Bosch ZU4 Magneto

The raised section on the points cover is for the cut out wire back to the inside of the vehicle. Amazing what you find when you clean things up.

Just pulled a couple out of the cupboard.

A German Bosch FU4 with impulse unit and a very early all brass DU4. I have another couple of early brass ones plus a Lucas somewhere.

Just had a thought on how generic magnetos are.

They were pretty standard for most engines on the rotation of the engine magneto drive vs crankshaft speed.

Nearly every mounting system could be modified to take any magneto.

Those with vernier couplings were capable of being timed to about a tenth of a degree.

They put out a huge spark with no need for any other form of electrical input. In fact the only wires needed were the plug wires and an earth wire to stop the engine if you did not want to just turn the fuel off or stall it with the clutch. Aircraft are always stopped by turning the fuel off, turning the ignition off is no-no.

Their big failing is they need rotational speed to create spark. Most magnetos require about 300rpm to work and are difficult starters on 12 volt systems and almost impossible with the old 6 volt cranking speeds. Later developments produced the impulse unit which caught on rotation, winding up a spring then released, spinning the magneto fast enough for a start.

The other idea was the "shower of sparks" used by Henry Ford in the Model T whereby 4 coils produced a spark to each plug via a vibrating set of points. This would often start the "T" even without turning the crank handle as a one of the pistons would be somewhere near the top of the compression stroke as the spark plug began firing madly. Many aircraft of the 30-60 period also had the shower of sparks system for instant start prior to switching to magnetos..

Once running the engine would be switched to magneto ignition. The Model T magneto was a part of the flywheel and he was so smart he used excess power from the magneto to charge the battery. Wow those lights were bright going down a hill at high revs, better than a modern high beam. Unfortunatelky two candles would do better if driving very slowly around the neighbourhood

The Model T could be driven either coils (battery) or magneto but aircraft had the shower of sparks circuit on a spring key or separate button and would not be flown on that system.

Very interesting that stopping the engine in aircraft is a no no to kill the spark, any reason why?

Impulse mechanism are found on some stationary engines and as you say it allows the starter usually on a crank handle to give the maggy a quick spin once the spring inside has would up and let go. Slightly off topic but with starters I never knew about inertia starters used on some planes. They have the best sound as you wind them up almost like an air raid siren.

Tanks with radial motors had the wind up starters ......and looking at U tubes of German tanks,they also had the wind up starters..........and I forgot to mention the US AAF Wreckers with Hercules motors also had wind up starters......from the side IIRC.

On a Victa mower, at least, if you shut it down on the spark, you have a carby full of oil next week and very difficult to start.
Stale fuel / dried fuel residue in the carb on a plane could be deadly, I suppose.
Lang will enlighten us...

Jarrod.

Have seen the wind up ones that seem to wind a spring up and once tensioned it lets go and starts the engine.The inertia one is a heavy flywheel you crank like buggery and when it is quick enough you engage it to turn the engine over. These things growl and groan like a siren.
I run my mowers dry on those with a fuel tap. Chainsaw, blower/vac and whipper snipper gets the tank emptied then started again to run dry.

Firstly

The reason you shut down an aircraft engine with the mixture control is unlike a car engine which stops within a revolution from idle, an aircraft has a huge gyroscopic flywheel on the front in the shape of a propeller. Remember watching any old airliners shut down and the prop takes a long time to finally come to a stop - in some of the big 4 bladed radials it can be 7 or 8 revolutions.

Each time you do this with normal carburettor operation without spark it washes the cylinder walls of oil and the cumulative effect of hundreds of shut downs is to increase wear and reduce engine life. So, Throttle Idle , Mixture Shutoff, leaving the spark going to keep the engine operating until the last burnable bit of fuel comes through. Once it stops Switches Off.

Next Starters

We are talking 3 different things.

1. Magneto impulse unit. This is a coil spring that is wound tight by the case of the impulse unit bearing on a projection and suddenly releases to spin the magneto faster than engine cranking speed. They work good.

2 Mechanical impulse starter. This is what is on your lawn mower. You wind up a spring then release it to throw out cams that spin the engine over for a start.

3. Inertia Starter
These involve a heave flywheel within the starter being spun up to a great speed either electrically or by a crank handle. When it reaches speed it is engaged in the starter gear to turn the engine over much like your lawn mower. Because of the quick dissipation of energy you can not "crank" the engine over for long periods like a direct drive starter.

These were common on aircraft engines (or vehicles like tanks using aircraft design engines) as they require smaller batteries. But just as importantly they have an emergency crank handle to spin them up meaning flat (or no) battery can not stop your progress. You see WW2 photos of both German and British crews standing on a wing madly cranking to get fighters off on a scramble. You could not possibly have a battery cart for every aircraft so this method got everyone going regardless of the squadron battery situation.

One of the DC3's I flew in PNG had a direct starter on one side and an inertia on the other - not normal and requiring two slightly different start procedures. We all had a go at climbing up on the wing to do the crank handle thing just for the hell of it but never did it in normal operations.

Lang

Starting a WW2 Panther tank

Here is an interesting article on the early hand cranked aircraft magneto. Instead of the model T system or variations thereof of battery and buzzer coils the pilot would crank like mad as the ground crew swung the prop to create the initial spark (circuit running through the main magneto terminals). As soon as the engine fired the main magneto took over.

I can remember my physics teacher getting us all in a circle holding wetted hands. The first and last students held the terminals of one of those old aircraft hand cranked magnetos (like the old hand cranked phone magneto but much more powerful). Great hilarity as ten people got a belt of electricity through them.

Probably could not do that today because a 15 year old in the group might have a heart pacemaker or maybe the teacher might be liable for an assault charge on the group.
www.themagnetoguys.co.uk/aircraft-starting-magnetos