High altitude, and low octane
robert at s-cars.org
Mon Jun 4 17:05:28 EDT 2001
At last we are on the same page, DeWitt. Now as to a lower octane rating
requirement at higher altitudes... In a NA (normally aspirated - not
turbo) engine the total oxygen partial pressure at the top of the
compression stroke will be lower than for the same engine at lower
altitudes. This will result in a slowing of the rate of propagation of the
flame front as a result of the Law of Mass Action (which sez, in essence -
the higher concentration you have of sumpin the faster it does its
thing). This would allow a lower octane fuel to serve, in most instances,
quite well. The critical Mass Action factor is a concentration term not a
mass term. One mass unit of fuel will require so many mass units of air
for complete combustion. This ratio will be essentially invariant between
sea level and a much higher altitude. The _concentration_ however will
vary significantly with altitude.
For a turbo engine, OTOH, the turbo makes up, at least in part, for lower
ambient air densities by compressing the incoming air before the
compression stroke. This effectively increases the concentration of the
oxygen and you are right back (almost) to the requirements at sea
level. This is probably something of an oversimplification but should be
more or less on the right track. Of course we could then talk about the
volumetric efficiency of a specific turbo and its tendency to grenade if
over spun, etc. I'd prefer to avoid that one. :-)
I will tell you this however, when I was at Pike's Peak with a buncha other
audinuts, I could feel a definite seat of the pants decrease in engine
output due to the combination of lower ambient air pressures and lower fuel
quality. I'm not positive that I could determine which of the two factors
was the controlling factor but I suspect that my ECU was retarding the
spark some to compensate for the fuel.
At 02:34 PM 6/4/01, DeWitt Harrison wrote:
>Gravity slightly biases the relative rates of diffusion of gases with
>different molecular weights tending to create a weak separation
>by altitude. O2 is 32, N2 is 28. I admit I grossly overstated this effect
>since the 50% number I carelessly grabbed was total pressure.
>Most folks are happy with the 21% approximation for any altitude.
>If I can find something definitive, I will supply it. Meanwhile, what
>is the rational for slightly reduced octane requirements at altitude
>when engines meter fuel according to air mass and not total
>At 04:01 PM 6/2/01 -0400, you wrote:
>>The kinetic-molecular theory sez that ain't gonna happen.
>>Additionally, an oxygen content below ~16% will not support fast enough
>>combustion to support flames. Ask any fireman. If the concentration of
>>oxygen were to be about 12 to 13% then it would be impossible for a
>>mountain climber to light a burner to cook a meal above 18K feet or
>>so. Now I haven't attempted that particular feat but I understand that
>>others do it regularly.
>>Nope. The percent oxygen will remain ~ 21% but its partial pressure will
>>decrease to well below its sea level value. That is because the total
>>pressure will be much lower. The O2 pressure will simply decrease
>>At 02:57 PM 6/1/01, DeWitt Harrison wrote:
>>>Sorry old chap. The partial pressure of O2 decreases with altitude.
>>>At sea level, O2 accounts for about 21% of the total pressure
>>>while at 18,000 feet O2 only provides about 12 or 13% of the total
>>>atmospheric pressure. If this was not so, then the reduction
>>>in octane with altitude wouldn't be reasonable at all. Cheers,
>>>At 12:51 PM 6/1/01 -0400, Robert Myers wrote:
>>>>At 11:53 AM 6/1/01, DeWitt Harrison wrote:
>>>>>I've been waiting for someone to bring up the relevant fact that,
>>>>>as altitude increases, not only does the total air density decrease
>>>>>but the composition of the gas mixture changes. Importantly,
>>>>>O2 thins out more rapidly than N2. At high altitude, there is
>>>>>proportionately less oxygen in a kilo of air so that a fuel metering
>>>>Ohno ohno ohno! The amount of oxygen in a kilogram of air is
>>>>(neglecting water vapor content) essentially constant regardless of
>>>>altitude. Oxygen does not "thin out" more than nitrogen as altitude
>>>>increases. Now if you had said something about the amount of oxygen in
>>>>a liter of air your statement would be OK. The relative partial
>>>>pressures of O2 and N2 remains constant regardless of altitude.
>>>>>system based on intake of air mass will inherently run richer
>>>>>than at sea level. This effects carbureted and injected engines
>>>>>alike and explains way 91 octane is generally adequate for
>>>>>Colorado cars. This still doesn't explain why we don't have 93
>>>>>octane at the pump if 91 is more costly.
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>>>> Rt. 4, Box 57, Fayetteville, WV 25840 USA WV tag Q SHIP
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>> Robert L. Myers 304-574-2372
>> Rt. 4, Box 57, Fayetteville, WV 25840 USA WV tag Q SHIP
>> '95 urS6 Cashmere Grey - der Wunderwagen ICQ 22170244
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Robert L. Myers 304-574-2372
Rt. 4, Box 57, Fayetteville, WV 25840 USA WV tag Q SHIP
'95 urS6 Cashmere Grey - der Wunderwagen ICQ 22170244
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