From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Detailed explanation of spark plug operation, heat range, and fouling characteristics.
Identifier | ExFiles\Box 58\2\ Scan016 | |
Date | 1st March 1931 guessed | |
Fig. 4 Of the various characteristics of the ignition spark, its appearance, that is, its fatness, color, etc., does not determine its ignition value. It is not possible to add to the power of an engine by adding condensers, retardation coils, auxiliary gaps and the like to the ignition system. This statement must be taken as referring to the spark, and not to the spark plug, for it is well known that the use of an improper type of spark plug may cause pre-ignition, and old and fouled spark plugs may cause missing and resultant loss of power. The spark plug, being exposed to the burning gases at its lower end, absorbs an amount of heat proportional to the surface exposed to the heat. Part of the heat thus absorbed must pass off through the seat of the plug, but enough should be retained to always keep the plug at a sufficiently high temperature to prevent fouling, as will be explained later. Fouling depends mainly on the temperature of the plug, as can be demonstrated by means of the apparatus shown in Fig. 1. Rods of different materials, with holes for thermo-couples in them at definite intervals, are heated in this apparatus, and oil is dropped on the side which is exposed to room temperature. The rod is inclined at such an angle that the oil will flow along it. If the furnace is maintained at 900 deg. C., the oil will begin to carbonize at a certain point along the rod, and will burn off at a point nearer the furnace, where the temperature is still higher. The points where carbonization and burning begin vary with different oils, but the differences are slight. If the rod is of a material of high heat-conductivity, the temperature gradient will be less and the carbon deposit will extend over a considerable portion of the length of the rod, within which there may be several thermo-couples. The above test shows the desirability of keeping the insulators of spark plugs at a temperature high enough to burn away the carbon, because carbon is a good conductor of electricity and when covering the end of the insulator will short-circuit the plug and cause missing. This is the reason why there is a limit to the so-called “coolness” of a plug. If a plug is so designed that it cannot hold enough heat to reach a temperature that will enable it to burn the carbon, it will give trouble from fouling. It is for this reason that a racing plug, which is designed to take care of abnormal heat, will make a very poor showing in a commercial engine, where it will cause fouling trouble. If thermo-couples are set into a plug designed to receive them (Fig. 2) in such a way that the temperature of the hottest part of the plug when the engine begins to pre-ignite can be read off, and the test is carried out with the various fuels on the market, the experiment will enable one to determine the temperatures which mark the limit of usefulness of the plug with the different fuels. Preferably the plug should always be hot enough to burn any carbon that may be formed, but not hot enough to cause pre-ignition. Unfortunately the temperature range marked by these two limits is comparatively narrow—only about 350 deg. C.—which explains why plugs may easily cause trouble. By referring to Fig. 3 it can be seen that the heat absorbed by the plug at its inner end, passes from the end of the center wire through the tip of the insulator, the body of same, the seat gasket, the spark plug shell, and the plug gasket into the cylinder wall and the cooling water. The threaded portion of the plug conducts only a negligible portion of the heat absorbed, and practically all of the heat must pass through the seat. There are numerous factors which tend to cause the temperature of the plug to vary, these including: Engine revolutions per minute; load on engine; cooling water temperature; engine temperature; kind of fuel; spark advance; air fuel ratio; air humidity; temperature of air around plug; draft around exposed part of plug. Considering the difference in engine revolutions per minute by using either cruising speed or top speed it is easy to see that the same plug cannot at the best be used in both cases. Fig. 4 shows plugs of different designs with paths of heat flow of different lengths, and illustrates how, by varying the resistance to heat flow, especially in the insulator, the working temperature of the plug can be controlled. To meet aviation requirements the AC Spark Plug Co., besides the aviation line already in the market for many years, has developed a set of plugs shown in Fig 4. With this set of plugs it is possible to take care of any aviation engine now in the market. This line of plugs is so designed that each member of it will, under given engine conditions, keep slightly cooler than the one which follows. Each plug is identified by a number and the higher the number the better the plug is adapted for use in a cool engine. | ||