Magneto application?

BOSCH ZR3 2SPK magneto, that is frame model ZR, 3 cylinder, fires 2 sparkplugs at the same time hence the 6 sparkplug leads. It is missing the cover on the shaftend and points cover. My question is what is the probable application for such a beast? The bevel gear on the input shaft speaks "aircraft" to me.

6 leads, two firing simultaneously? That means a 3-cylinder engine--surely an aircraft engine, but what one? 3-banger aircraft engines were pretty danged unusual.

Art Anderson

My old book written by Herschel Smith States this;

Three cylinder radial engines

Bristol-Lucifer 1925

Walter-Polaris 1928

Jacobs L-3 1929

Aeromarine 1931

This book is not complete enough to list magnetos.

M.L. Anderson

Dykes list Lenape as maker of sveral models of 3 cyl aero engine. No dates are given.

You can date the magneto, though, as Bosch consecutively numbered all magnetos manufactured, irrespective of size and type.

1900 1-6099

1901 to 9639

1902 14417

1903 24134

1904 42571

1905 75753

1906 123210

1907 185329

1908 251268

1909 434566

1910 646345

1911 933508

1912 1335331

1913 1567965

1914 1758030

1915 1798093

I am sorry my list goes no later than this, but it might be helpful to people in other cases.

I have a brand new Simms 4 aircraft magneto, very well made, but it has no advance/retard control. This 3 cyl 2spark has. Why would they run aero engines without being able to adjust the spark advance?

Ivan Saxton

This could be from a 6 cyl engine if it ran at crankshaft speed instead of off the camshaft.You get 3 power impulses per rev on a 6 cyl engine and the other 3 sparks would be on the exhaust stroke. Some modern cars do this with 3 ignition coils on a 6 cyl engine firing 2 sparks at once. They call the exh spark a waste spark.

Serial on this at over 3 million probably makes it 1920s. Have some literature on Anzani 3 cylinder but they do not show Bosch mags, neither does the Lawrence L-2 or Wright "Gale".

I am not sure about the waste spark application. we need to think about the arithmetic from simple basics. The rotation of a magneto armature breaks the magnetic flux across the poles twice per rev; each one a spark(in coordination with point opening, of course. So a magneto on a 4 cyl is run at crankshaft speed and takes two revs to give four sparks. The distributer gearing inside the mag has a 2:1 reduction. Magneto for a six has to spin 3 revs for 6 sparks, so the armmature has to run at 1.5 times crankshaft speed. That three would have to be geared for 1 1/2 revs per two turns of the crank, ie it should be geared at 3/4 crankshaft speed. If it was set to run a 6 cyl engine on a waste spark system, it would have to be twice as fast as that, ie 1 1/2 times crankshaft speed. So it should be possible to run it on a six: There is one reason it could be an advantage, but other compelling reasons that you would be better not to stretch the friendship.

The advantage would be that because the magneto is spinning faster it should have a more enthusiastic spark at low rpm for starting. This may be fine for a low speed engine, but at high speeds there may be things you would worry about. I had to make a new HT slip ring for the pick-up, and whereas the slip ring for a normal magneto has a bronze ring, my 2 spark ZR4 for the 1918 Mercer had two opposite segments inserted in the bakelite assembly. I cannot remember how I did it, but I do know it was a fiddle to make and insert them so they were loked in and would not make a lazy short circuit. You see, each segment connects to a different end of the secondary winding.

Other problem is that each spark has to jump two gaps simultaneously, and the higher the compression pressure, the harder it is to ionize the spark gap and fire the charge. And aircraft engines may cruise at a fairly wide throttle opening. (Of course you could cheat a little by running 50/50 water/ethanol "water injection" to give more polar component to the mixture to help the spark. It doesn't really appeal, and I doubt whether it would have been one that Herr Bosch would advocate. I have never had trouble fireing the Mercer, even on the armstrong starter with the 2-spark, so the possibility of hotter spark nfor starting is probably an illusiory advantage.

Ivan Saxton

Hello All, This is my first post here. I came across the picture of the magneto. I looks very similar to one I am working with. I would need more pictures to be sure.

Where did you get it? If it is the right thing I probably would want to buy it.

3 cylinder? No. 6 cylinder engine with TWO magnetos, two spark plugs per cylinder.

It may be from a BENZ 4 engine. I'm helping to restore one. It was used to power a WWI aircraft call an LVG CVI. She is a beautiful old beast. Weighs 450 kgs.

If I knew how to attach a picture (if you want) I could send/attach a picture.

Not many of these engines still exist - maybe about 12 worldwide. If you have more pictures please send and I'll look more carefully. We have full manual that explains how system electrics work.

Ivan, that list of Bosch serial numbers up to the mid teens seems to be readily available, but I have never been able to find any info from later years. We have a single cylinder Bosch mag on a lawn mower which we believe to be mid 1920s. It would be good to be able to date the mag which would then give a clue to the date of the mower.

Bosch magnetos are wonderfully easy to date up to 1915 regardless of which factory or country they were manufactured because the parent factory in Germany Licensed all production consecutively for example, the factory in the United States were Licensed to produce ten thousand magnetos in a given year and were allocated the serial numbers accordingly.

Unfortunately the parent factory lost it's Patents and Licensing rights during the argument of 1914 - 18

Noel

Do you still have this 3 cylinder magneto ? I'd be interested in buying it.