MSD and other CDI ignition half-truths

[Circlotron]

Obviously if the mixture fails to ignite then duration is an issue, but if it does ignite reliably then I don't really see a problem.

I suppose the mixture igniting is not just a black and white issue. The way I see it, things start as a tiny little bubble of flame, just like a baby in mummy's tummy. You want that bub(ble) to grow at a predictable rate so by the time TDC is reached the squish area pinches that flame bubble into the remainder of the yet to be burnt mixture and things really get moving so you reach peak pressure at approx 20 deg ATDC. Trouble is, it is a really rough environment and sometimes our little flame bubble pauses, staggers and stumbles on it growth to glory. A CDI just gives it a short sharp shove and then it is all on it's lonesome in the wilderness. An inductive discharge spark keeps feeding heat into the flame bubble while it is growing and so probably contributes to it's orderly and predictable expansion. Under ideal conditions there should be no difference between CDI and inductive because IIRC petrol vapour only needs about 2 millijoules to ignite it, but in the real world things can be far from ideal. Crazy turbulence, blobs of liquid fuel (especially if alcohol or with cold nitrous) exhaust gas dilution etc etc. That little flame bubble needs it's hand held along the way, not just booted in the bum and that's it.

[vlad01]

Obviously if the mixture fails to ignite then duration is an issue, but if it does ignite reliably then I don't really see a problem.

I suppose the mixture igniting is not just a black and white issue. The way I see it, things start as a tiny little bubble of flame, just like a baby in mummy's tummy. You want that bub(ble) to grow at a predictable rate so by the time TDC is reached the squish area pinches that flame bubble into the remainder of the yet to be burnt mixture and things really get moving so you reach peak pressure at approx 20 deg ATDC. Trouble is, it is a really rough environment and sometimes our little flame bubble pauses, staggers and stumbles on it growth to glory. A CDI just gives it a short sharp shove and then it is all on it's lonesome in the wilderness. An inductive discharge spark keeps feeding heat into the flame bubble while it is growing and so probably contributes to it's orderly and predictable expansion. Under ideal conditions there should be no difference between CDI and inductive because IIRC petrol vapour only needs about 2 millijoules to ignite it, but in the real world things can be far from ideal. Crazy turbulence, blobs of liquid fuel (especially if alcohol or with cold nitrous) exhaust gas dilution etc etc. That little flame bubble needs it's hand held along the way, not just booted in the bum and that's it.

just like staring a bonfire. half a bucket of fuel on it will explode with violence but cos its short it often fails to set the bonfire going, you need a long controlled ignition to get it going, either slow burning motor oil or a propane torch held at the bonfire and feeding it til its get hot enough to go itself.

[Circlotron]

Yep.
I sometimes say to people that you can whip your finger quickly through an oxy-acetylene flame and barely feel a thing but you can't do the same thing slowly through a match flame without noticing...
Another thing to consider is that if the mixture happens to be tumbling or swirling during the arc time then a stream of it is passing through the plug gap, so the longer you have the arc present the longer the strip of flame that exits the gap. Like setting a field on fire; you can light a patch in the centre and hope it spreads or you can drag a lighted torch along the length of the field and increase your chances of success greatly.

[vlad01]

Yep.
I sometimes say to people that you can whip your finger quickly through an oxy-acetylene flame and barely feel a thing but you can't do the same thing slowly through a match flame without noticing...
Another thing to consider is that if the mixture happens to be tumbling or swirling during the arc time then a stream of it is passing through the plug gap, so the longer you have the arc present the longer the strip of flame that exits the gap. Like setting a field on fire; you can light a patch in the centre and hope it spreads or you can drag a lighted torch along the length of the field and increase your chances of success greatly.

I know very well about this one, I am or was a high voltage nut years ago and made heaps of different high voltage apparatus.

passing a high speed stream of air though an arc actually drags it out a long way. Also that fire is electrically conductive to high voltage