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).
Chemical analysis of spark plug deposits and performance test results.
| Identifier | ExFiles\Box 150\1\ scan0181 | |
| Date | 28th August 1936 guessed | |
| 2. At 100 lbs. per sq.in. the plug was firing normally, the voltage required being 13,000. At 160 lbs. per sq.in., which represents a test far in excess of normal commercial practice, with cold air, 16,000 volts were supplied, which produced normal sparking across the electrodes and no sign of surface leakage. It is expected that the potential which must be applied to a plug firing mixture inside the engine is very much less than those figures which have been recorded in cold air, since the hot gases in the cylinder are ionised to a much greater degree. Chemical Analysis. The analysis of the deposits, both from the insulator and the barrel of the plug, was made at the chemical laboratory, the results of which are tabulated below:- Composition. Insulation. Barrel. Avg. weight of deposit per plug 0.0447 grms. 0.0772 grms. Carbon --- 14% Silica and Silicates 1% --- Lead Bromide 7% 58% Lead Oxide * 75% 22.5% Iron Oxide 2% 1% Alkali, etc. 7% 4% * Partly combined as sulphate (there was insufficient material to determine the sulphate quantitatively). The average weight of deposit found on each plug may be considered to be slightly higher than is normally found, but of course it is difficult to draw any definite conclusion owing to the lack of knowledge of the number of running hours and lead concentration to which the plugs have been subjected. It is rather unusual to find such a high proportion of carbon in the deposit removed from the barrel, and it is to be concluded from this that it was probably running rather on the cool side, which is also confirmed by the fact that the deposits on the insulation showed no evidence at all of fusing and were in every case easily removed by scraping. No reaction appears to have occurred between the lead deposit and the mica to form lead silicate, which occurs when the plugs have been subjected to excessive temperatures. As is usual, the deposit on the barrel consists largely of lead bromide, but the deposit on the insulation contains only a relatively small proportion of bromide but an appreciable amount of lead sulphate, the residue being presumably lead oxide. The latter condition is rather unusual but not by any means abnormal. | ||