Spark plug insulator structure

Details Spark plug insulator structure Spark plug insulator structure

1990-09-28

1992-07-07

Yusko, Donald J.

Electric lamp and discharge devices

Spark plugs

Plural part insulating means

313130, 313139, 313143, 313141, 313142, H01T 1316, H01T 1336

Patent

active

051285835

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to spark plugs for spark ignited internal combustion engines.
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 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 they have occurred, the present invention seeks to remove a major cause of these variations.


SUMMARY OF THE INVENTION

According to the present invention, there is provided a spark plug having a metal body, a center electrode and an insulator surrounding the center electrode, the end of the insulator nearer the spark gap being spaced by an annular gap from the metal body of the spark plug, characterised in that the annular gap is filled with an insulating material of low thermal capacity to prevent combustion gases from entering between the insulator and the body of the spark plug.
In a spark plug, it is important that the center electrode run at a high temperature, to prevent its fouling. It is also important to ensure that the surface path from the center electrode to the body of the spark plug should be as long as possible in order to prevent arcing or tracking along the surface cf the insulator, which would prevent the spark from flying across the spark gap. For these reasons, it is usual for the center 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 center electrode by the body of the plug to permit the plug to run at the required elevated temperature.
However, it is believed 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. It is known that the flow conditions and the mixture strength in the immediate vicinity of the spark gap are responsible to a great extent for the initiation of the flame and this in turns affects the speed of combustion and flame propagation in the burn cycle.
Various previously unexplained experimental facts suggest that the presence of an air gap in the spark plug is a cause of cycle to cycle variations. For example, it has been found that injecting air into the spark plug air gap reduces such variations. Also, measured values of mixture strength at the spark plug gap have been found to remain substantially constant despite significant changes in overall mixture strength. A still further fact in support of the conclusion that cycle to cycle variations are caused by the spark plug air gap is that high swirl combustion chambers, having better scavenging, are less prone to such variations.
The invention recognises this cause of variations between burn cycles and alleviates the problem by preventing gases trapped in the spark plug from participating in the combustion process.
There have been previous proposals to place an insert in the gap between the insulator surrounding central electrode and the body of the spark plug or to restrict the flow into the gap surrounding the insulator. However, none of the prior art arrangements reduces cycle to cycle variations and instead they are mostly concerned with the temperature characteristics of the spark plug or with the prevention of fouling.
U.S. Pat. Nos. 3,612,931 and 3,743,877 describe spark plugs incorporating a heat shunt to give the effect of a hot plug at lower operating temperatures. The heat shunt is a thermally conductive ring bonded on the insulator nose.
In GB-2 146 071B,

REFERENCES:
patent: 4289990 (1987-09-01), Mayumi et al.
patent: 4870319 (1989-09-01), Benedikt et al.

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