why humid is better!

[Roger Heflin]

Ludis Langens wrote:
> 
> rahmrh at cableone.net wrote:
> >
> > The humidity is not droplets, it is water vapor, and
> > the reason that it is bad, is that if you have 14.7psi of
> > stuff (or whatever is normal pressure for your location), if
> > the pressure of the water vapor is .3 psi, that is .3 psi
> > that contains *no* oxygen, whereas the other 14.4 psi
> > contains 21% oxygen.
> 
> How's this for an explanation (I'm not saying this is correct...):
> 
> The intake air is down to 14.4/14.7 of "normal" - the driver
> unknowningly opens the throttle a little bit more.  Now the engine gets
> as much oxygen as with dry air - plus a little bit of water vapor.  On a
> MAF engine (and a carb'ed engine), the results in a higher measured
> airflow and hence more fuel is injected.  On a MAP engine, the extra
> air/vapor (ie higher pressure) also leads to more fuel.  Presumably
> closed loop operation leans out the extra fuel.  However, ignition
> timing stays a bit advanced from normal (because of the higher MAF/MAP
> reading).  Hence, more butt-o-meter measured power.  The water vapor
> acts like water injection and keeps the engine from pinging.

The base issues the the pressure is constant (or mostly so ignoring
weather fronts which are mostly independent of the humidity), and 
the higher the humidity the more of that pressure is water vapor
and not air containing oxygen.

Here is a table from a thermo book, these numbers are for 100%
humidity at the listed temp and are pressures:
50F	0.1780
70F	0.3632
80F	0.5073
90F	0.6988
100F	0.9503

So at 100% humidity at 70F (does actually happen), you have .36psi
of the 14.7 psi total atmosphere that is pure water vapor, so originally
you had .21 (21% oxygen in the atmsophere) * 14.7 = 3.087 psi of oxygen,
with the water vapor displacing things you now only have .21*(14.7-.36)
or 3.011 psi of oxygen, you now only have 97.5% of the oxygen that you
originally did, so you will make less power that you did before, and
what makes this worse is the motor can only tell pressure so it does
not know what the humidity is and it detects 14.7 psi and sets up the
fuel based on that being "average" air, which makes things rich or lean
depending on what the current air is and what the definition of "average"
is.

If you get to 100% at 80F (only happens at a very few locations, watch
the weather channel maps for places that have 80F dewpoints and it
does not happen that much), you only have 14.19psi of air, or 2.98psi
of oxygen which means you are down to 96.5% of air that you would have
if you had 80F with no humidity.

				Roger
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