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).
The experimental testing of ignition coils, discussing voltage measurements, failures, and testing methods.
| Identifier | ExFiles\Box 164\5\ img027 | |
| Date | 28th January 1931 | |
| -4- EFC1/AD28.1.31 contd. We have produced experimentally, a coil with a larger number of secondary turns but on voltage test have not realised the proportional increase of voltage which was anticipated which has gone to show that coil quantities are not readily calculable, determinable or predictable on the mathematical basis we so fully worked out some years ago, owing, as already suggested, to the presence of parasitic surges which upset the pro rata basis of voltage in relation to turns. We have been able to measure the maximum voltage available from our standard ignition coils, utilising our ball gap spark distance voltmeter which has been calibrated by means of our H.T. A.C. transformer giving known secondary voltages inferred from the transformer ratio and the primary P.D. The resulting maximum figure varies appreciably from coil to coil, the average figure being roughly 8,000 volts regularly supplied, but surging voltages as high as 15,000 volts have been measured. In actual operation on accelerated pick-up on the 20/25 we have measured a voltage on the plug terminals as high as 9,000 at a time when misfires were occurring. Every care is now taken thoroughly to dry every item of coil make-up before putting these together - in spite of this breakdowns do appear to occur in circumstances in which they are least expected. We have described in a previous report our method of intensive testing of coils by periodically charging through a suitable resistance, a H.T. condenser of .15 m.f. capacity (now) which, arriving every time at its peak voltage discharges across a small adjustable gap through the primary of the ignition coil with the result that a stream of secondary sparks is given by the coil without the use of a contact breaker. The voltage on secondary discharge can be varied by altering the secondary gap and in this way we have been able to find out the weak points of ignition coils and also to prove the fact that our own coils are just as well insulated in their secondary windings as coils of other makes. When, however, we have attempted to improve the durability of ignition coils on this test, we have been much dissatisfied with the results in respect of their lack of consistency. | ||