Fuel

[Peva]

Peva,
Basic physics can not be avoided - the fuel tank must be subject to pressure equalisation, as the fuel level drops/atmospheric/diurnal changes occur - otherwise it will either collapse or attempt to expand (subject to construction limitations).
Atmospheric air and with it water vapour is being allowed to enter the system you have illustrated.
Further the above system is about reducing fuel vapours entering the atmosphere (exiting the tank) not about air entering the fuel tank, to replace the lost volume due to consumption and equalise internal pressure due to atmospheric and or temperature changes.

We're probably not going to agree.

I addressed the differences and quoted the factory manual explicitly stating that the system is closed off to the atmosphere with engine off by the n.c. (closed when de-energized, i.e., ignition off) valves.

With some further reading, I see that the diurnal temperature changes are - to some degree - accommodated by the volume of the canister allowing some pressure (or vacuum) buildup from vapor expansion/contraction without tank damage. I don't know how they handle larger pressure changes due to opposite-direction temperature extremes - they don't address that. The idea is that it *reduces* the exchange but within reason in case of unusual conditions. They are constantly adding requirements to evap. systems to squeeze every bit of planet-saving goodness out of our wallets. There may be two-way over-/under-pressure relief valves (lightly spring loaded bi-directional check valves or similar) to maintain some degree of cost and design practicality while accomplishing the major part of the goals.

The fuel level dropping with usage only means filling the tank space once with air with each tank full - not the same volume change multiple times through the twice daily temperature fluctuations if the car sits *unused* for several months. Compare that to an old school fuel system with nothing but a tank and an open hole in the gas cap where volatiles evaporate off and air gets freely pulled in with each temperature cycle allowing the fuel to degrade over a period of months with multiple air exchanges on the same tank of fuel. It was the same with cars with vented gas caps before evap. systems were required.

It is known that fuel will not go stale in a modern car system in anywhere near the relatively short time period that it will in lawn mowers unless the lawnmower has an evap. system similar to a modern car.

 

[Peva]

Peva,
Basic physics can not be avoided - the fuel tank must be subject to pressure equalisation, as the fuel level drops/atmospheric/diurnal changes occur - otherwise it will either collapse or attempt to expand (subject to construction limitations).
Atmospheric air and with it water vapour is being allowed to enter the system you have illustrated.
Further the above system is about reducing fuel vapours entering the atmosphere (exiting the tank) not about air entering the fuel tank, to replace the lost volume due to consumption and equalise internal pressure due to atmospheric and or temperature changes.

Taking one more pass at this - I think where we're talking past each other is that you think there has to be a valve or breather left open to allow for fuel usage emptying the tank and the diurnal temperature changes, while I see in the factory service manual that the valves are in fact closed with ignition off, but I see (read about) the canister adding volume to reduce the pressure changes due to temperature changes, and I suspect that there may be low pressure relief devices to handle extreme temperature changes - which would have a very different effect than a zero-restriction hole in the system, or vented fuel cap, or a normally open valve to atmosphere.


EDIT: I'm reading discussions that say that some cars have vented gas caps - and that in the context of evaporative emissions, a "vented" cap doesn't mean a cap with a hole in it, but the cap has a simple bi-directional overpressure valve built in that allows *some* pressure buildup *BUT*, beyond some amount of pressure, allows vapor/air to squeak by to reduce the in-tank to ambient-air pressure differential. Other cars have a sealed (non-vented) cap but have to have a similar over-pressure relief device built into the canister plumbing instead

So - we were both right. It allows partial pressure equalization, but not total free flow like a plain old hole would do. It's a compromise.

So the car whose FSM I was quoting from would have to have the vented gas cap since the evap. system valves do close with ignition off with no mention of the pressure relief device. I searched for some mention of gas cap being vented or not, but found none. I'm convinced it is vented (not with a simple hole, but with a simple pressure relief device built in).

 

[Skippydiesel]

We're probably not going to agree.

I addressed the differences and quoted the factory manual explicitly stating that the system is closed off to the atmosphere with engine off by the n.c. (closed when de-energized, i.e., ignition off) valves.

With some further reading, I see that the diurnal temperature changes are - to some degree - accommodated by the volume of the canister allowing some pressure (or vacuum) buildup from vapor expansion/contraction without tank damage. I don't know how they handle larger pressure changes due to opposite-direction temperature extremes - they don't address that. The idea is that it *reduces* the exchange but within reason in case of unusual conditions. They are constantly adding requirements to evap. systems to squeeze every bit of planet-saving goodness out of our wallets. There may be two-way over-/under-pressure relief valves (lightly spring loaded bi-directional check valves or similar) to maintain some degree of cost and design practicality while accomplishing the major part of the goals.

The fuel level dropping with usage only means filling the tank space once with air with each tank full - not the same volume change multiple times through the twice daily temperature fluctuations if the car sits *unused* for several months. Compare that to an old school fuel system with nothing but a tank and an open hole in the gas cap where volatiles evaporate off and air gets freely pulled in with each temperature cycle allowing the fuel to degrade over a period of months with multiple air exchanges on the same tank of fuel. It was the same with cars with vented gas caps before evap. systems were required.

It is known that fuel will not go stale in a modern car system in anywhere near the relatively short time period that it will in lawn mowers unless the lawnmower has an evap. system similar to a modern car.

Oaky - all sound a bit hopeful/jingoistic to me. I don't disagree. I get the principal just not the practicalities. Question effectiveness to effectiveness -especially if following practise not observed.

The fuel companies advice to me, regarding fuel going off/stail, is fill to 75 % or better (minimise air gap). This will slow evaporation & loss of volatile components. It will also reduce the amount of atmospheric air/cycling and terror hugely reduce chances of condensation.

Also, I don't think you can compare a car volume of fuel with what might be in a mower - the larger the starting volume, over a given time - the less effect volatile loss will impact on fuel quality/performance.

 

[Peva]

Oaky - all sound a bit hopeful/jingoistic to me. I don't disagree. I get the principal just not the practicalities. Question effectiveness to effectiveness -especially if following practise not observed.

The fuel companies advice to me, regarding fuel going off/stail, is fill to 75 % or better (minimise air gap). This will slow evaporation & loss of volatile components. It will also reduce the amount of atmospheric air/cycling and terror hugely reduce chances of condensation.

Also, I don't think you can compare a car volume of fuel with what might be in a mower - the larger the starting volume, over a given time - the less effect volatile loss will impact on fuel quality/performance.

Makes sense about keeping higher levels of fuel to keep *ratio* of volume of fuel to volume of open space (air + other vapors) in the tank relatively high.

On what you said about effects being different with fuel volume of car vs. less fuel volume with a lawnmower, I would say it's not volume of fuel per-se (say, 17 gallon capacity with actual average actual 13 gallons for car vs. 1.7 gallon capacity with actual average 1.3 gallons for a lawnmower, or scale the tank sizes for whatever car and lawnmower you want to compare) but the *ratio* of volume of fuel to volume of open (air + vapor) space in the tank, so in that sense the effects should be similar - but that of course would only be true if *assuming* both car and lawnmower have similar degree of "sealing", which, for this discussion we've been assuming *not* similar in that regard (i.e., an EPA compliant evaporative emissions system on the car, but a simple fuel tank plus gas cap with a simple unrestricted/free-flowing/breathing vent hole on the lawnmower).

I would argue that we've both done our best at honest discussion. I take exception to your use of the word "hopeful" as it implies that I'm trying to force the conclusion that I want, reality be damned. With the help of your experience and knowledge, I actually searched for and found explanations for the realities and adjusted my understanding accordingly - and I now understand the whole subject better than I did. Dare I say that the same could be said of you?

I don't understand your use of the word "jingoistic" with my understanding of the meaning (dictionary definition) of the word.