Method, apparatus and computer program product for providing a combustion chamber pressure signal

Abstract

Method for providing a filtered combustion chamber pressure signal for determining a quantity characterizing combustion in a cylinder (1) of an internal combustion engine, comprising the following steps: - providing a first digital combustion chamber pressure signal; - filtering the first digital combustion chamber pressure signal with a second low-pass filter (23) having a second cutoff frequency (fg2) to obtain a second digital combustion chamber pressure signal; - filtering the second digital combustion chamber pressure signal with a third low-pass filter (24) having a third cutoff frequency (fg3) lower than the second cutoff frequency (fg2) to obtain a third digital combustion chamber pressure signal; - determining the quantity characterizing combustion in the cylinder (1) using the second digital combustion chamber pressure signal or the third digital combustion chamber pressure signal.

Description

Technical field The invention relates to internal combustion engines, in particular internal combustion engines in which a combustion chamber pressure in a cylinder is detected by means of a combustion chamber pressure sensor, and a corresponding combustion chamber pressure signal for determining one or more quantities that characterize the combustion of an internal combustion engine. State of the art To improve the operation of internal combustion engines, knowledge of the combustion processes in the cylinders is helpful. This is especially true when these engines are operated using novel operating methods. Therefore, some internal combustion engines are designed to measure the combustion chamber pressure profile in at least one cylinder and, based on this profile, derive parameters that characterize the combustion in that cylinder. The combustion chamber pressure is measured using a pressure sensor located in a cylinder and a suitable analog-to-digital converter, which is typically integrated into the control unit's processor. Sampling occurs either at a predefined crankshaft angle interval, for example, every full 1° of the crankshaft angle, or at a predefined time interval, such as sampling frequencies of 50 kHz or 1 MHz, depending on the converter's design. The resulting combustion chamber pressure signal is then filtered in a corresponding control unit, usually with a predefined digital low-pass filter, for further interference suppression, or digitally low-pass filtered in downstream algorithms, as is the case, for example, in the calculation of the heating curve. A characteristic feature of combustion chamber pressure signals in internal combustion engines is that the usable bandwidth varies during a combustion cycle. This means that, for example, during the charge exchange phase, i.e., during the flow of gases into and out of the cylinder, the combustion chamber pressure signal undergoes only relatively slow changes and thus corresponds to a low-frequency signal with a low bandwidth. During the high-pressure phase, i.e., during the combustion phase, very rapid changes in the combustion chamber pressure signal occur, so that the combustion chamber pressure signal here corresponds to a high-frequency signal with a high bandwidth. However, conventional post-processing of the sampled combustion chamber pressure only uses a digital low-pass filter with a predetermined cutoff frequency, the design of which represents a compromise. Either the filtering of the digital low-pass filter for the combustion chamber pressure signal in the low-frequency range is too weak to suppress higher-frequency interference signals, or the filtering is too strong for the combustion chamber pressure signal in the high-frequency range. German patent application DE 42 23 649 A1 discloses a method for converting an analog signal from a sensor into a digital signal. This digitized signal is filtered by means of three parallel digital filters. The three filtered signals are then fed to a selection device, where a selection is made for further processing. Furthermore, the patent applications DE 10 2008 004 442 B3 and DE 103 05 656 A1 are known. It is therefore an object of the present invention to provide an improved filtering of the combustion chamber pressure signal, which achieves improved interference suppression in the low bandwidth areas and a higher usable bandwidth in the higher bandwidth areas of the combustion chamber pressure signal. Disclosure of the invention This problem is solved by the method for providing a filtered combustion chamber pressure signal according to claim 1 and by the device according to the dependent claim. Further advantageous embodiments are specified in the dependent claims. According to a first aspect, a method is provided for supplying a filtered combustion chamber pressure signal to determine a quantity characterizing combustion in a cylinder of an internal combustion engine. The method comprises the following steps: - supplying a first digital combustion chamber pressure signal; - filtering the first digital combustion chamber pressure signal with a second low-pass filter with a second cutoff frequency to obtain a second digital combustion chamber pressure signal; - filtering the second digital combustion chamber pressure signal with a third low-pass filter with a third cutoff frequency lower than the second cutoff frequency to obtain a third digital combustion chamber pressure signal; and - determining the quantity characterizing combustion in the cylinder using either the second or the third digital combustion chamber pressure signal. One aspect of the invention is to filter the combustion chamber pressure signal in the time or crankshaft angle ranges where the signal is low-frequency and therefore has a low bandwidth using a (third) low-pass filter with a lower cutoff frequency to provide sufficient interference suppression, and in the time or crankshaft angle ranges where the signal is high-frequency and therefore has a high usable bandwidth using a (second) low-pass filter with a higher cutoff frequency. This allows for significantly improved interference suppression. In particular, the high-frequency components relating to the time period of the cylinder's operating cycle during which combustion takes place are not affected by filtering the combustion chamber pressure signal with the higher cutoff frequency. The filtered combustion chamber pressure signal is provided either as the sampled combustion chamber pressure signal filtered by the low-pass filter with the lower frequency or as the sampled combustion chamber pressure signal sampled by the low-pass filter with the higher cutoff frequency. This improves the quality of the filtered combustion chamber pressure signal and allows for a more accurate calculation of the parameters characterizing the combustion process. Another advantage of the method for providing the filtered combustion chamber pressure signal is that, for example, high-frequency interference noises caused by the closing of the gas exchange valves or by the opening and closing of the injection valves, which are usually in the time or crankshaft angle range with low bandwidth, can be better suppressed. Furthermore, the first digital combustion chamber pressure signal can be obtained by sampling and analog-to-digital conversion of a sensor signal from a combustion chamber pressure sensor. In particular, the first digital combustion chamber pressure signal can be obtained by filtering the sensor signal digitized by the analog-to-digital converter with a first low-pass filter, wherein the first low-pass filter has a first cutoff frequency that is higher than the second cutoff frequency. According to another embodiment, the second digital combustion chamber pressure signal or the third digital combustion chamber pressure signal can be selected beforehand to determine the quantities characterizing the combustion in the cylinder. Furthermore, the first digital combustion chamber pressure signal can be undersampled before filtering with the second low-pass filter. It may be provided that the selection of the second digital combustion chamber pressure signal or the third digital combustion chamber pressure signal is carried out depending on a working cycle position. The selection of the second digital combustion chamber pressure signal or the third digital combustion chamber pressure signal can alternatively or additionally be carried out depending on an absolute combustion chamber pressure, wherein in particular the third digital combustion chamber pressure signal is used as the absolute combustion chamber pressure, wherein in particular a threshold comparison is carried out, the threshold of which for the absolute combustion chamber pressure depends on the speed and / or the load of the internal combustion engine and / or an additional predetermined correction value with respect to an offset of the combustion chamber pressure sensor. Alternatively or additionally, the selection of the second digital combustion chamber pressure signal or the third digital combustion chamber pressure signal can be carried out depending on an ignition timing for spark generation in the cylinder or depending on an injection timing of fuel into the cylinder. At least the second digital combustion chamber pressure signal can be delayed to compensate for the runtime of the third low-pass filter. It may be provided that the low-pass filters perform the filtering according to filter coefficients, with at least one of the low-pass filters used being speed-dependent. Furthermore, undersampling can be provided after filtering with the second digital low-pass filter and / or after filtering with the third digital low-pass filter. According to another aspect, a device is provided for supplying a filtered combustion chamber pressure signal for determining a quantity characterizing combustion in a cylinder of an internal combustion engine. The device comprises: - a second low-pass filter with a second cutoff frequency for filtering a supplied first digital combustion chamber pressure signal to obtain a second digital combustion chamber pressure signal; - a third low-pass filter with a third cutoff frequency for filtering the second digital combustion chamber pressure signal to obtain a third digital combustion chamber pressure signal; - a processing unit for determining the quantity characterizing combustion in the cylinder using the second digital combustion chamber pressure signal or the third digital combustion chamber pressure signal. According to another embodiment, a delay element can be provided to delay the second digital combustion chamber pressure signal in time, so that the second and third combustion chamber pressure signals do not have a time offset from each other, wherein in particular the duration of the delay depends on the rotational speed. According to another aspect, a computer program product is provided which contains program code which, when executed in a data processing unit, performs the above procedure. Brief description of the drawings Preferred embodiments of the present invention are explained in more detail below with reference to the accompanying drawings. These show: Fig. 1 a schematic longitudinal section through a cylinder of an internal combustion engine with a combustion chamber pressure sensor; Fig. 2 a flowchart illustrating the method for providing a filtered combustion chamber pressure signal; and Fig. 3 a flowchart illustrating a further embodiment of the method for providing the filtered combustion chamber pressure signal. Description of embodiments Fig. 1 shows a schematic longitudinal section through a cylinder 1 of an internal combustion engine. The cylinder 1 has a combustion chamber 2 in which a piston 3 is movably arranged. The piston 3 is connected to a crankshaft (not shown) via a connecting rod in order to drive it and provide the corresponding drive torque. An air / fuel mixture is supplied to cylinder 1 via an air supply line 5, and this mixture is admitted into the combustion chamber 2 of cylinder 1 through an intake valve 6. Correspondingly, an exhaust gas discharge line 7 is provided, which is connected to the combustion chamber 2 of cylinder 1 via an exhaust valve 8. Furthermore, cylinder 1 can include an ignition device 9, for example, in the form of a spark plug or the like. Cylinder 1 can also include an injection valve with which fuel can be injected directly into the combustion chamber. A combustion chamber pressure sensor 10 is also provided, which is arranged on the combustion chamber 2 to detect the combustion chamber pressure in the form of a combustion chamber pressure signal and to make it available for further processing. The further processing of the combustion chamber pressure signal can, for example, take place in a control unit 11, which also controls actuators of the internal combustion engine and the like for operating the internal combustion engine according to a four-stroke cycle. To operate the internal combustion engine, knowledge of the combustion process in cylinder 1 is helpful. Generally, the combustion in cylinder 1 is described by characteristic values ​​derived from the combustion chamber pressure signal detected by the combustion chamber pressure sensor 10. Examples of such characteristic values ​​are the combustion conversion points for 10%, 50%, and 90% of the energy converted, i.e., MFB10%, MFB50%, MFB90%, the start of combustion, and similar parameters. For further processing of the combustion chamber pressure signal, it is necessary to provide it in a suitably filtered manner to prevent high-frequency disturbances, for example, during periods when no combustion is taking place, from influencing the calculation of characteristic values. At the same time, however, the entire dynamic range of the combustion chamber pressure signal should be available for subsequent analysis while combustion is occurring. Fig. 2 shows a block diagram illustrating a device for providing a filtered combustion chamber pressure signal as a basis for determining quantities characterizing the combustion in cylinder 1. In an analog-to-digital converter 21, the sensor signal provided by the combustion chamber pressure sensor 10 is sampled. The analog-to-digital converter can, for example, be implemented as a delta-sigma converter. Sampling can be time-based or crankshaft angle-based. Before sampling, the sensor signal can be filtered with an analog low-pass filter to suppress aliasing effects. The analog-to-digital converter 21 provides a digitized combustion chamber pressure signal, which is filtered in a first digital low-pass filter 22 with a first cutoff frequency fg1. The first low-pass filtering is optional and serves to filter the digital combustion chamber pressure signal. Optionally, a subsampling unit can be provided downstream. The filtered first combustion chamber pressure signal is then filtered in a second digital low-pass filter 23 with a second cutoff frequency fg2. In a third digital low-pass filter 24, the second filtered combustion chamber pressure signal is filtered as the output signal of the second digital low-pass filter 23 with a third cutoff frequency fg3 in order to obtain a third filtered combustion chamber pressure signal. The cutoff frequencies fg1, fg2, fg3 of the digital low-pass filters 22, 23, 24 are chosen such that the first cutoff frequency fg1 is higher than the second cutoff frequency fg2, and this in turn is higher than the third cutoff frequency fg3. This ensures that a digitally filtered combustion chamber pressure signal with a relatively high bandwidth can be obtained as the second filtered combustion chamber pressure signal, and a combustion chamber pressure signal with a low bandwidth can be obtained as the third filtered combustion chamber pressure signal. Since the second and third filtered combustion chamber pressure signals can have different filter transit times, they are fed to a first and a second delay element 25, 26, respectively, and individually delayed to compensate for these transit times. Because the second combustion chamber pressure signal typically has a shorter filter transit time, the first delay element 25 for delaying the third filtered combustion chamber pressure signal can be omitted, and the transit time difference is compensated solely by the second delay element 26. As a rule, the delay elements 25, 26 can compensate for delays with a multiple of the sampling time or with a multiple of the crankshaft angle increment at which sampling is performed. Alternatively, delay elements 25, 26 with a fully variable delay time can also be provided. A decision element 27 is also provided, which receives the second filtered combustion chamber pressure signal and the third filtered combustion chamber pressure signal. The decision element 27 is configured to decide whether the second or the third filtered combustion chamber pressure signal is used for the current sample value and to supply only one of the two combustion chamber pressure signals to a subsequent processing unit 28. This can be done, for example, depending on the operating cycle position, i.e., the crankshaft angle for the cylinder 1 under consideration. Alternatively, decision element 27 can also select from the filtered combustion chamber pressure signals depending on the absolute combustion chamber pressure. The absolute combustion chamber pressure should be determined from the third filtered combustion chamber pressure signal, i.e., the one with the lower bandwidth, as this provides the best suppression of higher-frequency interference signals. For example, below 5 bar, the lower bandwidth signal can be used, and above 5 bar, the higher bandwidth combustion chamber pressure signal, i.e., the second filtered combustion chamber pressure signal, can be used. In this comparison, the threshold for the absolute combustion chamber pressure can also depend on the engine speed and / or load. Furthermore, the decision of decision element 27 can also be made depending on the ignition timing or an injection timing. For example, the second filtered combustion chamber pressure signal can be used from the ignition timing onwards, and from a working cycle stage at which combustion is definitely complete, the third filtered combustion chamber pressure signal with the low bandwidth can be used again. For a diesel engine, for example, the second filtered combustion chamber pressure signal with the high bandwidth can be used from the time of the first pre-injection or the first injection, and from a working cycle stage at which combustion is definitely complete, the third filtered combustion chamber pressure signal with the low bandwidth can be used again. The second digital low-pass filter 23 and the third digital low-pass filter 24 can be designed as either FIR or IIR filters. Optionally, all of the low-pass filters 22, 23, 24 can also include filters whose filter characteristics depend on the rotational speed. This means that the delays of the filter units can also depend on the rotational speed. Thus, it can also be provided that the delays of the delay elements 25, 26 are designed to be dependent on the rotational speed. Alternatively, decision element 27 can be omitted, and the second and third filtered combustion chamber pressure signals can be provided to the control unit simultaneously. The third filtered combustion chamber pressure signal can then be used, for example, to calculate characteristics such as the indicated mean effective pressure and the combustion position, and the second filtered combustion chamber pressure signal to calculate the maximum pressure gradient. Alternatively, the third filtered combustion chamber pressure signal can also be used, for example, to calculate the indicated mean effective pressure, and the second filtered combustion chamber pressure signal to calculate the combustion position. Furthermore, a subsampling unit can be provided between the second and third digital low-pass filters 23, 24. This makes it possible to perform sampling at high resolution and provide a high-resolution second filtered combustion chamber pressure signal. This high-resolution second filtered combustion chamber pressure signal is then subsampled, thereby reducing the sampling rate. The third digital low-pass filter 24 can then filter the second filtered combustion chamber pressure signal at the low sampling rate and provide the third filtered combustion chamber pressure signal at a low sampling rate. Naturally, in this case, the digital low-pass filters 22, 23, 24 should be designed in such a way that the aliasing conditions are adequately met.The decision element 27 can then select the second filtered combustion chamber pressure signal from the second digital low-pass filter 23 with the high sampling rate in the desired range and use the third filtered combustion chamber pressure signal from the third digital low-pass filter 24 with the low sampling rate in the remaining ranges. The decision can be made, for example, based on a crankshaft angle KW. Alternatively, the subsampling unit can also be provided after the third digital low-pass filter 24 in order to convert the high-sampled third filtered combustion chamber pressure signal to the lower sampling rate. Figure 3 shows a block diagram illustrating another embodiment of the device for providing a filtered combustion chamber pressure signal. In contrast to the embodiment shown in Figure 2, the second and third digital low-pass filters 23 and 24 are arranged in parallel. Both receive the first filtered combustion chamber pressure signal from the first digital low-pass filter 22 as their input. The cutoff frequencies of the digital low-pass filters 22, 23, and 24 correspond to those of the embodiment shown in Figure 2. As before, the second digital low-pass filter 23 provides the second filtered combustion chamber pressure signal, and the third digital low-pass filter 24 provides the third filtered combustion chamber pressure signal. Delay elements 25 and 26 are also provided.If the second digital low-pass filter 23 and the third digital low-pass filter 24 are provided with identical delays, compensation by delay elements 25, 26 is not necessary and can therefore be dispensed with.

Claims

Method for providing a filtered combustion chamber pressure signal for determining a quantity characterizing combustion in a cylinder (1) of an internal combustion engine, comprising the following steps: - providing a first digital combustion chamber pressure signal; - filtering the first digital combustion chamber pressure signal with a second low-pass filter (23) having a second cutoff frequency (fg2) to obtain a second digital combustion chamber pressure signal; - filtering the second digital combustion chamber pressure signal with a third low-pass filter (24) having a third cutoff frequency (fg3) lower than the second cutoff frequency (fg2) to obtain a third digital combustion chamber pressure signal; - determining the quantity characterizing combustion in the cylinder (1) using the second digital combustion chamber pressure signal or the third digital combustion chamber pressure signal. Method according to claim 1, wherein the first digital combustion chamber pressure signal is obtained by scanning and analog-to-digital conversion of a sensor signal from a combustion chamber pressure sensor. Method according to claim 2, wherein the first digital combustion chamber pressure signal is obtained by filtering with a first low-pass filter (22) of the sensor signal digitized with the analog-to-digital converter, wherein the first low-pass filter has a first cutoff frequency (fg1) which is greater than the second cutoff frequency (fg2). Method according to claim 3, wherein the first digital combustion chamber pressure signal is undersampled before filtering with the second low-pass filter (23). Method according to one of claims 1 to 4, wherein the second digital combustion chamber pressure signal or the third digital combustion chamber pressure signal is selected beforehand to determine the quantity characterizing the combustion in the cylinder (1). Method according to claim 5, wherein the selection of the second digital combustion chamber pressure signal or the third digital combustion chamber pressure signal is performed depending on a working cycle position. Method according to claim 5 or 6, wherein the selection of the second digital combustion chamber pressure signal or the third digital combustion chamber pressure signal is carried out depending on an absolute combustion chamber pressure, wherein in particular the third digital combustion chamber pressure signal is used as the absolute combustion chamber pressure, wherein in particular a threshold comparison is carried out, the threshold value of which for the absolute combustion chamber pressure depends on the speed and / or the load of the internal combustion engine and / or a further predetermined correction value. Method according to one of claims 5 to 7, wherein the selection of the second digital combustion chamber pressure signal or the third digital combustion chamber pressure signal is carried out depending on an ignition timing for spark generation in the cylinder or depending on an injection timing of fuel into the cylinder. Method according to any one of claims 1 to 8, wherein at least the second digital combustion chamber pressure signal is delayed to compensate for a runtime of the third low-pass filter (24). Method according to any one of claims 1 to 9, wherein the filter coefficients of at least one of the low-pass filters (22, 23, 24) used are speed-dependent. Method according to any one of claims 1 to 10, wherein subsampling is provided after filtering with the second digital low-pass filter (23) and / or after filtering with the third digital low-pass filter (24). Device for providing a filtered combustion chamber pressure signal for determining a quantity characterizing combustion in a cylinder (1) of an internal combustion engine, comprising: - a second low-pass filter (23) with a second cutoff frequency (fg2) for filtering a provided first digital combustion chamber pressure signal to obtain a second digital combustion chamber pressure signal; - a third low-pass filter (24) with a third cutoff frequency (fg3) for filtering the second digital combustion chamber pressure signal to obtain a third digital combustion chamber pressure signal; - a processing unit (28) for determining the quantity characterizing combustion in the cylinder (1) using the second digital combustion chamber pressure signal or the third digital combustion chamber pressure signal. Device according to claim 12, wherein a delay element (25, 26) is provided to delay the second digital combustion chamber pressure signal in time, so that the second and the third combustion chamber pressure signals do not have a time offset from each other, wherein in particular the duration of the delay depends on the rotational speed. Computer program product containing program code which, when executed in a data processing unit, performs the method according to any one of claims 1 to 11.

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