For you motorcycle freaks, the following portions of Kevin Cameron's
February 1995 TDC column in Cycle World may be familiar. It is by far
the best explaination of air/fuel ratios and jetting I've ever seen. I'm
just including snippets, so if you want the whole thing, you'll have to
get the back issue, or visit the magazine's forum on AOHell or something
like that. Better yet, subscribe, Cameron's columns are absolute gems
IMHO, YMMV. I am in no way associated with CW, blah blah blah blah.
Hell, I'll probably get sued for not obtaining permission to reprint
this.
Andy
TDC -- Weather by Kevin Cameron as it appeared in the 2/95 isue of Cycle
World
All our motoring activities occur within the highly variable atmosphere.
Our engines breathe the air, and are, directly or indirectly, cooled by
it. Therefore, local changes in air temperature, barometric pressure, and
moisture content can change how engines run.
>>SNIP<<
Power depends upon the weight of correct fuel-air mixture burned per
second. At 12,000 feet, the air pressure is only 65 percent of what it is
at sea level, so maximum power is also only 65 percent of sea level
power. If your engine has carburetors, the problem is worse. Carburetors
don't measure absolute pressure; they only compare venturi pressure with
outside pressure. Therefore, for equal throttle opening, they deliver
essentially the same amount of fuel at 12,000 feet as at sea level. The
result is that the fuel-air mixture becomes tremendously rich (too much
fuel in proportion to air) at high altitude; power is reduced in the
first place because of reduced air density. It is reduced even more
because every combustion cycle is cooled by all that extra fuel.
><><><><>Andy comment -- this explains why you will NOT get better
mileage at altitude, assuming your car is jetted for sea level or
thereabouts.
Power is at a maximum at a 12.5:1 air-fuel ratio, so race engine tuners
change carburetor jetting as local atmospheric pressure changes, whether
the change results from weather or from the altitude of the race track.
Standard atmospheric pressure at sea level is 29.92 inches of mercury. To
ensure that the engine receives the desired fuel-air mixture strength
even if local air pressure rises or falls, use simple proportion. Divide
the local air pressure by the standard pressure (29.92"), and multiply
that number times the main-jet number that we've found to work best at
standard pressure (the numbers on motorcycle main-jets are proportional
to flow capacity). Thus, if the barometric pressure goes down, we jet
down, and vice versa.
>>SNIP<<
Air also loses density as its temperature rises, so engines pump less
weight of air the hotter it gets. Since there is now less oxygen to
combine with fuel, power falls. As the weather cools, the air becomes
more dense, and the engine pumps a greater weight of air, and so needs
more fuel to go with it.
>>SNIP<<
Compensating for temperature is not quite so simple because zero degrees
on the thermometer does not represent the point at which gases cease to
gain density as they cool. Therefore, we must transform the temperature
scale to one referenced to absolute zero, the temperature at which all
molecular motion reaches minimum. For example, absolute zero on the
Fahrenheit scale is 460 degrees below the zero on our familiar
thermometers. If the air temperature was 85 degrees F at the end of
yesterday afternoon's practice session, that is 460+85=545 degrees
absolute. If this morning it is 62 degrees F, that is 460 +62=522 degrees
absolute. To find out how much richer we have to jet, we divide 545 by
522 to get 1.044. If yesterday our machine was close to the limit,
running really sharp on a 210 main-jet, this morning's jet ought to be
210x 1.044=219.3, so we'll put in 220 jets.
>>SNIP<<
With street motorcycles (>>>and LBCs too. a.r.<<<), all these peak-power
niceties are essentially irrelevant. A compromise jetting is adequate for
the range of air conditions usually encountered. The exception is the
case in which a machine jetted for sea level is then ridden mostly in or
around some high-altitude place like Denver or Mexico City (or vice
versa).
Again, this was written by Kevin Cameron in the TDC column of Cycle
World, 2/95. Even if you're not into bikes, Cycle World is worth the
price of subscription just for Cameron's monthly column! You'll learn
about things you never knew you needed to think about in the first place.
Cheers,
Andy
----------------Original Message Follows----------------
On Wed, 20 Dec 1995 BLECKSTEIN@SHELL.MONMOUTH.COM wrote:
> >The most important variable would be air resistance. You would use
less
> >gas per mile at the lower car speed, because the energy needed to
overcome
> >air resistance would be substantially less.
> >
> > Ray Gibbons Dept. of Molecular Physiology & Biophysics
> > Univ. of Vermont College of Medicine, Burlington, VT
> > gibbons@northpole.med.uvm.edu (802) 656-8910
> >
> >
> >Just to keep this thing complicated, obviously those west of the
Mississippi at
> higher altitude (thin air) get better mileage.
>
> Mike Leckstein(looking out the window at our end of fall blizzard.)
>
At high altitudes your mileage may drop because of the decrease in wind
resistance in the lower density air, but what is the effect of the lower
concentration of oxygen (say in g/l or in g/cubic ft) on engine
efficiency?
---------------- End of Original Message -----------------
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