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).
Report and examination on six K.L.G.850 plugs removed from a Bentley car.
| Identifier | ExFiles\Box 150\1\ scan0180 | |
| Date | 28th August 1936 | |
| ETHYL EXPORT CORPORATION. REPORT AND EXAMINATION ON SIX K.L.G.850 PLUGS EX BENTLEY CAR. Preliminary. Six K.L.G.850 plugs were sent to us by Messrs. Rolls-Royce (ref. their letter Hs{Lord Ernest Hives - Chair}/Wst.3/MA dated 7-8-36) for examination. These were removed from a Bentley engine and were stated to have caused misfiring. Unfortunately no details were given as to the past history of the plugs, such as the number of hours run or whether they had operated under bench test conditions, and no information as to the precise conditions under which the trouble was manifest was available. A brief visual examination failed to reveal anything abnormal such as fusion of deposits, etc. which might account for failure. It was difficult to pass an opinion on the amount of deposit present owing to the lack of knowledge of the duration of running to which the plugs had been subjected. It was deemed advisable, before carrying out a chemical analysis on the deposits, to make a physical examination and to test the insulation break down resistance. It was hoped, by this means, to ascertain whether one particular plug was at fault or whether the lead deposits had a deleterious effect equally on them all. Physical Examination. The first test made was to measure the voltage to produce a break down in the insulation of the body of the plug. For this purpose a plug was dismantled and a potential difference was applied to the central electrode and to the gland nut, which was progressively increased until failure of the insulation was noted. The results for the six plugs were as follows:- No. 1 12,200 volts - failure occurred inside the mica " 2 13,100 " " " " " " " " 3 13,700 " " " " " " " " 4 14,000 " " " " " " " " 5 14,600 " " " " " " " " 6 12,000 " " break down over surface, i.e. over lead deposit. One of the new plugs was also tested and broke down at 14,000 volts, the failure being inside the mica. It is believed that a break down voltage of a test of this nature, of 12,000 volts is the limit set by the Air Ministry, and all plugs will be accepted if no failure occurs at this voltage. The second test was to assemble the plug which showed the surface leakage and to test this under pressure. The plug was gapped to a normal gap of .015 to .018", which was the standard gap of the new plugs. | ||