[Diy_efi] false myths

gary gas-
Sun Apr 10 02:54:10 UTC 2005


Adam, I commend you on taking the time to go over (some of) 
the material JD wrote, as well as reading writings of the aerospace
engineer, the driving force behind GAMI.  From your perspective, 
sure sounds like you were wasting your time reading it.  I've read 
these articles twice, and learned some additional insight the second 
time around.  What was disliked by me, was the bits, like pieces 
of a puzzle, that were scattered about in several articles.  Have no 
idea if this was by design.  Anyway, not sure of any other engineers 
with the combustion chamber test/logging (in real time) equipment 
and experience, that GAMI has, let alone willing to make available 
to the general public, other info, if indeed a question did surface 
as to validity of GAMI's work.       
>
> ..... except for your failed analogy between a gas stove burner 
> and a combustion chamber regarding the molecules being "too
> far apart" to produce combustion....
>
Didn't EVER state that in that together, in that context, Adam, so 
I won't have you waste your time looking for it in my posts.

> >> ....and I know I haven't read anything about there being 
> >> no such thing as lean misfire, or your description of why 
> >> there is incomplete combustion from lean operation.  
> 
> > Start with the 'Back to the Future' series,
> 
> Read all three parts.  Nothing in there on combustion
> chamber events or how they occur.
>
What about the lean misfire?  Any reference to that?

It's a fair guess the author (professional pilot) was tutored (or 
had articles read) by the aerospace engineer noted above.  If
anything appeared grossly misleading or false, it would have 
been addressed.  So..... 

As far as violating any gas laws are concerned, not sure of any 
of JD's wording or 'assumptions' as you imply, (that were not 
part of my original posts BTW), that he printed, that would be 
any less accurate than anything believed or trusted to be fact, 
up to the time his articles were written.  Yes, 'Perfect Gas laws'
Nothin' like perfection.  Even Einstein revised some of the 
indisputable laws he established.  However, that is NOT to 
state I personally do not acknowledge them.  You are using 
some previous interpretation as a basis on which to 'grade' 
JD's work.  I've no problem with that.  You are entitled to believe, 
or acknowledge any persons work you wish.  Afterall, from a 
true engineer's POV, JD IS but a 'layman' himself.  However, 
my original post IIRC, was in regards to lean misfire, stoich as 
being the high point of EGT, and acknowledging a given AFR 
that causes the quickest flame travel.  Those are the findings of 
engineers with actual 'real time' test data.  I'll respect their 
findings, as accurate.  If you prefer to kill the messenger, so 
be it.  Or if you prefer to dispute the articles of JD, and/or 
his interpretation of anything, again, I don't have an issue with 
that either.  Actually, I do have a few related questions I could 
ask GAMI.  Actually, plan to do 'Oshkosh' this year, then I
could ask in person.  :)  Just a bit up the road for me.  Anything 
in the way of questions you'd like addressed, straight from the 
source?  As for JD, sure he can speak for himself.  BTW,
you referenced in one of your posts, the dialog you had with
JD, regarding his articles.  Did he answer any of your concerns?   

GAS     

----- Original Message ----- 
From: "Adam Wade" <espresso_doppio at yahoo.com>
To: <diy_efi at diy-efi.org>
Sent: Saturday, April 09, 2005 7:57 PM
Subject: Re: [Diy_efi] false myths


> --- gary <gas- at charter.net> wrote:
> 
> >> I've seen him talk extensively about spark advance,
> >> even fueling between cylinders, running LOP,
> >> measuring EGTs, and where engines have the best
> >> margin of safety from engine damage as well as
> >> where they run at best fuel economy. 
> 
> >> I don't recall him having addressed anything about
> >> the actual combustion process, and I know I haven't
> >> read anything about there being no such thing as
> >> lean misfire, or your description of why there is
> >> incomplete combustion from lean operation.  
> 
> > Start with the 'Back to the Future' series,
> 
> Read all three parts.  Nothing in there on combustion
> chamber events or how they occur.
> 
> > then reread #18,
> 
> Here he describes, in layman's terms, flame
> propagation in an infinite volume container with a
> totally non-homogenous mixture (the earth's
> atmosphere), with the burner of a gas stove.  If we
> were talking about a stratified-charge aircraft
> engine, then this might be generally applicable;
> however, to my knowledge, there are no
> stratified-charge aircraft engines, and the principle
> cannot be applied to an enclosed combustion chamber
> with a homogenous mixture in any way.
> 
> Further, his claim of "the fire leaping from molecule
> to molecule" is completely inaccurate, and even if it
> were, it could not be applied from an infinite volume
> container (the atmosphere) to an enclosed container (a
> combustion chamber), due to the basics of the Perfect
> Gas laws.  I'd have thought that was patently obvious
> to you.
> 
> His talk of flame propagation in the movies might hold
> true for the common range of atmospheric pressures and
> temperatures, and can be analogized somewhat for any
> other narrow range of pressure and temperatures, but
> neither pressure or temperature in an ICE during a
> complete operational cycle are anywhere close to so
> constant as those found in the earth's atmosphere. 
> This is where the analogy fails.  Even under the
> fairly consistent conditions from cycle to cycle in
> steady-state operation of a typical ICE, you can end
> up with detonation, or a failure to run reliably, with
> an identical mixture to that used when the engine is
> running strongly.  So the idea that flame propagation
> and speed of combustion vary based on AFR is
> fallacious.  It's whether there is enough heat
> generated at the flame front to propagate the flame
> steadily without the temperature of the end gas rising
> to the point of combustion (and therefore detonation)
> that is our goal in operating an ICE; mixture is, at
> best, a single part of that very complex reaction.
> 
> Aside from that, he has one mistaken reference to the
> "fuel molecules being close enough together" to
> continue the combustion process (which is totally
> wrong; it's the localized heat where there is fuel and
> oxygen that causes combustion, not proximity to
> anything else.  While there may be very little
> additional heat created before the combustion chamber
> volume increases from sparking a lean mixture, it's
> the lack of enough heat to continue the combustion
> chain reaction that prevents complete combustion of
> the available mixture, not proximity.  In fact, later
> on when he talks about detonation, he echoes the
> principle I just mentioned.  A closer examination on
> his part would have ferreted out that inaccuracy,
> since it's contradictory in his writings.
> 
> > #33,
> 
> He gets most of it right in this one.  His bits on
> flame front propagation are right on.
> 
> His final comments on detonation are incorrect,
> though.  Detonation quality changes based on how
> quickly the CC volume is increasing, the combustion
> temperature of the various bits of remaining mixture
> (which may or may not be homogenous), and how much
> mixture remains at the point of detonation.  If the
> volume increases rapidly enough, detonation can even
> be stopped once initiated; the rate of increase can
> control the quality and severity of detonation, but
> what happens during detonation is the same; the
> remaining mixture reaches the combustion point before
> the flame kernel reaches the mixture in question.  The
> detonation may be "spread out" in time slightly by
> variations in the mixture of the end gas, and
> therefore the auto-ignition temperature.  But again,
> it's the same exact principle, with some slight
> variations in action.
> 
> > and #43.
> 
> Oddly, he gets more of it right in this one (another
> inconsistency!)  Again, though, in his discussion of
> mixture, he talks only about conditions during the
> limited time available for combustion, which cannot be
> extrapolated into general theory.  There is not enough
> time for more complete combustion in stoichiometric
> mixtures before pressure and temperature int he
> combustion chamber drop to well below ignition
> temperature (and he doesn't mention that there will be
> a boundary layer of mixture against the cylinder wall
> that won't ignite due to the cooling of the mixture
> via contact with the cylinder wall, either, which is
> typically also a large contributor to unburned O2 and
> HC in the end gas; oddly, he mentions it later when
> explaining why the engine doesn't melt, but forgets
> about it when talking about unburned fuel and air!).
> 
> He also gets it wrong in saying that the cylinder
> walls donate heat to the mixture.  It's the
> compression that heats the mixture to where the spark
> ignites it and creates a stable flame front; the
> cylinder walls tend to ROB heat from the mixture,
> which is part of the reason a cold engine is harder to
> start and runs more roughly until it heats up.
> 
> Also, there is only one flame front, unless there is
> another point of ignition.
> 
> And he gets it wrong again in claiming that "some of
> the pockets of A/F mixture are a hair-trigger away
> from exploding."  Some mixtures will create a
> sustainable flame front at a certain temp and
> pressure, and some require higher temps or pressures
> to create such; the same goes for the
> "auto-combustion" point, where combustion is
> uncontrolled and closer to instantaneous.  It's about
> the temp required to make the mixture burn, or burn
> more rapidly, not the mixture itself being either
> "absolutely explosive" or "not explosive at all". 
> It's exactly analogous to the point of flash-over in a
> house fire, where the remaining things in the room
> auto-combust when the proper combination of fuel,
> oxidizer, and temperature are combined.
> 
> Oddly, near the end of this article, he outright
> states that mixture controls the speed, temp and
> pressure of combustion -- the very points he leaves
> out in the rest of the article!  He should have
> applied this to the pockets of end gas in the
> detonation discussion as well.
> 
> It's a matter of debate at this point whether the
> shock waves "scrub" the boundary layer during
> detonation, as he states, or whether there is simply
> enough heat and pressure to light off some or all of
> the mixture forming that boundary layer.  I'd be
> interested in seeing research to that end, if anyone
> has any. 
> 
> I hadn't read his bit on detonation yet.  Looks like I
> have more reading to do, and an email to send off to
> him as well regarding some of the errors in applying
> specific results to general principles.  But
> regardless, I don't see anything in any of those
> articles to back what you were claiming, except for
> your failed analogy between a gas stove burner and a
> combustion chamber regarding the molecules being "too
> far apart" to produce combustion; rather, there was
> not enough temperature and/or pressure as the piston
> fell to allow the flame front to continue propagating.
>  The mixture would happily burn if the temperature was
> raised, or pressure was increased/maintained, with no
> change in the spacing of fuel molecules in a given
> volume of air.






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