All,

 My 2 cents worth of knowledge to this conversation. This is my
experience.  YMMV.

 The only time I have seen an engine have a "waste spark" is in 6 and 8
cylinder automotive engines with
 electronic ignition and coil packs.  These engines are typically fuel
injected.  The way this works is
 there are two cylinders that are connected in series through the coil
pack. These two cylinders are 180
 degreess out of phase with each other.  When one is on the compression
stroke, the other is on the exhaust
 stroke.  The one under compression has a higher resistance than the one
on the exhaust stroke, therefore
 the one under compression receives the greater amount of voltage, which
fires the fuel-air charge.  The one
 on the exhaust stroke has much less resistance, so therefore receives
very little voltage.  On the next
 turn of the crank, the roles reverse.  The relative resistances of the
contents of the cylinders, in
 conjunction with the coil pack, is just your basic resistive voltage
divider.  Very simple in concept.

 The lawn mowers I have worked on throughout the years get ignition from
a magneto. They also have points
 that close just before the magnet get near the core, and then open at
the correct time to interrupt the
 current in the primary from the magnet passing the core as it nears
it's peak. This causes the primary
 field to collapse, thereby generating a higher voltage in the
secondary.  When the magnet passes the core
 when the points aren't operated, the secondary will have a high voltage
induced into it, but it is much
 lower and rather slow in rise and fall, therefore the voltages induced
is much lower.  So much lower, that
 it may or amy not jump the spark plug gap.  Either way, it isn't a
problem.
=20

Regards,

Jim

> -------- Original Message --------
> Subject: Re: [EE]: VW engine RPM question
> From: Isaac Marino Bavaresco <isaacbavaresco@yahoo.com.br>
> Date: Mon, March 07, 2011 4:51 pm
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
>=20
>=20
> Em 7/3/2011 19:12, AK escreveu:
> > Just to clarify, it's possible the VW engine has a waste spark during
> > the exhaust cycle, this is not all that uncommon with lawnmower
> > engines.
>=20
>=20
> I have seen some engines with this "waste" spark. I think it simplifies
> the design.
> The manufacturer says it helps to burn some fuel that may have remained
> unburned. To me they are just trying to "fancy" the facts.
>=20
> I have also seen at least one engine that produces sparks at the end of
> each stroke, even at the end of the intake stroke. It seems that the
> uncompressed mixture doesn't detonate even with the spark.
> This engine doesn't need a distribution system, each cylinder has a
> spark at each half-turn of the crank-shaft.
>=20
>=20
> Isaac
>=20
>=20
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