Partial credit to a couple of the previous writers. But water neither
displaces air nor increases oxygen content. For a given temperature and
atmospheric pressure a cubic foot of air contains the same amount of
oxygen no matter how much water it has in it. However, as an earlier
writer noted, when it rains it is usually cooler. Therefore the air
density is greater and more oxygen is available for combustion. By the
same augment your car should run better in the winter or anytime its cold.
A more significant effect is the benefit of having small amounts of water
in the intake charge. First of all, water absorbs a lot of heat.
Especially when it goes through a phase change such as liquid to gas.
Therefore, a little bit of water in the intake air can keep that charge
cool as it travels down through the carbs, intake runners and ultimately
the cylinder. This keeps the charge from heating up and expanding and
reducing the density. This effect is much more significant than just the
ambient temperature effect. The second benefit is a little more
complicated. As piston comes up toward top dead center on the compression
stroke the charge density is increased to the point at which the water
precipitates back out again (a localized rain storm in your cylinder ;-) )
. Since water in its liquid state is an incompressible fluid it takes up
space in the cylinder and effectively increases the compression ratio.
Finally on the power stroke the water slows down the combustion process by
cooling the flame front (the same effect as running higher octane gas).
Which is way the car doesn't knock, even with the compression ratio
increase from the moisture.
All these effects were well understood by World War II and many of the
fighter planes of the day used water injection systems to purposely add
water to the intake air. I believe the Auto Union GP cars of the late
'30's also used water injection.
Tom Gehring
Chemical Engineer & '65 MGB owner
----------
> From: SPB3430@aol.com
> To: mgs@autox.team.net
> Subject: humidity & how our cars run
> Date: Sunday, February 23, 1997 6:01 AM
>
> I think that when there is more moisture in the air that there is more
oxygen
> in the air too (remember that water is made up of 2 hydrogen & 1 Oxygen
> molecule)- more moisture in air means more Oxygen getting into your
carbs.
> Its kind of like a Supercharger - introducing more air to the mixture
makes
> things work better or something. I'm no scientist, I just remember
reading
> something about this somewhere. I think that one of the Listers here is a
> molecular scientist or something- maybe he can elaborate!
> Steve Bartley
> 1969 MGB Roadster
> NAMGBR 6-2585
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