Vapor Pressure

[John Dammeyer]

Hi guys,

Thanks for those equations.  Neither of those are probably appropriate for real
time calculations from cranking up to 7000RPM and 7"Hg to 30"Hg inside the
intake manifold with my little 8 bit processor. I'd have to create some lookup
tables.

But that does bring up an interesting point.  Al Grippo, in his paper where he
shows how do derive pulse width,  aludes to 3.1Kpa correction for vapor pressure
in the intake manifold, assuming 75% humidity and 85F temperature.  Now,  will
this be the vapor pressure of the gasoline as it changes state from liquid into
gas or water as it expands in the manifold?

My understanding of fuel injection is that the MAP sensor will detect the
pressure inside the intake manifold before fuel is injected.   Then the
appropriate calculations (or table lookups) are done and the fuel is injected.
Obviously, squirt an incompressable liquid into a closed volume and the pressure
would go up a bit but I doubt this impacts the manifold pressure significantly.
However,  once that gasoline turns into a vapour it must exert some extra
pressure.  Is that what is calculated in the formula?

Or,  given that RH is really a derived term and the proper variable for moisture
is dew point are we more concerned about the change in pressure caused by the
air mixture (containing some water) warms up inside the intake manifold.
Wouldn't the MAP sensor detect this?

In the Grippo (and Bowling) formulas the larger the vp the smaller the pulse
width.  This probably implies that some of the pressure measured by the MAP
sensor is due to water content vapor pressure and therefore there is less oxygen
in the air charge for combustion and so less fuel is needed.  Question is,  how
much of an impact would that really make if the fuel injection system is tuned
sitting at the edge of the Pacific Ocean with perhaps 80%RH and then run inland
where the RH for the same ambient temperature is only 30%.  For that matter,
what happens when the engine is run when the temperature equals the dew point
(fog in the air)?

I guess I have to pull out the books again and figure this out a molecular
level.

Cheers,

John


>Date: Mon, 13 Sep 1999 17:45:46 EDT
>From: A70Duster at aol.com
>Subject: Re: Vapor Pressure of Water
>
[snip]

>Date: Tue, 14 Sep 1999 11:55:50 +1000
>From: Stuart Baly <S.Baly at bom.gov.au>
>Subject: Re: Vapour Pressure
>


[snip]

>Cheers,
>Stuart Baly.
>Bureau of Meteorology, Australia.
>
>