From: rlr@bbt.com@INTERNET on Wed, Nov 27, 1996 12:01 PM
>> the slower burn rate of the higher octane fuel results in maximum
cylinder
>> pressure after the optimal time in the engines stroke, thus less
efficiency.
> If anybody has any information on burn rate of fuels with different
>octane ratings, I'd love to see it. Every reference I've seen seems
>to indicate that there is no significant difference in burn rate between
>lower vs. higher octane fuels (at least in the range available as
>pump gas).
There is no simple number that can be quoted but here's a quote form the
Mechanical Engineers handbook.
"Flame speeds in spark-ignition engines are low immediately after ignition,
attain maximum values when about half the combustion chamber has been
inflammed, and decrease toward the end of the process. Mean flame speeds are
a maximum for mixtures usually 10 to 20 percent richer than chemically
correct air-fuel ratio, and vary with fuels, engine speed, and turbulence.
At 900 ft/min mean piston speed, mean flame speeds with maximum-power
gasoline-air mixtures and optimum spark advance range from about 50 to 100
ft/sec."
Certainly does not help with what an octane increases will have on burn
rated but gives an idea on the range of burn rates. I guess if it was all
easy then engine designers would all be out of work because it would have
been figured out years ago.
Boy this is starting to make my head swim. I have not had to think about
this stuff since thermodynamics and mechanical power classes 25 years ago.
Dave Clement
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