> Sliped a decimal point somewhere.  There is only 10 ms / rev at

6000.  I

 > think the charts you referenced are wrong (or they may have meant
 > microseconds (us).
 >
 > Have to go with the simple approach here.  I have seen no evidence

of a

 > voltage shortage on NA or turbo rotaries so I can't get too excited

about

CD
 > ignition.  In addition, the long duration of inductive ignition is

a

benefit
 > on rotaries since the fuel-air mixture rapidly rotating by the

spark plug.

 > 'Corse I'm biased, I still have the scars from a failed CD ignition

system

 > :-)
 >
 > Tracy Crook
 > tcrook@rotaryaviation.com
 > www.rotaryaviation.com

 Could be. Would not be the first time that book was wrong.
 I'll research it some more.

 Paul Lamar

Is there any data available comparing inductive vs capacitive ignition
systems at very high cyclic rates? I have thought about running two

systems

on the leading plugs and firing one behind the other. The problems are

using

air gaps to keep the high voltage ends from shorting each other. It

gets a

bit foggy right here, but I have this feeling that there is some power
available on top with more sparks. I know that we can get a miss at

the top

if we are not running the alternator and the driver stalls the car a

few

times on the way to the grid. And this with the plugs gapped at .010".

MSDs

do not like low voltage. We run a garden tractor battery and it is

usually

enough to get in 45 minutes at full tilt. That should be between 7,500

and

9,500 RPMs or about 16 amps for both MSDs plus 6 amps for pumps. If

somebody

comes up with a way to do it, I will give it a try. The ideal (in my

opinion)

would be the amount of power required to dissolve the plugs in one

hour. Big

plug gaps (not small)
and delivery from several fully saturated coils. We got to the Moon

and back.

Why can't we do this simple thing?

I know what you mean about scars from a CD system. I had a pattern on

my left

wrist matching a cheap watch. I contemplated this while sitting beside

my

running race car with a timing light in my right hand, only seconds

away from

remembering my name. The real pain came just after that.
Lynn E. Hanover

Racing Beat loves CD systems. Jim is using one per spark plug on his
900 HP three rotor. He has a rack of six MSD systems.
A lot depends on the voltage converter design. Back in the early days we

used

a mechanical vibrator to change the 12 volts to 300 volts for our tube

type

CD ignition systems. They were prone to failure. Modern solid state

converters can

be made to work down to 5 volts or so.

Here is a screen capture of the pdf file system specs on a CDI system

designed by

John Clarke. Note the spark time in this system is 200 usec. This means
that at 6000 RPM there is still time to fire multiple sparks.
The H. Heisler chart is also shown again for time comparisons.

One thing John Clarke points out is that due to the rapid rise time
of the CDI spark it is essential to separate the spark plug wires or
you will get cross firing.

Paul Lamar

Another check on my numbers. 6000 RPM. 6000/60 seconds is 100 revs per

second.

1/100 rev per sec is .01 seconds or 10 milliseconds per rev.
Yep you are right Tracy as you very well should be since you deal with
this kind of stuff on a daily basis :-)>

10 ms is 10,000 usec.  10,000 / 360 degrees is 370 usec per degree of

e-shaft

rotation. So a 200 usec long CDI pulse from the John Clarke system
would consume only .54 degree rotation of the e-shaft.  Three to four

times

firing the MSD ign system would appear to be beneficial at 6000 RPM.
Please check my numbers again.

Here is another graphic illustrating the differences of the various types
of ignition systems.

Paul Lamar

Hmmm.. I figure it to be 27.77777 us  / degree at 6000 rpm.  That's why I
think the recovery time of a multi spark CD system has to be pretty fast to
do any real good at high rpm.  The Clark CD design will have a much faster
recovery than the old style example you showed but don't know if even that
is fast enough.  My crash & burn episode (many years ago) was due to a
failure of the SCR that discharged the capacitor (old style system).

Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com

Right again Tracy. I am really screwing up lately on my arithmetic.

Paul Lamar
 
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