Information
-
Patent Grant
-
6742508
-
Patent Number
6,742,508
-
Date Filed
Thursday, February 13, 200321 years ago
-
Date Issued
Tuesday, June 1, 200420 years ago
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Inventors
-
Original Assignees
-
Examiners
-
CPC
-
US Classifications
Field of Search
US
- 123 598
- 123 605
- 123 596
- 315 209 CD
- 315 209 SC
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International Classifications
-
Abstract
An ignition circuit comprising storage means to store electrical energy, first and second switching devices, means for charging the storage means and means for switching each of the switching devices such that charge on the storage means is transferred to cause firing of an igniter in such a way that the voltage across each switching device is only a fraction of the total applied to the igniter. In this way, standard solid state switches may be employed rather than the more costly devices required to handle the full voltage to be applied to the igniter.More than one such ignition circuit may be combined to provide a scanning multiple spark system with a plurality of igniters, each in conjunction with a pair of switching devices and means for switching each of the switching devices as described above.
Description
BACKGROUND OF THE INVENTION
This invention relates to ignition circuits.
High energy ignition systems employ a capacitor to store electrical energy which is then rapidly discharged to an igniter or spark plug to produce an intense spark sufficient to light a fuel-air mixture. A typical solid-state igniter may require up to 2000 volts to cause break-over. Once the spark has commenced, the igniter voltage collapses to near zero while a current of approximately 2000 amperes flows for the duration of the spark until the energy in the capacitor has dissipated. Normally this cycle of charging and discharging of the stored energy is repeated many times until satisfactory ignition of the fuel occurs.
Some high energy ignition systems employ a gas discharge tube, which breaks over at a point when the voltage on the charging storage capacitor reaches the desired level to ‘dump’ the accumulated charge into the igniter. For various technical reasons including, life expectancy, synchronisation, mechanical robustness and reliability, it is desirable to use solid state electronic switching of the discharge; the most suitable component for this is a thyristor. At present, suitable devices are not cheaply available to handle 2000 volts directly and the high currents in this application. However, devices to switch comfortably at 1000 volts are easy to obtain and are relatively low cost.
It is an object of the present invention to provide an alternative ignition circuit.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided an ignition circuit including storage means to store electrical energy, first and second switching devices, means for charging the storage means, and means for turning on each of the switching devices so that charge on the storage means is transferred to cause firing of an igniter in such a way that the voltage across each switching device is limited to a fraction of the total applied to the igniter. This makes it possible to use relatively inexpensive switching devices, capable of handling a moderate voltage, whilst providing the igniter with a sufficiently high voltage to cause sparking.
Preferably, the storage means comprises a double storage means and provides a reduced voltage point, compared to the total voltage applied to the igniter, which limits the voltage applied across each switching device.
The storage means preferably includes two storage capacitors. The voltage across each switching device is preferably substantially half the total voltage applied to the igniter.
BRIEF DESCRIPTION OF THE DRAWINGS
Several embodiments of ignition circuit will now be described, by way of example, with reference to the accompanying drawings, in which:
FIG. 1
is a diagram of a first embodiment of the circuit;
FIG. 2
is a diagram of a modified form of the circuit shown in
FIG. 1
;
FIGS. 3 and 4
are diagrams of further embodiments of the circuit; and
FIG. 5
is a diagram of a scanning multiple spark system employing the circuit.
DETAILED DESCRIPTION OF THE INVENTION
With reference first to
FIG. 1
, the circuit includes a transformer
1
, with a centre tapped secondary winding
2
. The transformer
1
may be either the output transformer of a switched-mode power converter, or the secondary of a mains step-up transformer. An input supply
3
is connected to the primary winding of the transformer
1
, this being current limited to prevent damage to the supply when momentary overloads are applied during the operating cycle of the system. The supply
3
preferably also has an overvoltage limit to prevent overcharge of the storage capacitors should discharge of ignition sparks not be requested or fail to occur. Two rectifier diodes
4
and
5
are connected in opposite senses to opposite ends of the secondary winding
2
. The cathode and anode of the diodes
4
and
5
are connected respectively to a series connection of two storage capacitors
6
and
7
, the junction
8
between the two capacitors being connected to a centre tapping
9
of the secondary winding
2
. The junction
10
between diode
4
and capacitor
6
connects to the anode of a first thyristor
11
. The junction
12
between the other diode
5
and capacitor
7
connects to a 0 volts reference point
13
. A second thyristor
14
is connected in series between the first thyristor
11
and a first output terminal
15
of the circuit. The circuit's other output terminal
16
is connected with the 0 volts reference point
13
. A resistor
17
is connected between the two output terminals
15
and
16
. A high current diode
18
is connected at its anode to the junction
8
between the two capacitors
6
and
7
. The cathode of the diode
18
connects both to a junction
19
between the two thyristors
11
and
12
and to one end of a resistor
20
. The other end of the resistor
20
connects to a junction between the resistor
17
and the output terminal
15
.
The circuit also includes a first trigger circuit
21
connected to the trigger of thyristor
14
and a second trigger circuit
22
connected to the trigger of thyristor
11
. The second trigger circuit
22
is operated in response to the output from a detector
23
connected across the thyristor
14
, which responds when this thyristor turns on.
The output terminals
15
and
16
of the circuit are connected across the electrodes of an igniter or spark plug
30
.
The trigger circuit
21
initiates the ‘request’ for a spark and may be either a free-running clock oscillator producing regular thyristor gate-pulses, or a pulse shaping circuit that produces a single output pulse in response to an external timing signal to initiate a spark.
In operation, assuming both thyristors
11
and
14
are initially off (open-circuit or high impedance), the alternating output current from the transformer winding
2
charges each storage capacitor
6
and
7
through diodes
4
and
5
. Typically, this voltage rises to 1000 volts on each capacitor
6
and
7
so that the voltage across their series extremities
10
and
12
is 2000 volts. The voltage across resistor
17
will be virtually zero at this time since its value is low enough to bleed away any thyristor leakage current and also the flow through the resistor
20
. The small current which does flow through the resistor
20
ensures that the diode
18
is forward biased and the voltage on its cathode is therefore virtually identical to that at the junction
8
of the capacitors
6
and
7
, that is, 1000 volts relative to 0 volts. By this means, the thyristors
11
and
14
have 2000 volts across them in total, but only 1000 volts across each device.
When the trigger circuit
21
requests a spark, it turns on the thyristor
14
so this becomes effectively a short circuit between its anode and its cathode. This directly connects the 1000 volts from the capacitor
7
, via the diode
18
and the thyristor
14
to the igniter
30
. Most igniters are unlikely to break down at this voltage so the resistor
17
provides a current path to maintain sufficient hold-on current for the thyristor
14
. It can be seen that the voltage across the thyristor
11
at this time remains safely at 1000 volts. The detector circuit
23
detects when the thyristor
14
turns on from the collapse of voltage across it; this causes the trigger circuit
22
to generate a trigger pulse for the other thyristor
11
. When the thyristor
11
turns on, the full 2000 volts, from the series arrangement of the two capacitors
6
and
7
, is applied to the igniter
30
resulting in the initiation of a spark. The diode
18
becomes reverse biased, which prevents current flowing back into the transformer centre tap
9
. The high current discharge through both thyristors
11
and
14
continues until virtually all the stored energy in the capacitors
6
and
7
is depleted and the thyristors both switch off because of the lack of hold-on current.
Since the capacitors
6
and
7
are of the same value, they tend to discharge together with only minor imbalance. However, there is always likely to be some tendency for one or other of the capacitors
6
or
7
to drain first and so exceed the zero limit and develop a reverse charge. Whilst this may not be disastrous, it puts severe strain on the associated rectifier diode
4
or
5
and in some converter circuits may cause saturation of the transformer core producing circuit failure. This effect can be reduced by adding some series resistance into the transformer secondary winding
2
, or by connecting a reverse protection diode across either half of the winding to clamp any reverse swing.
It is possible that the igniter
30
will break-over and commence sparking after the turn-on of the first thyristor
14
but before the firing of the second
11
. In this circuit, the resulting collapse in voltage across the capacitor
7
will simultaneously cause a step shift in the voltage at both ends of the capacitor
6
, so maintaining the 1000 volts charge from the capacitor
6
across the thyristor
11
, which it is able to withstand safely. Although the spark commences at an earlier point in the circuit's operation, the subsequent turn-on of the thyristor
11
still ensures that virtually all the stored energy from both capacitors
6
and
7
is available for the igniter
30
.
With reference now to
FIG. 2
, this shows a modification of the circuit of
FIG. 1
, equivalent components being given the same reference number with the addition of 100.
The circuit of
FIG. 2
differs from that of
FIG. 1
in that capacitor
106
is connected across the 2000 volts developed in the full winding
102
of transformer
101
and that the values and voltage ratings of capacitors
106
and
107
are adjusted appropriately. Capacitor
106
now becomes the single main energy store for the circuit. Since it is operating at the 2000 volts, for a given energy capacity its size and cost may be considerably reduced compared with the dual versions previously described. The other capacitor
107
provides only the initialising voltage for the igniter
130
and reservoir for the mid-rail voltage and will typically be only between 0.5% and 1% of the capacitance value of the main capacitor
106
.
In operation, assuming both thyristors
111
and
114
are off, the alternating output current from the centre tapped winding
102
of transformer
101
charges capacitor
106
to typically 2000 volts and capacitor
107
to 1000 volts through diodes
104
and
105
respectively. The series arrangement of the two thyristors
111
and
114
is subjected to the full 2000 volts from the main capacitor
106
but the voltage across each device is limited to 1000 volts as held by the voltage on the other capacitor
107
.
When the trigger circuit
121
turns on the thyristor
114
this directly connects the 1000 volts at the junction
119
of the two thyristors to the igniter
130
. The other trigger circuit
122
responds to breakdown of voltage across the thyristor
114
rapidly to turn on the other thyristor
111
. When the thyristor
111
turns on, the full 2000 volts from the main capacitor
106
is applied to the igniter
130
resulting in the initiation of a spark. The high current discharge through both thyristors
111
and
114
continues until virtually all the stored energy in the capacitor
114
is depleted and the thyristors both switch off because of the lack of hold-on current.
With this alternative unbalanced capacitor arrangement, the voltage stress on the thyristor
111
is significantly increased if the igniter
130
discharges after the thyristor
114
turns on but before the thyristor
111
turns on. However, since the turn-on of thyristor
114
immediately ‘requests’ the turn on of thyristor
111
, and because typical circuit resistances in leads and the like naturally cause a finite time for the voltage to increase across the main capacitor
106
, it can be arranged for the ‘crowbar’ effect of thyristor
111
to self limit the possibility of overvoltage.
Since there is only one main energy storage capacitor in this implementation, reversal of capacitor voltage is not likely and so an extra protection diode is usually unnecessary.
With reference now to
FIG. 3
there is shown another modification of the circuit in FIG.
1
. In this circuit equivalent components have been given the same reference number with the addition of 200.
In this circuit, the transformer
210
has two separate windings
202
and
202
′ in place of the single centre-tapped arrangement. Also, the thyristors
211
and
214
are not connected together directly. Instead, one thyristor
211
is connected in the series connection of the two capacitors
206
and
207
and the other thyristor
214
is connected between the terminal
210
at the end of the capacitor series and one output terminal
215
. The circuit has an additional diode
227
connected across the series connection of the capacitor
207
and the thyristor
211
directly to the output terminal
215
of the circuit. These two diodes
218
and
227
enable the circuit to operate whichever thyristor
211
or
214
is fired first, thus permitting an alternative thyristor drive arrangement. By way of example,
FIG. 3
shows a modified trigger circuit
221
that provides two virtually simultaneous outputs which are applied to the thyristors
211
and
214
together thus eliminating the need for a turn-on detector.
In operation, assuming that both thyristors
211
and
214
are off, the output of each transformer winding
202
and
202
′ will charge the associated energy storage capacitors
206
and
207
via diodes
204
and
205
respectively to 1000 volts. If, for example, thyristor
211
happens to turn on first in response to its signal from the trigger circuit
221
the 1000 volt charge on the capacitor
207
will be applied to the igniter
230
through thyristor
211
and diode
218
. Normally this is not sufficient to cause break over of the igniter
230
. The resistor
217
provides a path to ensure sufficient hold-on current for the thyristor
211
. When the other thyristor
214
turns on, in response to its own trigger pulse from the circuit
221
, the further 1000 volts charge on capacitor
206
is now added to that of capacitor
207
by nature of its series connection, thereby increasing the voltage applied to the igniter
230
to 2000 volts and initiating a spark. The high current discharge through both thyristors
211
and
214
continues until virtually all the stored energy in the capacitors
206
and
207
is depleted and the thyristors both switch off due to lack of hold-on current.
If the igniter
230
breaks over to commence sparking following the turn-on of the first thyristor
211
but before the firing of the second thyristor
214
, the circuit inherently avoids subjecting the second thyristor to any increase in voltage beyond the 1000 volt level, since the voltage on the capacitor
206
is unaffected by the turn-on of the thyristor
211
. If the thyristors are triggered so that thyristor
214
turns on before thyristor
211
, the diode
218
will carry the initial 1000 volt application from the capacitor
206
to the igniter
230
in place of the diode
227
.
With reference now to
FIG. 4
, there is shown another modified circuit. In this circuit equivalent components are given the same reference number as in
FIG. 1
but with the addition of 300.
This circuit has two secondary windings
302
and
302
′ and the two thyristors
314
and
311
are connected across respective windings via the diodes
304
and
305
. One capacitor
306
is connected between the junction
310
of the diode
304
with the thyristor
313
and an output terminal
315
of the circuit. The other capacitor
307
is connected between the two thyristors
311
and
314
. This circuit uses a ‘shunt’ method of high current switching. As in the circuits of
FIGS. 1 and 3
, individual storage capacitors
306
and
307
are employed at 1000 volts to govern the voltage imposed on each thyristor
311
and
314
. The capacitor
306
charges through the diodes
304
and
327
whilst the capacitor
307
charges through the diodes
305
and
327
. If identical value capacitors
306
and
307
are used, the diode
327
could in practice be replaced with a resistor. When the trigger circuit
322
turns on the thyristor
311
, the charge on the capacitor
307
is applied to the terminal
315
through the diode
318
. It should be noted that the voltage applied to the igniter
330
is negative in polarity relative to the 0 volts shown in the diagram. As before, when the thyristor
314
turns on, the additional 1000 volts on the capacitor
306
is added and applied to the igniter
330
to initiate a spark. This circuit provides the advantage that instantaneous protection may be provided against overvoltage of the transformer in the event of a disconnected or faulty igniter since triggering the thyristors will immediately clamp the winding voltages to zero. As in previous embodiments, the charging circuit associated with the transformer should be protected against over current in this condition.
FIG. 5
shows how circuits of the kind in the arrangement of
FIG. 1
can be used in a scanning multiple spark system. The components in
FIG. 5
equivalent to those in
FIG. 1
are given the same reference number with the addition of 400. The system has a single pair of storage capacitors
406
and
407
charged via diodes
404
and
405
from a transformer
401
and power supply
403
in the same manner as in the arrangement of FIG.
1
. Instead of having a single switching circuit, the system of
FIG. 5
has multiple switching circuits, in this example three circuits indicated A, B and C, which switch charge from the capacitors
406
and
407
to respective ones of three different igniters
430
A,
430
B and
430
C. Each circuit A to C has a pair of thyristors as in the arrangement of
FIG. 1
but these are triggered by signals from a single trigger circuit
421
common to the three circuits A to C. The trigger circuit
421
is a scanning trigger source with an individual output for each switching circuit A, B and C. The trigger circuit
421
triggers each switching circuit A, B and C in turn, one after the other. The interval between each trigger output is chosen to be long enough to allow replenishment of the stored capacitor energy. It will be appreciated that different numbers of switching circuits could be used to fire different numbers of igniters.
The present invention enables low cost electronic switching devices to be used without risk of damage.
Claims
- 1. An ignition circuit comprising storage means to store electrical energy, first and second switching devices, means for charging the storage means, and means for turning on each of the switching devices so that charge on the storage means is transferred to cause firing of an igniter in such a way that the voltage across each switching device is limited to a fraction of the total applied to the igniter.
- 2. An ignition circuit according to claim 1 wherein the voltage across each switching device is substantially half the total voltage applied to the igniter.
- 3. An ignition circuit according to claim 1 wherein the storage means comprises a double storage means and provides a reduced voltage point which limits the voltage applied across each switching device.
- 4. An ignition circuit according to claim 1 wherein the storage means comprises two storage capacitors.
- 5. An ignition circuit according to claim 4 wherein the two storage capacitors are of substantially equal capacitance.
- 6. An ignition circuit according to claim 4 wherein one storage capacitor is of greater capacitance than the other.
- 7. An ignition circuit according to claim 1 wherein the means for switching the switching devices comprises at least one trigger circuit.
- 8. An ignition circuit according to claim 7 wherein the trigger circuit controls the switching of both switching devices.
- 9. An ignition circuit according to claim 7 wherein the means for switching the switching devices comprises two trigger circuits, the first to control the first switching device and the second to control the second switching device.
- 10. An ignition circuit according to claim 9 wherein the means for switching the switching devices further comprises a detector circuit which detects the switching of the first switching device, thereby enabling the second trigger circuit to respond to switching of the first switching device.
- 11. An ignition circuit according to claim 4 which further comprises means for protecting the circuit from the adverse effects of reverse charging of the storage capacitors.
- 12. An ignition circuit according to claim 1 wherein the switching devices are solid state switches which, on switching on, allow current to pass through them in one direction only and which remain conductive once switched off, provided there is sufficient current passing through the switch.
- 13. An ignition circuit according to claim 12 wherein the solid state switches are thyristors.
- 14. An ignition circuit according to claim 12 which further comprises means to provide sufficient current to the solid state switches such that they remain on for a desired duration.
- 15. An ignition circuit according to claim 14 wherein the means to provide sufficient current to the solid state switches comprises at least one resistor.
- 16. An ignition circuit according to claim 13 wherein the means for switching each of the switching devices comprises free-running clock oscillators producing regular thyristor gate-pulses.
- 17. An ignition circuit according to claim 1 wherein the means for switching each of the switching devices comprises pulse shaping circuits which produce a single output pulse in response to an external timing signal.
- 18. An ignition circuit according to claim 1 further comprising at least one further pair of first and second switching devices and an igniter associated with each pair of switching devices and wherein the means for switching each of the switching devices is adapted to control all of the switching devices.
- 19. An ignition circuit according to claim 18 wherein the means for switching each of the switching devices comprises one trigger circuit.
- 20. An ignition circuit according to claim 19 wherein the trigger circuit is a scanning trigger source with an individual output for each pair of switching devices.
- 21. An ignition circuit according to claim 2 wherein the storage means comprises a double storage means and provides a reduced voltage point which limits the voltage applied across each switching device.
- 22. An ignition circuit according to claim 13 which further comprises means to provide sufficient current to the solid state switches such that they remain on for a desired duration.
Priority Claims (1)
Number |
Date |
Country |
Kind |
0203582 |
Feb 2002 |
GB |
|
US Referenced Citations (7)