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Date: Wed, 04 May 2005 11:18:25 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: EGT temps
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David Carter wrote:

> A point I like to think of is that 50 or 80 degrees "from" "peak" on 
> the rich side and on the lean side is, the same temperature.  For 
> piston engines, running 50 deg lean of peak is the same temp as 
> running 50 deg rich of peak, so the valves are seeing the same temp - 
> so why are some engine "gurus" so adament about cruising 50 deg rich 
> of peak?  It makes no sense to me.
>  
> So, I believe there is no such thing as "EGT too high".  There is only 
> "peak EGT", whatever it is for  your installation.
>  
> David 


The problem is with the delicate mechanical valves in the exhaust stream 
and the mangled fuel mix distribution that is necessitated by a 
carbeurated 4-banger.

The EGT as typically measured is an averaged value of the 4 cylinders.  
If you're lean on one, you may be rich on another, depending on how the 
fuel gets spread around in the intake.  There's really no way to tell 
without measuring it.  The valve in the first cylinder doesn't care 
about the temp in cylinder two.  Since all the valves have to stay below 
that critical temp, the solution is to build in some headroom. 

Going lean of peak will increase the chance of detonation.  The same 
rules apply.  The mix will not be consistent, so you have to provide a 
large margin of detonation prevention by retarding the combustion 
process with extra fuel.

The rotary deftly sidesteps the first problem by eliminating valves.  
The second problem is eliminated by proper design (and by leaving the 
turbo in the hangar).  As a side note, for maximum performance and 
economy, you want the engine to run as close to detonation without going 
over as you can get.


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