Spitfire electronic Ignition
The Bendix Scintilla WL-7A battery timer (distributor) was first introduced in 1934 on the Jacobs L4 engines, which used two of the timers to provide spark. As Jacobs added first the L5, then L6 engine models, both of these used the same battery timers. A few years later they added the L4MB engine model which utilized a battery timer in the right location and a magneto on the left. Continental Motors followed suit in 1939 with single or dual battery ignition available on their engines.
The advantage of the magneto/distributor combination was that the distributor is equipped with a mechanical retard/advance mechanism which retards the spark for easy starting. As the engine rpms are increased, the battery timer advanced the timing to increase power for normal operation. The weakness in the battery timer system was the occasional failure of the electrical components (condenser, coil, and contact points). The original coils and condensers were manufactured in ambient air, and moisture was sealed into these components. This moisture ultimately caused corrosion within these components with resultant failure. Also, as the points opened and closed, small amounts of metal were transferred from one contact to the other. This created point cratering and small changes in the internal ignition timing.
Radial Engines Ltd recognized these problems and in 2009 began experimenting with Hall Effect sensors that might replace the points and condensers in these distributors. We immediately saw multiple advantages to this system. We conducted low voltage tests of both the original battery timer and one that had been modified with a Hall Effect Sensor and matching coil.
The original unit began misfiring as the battery voltage approached 9 volts. We stopped testing the modified unit at 5 volts and the spark was still stable.
In addition to low voltage tests, we also conducted low and high temperature tests. Again, the modified unit performed better than the stock timer in the high temperature test.
The FAA granted approval on December 20, 2011, and as of this writing we have over 250 Spitfire electronic battery timers in service.