The invention relates, in general, to spark plugs for use within a main combustion chamber of an internal combustion engine. More particularly, the invention pertains to spark plugs having enhanced capabilities to transfer heat away from the firing tip of the spark plug into the body of the plug. This additional cooling of the firing tip enables the plug designer to use performance-improving features without the compromises inherent in currently used spark plug designs.
A spark plug is a device, inserted into the combustion chamber of an engine, containing a side electrode and an insulated center electrode spaced to provide a gap for firing an electrical spark to ignite air-fuel mixtures. The high-voltage burst from the coil via the distributor is received at the terminal of the spark plug and conducted down a center electrode protected by an insulator. At the bottom of the plug, which projects into the cylinder, the voltage must be powerful enough to jump a gap between the center and side electrodes through a thick atmosphere composed of the air-fuel mixture. When the spark bridges the gap, it ignites the fuel within the cylinder.
Typical spark plug designs, as illustrated in FIG. 1, employ a metallic center electrode that is slidingly fitted into a ceramic insulator to form the structure of the plug. This electrode is sealed in place with a glass seal/resistor material. The firing tip of the plug is heated by the combustion that occurs within the combustion chamber of the engine. The firing tip must be cooled below the ignition temperature of the air-fuel mixture. This is accomplished by heat being conducted through the insulator structure and the center electrode into the plug shell and eventually into the engine cylinder head and the coolant circulating within it. The insulator material is not as effective as a thermal conductor, as compared to the metallic center electrode. As a result, the center electrode carries away more heat than the insulator and the insulator is cooled by the electrode. Copper-cored center electrodes are typically used to promote this effect. The copper material, however, is somewhat expensive. A clearance is provided between the center electrode and the insulator. This simplifies assembly of the electrode into the insulator and compensates for expansion of the electrode during heating, as such expansion, if not compensated for, could result in cracking of the insulator. As a result of this clearance, the transfer of heat from the insulator is impeded.
Ideally, spark plug manufacturers would prefer to use a less thermally conductive material for the center electrode than copper-cored nickel. They would also prefer to use a center electrode whose diameter is physically smaller than that shown in FIG. 1. A smaller electrode would reduce the cost of the spark plug, as the cost of copper-cored nickel is relatively high. It also would be desirable to produce an electrode tip that has a small diameter so as to achieve a spark with lower applied voltages. Unfortunately, spark plug designers are hampered by the inability of these smaller diameter tips to pull heat away from the electrode tip in a quick enough manner. Additionally, an electrode having a thin diameter and/or one formed from cheaper materials would have insufficient heat transfer capabilities.
Another consideration in spark plug design is the fouling path length and how this length may be optimized with respect to the temperature of the tip. The fouling path area of the spark plug is the area of the insulator around the firing tip that gets covered with carbon and moisture. Excessive build-up of carbon and moisture prevents a spark from occurring, resulting in misfiring of the spark plug. A long fouling path length between the spark plug firing gap and the metal shell of the plug is desirable. Misfiring of the plug due to carbon deposits is less likely to occur with a longer fouling path because the carbon concentration becomes diluted due to the added length. Unfortunately, the length of the fouling path is limited by the ability to keep the firing tip of the insulator cool. This problem has been aggravated by the fact that some of the newer types of high compression or lean burn type of engines require more energy from the spark plug to cause ignition. As a result of this, the amount of carbon deposits that can be tolerated without causing misfiring has decreased. Care must therefore be taken to carefully balance the fouling path length of the plug with the resultant tip temperature in order to achieve optimum performance of the plug.
U.S. Pat. No. 5,877,584 to Kato et al. discusses the importance of maintaining a particular firing tip temperature while sufficiently reducing carbon-related deposits through optimization of a plurality of parameters. These parameters include the fouling path length or gap length (H) between the electrode tip and the insulator, the tip diameter (D1), and the width of the space (L) between the outer surface of the center electrode and the inner wall of the insulator.
Additionally, some of the newer types of engines require a spark plug that has a cooler tip surface. Due to these requirements, there are currently approximately six different spark plug designs that allow for different heat ranges for the plug. If the tip surface of a plug is too hot for a particular engine, then the air-fuel mixture can self-ignite. This would adversely affect control of engine timing, and could lead to failure of the spark plug and/or the engine. The cooler tip temperature of these spark plug designs is achieved by sacrificing the length of the fouling path. As a result, deleterious build-up of carbon deposits on the firing tips of these spark plugs is far too common.
It is therefore an objective of the invention to produce a spark plug having enhanced heat transfer capabilities.
Another objective is to produce a spark plug having enhanced heat transfer capabilities that allow for improvement of other spark plug performance characteristics.
A further objective is to produce a spark plug having a wire of small diameter as the center electrode so as to achieve a variety of advantages. These advantages include a cost savings in the reduction of materials required to form the electrode, the ability to produce a spark at lower voltages, and the ability to ignite leaner or more dilute air-fuel mixtures.
Yet another objective is to produce a spark plug having a thermally conductive filler material that provides intimate thermal contact between the center electrode and the body of the ceramic insulator.
An additional objective is to produce a spark plug having a thermally conductive material that is formed from a material whose coefficient of thermal expansion substantially equal to a coefficient of thermal expansion of the insulator body.
Yet another objective is to produce a spark plug that carefully balances the length of the fouling path with the temperature of the firing tip of the spark plug.
Still another objective is to produce a spark plug having a longer fouling path than those currently in production.
Still yet another objective is to produce a cold heat range spark plug having enhanced heat transfer capabilities without sacrificing the length of the fouling path.
In addition to the objectives and advantages listed above, various other objectives and advantages of the invention will become more readily apparent to persons skilled in the relevant art from a reading of the detailed description section of this document. The other objective and advantages will become particularly apparent when the detailed description is considered along with the drawing and claims presented herein.
Briefly, and in accordance with the forgoing objectives, the invention comprises a tubular insulator body, a center electrode and a thermally conductive filler material. The tubular insulator body has an inner surface, and the center electrode has an outer surface. The center electrode is positioned within the tubular insulator body such that a gap is formed between at least a portion of the inner surface of the tubular insulator body and at least a portion of the outer surface of the center electrode. The thermally conductive filler material has a high heat transfer coefficient. It is positioned within at least a portion of the gap so as to intimately contact the inner surface of the tubular insulator body and the outer surface of the center electrode. Positioned accordingly in the gap, the thermally conductive filler material enhances the ability of the spark plug to transfer heat away from the firing tip and into the body of the spark plug.