electronic ignition question

[jasonh]

I have a question about electronic CDI ignitions:

The CDI units designed for model aircraft typically have a hall effect sensor activated by a magnet attached to the crankshaft.

This makes perfect sense for a two stroke. When the piston hits TDC the magnet passes the sensor which gives a spark- good.

If the magnet is on the crankshaft and we have a 4 stroke we only want a spark every *second* time the magnet passes the sensor. Not only that, but the spark has to be generated on the right pass. ie- TDC before the power stroke, not TDC before the intake stroke.

How does this happen?

I see conversions of 4 stroke engines (E.g. Saito's) where the sensor/magnet is apparently on the crankshaft. I understand the electronics could do a divide by 2 easily enough, but how do you get the spark on the right stroke?

Am I missing something?

Thanks,
Jason H.

 

[duncanbojangles]

Those 4 strokes with the sensor and magnet on the crank do create a spark on both TDC before the power stroke and TDC before the intake stroke. And it's not really a problem. In one case, you've got a spark that ignites a fuel rich mixture like we want. In the other case, we get a spark in a mixture of exhaust gasses, which doesn't do anything. This is called wasted spark, and you can read more about it here: http://en.wikipedia.org/wiki/Wasted_spark I've dealt with plenty of motorcycles and scooters that employ this system.

 

[jasonh]

Ahh! That makes sense. Reading the article I didn't realize it was such a common practice - ie not just in the world of RC engine conversions. I learnt something today. Thanks for the answer.

Jason H.

 

[parisharma1]

thanks for sharing.