Method for detecting knocking combustion

Abstract

Method for detecting knocking combustion in an internal combustion engine caused by pre-ignition, wherein a knock signal is detected, wherein an exhaust signal is detected, and wherein the knock signal and the exhaust signal are evaluated to detect knocking combustion, wherein knocking combustion is detected when the knock signal is above a first reference value and when the exhaust signal is above a second reference value.

Description

[0001] The invention relates to a method for detecting knocking combustion according to claim 1 and a control unit for carrying out the method according to claim 9.

[0002] From DE 10 2009 058 578 A1, a diagnostic system and a method for engine knock sensors are known. The engine control system comprises a digital signal processing module that generates a fast Fourier transform of an engine knock signal produced by an engine knock sensor. An intensity determination module determines the engine knock intensity based on a maximum value or an average value of the fast Fourier transform. A status determination module determines the status of the engine knock sensor based on the engine knock intensity, several predetermined knock intensity thresholds, and the rotational speed of an engine crankshaft. If the knock sensor fails, a control module can control engine knocking based on inputs other than those from the knock sensor. For example,The control module can actuate fuel injectors and / or spark plugs based on predetermined settings and / or sensor inputs other than the knock sensor. For example, these other sensor inputs could include oxygen sensors in the exhaust stream.

[0003] US Patent 6,105,552 A discloses a method for detecting knocking combustion by evaluating knock signals and exhaust signals.

[0004] The object of the invention is to provide an improved method for detecting knocking combustion, in particular for detecting pre-ignitions, which are usually associated with knocking combustion.

[0005] The object of the invention is achieved by the method according to claim 1 and the control unit according to claim 9. An advantage of the described method is that not only a knock signal from a knock sensor is used to detect pre-ignition, but also an exhaust gas signal is taken into account. In this way, a more precise and accurate detection of a knocking process in the internal combustion engine can be achieved. Furthermore, a better differentiation between knocking combustion and pre-ignition can be achieved.

[0006] In one embodiment, the signal from a lambda sensor is used as the exhaust gas signal. This method offers the advantage that, as a rule, no additional sensors need to be installed, since lambda sensors are already fitted in most vehicles.

[0007] In a further embodiment, pre-ignition is detected when the knock signal exceeds a first reference value and when the exhaust gas signal exceeds a second reference value. This enables simple and precise detection of knocking combustion.

[0008] In a further embodiment, the method is improved by using a time gradient as a second reference value. Thus, the exhaust gas signal is compared with a predefined time gradient to detect knocking combustion. Tests have shown that during knocking combustion, deposits in the combustion chamber of the internal combustion engine are dislodged, releasing hydrocarbons into the exhaust gas. This causes a change in the exhaust gas signal, in particular a change in the lambda sensor voltage. This change in lambda sensor voltage can be used to detect knocking combustion with the help of the exhaust gas signal.

[0009] In another embodiment, the exhaust gas signal is evaluated and taken into account with a time delay of one gas transit time relative to the knock signal. This approach is necessary because the knock sensor immediately detects a corresponding knock signal during knocking combustion. In contrast, a change in exhaust gas caused by knocking combustion is detected later by a sensor located in the exhaust system.

[0010] Tests have shown that even a 5%, and especially a 15%, change in the exhaust gas signal compared to a target value indicates severe knocking combustion. The 5% or 15% change occurs within a defined timeframe, which is on the order of the duration of a combustion cycle.

[0011] In a further embodiment, at least one countermeasure against pre-ignition is implemented upon detection. This countermeasure can, for example, consist of mixture enrichment, injection cut-off, a change in the injection strategy, a change in the injection timing, and / or a change in mean effective pressure. Using the described countermeasures, further pre-ignition and an interruption of the intermittent engine operating state can be initiated.

[0012] The invention will be explained in more detail below with reference to the figures. They show Fig. 1 a schematic representation of an internal combustion engine with an exhaust system, Fig. 2 diagrams for a knock signal and an exhaust signal, and Fig. 3 a schematic representation of an exhaust gas signal.

[0013] Fig. Figure 1 shows a schematic representation of an internal combustion engine 1, which has six cylinders 2. The internal combustion engine 1 can, for example, be a turbocharged engine (bi-turbo), in which an increase in mean effective pressure is possible at low engine speeds. The internal combustion engine 1 is designed as a six-cylinder engine, with three cylinders 2 belonging to a first cylinder bank 4 and the other three cylinders 2 to a second cylinder bank 5. Furthermore, an exhaust system 3 is provided, which has a first exhaust pipe 6 and a second exhaust pipe 7. The first exhaust pipe 6 is connected to the cylinders 2 of the first cylinder bank 4, and the second exhaust pipe 7 is connected to the cylinders 2 of the second cylinder bank 5. A first exhaust gas sensor 10 is arranged in the first exhaust pipe 6. A second exhaust gas sensor 11 is arranged in the second exhaust pipe 7. The exhaust gas sensors 10 and 11 are connected to a control unit 8.Furthermore, a knock sensor 9 is provided for each cylinder bank 4, 5, which is also connected to the control unit 8. The control unit 8 is designed to control the combustion of the internal combustion engine 1, whereby in particular the fuel quantity, the injection timing, the ignition timing, etc. are adjusted according to a driver request and corresponding control programs stored in a memory 12. The memory 12 is connected to the control unit 8.

[0014] The knock sensors 9 are designed to detect structure-borne sound and transmit it to the control unit 8. The exhaust gas sensors 10, 11 are designed to detect an exhaust gas signal as a function of the concentration of a predetermined gas and transmit it to the control unit 8. For example, the exhaust gas sensors 10, 11 are designed in the form of oxygen sensors. In particular, the exhaust gas sensors 10, 11 can be designed in the form of lambda sensors. Depending on the chosen embodiment, only one exhaust gas sensor 10, 11 may be provided, which is arranged in a common exhaust pipe of the cylinder banks 4, 5. In addition, exhaust gas turbochargers 20 with compressors 21 are provided on the exhaust pipes 6, 7.

[0015] The knocking combustion triggered by auto-ignition differs significantly in intensity from controlled, externally ignited combustion due to its early mixture conversion point, both in terms of pressure and temperature. The knocking combustion generates increased structure-borne noise, which is detected by the knock sensor 9 and transmitted to the control unit 8.

[0016] A current trend of fuel-efficient displacement reduction, combined with increasing mean effective pressure requirements at low engine speeds in turbocharged combustion engines, promotes the occurrence of combustion anomalies such as auto-ignition and pre-ignition. These events, partly caused by deposits in the cylinder's combustion chamber, place significant stress on engine components and can lead to damage or even engine failure. Particularly with fuels containing a high number of low-boiling-point components, carbon buildup can occur in the cylinder's combustion chamber. Under higher engine loads, this can lead to prolonged periods of intermittent combustion, i.e., alternating between auto-ignition and spark ignition. This uncontrolled engine operating condition should be detected early and stopped or prevented by appropriate measures.

[0017] Besides increased structure-borne noise, knocking combustion leads to an enhanced cleaning effect in the combustion chamber. Hydrocarbons released from components within the combustion chamber can cause a sudden enrichment of the combustion gases in the exhaust system. Tests have shown that this can result in changes in the exhaust signal, particularly increases in the lambda sensor voltage of 5 to 15%.

[0018] The control unit 8 monitors the exhaust gas signal using the exhaust gas sensors 10 and 11 and detects a change in the exhaust gas signal as an indication of knocking combustion or intermittent combustion, in which there is a change between auto-ignition and spark ignition. To detect severe knocking combustion or pre-ignition, comparative values ​​for the knock signal and comparative values ​​for the exhaust gas signal are stored in memory 12. For example, a time gradient of the exhaust gas signal can be stored as a comparative value. In particular, the control unit 8 detects an indication of knocking or intermittent combustion in the short term with a change in the exhaust gas signal in the range of 5 to 15%.

[0019] The control unit 8 can detect pre-ignition or auto-ignition, i.e., knocking combustion, using bank-selective mixture analysis and cylinder-selective structure-borne sound analysis. Furthermore, the cylinder-selective structure-borne sound analysis allows the knocking combustion to be attributed to a specific cylinder. If the control unit 8 detects pre-ignition, various steps can be initiated to suppress the knocking combustion and, in particular, to prevent intermittent combustion.

[0020] For example, the control unit can selectively suppress individual injection processes in 8 cylinders, perform mixture enrichment selectively or globally, delay injection, change the valve timing, and perform a reduction in mean effective pressure, i.e., a load reduction.

[0021] Fig. Figure 2 shows a schematic representation in the upper diagram of a time course for a knock signal 13, which is detected by the knock sensor 9. At a zeroth time t0, the knock signal has a low value and rises above a first reference value 14 at a first time t1. As a result, the control unit 8 detects an indication of the occurrence of knocking combustion at the first time t1.

[0022] The lower diagram shows the time course of an exhaust gas signal 15. The time course of the exhaust gas signal 15 is synchronized with the time course of the knock signal 13. At the first time point t1, the exhaust gas signal 15 has an initial value 17, which only drops to a second value 16 at a later time point t2. The difference between the first value 17 and the second value 16 is detected by the control unit 8 and compared with a reference value. The reference value can, for example, be in the range of 5% of the first value 17, or in the range of 10% or 15% of the first value 17. If the control unit 8 detects at the second time point t2 that the exhaust gas signal 15 has dropped by more than the specified reference value, this indicates severe knocking or pre-ignition.Furthermore, the control unit 8 knows the transit time of the combustion gas between combustion in cylinder 2 and its arrival at the location of the exhaust gas sensor 10, 11. The control unit 8 now calculates the occurrence of knocking combustion in the exhaust gas signal 15, taking into account the transit time 18, back to the knock signal at the first time t1. Thus, both the exhaust gas signal and the knock signal can be considered to obtain an indication of pre-ignition.

[0023] Fig.Figure 3 shows a schematic representation of the temporal profile of an exhaust gas signal 15, which is detected, for example, by a lambda sensor. Instead of an absolute evaluation of the exhaust gas signal 15, the control unit 8 can also detect a temporal gradient and compare it with stored temporal reference gradients. For example, the control unit 8 detects the temporal gradient 19 of the exhaust gas signal 15 between the second and third time points t2 and t3. The detected temporal gradient 19 is compared with a reference gradient stored in memory 12. If the comparison shows that the detected gradient 19 is smaller than the stored negative reference gradient, the control unit 8 detects an indication of knocking combustion, which is triggered by pre-ignition.

[0024] The described method can also be used with an internal combustion engine with more or fewer cylinders.

Claims

[1] Method for detecting knocking combustion in an internal combustion engine caused by pre-ignition, wherein a knock signal is detected, wherein an exhaust signal is detected, and wherein the knock signal and the exhaust signal are evaluated to detect knocking combustion, wherein knocking combustion is detected when the knock signal is above a first reference value and when the exhaust signal is above a second reference value. [2] Method according to claim 1, wherein the exhaust gas signal is detected with a lambda probe. [3] Method according to claim 1 or 2, wherein the second comparison value represents a time gradient. [4] Method according to any one of claims 1 to 3, wherein the second comparison value represents a change in the exhaust gas signal. [5] Method according to any one of claims 1 to 4, wherein the exhaust gas signal is taken into account with a time offset of one gas transit time to the knock signal. [6] Method according to any one of claims 1 to 5, wherein the exhaust signal indicates knocking combustion when the exhaust signal changes by at least 5%, in particular by at least 15%, compared to a target value. [7] Method according to any one of claims 1 to 6, wherein, after detection of pre-ignition, at least one countermeasure against pre-ignition is carried out. [8] Method according to any one of claims 1 to 7, wherein, upon detection of pre-ignition, mixture enrichment, injection suppression, a change in injection strategy, a change in the timing of injection and / or a change in mean effective pressure is performed. [9] Control unit configured to perform a method according to any one of claims 1 to 8.

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