Message-ID: <52548863F8A5D411B530005004759A930126B4@QBERT>
From: George Braly <gwbraly@gami.com>
To: "'lancair.list@olsusa.com'" <lancair.list@olsusa.com>
Subject: RE: LOP/ROP debate
Date: Wed, 2 May 2001 15:44:45 -0500 
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Jim Scales:

You ask about two things.  TCM IO-550G operation LOP and the issue valve
oxidation and lubrication.

First, the IO-550G.  

	Here is my take on that (and similar) top down spider induction
engines.  TCM made a good attempt to make something better than the runner
log branch induction system on the traditional IO-520/550 engines.  As
sometimes happens,  the enemy of good turned out to be an attempt at better.
The short branch induction runners cause the engine to have a tuned
induction system.  But, ... while most people familiar with the conventional
wisdom might respond  "good"... the real answer is... well,  "... not
exactly."   

	We consistently see BETTER (tighter) cylinder to cylinder air flow
uniformity with the runner log branch engines across the spectrum of RPMs of
interest to normal engine operation than we see (measure) with the 550G
configuration induction system.  

	However,  the 550G engines do not suffer from the classic fuel
mis-distribution that the runner log branch engines suffer.  We know how to
correct the fuel component of the fuel/air equation.  Correcting the air
imbalance is... a bit more difficult.  On experimental engines, we are
almost always able to adjust the fuel/air ratios on the "G" (normally
aspirated) and the TSIO550 engines, and get them to operate smoothly lean of
peak.  

	On the normally aspirated engines,  I recommend that all takeoff,
climb and cruise operations be done with wide open throttle... (WOT) even at
cruise at 1000' MSL.  However, one can modulate the horsepower by using the
red knob and the blue knob.

Thus, when flying normally aspirated engines at 1000' MSL, I will typically
operate at WOT and 2500 RPM and lean the mixture to 60 to 90F lean of peak.
However, by the time I get to 10,000',  I will typically still be at WOT and
2500 RPM, but the mixture will then be only 10 or 15F lean of peak.  One can
nicely interpolate in between.  Above 10,000',  I will often elect to go to
75 ROP in order to get some speed back.

SECOND:

Gasoline is not a very good lubricant.  Oil is.  Gasoline, especially,
excess gasoline is a solvent for oil.   Oil is lubricant.  Why would you
wish to wash away the oil lubricant with an oil solvent by putting more fuel
than necessary into your  cylinder?

Now... re the "excess fuel" cooling the valve....   this is an almost
universally held belief that when you make the  mixture richer you "cool"
the cylinder by "washing" excess liquid fuel through the cylinder.  

It is seductively simple and a very easy explanation for why "richer" than
peak is "cooler".   It turns out that the excess fuel, per se,  has
virtually nothing to do with the cooling.  Ok... now that I have taken your
breath away... here is what happens:

If you do the calculations for the excess mass flow of fuel and the heat of
vaporization, etc... i.e., do the thermodynamic calculations,  you quickly
realize that there is no way at all that the tiny amount of additional fuel
can even begin to carry away enough BTU's of energy to make a measurable
difference in the CHT.  So... what is the explanation?

More complex... but based on real measurement, not supposition.  The primary
effect of a rich mixture is to slow down the burn time.  If you slow down
the burn time,  you delay the point in the combustion process at which the
combustion pressure event reaches a maximum value.  Going from  25F rich of
peak to 150F rich of peak may move the peak of the combustion pressure event
from 14d after top dead center out to 18 or 20d ATDC.  This later peak
pressure will be lower in magnitude (maybe 750PSI verses 850 PSI)  because
the piston will have traveled further down and created more volume over the
piston.  The lower peak pressure results in a lower peak combustion
temperature.  NOW... the routine EGT outside the cylinder is down around
1500F.  But at the peak of the combustion event, the internal bulk gas
temperature is up around 4000F !!!   The very high temperature results in a
very high rate of heat transfer from the combustion gases into the cylinder
head.  If you drop that very high peak combustion temperature from say,
4000F down to 3800F,  you reduce the RATE at which BTU's are transferred
into the cylinders by a LOT. 

Thus, rich mixtures result in cooler CHTs NOT because the fuel carries away
BTUs .... but because the excess fuel reduces the rate at which the burn
takes place and as a result, the peak internal combustion bulk gas
temperatures are lower and there is  a reduced rate of heat transfer from
the bulk combustion gases into the cylinder head.

Now... guess what?  LEAN mixtures burn more slowly than 25F ROP mixtures.
Guess why CHTS drop when you go 25F LOP ?  YES! The burn time is slower and
therefore there are lower peak bulk gas temperatures in the cylinder and
lower rates of heat transfer into the cylinder head.... and thus, lower
CHTs.

A mixture that is 25F LOP may burn about as slow as a mixture 250-300F rich
of peak.   THAT is why you get much more cooling effect from a modestly lean
mixture than you do from even a very rich mixture. 

Remember... there are hundreds of millions of hours of lean of peak
experience.  IT WAS UNIFORMLY GOOD EXPERIENCE WITH RESPECT TO  ENGINE AND
VALVE TRAIN DURABILITY.  

Regards,  George 

  
-----Original Message-----
From: JOSCALES@aol.com [mailto:JOSCALES@aol.com]
Sent: Wednesday, May 02, 2001 12:04 AM
To: lancair.list@olsusa.com
Subject: Re: LOP/ROP debate


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I have followed the debate and have learned a great deal.  However, I
believe 
the only engines I have seen discussed are turbocharged ones that can get
the 
horsepower back after leaning by increasing MAP.  I, unfortunately, (or, 
fortunately, depending on how you look at it) have a TCM IO550G in my
Lancair 
ES and can't seem to dial up more MAP on demand so I see a deterioration in 
air speed when I lean to 50 LOP.

Speed loss aside, am I to understand that all the details (except getting
the 
MAP back) that are being discussed apply to my engine and my situation?  
Probably a less than bright question but it seems to be the one I have at
the 
moment.

Also, I would like Mr. Braly's comments on the issue that was brought up
some 
time ago about valve stem lubrication and cooling by the excess fuel in a
ROP 
situation.  Also, there was the question of oxidation on the valves which 
supposedly is more likely to occur in a LOP situation.  I have no opinion on

either of these points but would sure like someone who knows the facts to 
express one.

I will now sit back and see where this goes.  Thanks to everyone who has 
contributed to this debate.  This is exactly what makes our 
sport/hobby/passion/business so much fun.

Jim Scales
98AF
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