MAKING SPARKS FLY
Thedevelopmentof theignitionsystemh, owit works,and a few maintenancetips. Jacquelinebickerstaf(faka Pub)explainsthe fizzicksbehindsparkingperformance. Part Four:flywheelmagnetoswith externalht coils(includingenergytransferand CD/) and Lucassrrotatingmagn
The development of the ignition system, how it works, and a few maintenance tips.
Jacqueline Bickerstaff (aka Pub)expla i ns the fizzicks behind sparking performance. Part Four: Flywheel magnetos with external HT
coils (including Energy Transfer and CDI) and
Lucassrrotating magnet magnetos
Following on from last time ...
Not all flywheel magnetos overwind their Low Tension ignition coils with the High
Tension winding. Another option eventually appeared, using the'ignition'winding to power an external Htcoil .The most obvious way was to connect as if it was a battery, and this was incorporated into the Villiers 225cc 1H ignition system possibly because they chose to use the'ignition' coil also for trickle charging. As suggested in part 1, it is a fairly obvious move.
However, from personal experience it is apparent that an ordinary DC ignition coil does not work very well, and Villiers developed the M2533 coil especially for it.
This system was not replicated for the later 250cc 2H single and 2Ttwins, which instead adopted an 'Energy Transfer' alternative. The generator, still referred to asaflywheel magneto on account of its big and heavy bronze wheel, dedicated two windings to lighting, and one or two to ignition, one per cylinder .The difference is that the HT coils are wired in parallel, not series,with the contact points (seethe cut down circuit of the 2T ignition). So how, readers may wonder, are the sparks generated with this coil carrying no current at the point of contact opening, and so no flux collapse, etc? However, the generator's ignition coil has meanwhile been producing current, shorted through the points , and that is what gets interrupted.
As mentioned in the basic principles explained in part 1, a coil wound onto a magnetic material gives the electrons I current a sort of inertia. Normally, when points open and current has to stop, it is that inertia that produces high primary and secondary voltages. But it was also pointed out that the reverse is also true, starting current flowing suddenly is equally hard and generates similar big voltages.
Returning to Energy Transfer layouts, whilst the points are closed the generator coil current flows through the points straight to earth, but the ignition coil current remains zero because it is shorted by them. Everything changes when the points open, and it is a case of the irresistible force meeting the immovable object. With nowhere else for current to go, the coil currents must equalise in a hurry one reducing and the other increasing. Both the generator and HT coils respond to these sudden enforced current changes with the
usual voltage spike, increased in the case of the HT coil by its turns ratio . However, for this system to work the generator and ignition coils do need to be reasonably matched for their purposes which is why fitting a standard battery ignition type coil usually does not work very well , it just overwhelms the generator coil . Fortunately the system was not unique to obsolete old Villiers engines, and was adopted for a lot of Continental and Japanese lightweights and so suitable coils are readily available on ebay (usually titled 'AC ignition').
Failed self contained flywheel magnetos can be reclaimed this way, when rewinding their ignition coil is impossible or unaffordable. Remove the Htwinding (which may have leaks or shorts if it does not work) , and bring out the Ltwire for connection in the Etmanner to an external coil from ebay.if the low tension winding is itself faulty, then with care these can be rewound by hand (but insulate well, and soak in a varnish to stabilise the winding against vibration).
Although this way of operating flywheel magnetos istechnically Energy Transfer,the term is more usually reserved for the infamous Lucasenergy transfer system. In principle this can be regarded asa rotating magnet magneto using energy transfer technique, or as an inside out flywheel magneto operating similarly . Sowhy the awful reputation?
Firstly, of course, the generator itself was optimised as an alternator,➤
not a spark maker, and that affects the poles and polepiece shapes,which on ignition instruments are sometimes optimised for rapid flux switching and voltage peaks not overall generated energy. But secondly, unlike most flywheel magnetos which operate on fixed ignition two strokes, the Lucas Etwas used on four strokes that really want advance retard to optimise both starting and running. A Lucas alternator, with 6 magnets and running at engine speed, produces AC half cycles every 60 degrees of engine rotation, only the middle bit of that being useful.
There is a further complication illustrated earlier, that as engine revs rise, so the optimum point for spark making delays,just asany advance retard mechanism moves it the other way. Flywheel magnetos do suffer these effects, but the manufacturers have done their best to find a general optimum for the fixed ignition, and even then service sheets show they did not always get it right, and made stepped keys or alternative cams.
On those two strokes, running fixed ignition, the cams were usually fixed on the mainshaft, whereas most four strokes drive their cams at half engine speed through gears or chains and that provides backlash, tolerances, and owner timing intervention, all to interfere with the 'magnetic timing'. It often did not end well, and the bikes using Etgot a bad reputation for starting, or running, or even both. Only the determined 'originality ' owner should replicate it on a rebuild, asa conventional alternator and
a 12volt conversion will serve much better (with Lucas2mc capacitor to replace the battery if it really eannot be tolerated).
There is a further class of'flywheel magneto ' that might be ment ioned, although it is moving beyond the traditional British classicera. Soon after the Japanesebroke into the British market, electronics and electronic ignitions reached maturity. The old inductive discharge and points system had always needed sparking at the points suppressing, and a relatively slow spark voltage rise resulted. With electronic switching
the speed could be significantly increased, giving lesstime for fouled plugs to leak the energy away before a spark established itself.
Capacitive Discharge Ignition (Cdi)was the way and became commonplace. One way or another a capacitor is charged to a high voltage (100 400 volts according to the system) then suddenly discharged through the HT ignition coil. One way of implementing this system, adopted by some Japanese makers, involved generating the high voltage directly in one of the alternator coils. A further coil, or coils ('pulse coils') then provided the signal to spark and incorporated non mechanical magnetic advance in the process. Since these are all part of the generator, they might be considered to be a modern version of the 'flywheel magneto'.
The high voltage coil requires a higher number of turns than usual for lighting purposes, and better insulation to suit. The pulse coils do very little work and may be quite small. In the system illustrated, the capacitor (much larger in value than the normal ones) is first charged up in series with the HT coil. There are two pulse coils, with three connections to the thyristor switch, providing one short late pulse, one short early pulse, and one long pulse, whose amplitude rises with generator speed . The spark is fired when the thyristor gate voltage passesa threshold . At low speed this occurs with the late short pulse, but at higher speeds the wide pulse reaches the threshold earlier and triggers the spark, ie. advancing it. The purpose of the short negative pulse is to subtract from the wide one , so that the threshold can never be triggered earlier (over advance).
Some older Japanese models did have a reputation for stator winding failures early in their lives, and Htcoils do occasionally fail too unfortunately in a gradual way leading to misfires and poor starting (test them against similar numbers as magneto coils) . Otherwise ignition troubles are likely to be wiring faults which are fixable, or electronic faults which are probably not except by module replacement.
Tocomplete the rotating magnet magnetos, the Lucas Srseries should be covered. These were made in platform fitting format, although adapters for the common triangular fitting were also available, but that permutation made them significantly more bulky than a conventional rotating armature type . As seen in the diagram, the single cylinder type required no brushes at all, but the adaption to twins involved a rotor arm and distributor inside the Bakelite end cover (but rotor arms should not make rubbing contact so should not wear). So everything isaccessible,and testable too, but rarely goes wrong anyway.
A racing version (Sr r)was made and fitted to some Manx Nortons, and 4 cylinder options too (Sr4,some with impulse starter drives for stationary engines, etc). The Srought to be completely trouble free, but manuals warn against removing the contact breaker cam from its shaft which may not be keyed and so will lose the 'magnetic timing; leaving the owner with a problem establishing the optimum place to refit it.
Although that is the end of rotating armature and rotating magnet types, it has not actually completely exhausted the magneto ignition possibilities . There may be US readers with Dixie magnetos, or Rudge enthusiasts still using their Maglitas. These are something different again but do you even want to know? Re