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RAB
05-03-2003 02:37:58
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Re: Re: Re: Re: How does battery ignition work? in reply to Steve, 05-02-2003 05:49:35
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Steve,
Any change in current/magnetic flux will induce a 'sympathetic' reaction in any other coil to which it is coupled. It is just a matter of how fast the magnetic flux changes. High rates of current change in the primary induce more volts in the secondary than slower rates. So any change of current in the primary will induce an opposing voltage in the secondary but any current flow is limited!, this in turn creating some magnetic flux which will affect the primary. It would not matter a bit were the primary current to rise (almost) to maximum, as even the rate of current change in the primary is now very small.
When the points open the primary current (at 12 volts)is stopped, so there is a very high rate of flux change in the coil(s). The primary will produce a much lower voltage than the secondary (turns ratio) but will dissipate a lot or all of energy accross the points if given the opportunity. This is where the condensor comes into use - it prevents an arc while the points are only just apart ( once you get an arc it would spark even at full points opening due to the air being highly ionised) and effectively do an electric welding job on the points! Yes, just like a stick welder - you have to make contact between electrode and item to be welded, then you can 'draw' the arc and weld away quite happily!
Meanwhile the high voltage in the secondary is just busting to jump anywhere it can, and accross the appropriate spark plug is where we allow it to take the easiest path, by means of the distributor.
Now just for a moment consider what would happen if current were flowing accross the points gap - change in current = change in flux so reducing flux available in the secondary.....
All these are going on in a fantastically short period of time, but basically the energy will take the easiest path and anything which doesn't go to the spark will (a) be lost and (b) will actively reduce the likliehood of ever reaching spark plug potential in the secondary.
Sorry but there are so many things to consider, all at the same instant, that it can easily get muddled. Generally like all electronics, if there is a factor of 10 difference the less likely option can be ignored, so some of the above arguments can be ignored - unless the system is much less than 100% efficient - which is when the faults show up clearly.
Your other question about sparks at the points - theoretically there should never be sparks at the points, practically there will be, but high voltage spikes, not welding currents we hope!
Regards, RAB
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