Fuel cap venting

[gmctd]

Fuel cap venting - basic fuel cap venting theory also applies to the gasoline-fueled vehicles

Barometric pressure is Atmospheric pressure, measured in pounds per square inch actual, or psia, 0 - 15psia (rounded up from 14.696psia mean) - 14.696psia BP, or one atmosphere, = 1 BAR

Vacuum is any reduction in atmospheric pressure, measured in inches on the mercury column, or "HG, where 0"HG vacuum = 15psia, and 30"HG vacuum = 0psia - 2"HG vacuum = 1psia, so 2"HG would be (15psia - 1psia) = 14psia

Gravity and atmospheric pressure on the surface of a liquid enhances an effect known as surface tension - aeration is ability of a liquid to foam - thus water boils at ~220* at sea level* - water boils at lower temperature as altitude increases, higher temperature as altitude decreases - your radiator cap is proof enuff of that, as greater pressure increases surface tension, raising coolant boiling point - also ensures the t-stat opens at the rated temperature, as IIRC, water boils at ~202*F in Denver, the mile high city - always use a thermometer in the pan of boiling water when verifying t-stat operation in the kitchen

Foaming is aeration you can see, the bubbles being large enuff to break thru the surface tension of the liquid - as the bubbles decrease in size, they have greater difficulty in breaking surface tension to emerge thru the surface, thus remaining submerged within the liquid

Diesel fuel foams easily - the hotter the ambient temperature, the warmer the fuel, the greater the foaming, the greater the evaporation, so the tank pressurizes easily in warm climate, but never more than 1psi above BP, if the fuel cap is functional - conversely, Diesel fuel foams less when cold, so the tank pressurizes less easily in cold climes

Gasoline, being more volatile than Diesel fuel, reacts to temperature more due to increased evaporization, thus pressurizing the tank more - the 1psia pressure vent is as important here as that for Diesel fuel

Actual fuel cap venting is (+/- 1psia) - 1psia = 2"HG vacuum, so your fuel cap will vent at (rounding Barometric pressure to 15psia) 16psia overpressure, also at 14psia underpressure

When tank is full of fuel, little free-air volume remains between the surface of the fuel and the top of the tank, so any hiss will be small - as fuel level drops, free-air volume above the fuel increases, so the hiss will be greater - this indicates greater volume of air in the tank, not greater pressure or vacuum, where 1ft3 @ 1psia vents quicker than 35ft3 @ 1psia

The tank and cap was designed to also maintain ~2"HG vacuum, intended to reduce aerated fuel, which is foaming you cannot see, the bubbles being too small to break thru fuel surface tension - since each bubble below the surface of the fuel is a tiny microcosm of atmospheric pressure, any reduction in BP above the surface will reduce surface tension, allowing the smaller bubbles to break thru, thereby reducing aeration

The fuel cap should be checked regularly to ensure it meets correct pressure\vacuum venting specs - can use a radiator-pressure hand pump and a vacuum hand pump

*Note: the boiling point of H20 is ~212*F\100*C at mean sea level, tho the actual temp depends on altitude, which relates to Barometric pressure - the downer you go, the upper the temp, for instance Death Valley - with global warming, the increasingly warm atmosphere is expanding, becoming less dense, such that mean Barometric Pressure is decreasing - AGA mean BP standard reference of 14.72xpsia at sea level was changed to 14.696, accordingly - mean sea level will be changing as polar ic caps melt, further decreasing BP as less ice means warmer air

 

[JeepJeepster]

Fuel cap venting - basic fuel cap venting theory also applies to the gasoline-fueled vehicles

Barometric pressure is Atmospheric pressure, measured in pounds per square inch actual, or psia, 0 - 15psia (rounded up from 14.696psia mean) - 14.696psia BP, or one atmosphere, = 1 BAR

Vacuum is any reduction in atmospheric pressure, measured in inches on the mercury column, or "HG, where 0"HG vacuum = 15psia, and 30"HG vacuum = 0psia - 2"HG vacuum = 1psia, so 2"HG would be (15psia - 1psia) = 14psia

Gravity and atmospheric pressure on the surface of a liquid creates an effect known as surface tension - aeration is ability of a liquid to foam - thus water boils at 220* at sea level - water boils at lower temperature as altitude increases, higher temperature as altitude decreases - your radiator cap is proof enuff of that, as greater pressure increases surface tension

Foaming is aeration you can see, the bubbles being large enuff to break thru the surface tension of the liquid - as the bubbles decrease in size, they have greater difficulty in breaking surface tension to emerge thru the surface

Diesel fuel foams easily - the hotter the ambient temperature, the warmer the fuel, the greater the foaming, the greater the evaporation, so the tank pressurizes easily in warm climate, but never more than 1psi above BP, if the fuel cap is functional - conversely, Diesel fuel foams less when cold, so the tank pressurizes less easily in cold climes

Gasoline, being more volatile than Diesel fuel, reacts to temperature more due to increased evaporization, thus pressurizing the tank more - the 1psia pressure vent is as important here as that for Diesel fuel

Actual fuel cap venting is (+/- 1psia) - 1psia = 2"HG vacuum, so your fuel cap will vent at (rounding Barometric pressure to 15psia) 16psia overpressure, also at 14psia underpressure

When tank is full of fuel, little free-air volume remains between the surface of the fuel and the top of the tank, so any hiss will be small - as fuel level drops, free-air volume above the fuel increases, so the hiss will be greater - this indicates greater volume of air in the tank, not greater pressure or vacuum

The tank and cap was designed to also maintain ~2"HG vacuum, intended to reduce aerated fuel, which is foaming you cannot see, the bubbles being too small to break thru fuel surface tension - since each bubble below the surface of the fuel is a tiny microcosm of atmospheric pressure, any reduction in BP above the surface will reduce surface tension, allowing the smaller bubbles to break thru, thereby reducing aeration

The fuel cap should be checked regularly to ensure it meets correct pressure\vacuum venting specs - can use a radiator-pressure hand pump and a vacuum hand pump

Who wrote that?

Gravity and atm pressure does not cause surface tension. Surface tension occurs from the attraction between the molecules of the liquid. Ever thought of what water looks like in space?

Boiling(bubbles) is the vapor of that liquid. With water the bubbles would be water vapor, not sure if you could call that aeration. Boiling occurs when the vapor pressure is higher than that of the atm pressure around it.

 

[CHUD]

With water the bubbles would be water vapor, not sure if you could call that aeration.

I have experienced water aeration with my pool pump. It occurs when there is a hole in the suction side of the pump that allows air to enter the pump & mix with the water. The water will then exit on the return side of the pump with a layer of fine white foam on the top. No boiling involved whatsoever.

 

[gmctd]

THIMK!

Liquids in the vacuum of space (that don't quickly freeze) quickly "boil off" to way less than even a fine mist, incl H2O - no gravity, no pressure - frozen liquids would still boil off, but at much slower rate - remember how a vacuum pump is used to also evaporate moisture (H2O) in closed refrigeration systems - p'raps yer thinking(?) of water droplets in gravityless flight, where the cabin is pressurized at some point near atmospheric pressure, such that the water remains cohesive with max droplet size formed according to pressure level - slowly reduce the pressure toward 0 and the water slowly disappears into finer and finer mist, even if gravity was keeping the decreasing liquid quantity pooled within some open container, such as a bottle

Altho, perhaps I should have used the term 'enhances' rather than 'creates'?

Besides, you may be missing the point - slight vacuum in the fuel tank reduces aeration - do not defeat the valves in the vented fuel cap because of the hiss when removing the cap - venting is one of the emissions tests in the DOT vehicle inspection procedures

p.s. - I corrected the post, thanks for calling that major faux pas to my attention

 

[tjkj2002]

Gassers don't create a vacuum in the gas tank,they pressurize it just slightly(fuel injected only),otherwise the EVAP system would not work(it would flow backwards defeating the purpose).

 

[gmctd]

Because gasoline does not foam like Diesel fuel, some points referencing vacuum in the post can be ignored - however, the gasser (patooie!) KJ's do have a vacuum pump connected to the fuel tank for on-board emissions testing, even having a vacuum-specific DTC for it - also, the fuel cap must pass a vacuum test or be replaced before the DOT emissions sticker can be issued

 

[Ry' N Jen]

Last time I checked, water boils at 212° Fahrenheit/ 100° Celsius and not 220° as you state!

 

[tjkj2002]

Because gasoline does not foam like Diesel fuel, some points referencing vacuum in the post can be ignored - however, the gasser (patooie!) KJ's do have a vacuum pump connected to the fuel tank for on-board emissions testing, even having a vacuum-specific DTC for it - also, the fuel cap must pass a vacuum test or be replaced before the DOT emissions sticker can be issued

The vacuum pump on the gas KJ's is used solely to test for EVAP system leaks.

 

[tommudd]

The vacuum pump on the gas KJ's is used solely to test for EVAP system leaks.

you forgot to say (patooie) gas KJs Troy what ever he means by that

 

[gmctd]

My bad on the boiling point of H20 (left out the squiggly ~, indicating approximately), tho the actual temp depends on altitude, which relates to Barometric pressure - the downer you go, the upper the temp, for instance Death Valley - with global warming, the increasingly warm atmosphere is expanding, becoming less dense, such that mean Barometric Pressure is decreasing - AGA mean BP standard reference of 14.72xpsia at sea level was changed to 14.696 - mean sea level will be changing as polar ic caps melt, further decreasing BP as less ice means warmer air

So, scene from an old western cowboy movie, transposed to current times and modern steeds:

Picture the grizzled old-timer lifer Diesel mechanic in grimy overalls, squinty-eyed, needing a shave, gray stubble enhancing his firm jaw-line, favorite chaw residing in his cheek, wiping his heavy-work callused hands with dirty red shop-towel while answering newbie questions about a newly-acquired Cummins\equiv - each time the debutante compares what he thinks he knows about gasoline-fueled engines to his recent Diesel-powered acquisition, the old-timer listens, squints, glancing up at the sky, looks at the knows-little dufus, utters a disgusted "gassers - patooie!" precisely aiming a stream of 'baccy-juice between the newbie's feet - would have a tendency to encourage putting the brain in gear before putting the mouth in motion, eh

And, that's the.....rest......of the story.....................

 

[Ry' N Jen]

Picture the grizzled old-timer lifer Diesel mechanic in grimy overalls, squinty-eyed, needing a shave, gray stubble enhancing his firm jaw-line, favorite chaw residing in his cheek, wiping his heavy-work callused hands with dirty red shop-towel while answering newbie questions about a newly-acquired Cummins\equiv - each time the debutante compares what he thinks he knows about gasoline-fueled engines to his recent Diesel-powered acquisition, the old-timer listens, squints, glancing up at the sky, looks at the knows-little dufus, utters a disgusted "gassers - patooie!" precisely aiming a stream of 'baccy-juice between the newbie's feet - would have a tendency to encourage putting the brain in gear before putting the mouth in motion, eh

And, that's the.....rest......of the story.....................

And just what does that have to do with the price of tea in China?
I think I'll smoke some of my Chemo weed and then read this all over again... Maybe then I can make some sense out of it!