GB2334549A

GB2334549A - Knock detection in a combustion engine using ionisation analysis

Info

Publication number: GB2334549A
Application number: GB9803519A
Authority: GB
Inventors: Bart Schreurs
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1998-02-20
Filing date: 1998-02-20
Publication date: 1999-08-25
Anticipated expiration: 2018-02-20
Also published as: GB2334549A8; GB9803519D0; GB2334549B

Abstract

Arrangement and method for detecting knocking condition in combustion engines using ionisation analysis having a measuring circuit with two low pass filters 10, 11, the signal form one filter is subtracted from another, the extracted frequency content within the limits defined by the cut off frequencies of the low pass filters is passed through a high pass filter 14 and then subjected to half wave rectification by diode 16.

Description

ARRANGEMENT AND METHODFORKNOCKDETECNON IN COMBUSTION ENGINES Technical Field The present invention refers to an arrangement for knock detection in combustion engines using ionisation analysis in accordance with the preamble of claim 1, and a method for knock detection by such a system in accordance with the preamble of claim 8.

Background of the Invention In ignition systems having integrated means for detection of the degree of ionisation in the combustion chamber, preferably via the spark plug gap, a number of combustion related parameters can be detected via the ionisation current. In one systems used in motor vehicles, i.e.. in the SAAB 2.3 litre four-cylinder petrol engines, an amplified analogue signal proportional to the degree of ionisation is sent from an ignition module mounted on the engine, or ignition cassette, to the ignition system's control unit. The knock intensity is then detected in the control unit by extracting from the amplified analogue ionisation signal a representative frequency content related to a knocking condition. The representative frequency content for a knocking condition is extracted from the amplified ionisation signal by using a first high-pass filter, a second band-pass filter, a rectifier and finally a windowed integrator. The high pass filter having a cut off frequency just below the typical knock frequency, and the band-pass filter with a centre frequency close to the knock frequency. The reminding rectified high frequency content is thereafter integrated during a knocking window, i.e. when knock typically occurs during the combustion process. The windowed integrator 17 is controlled by external signals W from an engine control module, in a conventional manner. The operating time for the integrator, i.e. the window, could be initiated upon a tirne basis or position in the combustion process, the latter given by the turning position of the crankshaft. The value from the integration thus obtained is an indicative measure for a knocking condition as well as the intensity of the knocking condition.

One problem with such signal conditioning is that occasional spikes generated in the ionisation signal, could pass through the signal conditioning process, and result in a high integrated value, thus indicating a knocking condition. The occasional spikes could be generated by some stray capacitance's in the measuring circuit being discharged, causing a superposed high voltage pulse of short duration and having a frequency above the cut off frequency of the high pass filter.

In systems where the ionisation signal from the combustion chamber is extracted via measurements over the existing spark plug, such spikes may be generated for example by voids in the insulator, i.e. ceramics, of the spark plug as such. The voids could not be completely avoided during manufacture of spark plugs, and spikes could then not be prevented from being generated. Spikes are generated in a random manner, typically after some 40-50 ignitions, and it is of major importance that these spikes are eliminated or not interpreted as a knocking condition. A knocking condition must be detected and counteracted as soon as possible, or otherwise may severe engine damages occur and the knocking condition may escalate in magnitude if not counteracted immediately. On the other hand, if knock counteracting measures are initiated also during non-knocking conditions, the engine will operate under less favourable conditions as of fuel consumption and efficiency.

Summarv of the Invention The invention has the objective of increasing the signal to noise ratio when detecting a knocking condition in combustion engines using ionisation information.

A further objective is to maintain as much information from the combustion process as possible, i.e. particularly information relating to a knocking condition, using integration of the high frequency content typical for a knock condition. Information related to occurrence of a knocking condition as well as the magnitude of the knocking condition could thus be maintained.

Yet another objective is to be able to mask out any occasional spikes generated in the measuring circuit, avoiding a false detection of a knocking condition, and thus preventing any knock counteracting measures from being initiated. The combustion engine could as a result be controlled near it optimum without any retardation of ignition timing due to false indication of knock.

The arrangement and method in accordance with the invention are distinguished by the characterising parts of claim 1 and 8 respectively.

By means of the arrangement and method in accordance with the invention it is possible to improve the signal to noise ratio for the ionisation signal. Occasional discharges in the measuring circuit, particularly discharges occurring in the spark plug, could be masked out efficiently, not causing any incorrect indication of a knocking condition and subsequent ignition tining retard.

Other special features and advantages of the invention are indicated by the other characterising parts in attached claims and in the subsequent description of a design example. The description of the design example is conducted with reference to the figures indicated in the following list of figures.

Enef Description of the Drawings The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows an ionisation signal conditioning circuit according the invention; Figure 2 shows a typical ionisation current signal from a combustion subjected to a knocking condition; Figure 3 shows a typical ionisation current signal from a combustion not subjected to a knocking condition, but where a discharge from stray capacitance's in the spark plug occurs during the measuring window used for knock detection, Figure 4 shows a measuring circuit for detection of ionisation as implemented in figure 1.

Description of the Preferred Embodiment The invention is applied in combustion engines having one measunng gap arranged within the combustion chamber of a combustion engine. In figure 1 is shown one embodiment where the spark plug gap is used as the measuring gap. A measuring circuit 7, preferably of a type shown in US,A,5676113, for detection of ionisation within the combustion chamber is arranged in one end of a secondary winding 5 of the ignition coil 3. The measuring circuit is shown in figure 4, including a capacitor 30 being charged by the ignition current to a voltage level defined by the break down voltage, i.e. zener voltage, of a zenerdiode 31 connected in parallel with the capacitor.

A signal proportional to the degree of ionisation is obtained over a resistance 33, having a protecting diode 32 connected in parallel. The Capacitor thus creating the bias voltage source for measuring the degree of ionisation in the combustion chamber via the spark plug gap 6.

The ignition spark in the spark plug gap 6 is induced by switching the current in the primary winding 4 of the ignition coil in a conventional manner. One end of the primary winding 4 is connected to a battery 1 and a circuit breaker 2 is arranged in the other end of the primary winding. The operation of the circuit breaker 2 is preferably controlled by an electronic control module(not shown) in a conventional manner, dependent of detected engine parameters such as load, rpm and temperature.

The inventive ionisation signal conditioning circuit, capable of detecting a knocking condition, extracts from the measuring circuit a signal representative for the degree of ionisation within the combustion chamber.

The signal delivered from the measuring circuit 7 could in principle correspond to the dotted curve shown in figure 2, obtained from a combustion within the combustion chamber subjected to a knocking condition.

On the vertical axis is the amplitude ION of the ionisation signal plotted versus time T on the horizontal axis. After generation of the spark at the spark plug gap could the ringing effect of the ignition coil discharge be observed at time tt, which generate the first peak P1 on the plotted curve. Shortly thereafter is the so called flame ionisation phase developed, which corresponds to the development of a burning kernel at the spark plug gap, generating a second peak P2 on the plotted curve. The ION-signal decreases as the kernel moves away from the spark plug gap, but starts to increase as an effect of the increase of combustion pressure and temperature during the so called post ionisation phase. A third peak P3 occurs typically during the post ionisation phase. This third peak could, as described in US,A,5676113 or SE,A,9501801-6, be used in order to detect location of the pressure peak position.

In a typical approach for knock detection is the ION-signal processed in a signal conditioning circuit containing a first high-pass filter, a second band-pass filter, a rectifier and finally a windowed integrator in that particular order.

The first high-pass filter have a cut off frequency just below the knocking frequency of the engine in concern. For a 1.6 Litre, four cylinder engine, this knocking frequency is determined by the geometry of the combustion chamber, and could typically correspond to a frequency in the order of 6300-6400 Hertz. The second band-pass filter have a centre frequency close to the knocking frequency of the combustion chamber in concern. The signal obtained after the first two filters is then representative for a any frequency content corresponding to the knocking frequency of the combustion chamber. The rectifier serve the purpose of rectifying the signal, in order to be able to integrate the signal in the last stage. The integrated value thus obtained from the windowed integrator could then be used as detector for a knocking condition as well as the intensity thereof.

One disadvantage with such an approach is that also electrical discharges in the measuring circuit, such as those generated by voids in the spark plug, having a short duration and occurring during the combustion when a knocking condition could be detected, is passing through the filters and generates an substantial signal output from the windowed integrator. Such a random discharge would then result in a false indication of a knocking condition, and knock counteracting measures may be initiated even though not needed. The engine will then as a consequence be operated in a non-optimal manner and will loose efficiency with an increase in fuel consumption.

In order to avoid false indications of a knocking condition is the inventive ionisation signal conditioning circuit including means 1S16 to block out any occasional discharges from generating a knock indicative output. These means will be described in detail in following parts of the description. The embodiment chosen is designed for a 1.6 Litre, four cylinder engine having its knocking frequency in the order of 6300-6400 Hertz, unequivocally dictated by the geometry of the combustion chamber in concern.

As a first measure is the raw ionisation signal passed to two Low-Pass filters LPF1 and LPF2 arranged in parallel, both preferably of the analogue Butterworth type.

The first Low-Pass filter LPF1 is chosen such that the cut off frequency is in the order of 20-60% of the knocking frequency, preferably 37%. For the 1.6 Litre engine mentioned, preferably 2400 Hertz.

The second Low-Pass filter LPF2 is chosen such that the cut off frequency is slightly above the knocking frequency, in the order of 1.5 10% of the knocking frequency, preferably 2%. For the 1.6 Litre engine mentioned, preferably 6500 Hertz.

The output obtained from the first Low-pass filter LPF1 would then correspond to a typical smoothed ION-signal, without any traces of knock or any other sudden changes in the ionisation signal.

The output obtained from the second Low-Pass filter LPF2 would then correspond to an ionisation signal, and still containing any frequency content corresponding to the knock frequency. However, higher order of frequencies are efficiently blocked out, and a band-pass filter is not needed in order to block out frequencies of a higher order.

Due to the subtraction, the biggest amount of DC content is already discarded. The following remaining DC component can now be taken away with a HPF with a higher cut-off frequency which is less prone to ringing effects than in the case where all the DC component had to be removed with a HPF.

For the following phase in the signal conditioning it is important that the parallel filters LPF1 and LPF2 have a similar characteristics as of phase shift. Both Low-Pass filters being of the Analogue Butterworth type with cut off frequencies as close as possible would fulfil this requirement, but other filters could be used as long as the output signal from each filter have the same or substantially the same phase shift.

Both outputs from LPF1 and LPF2 are passed to a subtraction stage 12, wherein the output from Low Pass filter LPF1 is subtracted with the output from Low Pass filter LPF2. The result after the subtraction would correspond to an inverted reminder of the signal within the frequency range defined by the limits corresponding to the cut off frequencies of the two Low Pass filters.

The result from the Parallel signal processing in filters LPF1 and LPF2 with subsequent subtraction would not be equivalent to a resulting signal from a Band Pass filter. A Band Pass filter would, as an effect of its inherent characteristics, distort signals such that a single positive half wave with a frequency within the range of the Band Pass filter, or single negative half wave, results in an output from the Band Pass filter having a ringing effect with both negative and positive half waves at a successively declining amplitude. This is an effect of the "self-resonance" effect of band pass filters.

A spike SP such as the one shown in figure 3, would with the inventive circuit still come out from the subtraction as a single half wave spike, in contrary to what could be obtained from a signal conditioning process using a Band Pass filter, the latter resulting in a ringing signal with alternating positive and negative half waves at declining amplitude when subjected to a spike SP.

The result from the subtraction is thereafter passed to an amplifier 13, and a High Pass filter HPF/14. The High Pass filter 14 having a cut off frequency in the order of 160 Hertz, thus discarding any reminding DC components in the signal.

Before further signal conditioning is the signal amplified in amplifier 15. In the last signal conditioning stage is the signal passed through a half-wave rectifying means 16, which in its most simple form could be realised in form of a diode. In practice would such a diode need some offset correction due to that the signal from amplifier 15 still may have some DCcomponent. The offset correction could also be used to change the sensitivity of the circuit, The amplifier 15 can changes the slope of the sensitivity and the offset correction can change the offset ; the combination off both gives a very flexible and complete sensitivity adaptation . In following parts of the description are the signal obtained from the amplifier considered as compensated as of any offsets, i.e. having an idealized characteristics without any DC-components.

The half wave rectification is implemented such that all half waves having the same sign as the spike are discarded, i.e. those half waves being correlated to a momentary increase of the combustion related signal.

This will result in the fact that the spike and the effects of the spike will be eliminated from the remaining signal content. Only half waves having an opposite sign in relation to the spike would pass through. This will lead to that all half waves emanating from any knock frequency content, said half waves having an opposite sign of the spike, will pass.

The signal emanating from the signal conditioning stages 10-16 will then contain information relating to any knocking condition in the combustion chamber. In order to be able to establish the order of intensity of the knocking condition is the signal obtained after the half wave rectifying means 16 passed on to an integrator 17. The value from the integrator 17 could then be used as a representative value KS for the knocking condition, as of occurrence of knock as well as of the intensity of knock.

The invention can be modified in a number of ways within the framework of attached claims.

The second Low Pass filter LPF2, 11, could be substituted by any kind of Low Pass filter with a higher order of cut off frequency. If for example a cut off frequency in the order of some 10.000 Hertz is chosen for LPF2, then a phase compensation is most likely needed in order to adjust the outputs from Low Pass filters to such an extent that they have substantially the same phase shift.

The first Low Pass filter LPFl, 10, could also be substituted by any kind of Low Pass filter with a lower order of cut off frequency. If for example a cut off frequency in the order of some 500-1000 Hertz is chosen for LPF1, a similar type of phase compensation is most likely needed in order to adjust the outputs from Low Pass filters to such an extent that they have substantially the same phase shift.

The essential part for the parallel filter or signal processing in filters 10 and 11 are that the resulting signals have substantially the same phase shift.

The outputs from LPF1 and LPF2 could in subtraction stage 12, be subtracted the other way around, i.e. such that the output from Low Pass filter LPF2 is subtracted with the output from Low Pass filter LPF1. The result after the subtraction would correspond to an non-inverted reminder of the signal within the frequency range defined by the limits corresponding to the cut off frequencies of the two Low Pass filters. If this method for subtraction is used, then the rectifying means 16 shown in figure 1, i.e. the diode, should be reversed. The effect would be that only half-waves having a negative sign would pass through.

In signal conditioning circuits as the one shown in figure 1, where an inversion of the signal is made throughout the signal condition process, then half waves from rectifying means having opposite sign as the spike in the combustion related signal must be discarded. In signal conditioning circuits where no inversion of the signal is made throughout the signal condition process, then half waves from rectifying means having the same sign as the spike in the combustion related signal, must be discarded.

It is advantageous that the high frequency content in the signal, i.e. frequencies well above the knock frequency, is discarded as soon as possible in the signal conditioning circuitry, preferably by a Low Pass filter such as LPF1 in the embodiment shown. High frequency noise is likely to be generated in the combustion engine environment, and could result in increase in integrator value and thus false indications of knock, if not discarded as soon as possible during the signal conditioning process.

Claims

Claims I. Arrangement for detecting a knocking condition in combustion engines using ionisation analysis within the combustion chamber of the engine, said knocking condition generating a oscillation in the combustion chamber having a knock-frequency unequivocally dictated by the geometry of the combustion chamber, wherein the arrangement includes; -a measuring gap (6) arranged in the combustion chamber, -bias means (30,31) connected to the measuring gap thereby applying a bias voltage over the measuring gap, -a measuring circuit (7) connected to said measuring gap which at an output from the measuring circuit deliver a signal proportional to the degree of ionisation in the combustion chamber as developed in the measuring gap by said bias means, characterised in that the signal proportional to the degree of ionisation delivered from the measuring circuit at its output is connected to signal conditioning means including; -a first and second Low-pass filters (10,11) connected in parallel, each Low-Pass filter filtering the signal proportional to the degree of ionisation respectively, and wherein the first Low-pass filter (10) have a cut off frequency in the order of a part of the knock frequency, and the second Low-Pass filter (11) have a cut off frequency exceeding the knock frequency, -a subtraction stage (12) connected to the outputs from the Low-Pass filters (10,11) and receiving the filtered output from the first and second Low-Pass filters, which subtraction stage at its output deliver a signal corresponding to the result of subtraction between the signals received from the first and second Low-pass filters, -a DC component discarding means (14) connected to the output from the subtraction stage, which DC-component discarding means deliver a signal which is substantially free from any DCcomponents in the signal -a half wave rectifying means (16) connected to the output from the subtraction stage, wherein the half wave rectifying means is discarding half-waves correlated to momentary increase of the combustion related signal, which half wave rectifying means deliver a signal proportional to the order of the knocking condition.
2. An arrangement in accordance with claim 1 characterised in that the rectifying means (16) is realised in form of a diode.
3. An arrangement in accordance with claim 1 or 2 characterised in that the signal conditioning circuit includes a first and second amplifier (13,15) each connected before the DC component discarding means (14) and the half wave rectifying means (16) respectively.
4. An arrangement in accordance with claim 3 characterised in that the output from the half wave rectifying means (16) is connected to an integrator (17), preferably a windowed integrator being able to start integration during selected events during the combustion, wherein the value (KS) obtain from the integrator is present at an output from the integrator and used as an indicator of occurrence of as well as intensity of a knocking condition.
5. An arrangement in accordance with claim 3 characterised in that the DC component discarding means (14) is realised in form of a high pass filter, having a cut off frequency at a part of the cut off frequency of the first low pass filter (10).
6. An arrangement in accordance with claim 5 characterised in that the cut off frequency of the high pass filter (14) is below 10 % of the knock frequency of the combustion chamber.
7. An arrangement in accordance with claim 1 characterised in that the cut off frequency of the first Low-pass filter (10) is iii the order of 20 60 % of the knock frequency of the combustion chamber, and that the cut off frequency of the second Low-pass filter (11) exceeds the knock frequency of the combustion charnber in the order of 1.5-10%, preferably 2%.
8. Method for detecting a knocking condition in combustion engines using ionisation analysis within the combustion chamber of the engine, said knocking condition generating a oscillation in the combustion chamber having a knock-frequency unequivocally dictated by the geometry of the combustion chamber, and where a combustion related signal is obtained being proportional to the degree of ionisation in the combustion chamber; characterised in that the method includes sequential signal processing stages for the combustion related signal consisting of; - a first signal conditioning stage wherein the high frequency content of the combustion related signal is discarded generating a high frequency filtered combustion related signal, said high frequency content corresponding to frequencies above said knock frequency; - a second signal conditioning stage wherein the low frequency content of the high frequency filtered combustion related signal is discarded, and as a result from said first and second signal conditioning stages being able to extract a selected range of frequencies signal around said knocking frequency from said combustion related signal, - a third signal conditioning stage wherein a half wave rectification is made upon the selected range of frequencies signal, said half wave rectification discarding all half waves contained in the selective range of frequencies signal, which discarded half-waves are correlated to momentary increase of the combustion related signal, thus generating after said half wave rectification output signals being proportional to momentary decrease of the combustion related signal within the selected range of frequencies signal.
9. A method in accordance with claim 8 characterised in that the first signal conditioning stage includes parallel processing of the combustion related signal in a first and second filtering stages, wherein the first filtering stage includes the process of discarding frequency content in the combustion related signal above a first threshold frequency equivalent to a part of the knocking frequency, and the second filtering stage includes the process of discarding frequency content in the combustion related signal above a second threshold frequency exceeding the knocking frequency, said second threshold frequency preferably only exceeding the knocking frequency by some percent of the knocking frequency, and that the filtered results from the first and second filtering stages are subjected to subtraction in order to generate the high frequency filtered combustion related signal.
10. A method in accordance with claim 8 or 9 characterised in that the filtered result from the first filtering stage is subtracted by the result from the second filtering stage, and that the third signal conditioning stage is discarding all half waves having a negative sign.