Optimizing A/F Mixture & Quality

Gregory R. Travis greg at indiana.edu
Wed Jun 11 16:39:58 GMT 1997


On Tue, 10 Jun 1997, Shane Moseley wrote:
> 2. Re: Optimum conversion of fuels potential for energy (combustion)
> 
> After being intrigued for many years about engine intake designs -
> nothing mixes up conversation better than Smokey Yunick (& other
> Otto-cycle designs) and his miracle engines that got 50-60 mpg with no
> performance loss (40% gains instead 8-) by HEATING the incoming A/F
> mixture to around 400 degrees.  Most say 'yeah right - then why are the
> factories all producing intercoolers?'  Well, I have read many of
> Smokey's articles and have the one with the design drawings of his Fiero
> experiment in Hot Rod June '84.  I understand that internal combustion
> engines probably average around 25% efficiency of converting fuels
> potential for energy (BTU's?) into actual usable energy (read flywheel
> horsepower).  And that current designs might be between 30-40 percent in
> optimum conditions (read hardly ever).  Seems to be alot of room for
> improvement here!  This is the basis for Smokey's design.  Further -
> according to several sources (nice one at
> http://www.autoshop-online.com/auto101/fuel1.html about all sorts of
> things including description of operation of Chrysler X-Ram intakes) the
> optimum condition of the incoming mixture are something like:

I'm not real familiar with Smokey's work here - I have a couple of his books.
He's, well, different.

But isn't his scheme (heat the intake air) basically a straight-forward
application of the Carnot cycle?

If my understanding of the Carnot cycle is correct, engine efficiency is
given by:

	Efficiency = 1 - (Heat Out / Heat In)

		(where both Heat Out and Heat In are expressed in an
		 absolute temperature scale)

[Therefore, a 100% efficient heat engine would have to take in energy from
a heat source that was infinitely hot and convert all of it to an infinite
amount of energy all the while producing exhaust at absolute zero.  This
is not possible so 100% efficiency is impossible.  Unless you use Slick 50
and Splitfire plugs.]

One of the conclusions that you can draw from this law is that anything you
can do to lower the exhaust temperature of your reciprocating internal
combustion engine, everything else being equal, will raise its fuel
efficiency.  (The converse of the law is that the more efficient engine
will have a lower exhaust temperature.  This is exactly what happens when
you raise the compression ratio of an engine)

One way of doing this is to place a device in the exhaust stream which
absorbs exhaust heat.  The resulting exhaust is then cooler.  But you now
have to do something with that absorbed heat that doesn't involve simply
transfering it to the atmosphere.  That, after all, would be exhaust!

The only thing you can do with the heat then is to transfer it BACK
to the input of the engine.  This both increases the "Heat In" in the
equation and lowers the "Heat Out".  According to Mr. Carnot, that will
increase the engine's efficiency.

This property is put to real-world application all the time in the turbine
world.  Gas turbines exhaust (excuse me) a tremendous amount of waste heat,
relative to piston engines, and are relatively fuel inefficient.

To recover this energy, it's popular to put a "regenerator" in the exhaust
stream.  Physically this consists of some extreme high-temperature material
(ceramic matrix, etc.).  The regenerator then PHYSICALLY moves the heat
from the exhaust to the turbine's intake.

For example, Chrysler's gas turbine automobile experiment of the 1960s
involve disk-shaped regenerators which rotated slowly.  The gas turbine's
exhaust was directed at 180 degrees of the disk and the intake was taken
through the other 180 degrees.  Since the disk slowly rotated, the hot half
of the disk (the half exposed to the exhaust) was constantly being rotated
over to the intake side where the heat was taken back out.

This regeneration allowed the Chrysler turbine to obtain fuel specifics in
the 0.4x range - incredible for a gas turbine.  Allison's auto engine,
without a regenerator, had fuel specifics above 1.0

Interestingly, a regenerator was part of Lycoming's recent piston-engine
GAP proposal.  In Lycoming's piston diesel, a "disk on a stick" regenerator
occupied the cylinder along with the piston itself.  The regenerator was
driven from above (with the regenerator shaft travelling through and out
of the cylinder at the head (and with appropriate seals)).  The regenerator
disk was to sweep through the cylinder, absorbing extra exhaust heat, while
the piston itself was on the downward expansion (exhaust) stroke.

Then, after the piston returned to top-dead-center (the regenerator disk
(hopefully) having already returned there), the regenerator would follow
it back as it went down on the intake stroke - transferring the heat BACK
to the intake charge.

Lycoming (actually the ReJen corporation which holds the patents on the
mechanism) was shooting for fuel specifics in about 0.27 range as a result.

NASA thought it a bit too far out...

greg




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