Need some details on the ignition system

I like the sound of that, but largely don't understand it, so I'm even more impressed Aero

The Pope loves hotting things up - his Pope-mobile does 0-100 in 6 secs, not bad with elevator recliners in the back and bullet proof glass

I was lucky enough to be on a road into the city behind a vatican motorcade this week, no bull (far enough back to not look like i was interested)
The usually 60 kph stretch with 10 sets of lights, was all green and 110 kph the whole way of empty four lane I'm glad he came ;D

My SR's wiring is not much good. Keep us posted

Don't know what sort of speeds you can get from your lathe, my Boxford cetainly won't produce anything more then 1,400 rpm - just off idle on the SR. Plus it won't heat the coils. You do need to check that you've capped the advance and that it doesn't keep on going, 40 or 45 degress of advance on road bike - i would reckon that would wreck the piston. I thought about this and decided to buy a fan to put in front of the engine, to build something that would spin the generator at 7,500 rpm would not be easy. Its quite scarey running the engine up to the red line like this - I hope not to have to do it too often and collect the data I want in just a few goes. I'm glad I have a motad silencer though!

You'd have to use a 1uf capacitor because of the ignition coil. Thats a heavy current SCR you've choosen and somewhat at odds with the choice of diodes, I'm not sure if it'll work - remember the SCR can also 'control' the timing if you let it. Its that what Yamaha used? More importantly in a tuned circuit the voltage at the capacitor is going to be X mulitply the input put voltage. You'd have to sit down and work that one out with someone who knows more then I do to get the exact figure!

Just a warning...please drink 6 large beers and take some mushrooms before reading!


I'll explain this all I'm going to, then its down to you to work it out. When I've been at uni or studied something, the professors or people running the courses always explain stuff then tell you to go and work it out for yourself and show them what you've learnt.

There are two parts to this system. The capacitor charging section and the signal conditioning section. The operate independantly from each other.

The capacitor is in fact connected to the primary of the ignition coil and therefore forms a tuned circuit. A capacitor and a coil makes an oscilator. You don't want an oscilator so you fit an 'anti ring diode' to damp the circuit and stop it from ringing (or free running for the German speakers). As I don't know what the impedence of the ignition coil (its not 1 ohm) I haven't worked out out the frequency this would happen, but as an uneducated guess I'd say it will probably be around 150 hertz.

The system has to use a high voltage to get enough 'go' to fire the spark plug. Since there is no Dc to DC converter in this system the generator has to provide the power. I think the voltage supplied to the CDi box to charge the capacitor is around 400 volts above idle. That is rectified through a full wave rectifier to give a DC level to charge the capacitor. If you understand the different sort of rectifiers you'll understand what level of voltage is being used to charge the cap. However tuned circuits don't behave in a conventional manner and the circuit input voltage at the capacitor will be multiplied by a certain factor. You could sit down and work this factor outbutthe fact that yamaha used a different diode here speaks volumes. The laid back approach would be to say that the yamaha designers had the same range of components available as we do, (30 years ago I'd say less choice). So there is nothing out of the ordinary required.

Also remeber while their are some high voltages, there is very little current.

And here's where we tie the two parts together. To get a spark you need to switch on and off the capacitor to cause the field in the coil to break down. And what's controlling ths spark? Basically a seperate AC generator. It uses the principle that the voltage level generated will rise as engine speed increases to advance the timing. Well it advances nothing, it just reaches the gate's trigger voltage quicker as engine speed increases, hence its a synthetic timing curve. The AC coming off the pulse coil is rectified and limited to give the timing curved required.

I think I have fully explained what took me a lot of working out. I won't be posting any more on this subject and mods feel free to lock this thread.

I warned you

Me too!

On the web try googling capacitor discharge ignition. However you may find that many of the articles now require a subcribtion to view them. A trip to your local library may turn up more free info. If you have seen something you think is good, and its been published, chances are you can order it for next to nothing from your library. Most articles tend to dive off into all sorts of theory that you don't need, just stick to the basics.

I found this to be useful.
www.transmic.net/fichiers/cdi4347.pdf

Great work guys!

So are either of you guys going to start replicating these? I need a CDI.