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).
Test report on magneto performance, comparing spark gap characteristics between Bosch and Watford models.
| Identifier | ExFiles\Box 4\5\ 05-page188 | |
| Date | 1st May 1921 | |
| Hsl/LGS 5.21. -2- Contd. always prefer to jump the safety spark gap if the engine is compressing air and not mixture. During the tests on the bench, we proved that if we warm the air, the spark would pass more easily at the plug points. The worst condition is when the spark occurs exactly at the top of the stroke, which is the point of maximum compression. If the spark takes place earlier or later, it can more easily pass at the plug points. Under normal conditions of engine running, the spark, of course, always occurs before the top of the stroke. Therefore, the worst condition for misfiring is to run with the magneto retarded. We carried out similar tests with a pre-War Bosch magneto. On there the safety spark gap is set at .400" compared with .300" for the Watford. We found with the Bosch magneto that with the central electrode positive, we could open the plug until there was a .025" gap before the sparking occurred at the safety spark gap. When the central electrode is negative, we could open it to .040" before it would jump the safety gap. We tried opening out the safety spark gap on a Watford magneto to .400" but we then get the sparking jumping up the H.T. collector brush holder. We carried out a further set of tests with the engine running with its own induction pipe and carburetter. We found with the E6 Watford magneto that it was possible under certain conditions, when the engine was running, to get sparking across the safety spark gap rather than at the .020" gap at the sparking plug points. | ||