The present invention relates to a method for generating a reference signal in a knock detector connected to an internal combustion engine, the output signal of which represents the degree of knocking in the engine at any given time.

In the case of engines fitted with a knock detector, the output signal of said detector is so arranged as to control at least one entity which regulates the combustion process of the engine. Already disclosed is the principle of taking as a reference value for the normal level of vibration of the engine the output signal from an accelero eter incorporated in the knock detector, said output signal preceeding by a short interval the period of time during which any knocking which is present is monitored. This method may be used only up to a certain level of rotational speed, of around 4000 rp , since disturbing vibrations from ignition and similar impulses will produce incorrect readings for the reference values over and above a certain level. The aim of the present invention is to provide a simple and reliable method for producing a reference value for the level of vibration of the engine under normal conditions of combustion, said method enabling a representative reference value to be produced even when

the engine is running at high rotational speeds. In achieving this, the present invention is characterized in that it is the rotational speed of the engine which is monitored, and in that the reference signal which is generated is proportional to the rotational speed.

3y taking the rotational speed of the engine as the source of the reference value, it is possible to produce a simple signal by utilizing the ignition distributor which is part of the ignition system of the engine. The pulsating signal from the distributor is totally free from any disturbing signals which may affect the number of pulses and may be used without complication for both digital and analogue signal processing.

In one preferred embodiment of the invention, the value of the reference signal remains constant up to a certain predetermined rotational speed of the engine, whereas the value of the signal at speeds above the predetermined rotational speed exhibits an essentially proportional relationship to the rotational speed of the engine. Thus a break-point is produced on the curve which represents the value of the reference signal as a function of engine speed. In this way the curve may be made to agree quite well with the vibrations which occur in an engine which is running normally. In another preferred embodiment of the invention, the aforementioned constant relationship ^" between the value of the reference signal and the rotational speed of the

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engine is made up of a number of different, constant relationships. Below a predetermined speed the reference signal will exhibit an essentially constant value, whereas at speeds over and above the aforementioned predetermined speed the reference signal will exhibit a number of relationships which are proportional to the speed of rotation, and whic increase in stages as the speed increases. In this way it is possible to adapt the reference signal advantageously to suit the actual level of vibration of the engine at different rotational speeds. Other characteristic features of the present invention may be appreciated from the following description of a typical embodiment of the invention and from the follov/ing Patent Claims. The description is made with reference to the attached figures, of which

Figure 1 shows a block diagram for the application of the method in accordance with the invention Figure 2 shows a wiring diagram for a preferred embodiment; and Figure 3 shows graphs which illustrate the generation of reference signals for the detection of knocking in an internal combustion engine. The typical embodiment illustrated in the figures may be implemented in the form of an arrangement in accordance wit Swedish priority patent application

7903944-7. The aforementioned patent application relates to a knock detector with an output ^' signal so arranged as

to -activate a subsequent timing valve for the control of an entity which regulates the knocking and influences the combustion of an engine. The present invention is not however restricted to regulating knocking with the help of a timing valve, but may also make use within the scope of the idea of invention of other, previously disclosed means for the control of an entity to restrict knocking.

In a wiring diagram as shown in Figure 1 , the knock detector incorporates an accelerometer 2 installe on a vehicle engine 1 for the purpose of monitoring the level of vibration of the engine. The piezoelectrical principle is used to produce a voltage in the acceleromet 2 which is led via a cable 3 to an amplifier 4 before the signal which has been amplified in this way is then led via a cable 5 to a comparator β . The aforementione signal is compared in the latter wit a reference signal which is led via a cable 7 to the comparator 6 from a reference value generating circuit 8 in accordance with the invention. Said circuit S is fed via a cable 9 with, a pulsating signal which is generated in the ignition distributor 10 of the engine in such a way that the frequency of the pulses is directly proportional to the rotational speed of the engine. The output signal from the comparator 6 represen the level of vibration of the engine at any given time and is led via a cable 11 to an integrator 12 , in which

the signal is totalized so that the output signal from the integrator in the cable 13 may be considered to be equivalent to an average level of vibration in the engine 1 . Said output signal is adapted in such a way as to cause an implementing organ to activate an entity which will cancel out any knocking which is present in the engine.

A reference value generating circuit 8 in accordance with the invention is illustrated in greater detail in Figure 2. The succession of pulsating signals from the ignition distributor 10 is led via a cable 9 to a transducer 14 of familiar design. Here the succession of pulsations is converted into a voltage, the level of which corresponds to the frequency of the pulsations and thus to the rotational speed of the engine. Said voltage is led via a cable 20 to a reference value modulator 19 in accordance with the invention. This incorporates a so-called voltage sensor 21 , which in a manner with which we are already familiar will prevent any undesired effect from the signal between the transducer 14 and the signal processing circuit which follows on from the voltage sensor 21 .

From the voltage sensor 21 the reference signal is led via a cable 24 fitted with a resistance 23 to the negative input of an operation amplifier 25. A resistance 26 is located in a secondary cable 27 which connects the output of the amplifier 25 to its negative

input. The relationship between said resistances 23 , 26 determines the signal amplification of the amplifier 25 .

To the positive input of the amplifier 25 is applied a fixed voltage present between two resistances 28 , 29 in a cable 30 which connects a voltage source with earth. As long as the voltage in the cable 24 has a low value, then the output signal of the amplifier 25 will have a high potential. The output from the amplifier 25 will adopt a potential which will correspond to any increase in voltage in the cable 24 . Thus the amplifier 25 will have an inverting function where a lov/ reference signal gives rise to a high output signal, and vice versa.

The output signal from the amplifier 25 is led via a cable 34 and a resistance 33 connected in said cable to the negative input of an operation amplifier 35 Said amplifier 35 is wired up in an identical fashion to the amplifier 25 , with the result that the resistance 36 corresponds to the resistance 26 , etc. From the functional point of view, this means that the amplifier 35 v/ill also have an inverting function, i.e. a high voltage, when the cable 34 produces a lov/ output signal, and vice versa. This means that the reference signal v/iil again be rectified at the output of the amplifier 35 so that a lov/ output signal from the transducer 14 representing a lov/ rotational speed of the engine v/ill also be accompanied by a low output signal from the reference value generating circuit 19 ..

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Figure 3 presents the voltage as a function of the rotational speed of the engine, v/here curve A relates to the reference signal from the transducer 14 , curve B relates to the reference signal from the reference value modulator 19 and curve C relates to a reference signal from another embodiment of the reference value modulator 19 . Said curves A-G may be plotted in relation to an optimum reference signal curve which represents the actual level of vibration v/hich occurs in the engine under normal conditions of combustion.

The position of the break-point in curve B along the rotational speed axis may be changed for the reference value modulator shown in Figure 2 by varying the potential at the positive inputs of the amplifiers 25 , 35 ΏJ means of the resistances 28 , 29 and 38 , 39 respectively. The voltage level for the constant part of the curve B may similarly be influenced by varying said resistances.

The angle between the reference signal curves is an indication of signal amplification, v/hich may be varied in the case of the present invention by selecting different relationships between the resistances ^' 23 , 26 and 33 , 36 respectively shov/n in Figure 2.

A reference signal in accordance with curve C i Figure 3 may easily be produced with a modulator v/hich, in addition to what appears in Figure 2, v/ill also include an additional operation amplifier wired up as a comparator.

This will then be. so arranged as to monitor the voltage level at the output from the modulator sc that the comparatur v/ill switch over the output signal v/hen the desired voltage level for the second break-point on curve C is reached. An electronic switch (circuit-breaker) v/ill then influence the number of resistances v/hich are connected together in a potentiometer on the output side of the transducer 14 . In this v/ay the potential on the input side 20 of the modulator may be varied in stages. An increase in potential on the input side 20 as the result of a change of this kind will cause the output signal from the modulator to be amplified, v/hich v/ill be manifested in the form of an increased angle of the curve C after its second break-point. Using the same principle, other break-points may be created on the reference signal curve in order to better suit it to fit curve D .

The typical embodiment of the invention described above should not be regarded as being binding for the invention, v/hich may be modified within the context of the follov/ing Patent Claims to produce a number of alternative embodiments.

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