GB2147049A - Spark plug for internal combustion engines - Google Patents

Abstract

The spark plug is a structural combination of an ignition spark gap extending into the combustion chamber of the internal combustion engine, a capacitor (14) dischargeable through the ignition spark gap (13), via an auxiliary spark gap (12), an electrical insulator (15), and a metal housing (16). The capacitor is a foil capacitor (14) and is located in a cavity formed between upper and lower portions (18, 17) of the insulator (15). A relatively high capacitance value is thereby obtained and enables the production of a powerful ignition spark. <IMAGE>

Description

SPECIFICATION Spark plug for an internal combustion engine The invention relates to a spark plug for an internal combustion engine and in particular a spark plug which comprises an ignition spark gap, a capacitor dischargeable by way of the spark gap, an electrical insulator and a metal housing which holds the individual parts together, the spark plug being in the form of a structural unit.

A spark plug of this kind is already known (from, for example, German Offenlegungsschrift No. 2,363,804 or from German Offenlegungsschrift no. 2,810,159). When in use, this known spark plug is distinguished particularly by the fact that even lean fuel-air mixtures are ignited with adequate certainty, thereby resulting in a good yield of energy from the fuel. Also complete combustion of the fuel-air mixture takes place, hence resulting in favourable exhaust gas values, and also the ignition voltage pulses from conventional ignition systems can be used for ignition. This results from the fact that a particularly large amount of energy is converted in the initial phase of the breakdown phase of the ignition spark.It has now been found that it is difficult to realise the capacitor in practice, since the required capacitance value is relatively high and must also be sufficiently accurate.

It is an object of the present invention to overcome the above difficulties to a satisfactory extent.

In accordance with the present invention there is provided a spark plug for an internal combustion engine, which spark plug is formed by structurally uniting an ignition spark gap extending into the combustion chamber of the internal combustion engine, an ignition capacitor dischargeable by way of the ingition spark gap, an electrical insulator and a metal housing which holds the individual parts together, wherein the ignition capacitor is a foil capacitor and is located in a cavity in the insulator.

Preferably, the capacitor is a wound hollow cylinder whose ends are used to establish its electrical connections.

Advantageously, the insulator comprises a top part and a bottom part which overlap one another coaxially by means of sleeve-shaped portions. Such that a gap of annular crosssection remains for accommodating the capacitor.

The invention will now be described further hereinafter, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of an ignition system having a spark plug constructed in accordance with the invention; and Figure 2 is a sectional view of the construction of such a spark plug.

The ignition system illustrated in Fig. 1 is intended for an internal combustion engine (not illustrated) of a motor vehicle (also not illustrated). This ignition system is fed from a source of direct current 1 which may be the battery of the motor vehicle. A lead 2 is connected between the negative pole- of the current source 1 and earth, and a supply 4 lead incorporating an operating switch (ignition switch 3) is connected to the positive pole of the current source 1. The supply lead 4 is the starting point for a circuit branch which leads first by way of the primary winding 5 of an ignition coil 6 and then to the earth lead 2 by way of a circuit breaker 7.

The circuit breaker 7 has a spark-suppressor capacitor 8 connected in parallel therewith and can be opened, that is to say, it can be brought into its non-conductive state, at the instant of ignition by a cam 9 driven by the internal combustion engine. One end of the secondary winding 10 of the ignition coil 6 is connected to the lead between the primary winding 5 and the circuit breaker 7, and the other end of the said secondary winding is connected to a high-voltage terminal 11 of a spark plug.

It will be appreciated that a transistorized ignition system may be used instead of the ignition system described hitherto.

The high-voltage terminal 11 is connected to earth firstly by way of a series spark gap 1 2 and then by way of an ignition spark gap 13. An ignition capacitor 14 is connected in parallel with the series combination comprising the series spark gap 1 2 and the ignition spark gap 1 3.

The high-voltage terminal 11, the series spark gap 12, the ignition spark gap 1 3 and the ignition capacitor 14 are combined to form a structural unit which is screwable as a spark plug into the cylinder block of the internal combustion engine in a conventional manner. An insulator 1 5 and a metal housing 1 6 holding the individual parts together are also essential parts of the structural unit. The essentially rod-shaped insulator 1 5 is of circular cross section and comprises a bottom part 17 and a top part 18. The two parts 17, 18 overlap one another coaxially by means of sleeve-shaped portions 1 9, 20 over a length a, such that a space of annular cross section remains for accommodating the capacitor 1 4.

The capacitor 1 4 is a foil capacitor and advantageously constitutes a wound hollow cylinder and whose end faces are used to establish its electrical connections.

The high-voltage terminal 11 is in the form of a pin 21 which is rigidly mounted in the top part 1 8 and which has a screw-threaded portion 21' extending out of the part 18. The first portion 22 of a central electrode 23 within the top part 1 8 is contiguous to the high-voltage terminal 11. One end of the first portion 22 of the central electrode 23 is connected to the high-voltage terminal 11, and its other end is connected to one electrode 24 forming part of the series spark gap 1 2. The other electrode 25 of the series spark gap 12 is connected to the end of a second portion 26 of the central electrode 23, and the other end of the portion 26 projects from that end of the bottom part 1 7 which is remote from the high-voltage terminal 11.

This projecting end and a horn-shaped counter-electrode 27 form the ignition spark gap 1 3. The horn-shaped counter-electrode 27 projects from that end face of the sleeveshaped metal housing 1 6 which is remote from the high-voltage terminal 11. The metal housing 16 surrounds the bottom part 1 7 and the bottom of the top part 18, such that the individual parts of the spark plug are rigidly held together.

One end of the capacitor 14 is electrically connected to the electrode 24 of the series spark gap 12, and its other end is electrically connected to the metal housing 1 6 by means of a contact 28 indicated by bold line.

The threaded pin 20 forming the highvoltage terminal 11, the first portion 22 of the central electrode 23, the cylindrical electrodes 24, 25 of the series spark gap whose mutually facing ends are rounded, and the second portion 26 of the central electrode 23 are disposed in the insulator 1 5 such that their longitudinal central axes coincide with the longitudinal central axis of the insulator 15.

The distance between the electrodes 24, 25 of the series spark gap 1 2 is determined by a sleeve 29.

The discharge circuit comprising the elements 13 to 14 must be designed from the viewpoint of the maximum frequency, that is to say, that it must have low inductance and also a low ohmic resistance.

When using the spark plug in accordance with the invention, the mode of operation of the ignition system just described is as follows: The ignition system is capable of operation when the operating switch 3 has been closed.

By opening the circuit breaker 7 or, in the case of a transistorized ignition system, by actuating the trigger means, the current flowing through the primary winding 5 is interrupted and a high-voltage pulse is induced in the secondary winding 10 and is applied to the high-voltage terminal 11. The capacitor 14 is thereby charged up to the electrical breakdown of the series spark gap 12. This breakdown produces a powerful ignition spark at the ignition spark gap 1 3 within a few nanoseconds. This results in reliable ignition of even a lean fuel-air mixture, a good yield of energy from the fuel, complete combustion of the mixture, and hence reasonable values of the exhaust gas emitted.

By virtue of the fact that a foil capacitor is used as the capacitor 14, a relatively high value of the capacitance can be chosen and can be established with sufficient accuracy.

The structural arrangement of the capacitor 14 enables the spark plug to be assembled without assembly being impeded by the insertion of the capacitor 14.

Claims (3)

1. A spark plug for an internal combustion engine, which spark plug is formed by structurally uniting an ignition spark gap extending into the combustion chamber of the internal combustion engine, an ignition capacitor dischargeable by way of the ignition spark gap, an electrical insulator and a metal housing which holds the individual parts together, wherein the ignition capacitor is a foil capacitor and is located in a cavity in the insulator.

2. A spark plug as claimed in claim 1 where the metal housing also contains a spark gap in series with the ignition spark gap.

3. A spark plug constructed and adapted to operate substantially as herein described, with reference to and as illustrated in the accompanying drawings.

3. A spark plug as claimed in claim 1 or 2 wherein the ignition capacitor is a wound hollow cylinder whose end faces are used to establish its electrical connections.

4. A spark plug as claimed in claim 1, 2 or 3, wherein the insulator comprises a top part and a bottom part and these two parts overlap one another coaxially by means of sleeve-shaped portions such that a gap of annular cross section remains in which the ignition capacitor is accommodated.