80 Fuel tank breather

[Frits Metz]

Interesting discussion here.

Perhaps I've missed it here, but does anybody know what the designed fuel tank pressure is? That means the strength of the spring in the check valve on the emission line?

Has anybody toiled (tampered) with this spring to reduce the tank pressure?

There are some notions mentioned briefly but not quantified, for instance:
- when taking cold fuel from underground storage at say 16°C, and allowing it to "climatise" to say 28°C, how much vapour is likely to be vented off from say a full tank?
- Understanding the flashpoint of petrol to be -43°C and the boiling point being +95°C, how are these increased by an increased tank pressure?
- What is the change in latent heat of evaporation for the changing fuel tank pressures, particularly in view of a sudden drop in pressure (when the filler cap is removed), resulting in a certain amount of superheat in the flashed fuel? Does anybody here have a Mollier Diagram for Petrol?

Then of course we can start evaluating the effects of blocked emission vents (check valves). How much pressure can we tolerate on a tank. After all, there is no certified relief valve on the system ...

How much thought has gone into thermal insulation of Petrol Tanks (Diesel tanks don't have such a big problem)?

 

[Frits Metz]

I've spent quite a bit of time reading up about this is the last couple of hours, and there is definitely something to be said for insulating a tank IMO (for non-EFI) for hot climates, and for NOT removing the check valve.

For starters, the RVP of gasoline is (in the US) supposed to be 7.0 psi (about 47kPa). This means that the gasoline should remain in liquid form below that pressure at 100°F or 37.8 °C.

This means that if the fuel in your tank is stable around 37.8 °C, and your tank does not have a vent, the pressure will be around 47kPa, and stay there.

Obviously that is quite a high pressure, and we don't keep the pressure there. In fact if the spring loaded check valve (pressure setting unknown) is replaced by a filter, there is zero pressure in the tank, which means it will continue vaporising fuel.

It must also be understood that since gasoline is a compound of various hydrocarbons, with various individual vapor pressures, the "lighter" ends obviously flash off first, and are vented off through the vent. Over time thus the inventory will "go flat" as the light ends are flashed off, and the RVP actually goes up (due to compositional changes).

Next thing is the temperature issue; as the temperature goes up, the RVP goes down. This means that the hotter the gasoline, the more vapour is formed and vented. This obviously means more inventory loss. This does however illustrate the importance of having a functional vent.

Other issues to consider:
- Fuel injection cars recycle huge amounts of hot fuel back to the tank, making tank insulation less beneficial.
- Leaving a car standing for a long time with a lot of fresh fuel, is not a good idea, as the calorific value depreciates as it flashes off.
- A pressure on your tank is beneficial for retaining your fuel quality, short of suggesting keeping the pressure as high as possible, one should definitely not remove the spring from the vent check valve.
- Agitation (through driving) also induces vaporisation of fuel, which can increase tank pressure.

From: http://www.epa.gov/oms/models/nonrdmdl/nr-001.pdf

..a day with an average temperature of 90F instead of 75F results in twice as much diurnal emissions, whereas in ARB's OFFROAD model the 90F day would increase diurnal emissions by a factor of 3

 

[Dave2000]

Well I found the valve often gets caked with mud ect and have seen this on quite a few 80's that are used 'seriously', none of them showed any fault during driving (petrol or diesel) before or after although my experience is more diesel led. On mine and a few other diesels I have removed the valve altogether and fed it along the chassis rails and up alongside the bulkhead, a push in 'inline' fuel filter keeps out any rubbish. I am not sure if it is a good idea to vent petrol into the engine department, might be a tad bad if the car rolls and drains the fuel onto a hot exhaust ect but I do not see diesel as being a such a problem, when I eventually get the snorkel fitted I will run the tubes up the inside as per my last Discovery, it is tidy and stays clean.

regards

Dave

 

[Frits Metz]

Dave, diesel doesn't exhibit the same problem, because it has a vapour pressure of +62°C, and an even higher RVP.

 

[Dave2000]

Dave, diesel doesn't exhibit the same problem, because it has a vapour pressure of +62°C, and an even higher RVP.

Thanks for the tech info about 'why' it is not a problem

regards

Dave

 

[Dave Docwra]

Dave, It would be less off a risk to vent petrol onto a hot manifold than diesel.

 

[Dave2000]

Dave, It would be less off a risk to vent petrol onto a hot manifold than diesel.

Very possible but add a few sparks from the ignition system and you have all you need for a fire with all that vapour drifting around under a hot bonnet. I would not fancy venting my petrol tank into my engine bay but have done it with diesels and never had a problem. Of course we also know that diesel tends to burn slowly when compared to petrol and is a lot less volatile/lower flash point.

regards

Dave

 

[Dave Docwra]

Petrol will not ignite on a hot manifold it would evaporate but diesel will ignite, so many people under estimate how volatile diesel is when it comes into contact with a heated surface, obviously a spark is a different fire starter altogether but you did refer to a manifold earlier.

You would be amazed how many people have their insurance claim rejected after stating a cigarette they were smoking caused the vehicle fire whilst working on the petrol system.

Regards Dave.

 

[Dave2000]

My apologies I should not have generalised,

regards

Dave

 

[nielsc]

Hey all, I did not want to open a new topic so I will post here:

I have also noticed when filling up my 80 that when I unscrew the tank cap i get alot of suction noise, and after being filled up, my engine picks up noticably better when leaving the fuel pump.

I do approx. 300 km to when the fuelgauge is halfway, then 150 - 200km untill its empty.
Could it be that I have a bad fuel economy (18MPG) due to the fuel tank being at a vacuum when the tank gets more empty?
I have just cleaned the breather in the filler cap and the little mesh at the top was really dirty (black).

 

[Towpack]

Petrol will not ignite on a hot manifold it would evaporate
Regards Dave.

Don't count on it. I've seen it happen with an overfilled motorcycle petrol tank with spillage on to the hot headers. Diesel has a higher flashpoint than petrol but a lower autoignition temperature (the temp at which it will ignite without an ignition source) but the autoignition temp of petrol is easily reached by a hot exhaust manifold IMO.

 

[clivehorridge]

Don't count on it. I've seen it happen with an overfilled motorcycle petrol tank with spillage on to the hot headers. Diesel has a higher flashpoint than petrol but a lower autoignition temperature (the temp at which it will ignite without an ignition source) but the autoignition temp of petrol is easily reached by a hot exhaust manifold IMO.

Notwithstanding the chemistry of it all, which is very interesting, I had an old Chrysler 180 once (looked like a Hillman Avenger on steroids) and one morning the auto-choke on the carb stuck on. It was pulling neat petrol through for a while and because most of that was going un-burned straight into the exhaust, leaks in the exhaust manifold/head gasket had allowed liquid petrol to gather on the outside of the top of the exhaust manifold in pools.

Of course the manifold got hotter in the 10 minutes it took to get to running temp, and just at the optimum moment, the pools of petrol ignited.

The ensuing open fire under the bonnet burned out the main wiring harness as it came through the bulkhead and also melted the plastic clutch fluid pipe going from the master to the slave.

Needless to say, it all got rather hot under there especially when I "cluched" to change gear and injected hot clutch fluid into the inferno.

Be it diesel or petrol, when its in the wrong place at the wrong time and it's hot, something's gotta give and it won't be pleasant. JMHO...

 

[Bat21]

Just trawled through this thread and can't find the answer I'm looking for.
Where about is the tank breather located? Is it under the truck or in the engine bay somewhere?

 

[Towpack]

I believe it’s rubber pipe underneath the truck and vents into one of the chassis/body rails.

 

[Bat21]

I believe it’s rubber pipe underneath the truck and vents into one of the chassis/body rails.

Thanks TP. Can anyone be more specific as to were this is?.... I will have a crawl about tomorrow maybe and have a look.

 

[Towpack]

Well, on mine, the breather pipe exits the rear of the tank, more or less alongside the filler pipe, goes straight into what must be the check valve and then into a hole in the chassis cross member just forward of the rear axle.

 

[Bat21]

Nice one... that gives me an idea where to look.

 

[Bat21]

Had a play about with my tank and breather earlier and it seems the tank does indeed run under positive pressure.

Really confused now as to where it gets this pressure from as the fuel returning back is going to be less than is drawn off.