Fw: Types of Ignitions

[Bruce]

Since the recent post about CDIs, and having just posted this on another
list, it seems appropriate to mention some things about the different
igniton systems, ie the good and back of the various ones.
Bruce



From: "Bruce" <nacelp at bright.net>
Subject: Types of Ignitions
 Again, this is how I see things.

 You have lots of combinations.
   Inductive, this goes back to the days of point systems, and to the MY
2002
 cars.  The spark energy is *stored* in the coil and released on command.
   CDI (capacitive discharge ignitions), these generate a primary voltage in
 the range of 400 volts, to fire the coil rather then the 14 volts of an
 inductive system.

 Upsides:
   Inductive are easy to design, and relatively cheap.  They generate
reserve
 voltage usually high enough to fire the plug all the time, and do it with a
 fair amount of amperage, which generates a *long* (timewise) spark.
   CDI, if you look at a coil they generally have a 100:1 ratio.  So on a
14v
 system that means you can only generate 1,400 volts.  Trouble is they
 generate 30,000 volts.  So how does this happen?.  When the *points* first
 open and the magnetic field collapses, there isn't enough voltage to bridge
 any gaps.  So the 1,400 volts of the secondary collapes again thru the
 primary, this time, generating a much higher secondary voltage as the
 primary again collapeses, to then the secondary is eventually high enough
to
 cross all the gap(s) and generate a spark.   Now, with the CDI, and a 400
 volt primary, when you figure that with a 100:1 coil, you can see that it
 instantly (well almost) can make 40,000 volts on the secondary side to
 strike a spark.   SO THAT HAS TO BE GOOD, RIGHT, well not really. In this
 being able to do it instantly means that the spark only lasts for an
 instant, the most often used strategy to over come this, is using several
 sparks or rather coil firings to get the reaction going in a proper manner.
 In a high turbulence chamber, this might even be a good thing, but, your
 still dialing with HAVING to have seveal sparks.

 Good Misses:
   A good miss?..........
   Yep,  there are a GOOD SIGN.
   Ya, right, about now you've probabl thought I am as crazy as I sound.
 But, bear with me.  A miss can be a symptom of an engine problem.  When
 would you like to find a problem, too early?, or too late?.   While a miss
 *early* miss might be a PITB to diagnose, long term it can be cheaper then
 having such a small miss that it goes unattended, and then you find a burn
 valve, rather then tight clearance one.   you can also get into weird
 circumstances, where you wind up tuning for compression ignition at WOT
 rather then spark ignition.  Now you're probaby thinking I'm really gone.
 But, you can add enough fuel and timing, to min detonation, and the chamber
 is ready to expode on it's own, and the spark is just used as the straw to
 break the camels back.  Doubt me?, as an extreme, look at a top fuel
 dragster, and how they do things.   Every note some of the old chip designs
 used a ton of fuel at WOT?.   Just one way around a limited igintion
system.

 Controlling the min firing voltage:
   Why even have a min firing voltage?.
   Well, for openers, it's about obsolete, but I'm on a roll so bare with
me.
 Until lately cars used a distributor, to spread the spark aound, meaning
 there was an air gap and spark that had to build to cross that gap before
 getting to the plug.   While you had to have a gap there, folks figured out
 by controlling that gap they could set the min firing voltage so like on a
 cold day with full choke the plug would fire rather then foul since it HAD
 to use the min firing voltage anyway.  Now thur the advent of nice
 electronics, they can control the min firing voltage with being able to
 ***instantly*** turn off the coil.   This being able to instantly turn off
 the coil means that you can get the *ringing* of the coil oscillations done
 much faster, and get the voltage built up in the secondary side much
faster.

 Misc stuff:
    With a DIS, its the capacitance of the none firing plug sets the max
 voltage available for the firing plug.  Disturb that, and you rob the
firing
 plug of it's reserve.   Some will want to argue this point, but like I said
 it a matter of robbing the firing plug of it's reserve, the engine will run
 with a 0 gap plug, but it does effect things.
    The longer the secondary plug wires, the more prone to failure they are.
    Ain't no such thing as too much spark energy.

 Now, that you have a fair amount of info., you can look at points, HEI,
DIS,
 CDI, Inductive, and CNP in a whole new way.

 Hope ya'll find this amusing, or informative.
 Bruce
    87 GN, just so happens to have an eDIST (CNP).


----- End of forwarded message from owner-diy_efi at diy-efi.org -----
----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org