Any Chemical Engineers out there?

[C-130 Hercules 556]

I understand that as the elevation increases, the atmospheric pressure, and therefore vapour pressure, decreases, which means the boiling point (e.g for water) is lower on top of a mountain. Considering this now for the flashpoint of a flammable liquid, is it right to assume that this would be lowered as well at altitude (or am I missing something somewhere)?

Now trying to make this concept relevant to this forum – if above holds true, it would mean that if I take 98 Octane, sold in Dunedin for example, to the Crown Saddle (about 1000m) it would behave more like 99-100 Octane? Trade off being that due to reduced atmospheric pressure, less mixture would enter the cyclinders...

[lord_jagganath 421]

only if the car was a gravity fed carburetor, most cars have a positive displacement fuel pump that will pump a constant volume of fuel no matter the atmospheric pressure. you'd have less oxygen in the chambers so in fact, the performance is slightly reduced.

as for it behaving like 100 octane, i'd say it is possible but barely.

[yng_750 247]

Octane rating is a measure of a fuels resistance to explosion. (engine need a slow burn)

as you get closer to a fuels flash point it gets easier to explode rather than burn.

So if the lower pressure lowered the flash point it would likely lower the octane rating.

that being said the fuel is ignited in the engine where it has compression so the outside air pressure will have no affect on the octane etc.

The fact that much less oxygen will be entering the engine will have a markable negative effect on power.

[JoKer 58]

the performance is slightly reduced.

yea think along the lines of Pikes peak (quite a climb) cars loose up to a third (may be grossly exaggerated : need researching/google) of their power from the start line to finish simply due to the lower O2 levels at higher Altitude

[_ethrty-Andy_ 2122]

also why turboprop planes and similar have the turbo or supercharger

[C-130 Hercules 556]

Thanks all - so, leaving the vehicle aspect aside for a moment, you would agree that the flaspoint for a flammable liquid is reduced as one ascends? Or is this effect negated by the fact that less oxygen is available?

Sorry, just trying to get my head around it. I know it is not the most animated topic. Maybe I should join my local chemistry club?!

[MLM 57]

My 2c

Flash point is the temperature required to support a vapor pressure which will ignite with an ignition source. If the atmospheric pressure drops the ability for the 'fuel' to boil off increases (increase vapour pressure) . Seeing as the air is less dense at a lower atmospheric pressures you will approach the required air fuel ratio (flash point) sooner for a given temperature. There would be limits to this such as a very low atmospheric pressure (zero or just above) there would not be enough air to support ignition or said another way the ratio of fuel exceeds the air available it wont burn.

If this is part of an exam question your studying i would do some more reading.

[C-130 Hercules 556]

If this is part of an exam question your studying i would do some more reading.

Thanks! No exam, just for my own interest believe it or not

[c.robertson00 11]

also why turboprop planes and similar have the turbo or supercharger

Turboprops are a gas turbine engine so no turbos or superchargers are required there.

[yng_750 247]

yes flash point decreases with altitude

this graph is from a test of jet fuel used in an CAA investigation.

[C-130 Hercules 556]

Lovely. Just what I thought.

[yng_750 247]

boyles law refers to gas not vapors.

[MLM 57]

To add to the confusion

My understanding is:

Flashpoint is the temperature at which a volitile liquid reaches a concentration which can be ignited which is measured at 1 ATM.

The temperature at which liquid becomes a gas is the boiling point measured at 1 ATM.

[MLM 57]