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).
Comparison of spark plug performance, contrasting cemented K.L.G./Lodge plugs with A.C. plugs using a silver centre electrode.
| Identifier | ExFiles\Box 27\3\ Scan256 | |
| Date | 10th October 1939 guessed | |
| - 2 - of plugs which had completed from 50 to 100 hours running. These plugs were carefully opened and the insulators were split so that a view of the condition of the central electrode pin could be obtained (see photograph). It will be noted on examination of the photograph that the centre pins of the K.L.G. and Lodge plugs show considerable signs of oxidisation for at least half their length. The cement which is used for retaining thepins in position is also blackened and crumbled due to the influx of combustion gases through the cement. It is obvious therefore that the heat conductivity of the centre pin has been greatly impaired due to the poor thermal contact between the pin, via the cement, to the insulator. These pins therefore due to their inability to conduct away the heat from the tip of the plug are liable to overheat and cause violent preignition (see heat flow paths Fig.1.) If we now examine the condition of the A.C. plug we see that quite a different state of conditions exists. It will be noted that the silver central electrode is still untarnished and that excellent thermal contact is maintained between the insulator and the electrode. This is largely due to the fact that no cement is used to seal this electrode it being cast into the insulator i.e. molten silver is poured into the plug and moulds itself in close contact with the insulation, furthermore a gas tight seal is obtained at a short distance from the plug nose, whereas in the case of the Lodge and K.L.G. the gas seal is made at the top of the insulator i.e. underneath the top terminal of the plug (See Fig.2.) It is obvious that the advantages to be gained from the use of the silver centre electrode are many, particularly when it is realised that a cool running plug may be evolved with an ample length of insulator nose to overcome oiling troubles at low power conditions. It seems probable also, that this type of plug would promote the transference of heat back from the engine to the plug under idling or gliding conditions thereby reducing the possibility of oiled plugs, and at the same time under high duty conditions the silver electrode will hasten the flow of heat away from the nose of the plug towards the cooler plug wall and seatings. | ||