GB2219041A

GB2219041A - Spark plug

Info

Publication number: GB2219041A
Application number: GB8812772A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1988-05-28
Filing date: 1988-05-28
Publication date: 1989-11-29
Also published as: DE68914453D1; DE68914453T2; EP0418284A1; GB8812772D0; WO1989011745A1; US5128583A; EP0418284B1

Description

i SPARK PLUG 221904 1 The present invention relates to spark plugs for

spark ignited internal combustion engines.

11 Considerable variations in combustion are noticed from one cycle of operation of an internal combustion engine to another even when there is no irregularity in the breathing or fuelling of the engine. Some cycles have rapid flame propagation while others have slower flame propagation making it difficult to achieve precise ignition timing in all cycles.

This effect has previously been noted and attempts have 15 been made to predict fast and slow burn cycles in order to adjust the ignition timing accordingly but these ignition systems have not proved simple to implement.

Rather than combat the cycle to cycle variations after 20 they have occurred, the present invention seeks to remove a major cause of these variations.

According to the present invention, there is provided-a spark plug having a metal body, a centre electrode and an insulator surrounding the centre electrode, the part of the insulator disposed within the combustion chamber being separated by a gap f rom the metal body of the spark plug, wherein means are provided for preventing or restricting gas movement in the gap between the insulator surrounding the centre electrode of the spark plug and the metal body of the spark plug.

In a spark plug, it is important that the centre electrode should run at a high temperature, to prevent its fouling. It is also important to ensure that the surface path from the centre electrode to the body of the spark plug should be as long as possible in order to prevent arcing or tracking along the surface of the 1 1 insulator, which would prevent the spark from flying across the spark gap. For these reasons, it is usual for the centre electrode to be surrounded by an insulator which is itself spaced from the body of the plug. This configuration prolongs the surface of the insulator to prevent tracking and also creates a thermal barrier preventing the cooling of the insulator surrounding the centre electrode by the body of the plug to permit the plug to run at the required elevated temperature.

However, it has been found that the poor scavenging of gases from this small gap, which forms part of the combustion chamber, results in an erratic flow in the vicinity of the spark gap, and it is known that the flow conditions and the mixture strength in this immediate vicinity of the spark gap are responsible to a great extent for the initiation of the flame which in turns affects the speed of combustion and flame propagation in the burn cycle. The present invention is predicated on the appreciation of this cause of variation between burn cycles and seeks to alleviate the problem by restricting the flow of gases into and out of this gap.

In the preferred embodiment of the invention, the gap between the insulator and the body of the spark plug is filled with a cellular filler of poor thermal conductivity capable of withstanding the combustion temperatures and pressures yet able to accommodate thermal expansion. A paint formed of micro-balloons of ceramic material is known which has extremely poor thermal conductivity. A filler formed of such microballoons suspended in a binder can ensure that the centre electrode is not cooled by heat flow through the f iller.

1 7_ 1 i The invention will now be described further, by way of example, with reference to the accompanying drawing which is a partial section through a spark plug of the invention.

The spark plug in the drawing comprises a metal body 18, and a centre electrode 16 spaced from the body 18 by an insulator 14. The earth electrode which defines the spark gap lies in a plane different from that of the drawing and is not shown. - In a conventional spark plug, the gap between the insulator 14 and the metal body 18 is empty. This, as earlier described, was done in the prior art to maintain a desired temperature of the insulator surface. to prevent fouling and to ensure that tracking does not occur over the surface of the insulator 14.

It has now been found that the gap within the spark plug forms a part of the combustion chamber which is poorly scavenged. Burnt gases remain in this gap at the end of the combustion cycle and can interfere with the initiation of combustion in the next cycle. In the present invention, it is required to prevent gases from entering the gap between the insulator 14 and the body 18 of the spark plug, without at the same time cooling the centre electrode or providing a short circuit path over which tracking can occur.

To meet these objectives, the gap between the insulator 14 and the body 18 is filled with a filler 12 having very poor thermal conductivity. The insulator 14 is still allowed to heat up as little heat flows across the filler 12 and furthermore the surface of the filler 12, being poorly cooled, attains high temperatures (essentially the same temperature as the combustion gases) and prevents any carbon deposits which can cause tracking.

a A paint (a. spin-off from space technology) has recently become available which exhibits extremely poor thermal conductivity. A red hot sheet of steel having a coating of such a paint at one end, can safely be hand held at its painted end. This paint is believed to comprise micro-balloons of a ceramic material, the trapped air being the cause of the poor conductivity.

If such micro-balloons are mixed with a binder to form a filler, then the resulting filler should have comparable thermal conductivity and should be particularly suitable for f illing the gap between the insulator 14 and the body 18 of the spark plug.

Apart from the presence of the filler 12, the spark plug is essentially conventional and for this reason further explanation of the construction and the manufacture of the spark plug is not deemed necessary.

1 W Q1 ir 1

Claims

1. A spark plug having a metal body, a centre electrode and an insulator surrounding the centre electrode, the part of the insulator disposed within the combustion chamber being separated by a gap from the metal body of the spark plug, wherein means are provided for preventing or restricting gas movement in the gap between the insulator surrounding the centre electrode of the spark plug and the metal body of the spark plug.

2. A spark plug as claimed in claim 1, wherein the gap between the insulator and the body of the spark plug is filled with a filler capable of withstanding the combustion temperatures and pressures yet able. to accommodate thermal expansion.

3. A spark plug as claimed in claim 2, wherein the filler is formed of a binder, and a suspension of micro- balloons of ceramic material.

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

Published 1989 atThe Patent Office, State House, 66/71 High Holborn, London WCIR 4TP. Further copies maybe obtainedfrOM The Patent Unice. Wes Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1/87