Can someone explain the “shower of sparks” ignition system, that uses a second set of points in the starting magneto? I’m beginning to understand mags now.
John, here is my understanding of the “shower of sparks” system. It is nothing more than an electronic “vibrator” that rapidly “makes and breaks” the circuit through the coil, if the circuit is completed through the spark-retard point set in the mag. It is activated only in the “start” position of the ignition switch. It basically substitutes the “pulsating DC” signal from the vibrator for the normal pulse that is generated in a spinning mag. Since RPM is so low during starting, there is plenty of time to generate multiple sparks, if there is a means to do it. The mag itself can’t do it, since it is geared to engine speed. So instead, the SOS box does it electronically. You have to have good battery condition in order for the SOS box to have enough power to work (since the starter is also being driven). The SOS box is using the battery to generate the signal to the spark coil (via the vibrator), rather than using the pulse generated within a spinning mag.
If you are familiar with the various auto electronic ignitions such as the MSD system, they do much the same thing but with far more modern electronics. Another somewhat distant example might be the vibrators that were used in the original tube-type car radios. Those circuits needed a higher DC voltage (much more than 12V) in some circuits, in order for the tubes to function. They used a vibrator to generate high-frequency DC “pulses”, which were then fed through a voltage boosting coil (much like an ignition coil, but designed with a lower winding ratio to give less voltage with more current. Since the coil “saw” a rising and falling primary voltage (pulsing DC), it could generate a secondary voltage.
While most people tend to think that you have to have AC voltage for a transformer to work (if they know electricity at all), an ignition coil is just another type of transformer. Pulsating DC works just as well as AC in a transformer; it’s just that, for a given input voltage, you only get half the output current that you would otherwise get if the input was AC rather than DC (since DC only gives the positive half of the signal). Look at an image of an AC sine wave, and picture DC as being only the upper half of the sine wave.
OK, I Googled it. These mags have no impulse coupling. They have a second set of retarded points for starting. The box is a buzzer. It uses battery power to energize the mag. Since there is no impulse coupling, the mag is not turning fast enough to provide energy by itself. The buzzer converts the DC into pulsating DC, which can be transformed into high voltage by the transformer inside the mag. Transformers can’t operate on pure DC. Such a buzzer is basically a relay that turns itself off. The information came from the following URL:
Bob Steward, A&P IA
As Jerry mentioned, the “Shower of Sparks” system uses a starting vibrator to make a pulsing DC to energize the magneto coil.
It is changing the method of operation from “magneto” to points and coil type ignition. In a magneto the spinning magnets on the rotor, which is driven synchronously by the engine, induce an electric current in the coil (the magnets and coil are a generator). Because the voltage induced is dependent on the speed that the magnetic lines of force are cut by the wires in the coil AND the strength of the magnetic field, at low RPM the magneto produces a very low voltage in the primary windings of the coil, and this then causes a very low voltage in the secondary windings which are supposed to step up the voltage and create enough voltage to jump the gap on the spark plug.
An impulse coupler delays the magneto rotor and winds a spring which at the trip point is released, causing the rotor to spin rapidly to catch up with the timing of the engine, and this rapid “snap” of the rotor cuts the magnetic lines of force very quickly and creates *1* delayed spark. If everything is right (compression, fuel mixture, spark timing and plug gap) the spark will occur and light the air/fuel mixture causing that one cylinder to fire, and drive the engine towards the NEXT cylinder’s compression stroke and the process repeats all over again. Once the RPM of the engine reaches 400-500 RPM the centrifugal force causes the impulse coupling to disengage and the spark timing is not retarded and the mag rotor is spinning fast enough to cut the lines of force fast enough to generate a strong spark. Should the battery be dead, or too low to spin the starter, one COULD hand prop it and because of the spring wind up and release of the impulse coupler the engine can be started by just moving the prop in the proper direction. It need not be turning very fast, which is why a “hot prop” is dangerous.
Back to “Shower of Sparks”. The SoS system is using BATTERY power to send the coil a series of on-off-on-off pulses as the vibrator makes and breaks the connection. Because it has the 12V from the battery and because the pulses are very rapid, a “shower” of sparks occur when the magneto points open. This is the reason for the RETARDED points, so that the shower of sparks occur AFTER Top Dead Center, so that the engine does not fire and push the piston backwards down the compression stroke.
The only real difference between “shower of sparks” and the point and coil system on your pre-electronic ignition car is that the car system didn’t have a starting vibrator and would only make 1 spark per timing event. Back in the late 60’s lots of companies (like JC Whitney) were selling an electronic version of shower of sparks that ran all the time. It was just an electronic oscillator that sent pulsing current to the coil. When the points opened the pulses made several sparks.
Now if I managed to explain that well enough, you may have caught the fact that the SoS needs battery power to work, and since the SoS is only engaged when the ignition switch is in the START position, if the battery is dead or so low that the starter won’t turn the engine over, you CAN NOT hand prop a SoS equipped plane. Because there is no impulse coupler to speed the magneto rotor, and without power to the SoS starting vibrator, it won’t send ANY sparks to the engine!
Someone with an intimate knowledge of the system COULD re-wire it under the panel to send power to the starting vibrator without the starter turning, but that is in the realm of being stranded a hundred miles from civilization (in the bush?). Certainly not what one would consider doing Sunday AM when you find the battery dead from leaving the Master on after the last flight.
For those wondering, we COULD have an electronic version of the SoS today, if it were not for the Trial Lawyers, the FAA bureaucracy and the fact that Bendix is owned by Teledyne Continental now, and they are hard at work on the FADEC system with electronic ignition, and don’t want to invest in the antique magneto systems.
There are places that have built and offered electronic SoS for experimentals, but they are not approved for our planes. 🙁