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).
The stagnation in spark plug design and the problems with various insulating materials.
| Identifier | ExFiles\Box 33\3\ Scan159 | |
| Date | 13th September 1921 guessed | |
| SPARKING Plug design has for many years been stagnant. The explanation of this is undoubtedly to be found in the limited properties of materials from which insulators have been produced; none has been capable of withstanding the high temperatures necessarily encountered. * * * Compromise has been the watchword throughout, and many schemes have been evolved to slightly better the results gained, as more and more has been demanded from the ignition apparatus to meet the call for higher, and still higher, efficiency motors. * * * The plugs of to-day differ but slightly from the original plug used in the first electrically ignited motor engines. * * * Porcelain has given way to steatite, which is the material more generally employed to-day, but trouble has been experienced in securing gas-tightness, in overcoming which, difficulties have arisen from cracks, with consequent breakdown of insulation, caused by crushing, and the high temperatures at which modern engines work. * * * This is due in some cases to the difference of the expansion ratios of the steatite insulator and the metal parts of the plug, and in others, to the screwing down operation of the plug gland. * * * Another insulating material in common use is mica, the change in the qualities of which, under heat, and the necessity to bend it and build it up in many pieces renders it unreliable as an insulator. If it gets too hot it chars to a chalky-white, opaque condition, becoming absorbent to moisture and oil. Once the oil has saturated into the surface it carbonises and breaks down the insulation to a great degree, weakening the spark sufficiently to bring about that very irritating trouble of intermittent misfiring. * * * Pre-ignition is again another bugbear of motor users, and frequently plug heat causes that pinking on hills which has so often been unfairly blamed to carbon in the combustion chamber. * * * There has never been any serious attempt made to produce a plug of which every part is easily replaceable. Pretensions to the provision of this advantage have been made, but experience has proved the fallacy of them. * * * Many plugs have been placed on the market with spark gaps without and within the plug, but a visible outside spark does not prove that the plug is firing the charge or even that the inside spark is jumping the proper gap. The essential point to be indicated is, whether the explosive mixture is being fired all the time; nothing else matters. | ||