Motorcycle Repair: Magneto ignition system

Question

Pickup coil  
Magnet   QUESTION: Hello Sir,
I am actually having a doubt in magneto ignition system used in bikes.
I have seen that the magnet which is actually the flywheel, is having two raised parts on it, might be which acts as trigger for the pickup coil. Now in 4 stroke engine for every two revolutin of cranckshaft we require a spark only one time. But the triggering takes place in each revolution. So I want to ask how does the CDI understands that in which revolution it needs to spark?

ANSWER: Hello Jaimeen,

First of all, the system shown in the picture isn't a magneto, it is an A.C. alternator which charges the battery, which drives a D.C.-C.D.I. ignition. Magnetos do not use a pulser/pick-up coil, but a high voltage coil that serves both as power source and trigger.

Now, as for the main part of your question...
Older bikes, use the "Wasted Spark System", which means that we have spark every time the piston reaches Top Dead Center, but only on compression stroke is giving ignition.
The other, on exhaust stroke, has no effect, thus is wasted.
On multi-cylinder engines, that have one ignition coil for a pair of cylinders, let's say
1 and 4 in a in-line four, when spark at #1 gives ignition, spark at #4 goes wasted and vise-versa.

Later bikes, with an ignition coil on every cylinder, have a CPS (camshaft position sensor)
along with a multi-point pulser rotor, which let's the Ignition Box "know" when and where to give a pulse/spark.

Best Regards and Greetings to lovely India,
Lazaros.  

---------- FOLLOW-UP ----------

QUESTION: Thank you so much sir for answering.
Now I want to ask that this is a single cylinder engine (Make: Honda; Model: Stunner) and it do not have any camshaft position sensor. So as per your answer this engine will create a spark every time the piston reaches TDC?
And if it happens then is there any possibility that the incoming charge from carburetor gets ignited in the suction stroke itself? Because the piston at the end of 4th stroke reaches TDC (after the exhaust stroke) and the inlet valve is made advance about 10 degrees. So the the inlet valve opens before the piston reaches TDC and the spark will be generated. Can this cause the burning of incoming charge as it contains fuel and air both?

Answer
Hello again,

On a typical motorcycle four-stroke engine, the spark is given, from 10 to 35 (or even more) degrees BTDC (Before Top Dead Center), thus the exhaust stroke is still in progress.

Furthermore, the mixture requires compression in order to ignite by a spark.

Even on the compression stroke, where it normally should, if it isn't sufficiently compressed
it will not ignite.

Only in conditions of extremely rich or lean mixture, in which we have misfiring and the exhaust gases are also rich in unburned fuel, we have ignition on the exhaust stroke and firing through the exhaust and that's only possible because its temperature is already high and no compression is required.   

Best regards,
Lazaros.