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).
Magneto ignition trouble on the 40/50 chassis due to increased compression ratios.
| Identifier | ExFiles\Box 61b\4\ scan0185 | |
| Date | 9th June 1932 | |
| X6105 To R.{Sir Henry Royce} from Hs{Lord Ernest Hives - Chair}/Wst. c. to Sg.{Arthur F. Sidgreaves - MD} Wor.{Arthur Wormald - General Works Manager} c. to E.{Mr Elliott - Chief Engineer} Da.{Bernard Day - Chassis Design} c. to By.{R.W. Bailey - Chief Engineer} Hs{Lord Ernest Hives - Chair}/Wst.2/KT.9.6.32. X6098 MAGNETOS ON 40/50 CHASSIS. Sometime ago it was agreed that we should drop the magneto on the Phantom chassis and substituting instead a double R.R. battery ignition system. Sg.{Arthur F. Sidgreaves - MD} was not in favour of this policy at that time however. We have now run into further trouble with magneto ignition due to raising the compression ratio. The magneto now misfires on the 'pick up' in a similar way to the battery ignition on the 20/25 before we improved the voltage output of our present standard ignition coil. We traced the trouble to the safety gap of the magneto which we found operated when the plug voltage increased due to the engine picking up under load on full throttle. It was found necessary to increase the safety gap from 6.5 mm. to 11 mm. before misfiring was eliminated. This opening out of the safety gap of course increases to the danger margin, the breakdown capabilities of the magneto winding insulation, and we are in touch with Messrs Norths with a view to getting them to increase the safety margin of their insulation so that we can run with the larger safety gap. We are afraid that Messrs Norths will find it very difficult to comply with our requirements as the winding on the E.W.6 Watford already occupies all the available space on the armature. We do feel, however, that this trouble will follow us whenever we still further increase the compression ratio and we cannot see how we are going to get a better magneto without increasing the size, weight and expense of the same. With the battery ignition we are much happier as we have a much better insulation space factor on our ignition coil. We think that this inherent defect with magnetos has been a large deciding factor, next to the price, in its decrease in popularity. | ||