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).
Investigation into battery ignition misfiring on the 20/25 HP engine, leading to a reduction in sparking plug gap size.
| Identifier | ExFiles\Box 54\5\ Scan342 | |
| Date | 7th December 1930 | |
| R.{Sir Henry Royce} from EFC. c. Sg.{Arthur F. Sidgreaves - MD} c. HY.{Tom Haldenby - Plant Engineer} c. Wor.{Arthur Wormald - General Works Manager} E.{Mr Elliott - Chief Engineer} X5680. EFC/ADL/7.12.30. X.5680. 20/25 HP. BATTERY IGNITION. X.699. The use of the high compression on the 20/25 engine has caused the battery ignition to be working near its limit of available sparking plug voltage i.e. with .025" sparking plug gaps, unless these are newly cleaned and adjusted. A certain amount of misfiring has been complained of. At first it was thought possible that the available voltage of the coil was sufficient but that the sparking over in the distributor was taking place concurrently with the misfiring. It will be remembered that it had sometime back been proved that sparking over took place in the distributor at a lower voltage than would be expected from the apparent spark over distance, the sparks not going straight across but partly tracking down the inside surface of the distributor head. By making observations on a with a distributor head provided with windows on a 20/25 chassis, we proved that this was not the case with the misfires in this instance, but that, though the coil itself is capable of giving sufficient voltage to cause such spark over in the distributor in the absence of the connection of the sparking plug leads, the limit with the whole system connected up was that of the voltage available from the coil itself not being invariably able to raise the high tension system to sufficient voltage to cause the jump over of the .025" sparking plug gap under conditions of low temperature of charge with high compression. As a result of the above we decided to make some chassis tests utilising a calibrated sphere spark gap in parallel with the sparking plugs in order to arrive at the actual sparking plug voltages in different circumstances. In the meanwhile it has been arranged for the sparking plug gaps on production to be reduced to .020", this small change making the ignition satisfactory with only a slight effect on the slow running capabilities. | ||