What/how does compression ratio affect an engine?

On Mon, 10 Dec 2007 20:38:01 -0800, Bob Fry
wrote:

"DA" == Dale Alexander writes:

DA Now, what would 22K BTU stuff do in an 7 1/2 to 1 aircraft
DA engine? Probably nothing as the lower compression would limit
DA the amount of work actually being done.

If I can reword your statement to "what would higher octane stuff
do..." then I can answer not probably, but definitely, it will not do
anything different, if the only difference between the two fuels is
octane.

DA But it will still burn
DA hotter than a fuel with a lower BTU content.

Where did the difference in "BTU content" (i.e. unit chemical energy
content) come from? Not from a mere octane enhancer. Perhaps the
fuels used in your road racing experience had not only different
octanes, but also different unit energies.

Racing fuel, generally, is not even CLOSE to gasoline. It is a very
complex witches brew, and generally significantly higher specific
gravity, as well as higher BTU per gallon (not necessarily more BTU
per lb, hence the higher SG)

Eh? Where's this "extra heat" come from? There is no
practical unit energy difference between different octane
fuels. --


Exactly. "Octane" is, by definition, a measure of a fuel's
resistance to knock under specific conditions. "High Octane"
fuel does not burn any hotter, generate any more power, or
improve your fuel economy (note: see exception below). Higher
octane fuel lets the engine designer use a higher compression
ratio, or more spark advance, etc. without triggering knock. It
is the compression / spark changes that result in more power,
etc.



--
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