> From: tooze@vinny.cecer.army.mil (Marcus Tooze)
>
> All this talk of electronic 'trouble free, adjustment free' ignitions has got
> me interested.
>
> How does an electronic ignition work?? Does it still rely on the distributor
> turning in time with the cam/engine, or is the dist. just left on the engine
> to plug the hole?? Obviously it replaces the points somehow, but how????
I have been looking at putting one of these in my Fiat 124 (sorry-
non britcar application) and have done a whole lot of reading about ignition
systems. Here are 5 common types of ignition systems (increasing complexity) :-
* Kettering (standard) system with points, condensor and a coil
* Transistor Assisted Ignition, using standard points.
* Transistor Assisted Ignition, using electronic sensor.
* Capacitive Discharge Ignition, using standard points.
* Capacitive Discharge Ignition, using electronic sensor.
Your stock system wears out, mainly due to the current being
switched through the points (in fact, the condensor prevents it being worn
out really quickly).
A TAI system uses either the existing points, or an electronic
sensor, to trigger a set of transistors which switch the current through
the low tension side of the coil. These often have electronics to limit
the duration of the current flow out of the coil, so that it stores more
energy between firing.
A CDI system is similar to TAI, except that it stores energy
in a high voltage capacitor which is discharged into the coil. This results
in a much more powerfull spark. Initially, the systems dumped too much
energy into the coil, which resulted in the spark firing in more than one
of the plugs at once. Current systems should fix this.
Either of the types which use your existing points increase the
maintenance time between point changes, since the points will no longer
have a 300V arc across them. Wear on them does, however, still occur, due
to the distributor lobe follower (the little plastic or bakelite bit)
rubbing on the lobe. As an added bonus, in case of malfunction, the system
can easily be put back to skock.
The types of system which use electronic sensors replace the points
(or maybe the whole distributor) with a maintenance free electronic sensor.
These usually have a vane (like a windmill) which interrupts a beam of
light or magnetic sensor, which in turn triggers the electronic ignition
box.
All of these systems promise better performance. They do this by
generating a stronger and more accurately timed sparc, though you won't
notice much difference until the top half of your rev range. Compared to
a freshly tuned conventional system, maybe 5% more efficiency is to be
expected. Maintenance is, however, dramatically reduced.
More exotic systems :-
* MSD Multiple Spark systems. These generate a number of spark
pulses instead of one. Some people swear by them. In fast-burn
head designs (like BMC A series engines), they offer only a
slight decrease in unburnt HC emissions.
* Multiple coil systems. Replace the distributor with one coil
for each spark plug, and trigger these electronically. Probably
the ultimate, but quite complex.
* Magneto systems. A bit before my time. Someone else will have
to explain these.
--
Nigel Pearson, research dog's-body @ University of Technology, Sydney.
AARNet: nigel@socs.uts.edu.au Talk: (Sydney) 330 1799 or 330 1816
Hardcopy: Nigel Pearson, SOCS, UTS, PO Box 123, BROADWAY, Australia, 2007
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