High altitude, and low octane

[DeWitt Harrison]

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
pressure.

DeWitt Harrison
88 5kcstq

At 04:01 PM 6/2/01 -0400, you wrote:
>Sorry DeWitt,
>
>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 
>proportionately.
>
>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,
>>
>>DeWitt Harrison
>>88 5kcstq
>>
>>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.
>>>>
>>>>DeWitt Harrison
>>>>88 5kcstq
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>---
>>>>Incoming mail is certified Virus Free.
>>>>Checked by AVG anti-virus system (http://www.grisoft.com).
>>>>Version: 6.0.255 / Virus Database: 128 - Release Date: 5/17/01
>>>
>>>Bob
>>>*****
>>>  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
>>>  http://www.cob-net.org/church/pvcob.htm
>>>*****
>>
>>
>>
>>
>>
>>---
>>Incoming mail is certified Virus Free.
>>Checked by AVG anti-virus system (http://www.grisoft.com).
>>Version: 6.0.255 / Virus Database: 128 - Release Date: 5/17/01
>
>Bob
>*****
>  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
>  http://www.cob-net.org/church/pvcob.htm
>*****