Sender: <marv@lancaironline.net> (Marvin Kaye)
To: flyrotary
Date: Sun, 30 Mar 2003 22:05:32 -0500
Message-ID: <redirect-2093033@logan.com>
From: Lehanover@aol.com
Subject: Re: [FlyRotary] Re: Spark Plugs
MIME-Version: 1.0
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: 7bit

In a message dated 3/29/2003 11:13:04 AM US Eastern Standard Time, 
alventures@email.msn.com writes:

> OK, Lynn; now you've done it.  I understand that concepts that
>  preignition is ignition of the compressed fuel/mixture prior to the
>  normal ignition by the spark, caused by some hot spot or something; and
>  detonation is a sort of flash, or spontaneous combustion ahead, or away
>  from, the normal flame front caused sonic compression waves, or
>  whatever.  How can you tell the difference?  And what causes one or the
>  other? And does one lead to the other?  When the piston engine in my car
>  "pings" because the timing is too early or the octane is too low, which
>  is it.  I've never heard preignition or detonation in a rotary, but if I
>  ever do, now I'm going to want to know which it is.
>  
>  Whenever you find the time.  Thanks,
>  
>  Al Gietzen

You have it right.
It is not impossible in a non turbo. Just real difficult. The surface area of 
the combustion chamber at TDC is so large that much of the heat of 
compression is carried away before the plugs fire. This is lost efficiency, 
but a cooler mixture is less likely to detonate. Also the clearance between 
the housing and the rotor face near the apex seals is so small when near TDC, 
that the flame front tends to go out before consuming all of the fuel air 
mixture. So unburned hydrocarbons is the big pollution problem and adds to 
exhaust gas temperature. Also the compression ratio is on the low side and 
that keeps the flame speed down. So you can run some dreadfully poor (low 
octane) fuel without a problem. You could go up to 30 degrees of timing 
without a problem. All of this if your water and oil temps are under control. 
Another thing to think about is that you can exceed 100% cylinder filling. 
With a good long intake runner and a good exhaust system, it happens. That 
would be RPM sensitive and more likely near the lower end of the power band, 
so at that RPM, the effective compression ratio is higher than the advertised 
9.4 :1 or whatever. We recently saw some fine wire plugs with some of the 
wire gone. If the engine had already been overheated, that can cause 
detonation by adding a bit more heat to the mixture. Plus the glowing fine 
wire electrode can cause preignition by themselves.You could hear this in a 
street car but any loud application will cover the sound. Detonation quickly 
raises the chamber temps and preignition can follow. You won't hear that 
either but it will cause a very noticeable power loss. If you pull off some 
power, thus reducing heat load, and the power returns, that may have been an 
episode of preignition. This is much more likely on a turbo engine where the 
effective compression ratio changes with boost.  

Lynn E. Hanover