[Diy_efi] false myths

[Adam Wade]

--- 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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