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[SOLVED] CDI ignition timing design

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Full Member level 4
Dec 12, 2008
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Hello All,
In CDI, the magneto-flywheel assembly along with pickup coil generates ignition timing signals which is one negative and one positive pulse separated by some angular distance used to decide the ignition time for spark. I wanted to know these:
1. which pulse should be used to fire the spark?
2. Does these pulses occur at like TDC and some x degrees befre TDC?
3. I heard of 3 or more pulse output per cycle, are these pulses too occur like at TDC and some angle before TDC?
4. Does BDC also generate some pulse in this pulser coil?
5. What is the significance of generating two pulse per cycle one at TDC and another some degree before TDC?

Also, please tell me if I have any misconception about CDI engine.

Thanks for answers

Thanks for answers. I found the answers to the questions.
1. The first positive pulse is reference pulse from where firing angle is determined to fire the spark plug.
2. None of the pulses occur at TDC. All the pulses are some x degrees before TDC.
3. Just explained in answer of 2, all pulses occur before TDC and after BDC.
4. No
5. No pulse at TDC as mentioned above. A pulse system is a coil mounted statically. A flywheel assembled to magneto shaft is mounted with one or more magnets. Each magnet has 2 opposite poles. SO, when these poles passes through pulser coil, two alternating pulses are generated i.e. one positive and second negative. The distance between two alternating pulses depends on diamention of magnet and flywheel and is always fixed in terms of angular distance. Hence, in single pulse system, there is only one alternating pulse. The positive pulse determines the reference point from where firing angle should be determined and negative pulse is the lower threshold of firing angle and is usually used by analog CDI to fail safe fire in case digital CDI fails to fire. All these pulses occurs before TDC as already mentioned. In multi pulse system, there more than one alternating pulses. The CDI engine manufacturer sheet determines the angle from TDC and programmer decides the angle at which firing should be done based on reference point given by engine manufacturer.


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