Bob Allen Wrote:
<< I can't wait to hear the next chapter from Morrison on how a 45 degree,
single-crankpin V-twin can have a single spark hit TDC on the compression
stroke of one cylinder while the other fires at TDC on the exhaust stroke>>
And the answer it --- it doesn't. Rick is correct in saying that Harley's
use the "waste spark" principle in their ignition. Bob is also correct in
stating that the two power pulses from a Harley are not 360 degrees apart.
They are actually 315/405 since there is a 45 degree offset between TDC.
To make this work, Harley times the coil such that the good spark fires at
the right time for each cylinder but the waste spark fires 45 degrees ways
away from TDC on the exhaust stroke- early on one cylinder and late on the
other. The 45 degrees early ignition pulse does no harm although if you
are running real rich it could cause a backfire. The 45 degree late pulse
fires well into the intake cycle. It make be responsible for some of the
"chuffing" Harley's do at low rpm. Several aftermarket firms offer "single
fire" ignitions to eliminate the waste spark and they claim more power and
smoother operation as benefits.
Personally I have always been a little uncomfortable with the whole "waste
spark" concept. Firing the plug with the intake valve open just does not
sound like a good idea to me. Modern emission controlled engines have very
little valve overlap but still it makes me nervous.
Also, one should not lump all "crank fired" ignitions in one group. While
2 cylinder per coil, no distributor approach is the one favored be Detroit,
it is not the only way to skin a cat. Where the precision is gain is by
using the crankshaft motion directly for position reference. All other
design considerations are cost vs benefit. Many drag racers, who as a
group consider the proper delivery of spark to be very important, use a
crank sensor and single coil plus a distributor that only chooses where the
spark goes, not how it is timed. This system has more wear parts
(distributor, cap, rotor) but is no less precise than other systems. Other
systems use a coil for each plug. This is probably the ultimate since it
gets rid of the waste spark concerns plus it gives each coil the maximum
amount of time between firings to recover during high rpm operation.
Regards,
Bill Eastman
61 MGA with stock ignition and, as such, absolutely no extra spark to waste
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