GB2327984A

GB2327984A - A capacitor discharge ignition circuit

Info

Publication number: GB2327984A
Application number: GB9814673A
Authority: GB
Inventors: Richard Arthur George Kinge
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1997-08-01
Filing date: 1998-07-08
Publication date: 1999-02-10
Anticipated expiration: 2018-07-08
Also published as: GB9814673D0; GB2327984B

Abstract

An ignition system has a first capacitor 1' connected by a switch 7' to primary windings 5' of a transformer 6'. Secondary windings 8' of the transformer 6' are connected via a rectifier circuit 21 to a second capacitor 20. An igniter 11' is connected to the second capacitor 20 by a cable 12' that also has its own capacitance. In use, the combined charge on the second capacitor 20 and the cable 12' is gradually increased each time that the switch 7' is closed and charge on the first capacitor 1' is transferred to the transformer 6'. The charge on the second capacitor 20 and cable 12' keeps increasing until it is sufficient to fire the igniter 11'.

Description

IGNITION SYSTEMS This invention relates to ignition systems Conventional ignition systems employ a capacitor charged from a voltage source.

When the charge on the capacitor has reached the necessary level, a switch is closed and the charge is applied to the primary windings of a transformer. The transformer acts to step up the voltage, the secondary windings being connected to a cable extending to an igniter mounted in a burner or the like. The igniter is often located some distance from the ignition system and is connected to it by a high voltage coaxial cable. Long cables of this kind act as transmission lines and seriously attenuate the fast pulses sent to the igniter electrodes. Furthermore, the charging of the self-capacitance of the cable absorbs much of the available energy and may result in insufficient energy at the igniter electrodes to produce reliable ignition.

It is an object of the present invention to provide an improved ignition system.

According to the present invention there is provided an ignition system including a first capacitor, means for charging the first capacitor, means for applying the voltage on the first capacitor to primary windings of a transformer, and means for charging a second capacitor from the voltage on the secondary windings of the transformer, the system being arranged such that the voltage on the second capacitor increases progressively each time the charge on the first capacitor is applied to the transformer until the charge on the second capacitor is sufficient to cause discharge at the igniter.

The means for applying the voltage on the first capacitor to the primary windings preferably includes a switch. The system preferably includes a cable having capacitance extending between the second capacitor and the igniter, the charge on the secondary winding being supplied to charge both the second capacitor and the capacitance of the cable. The ignition system preferably includes a rectifier circuit between the secondary windings and the second capacitor.

A conventional ignition system and one according to the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of the conventional system; and Figure 2 is a circuit diagram of the system of the present invention.

With reference first to Figure 1, the conventional system has a storage capacitor 1 connected across a voltage source 2 via a resistor 3 and a diode 4. Opposite plates of the capacitor 1 are connected across opposite ends of a primary winding 5 of a transformer 6 via a series-connected switch 7, which may be a mechanical or solid state switch, such as a thyristor. The secondary winding 8 of the transformer 6 is connected across the electrodes 10 of an igniter 11 via a coaxial cable 12. Figure 1 shows the electrical equivalent circuit of the cable 12, which comprises three series-connected inductors 13 in both conductors 14, and three capacitors 15 connected in parallel between the two conductors at junctions between the inductors.

With reference now to Figure 2, there is shown an ignition system according to the present invention. Components in the circuit of Figure 2 equivalent to those in Figure 1 are given the same reference numeral with the addition of a prime '. That part of the system on the input/primary side of the transformer 6' is identical to that of Figure 1, except that the capacitor 1' is generally smaller than that in the conventional system, so this part will not be described again here. The part of the system on the output/secondary side of the transformer 6' differs from the equivalent part in Figure 1 in that a secondary capacitor 20 is connected across the secondary winding 8' via a rectifier circuit 21 and at the input end of the cable 12'.

The rectifier circuit 21 is a half-wave device but could include a full-wave circuit so that it acts bidirectionally to recover more efficiently energy that may be lost in the "fly back" or ringing of the transformer 6'. The secondary capacitor 20 is connected across the two conductors 14' at the input of the cable 12'.

This circuit does not discharge sparks at the igniter 111 each time that the switch 7' is closed, but only after several cycles of charging and discharging the primary capacitor 1'.

When the switch 7' is closed, energy is transferred to the secondary circuit of the transformer 6', as before, but the energy is applied across the secondary capacitor 20. The rectifier circuit 21 prevents the capacitor 20 discharging through the secondary windings 8' when the voltage across the windings drops, so the charge in the capacitor is built up each time the switch 7' is closed. As the voltage builds up on the capacitor 20 it also builds up on the distributed capacitance 15' in the cable 12', which effectively forms a part of the secondary capacitor.

Fast voltage pulses no longer travel down the cable 12', so the available voltage is not attenuated. Each time that the switch 7' is closed, the voltage on the capacitance 20 and 15' of the secondary circuit will increase progressively. When this voltage exceeds the breakdown voltage of the igniter 11', the charge on the capacitances 20 and 15 is discharged across the igniter electrodes 10' to ignite the surrounding fuel/air mixture.

The system of the present invention gives a very reliable discharge of sparks at the end of a highly capacitive cable. It can be seen that the system progressively increases voltage until discharge occurs, in contrast with previous systems where the voltage applied is of a set value and may be insufficient to cause ignition in some circumstances. The present invention is, therefore, particularly useful for igniting fuel mixtures with a high dielectric strength, which are reluctant to ionize.

The circuit can be varied in various ways. For example, the primary circuit may be of various different kinds. Also, the secondary capacitor need not be located at the input end of the cable but could be located at some point along the cable, or at the igniter electrodes themselves.

Claims

1. An ignition system including a first capacitor, means for charging the first capacitor, means for applying the voltage on the first capacitor to primary windings of a transformer, and means for charging a second capacitor from the voltage on the secondary windings of the transformer, wherein the system is arranged such that the voltage on the second capacitor increases progressively each time the charge on the first capacitor is applied to the transformer until the charge on the second capacitor is sufficient to cause discharge at the ignite.

2. An ignition system according to Claim 1, wherein the means for applying the voltage on the first capacitor to the primary windings includes a switch.

3. An ignition system according to Claim 1 or 2, wherein said system includes a cable having capacitance extending between said second capacitor and said igniter, and wherein the charge on said secondary winding is supplied to charge both said second capacitor and the capacitance of the cable.

4. An ignition system according to any one of the preceding claims, including a rectifier circuit between said secondary windings and said second capacitor.

5. An ignition system substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

6. Any novel and inventive feature as hereinbefore described.