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Multi cylinder ignition systems

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Vixen:
Peter,

I am not sure you have got all that Hall trigger stuff quite right.

Lets go back to first principles. The image below represents the magnetic field surrounding a simple bar magnet (dipole). Note: the magnetic field is normal (vertical) to the two end faces (poles) of the magnet, but only in a very small part near the centre of that face. Everywhere else, the magnetic field is inclined at an angle. A Hall effect sensor acts as an electronic switch. It will switch ON, ONLY when an applied magnetic field is a) strong enough  b) normal (vertical) to the face of the sensor c) of the correct magnetic polarity.  One face of a Hall effect switch will only respond to the North pole of a magnet. Flip the Hall sensor over and the reverse face will only respond the the South Pole. Therefore the Hall sensor and magnet's orientation is vital.





We know it is possible to make a Hall effect multi cylinder ignition trigger systems by rotating an appropriate number of small rare earth past a single Hall sensor. Alternatively it is possible to use a single static magnet and Hall sensor with a rotating iron/ steel shutter wheel between. Both will 'sorta' work, but the shutter when has the potential for more precise timing. It seems unreasonable to assume that the 8 'cheapo' rare earth magnets in the rotating disc will all have identical magnetic fields. Some timing dither is to be expected. Which method you use depends on the design restraints and space available for your ignition trigger. The rotating disc could perhaps be smaller, or fit the available space better than the shutter disc. Sometimes it's the other way round.

One advantage of the shutter wheel (as George pointed out) is the ability to control the mark/space ratio (blade to gap ratio) of the output pulse from the Hall effect sensor. The mark/space ratio can be used to alter the 'dwell angle' of your ignition device. The 'dwell angle' gives time, especially at higher RPMs, to fully charge the capacitor in a CDI unit, or time for the field to fully build up in an ignition coil.  However, you need to be cautious. Coil systems fire when the points open i.e. the Hall sensor switches state. Commercial CDI units can fire either when the Hall switches On or when it switches to the OFF state. You never know which until you buy and test one. It's generally not a problem, as you can usually alter the angle where the Hall sensor triggers to suit your chosen CDI or coil.

When it comes to choosing the mark/ space ratio for your shutter wheel, you need to consider how and when the chosen coil or CDI fires. One will favor a longer mark to space ratio, whereas the another will favor a shorter mark and a longer space to give the desired 'dwell angle'. It therefore pays to optimise your design for ONE type of ignition system and stick with it. Swapping between ignition systems and hoping, may not always produce the best results.

Cheers

Mike

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