Why run it hot?

[Chris Howard]

At 08:02 17/09/96 -0400, you wrote:
>On Mon, 16 Sep 1996, Thor Johnson wrote:
>
>> 
>> A lot of traffic relating to ceramics/keeping exhaust hot has been 
>> passing through...  Why would I want to do this?  Is it just for 
>> TurboCharging, or is it useful in other respects?I thought gas blew up 
>> best when cold... enlighten me f I'm wrong.
>> 
>
>Hotter exhaust gas translates to higher gas velocity, which aides 
>scavanging during the exhuast stroke. It's a racer trick to get a little 
>more power out of the engine. Also, using an insulating header wrap 
>reduces temperatures under the hood, which also helps to make a little 
>more power by keeping fuel lines cooler. As Todd has pointed out, things 
>aren't always what they seem, and a good idea can lead to serious 
>consequences depending on the application.
>

Actually the main reason to do this is to increase the thermal efficiency of
the engine. For a theoretical reversible (Carnot) engine, thermal efficiency
is given by:

        Tl
n = 1 - --
        Th

where n = thermal efficiency
Tl = temperature of low temp. reservoir ( Atmosphere in this case )
Th = temperature of high temp. reservoir ( Combustion temp )

"The thermal efficiency of actual heat engines can be maximized by supplying
heat to the engine at the highest possible temperature (limited by material
strength) and rejecting heat from the engine at the lowest possible
temperature."

Source:

Cengel y, & Boles M. 
Thermodynamics: An Engineering Approach

Get hold of any elementary thermodynamics textbook for more info.

Chris Howard
Computer Science
University of New South Wales
(was Mech. Eng @ Sydney University)
email: s2184002 at cse.unsw.edu.au
Web: http://www.usyd.edu.au/~choward