The Aspirator said:
Thomas, this whole ignition thing has been intriguing me for some time now. Duration sounds like a good start to a proper flame kernel, but what about when using a multiple discharge igintion system like MSD's? I read it can discharge up to 6 sparks per event, these would obviously be short duration sparks though. But would that in theory start 6 flame kernels? Or be better in any way than a single long duration spark? I do have some more questions, but they can wait till I hear back.
Thanks,
John
John,
Ionization of the gap is the good start to begin the flame kernel development. And that requires a certain voltage level at a certain amperage level. The width of the gap, and the turbulence and pressures in the combustiion chamber greatly affect what those voltage and amperage values have to be; or what they need to be. The ignition coil has to be able to supply the required voltage at the needed amperage flow, as quickly as it can, once it has been triggered to do so. That is known as 'rise time'.
Once the ignition coil has produced and delivered that required voltage pressure and amperage flow to jump the gap across the plug electrodes, the spark commences. In other words, the ignition coil is triggered, the magnetic field collapses, the induction of high secondary voltage commences [voltage rising], and the gap is starting to tingle in anticipation [ionize] for what is coming real soon. When the spark actually completes the jump, the ionization is complete: we have a spark.
The width of the gap has its effects on the quality and the intensity [heat] of the flame kernel that develops as the spark is sizzling across the gap. A wider gap will require more voltage/amperage to make the spark actually happen, so that can be a good thing. If you can jump the gap, that means that you had to have some heat there to get it to happen.
BUT, if you lack fuel for the flame kernel to feed on, and grow with, the flame kernel can extinguish. That is where spark duration comes in.
By having the spark last longer time-wise, that sizzling continues longer and keeps that flame kernel going longer and hotter so that it can find the fuel it needs to keep burning and growing.
The duration of the spark affects the quality and intensity [heat] of that developing flame kernel; and can affect whether or not the flame kernel continues to grow.
So, what you need is an ignition coil that can develop plenty of secondary [high] voltage and amperage, and can do that FAST.
'Multiple spark discharge', as it is often called, or 'pre-ionizing discharge', as it is sometimes called, is a method that supposedly improves the ionizing of the spark plug gap. It was a way to get around the need for a fast rise time in the ignition coil. Or, another way to say that is: when you don't have the time for 'rise time' then you can use multiple spark discharge to get the ionization going.
I realize that that sounds like double speak. But think of it in the context of a motor turning at speeds over 10,000 RPM: you don't HAVE a lot of time for coil rise time. Especially on a motor with 6 or 8 or 10 or 12 cylinders. At 10,000 RPM, everything has to happen REAL QUICK.
So, you can go the MSD route, or go multiple coils, or do both. The Formula 1 people are the ones that get buggy about such things. To a lesser extent, the top classes of drag racing worry about that as well.
In the world I live and work in, things like MSD are unnecessary. Unnecessary to the point of being a gimmick.
If I can get a single ignition coil that can put out over 45,000 volts at over 6500 RPM on a V8, and do it reliably and consistently, then it can more than adequately do the job on my four banger...unless I wanted to rev it to 13K or more...
....nah, I don't think so. I don't do Formula 1.
What I am saying is this: if someone wants to play with things like MSD, that's fine with me. I don't see a need or a justification for it. But, whatever floats their boat.
HTH; and remember: the above was a brief and very abbreviated comment. I know there are several aspects of MSD theory that I did not address; but which I don't care enough about to elaborate on. I figured out years ago that I don't need anything that MSD supposedly offers; but that's my opinion and evaluation.
Thomas Fritz
...the stealth FTi