient, exhaust gas recirculation, crankcase ventilation, etc.). This narrows the cross-section, and less air can enter the combustion chamber for the same valve lift. These deposits are particularly critical at idle, as the cross-sections are smallest at this time. In the worst case, this can lead to rough idling and misfires. To prevent this, a valve lift correction is determined using the correction value. This can compensate for the negative effects of the deposits to a certain extent.

[0021] Furthermore, an initial quantity deviation test is performed. This test determines an initial air-fuel ratio value, which is calculated from a lambda value measured during the test and a desired lambda value for fuel combustion in the combustion chambers of the internal combustion engine. During this initial quantity deviation test, the internal combustion engine's load is controlled using variable intake valve lift.

[0022] Following the first quantity deviation test, a second quantity deviation test is performed. This second test determines a second air-fuel ratio value, which is calculated from a lambda value measured during the second test and a desired lambda value for fuel combustion in the combustion chambers of the internal combustion engine. During the second quantity deviation test, the engine's load is controlled using the throttle valve located in the intake manifold.

[0023] After performing the first and second quantity deviation tests, a comparative result is calculated from the first and second air-fuel ratio values. This comparative result provides an indicator of the presence of a fault in the intake tract of the internal combustion engine.

[0024] The first and second quantity deviation tests can be performed after the correction value has been determined. They can also be performed before the correction value has been determined. Finally, the first and second quantity deviation tests can be performed concurrently with the determination of the correction value.

[0025] Determining whether carbon deposits are present in the intake manifold of an internal combustion engine is achieved by combining the comparative results with the correction value. Each of these two criteria provides an indication of carbon deposits in the intake manifold. The combined evaluation allows for a more reliable assessment of whether carbon deposits are present in the intake manifold.

[0026] During the first and second quantity deviation tests, no changes are made to the fuel path, i.e., the supply of the required fuel mass for combustion. In contrast, the air path is altered by the different load control methods, once via variable intake valve lift control and once via the throttle valve, if a fault or carbon buildup is present in the intake tract.

[0027] The method according to the invention allows for the objective detection of coking in the intake tract of an internal combustion engine with direct fuel injection. This objective detection is achieved by measuring and utilizing other parameters present in the vehicle, instead of through subjective visual assessment. Cleaning or repair is therefore only carried out when objectively necessary.

[0028] Disassembly of the internal combustion engine solely for the visual assessment of carbon buildup is not required when using the method described herein. Carbon buildup can be detected very quickly. For example, the time required to determine whether a fault exists in the intake tract of the internal combustion engine, particularly carbon buildup, can be reduced to approximately ten minutes instead of an hour or more when disassembling the engine. Using the described method, the engine no longer needs to be partially disassembled, thus minimizing further potential causes of future repairs to the absolute minimum. Furthermore, the accuracy of carbon buildup detection using a mixture deviation analysis method performed according to the invention is significantly higher than in the prior art.

[0029] The correction value is read from the engine control unit in a suitably configured system. Generally speaking, the higher the absolute value of the correction value, the more likely it is that extensive deposits are present on the intake valves.

[0030] The correction value is, in particular, a value determined by the engine control unit during the combustion engine's operating time. Furthermore, the correction value is a value adapted by the engine control unit during the combustion engine's operating time, whereby this adaptation is based on a theoretically calculated air mass value of the air flowing into the combustion chambers of the combustion engine and a measured air mass value.

[0031] The correction value is compared with a predefined limit value, either for or within the context of the combined evaluation. If the correction value exceeds the predefined limit value, a fault in the intake tract is inferred. The predefined limit value can be written to a memory of the engine control unit, for example, after the combustion engine has been manufactured. Subsequently, the correction value can be adapted during the combustion engine's operating time, as is already the case with combustion engines featuring intake valve lift control. The currently valid correction value is processed to carry out the method according to the invention. Optionally, it can also be provided to process changes in the correction value over time, for example, since the vehicle's initial commissioning.

[0032] Exceeding the predefined limit value alone is merely an indication of coking. An error signal representing coking is only appropriately issued when the first and second air-fuel ratio values ​​also differ. The combined occurrence of both conditions at the same time allows for a highly reliable error message indicating coking.

[0033] The first and / or second quantity deviation test is best performed with the combustion engine idling. Control is achieved, as described above, by an external engine test device or the vehicle's engine control unit.

[0034] The first quantity deviation test is expediently performed with the throttle valve open and the intake valve lift small or even minimal. In other words, the first quantity deviation test is carried out in the so-called unrestricted state, where the amount of air introduced into the combustion chamber depends solely on the intake valve lift. Due to the selected small or minimal lift, the amount of air introduced into the combustion chamber will be noticeably reduced compared to the expected air volume if carbon deposits are present in the intake port or on the intake valve. Since the desired lambda value (i.e., the target lambda value) deviates more significantly from the measured lambda value, this manifests itself in a certain initial, excessively rich air-fuel ratio value.

[0035] In contrast, the second air-fuel ratio test is performed with the intake valve at maximum lift and the throttle valve largely closed. Load control is thus achieved via the throttle valve (so-called throttled operation). If carbon deposits are present in the intake port or on the intake valve, the second air-fuel ratio value will not deviate as much as in the first air-fuel ratio test, because the cross-section through which the air can flow into the combustion chamber is proportionally less affected by the carbon deposits than during the first air-fuel ratio test.

[0036] A fault in the intake tract, i.e., in the air path of the internal combustion engine, can therefore be inferred if the first air-fuel ratio and the second air-fuel ratio values ​​differ. In particular, the presence of carbon buildup in the intake tract is inferred if the first air-fuel ratio value is less than a predetermined first threshold value and the second air-fuel ratio value is greater than or equal to the predetermined first threshold value.

[0037] The first air-fuel ratio is calculated by dividing the lambda value measured during the first quantity deviation test by the desired lambda value. The second air-fuel ratio is calculated by dividing the lambda value measured during the second quantity deviation test by the desired lambda value.

[0038] Due to the preferred method of determining the first and second air-fuel ratio values, coking is indicated when the first air-fuel ratio value is lower than the specified threshold and therefore lower than the second air-fuel ratio value. This results from the fact that during the first quantity deviation test, with a given level of coking, a smaller quantity of air can flow into the combustion chamber than would be the case without coking. The measured lambda value is thus lower than the desired lambda value. In contrast, in the second quantity deviation test, the difference between the measured and the desired lambda value is not as large, since coking does not alter the volume of air flowing into the combustion chamber to such a significant degree.

[0039] The first threshold is, in particular, equal to 1.

[0040] Furthermore, a computer program product is proposed that can be loaded directly into the internal memory of a digital computer and comprises software code sections that execute the steps of the procedure described herein when the product is running on a computer. The computer program product can be in the form of a data carrier, such as a DVD, a CD-ROM, a USB flash drive, and the like. The computer program product can also be in the form of a signal that can be loaded via a wireless or wired network.

[0041] A second aspect of the invention relates to an engine test device. This engine test device is designed to detect carbon buildup in the intake tract of an internal combustion engine with direct fuel injection, a throttle valve, and variable intake valve lift control. The device can be connected to the vehicle to control its operation and receive internal vehicle measurements, for example, via an electrical or radio connection. The engine test device controls the internal combustion engine via the engine control unit (ECU) so that the method described above is carried out.

[0042] The invention is described below with reference to the drawings and an exemplary embodiment. It shows: Fig. 1 An exemplary flowchart for a method according to the invention for detecting coking in the intake tract of an internal combustion engine.

[0043] In Fig. Figure 1 shows an exemplary flowchart of the inventive method for detecting coking in the intake tract of an internal combustion engine. The internal combustion engine has one or more cylinder banks, each cylinder bank comprising several cylinders, each with a combustion chamber and at least one injection nozzle. In particular, exactly one injection nozzle is provided in each combustion chamber. A common air mass flow is supplied to the combustion chambers of each cylinder bank. Likewise, a common exhaust gas flow is discharged from the combustion chambers of each cylinder bank. The internal combustion engine has direct fuel injection; in particular, it is a gasoline engine with direct injection and fully variable valve timing. The internal combustion engine is controlled by means of various control parameters stored in an engine control unit.

[0044] The internal combustion engine incorporates a variable intake valve lift control system in addition to a conventional throttle valve in the intake tract. This is a fully variable mechanical valve control system. As is well known to those skilled in the art, and therefore will not be elaborated upon further, this system regulates the load control not by the throttle valve, but by a valve lift curve of the intake valves. The variable valve lift allows the amount of air entering a combustion chamber of the internal combustion engine to be regulated, so that the throttle valve upstream of a cylinder bank is no longer needed during normal operation. The throttle valve is only used in special operating conditions, such as emergency running mode. This significantly reduces charge exchange losses during partial load operation of the internal combustion engine, thus achieving fuel consumption improvements.

[0045] In a combustion engine with variable valve lift control, the intake valve lift control processes a correction value (KW) provided by the engine control unit. The intake valve lift control calculates the correction value (KW) as an offset value with a preset valve lift. The correction value (KW) accounts for the fact that deposits can form on the intake valves, for example, due to contaminated incoming gases (from the environment, exhaust gas recirculation, crankcase ventilation, etc.). This narrows the cross-section, and less air can enter the combustion chamber for the same valve lift. These deposits are particularly critical at idle, as the cross-sections are smallest at this time. In the worst case, this can lead to rough idling and misfires. To prevent this, a valve lift correction is determined using the correction value.This can offset the negative effects of the deposits to a certain extent.

[0046] The correction value KW is a value determined by the engine control unit (ECU) during the combustion engine's operation. This value is typically adapted over time by the ECU, based on a theoretically calculated mass airflow value of the air flowing into the combustion chambers. The KW correction value is determined as follows: A theoretical mass airflow (m_theo) in the cylinders is calculated using an operating model of the intake air system, taking into account all involved mass flows and operating conditions. An actual mass airflow (m_real) is measured either directly or indirectly, for example, using a hot-film air mass meter (HFM). From the determined mass airflows, a ratio V = m_real / m_theo is calculated.The ratio V is the reference variable for the adaptive control, with a target value of V_target = 1, so that the actual and theoretical air mass flow rates are identical. The adaptive control (so-called adaptation function) modifies the air mass control elements as a manipulated variable, in this case the intake valve lift EV_lift_offset, with the resulting difference corresponding to the correction value KW. A positive value of the intake valve lift EV_lift_offset describes an additional opening of the intake valve by x [mm]. The currently valid correction value KW is processed for the implementation of the method according to the invention.

[0047] The correction value KW is determined in step S11 by reading it from the engine control unit. Generally, the higher the absolute value of the correction value, the more likely it is that extensive deposits are present on the intake valves. The correction value KW is determined as follows: KW=(EV_hub_offset / EV_hub_offset_Limit)*(V_Limit / V).

[0048] Here, "EV_hub_offset_limit" is a limit value for monitoring the valve lift adaptation, and "V_limit" is a limit value for monitoring the remaining control deviation of the air masses. The correction value KW is used according to the following logic: In step S12, the correction value KW is compared with a predefined limit value GW. The limit value GW can be written to a memory, such as the engine control unit or a database, either vehicle-specifically or uniformly for a combustion engine type, for example, after the combustion engine has been manufactured. The limit value GW can, in particular, be a limit value that takes into account manufacturing tolerances of the combustion engine. If the limit value GW is chosen to be, for example, GW = 1, then the following applies: If the correction value KW is greater than or equal to 1 (KW ≥ 1), a critical situation exists. There is a suspicion that coking is present. This suspicion can be validated by further tests, which are described below in steps S21, S22, and S23. These tests are referred to as quantity deviation tests. If the correction value KW is less than 1 (KW < 1), a non-critical situation exists. It is not assumed that coking is present.

[0049] In step S12, it is determined whether the correction value KW exceeds the limit value GW (i.e., KW >= GW) or not (i.e., KW < GW). The result of the comparison is further processed in step S30. Exceeding the predefined limit value (i.e., KW >= GW) alone is an indication of the presence of coking, which is subsequently verified by carrying out the following steps S21, S22, and S23.

[0050] Steps S21, S22, and S23 can be executed before or after steps S11 and S12. Steps S21, S22, and S23 can also be performed in the same way as... Fig. Figure 1 shows that steps S11 and S12 are executed in parallel.

[0051] In step S21, an initial quantity deviation test is performed, which determines an initial air-fuel ratio value w1. The air-fuel ratio value w1 is calculated as the quotient of the lambda value λ measured during the initial quantity deviation test. real, 1 and the desired lambda value λ soll, 1 (i.e., the target lambda value) of the fuel combustion in the combustion chambers of the internal combustion engine. In the first quantity deviation test, the internal combustion engine is load-controlled by means of variable intake valve lift control (VVT).

[0052] In step S22, a second quantity deviation test is performed, which determines a second air-fuel ratio value w2. The second air-fuel ratio value w2 is calculated as the quotient of the lambda value λ measured during the second quantity deviation test. real , 2 and the desired lambda value λ soll, 2(i.e., a target lambda value) of the fuel combustion in the combustion chambers of the internal combustion engine. In the second quantity deviation test, the load control of the internal combustion engine is carried out by means of the throttle valve in the intake tract of the internal combustion engine.

[0053] In step S23, the previously determined first and second air-fuel ratio values ​​w1 and w2 are compared. Based on the comparison result VE (i.e., w1 < w2 or w1 = w2 or w1 > w2), a conclusion can be drawn about the presence of a fault, in particular the presence of coking, in the intake tract of the internal combustion engine. The comparison result VE is further processed in step S30.

[0054] The first and second quantity deviation tests are performed one after the other while the combustion engine is idling.

[0055] The first quantity deviation test performed in step S21 is carried out with a small or minimum intake valve lift and the throttle valve, located in the intake tract of the internal combustion engine, open. In other words, the first quantity deviation test is performed in the conventional operating mode of an internal combustion engine equipped with variable intake valve lift control.

[0056] In contrast, the second quantity deviation test in step S22 is performed with the intake valve at maximum lift (i.e., the intake valve is fully open) and the throttle valve largely closed. This operating mode corresponds to a limp-home mode, in which, for engines with the variable intake valve lift control disabled, load control is achieved via the throttle valve.

[0057] Depending on whether coking is present, characteristic first and second air ratio values ​​w1, w2 result (i.e. w1 < w2 or w1 = w2 or w1 > w2), where their ratio to each other is an indication of the presence of coking.

[0058] A general fault in the intake tract can be inferred if the first air-fuel ratio value w1, determined in step S21, and the second air-fuel ratio value w2, determined in the second step S22, are different (i.e., w1 <> w2). This results from the fact that the air-fuel ratio values ​​w1 and w2 should then have the same value if the lambda values ​​λ measured in steps S21 and S22, respectively, were equal. real , 1 and λ real, 2. The same amount of air should be introduced into the combustion chambers under different load control conditions, which should be reflected in the same measured lambda value. However, if the amount of air introduced into the combustion chambers differs between steps S21 and S22, this leads to differences in the lambda values ​​λ measured in steps S21 and S22. real , 1, λ real, 2 , while the desired lambda values ​​λ soll, 1 , λ soll, 2 remain unchanged.

[0059] Coking leads to the deposition of a carbon-like mass in the intake, particularly in the intake port and / or on the intake valve. The gradual accumulation of this carbon, especially during step S21 where the valve is only minimally open, disproportionately reduces the otherwise available airflow cross-section. This results in a smaller volume of air entering the combustion chamber, thus affecting the measured lambda value λ. real, 1 becomes smaller. This is noticeable as a decrease in the first air-fuel ratio value w1. If the first air-fuel ratio value w1 is therefore smaller than the second air-fuel ratio value w2 (i.e., w1 < w2), coking can be inferred. The comparison can be made particularly against a predefined threshold value chosen to be 1, since the respective air-fuel ratio values ​​w1 and w2 correspond to the value 1 when there is no error, as the measured and the desired lambda values ​​are then approximately the same.

[0060] The reverse case, in which the first air ratio value w1 is greater than the specified threshold value and greater than the second air ratio value w2 (i.e. w1 > w2), suggests a leak in the intake tract, since, due to the flow conditions in the second step S22, in which the combustion engine is operated at a throttled speed, less air enters the combustion chamber.

[0061] It is particularly useful to perform the test described above on a cylinder-by-cylinder basis. For this purpose, the first air-fuel ratio value w1 and the second air-fuel ratio value w2 are determined for each cylinder individually, and a comparison is also made on a cylinder-by-cylinder basis. This makes it possible not only to make a general statement about the presence of a defect or coking, but also to identify the cylinder exhibiting the defect or to determine the defect intensity per cylinder.

[0062] In what way are the measured lambda values ​​λ used in the first step S21 and the second step S22? real , 1 or λ real , 2 and the desired lambda values ​​λ soll, 1 or λ soll, 2 The method for determining this is generally known to those skilled in the art. One possible procedure is described, for example, in the applicant's WO 2016 / 041 742 A1.

[0063] Furthermore, other procedures for determining a measured and a desired lambda value for each cylinder are known to those skilled in the art, so that a detailed description of the determination is omitted in the present description.

[0064] If, in step S30, it is determined that the correction value KW is greater than the limit value GW (KW > GW) and simultaneously the comparison result VE indicates that w1 < w2, then coking is suspected, since two independent methods each provide evidence of coking. In this case, an error message can be issued via the engine test device. In all other cases, an error message can be suppressed and, if necessary, a warning issued. Reference symbol list S11 Procedure step S12 Procedure step S21 Procedure step S22 Procedure step S23 Procedure step S30 Procedure step λ real , 1 measured lambda value in the first quantity deviation test λ real , 2 measured lambda value in the second quantity deviation test λ soll, 1 desired (target) lambda value in the first quantity deviation test λ soll, 2 desired (target) lambda value in the second quantity deviation test w1 first air ratio value w2 second air ratio value KW correction value VE comparison result GW limit value

Claims

[1] Method for detecting coking in the intake tract of an internal combustion engine with direct fuel injection, throttle valve and variable intake valve lift control, comprising the steps: - Determining a correction value (KW) which is calculated by the intake valve lift control as an offset value with a preset valve lift; - Performing an initial quantity deviation test, by which an initial air-fuel ratio value (w1) is determined from a lambda value (λ) measured during the initial quantity deviation test. real, 1 ) and a desired lambda value (λ soll, 1 ) the fuel combustion in the combustion chambers of the internal combustion engine, whereby in the first quantity deviation test a load control is carried out by means of the variable intake valve lift control; - Performing a second quantity deviation test, by which a second air-fuel ratio value (w2) is determined from a lambda value (λ) measured during the second quantity deviation test. real, 2 ) and a desired lambda value (λ soll, 2 ) the fuel combustion in the combustion chambers of the internal combustion engine, whereby in the second quantity deviation test a load control is carried out by means of the throttle valve; - Determining a comparative result (VE) from the first air ratio value (w4) and the second air ratio value (w2); - Determine whether coking is present in the intake tract of the internal combustion engine by combining the comparison result (VE) and the correction value (KW). [2] Method according to claim 1, wherein the correction value (KW) is read from an engine control unit. [3] Method according to claim 1 or 2, wherein the correction value (KW) is a value determined by the engine control during the running time of the internal combustion engine. [4] Method according to one of the preceding claims, wherein the correction value (KW) is a value adapted by the engine control during the running time of the internal combustion engine, wherein an adaptation is made from a theoretically calculated air mass value of air flowing into the combustion chambers of the internal combustion engine and a measured air mass value. [5] Method according to any of the preceding claims, wherein the correction value is compared with a predefined limit value (GW), and the presence of a fault in the inlet tract is inferred if the correction value (KW) exceeds the predefined limit value (GW). [6] Method according to claim 5, wherein an error signal representing coking is output when, in addition, the first air ratio value (w1) and the second air ratio value (w2) are different. [7] Method according to one of the preceding claims, wherein the first and / or the second quantity deviation test is carried out in the idle operation of the internal combustion engine. [8] Method according to one of the preceding claims, wherein the first quantity deviation test is performed with a small or minimal stroke of the inlet valve and an open throttle valve. [9] Method according to one of the preceding claims, wherein the second quantity deviation test is performed with the inlet valve at maximum stroke and the throttle valve largely closed. [10] Method according to one of the preceding claims, wherein the first air ratio value (w1) and the second air ratio value (w2) are determined cylinder-specifically, and the comparison result (VE) is determined cylinder-specifically. [11] Method according to one of the preceding claims, wherein the first air ratio value (w1) is determined by the quotient of the lambda value (λ) measured during the first quantity deviation test. real, 1 ) and the desired lambda value (λ soll, 1 ) is formed and the second air ratio value (w2) is determined by the quotient of the lambda value (λ) measured during the second quantity deviation test. real, 2 ) and the desired lambda value (λ soll, 2 ) is formed. [12] Method according to one of the preceding claims, wherein the presence of coking in the inlet tract is inferred if the first air ratio value (w1) is less than a predetermined first threshold value and the second air ratio value (w2) is greater than the predetermined first threshold value. [13] Computer program product that can be loaded directly into the internal memory of a digital computer and comprises software code sections with which the steps according to any one of claims 1 to 12 are performed when the product is running on a computer [14] Engine test device for detecting faults in the intake tract of an internal combustion engine, which is designed to perform the method according to any one of claims 1 to 12.

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