Lean Burn & A/F ratio

[Phil Lamovie]

Hi All,

Lean burn is more of a catch cry than a reality for lots of engines.

The real devil is chaos.  (no I don't mean the one that '86 & 99 had
to deal
with) this one is a whole lot less predictable.

When we talk of air fuel ration we do it in mass terms and that should
be a
clue as to what we are certain of and what not.

Lets take a cup of coffee for example. We are certain we added hot
water
a spoon of coffee and two sugars perhaps we even have accuracy to the
microgram. What can we assume it tastes like. Sweet ? Some of you
conspiracy buffs will be thinking AH !! you didn't say if it was
stirred.

Worse than that in the case of A/F mixing it really should be
expressed as
Air/Fuel Vapor ratio. Raw fuel is completely incombustible. Only the
vapor can ignite. As we spray our fuel into the manifold we have an
extremely small period of time in which to stir our coffee so to
speak.

Even at idle we have at most 70 ms. This time factor is critical in
all engine
management issues. Some of the droplets that leave the injector are
several
hundred times larger than others. If we took several samples of the
mixture at
any time after the intake valve closes  we would inevitably get
different answers.

The chaotic nature of droplet formation and vaporization and then
mixing with
the incoming air means that there are always parts of the physical
space that
are richer and leaner.

A good example here would be to drop a thousand ping pong balls from
a good height above a basketball court. Now after they settle we
measure the
distance between all adjacent balls. What do we imagine the difference
will
be between the closest pair and the farthest pair.

Now think of two tiny drops side by side that vaporize during the
compression
stroke. They will give rise to a local A/F of double what a single
drop might.

Now think of the empty space that was left when the two drops were
side by side.

Now when you talk of A/F of 14.7 what do you imagine the leanest bit
is at.
How about 15.3 or 16.1 or 23.4 ?  It all depends on the sample zone.

The reason we dump all that fuel into hypo engines is that it is hard
work
to use up all the available oxygen. So an extra 35% fuel  is not
considered
excessive. The same applies at the lean end, there are still rich
pockets
that are easily combustible and leaner pockets that are barely
combustible.

The Japanese have engines on the market with electronic direct
injection at
about 3500 psi. It's all about droplet size and the big/small ratio.
Then the
manifold and porting have a swirl and tumble job to do. Towards the
end there is
still some squish to do and then you go for ignition.

To give you an idea of how chaotic this all is. Toyota installed a
color camera
in a central spark plug (Kinsler do them as well) and took multiple
frame
exposures at very high speed (approx. every 5 degrees of crank) for
600 cycles of
combustion. The best way to describe the difference between the flame
front
progression and shape and color would be to think of snowflakes. They
are
as alike as 600 randomly chosen human beings.

Worse than that there were 3 critical lean misfires. All without
varying the A/F ratio
at any time. Each lean misfire caused a noticeable effect on the next
induction and
that resulted in a rich condition. Not fair I hear you say.

All the engine tuner can do is use the smallest injector and the
highest pressure
and the most tumble, swirl and squish that he/she can afford. Heat as
energy
added to the equation is always capable of adding something to the
speed of
mixing. Hot air or fuel or higher compression or egr can all be
useful.

After that you use an A/F ratio that works at that load on that engine
and you
map to suit.

The real art is to pick the fudge factor. You can either be
responsible for making
less HP than possible or making an engine that goes great until it
explodes.
The secret is to pick the right one for the right state of tune.

Even the NRHA fines competitors for an engine blow up or oil down as
they call
it. Those 5000 hp engines can make 5100 sometimes....


Happy mapping,

phil


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