Craig,
This is part of the paradox. Gasoline in a homogenous mixture, with no air
turbulence, will burn at the rate of approximately 2 cm per second. If you
think of a piston being 6 cm in diameter, and a spark plug to one side...
this means gasoline needs 3 seconds to burn across that distance! No
way, can't be. That 2 cm per second for a burning rate seems all wrong...
but it is correct. Remember there is NO air turbulence at all, just a static
burn. Actually there are chemical engineers and experiments to confirm
this burn rate.
Now enter turbulence and eddy currents. If you think of a 16 in rollers in
a 4x4 pattern. Drop ink on the top roller, and start rotating it. Since all the
rollers are turning at the same time, the ink will spread quickly to all 16.
Darn wish I could put a drawing in here... maybe Van can make a sketch
for us. <grin>. This is what happens in a combustion chamber, is the eddy
currents (little circles) will cause the burning to spread much more rapidly.
So for gasoline it really is the same blend of compounds for all three grades
of fuel. The higher octane fuel gets a improved knock reducer, and usually
additional detergent compounds. Burn rate is still the same as the regular
fuels.
Prof. Assanis was excellent, and quite tolerant of my questions. That myth of
the burn rate being different is a hard one to kill. Of humor was we had a
good 'discussion' about wankel engines, hybrid vehicles, and electric vehicles.
His entire research and area of expertise is of internal combustion engines and
doing model based systems. I also have an invite from him to come up to
talk with his graduate students & see the lab in Ann Arbor, MI. Not sure
when I will get up that way, but you can be sure I will show up!
Cheers,
Tom Walter
> I've scratched my head on this a few times myself. Question...Gas CAN be
> refined to a higher octane (anti knock) without additives. So, isn't
> detonation (explosion) just a degree of the same process of rapid burn? So
> wouldn't the higher octane 'resist' the increase in rate of burn (burn
> slower) compared to a non-additive standard, as the ignition advances and
> pressure goes up?
>
> Craig Carter
>
> In a message dated 9/22/00 7:53:20 PM Pacific Daylight Time,
> ra0618@email.sps.mot.com writes:
> << I have to admit some old myth's die hard. I had for ages believed that
> premium
> fuel burned slower. Not so, all the grades burned the same. Premium just had
> the additives to prevent knock, so you can run a higher spark advance (which
> creates a higher pressure, hence better performance). Of course you need
> higher compression to take advantage of the advanced spark to gain the higher
> combustion pressure. >>
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