High altitude, and low octane
de at aztek-eng.com
Mon Jun 4 13:34:18 EDT 2001
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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