Re: low-octane gas

[John Denker <jsd@MONMOUTH.COM>]

At 09:07 AM 6/27/00 -0400, I wrote:
> Carnot tells us that high
> efficiency (for engines in this category) requires a high ratio of fire
> temperature to exhaust temperature.

I stand by that assertion.  That's not the only thing that efficiency
requires, but that's one of the things it requires.  In context, that
statement was appropriate and no further detail would have been appropriate.

At 12:13 PM 6/27/00 -0500, brian whatcott wrote:
> flame temperature is relatively insensitive to compression

Really?

ISTM if that were true, a high-compression engine would have less power and
less efficiency than a low-compression engine (other things [such as
displacement] being equal).

At 10:32 PM 6/28/00 -0500, brian whatcott wrote:
> the thermodynamic efficiency
> of the four stroke is controlled by the adiabatic temperature change
>  of the compression stroke - or in brief, by its compression ratio
> (rather than the flame temperature, as some folks think

Temperature change is mostly the right idea.  I just don't see how Brian
plans to get a large temperature change without a large temperature.

============

There is of course more to the story.  Efficiency exhibits a first-order
dependence on
 -- nominal compression ratio
 -- chemical properties of the raw fuel
and under normal conditions a significant second-order (sometimes even
first-order) dependence on
 -- stoichiometry
 -- RPM
 -- structural details such as
   --- shape of the cylinder,
   --- placement of the spark plug
   --- et cetera
 -- ignition timing
 -- valve timing
 -- various other things.

The peak temperature has a similar dependence on the same factors (except
possibly valve timing).