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).
Ignition system tests, analyzing spark plug voltages and the performance of ignition coils.
| Identifier | Morton\M13.2\ img013 | |
| Date | 17th December 1930 | |
| -4- EECI/AM/17.12.30 contd. (b) Plug gaps .030" Water temperature 70°C. Cylinder No. Flux voltage. 1 8500 2 9700 3 9200 4 9700 5 7550 6 7550 Average maximum voltage per plug 8870. The results of the above tests illustrate the high sparking voltages resulting from the severe ignition conditions on the 20/25 chassis, particularly if the sparking plug points have been allowed to oxidise and/or their gaps have increased due to burning away as is almost invariably the case initially after a short mileage. For slow running purposes sparking plug gaps of .030" would be desirable to work with the battery ignition on our engines, but with these gaps the 20/25 condition would be too severe for regular firing with the voltage available from our present standard ignition coil. Quoting from my assistant's report regarding the voltage available from the coil we have :- "The maximum regular sparking voltage generated on our coil at 12 volts is 10,500, but after the coil has been warmed up by running it in an oven temperature of 80°C. on a voltage of 17.5 for two hours this voltage is reduced to about 9000, so that misfiring on the "pick up" would occur. Presumably the ignition would fail altogether with .030" plug gaps on the pick up with the coil hot, but these conditions are ameliorated by the fact of the plug voltage being reduced due to the plug electrode temperature also being high. On both 19-EX and the 20/25 chassis, missing occurred on the pick up with .030" gaps, being worse on the 20/25". As a result of the above we are putting in hand for experimental test two ignition coils with increased number of secondary turns i.e. from 18,000 (present standard) to from 20,000 to 21,000, and with the 20% increased voltage expected from this change we | ||