How does the CDI know when to fire spark?

arguevera

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Jul 16, 2017
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This is more of an engineering question, but I am curious, given that the CDI only takes in two wires (for what I assume is positive and neutral), how does it know when the piston is located to fire a spark?
 

2door

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Sep 15, 2008
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Wheelbender is correct. There is a magnetic signal provided to the CDI generated from a rotor, or rotating magnet on the crankshaft. The magnet, or rotor is indexed to the crankshaft which determines piston position.

Tom
 

ivan H

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Oct 8, 2011
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Ok, a 2 wire CDI works like this. The magneto produces an AC voltage. The positive going (upper) half of the waveform charges the CDI's charge storing capacitor. As the waveform goes negative (lower half of the wave), it triggers the CDI to fire. It is very early in the negative going half of the wave (like a few volts negative potential) that this happens. There's a bit happening to make the spark at your plug, but that is the gist of how its "timed". Hope this helps. Cheers

Edit; I know this thread was dormant for a couple of months, but seeing as it hadn't been answered how a "two wire" CDI, where there is no crank sensor or firing coil is made to fire I thought I'd explain it, in very basic terms, without going into the electronics of the CDI. Cheers
 
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Russell

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Apr 19, 2009
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Ok, a 2 wire CDI works like this. The magneto produces an AC voltage. The positive going (upper) half of the waveform charges the CDI's charge storing capacitor. As the waveform goes negative (lower half of the wave), it triggers the CDI to fire. It is very early in the negative going half of the wave (like a few volts negative potential) that this happens. There's a bit happening to make the spark at your plug, but that is the gist of how its "timed". Hope this helps. Cheers

Edit; I know this thread was dormant for a couple of months, but seeing as it hadn't been answered how a "two wire" CDI, where there is no crank sensor or firing coil is made to fire I thought I'd explain it, in very basic terms, without going into the electronics of the CDI. Cheers
Thank you for clearing this up in basic understandable terms. Also thanks to others that helped also.