Alpine clan,
Along the same lines of ignition trivia, here is some more.
Most of todays vehicles use one coil to differentially drive
two cyls at a time. This cleans the non-powered plug every
other revolution, These type systems can run for over
100,000 miles without even a change of plugs.
These systems (DIS) do not use a distributor, and the spark
current goes from coil to plug to block to other plug to the other
side of the coil. The coil secondary is isolated from the chassis.
On some engines, the spark voltage is monitered, as an
instantaneous indication of mixture quality and detonation.
Such vehicles like the newer SAABs have much better
control of the entire engine, and can run alot more
advance and leaner mixtures than can more conventional
engines.
The ionization voltage across the plug is much that of a
zerer diode in electronics, where the 30kv to 50kv potential
is forced to drop down to the voltage that the combution
will allow. When the load is higher, the spark voltage reduces
to whatever the ionization voltage is to start the spark.
At lower plug voltages, the current is somewhat higher,
and the overall energy is about the same.
Todays coils produce around 200 milla joules of
potential spark energy.
The plug voltage is typically less than 10KV, but under
some conditions, the voltage needs to be very high.
Such conditions are during cruising, where a higher manifold
vacuum and low combustion pressures dont lend themselvs
to lean AF ratios. the ionization voltage is higher, but the spark
current is relatively low.
The remainder of the coils potential is dropped across the
plug wires and the output impedance of the coil itself.
Todays ignitions are intollerant to solid wires, and non-resistive
spark plugs. The amount of energy available in the coils is
so much higher that the resultant RF wreaks havoc on all the
local electronics, and the higher current loads on the coil result
in higher coil temps and more control module killing inductive
kickback.
High performance engines seldom need more voltage, but
the richer mixtures require more energy or current.
Since the pug voltage goes to whatever is needed to start
and sustain combustion, the only way to get more energy
to the air fuel, is more current, or multiple spark per combustion
event. At low RPMs, MSD "multi spark" systems can make
more power only if a conventional system has poor
combustion efficiency. Big cams, high compression ECT
lend themselves well to MSD ignitions. At higher RPMs,
MSD systems can not get enough energy into the coil before
the next spark cycle and thus have fewer and fewer sparks
per cycle, down to the point of only having a single spark
per cyl cycle. At this point, you have lost any sort of advantage
in running an MSD ignition.
CD ignitions used to be very popular as replacements to
points. These systems are costly, but have the advantage
of running a constant energy spark all the time. These
systems have a different basic opperation in that the coil
is driven off a generated high voltage supply instead of
the field collapse of the primary as in a kettering type
ignition. This gives the advantage of a faster rise on
the spark voltage, and less mechanical advance is
needed.
Thats about all I know about spark.
Jarrid Gross
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