why humid is better!

[Greg Hermann]

At 8:40 AM 8/21/01, Gary Heuston wrote:
>can anyone provide any actual data or even proven scientific theory as to why
>humidity would be either good or bad for power?  Personally I think the
>car runs
>better when the air is dry but I would like to hear from some of the
>experts out
>there...(i.e Greg could you elaborate on that please? =D  )
>
>Gary
>gary.heuston at afwa.af.mil

-----------------

Air at the same total pressure with higher humidity is slightly less dense
than dry air--because of the lower molecular weight of H2O (18) as opposed
to the average molecular weight of air (29).

Water is a good chemical anti-detonant.

Net effects--

A bit less oxygen going into the engine with higher humidity, but also a
bit less pressure drop in the ports, etc. (All due to the lower density)

Complex effects on a turbo--will need a slightly higher compressor rpm to
make the same boost because of the lower density. Will probably spool
quicker, tho, because of the lower power requirement vs. compressor rpm
(also due to the lower density).

Lower air resistance to the forward motion of the vehicle (due to the lower
air density)

The temp rise of the more humid air is greater during the compression
stroke (due to the lower molecular weight of the H2O relative to dry air),
therefore the peak cycle temp is a bit higher, and higher Carnot efficiency
is therefore available.

The anti-det properties of the water probably compensate some for the
average hot-rodder's tendency to run excessive amounts of ignition advance.

----------------

The guy was basically correct in saying that there are potential positive
effects of finely atomized water in intake charge--so long as it doesn't
evaporate, and thus displace O2, before the intake valve closes. In this
form, water acts as an internal coolant, and greatly reduces the negative
work needed by the piston during the compression stroke--tends to make the
compr. process look more like an isothermal process than an adiabatic one.
The lower temps and pressures (all through the cycle) greatly inhibit
detonation (on top of the chemical inhibition properties of H2O). The lower
temps and pressures improve mechanical reliability. Also lowers Carnot
efficiency considerably (although this is compensated for by the reduction
in comp. stroke negative work). Another big gain comes from reduced heat
rejection to the water jackets because of the lower peak cycle temps.

Greg


>
>Greg Hermann wrote:
>
>> At 11:25 PM 8/20/01, KasaRyan at aol.com wrote:
>> >Here's my whack on it.  The water vapor in the air is in a finely divided
>> >liquid state, not a gas, so when you heat it, it goes thru a phase change
>> >into steam.  The resulting increase in volume is larger than if you took the
>> >same amount of combustion calories and put it into air, which is already
>> >behaving according to gas laws.  More volume=more push on the pistons= go
>> >faster!
>> >
>> Nope!!


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