EP0218029A1 - High voltage capacitive ignition device for an internal-combustion engine - Google Patents

Abstract

High-voltage capacitor ignition device for internal combustion engines with a voltage source that can be switched off in the rhythm of the ignition pulses, which is designed as an audio frequency push-pull flyback converter, which is controlled by a pulse width modulator and generates a high voltage charging an ignition capacitor from an on-board electrical system low voltage, and with one the ignition capacitor via an ignition coil discharging thyristor, which is triggered by the ignition pulses at its gate electrode and blocked by switching off the voltage source. At least one comparator (CP1, CP2) receives the voltage occurring at the gate electrode as a parameter of the operating state of the thyristor (THY), compares it with a reference voltage and generates an output signal that switches off the voltage source only as long as the thyristor (THY ) is in its conductive state. The primary winding of the ignition coil (SP) is bridged with a series circuit, which in turn is formed from a diode (D9) and the parallel circuit from a choke coil (DR) and a resistor (R16).

Description

The invention relates to a high-voltage capacitor ignitor of the type mentioned in the preamble of claim 1.

Such an igniter is e.g. known from the book "Modern Electronic Circuits Manual" McGraw-Hill Book Company, 1980, page 83. This known ignition device uses a flyback converter which can be switched off and which oscillates at f = 10 kHz and which is formed from NAND elements. The primary winding of a transformer is connected in series with the thyristor discharging the ignition capacitor, the secondary winding of which drives a further thyristor in order to generate a pulse for switching off the voltage converter. The duration of this switch-off pulse must be so long that an early restart of the voltage converter is delayed as long as the main thyristor is conductive or ionized. If the voltage converter is switched on again too early, the thyristor immediately carries current again, without this having to be triggered by an ignition pulse at its gate electrode. The switch-off time for the voltage converter must therefore be chosen so long that the thyristor certainly deionizes again after the maximum possible high voltage has been discharged at the ignition capacitor, i.e. after the maximum possible current has flowed through the thyristor, i.e. is completely blocked before the voltage converter can be switched on again to recharge the ignition capacitor.

However, this limits the maximum ignition frequency up to which a sufficient high voltage can still be generated at the ignition capacitor. In the known ignition device, the duration of the switch-off pulse for the voltage converter is changed as a function of the ignition frequency, so that the switch-off duration of the voltage increases as the frequency increases to reduce transducer. However, this takes place via the transformer in the circuit of the thyristor, which, as soon as a pulse has been triggered, triggers the second thyristor, whereupon this transmits a switch-off pulse delayed by an RC combination to the NAND elements. The switch-off time is shorter with a higher frequency, but it can only represent an approximation of the actual conditions at the main thyristor, which, with some safety margin, must always be greater than the physically absolutely possible minimum time. Since, as is well known, the transformer can only transmit current changes, it is not possible to see in this known circuit how a faulty tripped thyristor should come out of its permanently on state, provided its holding current is not interrupted by switching off the voltage source. It should also be noted that the discharge time of the ignition capacitor depends essentially on the impedance of the ignition coil. In the case of a circuit as published here, the switch-off time would either have to be based on the greatest possible impedance of an ignition coil that might be used, or the device had to be optimized each time for the ignition coil to be used, production tolerances of the ignition coil also playing a role here . In addition, if the spark output is changed by changing device components, the switch-off time must be adjusted each time by RC tuning.

The object of the invention is to develop an ignition device of the type mentioned in the preamble of claim 1 in such a way that the voltage source is only switched off for as long as the thyristor actually, even under constantly changing working conditions, in its conductive or still ionized operating state is, which should be as short as possible, and that changes in performance-determining components of the device and external ignition system have no influence on the optimal switch-off time of the voltage source, since the switch-off time adapts at any time.

In an igniter of the type mentioned, this object is achieved by the features specified in the characterizing part of patent claim 1.

In the ignition device according to the invention, the voltage occurring at the gate electrode of the thyristor is directly monitored with the aid of at least one comparator and compared with a reference voltage.

The invention makes use of the knowledge that the voltage occurring at the gate electrode provides information at all times about the current operating state of the thyristor. This is because the voltage occurring at the gate electrode only reaches its constant quiescent voltage of approx. <1 mV when the thyristor deionizes, i.e. is completely locked. In this state, the thyristor can only be turned on again if it receives a sufficiently large pulse at its gate electrode. With the aid of the at least one comparator, an output signal that switches off the voltage source can therefore only be generated for as long as the thyristor is conductive or ionized. However, the voltage source is only switched off for as long as is absolutely necessary for physical reasons. These shortest possible switch-off times of the voltage source, however, extend the usable charging times for the ignition capacitor, so that higher ignition frequencies or a higher ignition voltage UC can be achieved in the upper speed range. Furthermore, the switch-off time adjusts component changes optimally, e.g. after changing the ignition coil impedance and the associated change in spark duration.

In addition, the ignitor according to the invention is also very insensitive to interference, since the conductive monitoring of the gate electrode detects a conductive or ionized state of the thyristor even if this state should be initiated incorrectly, i.e. not due to an ignition pulse given to the gate electrode. A faulty thyristor that is permanently conductive and thus short-circuits the voltage source has a voltage of approximately 600 millivolts on its gate electrode. Even in such a state, the output signal of the at least one comparator switches off the voltage source, as a result of which the holding current for the thyristor is interrupted and the thyristor is therefore blocked.

The thyristor can be switched off in an accelerated manner by a series circuit bridging the primary winding of the ignition coil, which is formed from a diode and a parallel circuit from a resistor and a choke coil. This takes place in detail in that a small pulse is derived from the positively polarized return pulse generated by the ignition coil, which in turn charges the ignition capacitor in reverse polarity. After this small pulse has subsided, the ignition capacitor with the opposite polarity is applied to the anode-cathode path of the thyristor. This causes accelerated deionization or blocking of the thyristor. The majority of the return pulse, however, is fed back to the ignition coil via the diode and the very low-resistance DC resistance of the choke coil and the resistor connected in parallel. Despite taking part of the energy from the return pulse, the ignition spark can therefore continue to burn without interruption as a DC ignition spark.

Embodiments of the invention are specified in the subclaims.

• Fig. 1 den Stromlaufplan eines bevorzugten Ausführungsbeispiels des erfindungsgemäßen Zündgerätes,
• Fig. 2 ein Oszillogramm der am Zündkondensator auftretenden Spannungen,
• Fig. 3 Oszillogramme der an der Zündspule und der Drosselspule auftretenden Spannungen,
• Fig. 4 ein Oszillogramm der an der Gate-Elektrode auftretenden Spannung und
• Fig. 5 ein Oszillogramm der am Thyristor zwischen Anode und Kathode auftretenden Spannung.

An embodiment of the invention is explained with reference to the drawing. In detail shows:

• 1 shows the circuit diagram of a preferred exemplary embodiment of the ignition device according to the invention,
• 2 shows an oscillogram of the voltages occurring at the ignition capacitor,
• 3 shows oscillograms of the voltages occurring at the ignition coil and the choke coil,
• Fig. 4 is an oscillogram of the voltage occurring at the gate electrode and
• 5 shows an oscillogram of the voltage occurring at the thyristor between the anode and cathode.

As can be seen from the circuit diagram shown in FIG. 1, the voltage source has an audio frequency push-pull flyback converter, f = 10 kHz, with two power transistors TR1 and TR4 and two driver transistors TR2 and TR3. These are connected in the usual way with resistors R3, R7, R21 and diodes D5 and D6 and connected to the primary windings of a transformer T1, the secondary side of which works on a rectifier bridge D1 to D4. The flyback converter is controlled by a pulse width modulator PWM in the form of a commercially available integrated circuit. The modulator outputs a DC reference voltage of 5 volts at its connection 16, which is divided into 2.5 volts with the aid of a voltage divider formed from resistors R1 and R2, and is at the comparator connection 2 as its reference. At the other comparator terminal 1 there is a voltage divider formed by a resistor R8, R9 and R4 from the rectifier circuit. Bridge delivered high voltage UC divided voltage. A resistor Z9 designed as a thermistor or NTC resistor is connected in parallel with a Zener diode Z1. Capacitors C1, C2 and C3 complete the switching of the voltage source in the manner shown.

The high voltage UC emitted by the rectifier bridge is connected to an ignition capacitor C4, which is connected in series with an ignition coil SP and is connected to ground or the negative pole of an on-board electrical system battery via a connection shown as a thinly drawn line for charging it. The other connection of the ignition capacitor C4 is connected to the ignition coil SP via a thyristor THY. The ignition coil SP is connected in series from the diode D9 with a parallel connection of a choke coil DR and a resistor R16. A resistor R17 is connected in parallel with the diode D9. The diode D9 is switched against the discharge direction of the ignition capacitor C4.

A connection which can be connected to a conventional interrupter or another ignition pulse generator is connected via a diode D10 and a resistor R19 to the inputs of two comparators CP3 and CP4, the other inputs of these two comparators each receiving the above-mentioned reference DC voltage of 5 volts. The two other connections of the resistor R19 are each connected to a capacitor C6 and a Zener diode Z2, which in turn are connected to ground or the negative pole of the battery. A resistor R20 supplies an interrupter contact with working current. The Zener diode Z2 limits the incoming ignition signals to a constant amplitude. The interconnected outputs of the comparators CP3 and CDM are connected to the gate electrode of the thyristor THY via a capacitor C5 and a diode D8. Resistors R13 and R14 define the two connections of the capacitor C5 in terms of DC voltage. on the Resistor R18 is given a part of the above-mentioned DC reference voltage to the gate electrode of the thyristor THY in order to raise its idle potential slightly, for example to 150 mV. It should be emphasized that this increase in the quiescent voltage occurring at its gate electrode when the thyristor is blocked, which otherwise is essentially only <mV, to preferably 150 mV, does not influence the operating behavior of the thyristor.

The gate electrode is connected to inputs of comparators CP1 and CP2 via a resistor R14 in order to pass the voltage occurring at the gate electrode of the thyristor THY to these comparators. The other inputs of these comparators receive a reference voltage of 50 mV and 380 mV, respectively. These reference voltages are derived from the aforementioned DC reference voltage with the aid of a voltage divider formed from resistors R10, R11 and R12. A diode D7 connected to the inputs of the comparators protects them from excessive input voltages. The interconnected outputs of the comparators CP1 and CP2 are connected to the compensation connection 9 of the pulse width modulator PWM, which is particularly useful in this application for switching off the modulator.

The ignitor shown in Fig. 1 works as follows:

When the thyristor THY is blocked and the voltage source is switched on, a high voltage of a maximum of 450 volts is generated at the output of the rectifier bridge D1 to D4, to which the ignition capacitor C4 is charged, the charging current from the positive output of the rectifier bridge through the capacitor C4, the ignition coil SP and, parallel to this path, flows through the diode D9 and the choke coil DR to the negative output of the rectifier bridge. The High voltage UC is divided using the voltage divider R8, R9 and R4 down to a value of the order of 2.5 volts, this value being compared to the reference direct voltage of 2.5 volts present at the comparator connection 2 of the pulse width modulator PWM. The pulse width modulator strives to control the flyback converter so that the value at the comparator connection 1 is brought to 2.5 volts. Since the resistor R9 is designed as a thermistor and is therefore temperature sensitive, the high voltage UC is controlled in accordance with the temperature prevailing at the thermistor R9. As can be seen in FIG. 2, the high voltage changes inversely proportional to the temperature T. The lower the temperature, the higher the high voltage UC. The thermistor R9 is preferably exposed to the temperature of the ignitor itself, which is determined by the heat loss of the igniter, but also by the surroundings thereof. In the case of a cold ignition device and thus also a cold internal combustion engine, a maximum high voltage UC is therefore generated, which is limited by the Zener diode Z1, comparable to a kind of "automatic cold start" which supplies the internal combustion engine with ignition sparks of greater energy content. The igniter is preferably designed and arranged so that it heats up in approximately the same way as the warm-up internal combustion engine. As a result, ignition sparks with increased energy and duration are supplied to the internal combustion engine in its cold state, in order to also reliably ignite mixtures which are not intended to ignite. If the internal combustion engine and thus also the ignition device reach their operating temperatures, the high voltage UC is reduced accordingly, since ignition sparks of lower energy and duration are sufficient and the ignition device is only operated with the required power.

The thermistor R9 also serves as overload protection for the igniter, since this is high Power consumption heated up more due to the heat loss, which reduces the high voltage UC and also limits the power consumption by the ignitor.

If an ignition pulse is sent to the comparators CP3 and CP4 from the connection connected to the ignition pulse generator, a pulse appears at the outputs of these comparators whenever the ignition pulse exceeds the reference DC voltage of 5 volts. Ignition pulses that do not represent a real ignition pulse, e.g. can be generated by contact bouncing an interrupter contact are suppressed by the circuits Z2, R19 and C6. A real firing pulse arrives at the gate electrode of the thyristor THY via the capacitor C5 and the diode D8 in order to switch it on. When the thyristor is conductive, the ignition capacitor C4 is discharged via the ignition coil SP, the negatively polarized voltage pulse USP shown in FIG. 3 occurring.

A small part of the very powerful (up to 65 A!), Reflected by the ignition coil SP, positive polarized return pulse, which follows the negatively polarized initial pulse, is derived via the circuit formed from the diode D9, the choke coil DR and the resistor R16 The amplitude of the resistor R16 is limited to a maximum of approximately 40 volts and the temporal length of the choke coil DR is limited to approximately 40 us. When using an extremely low-resistance high-performance ignition coil, the choke coil DR should have a value of preferably 20 pH, with a direct current resistance of at most 100 mOhm, and the resistor (R16) should have a value of 600 mOhm.

The above-mentioned small pulse UDR derived from the return pulse (see FIG. 3) represents a voltage source for its duration, which in turn, from the ignition coil connection SP +, briefly generates a charging current burst through the capacitor C4 and through the thyristor THY sends. When this small pulse subsides, the ignition capacitor C4 is connected to the main electrodes of the thyristor THY with a reverse polarized, reduced voltage and accelerates it from its still conductive state, while the main part of the return pulse via the diode D9 and the very low-resistance DC resistor of the choke coil DR is returned to the ignition coil SP and here the ignition spark continues to burn without interruption.

When the thyristor THY becomes conductive, the voltage UG occurring at its gate electrode also changes (see FIG. 4), which is applied to the inputs of the comparators CP1 and CP2. 4, the two reference voltages of the two comparators CP1 and CP2 are also indicated with dashed lines, the reference voltage of the first comparator CP2 being above the gate open-circuit voltage which has already been raised to 150 mV on the far right in FIG occurs when the thyristor is blocked or no longer ionized. The reference voltage at the second comparator CP1, however, is below this quiescent voltage. As can be seen from FIG. 4, the comparators CP1 and CP2 emit signals at their outputs as long as the voltage occurring at the gate electrode exceeds or falls below the reference voltages. It should be noted that the thyristor THY is still conductive or ionized even if the pulse below the second reference voltage occurs. This means that even in this case the thyristor will immediately be fully current again if the voltage source is switched on.

When the thyristor THY is conductive, the output signals of the comparators CP1 and CP2 reach the connection 9 of the pulse width modulator PWM, used here as a switch-off option, which is thus immediately blocked, which in turn the voltage source is immediately switched off, ie no more high voltage UC is generated. As can be seen from FIGS. 1 and 4, the normally open voltage at the gate electrode of the blocked thyristor THY, which is normally somewhat less than 1 mV above zero potential, is increased to 150 mV, for example, in order to also have the second comparator CP1 with a positive reference voltage of eg to be able to have 50 mV operated. This measure is necessary because the comparator CP1 is fed by only a positive voltage and is therefore not able to compare voltages that are close to the zero potential or even negative. The slight increase in the no-load voltage at the gate electrode has no influence on the operating behavior of the thyristor THY.

As can be seen from FIG. 4, the second comparator CP1 determines that the second reference voltage has been exceeded after approximately 105 μs, without the first reference voltage of the first comparator CP2 also being exceeded. However, this means that the thyristor is no longer ionized. After the voltage source is switched on again, the thyristor can therefore only be turned on again if it has a sufficiently large drive pulse at its gate electrode, i.e. a real ignition pulse.

Immediately after deionization of the thyristor THY, the output signal at the compensation connection 9 of the pulse width modulator PWM therefore disappears, as a result of which the voltage source is switched on again and the ignition capacitor C4 can thus be recharged. The above-mentioned, shortest possible switch-off time of only about 105 μs is also entered in FIG. 2 and relates to the maximum voltage of UC. With a lower voltage UC, the switch-off time of the voltage source is reduced by approximately 5 ps to approximately 100 μs at the minimum voltage of UC. As the speed of the internal combustion engine increases, the ignition frequency naturally also increases, as a result of which the charging times for the ignition capacitor C4 become shorter and shorter. This state of affairs is also shown in FIG. 2, so that the high voltage UC occurring at the ignition capacitor C4 then decreases inversely proportional to the ignition frequency. On the other hand, however, such a decreasing charging voltage at the ignition capacitor C4 is still large enough for reliable ignition of the mixture of the internal combustion engine, since at high and highest engine speeds and thus the piston stays in the top dead center area for a much shorter time, a correspondingly lower energy and again so that a shorter spark is still sufficient to ignite the mixture. On the other hand, since the switch-off time of the voltage source is as short as possible due to the direct monitoring of the voltage occurring at the gate electrode and the accelerated deionization or blocking of the thyristor, the charging time available for charging the ignition capacitor C4 is even at the highest speeds still maximum.

Through the gate monitoring by comparators, as well as through the small positive pulse UDR derived from the return pulse, which reverses the voltage UA, THY (see FIG. 5) on the anode-cathode path of the thyristor and thereby accelerates it deionized or switches off, a maximum ignition frequency of over 1 kHz can be achieved. It being to be noted that 800 Hz corresponds to a four-stroke Otto engine with eight cylinders at a speed of 12,000 rpm ^-1. It should also be mentioned that in connection with a modern, commercially available high-performance ignition coil as an ignition transformer at maximum voltage UC, an ignition spark duration of effectively 300 us can be achieved. With other commercially available higher-resistance ignition coils with reduced steepness of rise Voltage USP (see Fig. 3), 600 to 700 us effective spark burn time reached.

4 and 5 of the voltage U _G at the gate electrode of the thyristor and the voltage U _A , THY measured across the anode-cathode path of the thyristor, the temporal and amplitude relationship between them is particularly clear to recognize both tensions. Figure 5 shows as a "magnifying glass section" part of the very steep voltage drop when the thyristor becomes conductive. At the same time, the voltage occurring at the gate electrode drops sharply to below zero potential. Since at this point in time the thyristor then receives the small pulse derived from the back pulse, the voltage at the anode-cathode path of the thyristor rises again slightly and briefly, after which it reverses in polarity and for accelerated deionization or accelerated blocking of the thyristor leads. At the same time, the voltage occurring at the gate electrode drops below zero potential. This voltage then decays in the manner shown in FIG. 4 to the quiescent gate voltage shown there on the far right. As stated, this quiescent voltage is reached after a period of about 110 microseconds. As already explained above, the output signals at the outputs of the comparators CP1 and CP2 then disappear, as a result of which the pulse width modulator PWM and thus also the voltage source are switched on again.

It can thus be seen very clearly from the oscillograms of FIGS. 4 and 5 explained in more detail above that the voltage occurring at the gate electrode of the thyristor is an exact image of the respective operating state of the thyristor.

Although in the embodiment shown in FIG Ignitor two comparators CP3, CP4 are provided for the control of the thyristor THY, only a single one can be used for control. The use of two comparators connected in parallel increases the strength of the drive pulse or relieves the load on the individual comparator output transistor and is also useful since a widespread form as a quadruple comparator is already available which contains the comparators CP1 to CP4 in the exemplary embodiment shown.

Claims (12)

1.High-voltage capacitor ignition device for internal combustion engines with a voltage source that can be switched off in the rhythm of the ignition pulses, which is designed as an audio frequency push-pull flyback converter, which is controlled by a pulse width modulator and generates a high voltage charging an ignition capacitor from an on-board electrical system low voltage, and with a thyristor discharging the ignition capacitor via an ignition coil, which is triggered at its gate electrode by the ignition pulses and blocked by switching off the voltage source, characterized in that at least one comparator (CP1, CP2) connected to the gate elec trode occurring voltage as a parameter of the operating state of the thyristor (THY) directly monitored and an output signal that switches off the voltage source only as long as the thyristor (THY) is in its conductive state, and that the primary winding of the ignition coil (SP) one Circuit (D9, R16, DR) is assigned, with which the thyristor (THY) accelerating blocking pulse can be derived from the back pulse of the ignition coil (SP). 2. High-voltage capacitor ignition device according to claim 1, characterized in that the circuit is a series circuit formed from a diode (D9) and a parallel circuit comprising a resistor (R16) and a choke coil (DR), which is the primary winding of the ignition coil (SP ) is connected in parallel. 3. Ignition device according to claim 1 or 2, characterized in that the reference voltage of a comparator (CP2) is greater than the quiescent voltage occurring at the gate electrode when the thyristor (THY) is blocked and that this comparator (CP2) switches off the voltage source Signal generated whenever the voltage occurring at the gate electrode is greater than the reference voltage. 4. Ignition device according to one of claims 1 to 3, characterized in that the reference voltage of a second comparator (CP1) is lower than the quiescent voltage occurring at the gate electrode when the thyristor (THY) is blocked and that this second comparator (CP1) does this generates the output signal that switches off the voltage source whenever the voltage occurring at the gate electrode is less than the reference voltage. 5. Ignitor according to claim 4, characterized in that the gate electrode of the thyristor (THY) is connected via a resistor (R18) to a reference voltage (5 V) in order to raise the quiescent voltage slightly above the zero potential, preferably 150 mV , so that the second comparator, which is fed by only a positive voltage, can detect a drop below this gate quiescent voltage. 6. Ignitor according to one of claims 1 to 5, characterized in that the gate electrode of the thyristor (THY) is controlled via at least one further comparator (CP3, CP4), which receives a reference DC voltage (5V) at its other input. 7. Ignition device according to claim 6, characterized in that the at least one further comparator (CP3, CP4) is controlled by a series connection of a resistor (R19) and a diode (D10) of ignition pulses, the one connection of the resistor ( R19) is connected to ground via a capacitor (C6) and the other connection of the resistor (R19) is connected to ground via a Zener diode (Z2), as a result of which unwanted ignition, for example is prevented by bouncing a breaker. 8. Ignition device according to one of claims 1 to 7, characterized in that the pulse width modulator (PWM) limits the high voltage (UC) emitted to the ignition capacitor (C4) by the voltage source to a value corresponding to the temperature and keeps it constant. 9. Ignition device according to claim 8, characterized in that a thermistor (R9 NTC) 'is connected to the comparator connections (1, 2) of the pulse width modulator (PWM) such that the high voltage (UC) emitted by the voltage source is controlled in inverse proportion to the temperature of the thermistor. _10th Ignition device according to claim 9, characterized in that the temperature measured by the thermistor (R9 NTC) is adapted to the operating data of an internal combustion engine, in that the heating of the ignition device, for example as a result of its heat loss, serves as a reference variable. 11. Ignitor according to claim 10, characterized in that the thermistor (R9 NTC) forms a thermal overload protection for the igniter. 12. Ignitor according to one of claims 8 to 10, characterized in that the thermistor (R9 NTC), a Zener diode (Z1) is connected in parallel, which limits the high voltage (UC) to a maximum value.

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