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).
Article discussing spark plug problems, their relation to heat characteristics, and diagnosing engine conditions.
| Identifier | ExFiles\Box 58\2\ Scan015 | |
| Date | 16th March 1929 | |
| Spark Plug Problems The Relation of Heat Characteristics to Operating Conditions By HECTOR RABEZZANA Chief Spark Plug Development Engineer, A.{Mr Adams} C. Spark Plug Co. Fig. 1 (Diagram labels: BURETTE, PORCELAIN ROD, OIL, THERMO COUPLES, CARBON, FURNACE) Fig. 2 (Illustration of an AC spark plug) WHEN a spark plug performs unsatisfactorily it is often due to use of a plug intended for another type engine, or a different operating conditions. No matter how good a spark plug may be, it will not give satisfactory service if it is of the wrong type. Generally the public fails to realize that there is a lot of difference between spark plugs—not only in the quality of the material and workmanship used in their manufacture, but also in the heat characteristics of the plugs themselves. Spark plugs face three problems, pre-ignition, rapid wearing away of the electrodes and fouling. Pre-ignition is the result of the plugs becoming red hot and firing a mixture too early, causing pinging and a sharp decrease in the engine’s power. The fast wearing away of electrodes results from the plugs operating at too high a temperature. The fouling of a plug is due to two causes —liquid fuel and oil reaching the gap and combustion deposition over the insulator. Fouling trouble, due to oil or liquid fuel, very rarely occurs in present-day engines, and when it does appear it is generally referred to as “an engine pumping oil.” Not only will there be trouble under such a condition, but also valve trouble and excessive carbon deposition. Even clean oil will cause trouble through softening the carbon on a plug that may be slightly carbon coated. Also a new plug can be short-circuited by oil splash in which floats a small amount of carbon and metallic dust. New oil before it has been placed in the crank-case and become impregnated with carbon is a non-conductor of electricity, but as soon as it becomes filled with carbon, it is then a conductor of electricity. The black coat of so-called carbon which forms over the insulator of a plug is the result of too low operating temperature for that plug. When the engine reaches a higher temperature, the insulator also will raise its temperature and automatically burn away the carbon which has piled up during the cold running period. But, on the other hand, if the temperature reaches too high a point, it will cause the plug to pre-ignite, or cause detonation. These limits of temperature are all worked out by the physical dimensions of the part of the insulator which is exposed to the combustion gases and the overall design of the plug. If the insulators of the plugs become a brownish color, this coat usually has an iron-oxide base, and when this coat darkens in color, because it becomes thicker, it is advisable to change plugs, as they will cause missing at wide open throttle. If a spark plug develops leakage between the shell and the porcelain, the insulator cracks, or the electrode wires wear out too fast, this is a sure sign that the plug used becomes too hot in that engine. A plug made to run cooler has to be used, as will be explained in the second part of this article. It is almost impossible to get one type of plug to meet the requirements of different engines, or the different conditions under which the same engine may be used. The spark plug, aside from its service as an igniter, is the only part of the engine that, with visual inspection, can give an exact diagnosis of the condition of the engine. Let’s analyze a few of them. If a new set of plugs is installed in an engine, and after a few hours of running, without any excessive idling at the end of the run, the plugs are removed and examined, and show a black coat, this will indicate that the type of plug used is too cold for that engine. These should be changed for ones with long insulation, which will allow a lower temperature working range so the carbon will burn away, thus preventing the plug from becoming fouled. If the plug shows a very white insulator, after a few hours of running, this type of plug is too hot, and it may cause preignition. In this case a plug with a shorter insulation should be used. In setting the carburetion, use a spark plug that is known to work right, and if the insulator stays white after a few hours of running, it means that the carburetion is too lean and back-firing in the carburetor may happen very easily. If the plug reaches a brown color with black spots after a few minutes’ run, that means that the carburetion is too rich and it will have a tendency to foul the plug quickly. If the porcelain becomes a dark brown color in a very short time and everything else in the engine appears to be all right, including carburetion, most likely the fuel used should be discarded if possible, because not only the plugs will be short-lived, but this fuel will also cause the engine to deteriorate. In altitude flying or on every long hops, the spark plug gaps have to be reduced in size to prevent flash-over on the outside of the plugs. If the wearing of the electrodes is not due to excessive engine temperature, or to long use of plugs, it is always traceable to poor fuel. Fuel with a high percentage of sulphur should not be used, because it may cause the electrodes and valves to become corroded so quickly that it may put an end to a flight in an unreasonably short time, due to the failure of either the electrode wire or the valves. Lubricating oil also has to be watched in order to avoid the same trouble because the sulphur content may produce sulphuric acid when combined with water, either as a result of condensation or combustion residue. In this case the inside of the engine, especially the bearings, will suffer considerably. When we once learn how to judge engine conditions from analysis of the plugs, it is easy to avoid trouble by choosing the right plug for a given job. Reprinted from AVIATION, March 16, 1929 | ||