WB Instruments

[Brian L Massey]

On Fri, 13 Jul 2001 08:37:01 -0400 "Bruce" <nacelp at bright.net> writes:
> 
> You want to *read* the combustion exhaust gases.  The other stuff is 
> from
> overlap, ie when both the valves are open, poor sealing valves etc.. 
>  There
> is a time between exhuast events when the flow tries to slow way 
> down,  yet
> the column of moving air is trying to keep it moving so, there are 
> low
> pressure areas, and if the O2 refeneces to atmosphere then that will 
> affect
> things.  What the *other* stuff is doesn't matter one whit to what 
> you as a
> tuner are trying to do.  It's the peak of the pulse you want to 
> concentrate
> on.

Thanks Bruce, I was hoping to hear some theories on your readings and
peeking effect to get into it a little bit more. The two ideas you say
above are valve overlap and pressure effects. Its from my mech bent
again, but either of these theories I wonder about. The valve overlap is
at the end of the blowdown, and relative to a whole four cycles, isn't
very long, gas transport wise anyway. If that's what is causing the
peaks, then I would think you would want to *ignore* those. Relative to
the overall background afr in the exhaust pipe between blowdowns, I can't
imagine how overlap would explain the afr signal *peaks* being the thing
you want to concentrate on.

The other theory about there being a pressure effect would also call into
question whether the actual peak you see is really the afr value your
looking for. From my gas dynamics courses a *long* time ago, IIRC the
pressure wave will hit the sensor way before the mass transport brings
the actual exhaust gas plug to the sensor. That's because the waves
travel much faster than the gas itself. (like waves in the ocean, is the
analogy that's used...the waves travel way faster than the actual water
is transported). Plus there are lots of pressure waves occuring from
reflections in runner to header lengths, etc., which is how tuned exhaust
systems work, so if your seeing significant pressure effects, they could
in theory really mess up the works of picking out the *real* afr.

Im not dismissing either idea, just thinking out loud about them, and
curious to hear any more inputs on this. One thing I guess I *am*
crossing off is the idea that dilution from outside air could be
involved. I would think at least at any reasonalbe rpm well above idle,
that the gas flow out of the engine would completely overwhelm any small
air leaks there might be in the exhaust piping itself. Would you agree?
Tnx.

Brian
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