From:Paul <rotaryeng@earthlink.net>
05/04/1999 18:40
Subject: Long spark durations......
To: zz <rotaryeng@earthlink.net>
Tracy Crook wrote:
bryan.wilkinson wrote:
Paul ,
Lookd over this old article on off the peg VW cconversions { beefier
crankshaft mainly & shimming of cylinders } can't remember company
name
but from an old 1991 magazine.
Anyway - it seems magneto's give a longer duration spark than EI - so
how do you get this with electronic ignition ?:^/ I would have thought
this would be standard stuff these days for emissions purposes anyway
.......
Bryan
Its called Multiple Spark capacitor Discharge.
There is a MSD brand of electronic ignition system here in the states
that is very popular.
Paul Lamar
At the risk of injuring some sacred cows I'll put in my $.02 here.
True, MSD (multiple spark discharge) is the only way to get long duration
with CD type ignition and MSD (the company) makes a fine one. This is as
compared to "regular" CD which gives a very short duration spark. The
price
you pay is the weight of this system and high power consumption as well
as
a pretty hefty price.
It should be pointed out that electronic ignition is not synonymous with
CD.
Most all cars are equipped with "EI" these days but this just means that
the
timing and switching of the coils is done with electronics instead of
mechanical points.
Inductive Coil ignition (as opposed to CD) inherently gives a nice long
duration spark which is important when doing lean burn combustion (what
you
should be doing 95% of the time in an airplane). Magnetos give a longer
spark than straight CD but usually less than inductive coil ignition.
Another factor in ignition is energy in the spark. CD (especially msd
type)
is potentially the highest here but there are exceptions. The Ducati CD
used on Rotax engines is so weak that its hard to see the spark in
daylight.
I know people who have spent days trouble shooting this ignition system
(because they couldn't see the spark) when there was nothing wrong with
it.
The energy in Inductive coil & magneto type systems depends on the
particular coil involved. Magnetos are usually at a disadvantage here
because of the small coil size. The 86 - 92 Mazda coils are relatively
high energy type (and this is reflected in their weight).
My advice to those using the Mazda rotary for aircraft use is to use the
stock coils unless you have a very compelling reason not to. In the RPM
ranges we are interested in (about 6500 rpm max), there is very little
advantage to using anything else. Those who claim that there is a world
of
difference when using something else are either talking about operation at
the far end of the performance spectrum or they are dreaming. Gains due
to
ignition changes are purely due to improvements in efficiency (which is
wonderful) so don't you think Mazda would have done this is the
improvements
were this dramatic? They were so desperate for efficiency gains during
the
design of the second generation RX7 (1986 - 1992) that the project was
going
to be cancelled if they didn't find a way to reduce the car weight by ONE
GRAM per design team member. They sweat bullets and spend millions to do
that. Trust me, if a better ignition system would have done the job, they
would have done that instead.
True, there IS a big difference if you are going to run at 9000 rpm
(like
the Racing Beat 3 rotor project) but the average airplane builder would be
wasting their time (& money) going on a quest for the ultimate ignition
system. But, (as Dennis Miller says) that's just my opinion, I could be
wrong :-)
Tracy Crook
rws@altavista.net
Web site: http://www.rotaryaviation.com