DIS Explainations

[Gregory A. Parmer]

On Thu, 19 Mar 1998, Bruce Plecan wrote:
> On these waste spark ignitions I've heard all kinds of explainations
> for how they work, and the best logical one I've heard is that the
> one cylinder at overlap doesn't fire because it's still surrounded
> by exhaust gases.  But, as you retard the timing you get to the
> stage that there is enought intake valve opening that it can light, that
> cylinders mixture.  Soo given enough cam timing, or boost then you
> can have a problem.  Or run a 0 overlap cam if running a turbo/
> supercharger, to avoid this.
>   For there to be a spark there first has to be a path of ionized gas
> right???..  Or can you have a spark in  an environment of inert gas?? Bruce
> Slippery slopes, are those the sides of a Cone Shaped Hat?


OK, I'll bite...
The "wasted" spark only occurs on the cylinders that are exactly one
revolution out of sync. On a chebby v-8 w/ a firing order of 18436572
plugs 1&6, 8&5, 4&7, and 3&2 can fire at the same time (damn I hope
that's the right sequence :)  ).
                             TDC       BDC         TDC
4 cycle =        Compression  |  Power  |  Exhaust  |  Intake.
(piston travel)    (up)         (down)      (up)       (down)
                   BTDC          ATDC       BTDC        ATDC
(spark)                     S                      S(wasted)

Say the coil fires at 15deg BTDC. One crankshaft revolution 
later it is firing again at 15deg BTDC on  *exhaust*. It's hard 
enough to ignite a proper mixture. A mostly burned mixture of 
exhaust fumes ain't gonna do diddly. Look at it this way--as recently
as one stroke ago it was in full flame and the fire has since
gone out. What good is a tiny spark in this atmosphere?  The 
area of valve overlap happens mostly on the Intake stroke well 
after the point of spark, even after TDC(?), as I understand it. 
You've gotta get pretty far off before it's a problem. 

BTW--the spark is still there...is it allowable to use 
"inert gas" in a dialog about an internal combustion engine?  :)

Donning my flameproof conehat,
-greg