Ion current detector

In order not to deteriorate the detection properties of an ion current that monitors a combustion state even when a smolder leak is present in an ignition plug of an internal combustion engine, an ion current detector includes an ignition plug of an internal combustion engine, an ignition coil which supplies a high voltage to the ignition plug, a controller which sends an ignition command, a bias capacitor which supplies a bias voltage to the ignition coil, an ignition capacitor which is connected to a primary side winding of the ignition coil, a DC power supply which charges the ignition capacitor and the bias capacitor, a current detecting unit which detects a current flowing through the ignition plug, an ion current detecting unit which detects an ion current from the current detected by the current detecting unit, and a Zener diode which restricts a charging voltage of the bias capacitor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ion current detector which detects an ion, for example, generated by combustion in an internal combustion engine.

2. Description of the Related Art

In recent years, the problems of environmental conservation and fossil fuel depletion have been raised and it becomes an urgent need to deal with these problems also in the automotive industry. As an example dealing with this, there is ultra-lean combustion (stratified lean combustion) operation of an engine which utilizes a stratified fuel-air mixture. In the stratified lean combustion, the concentration of the fuel-air mixture may vary; and therefore, in the case of combustion in a locally rich state, soot is generated and carbon adheres to an ignition plug. Accordingly, a leak caused by a smolder is generated.

Under conditions where the smolder leak is generated, there is a high possibility that fails to ignite the fuel-air mixture; and from this standpoint, a technique for monitoring whether or not intended combustion is obtained is desired. As one of such methods, methods which detect an ion generated by the combustion as a current and monitor the combustion state from generation conditions of the ion current are proposed in large numbers.

As a device for detecting such ion current, for example, in an internal combustion engine ignition device disclosed in Patent Document 1, an ignition circuit of an ignition plug is configured so as to perform multiple discharge by so-called a capacitor discharge ignition (CDI) system which includes an ignition coil in which an ignition plug is connected to a secondary side winding L2, a capacitor which is connected in series to a primary side winding L1of the ignition coil, a boosting circuit which charges the capacitor by boosting a voltage of a battery BT, a thyristor which intermits a closed-loop formed by the capacitor and the primary side winding L1, and a timer circuit which repeatedly outputs a trigger signal TG that makes the thyristor turn on at a constant discharging interval during the time defined by an ignition signal IG that is set depending on an operating state. The capacitor is charged by utilizing a secondary current flowing through the secondary side winding of the ignition coil and a voltage (bias voltage) across the ends of the charged capacitor is applied to a central electrode of the ignition plug via the secondary side winding of the ignition coil; and accordingly, an ion current that flows by an ion generated by combustion of a fuel-air mixture is detected to get a grasp of the combustion state.

However, in the internal combustion engine ignition device of Patent Document 1, a method for charging a capacitor by utilizing the secondary current is adopted; and there are concerns that the capacitor cannot be sufficiently charged to a necessary voltage when capacitance of the capacitor is set to be large, and energy is consumed for charging the capacitor and therefore energy for spark discharge is reduced to deteriorate ignition properties to the fuel-air mixture. Therefore, a capacitor whose capacitance is relatively small is used for an actual device in consideration of such circumstances.

But, in the case where the smolder leak of the ignition plug is generated, the smolder leak being apt to generate in the stratified lean combustion, problems exist in that energy stored in the capacitor flows out as a leakage current from a path of the leak caused by the smolder and therefore a bias voltage for detecting an ion current is reduced to deteriorate the detection properties of the ion current, and the proportion of the amount of energy flow with respect to the capacitance of the capacitor increases and therefore the leakage current associated with the lowering of the bias voltage has a waveform shape that simply attenuates during one ignition cycle and it becomes difficult to discriminate between the ion current and the leakage current.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made to solve the problem described above, and an object of the present invention is to provide an ion current detector which does not deteriorate the detection properties of an ion current that monitors a combustion state even when a leak caused by a smolder is present in an ignition plug of an internal combustion engine.

In order to solve the foregoing problem, an ion current detector of the present invention includes a bias capacitor which supplies a bias voltage to an ignition plug of an internal combustion engine, a direct current (DC) power supply which charges the bias capacitor, and an ion current detecting unit which detects an ion current included in a current flowing through the ignition plug.

According to the ion current detector of the present invention, the bias capacitor which supplies the bias voltage to the ignition plug is charged by the DC power supply, whereby there is an effect in that the detection properties of the ion current does not deteriorate even in the case where a path of a leakage current caused by a smolder or the like of the ignition plug is formed.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an ion current detector according to a preferred embodiment of the present invention will be described with reference toFIG. 1,FIG. 2, andFIGS. 3A to 3D.

FIG. 1is a block diagram showing the whole of an ignition system of an internal combustion engine including an ion current detector according to a preferred embodiment 1;FIG. 2is a circuit configuration diagram of the ion current detector of the preferred embodiment 1; andFIGS. 3A to 3Dare various types of waveform diagrams in the ion current detector.

As shown inFIG. 1, the ignition system of the internal combustion engine in the preferred embodiment 1 includes an ignition plug102of the internal combustion engine, an ignition coil103which supplies a high voltage to the ignition plug102, a controller101which sends an ignition command to the ignition coil103, a bias capacitor104which supplies a bias voltage to the ignition plug102, an ignition capacitor105which is connected to the primary side of the ignition coil103, a DC power supply106which charges the ignition capacitor105and the bias capacitor104by electric power of a battery Es, a current detecting unit107which detects a current flowing through the ignition plug102, an ion current detecting unit108which detects an ion current from a current including the ion current detected by the current detecting unit107, and a charge limiting unit109which restricts a charging voltage of the bias capacitor104. Among those, the ion current detector100is a portion composed of the bias capacitor104, the ignition capacitor105, the DC power supply106, the current detecting unit107, the ion current detecting unit108, and the charge limiting unit109, those of which are shown by a dashed line inFIG. 1.

Next, the basic operation of the ignition system and the ion current detector100of the internal combustion engine will be described with reference toFIG. 1andFIG. 2.

First, voltage of the battery Es is boosted by the DC power supply106to charge the bias capacitor104. This supplies a stable bias voltage Vb for detecting an ion current from a current flowing through the ignition plug102. The bias voltage Vb is applied to a central electrode102aof the ignition plug102via the ignition coil103.

On the other hand, the ignition coil103generates a high voltage (secondary voltage) for generating spark discharge by an ignition command sent from the controller101. The secondary voltage is applied to the central electrode102aof the ignition plug102to generate the spark discharge between the central electrode102aand the earthing side electrode102bof the ignition plug102. A combustible fuel-air mixture in a combustion chamber of the internal combustion engine is ignited by the spark discharge. This starts combustion and a combustion ion is generated associated with the combustion.

The bias voltage Vb has been already applied to the central electrode102aof the ignition plug102; and therefore, current flows via the combustion ion. This current is generally referred to as an ion current. In this case, the ion current flows through the earth→the bias capacitor104→the current detecting unit107→a secondary side winding103bof the ignition coil103→the ignition plug102; and the ion current further flows to the earth via the combustion ion.

The ion current detecting unit108monitors a current waveform of the current flowing through the ignition plug102, the current being detected by the current detecting unit107; and the ion current detecting unit108detects main parameters such as the level of the ion current and generation/completion timing. The controller101determines a combustion state using the main parameters of the ion current obtained by the ion current detecting unit108and controls various types of actuators including the ignition coil103as needed.

Next, the detail operation of the preferred embodiment 1 will be described with reference to a specific circuit diagram of the ion current detector100shown inFIG. 2and waveform examples (each arrow denotes a zero point) ofFIGS. 3A to 3D. In this case,FIG. 3Ashows a control signal waveform,FIG. 3Bshows a voltage waveform to be applied to the ignition plug102,FIG. 3Cshows a current detection waveform in the case where a smolder leak is not present, andFIG. 3Dshows a current detection waveform in the case where the smolder leak is present.

First, description will be made in the case of a fundamental operation in a clean state where the smolder leak is not present in the ignition plug.FIG. 2shows an ignition device generally referred to as a direct current capacitor discharge ignition (DC-CDI) system and represents a configuration in which the DC-CDI system is combined with the ion current detecting unit.

A boosting type DC/DC converter is used as the DC power supply106and boosts a voltage (approximately 12 V) of the battery Es to not lower than approximately 200 to 300 V to charge the ignition capacitor105and the bias capacitor104for detecting an ion current.

As shown inFIG. 3A, when a High ignition command signal301is sent from the controller101as a control signal, an insulated gate bipolar transistor (IGBT)202serving as a switch becomes an ON state and a primary current flows from the ignition capacitor105to the earth via a primary side winding103aof the ignition coil103. When the current begins to flow through the primary side winding103a, high voltage induced electromotive force302(solid line) shown inFIG. 3Bis generated in the secondary side winding103bof the ignition coil103that is in a magnetically coupled state because the number of windings of the secondary side winding103bis larger than that of the primary side winding103a; and a dielectric breakdown is caused between the central electrode102aand the earthing side electrode102bof the ignition plug102. This causes a secondary current303shown inFIG. 3Cto flow through the secondary side winding103bof the ignition coil103by way of the earthing side electrode102bof the ignition plug102→the central electrode102aof the ignition plug102→the secondary side winding103bof the ignition coil103→a Zener diode203→a Zener diode204→the earth.

Each voltage across the respective capacitor ends of the ignition capacitor105and the bias capacitor104is always monitored, and if both voltages reach a predetermined target charging voltage, the boost of the battery Es by the DC/DC converter106is stopped. The target charging voltage of the bias capacitor104and that of the ignition capacitor105are set to be the same value, and a breakdown voltage of the Zener diode204is set to be the same or more than the target charging voltage of the bias capacitor104. Ideally, the breakdown voltage of the Zener diode204is preferably set to be the same value as the target charging voltage of the bias capacitor104; however, realistically, the breakdown voltage of the Zener diode204is set to be slightly higher in consideration of variations in the characteristics of the bias capacitor104. This is because that there is a case where if the breakdown voltage of the Zener diode204is lower than the charging voltage of the bias capacitor104, the boost cannot be stopped. For example, the target charging voltage is set to 200 V and the breakdown voltage is set to 210 V. The breakdown voltage of the Zener diode204serves as the charge limiting unit109which restricts the charging voltage of the bias capacitor104.

Alternatively, if only the target charging voltage of the ignition capacitor105is set and the voltage across the ignition capacitor105reaches the target value, even if the boost of the battery Es by the DC/DC converter106is stopped, substantially similar effects can be obtained. This is because it is considered that, actually, an increase or decrease in energy of the ignition capacitor105is more predominantly larger and capacitance of the ignition capacitor105is also generally set to be larger than the bias capacitor104; and therefore, if the ignition capacitor105satisfies the target charging voltage, the bias capacitor104also satisfies the target charging voltage. Furthermore, the bias capacitor104is maintained in a state near full charge by the DC/DC converter106; and therefore, the secondary current303flowing through the secondary side winding103bof the ignition coil103shown inFIG. 3Chardly flows through the bias capacitor104. Thus, energy loss of the spark discharge hardly generates, the consumption of unnecessary electric power can be suppressed, and the deterioration of ignition properties can be suppressed.

On the other hand, the bias voltage Vb by the bias capacitor104in which charging has been completed is applied to the ignition plug102via the current detecting resistor107and the secondary side winding103bof the ignition coil103, that is, as shown inFIG. 3B, approximately 200 V is always applied to the central electrode102aof the ignition plug102as the bias voltage Vb; and accordingly, a plug voltage304to be applied to the ignition plug102is maintained at a constant value. Therefore, if a combustion ion due to ignition and combustion of the fuel-air mixture caused by the spark discharge is present near the central electrode102aof the ignition plug102, the combustion ion serves as a medium and accordingly, as shown inFIG. 3C, an ion current305flows through the secondary side winding103bof the ignition coil103by way of the earth→the bias capacitor104→the current detecting resistor107→the secondary side winding103bof the ignition coil103→the central electrode102aof the ignition plug102→the earthing side electrode102b.

As a result, a potential difference between both terminals of the current detecting resistor107is monitored by the ion current detecting unit108; and accordingly, it is possible to get a grasp of what sort of the ion current305flows. In this regard, however, during the time when a high voltage is generated by induction of the ignition coil103, the high voltage of opposite polarity negates the bias voltage Vb; and therefore, the ion current305cannot be substantially detected. For this reason, as the ion current detecting unit108, the preferred embodiment 1 copes by using a differential amplifier corresponding to high common mode voltage as shown inFIG. 2.

Incidentally, as the configuration of the ion current detector100, it is preferable that those surrounded by a solid line shown inFIG. 2are in the same package; alternatively, the ion current detector100and the controller101are in the same package.

Furthermore, in the ion current detector100shown inFIG. 2, all constituent elements except for the switch (IGBT)202can be used in common for a plurality of cylinders by a single circuit. That is, an ion current generated in the plurality of cylinders can be detected by one circuit of the differential amplifier of the ion current detecting unit108. In the case where a leakage current caused by a smolder to be described later is generated, it is difficult to specify a cylinder in which the leakage current is generated; however, the ion current of the plurality of cylinders can be confirmed by one common signal wire and thus reduction in size and reduction in cost of the ion current detector100can be achieved.

Next, description will be made on operation in the case where the smolder leak is present in the ignition plug102while comparing the conventional ion current detector (configuration in which the DC/DC converter106of the DC power supply connected to the bias capacitor104is not present in the ion current detector100of the preferred embodiment 1 described inFIG. 2) with the ion current detector100according to the preferred embodiment 1 of the present invention.

In the conventional ion current detector, in the case where the smolder leak is present, a voltage308to be applied to the central electrode102aof the ignition plug102is as shown by a dashed line inFIG. 3B. The bias capacitor104is only charged by the secondary current flowing through the secondary side winding103bof the ignition coil103; and therefore, energy continues to flow out from the path of the smolder leak after the completion of the ignition. Thus, the voltage308to be applied to the ignition plug102follows only one track of lowering as shown inFIG. 3B.

At this time, a current waveform detected by the current detecting resistor107is as shown by a dashed line inFIG. 3D. This is because that the leakage current flows from the path of the smolder leak. As described before, the current simply attenuates associated with the lowering of the bias voltage Vb. An ion current306as shown inFIG. 3Dis superimposed on the current waveform. However, the bias voltage Vb also lowers; and therefore, the waveform of the superimposed ion current306is smaller as compared to the waveform of the ion current305in the case where the leakage current caused by the smolder is not present and the waveform is like a crushed shape. In such a state, by the occurrence of the leakage current caused by the smolder, it becomes difficult to extract the ion current waveform from the current waveform detected by the current detecting resistor107and it becomes difficult to assess generation timing, completion timing, or the like of the ion current; and therefore, a problem exists in that such difficulties influence the determination of the combustion state or the like of the internal combustion engine that utilizes the ion current.

In the ion current detector100of the preferred embodiment 1 of the present invention, the bias capacitor104is charged by the DC/DC converter106of the DC power supply; and therefore, the current waveform detected by the current detecting resistor107has not a change as shown by a solid line inFIG. 3D. As in the conventional ion current detector, the leakage current caused by the smolder flows; however, the DC/DC converter106is connected and accordingly the bias capacitor104is always in a full charge state. Therefore, the bias voltage Vb does not lower as in the conventional ion current detector, the current detected by the current detecting resistor107becomes a constant value, and an ion current307is superimposed on this as shown inFIG. 3D. The lowering of the bias voltage Vb is not present; and therefore, the waveform of the ion current307becomes a similar current waveform to the waveform of the ion current305in the case where the smolder leak is not present.

Accordingly, the bias capacitor104is charged by the DC/DC converter106; and therefore, even in a state where the smolder leak is present, the current waveform detected by the current detecting resistor107can be the same sort of a waveform change whose base is offset by a constant value. Thus, there is little effect on accuracy which assesses the generation timing, completion timing, or the like of the ion current.

As described above, according to the ion current detector in the preferred embodiment 1, there is an effect in that the DC power supply, which charges the bias capacitor that supplies the bias voltage to the ignition plug, is provided; and accordingly, even in the case where the leakage current caused by the smolder or the like of the ignition plug is present, the detection properties of the ion current due to the ion generated during combustion do not lower.

Furthermore, in the ion current detector in the preferred embodiment 1, the bias capacitor is maintained in a state near full charge by the DC power supply; and therefore, the secondary current flowing through the secondary side winding of the ignition coil hardly flows to the bias capacitor. Thus, there is also an effect that energy loss of the spark discharge hardly generates, the consumption of unnecessary electric power can be suppressed, and the deterioration of ignition properties can be suppressed.

In addition, inFIG. 2, the description has been made on the case where the DC power supplies, which charge the bias capacitor and the ignition capacitor respectively, are provided; however, similar effects can be exhibited even when these capacitors are charged by one DC power supply.

Besides, the same reference numerals as those in the drawings represent the same or corresponding elements.

The ion current detector according to the present invention is applied to an ignition device of the internal combustion engine; and accordingly, a combustion state of an internal combustion engine mounted on an automobile, a two-wheeled vehicle, an outboard motor, other special machine, and the like can be correctly monitored; and therefore, it becomes possible to efficiently operate the internal combustion engine and it is useful for fossil fuel depletion problem and environmental conservation.100Ion current detector101Controller102Ignition plug103Ignition coil104Bias capacitor105Ignition capacitor106DC power supply (DC/DC converter)107Current detecting unit (Current detecting resistor)108Ion current detecting unit (Differential amplifier)109Charge limiting unit204Zener diodeEs Battery