Fuel System Design Question

[pwratch]

Are there any issues associated with placing a fuel tank significantly above the engine and filter system? Something on the order of 6ft above?

[Captain Bill]

It depends on the size of the tankage. A large enough tank will raise your center of gravity significantly when full, which would affect the vessel's stabilty and ride comfort.

[Morganministry]

Shouldn't be a problem. This will allow you to "gravity feed" your fuel system. Make sure to put a shut off valve at the tank to stop fuel flow when needed.

Scott

[s/v Jedi]

Well, someone should do the math to find how much pressure that would be down at the engine. I think 7 psi is the maximum safe pressure. Also, the return-line will be pressurized at that same level, something that needs to be checked with the engine spec's.

When I pressurize my fuel system at 7 psi without the engine or genset running, I hear diesel fuel flowing back into the tank from the return-line, meaning it gets pushed through engine lift- and injection-pumps.

cheers,
Nick.

[captain58sailin]

Head pressure with water is .433 psi per foot of elevation, with diesel it will be a small amount less, so your head pressure would not be an issue, I have never heard of a head pressure problem regarding diesel delivery, usually it is a starvation issue.

[DHHong Kong]

The potential problem is the head of fuel causing fuel leakage thru any one (or more) of the injectors, partially filling the cylinder space above the piston(s). When the engine is cranked and this fuel cannot be compressed you will get a hydraulic lock, with a probable bent con-rod or worse, if the engine fires on a couple of cylinders first, a rod thru the side of the block. It is a bit of a gamble though shut off valves, if used each time the engine is stopped, should assist.
Good luck!!
Paul

[klem]

Having a fuel system with head pressure used to be common before modern fuel pumps although the head was usually less than 6'. Depending on the engine, different pressure at the injection pump (or high pressure pump in the case of common rail) is desirable. Older mechanical engines often run between 20 and 40psi whereas newer common rail engines are often under 10psi. A 6' head would not create enough pressure to be a problem on any engine that I can think of but it could be a problem with more head since this is a pressure that cannot be dealt with by your overflow valve.

The issue with fuel being delivered when you do not want it is engine specific but should not be a problem on an engine in good conditions. Provided that your injectors seal well, you will never reach pop off pressure (or they will not be actuated for common rail). Fuel pumps will allow fuel flow at these pressures but a well designed IP will not. It is something to think about if you are considering this system.

This does beg the question, why is the tank going so high? Unless the boat is really deep with the engine mounted really low, you could have problems such as stability, height of deck fill, etc. So what is the engine and application?

[r.furborough]

Are you talking about a diesel tank or gasoline tank?

If gasoline and a carburetted engine like an Atomic 4 or Palmer P60, then NO, do not do this.

If a diesel/fuel injected engine, then as long as the fuel delivery pressure is within the manufactureres specs for the engine you should be OK.

How big is the tank and how big is the boat? Be careful of raising the center of gravity too much.....

[ConradG]

A gravity feed day tank is a great thing as far as reliability goes. The onlt problem becomes if you have a leak in the lines. It will drain right out. The other problem is that the athorities don't like it and niether will your insurance company. Most won't know or look. A tank that holds enough fuel for a given amount of time (your choice) sort of lessens the potential clean up problem and keeps the ctr of gravity issue under control especially if mounted on the centerline. Just remember to fill it as part of your daily check or if using for a long period. You can also use a simple shut off valve for safe keeping but it has to be remembered, obviously. The isssue then becomes the fuel return. If your engine returns alot of fuel it can become rather hot in hot climates especially. Most mechanical ips don't move that much fuel. Some comonrails do. Some engines are finicky with hot fuel. Hot fuel will also be more prone to lighting if there is a leak. Think about tank placement.

[Laidback]

Originally Posted by pwratch

Are there any issues associated with placing a fuel tank significantly above the engine and filter system? Something on the order of 6ft above?

Is this tank the only fuel tank? Or is it a header tank which filled from the main fuel tank located at a similar level to that of the engine?
If it is a header tank, then excess fuel is returned to the main tank.

[David M]

Daytanks are in common use in commercial vessels which have to meet higher safety standards than non-commercial vessels.

[pwratch]

Thanks for the responses! The head pressure "problem" is one that occurred to me as a result of a recent experience with a gas outboard that flooded sometimes when the gas tank was filled early in the morning and then left at the dock with the fuel vent closed to prevent the now-full tank from spilling. The pressure apparently forced gas into the system and flooding, of course, was the result. More a nuisance than a real problem but it caused me to think again about a proposed diesel setup. The system under consideration would have a single tank (no header) of 25-30 gallon capacity and a modern diesel as client. The shut-off valve is a good idea, of course and I shall research the output specs (if avail.) for the return system. Thanks again for the insights.

[ConradG]

Depending on your fuel burn and your personal preferances, you can get by with a relatively small tank, say 10 gal, as your main tank that suplies your engine and that your engine returns to. If you burn a galon/hr and regularly use it only for a few hrs/day there's no problem. If you are using it for longer than 6 hr stints and your fuel gets too hot to keep your engine happy you can take a few strokes on the hand pump to fill it back up with fuel from your storage tanks. Another advantage can be that if you have two storage tanks, a comon setup for weight distribution, and a selector valve to your fill pump, if you get contaminated fuel somewhere, you can choose the other tank to fill from and to clear out any that gets in the raised day tank is simple and a small amount. But don't forget that shut off valve on the elivated tank. It is needed to change a filter or to do any work on the lines!

[Chief Engineer]

On the vessel that is my avatar...we have "day tanks" for the Mains and Gensets.

They are about 10 feet above the fuel pumps on the engines.

On one tug I worked on we had a day tank for a Detroit Diesel Genset.
Didn't have a fuel pump on the engine.....the head pressure of the fuel did all the work.

I might suggest two fuel valves one at tank one before fuel filter.

Post #2345...wierd....

[Morganministry]

Originally Posted by r.furborough

If gasoline and a carburetted engine like an Atomic 4 or Palmer P60, then NO, do not do this.

What would be the issue with a carburated engine?

I know of a couple of times where a fuel pump went out on an old Scout that we had. We rigged a fuel line to a 5 gallon container that was set on the hood. The line went strait to the carburator. The engine ran fine and we drove home with the jug strapped to the hood.

Scott