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).
General comments and conclusions from an investigation into spark plug performance and failure, particularly with doped fuel.
| Identifier | ExFiles\Box 35\4\ scan 008 | |
| Date | 12th March 1928 guessed | |
| E.S.344. 6. 7. General Comments. The general inference to be drawn from this investigation is that a plug running slightly hotter than the R.11.V. is required to cope with the deposit formed on the insulator during the warming up of the engine. This raises a serious difficulty, in that to do this, some sacrifice of the heat resisting properties of the plug at the upper range would be involved. It is not known how far the R.11.V. type of plug is outside the range of pre-ignition on the "Falcon XI" engine, but on the assumption that the plug is satisfactory in this respect under full power conditions on the test bed, it is safe to predict that under flight conditions the plug will run somewhat cooler and will therefore possess reserve heat resisting capacity. This reserve could be drawn upon and the working temperature of the plug at the lower end of the scale raised by suitably increasing the volume of the gas space and the area of the insulator exposed to the flame. 8. Conclusions. (a). Four of the six plugs examined were defective as regards insulation resistance; the remaining two sparked satisfactorily under air pressure of 200 lb/sq.in (cold). (b). The failure of the four plugs is attributable directly to the effects of T.E.L. dope used in the fuel. At moderate temperatures the deposit on the insulator was found to be of a highly conducting nature and formed a low resistance path to earth for the H.T. current. (c). The tests show that taking the plugs through a temperature cycle changes completely their effective insulation resistance. Plugs, which as received were virtually short-circuited by deposit on the insulator were found on heating to a critical temperature in the region of 150°C. to lose conductivity very rapidly and reach insulation resistances comparable with those given by new plugs. (d). This change, which was found to be irreversible and permanent in character, may be due to disintegration of the deposit on the porcelain insulator arising from possible differences in the relative co-efficients of expansion, or to a chemical change in the nature of the deposit itself at a critical temperature. (e). The data obtained from this investigation is regarded as of considerable importance, particularly in relation to the operation of porcelain insulated plugs on engines using doped fuel. (f). It is recommended that a similar type of plug designed to run slightly hotter at the lower end of the scale but still outside the range of pre-ignition should be tried in the "Falcon XI" engine. Attached:- Drawings Nos, E.S.1651, 1668, 1669, 1670 and 1671. | ||