Spark plugs are conventionally furnished in various heat ranges, the heat range being a measure of the thermal capacity of the spark plug. Because different engines develop different amounts of heat in their combustion chambers during their various phases of operation, they are fitted with spark plugs whose heat ranges are most appropriate for the normal operating conditions of the engine. In operation, the electrodes and insulator of the spark plug adjust to a mean temperature corresponding to the engine load. The insulator nose should be at an operating temperature of between 450.degree. C. (842.degree. F.) and 850.degree. C. (1562.degree. F.) for proper operation. Temperatures above 450.degree. C., the self-cleaning temperature are desirable because at those temperatures the deposits on the insulator in the form of soot, carbon residue, and other compounds are burnt off or undergo chemical change. At idle or part load such as during city driving, at intersections, or on downgrades or the like, the insulator nose temperature often drops below 150.degree. C. (302.degree. F.). If the amount of engine lubricating oil entering the combustion chamber due to worn piston rings, intake valves, etc.; if the fuel-air mixture is not too rich; and if this low temperature is not maintained for too long a time; the spark plug will usually still function properly. A temperature of 850.degree. C. at the insulator should not exceeded, because above 900.degree.C. (1652.degree. F.) auto-ignition can occur, and at very high temperatures the electrodes are also subjected to increased attack or destruction by sulphur and other aggressive compounds resulting from the combustion process.
Conventional spark plug characteristics can thus be defined as follows:
The lower the heat range index of a spark plug, the higher its resistance to auto-ignition, but the lower its resistance to fouling.
The higher the heat range index of the spark plug, the lower its resistance to auto-ignition, but the higher its resistance to fouling.
Because of the wide range of operating conditions an engine can encounter, the selection of the proper heat range for a spark plug for a specific engine necessarily is a compromise. Thus, there is always the possibility that the engine will be operated under conditions that are wrong for the heat range of the spark plug installed therein. This possibility can lead to poor performance and engine damage with consequent owner dissatisfaction. To overcome these and other serious disadvantages of prior art spark plugs, it is thus a principal object of this invention to provide a multiple heat range spark plug whose operating temperature is varied automatically such that it operates at an optimum temperature during all phases of engine operation. It is a concurrent object of this invention to provide a heat pipe in the spark plug to maintain it automatically at an optimum operating temperature, with the centerbore of the spark plug insulator itself forming the walls and incorporating the capillary means of the heat pipe such that its most effective heat transfer characteristics are obtained. A further object is to provide a spark plug whose insulator nose operates at a self-cleaning temperature at all times to thereby reduce the danger of combustion misses and misfiring which cause sharp increases in the emission of hydrocarbons, to thus yield advantages with respect to exhaust-gas values in low load ranges while at the same time improving fuel economy.