A METHOD FOR DETECTING AN ION CURRENT

Abstract: The invention relates to a method for the generation of a low tension for the purpose of detecting an ion current in the spark gap of an internal combustion engine. The tension is generated by a controllable ignition magneto (5) arranged in order to charge (2) an ignition capacitor (4). Said tension is connected (3) to the primary side of the ignition of a spark and after the decay of the spark, after which the ion current is detected (11) on the secondary side of the ignition device.

Description: A METHOD FOR DETECTING AN ION CURRENT DESCRIPTION Technical Field The present invention relates to a method for the generation of a voltage for the purpose of detecting an ion current in the spark gap of an internal combustion engine. The detection is supposed to take place after the ignition of the spark and after the decay of the spark. Prior Art It is known that the combustion of an air/fuel mixture in an internal combustion engine results in the production of ions. These ions can be detected by applying a voltage across the spark gap with the result that an ion current is generated. This ion current can be measured and used for the detection of misfire, knock, missing combustion, combustion quality and so on, of the engine. The measurement of the ion current attained in the spark gap can take place either on the high tension side of the spark device or on the low tension side. On the high tension side a measurement problem is the difficulty of handling the generated voltage (up to about 50 kV) by means of commercially available electronic components. Due to these problems the ion current measurement takes place on the low tension side of the spark device today. According to this method there are problems as well, that is to say component tolerance problems and leakage currents coming into existence in components and coils and causing interpretation uncertainty of the measurements carried out. Furthermore, the spark itself disturbs the measurements of the ion current when the spark current and ion current are time- connected to each other, and the differences of the amplitudes are about 1000 times. Another problem is that the ion current amplitude is influenced by petrol additives. The technique of today for the purpose of measuring an ion current is based on the discharge of a DC voltage of about 100 V being stored in a capacitor arranged for that purpose in the secondary circuit of the ignition device, which DC voltage is discharged via the spark device in connection with the generation of the spark. This voltage gives rise to a varying ion current, where the ion current level depends on the number of free ions. A change of the number of the ions changes the conductivity between the electrodes. Ignition knock, misfire, combustion quality and so on can be read from the ion current by means of signal processing, such as frequency separation and other mathematical signal processing. Brief Description of the Invention The object of the invention is to generate an ion current in the spark gap of an internal combustion engine and solve the problems mentioned above relating to the electronic components and the effect from the spark current. After signal processing the detection of knock, misfire, combustion quality and so on can be accomplished by means of this ion current. According to the invention the ion current is generated by applying a low voltage across the spark gap, which has to be done after the decay of the generated spark so that the spark does not disturb the measurement of the ion current. The voltage is applied by means of an ignition magneto, for example a high frequency oscillator. It is known to arrange an ignition magneto in a capacitive ignition system in order to charge a charging capacitor. See our Swedish patent application No. 9501259-7. According to the invention this ignition magneto is also used to generate said voltage for the purpose of generating an ion current. The voltage is applied across the spark gap by means of the secondary coil of the ignition device or across a specially arranged winding. The ion current generated is detected on the low tension side of the secondary side of the ignition device. Additional features are indicated in the accompanying claims. The invention will be explained by means of examples of embodiments shown in the drawings. Figure 1 indicates a system for the generation of a tension according to the invention. Figure 2 indicates an ignition coil and a measuring circuit for the ion current according to the invention. Description of Embodiments of the Invention Fig. 1 indicates a capacitive ignition system of an internal combustion engine. The invention can also be used in inductive ignition systems. 1 indicates an ignition coil with a connection 2 to a first primary winding A and a connection 3 to a second primary winding B, which is arranged specially for said purpose. A charging capacitor 4, preferably having a low capacity, is connected to the connection 2 of the first primary winding. The charging capacitor 4 is also connected to an ignition magneto 5, for example a high frequency oscillator, in order to give a short high energy spark being able to ignite the fuel mixture. The connection 3 of the second primary winding B is connected to high frequency oscillator 5 to make it possible also to use the high frequency oscillator as a low tension source for the generation of the ion current. The discharge of the charging capacitor 4 is controlled by a thyristor 6 or the like, the control electrode 6 S of which is connected to an electrical control unit 7. The control unit 7 is also connected to the high frequency oscillator 5. The mentioned components are known as such, and therefore their constructions or functions do not have to be described here. On the secondary side of the ignition coil 1 there is a connection 8 on the high tension side to a spark plug 10, and on the low tension side there is a connection 9 to earth with measuring circuits 11 for the measurement of the ion current. The system works as follows. The charging capacitor 4 is discharged by triggering the thyristor 6 which is controlled by means of the control unit 7. The discharge results in a spark in the spark plug after which ions are produced at the combustion of the air/fuel mixture in the combustion space. After the decay of the spark an oscillating low tension is applied to the primary side of the ignition coil, by means of the high frequency oscillator 5, to a special winding B connected to the ignition coil. The reason for using different primary windings A, B is to increase the accuracy of the measuring signal, which signal thereafter is measured of the secondary winding of the ignition device. If the primary/secondary ratio is 1/100 an eventual inaccuracy is amplified about 100 times when controlling the primary voltage. The applied low tension produces a current which depends on the number of ions produced in connection with the combustion. Both the charging circuit 4, 6 and the ignition coil 1 must be very fast and therefore high frequency can be used in the charging circuit. The amplitude of the ion current is influenced by additives in the petrol. By changing the applied ion measuring voltage the ion current can be adapted to the right basic level for all types of fuel. A control of the amplitude of the applied low tension for the generation of an ion current is accomplished by the control unit 7. A control of the duration and the time of the application, i.e. the time for the "connection" of the ion current, are also arranged by the control unit 7. This time must be chosen so that disturbances of the measurement do not arise from the oscillating spark current generated by the ignition of the spark. So, the spark current should be decayed prior to the connection of the measuring tension. The generated ion current is detected on the low tension side 9 of the spark device in a separate measuring circuit 11 which is arranged to the connection 9. The ion measuring voltage can be rectified (D) and smoothed by means of distributed capacities (C) occurring in the ignition coils of the ignition device, or by means of separate distributed capacities specially placed in the coil. It is obvious for the man skilled in the art that the embodiment shown is only an example of the invention. The invention is only restricted by the characteristics given in the claims.

Claims: CLAIMS 1. A method for the generation of a voltage for the purpose of detecting an ion current in the spark gap of an internal combustion engine, characterized in that a controllable ignition magneto (5) or the like is arranged (2) on the primary side of the ignition device in order to charge an ignition capacitor (4), and that after the ignition of a spark and after the decay of the spark said ignition magneto is connected to a special primary winding (B) on the primary side, as a low tension source (3), so as to generate an ion measuring tension, after which the ion current is detected (11) on the low tension side of the secondary side of the ignition device. 2. A method according to claim 1, characterized in that the amplitude of the ion current/ion voltage is controllable (7). 3. A method according to any of the claims 1-2, characterized in that the duration of the ion current is controllable (7). 4. A method according to any of the claims 1-3, characterized in that the time for the connection of the ion current is controllable (7) so as to eliminate measurement disturbances originating from said spark and said decay of the spark. 5. A method according to any of the claims 1-4, characterized in that the ion measuring tension is maintained on a DC level by means of any of the distributed capacities in the ignition coils of the ignition device. 6. A method according to any of the claims 1-5, characterized in that special distributed capacities are created to be used for the generation of the ion measuring tension.

Inventors: BENGTSSON JOERGEN, OTTOSSON LARS-OLOF, BENGTSSON, JOERGEN, OTTOSSON, LARS-OLOF
Applicants: BENGTSSON JOERGEN, OTTOSSON LARS OLOF, SEM AB, BENGTSSON, JOERGEN, OTTOSSON, LARS-OLOF
Application year: 1997
Main classification: F02P5
IPC codes: F02D45, F02P17, G01M15