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Date: Thu, 24 Sep 1998 10:03:03 -0500
From: Snowbird <snbird@ibm.net>
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Subject: Re: Another silly question: why do leaner engines run hotter?
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up_in_the_air@my-dejanews.com wrote:

> In article <M8jO1.345$f97.1158267@news4.mco.bellsouth.net>,
>   "Andy Engle" <aengle@xnospamx.bellsouth.net> wrote:
> > I don't understand why a leaner piston engine runs hotter.  It 
> > would seem to me that more fuel would produce a "bigger bang" and 
> > lead to increased power/hotter cylinder head temperatures.
> <snip>
 
> Fuel requires oxygen to burn.  Leaning increases the ratio of oxygen 
> to fuel. A rich mixture does not have enough oxygen for a complete 
> burn and hence runs cooler. A properly leaned mixture provides the 
> best burn, a by-product of which is more heat.

True.  But begs another question.  We're told that over-leaning
damages the engine.  By causing it to run too hot?  Well, Figure
3-1 in my Lycoming Operator's Manual for the O-360 shows how
CHT (cylinder head temperature) and EGT (exhaust gas temperature)
vary with mixture.  As one might expect, there is an inverted
V-shape curve of CHT vs mixture; the engine runs first hotter as the
mixture gets leaner (and combustion more complete) then the engine
cools off again (as the mixture becomes too lean to burn completely).

But the entire variation of CHT with mixture is only about 30 
degrees, and the difference between CHT at recommended max power 
leaning and the maximum CHT is only about 10 degrees.  I get a bigger
variation in CHT if one teeny tiny little piece of engine baffling
is loose, or one piece of baffle seal folded backwards.

This suggests to me that if running too lean damages the engine, 
the mechanism is not excess CHT.  Does anyone know the mechanism?

Snowbird

