Vapour Pressure

Stuart Baly S.Baly at bom.gov.au
Wed Sep 15 01:06:07 GMT 1999


>Date: Tue, 14 Sep 1999 09:00:56 -0700
>From: "John Dammeyer" <johnd at autoartisans.com>
>Subject: Vapor Pressure

[snip]

>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?

At 85F and 75% RH, the water vapor pressure is indeed 3.1kPa. But I'd 
almost wager that in the manifold, the vapor pressure will be less than 
ambient i.e. it will be 3.1*(MAP/atmospheric pressure), given that water 
vapour is throttled just the same as the rest of the air.

[snip]

>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)?

This is something I hadn't thought of before.
e.g. at 0C, RH=60%, water vapour pressure=0.37 kPa
at 35C, RH=100%, water vapour pressure=5.6kPa
in other words, about 5% difference in the oxygen content of the air.
You don't actually need fog for the RH to be 100%. The RH will be 100%, or 
very close, every time it rains.

Cheers,

Stuart Baly.

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