DIS Explainations

[Clive Apps Techno-Logicals 416 510 0020]

> 
> 
> > 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.
> 
> 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

on radical cam profiles there is enough intake flow into the cylinder
at 345^ ATDC that the extra spark can ignite the mixture under some
heavy load, low RPM, and high heat conditions
this is especially bad if the flame front travels up the intake and 
blows the top off the engine

many GMs now use a simlar system
with the factory cams there is not enough combutible gas
in the cylinder this early to be a problem
with a high RPM cam there can be
it should not be a problem for most applications

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

exhaust gases are pretty close to inert in a properly tuned engine
CO2 and H2O should be mostly what is left

Clive