Oil & Fuel Fuel Tank and I am CONFUSED!

[rock2610D]

Did some rototilling last weekend and was somewhat confused about my fuel tank and what it did.

I was under the impression that the injection pump is gravity fed on my 2610D. But after about an hour in 70 degree weather of tilling I noticed my red fuel cap was about 3 to 4 inches LOWER than normal. I unscrewed the red fuel cap 1/2 turn(Aftermarket with no bleed hole) and the tank sucked air in and the fuel cap rose to normal location!!!

Can anyone tell how this can happen on a gravity fed fuel system. Something must have applied a signifigant vacuum to the fuel tank. Could the injector returns cause this?? It is apparent I have no fuel leaks or no vacuum would have been present.

If anyone can explain this to me I would appriciate it.

THANKS

 

[mark777]

Rock,

You need a vented cap.

 

[rock2610D]

I figured that but how on earth does so much vacuum get to fuel tank. I would have thought that at a certain point fuel would have just stopped flowing to pump. It did not, the engine just kept collapsing the fuel tank.

I would not think Hoyles would sell a cap that does not vent for a tractor that needs a vented cap.

Still wondering

 

[mark777]

Well Rock,

The truest analogy I can think of and relate to is a full sized fuel truck dumping fluid into underground storage tanks. I have witnessed the damage from suction to tankers. Almost always from the operator forgetting to pop the vent at the top of the tanker.
I'm not up on fluid science really, but the damage to these tanks is devastating and total, long before the fuel stops running from gravity.

 

[Tom_Veatch]

...
I was under the impression that the injection pump is gravity fed on my 2610D.
...

Can anyone tell how this can happen on a gravity fed fuel system. Something must have applied a significant vacuum to the fuel tank.
...
If anyone can explain this to me I would appreciate it.

I'll try.

"Gravity fed" is a somewhat misleading term. Fluids, such as diesel fuel, move in response to pressure differences, flowing from a location of higher absolute pressure to one of lower absolute pressure.

Assuming that pressure loss in the fuel line is negligible, the absolute pressure at the pump inlet is the sum of the absolute pressure on the surface of the fuel in the tank plus the gravity head due to the difference in height between the fuel level and the pump inlet.

That pressure forces fuel from the tank through the lines to the pump inlet. If the tank is not vented the pressure on the surface of the fuel is reduced below atmospheric as fuel flows from the tank. But, although the pressure in the tank is less than the pressure outside the tank, it doesn't mean that there is no pressure on the surface of the fuel. It just means that it's less than the normal atmospheric pressure of 14 to 15 PSI.

So, there is still pressure forcing fuel out of the tank into the pump inlet and fuel continues to flow until such time as the pressure in the tank is reduced to equal the vapor pressure of the fuel. At that point, the fluid begin to boil off into vapor.

It is not the same situation that would exist if the fuel line were disconnected from the pump inlet and opened to the atmosphere. In that case, fuel would stop flowing when the tank pressure plus gravity head became equal to the atmospheric pressure. However, in this case, the fuel at the pump inlet is not exposed to atmospheric pressure so the fuel just keeps flowing due to the residual pressure in the tank plus the gravity head.

If that's confusing, think of it this way:

You don't have any real difficulty turning a plastic bottle of water up and sucking the water out of it even though there is no vent allowing air to replace the water that's removed. The plastic container simply collapses just like your fuel tank collapsed. Even if the container were rigid - like one of the old glass coke bottles - you could still suck a significant amount of water out of the bottle before the pressure differential became greater than what your cheeks and jaw (which is exposed to atmospheric pressure) could overcome.

 

[Eddie]

On the 1700 I had there was a strainer that was under the fuel cap. It prevented the cap from sealing completely letting it vent. May be the same on the 2610.

 

[Eddie]

Like This:

Fuel Tank Screen: Yanmar Tractor Parts

 

[rock2610D]

Thanks for the detailed responses. Its nice of you guys to help my poor brain. It makes sense now! Basically the weight of the fuel was overcoming the ability of the plastic tank to maintain its geometric shape.

My tractor originally had a vented cap with a fuel level gauge on it. Guess I will have to purchase the strainer, or a new cap that will vent.

Live and Learn,,,, THANKS GUYS!!

 

[MJPetersen]

Or make a vent in the cap that you have. I have seen that done in a couple ways. (1) remove with a drill a small section 1/4" of the treads on the inside of the cap (could also be the outside tank threads, but better on the cap. (2) drill a hole in the side of the cap near the top. (3) Put a vented plug in the top of the cap.

Happy tilling. How is your turbo working. Are you still happy with it?

Mike

 

[Underwaterdog]

It never occurred to me that a gravity feed from a 1/4" line could implode a fuel tank. But the math tells the tale. Since I have my 2500 broken apart to replace the rear main seal, I easily got a rough measurement of the fuel tank, and the 2610 is roughly the same frame as the 2500 except for the powershift.
Its is late and I should not be doing this, but since we are getting technical here I can't resist. These are rough estimates but it still illustrates the "gravity" of the situation.
I measured my fuel tank and it is about 17" x 12" x 11". That relates to approximately 1036 square inches of surface area of the tank.
Pressure (PSI) = feet head x 0.433 x specific gravity
Specific Gravity of Fuel Oil = 0.85 approximately
Assume the tank was about 3/4 full. 12" is the height of the tank, so lets say we had 8" of fuel in the tank. Add another 4" from the bottom of the tank to the inlet of the injector. So now we have a nice round 1 foot of head to work with.
That gives us .368 lbs per square inch of pressure at the injector. Mulitply that times the surface area of an unvented tank 1036 sq. in., = 381.35 lbs., and multiplied times atmospheric pressure of 14.7lbs= 5605.84 lbs of total pressure pushing in on the outside of the tank. Since I remember from Rock's previous turbo charging post that he estimates his actual atmospheric pressure is 11 PSI due to altitude, I will say his actual atmospheric pressure is 74% of sea level pressure of 14.7 PSI. So it should be reasonable to reduce the 5605.84 lbs of total pressure to 74% or 4148.33 lbs of total available force to imploding the unvented tank. It obviously never reached anything even close to that figure, because the plastic tank was not capable of actually withstanding anything remotely close to that, and it imploded long before the pressure reached that level. Just be thankful that the tank was plastic and not metal, and was able to return to its original shape, or it would have been a useless crumbled mess. And there is always the chance that the fuel flow would have reduced to the point the engine would have died when the inlet pressure dropped. I can't help but smile when I think of the look on Rock's face when his fuel tank cap disappeared into the cowl . But, I did learn something here too. An unvented or blocked cap in a gravity feed application can certainly cause the fuel tank to implode. The line coming back to the tank from the injectors is supposed to be a return line. If I overfill my tank it leaks a little. That would probably be enough to vent. But apparently Rock's doesn't leak like mine. Now all my math may be bu##%*it, but it at least it sounds good. And if I made an error in the calculations I would love for someone, (Tom) to correct me.