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 testing of plug insulation resistance at high temperatures and pressures.
| Identifier | ExFiles\Box 35\4\ scan 006 | |
| Date | 17th March 1928 guessed | |
| E.E.341. 4. working conditions i.e. 200-300°C. The method adopted in this test was as follows:- The plug was screwed into an electrically heated cylinder supplied with air pressure at 500 lb/sq.in. in order to suppress sparking at the electrodes. The effective resistance was then measured by means of a calibrated magneto, the plug under test forming the shunt resistance on the H.T. circuit. Two plugs were tested in this manner and in each case a curious phenomenon made itself apparent; the effect of raising the temperature of the plug to 200° or 300°C. being to change entirely the effective insulation resistance as measured at atmospheric temperature. The two plugs in question, Nos. 1 and 6 which, as shown in Table I originally had effective insulation resistances of 25,000 and 45,000 ohms respectively now gave resistances of 1.4 megohms and greater than 2 megohms, respectively. A further increase in effective resistance was observed on Plug No.1 after it had cooled down to atmospheric temperature, the value then being greater than 2 megohms. In view of this peculiarity and in order to determine the effective resistance characteristic over a range of temperatures, two further plugs, Nos.2 and 4, were slowly raised in temperature up to 300°C. Measurements of the regular sparking voltage of the calibrated magneto associated with the plug under test were made at frequent intervals and the effective resistance deduced from the values so obtained. Figs.1 and 2 show very clearly that the voltage which the magneto is capable of delivering at the plug electrodes, increases very rapidly at a critical temperature in the region of 150°C. Similarly, Fig.3 and 4 show the corresponding rapid increase in effective resistance of the plug beyond the critical temperature. After cooling down to atmospheric temperature both plugs showed effective resistances greater than 2 megohms and were then capable of giving regular sparking at 10,000 volts. These results, which are shown on the following table, confirm conclusively the existence of the phenomenon observed in the tests on Plugs Nos.1 and 6 and show that plugs originally defective owing to conducting deposit on the insulation become serviceable when raised to a temperature of about 150°C. This behaviour is entirely opposed to that observed on certain defective mica and quartz insulated plugs but it should be pointed out that the relevant factors in each case are not entirely the same. (R.A.E.Reports Nos.E.E.276 and E.E.280). Table/ | ||